GENERAL BUILDING HEIGHTS AND AREAS

Section 501 General

Section 503 General Building Height and Area Limitations

Section 504 Building Height and Number of Stories

Section 505 Mezzanines and Equipment Platforms

Section 506 Building Area

Section 507 Unlimited-Area Buildings

Section 508 Mixed Use and Occupancy

Section 509 Incidental Uses

Section 510 Special Provisions

Key Points

Section 501 General

501.2   Address identification.   In this section, the International Building Code® (IBC®) intends that buildings be provided with plainly visible and legible address numbers posted on the building or in such a place on the property that the building may be identified by emergency services such as fire, medical, and police. The primary concern is that responding emergency forces may locate the building without going through a lengthy search procedure. In furthering the concept, the code intends that the approved street numbers be placed in a location readily visible from the street or roadway fronting the property if a sign on the building would not be visible from the street. Address numbers may be required in multiple locations to help eliminate any confusion or delay in identifying the location of the emergency. The fire code official can require, and must approve, additional address identification locations. Regardless of the sign’s location, the minimum height of letters or numbers used in the address is to be at least 4 inches (102 mm) and contrast to background itself. Additional criteria are provided to provide consistency and clarity of the identifying numbers.

Section 503 General Building Height and Area Limitations

The IBC regulates the size of buildings in order to limit to a reasonable level the magnitude of a fire that potentially may develop. The size of a building is controlled by its floor area and height, and both are limited by the IBC. Whereas floor-area limitations are concerned primarily with property damage, life safety is enhanced as well by the fact that in the larger building there are typically more people at risk during a fire. Height restrictions are imposed to also address egress concerns and fire department access limitations.

The essential ingredients in the determination of allowable areas are:

1.   The amount of combustibles attributable to the use that determines the potential fire severity.

2.   The amount of combustibles in the construction of the building, which contributes to the potential fire severity.

In addition to the two factors just itemized, there may be other features of the building that have an effect on area limitations. These include the presence of built-in fire protection (an automatic fire-sprinkler system), which tends to prevent the spread of fire, and open space (frontage) adjoining a sizable portion of the building’s perimeter, which decreases exposure from adjoining properties and provides better fire department access.

A desirable goal of floor-area limitations in a building code is to provide a relatively uniform level of hazard for all occupancies and types of construction. A glance through Table 503 of the IBC will reveal that, in general, the higher hazard occupancies have lower permissible areas for equivalent types of construction and, in addition, the less fire-resistant and more combustible types of construction have more restrictive area limitations.

The IBC also limits the maximum height and number of stories based on similar reasons discussed for area limitations. In addition, the higher the building becomes, the more difficult access for firefighting becomes. Furthermore, the time required for the evacuation of the occupants increases; therefore, the fire resistance of the building should also be increased.

The code presumes that when the height of the highest floor used for human occupancy exceeds 75 feet (22,860 mm), the life-safety hazard becomes even greater because most fire departments are unable to adequately fight a fire above this elevation from the outside. Furthermore, the evacuation of occupants from the building is often not feasible. Thus, Section 403 prescribes special provisions for these high-rise buildings. Similar concerns for buildings with occupied floors well below the level of exit discharge are addressed in Section 405 for underground buildings.

Coming back to this section, the code specifies in Section 504 both the maximum allowable height in feet (mm) and the maximum number of stories. The maximum allowable height in feet is regulated by Table 504.3 primarily by the building’s construction type, with limited regard for the occupancy or multiple occupancies located in the building. However, the maximum allowable height in stories can vary significantly based on the occupancy group involved as set forth in Table 504.4. Where multiple occupancies are located in the same building, and the provisions of Section 508.4 for separated occupancies are utilized, each individual occupancy can be located no higher than set forth in the table. See Figure 503-1. Where the nonseparated-occupancies provisions of Section 508.3 are applied, the most restrictive height limitations of the nonseparated occupancies involved will limit the number of stories in the entire building. See Figure 503-2. In general, the greater the potential fire- and life-safety hazard, the lower the permitted overall height in feet (mm), as well as the fewer the number of permitted stories.

Figure 503-1   Height limitations—separated occupancies.

Figure 503-2   Height limitations—nonseparated occupancies.

503.1   General.   Once the occupancy group(s) and type of construction have been determined, the limitations in Tables 504.3, 504.4, and 506.2 must be consulted individually. Allowable building height, as set forth in Table 504.3, is based on height in feet above grade plane. Both “building height” and “grade plane” are defined in Chapter 2 to ensure consistent application of the provisions. The table includes the increased height allowances that are available for most types of sprinklered buildings. The allowable number of stories is addressed in Table 504.4, presented in a format generally consistent with that used for Tables 504.3 and 506.2. Again, definitions are very important, and in this case the term “story above grade plane” must be fully understood to apply the provisions appropriately. An increase in allowable stories for most types of sprinklered buildings is also applied within Table 504.4.

The determination of maximum allowable building area is initiated in Table 506.2 through the identification of the appropriate allowable area factor. The factor varies based upon occupancy classification, construction type, installation of an automatic sprinkler system, and number of stories in the building, and is applied on a per story basis. Unlike the determination of building height and number of stories, the limitation in the table can be further increased due to the presence of adequate frontage at the building’s perimeter. A more detailed analysis can be found in the discussion of Sections 504 and 506.

In this section, the IBC also indicates that fire walls, in addition to exterior walls, create separate buildings when evaluating for allowable height and area. Defined and regulated under the provisions of Section 706, the function of a fire wall is to separate one area of a building from another with a fire-resistance-rated vertical separation element. Where a fully complying fire wall is provided, it provides two compartments, one on each side of the wall, which may each be considered under the IBC to be separate buildings. Multiple fire walls may be utilized to create a number of separate buildings within a single structure. One of the resulting benefits of the use of a fire wall is that the limitations on height, number of stories, and area are then addressed individually for each separate building created by fire walls within the structure, rather than for the structure as a whole.

503.1.1   Special industrial occupancies.   This special provision exempts certain types of buildings from both the height limitations and the area limitations found in Sections 504 and 506. Thus, the type of construction is not limited, regardless of building height or area. It is also not necessary to comply with the provisions of Section 507 for unlimited-area buildings to utilize this provision. Applicable to structures housing low-hazard and moderate-hazard industrial processes that often require quite large areas and heights, the relaxation of the general provisions recognizes the limited fire severity, as well as the need for expansive buildings to house operations such as rolling mills, structural metal-fabrication shops, foundries, and power distribution. It is not the intent that buildings classified as Group H occupancies be addressed under the allowances of Section 503.1.1.

503.1.2   Buildings on the same lot.   Where two or more buildings are located on the same lot, they may be regulated as separate buildings in a manner consistent with buildings situated on separate parcels of land.

As an option, multiple buildings on a single site may be considered one building, provided the limitations of height, number of stories, and floor area based on Sections 504 and 506 are met. The height and number of stories of each building and the aggregate area of all buildings are to be considered in the determination. Under this method, the provisions of the code applicable to the aggregate building shall also apply to each building individually. Further regulations for buildings on the same lot are discussed in the commentary for Section 705.3.

Section 504 Building Height and Number of Stories

Because automatic fire-sprinkler systems have exhibited an excellent record of in-place fire suppression over the years, Tables 504.3. and 504.4 allow height increases where an automatic fire-sprinkler system is installed throughout the building. The tables typically reflect an increase of one story in the number of stories, and 20 feet (6,096 mm) in building height, where the building is provided with an automatic fire-sprinkler system throughout.

There are basically four variations to the general requirements for height and story increases:

1.   Extended height and number of stories allowances are not permitted for Group I-2 occupancies of Type IIB, III, IV, or V construction, or for Groups H-1, H-2, H-3, and H-5 occupancies of any construction type. These occupancies present unusual hazards that limit their heights even where a sprinkler system is present.

2.   One-story aircraft manufacturing buildings and hangars may be of unlimited height when sprinklered and surrounded by adequate open space. Such uses require very large structures and through the safeguards provided, should be adequately protected.

3.   For Group R buildings provided with an NFPA 13R sprinkler system, the tables reflect that the increases in height and number of stories apply only up to a maximum of 60 feet (18,288 mm) and four stories, respectively. The limitation of four stories and 60 feet for buildings sprinklered with a 13R system cannot be exceeded under any circumstances. In those residential buildings where an NFPA 13, rather than an NFPA 13R system, is installed, the limitations of 60 feet (18,288 mm) and four stories are not applicable.

4.   Roof structures such as towers and steeples may be of unlimited height when constructed of noncombustible materials, whereas combustible roof structures are limited in height to 20 feet (6,096 mm) above that permitted by Table 504.3. In all cases, such roof structures are to be constructed of materials based on the building’s type of construction. These requirements are not based on the presence of the sprinkler system. Additional requirements for roof structures can be found in Section 1510.

Section 505 Mezzanines and Equipment Platforms

A mezzanine is defined in Chapter 2 as an intermediate floor level within a room or space. As long as the area of the mezzanine is limited in size, an intermediate floor without enclosure causes no significant safety hazard. The occupants of the mezzanine by means of sight, smell, or hearing will be able to determine if there is some emergency or fire that takes place either on the mezzanine or in the room in which the mezzanine is located. However, once portions of or all of the mezzanine is enclosed, or the mezzanine exceeds one-third the area of the room in which it is located, life-safety problems such as occupants not being aware of an emergency or finding a safe exit route from the mezzanine become important. Therefore, the code places the restrictions encompassed in this section on mezzanines to ameliorate the life safety that can be created.

505.2   Mezzanines.   By virtue of the conditions placed on mezzanines in Section 505, a complying mezzanine is not considered to create additional building area or an additional story for the purpose of limiting building size. The floor area of a complying mezzanine need not be added to the area of the floor below for the purpose of limiting building area by Section 506. This allowance essentially provides for free floor area in the comparison of the total actual area to the total allowable area. As previously mentioned, complying mezzanines also do not contribute to the actual number of stories in relationship to the allowable number of stories permitted by Table 504.4. The limitations imposed on mezzanines are deemed sufficient to permit such benefits.

In contrast to the above allowances, the floor area of mezzanines must be included as a part of the aggregate floor area in determining the fire area. Because the size of a fire area is based on a perceived level of fire loading present within the building, the contribution of a mezzanine’s fire load to the fire loading in the room in which the mezzanine is located cannot be overlooked. Figure 505-1 depicts the proper use of these provisions. The clear height above and below the floor of the mezzanine is also regulated at a minimum height of 7 feet (2,134 mm).

Figure 505-1   Mezzanine height and area.

505.2.1   Area limitation.   There is no limit on the number of mezzanines that may be placed within a room; however, the total floor area of all mezzanines must typically not exceed one-third the floor area of the room in which they are located. See Figure 505-2. As illustrated in Figure 505-3, any enclosed areas of the room in which the mezzanine is located are not to be utilized in the calculations for determining compliance with the one-third rule.

Figure 505-2   Mezzanine area.

Figure 505-3   Mezzanine area.

Where two specific conditions exist, the aggregate floor area of mezzanines may be increased up to two-thirds of the floor area of the room below. First, the building must contain special industrial processes as identified in Section 503.1.1, and second, the building shall be of Type I or Type II construction. Intermediate floor levels are very common in buildings of this kind because of the nature of their operations. By limiting the increased mezzanine size to noncombustible buildings housing primarily noncombustible processes, fire safety is not compromised.

A second exception also permits an increase in allowable mezzanine size, up to a maximum of one-half of the area or room in which the mezzanine is located. See Figure 505-4. The increased size takes into consideration the enhancements of noncombustible construction, automatic sprinkler system protection, and occupant notification. By limiting construction to Type I or II, there is no contribution to the fire hazard that is due to the materials of construction. The automatic sprinkler protection will increase the potential for limiting fire spread. In addition, the occupant notification system increases occupant awareness of a fire condition and allows for evacuation during the early fire stages.

Figure 505-4.   Maximum floor area of mezzanine.

505.2.3   Openness.   As a general rule, a mezzanine must be open to the room in which it is located. Any side that adjoins the room will be considered open if it is unobstructed, other than by walls or railings not more than 42 inches (1,067 mm) in height, or columns and posts. There are, however, five exceptions to the requirement for openness that result in most mezzanines being permitted to be partially or completely enclosed. If in compliance with any one of the five exceptions, the mezzanine need not be enclosed.

1.   Illustrated in Figure 505-5, this criterion is that the enclosed area contains a maximum occupant load of 10 persons. The limitation on occupant load is based on the aggregate area of the enclosed space. This exception is consistent with other provisions of the code that relax the requirements where the occupant load is expected to be relatively small. This exception may result in the enclosure of an entire mezzanine or just a portion of it.

2.   A mezzanine may also be fully or partially enclosed if it has two means of egress. No further conditions are established under this option.

3.   Also depicted in Figure 505-5, up to 10 percent of the mezzanine area may be enclosed. This is usually done for toilet rooms, closets, utility rooms, and other similar uses that must, of necessity, be enclosed. As long as the aggregate area does not exceed 10 percent of the area of the mezzanine, the enclosure is permitted by the code.

4.   Those mezzanines used for control equipment in industrial buildings may be glazed on all sides. This exception is necessary because of the delicate nature of much of today’s control equipment and the fact that it may require a dust-free environment.

5.   This exception allows enclosure of the entire mezzanine, provided specified egress and sprinkler provisions are met. Similar to the conditions of Exception 2, this exception requires a minimum of two means of egress from the mezzanine. However, it differs in that a number of additional conditions are required. Therefore, there is no application of this exception as Exception 2 satisfies the condition for the enclosure of mezzanines without any other conditions of compliance.

Figure 505-5   Enclosed mezzanines.

505.3   Equipment platforms.   In buildings containing platforms that house equipment, such platforms need not be considered stories or mezzanines, provided they conform with the provisions of this section regulating platform size, extent of automatic sprinkler protection, and guards. In addition, the equipment platforms cannot serve as any portion of the exiting system from the building. Complying platforms are not deemed to be additional stories, do not contribute to the building floor area, and furthermore, need not be included in determining the size of the fire area.

Section 506 Building Area

506.1   General.   Building area is one of the three limiting factors in the IBC when it comes to determining maximum allowable building size based on occupancy and type of construction classifications. The presence of automatic sprinkler protection and/or the amount of building frontage also contribute to this determination. The process is initiated in Table 506.2 where the allowable area factor is identified, and continued through the application of various modifiers. A general overview of the concept regarding allowable building area can be found in the discussion of Section 503.

There are two additional approaches to allowable area compliance that modify or exempt compliance with Section 506. For those buildings constructed under the provisions of Section 507, there is no limit on floor area, provided specified conditions are met. In addition, modifications and exemptions to height and area limitations of Chapter 5 are set forth in Section 510. Although there is no requirement to apply Section 507 or 510 for allowable area determination, the allowances provided in these sections provide alternative methods for compliance.

The code intends that a basement that does not exceed the allowable floor area permitted for a one-story building need not be included in determining the total allowable area of the building. This provision is a holdover from many years back when basements were most commonly used for service of the building. Today, it is not uncommon to find basements occupied for the same uses as the upper floors. There apparently has been no adverse experience for cases of this type (most likely because of the fire-sprinkler protection required in many basements), and therefore the code provision appears to be satisfactory. The code does not address how to handle a basement that exceeds the area permitted for a one-story building. However, as the code does not permit any story to exceed that permitted for a one-story building, it would seem logical that a basement should also be limited to the same area. If anything, a fire in a basement is more difficult to fight than one in an above-ground story. Thus, it does not seem reasonable or appropriate to permit a single basement with an area exceeding that allowed for a one-story building. Multiple basements are also exempt from inclusion in the building area calculations, but only where the aggregate area of such basements does not exceed the area permitted for a single-story building. Additional requirements for basements extending well below grade level may be found in Section 405 for underground buildings.

506.2.1   Single-occupancy, one-story buildings.   The most straightforward determination of allowable area is applied to those single-story buildings that contain only one occupancy classification. The formula for the calculation of the maximum allowable area for these types of buildings (in square feet), Equation 5-1, is additive. It is based on the sum of the tabular allowable area factor shown in Table 506.2 and any increase that is due to building frontage per Section 506.3. A simple example is shown in Application Example 506-1.

Application Example 506-1

How does the installation of an automatic sprinkler system relate to a building’s allowable floor area? The tabular allowable area factors established in Table 506.2 include any increases that may be appropriate due to the installation of an automatic sprinkler system. Because of the excellent record of automatic sprinkler systems for the early detection and suppression of fires, the IBC allows quite large floor-area increases where an automatic fire sprinkler system is installed throughout the building. As evidenced in Table 506.2, the maximum allowable area of a fully sprinklered one-story building is typically four times the amount for a sprinklered building; and for a building of two or more stories in height, the tabular allowable area is typically three times the floor area permitted for a nonsprinklered condition. The lesser sprinkler benefit for multistory buildings is based on the assumption by the code that fire department suppression activities are still going to be required even where an automatic fire-sprinkler system is installed. Therefore, a multistory building presents more problems to the fire department than a one-story building, and a smaller floor area allowance is permitted.

Table 506.2 reflects increased floor area allowances for buildings protected throughout with an NFPA 13 sprinkler system, but not those buildings protected with an NFPA 13R system. In addition, the increased floor areas are not applicable to Group H-1, H-2, or H-3 occupancies. The code considers that the conditions requiring the installation of sprinkler systems in these three high-hazard occupancies are such that the sprinkler system should not also be used to provide additional allowable area. In a mixed-occupancy building, this restriction is only applicable to the Group H-2 and H-3 portions. Other occupancies within the building are permitted to utilize the allowable areas established for building protected by an automatic sprinkler system as indicated in Section 506.2.2.1.

What other conditions affect the determination of allowable floor areas? As expected, there are other situations that can arise involving the determination of the allowable area for a building. It should be noted that, as illustrated in Application Example 506-2, the introduction of a fire wall in a large-area building will result in the loss of a portion of the open space at the building perimeter. Application Example 506-3 illustrates the permitted frontage increase for an open space shared by two buildings on the same lot.

506.2.2   Mixed-occupancy, one-story buildings.   For allowable area purposes, the introduction of multiple occupancies into the building requires an additional level of scrutiny. The methods for determining allowable area compliance in mixed-occupancy buildings differ based on the manner in which the various occupancies are addressed. Where the accessory occupancy method is utilized, the allowable area of the main occupancy will govern. Where occupancies are regulated under the nonseparated-occupancy method, the allowable area is limited to that of the most restrictive occupancy involved. In the case of separated occupancies, the unity formula must be used to determine if the allowable area is exceeded. Where a single-story, mixed-occupancy building is being evaluated for allowable area compliance, the process is addressed in Section 508.1. Further discussion can be found in the narratives of Section 508 for each of the three mixed-occupancy methodologies.

506.2.3   Single-occupancy, multistory buildings.   In addition to the floor-area limits placed on each story of a building, the entire building is also limited in size. Equation 5-2 sets forth a procedure for determining the allowable area for a multistory building containing only one occupancy classification. For multistory buildings other than those consisting of only two stories above grade plane, the IBC permits the total combined floor area to be three times the total allowable area permitted per story. Structures two stories above grade plane are limited in size to a total combined floor area of twice that permitted per story. Examples of these limitations are shown in Figure 506-1. It is important to again state that each story in a multistory building is individually limited in size.

Figure 506-1   Building area limitations.

Application Example 506-2

Application Example 506-3

The exception to this general rule can be found within Equation 5-2. Four-story residential occupancies provided with an NFPA 13R automatic sprinkler system are permitted additional allowable area beyond that permitted under the general provisions. As the installation of a 13R sprinkler system in a residential occupancy does not provide for an increase in allowable area for sprinklered buildings, and the use of an NFPA 13R system is limited to buildings no more than four stories in height, it is considered appropriate to permit the maximum allowable area per story for each of the stories in the residential building. This would include four-story structures as depicted in Figure 506-2.

Figure 506-2   Group R area determination.

506.2.4   Mixed-occupancy, multistory buildings.   Similar to the approach described in Section 506.2.2 for one-story buildings, a mixed-occupancy building with more than one story above grade plane must be analyzed on a story-by-story basis and each individual story must be shown to be in compliance with the applicable provisions of Section 508.1. In all cases, regardless of the building’s number of stories, each individual story must comply for allowable area purposes. For those buildings that are two and three stories in height, if each such story complies with the allowable area limitations, then the entire building complies. However, the approach to buildings four or more stories in height above grade plane differs from that used for two-story and three-story conditions. Again, each story in the building must comply individually. In addition, the aggregate sum of the actual areas of each story divided by the allowable areas of such stories cannot exceed 3. The procedure is illustrated in Application Example 506-4.

In all cases for mixed-occupancy conditions, reference is made to the provisions of Section 508.1, which identify the three options for mixed-occupancy determination (accessory occupancies, nonseparated occupancies, and separated occupancies). Since each story is to be evaluated independently in a multistory condition, it is possible for the designer to utilize different mixed-occupancy options on various stories within the building.

Application Example 506-4

506.3   Frontage increase.   The initial requirement of the code, insofar as a frontage increase is concerned, is that it adjoin or have access to a public way. Thus, the structure could extend completely between side lot lines and to the rear lot line, and be provided with access from only the front of the building, and still potentially be eligible for a small frontage increase. Therefore, it follows that if a building is provided with frontage consisting of public ways and/or open space for an increased portion of the perimeter of the building, some benefit should accrue based on better access for the fire department. Also, if the yards or public ways are wide enough, there will be a benefit that is due to the decreased exposure from adjoining properties.

Because of the beneficial aspects of open space adjacent to a building, the IBC permits increases in the tabular areas established from Table 506.2 based on the amount of open perimeter and width of the open space and public ways surrounding the building. For any open space to be effective for use by the fire department, it is mandated that it be accessed from a public way or a fire lane so that the fire department will have access to that portion of the perimeter of the building that is adjacent to open space. See Figure 506-3.

Figure 506-3   Open space access.

Open space and public ways—what can and cannot be used. In addition to allowances for public ways, the IBC uses the term open space where related to frontage increases in the determination of allowable floor areas. Although the term open space is not specifically defined in the IBC, the definition of a yard is an open space unobstructed from the ground to the sky that is located on the lot on which the building is situated. It is logical that this definition is consistent with the intended description of open space. This definition seems to preclude the storage of pallets, lumber, manufactured goods, home improvement materials, or any other objects that similarly obstruct the open space. However, it would seem reasonable to permit automobile parking, low-profile landscaping, fire hydrants, light standards, and similar features to occupy the open space. These types of obstructions can be found within the public way, so their allowance within the open space provides for consistency. Because a yard must be unobstructed from the ground to the sky, open space widths should be measured from the edge of roof overhangs or other projections, as shown in Figure 506-4.

Figure 506-4   Measurement of open space.

Regarding the use of public ways for providing frontage increases, the width of public way that should be used for determining area increases seems to cause confusion. Should the full width of the public way or only the distance to the centerline be used? The confusion evolves from the definition of fire separation distance as established in Chapter 2, which states that fire separation distance is measured from the building face to the centerline of a street, alley, or public way. However, the requirement to use the centerline is limited to fire separation distance and is not applicable to Section 506.3. For determining frontage increases for open space, the full width of the public way may be used by buildings located on both sides of the public way as expressed in Item 2 of Section 506.3.2.

The following type of question is also sometimes asked: “Why cannot I use the big open field next door for area increases?” Section 506.3.1 specifically mandates that open space used for a frontage increase must be on the same lot as the building under consideration, or alternatively, dedicated for public use. There is a good reason for this limitation, insofar as the owner of one parcel lacks control over a parcel owned by another and, thus, the open space can disappear when the owner of “the big open field” decides to build on it. One method by which some jurisdictions have allowed such large open spaces to be used is by accepting joint use of shared yards. It is typically necessary that a recorded restrictive covenant be executed to ensure that the shared space will remain open and unoccupied as long as it is required by the code. The creation of a no-build zone does not seem unreasonable insofar as the aim is to maintain open spaces between buildings. Any covenant should be reviewed by legal counsel to be sure it will accomplish what is intended. In addition, it should clearly describe the reason and applicable code section so that any future revisions or deletions may be considered if the owners wish to terminate such an agreement. In such an event, each building should be brought into current code compliance, or the agreement would be required to remain in effect.

Whereas use of a public way as open space is permitted by the IBC, other publicly owned property is generally not, because the building official usually has no control over the long-range use of publicly owned property, and there is little assurance that such property will be available as open space for the life of the building. Remember that what is today’s publicly owned open parking lot could become tomorrow’s new city hall, and the open space used to justify area increases would no longer exist. Whereas Section 506.3.1 allows publicly owned property to be considered open space, the intent is such that the property be permanently dedicated for public use and maintained as unobstructed. The term public way was used in place of streets because its definition allows the use of a broader range of publicly owned open space while still allowing the building official some discretion as to the acceptability of a particular parcel. Public way usually conjures up visions of streets and alleys, but how about other open spaces such as power line right-of-ways, flood-control channels, or railroad rights-of-way? Many such open spaces are generally acceptable, provided there is a good probability that they will remain as open space during the life of the building for which they will serve. Power lines and flood-control channels are usually good bets for longevity, but railroad routes are often abandoned and, therefore, may not be as good a bet. There is also an expectation that the public way is maintained in an unobstructed condition to allow for fire department access, which potentially would disallow the use of waterways and similar features. If the public way does not provide for fire department access, its use for a frontage increase is prohibited. It should be noted that the definition for public way requires any such public parcel of land, other than a street or alley, to lead to a street. Figure 506-5 provides a visual summary of open space and public ways that could be used for open-space area increases.

Figure 506-5   Yards and public ways available for area increases.

How much increase? In the case where public ways or open space adjoin more than 25 percent of the building’s perimeter, the code permits an increase in the building area per story as shown in Table 506.2. The amount of the increase is based on the percentage of open perimeter having a width of at least 20 feet (6,096 mm). By utilizing the formula shown below, the area increase that is due to frontage (I_f) can be determined by Equation 5-5:

Based on this method of calculation, the maximum area increase permitted will typically be 75 percent, as shown in Application Example 506-5. This is based on the general requirement of Section 506.3.2 that requires a value of 30 feet (9,144 mm) to be used for the value W in those cases where W exceeds 30 feet (9,144 mm). As this figure illustrates, the entire perimeter of the building must adjoin a public way or open space having a width of at least 30 feet (9,144 mm). Where less than the entire perimeter has adequate open area, the area increase for frontage will be reduced as illustrated in Application Example 506-6.

Application Example 506-5

Where the open space at the building’s perimeter is between 20 feet (6,096 mm) and 30 feet (9,144 mm) in width, the code permits the use of the weighted average of such width in relation to the entire perimeter. This approach allows for the width W in Equation 5-5 to be more representative of the availability of open space around the building, rather than basing the frontage increase on simply the smallest open space of 20 feet (6,096 mm) or more.

Application Example 506-6

Whereas in most cases the increase available based on the weighted average method is minimal, it does provide for some degree of allowable area adjustment. An example of calculating W by weighted average is shown in Application Example 506-7 based on Equation 5-4.

Whereas 75 percent is generally the largest allowable frontage increase, a greater area increase is permitted for those buildings that comply with all of the requirements for unlimited-area buildings as described in Section 507, other than compliance with the 60-foot (18,288-mm) open space or public way requirement.

A maximum frontage increase of just less than 150 percent can be achieved based on the entire perimeter being open with a minimum width of slightly less than 60 feet (18,288 mm). Once 60 feet (18,288 mm) of accessible open space and public ways is obtained for 100 percent of the building’s perimeter, the provisions of Section 507 are applicable and the frontage increase formula is not to be used. An example of the increased frontage increase is shown in Application Example 506-8, which also includes the calculated increase based on weighted average.

Application Example 506-7

How must access be provided to an open space? The IBC provides no details as to the degree of fire department accessibility required in order to consider open space for an allowable area increase; it only mandates that access be provided from a street or approved fire line. It is clearly not the intent of the provisions to mandate a street or fire line completely around a building in order to acquire the maximum frontage increase. However, fire personnel access from such streets or fire lanes is necessary. Although it is not a requirement to provide access around a building for fire department apparatus, other than that required by IFC Section 503.1.1, the frontage increase is based on the ability of fire personnel to physically approach the building’s exterior under reasonable conditions. For example, where the space adjacent to the building is heavily forested or steeply sloped, the frontage increase addressed in Section 506.3 is not permitted. The presence of a lake or similar water feature next to a building would also prohibit an area increase. The evaluation of each individual building and its site conditions is necessary to properly apply the code for fire department access.

Application Example 506-8

Section 507 Unlimited-Area Buildings

There are many cases where very large undivided floor areas are required for efficient operation in such facilities as warehouses and industrial plants. Through the use of adequate safeguards, the IBC recognizes this necessity and allows unlimited areas for these uses under various circumstances. Large open floor space is also desirable for other applications; therefore, such allowances are also permitted for business and mercantile occupancies, as well as specific assembly and educational uses. The use of this section is typically intended to eliminate fire-resistive construction of the building that would be mandated based on the area limitations of Section 506. Contrary to the general philosophy that as a building increases in floor area the allowable types of construction become more restrictive, many of the unlimited-area uses permit the use of any construction type.

507.1   General.   Historically, structures constructed under the provisions for unlimited area have performed quite well in regard to fire and life safety. A number of occupancy groups, particularly those relating to institutional, residential, and high-hazard occupancies, are excluded from the benefits derived from the provisions for unlimited-area buildings. Such occupancies pose unacceptable risks that are due to their unique characteristics. As a general rule, only those occupancy classifications specifically identified in this section are permitted to be housed in buildings allowed to be unlimited in area by Section 507. For example, Group I occupancies are not specifically permitted by any of the provisions addressing unlimited-area buildings. Therefore, it would appear no amount of Group I is permitted in such structures. However, Section 507.1.1 identifies the method that allows for a limited degree of such prohibited occupancies. Section 508.2.3 indicates that the allowable area for an accessory occupancy is to be based on the allowable area of the main occupancy. If the main occupancy is permitted by Section 507 to be in an unlimited-area building, a complying accessory occupancy also enjoys the same benefit. Figure 507-1 illustrates this condition.

Figure 507-1   Accessory occupancy in an unlimited-area building.

It is also not uncommon for two or more occupancies regulated under the provisions of Section 507 to be located within the same building. For example, assume a one-story building contains a Group M furniture store and its associated S-1 warehouse. Because both Group M and S-1 occupancies are permitted in an unlimited-area building complying with Section 507.3, both occupancies are permitted to be located in the same unlimited-area building. See Figure 507-2. Any fire-resistive separation requirement would be based on the applicable mixed occupancy method of Section 508 applied to the building.

Figure 507-2   Mixed-occupancy unlimited-area buildings.

All buildings regulated under the unlimited-area building provisions of Section 507 are required to be surrounded by public ways and yards of substantial width. This continuous open space provides a means for the fire service to access the building as necessary from the exterior, while at the same time maintains a sizable separation from any other structures on the site. The required open space is to be measured from all points along the building’s exterior wall in all directions, ensuring that the full perimeter of the building is provided with continuous open space. This differs somewhat from the right-angle method established for an allowable area frontage increase as set forth in Section 506.2 where such continuity of open space is not required at the building corners. See Figure 507-3. Consistent with the measurement method of Section 506.2 to gain a frontage increase, the open space width adjacent to those exterior walls fronting on a public way is permitted to include the entire width of the public way.

Figure 507-3   Open space measurement for unlimited-area building.

507.2.1   Reduced open space.   There may be situations where the full 60 feet (18,288 mm) of open space or public ways surrounding an unlimited-area building cannot be obtained or is undesirable. The IBC permits a reduction in the required open space under very specific conditions, as illustrated in Figure 507-4. In no case may the permanent open space be reduced to less than 40 feet (12,192 mm) in width. By limiting the amount of reduced open space, requiring a high degree of exterior wall fire resistance, and mandating opening protectives in all openings in the exterior wall facing the reduced open space, the code provides protection equivalent to full 60-foot (18,288-mm) yards or public ways. A final point: The permitted reduction in open space is only applicable to specific portions of Section 507. Although the allowance is not identified as acceptable for buildings complying with Section 507.7, it would seem the reduction could also be viewed as appropriate since Section 507.7 is a companion provision to Section 507.6.

Figure 507-4   Reduced open space.

507.3   Nonsprinklered, one-story buildings.   This section addresses a Group F-2 or S-2 occupancy in a one-story building of any type of construction. Both Group F-2 and S-2 occupancies by definition are low-hazard manufacturing or storage uses, which the code considers to be low fire risks. Fire risk is further reduced by requiring that the building be surrounded by yards or streets with a minimum width of 60 feet (18,288 mm). The relatively low fire loading expected in such occupancies is why the code does not require the installation of an automatic fire-sprinkler system for this application. The use of this provision is applicable to buildings of all construction types; however, it is anticipated that the structure will mirror the contents in the absence of combustible elements.

507.4   Sprinklered, one story buildings.   Specific moderate-hazard occupancies, limited to Groups B, F, M, and S, are permitted single-story buildings of unlimited area where the building is completely surrounded by streets or yards not less than 60 feet (18,288 mm) in width and the entire structure is protected by an automatic fire-sprinkler system. The limitation of one story does not apply where the building is utilized for rack storage, provided the building is of noncombustible construction, and is not intended for public access. This unlimited-area storage facility, required to conform with Chapter 32 of the IFC, is permitted to be of any height.

In most applications, the use of the unlimited-area provisions simply means that the type of construction is not regulated, regardless of the size of the building’s floor area. The code assumes that the amount of combustibles and, consequently, the potential fire severity are relatively moderate. In addition, the protection provided by the automatic fire-sprinkler system plus the fire-department access furnished by the 60-foot (18,288-mm) yards or streets surrounding the building reduce the potential fire severity to such a level that unlimited area is reasonable.

One-story Group A-4 occupancies are also permitted to be of unlimited area where a sprinkler system is provided throughout and a minimum 60-foot (18,288-mm) open space surrounds the building. However, because of the increased risk posed by the anticipated high number and concentration of occupants in such a structure, the construction type of the building is limited to Type I, II, III, or IV. The automatic sprinkler system required in an unlimited-area building housing a Group A-4 occupancy may be omitted in those specific areas occupied by indoor participant sports, including tennis, skating, swimming, and equestrian activities. Such an omission mandates that exit doors from the participant sports areas lead directly to the outside, and the installation of a fire-alarm system with manual fire-alarm boxes is required. It is anticipated that such sports areas will have little, if any, combustible loading if the uses are limited to those described in the code. If there is a reasonable expectation that other types of uses could occur, it would be inappropriate to omit the sprinkler system in such areas.

Group A-1 and A-2 occupancies are permitted in a mixed-occupancy building when in compliance with the general limitations of Section 507.4 plus four additional criteria as established in Section 507.4.1. This allowance does not grant the designated Group A occupancies unlimited area, but rather allows such assembly occupancies to be located within a Group B, F, M, S, or A-4 unlimited-area building under specified conditions. The building must be classified as Type I, II, III, or IV construction. In addition, the Group A assembly occupancies must be separated with fire barriers from other occupancies within the building, in accordance with the separated occupancy provisions of Section 508.4.4. For example, in an unlimited-area retail sales building, a Group A-2 restaurant would be required to be separated from the Group M sales area by a minimum 2-hour fire-resistance-rated fire barrier. No reduction in the minimum required fire-resistance rating of the fire barrier is permitted for the presence of an automatic sprinkler system. Each individual Group A tenant would also be limited in area by the provisions of Section 506. No additional height increases would apply to the Group A occupancies because of the specific Section 507.4 limitation of one story for the entire building. An additional requirement mandates that all required means of egress from the assembly spaces exit directly to the exterior of the building. See Figure 507-5. Application of this provision does not require the assembly occupancy to be accessory to the major use of the building, nor is the assembly floor area limited to 10 percent of the floor area. As an additional note, it would seem appropriate that the provision also extend to Group A-3 occupancies, as such uses are typically considered equal or lesser in hazard level to the Group A-1 and A-2 classifications.

Figure 507-5   Group A occupancies in unlimited-area buildings.

507.5   Two-story buildings.   In Groups B, F, M, and S, the unlimited-area provisions also apply to structures that are two stories in height. Minimum 60-foot (18,288-mm) open space or public ways must surround the building, and an automatic fire-sprinkler system is required throughout the structure.

507.6   Group A-3 buildings of Type II construction.   Although most assembly occupancies are viewed as relatively high hazard because of the concerns associated with the number and concentration of the occupants, certain types of uses classified as Group A-3 occupancies are considered as moderate hazard. Therefore, it is possible to utilize the unlimited-area provisions for specific Group A-3 uses where the specified criteria are met.

Such buildings allowed to be of unlimited area are limited to one story in height, must be of Type II noncombustible construction, and contain only those specific types of assembly uses listed in the code. By limiting the types of buildings to places of religious worship, gymnasiums (without spectator seating), lecture halls, and similar uses, it is anticipated that the fire loading is relatively low. Buildings such as libraries, museums, and similar uses pose a higher risk that is due to the large amount of combustibles expected to be present. The potential for combustible loading is further reduced by the prohibition of a stage as a part of the use, although a platform is acceptable. Installation of an automatic sprinkler system is mandated, as is the presence of a minimum 60-foot (18,288-mm) open space around the building.

507.7   Group A-3 buildings of Types III and IV construction.   Buildings of Type III or IV construction are also granted unlimited-area status when housing specified Group A-4 occupancies, as also identified in Section 507.6. The requirements for sprinkler protection and adequate open space are also applicable. In addition, the assembly use must be located relatively close to the exterior ground level to expedite the exiting process. As a part of this requirement, any elevation change from the building to the grade level must be accomplished by ramps rather than stairs. This further provides for an efficient means of egress from the assembly building. The additional limitation regarding floor-level location is mandated due to the combustible nature of the building’s construction.

507.8   Group H occupancies.   Because many large industrial operations, both manufacturing and warehousing, have a need to utilize a limited quantity of high-hazard materials in some manner, it is necessary that Group H-2, H-3, and H-4 occupancies be permitted to a small degree in Group F and S unlimited-area buildings. Because of the allowances given to buildings of unlimited area, it is critical that the high-hazard occupancies be strictly limited in floor area and adequately separated from the remainder of the building.

There are four factors that limit the allowable floor area of the permitted Group H occupancies: the specific classification of the Group H occupancy, the building’s type of construction, its floor area, and the location of the high-hazard uses in relationship to the building. Where high-hazard occupancies are located on the perimeter of the building, fire-department access is enhanced, and exposure from interior areas is reduced. Accordingly, the permitted floor area of the Group H occupancies located on the building’s perimeter is considerably greater than that allowed for such high-hazard uses completely surrounded by the unlimited-area building.

Where Group H-2, H-3, and H-4 occupancies are located at the perimeter of the unlimited-area Group F or S building, their size is restricted by the area limitations of Section 506. In a condition where the high-hazard occupancy is totally enclosed by the unlimited-area building, the size of the occupancy is limited to only 25 percent of the area limitations specified in Section 506. Both of these conditions are shown in Application Example 507-1. The example also illustrates that multiple Group H occupancies that are not located at the perimeter of the building are limited in size based on the aggregate floor area of such occupancies. Similarly, where multiple Group H occupancies do occur on the building’s perimeter, the maximum permitted size is also based on the total of all such occupancies. In all situations, the appropriate fire-barrier assemblies mandated by Table 508.4 must be provided. Regardless of the location of the applicable Group H occupancies, in no case may such aggregate floor areas exceed 10 percent of the area of the entire building. The provisions addressing liquid use, mixing and dispensing rooms, liquid storage rooms, and spray-paint booths simply replicate those of Section 415.6, Exceptions 1 through 3, in order to remind code users of the limitations and allowances placed on such rooms and areas.

Application Example 507-1

507.9   Unlimited mixed-occupancy buildings with Group H-5.   The Group H-5 classification was created some time ago to standardize regulations for semiconductor manufacturing facilities. The Group H-5 category requires engineering and fire-safety controls that reduce the overall hazard of the occupancy to a level regarded to be equivalent to that of a moderate-hazard Group B occupancy. The mitigating provisions of Section 415.11 have effectively equalized the two occupancies in terms of relative hazard such that, in concert with the three additional criteria established in this section, the allowance for Group H-5 unlimited-area buildings is deemed to be appropriate.

A unique condition not applied to other unlimited-area buildings regulated by Section 507 is the need to limit the unseparated floor area of Group H-5 use. Although an unlimited amount of Group H-5 floor area is permitted within a complying unlimited-area building, it may be necessary to subdivide the floor area using minimum 2-hour separations due to the limits of Item 3. An application of the provisions and the required fire barrier separations is illustrated in Figure 507-6.

Figure 507-6

507.10   Aircraft paint hangar.   The provisions of Section 412.6 address aircraft painting operations where the amount of flammable liquids in use exceeds those maximum allowable quantities listed in Table 307.1(1). Classified as Group H-2, such aircraft paint hangars must be fully suppressed in accordance with NFPA 409 and of noncombustible construction. One-story hangars may be unlimited in floor area where complying with Section 412.6, provided they are surrounded by public ways or yards having a width of at least one and one-half times the height of the building.

507.11   Group E buildings.   Because of the various fire- and life-safety concerns associated with educational occupancies, buildings housing uses classified as Group E are typically not eligible for consideration as unlimited-area buildings. Only when the following six criteria are met does the IBC permit the area of a Group E educational building to be unlimited:

1.   The building is limited to one story in height.

2.   The building is of Type II, IIIA, or IV construction.

3.   Two or more means of egress are provided from each classroom.

4.   At least one means of egress from each classroom is a direct exit to the exterior of the building.

5.   An automatic sprinkler system is provided throughout the building.

6.   The building is surrounded by open space at least 60 feet (18,288 mm) in width.

507.12   Motion-picture theaters.   Because of their limited combustible loading, motion-picture theaters are granted unlimited floor areas in a manner relatively consistent with other moderate-hazard uses. This specific allowance is not extended to the other uses classified as Group A-1, such as performance theaters, because of their higher fire-severity potential.

In order to address the concerns related to the high-density, high-volume occupant loads often encountered in motion-picture theaters, unlimited area is only permitted where the building is of Type II noncombustible construction. This restriction further limits the fire load contained within the building construction. In concert with Section 507.4, a fire-sprinkler system must be installed throughout, and minimum 60-foot (18,288-mm) open areas must completely surround the building.

The application of this provision differs from the allowance granted in Section 507.4.1. That provision permits any Group A-1 occupancy, including motion-picture theaters, to be located in an unlimited-area building complying with Section 507.4, provided the limitations of the exception are met. However, it does not allow Group A-1 occupancies themselves to be unlimited in area. On the other hand, this section permits a Group A-1 theater complex to be unlimited in area, provided it is fully sprinklered, of Type II construction, and surrounded by adequate open space.

507.13   Covered and open mall buildings and anchor buildings.   The provisions of Section 402.4.1 for unlimited-area covered and open mall buildings are referenced for convenience purposes. Note that although the reduction in open space permitted by Section 507.2.1 is not applicable to covered or open mall buildings, a similar reduction is permitted by the exception to Section 402.1.1.

Section 508 Mixed Use and Occupancy

Multiple uses commonly occur within a single building. Each use creates its own distinct hazards, many of which are addressed by the code. However, many of the hazards are similar in nature, which allows the varied uses to be grouped into categories that recognize the common concerns. These categories are identified in Chapter 3 as occupancy groups. Where two or more occupancy groups share a single building, it is necessary to evaluate their relationship to each other as a mixed-occupancy condition. This section provides various methods to address such relationships in regard to occupancy classification, allowable height and area, and fire-resistance-rated separation.

508.1   General.   A mixed-occupancy condition exists where two or more distinct occupancy groups are determined to exist within the same building. In fact, it is quite common for a building to contain more than one occupancy group. For example, hotel buildings of various sizes not only house the residential sleeping areas, but may contain administrative offices, retail and service-oriented spaces, parking garages, and, in many cases, restaurants, conference rooms, and other assembly areas. Each of these uses typically constitutes a distinct and separate occupancy as far as Chapter 3 of the IBC is concerned. Because this situation is not uncommon, the code specifies requirements for buildings of mixed occupancies. Under such circumstances, the designer has available several methodologies (accessory occupancies, nonseparated occupancies, and separated occupancies) to address the mixed-occupancy concerns. The methods that have been established represent a hierarchy of design prerogatives that may be utilized at the discretion of the design professional. Although compliance is required with only one of the three mixed-occupancy methods, it is acceptable to utilize two or even all three methods in the same building, as shown in Figure 508-1. The common format utilized in presenting the requirements for each of the methods allows for a comparison of the provisions. This should assist in determining the most appropriate method, or methods, for the building under consideration. A simple comparison of the three mixed-occupancies methods is shown in Figure 508-2.

Figure 508-1   Combination of methods.

Figure 508-2   Summary of mixed-occupancy methods.

It is important to recognize that there is no relationship between the mixed-occupancy provisions of Section 508.4 and the fire area concept utilized in Section 903.2 for automatic sprinkler systems. Compliance with any of the three mixed-occupancy methods does not relieve the responsibility to comply with Section 901.7 and Table 707.3.10 regarding the proper separation of fire areas. An example is shown in Figure 508-3.

Figure 508-3   Occupancy separation versus fire area separation.

508.2   Accessory occupancies.   Those minor uses in a building that are not considered consistent with the major occupancy designation can potentially be considered accessory occupancies. They often are necessary or complimentary to the function of the building’s major use, but have few characteristics of the major occupancy in regard to fire hazards and other concerns. Therefore, accessory occupancies must each be assigned to an occupancy group established in Chapter 3 based on their own unique characteristics. While maintaining the philosophy of a mixed-occupancy building, this section permits such relatively small accessory uses to be considered merely a portion of the major occupancy for fire separation purposes. A good example would be a lunchroom seating 120 persons and located in a large manufacturing facility. Whereas the individuals using the lunchroom are generally the same individuals who work elsewhere in the factory, the hazards encountered while they are occupying the lunchroom are quite different from those created in the Group F-1 manufacturing environment. Therefore, the lunchroom must be appropriately classified as a Group A-2 occupancy, creating a mixed-occupancy condition. However, through compliance with the accessory occupancy provisions of this section, the need for a fire-resistance-rated separation between the lunchroom and manufacturing area is eliminated. In fact, no physical separation of any kind would be mandated. It is important to note, however, that in spite of the absence of a fire separation, the two areas would maintain their unique occupancy classifications. They would continue to be classified as a Group A-2 and F-1, respectively, and the building would be considered a mixed occupancy.

As previously indicated, consideration as an accessory occupancy is only possible where the use is subsidiary to the main occupancy of the building. There are several additional criteria that must also be met in order to utilize the accessory occupancy method. The occupancy under consideration cannot exceed 10 percent of the floor area of the story on which the accessory occupancy is located, nor more than that permitted by Table 506.2 for nonsprinklered buildings. See Figure 508-4. It is specified that the 10 percent limitation is based on the aggregate floor areas of all accessory use areas, not individually. Multiple minor uses that cumulatively make up more than 10 percent of the total floor area could pose a hazard that the code does not anticipate. There are unique situations—such as where minor uses are adequately separated spatially or are of such different types of uses that their aggregate area is not relevant—where the regulation of minor uses as individual areas could potentially be considered. See Figure 508-5. The application of these limits is subject to the interpretation of the building official, based on conditions unique to each building under consideration. The general limitations applied to accessory occupancies are shown in Application Example 508-1. There is no guidance in the code as to the determination of the allowable location of an accessory occupancy in a multistory building. The provisions address the building as a whole, with a reference to Section 504. In applying Section 504.2 addressing all mixed occupancies, it follows that the accessory occupancy cannot be located higher that permitted by Table 504.4 for that specific accessory occupancy as illustrated in Figure 508-6.

Figure 508-4   Aggregate accessory occupancies.

Figure 508-5   Individual accessory occupancies.

Figure 508-6   Maximum height of accessory occupancy.

Although the primary allowance provided by the accessory occupancy provisions is the lack of a required fire separation between the accessory occupancy and the remainder of the building, there is also a potential benefit that is due to the manner in which allowable area is regulated. The code calls for the allowable area of the accessory occupancy to be based on the main occupancy of the building. This approach typically allows for a greater allowable area than would be permitted under the conditions for both nonseparated occupancies and separated occupancies. Another important benefit provides for occupancies not normally permitted in unlimited-area buildings, as regulated by Section 507, to be located in such buildings. This allowance is established in Section 507.1.1. For example, a Group A-2 lunchroom considered an accessory occupancy may be located in a two-story Group B unlimited-area building complying with Section 507.5 with no separation required between the two occupancies. This allowance, along with the limitations for accessory occupancies, is shown in Application Example 508-2.

Application Example 508-1

Two exceptions indicate those conditions related to accessory occupancies under which some degree of fire-resistance-rated separation is mandated. The first exception indicates that the fire separation cannot be eliminated where the accessory occupancy is classified as Group H-2, H-3, H-4, or H-5. Where such occupancies occur in a mixed-occupancy building, it is necessary to apply the separated occupancy provisions of Section 508.4. The second exception mandates that the separation elements addressed in Section 420 for buildings containing dwelling units and sleeping units be provided.

508.3   Nonseparated occupancies.   This section presents another of the available methods addressing the relationship between different occupancies in a mixed-occupancy building. Under the specific conditions of this methodology, fire-resistance-rated separations are not mandated between adjacent occupancies. The fundamental concept behind this provision assumes that if the building is designed in part to address the most restrictive and most hazardous conditions that are expected to occur based on the occupancies contained in the building, a fire-resistance-rated separation is not needed. In fact, no physical separation of any type is required.

Utilizing the nonseparated-occupancy method, the building must be individually classified for each unique occupancy that exists. The height and area limitations for those occupancies will be used to determine the required type of construction for each occupancy, with the most restrictive type of construction required for the entire building. In addition, the most restrictive fire-protection system requirements (automatic sprinkler systems and fire alarm systems) that apply to an occupancy in the building shall apply to the entire structure. For the application of other code provisions, each individual occupancy will be regulated by only the specific requirements related to that occupancy. A special condition mandates that if a high-rise building is regulated under the nonseparated-occupancy provisions, even those portions of the building that are not considered high rise must comply with the high-rise requirements. See Figure 508-7. Another important limitation is that the use of the nonseparated-occupancy method is not permitted for Group H-2, H-3, H-4, and H-5 occupancies. In addition, the nonseparated-occupancy provisions cannot be utilized to eliminate any required dwelling unit or sleeping unit separation required under the special provisions of Section 420.

Figure 508-7   Nonseparated occupancies in a high-rise building.

Application Example 508-2

The approach to understanding the rationale for the nonseparated-occupancy method may be better understood by viewing the structure as multiple single-use facilities. In evaluating the building described in Application Example 508-3, assume that the building is entirely a Group B occupancy. Based on that assumption, determine its maximum allowable height and area, as well as any required fire protection features. Now evaluate the building as if it were entirely a Group E occupancy, again addressing the maximum allowable height and area, along with the requirements for fire protection systems. By applying the most restrictive height, area, and fire protection provisions of both Group B and E occupancies to the entire building, the conditions for nonseparated occupancies can be determined.

508.4   Separated occupancies.   Using this method of addressing mixed-occupancy buildings, the code directs that each portion of the building housing a separate occupancy be individually classified and comply with the requirements for that specific occupancy. Furthermore, the code intends that each pair of occupancies be evaluated through Table 508.4 as to the relationship of the hazards involved, often mandating a fire-resistance-rated separation between them. The allowable number of stories for each occupancy is independently regulated based on Table 504.4 based on the type of construction of the building. See Figure 508-8.

Figure 508-8   Maximum height of separated occupancies.

For allowable floor-area considerations in a building having multiple occupancies, the code uses a formula that is very similar to the interaction formula used in structural engineering where two different types of stress are imposed on a member at the same time.

In the case of a mixed-occupancy building, the code uses this type of formula for the calculation of the allowable building area for each floor. For example, if there are three different occupancies in a building, the formula is as follows:

a1/A1 + a2/A2 + a3/A3 < 1.0

WHERE: a1, a2, and a3 represent the actual areas for the three separate occupancies, and A1, A2, and A3 represent the allowable areas for the three separate occupancies.

See Application Examples 508-4 and 508-5 for examples of this computation.

Application Example 508-3

Application Example 508-4

Application Example 508-5

This formula essentially prorates the areas of the various occupancies so that the sum of percentages must not exceed 100 percent.

It is also appropriate to utilize a variation of this formula for the determination of the allowable area for the total building as evidenced by the provisions of Section 506.2.4. See Application Example 506-4. For all practical purposes, the need to evaluate the building as a whole is only necessary in buildings of four or more stories above grade plane. For two- and three-story buildings, if each story is compliant for allowable area purposes, the entire building will always comply.

Table 508.4 Required Separation of Occupancies.   The code has established several alternative methods for addressing mixed-occupancy buildings regarding fire separations between the various occupancies involved. Both Sections 508.2.4 and 508.3.3 for accessory occupancies and nonseparated occupancies, respectively, do not require any fire-resistance-rated separation between occupancies. However, Table 508.4, establishing fire separations under the separated occupancy method of Section 508.4, indicates varying degrees of separation. Fire-resistive separations of 1, 2, 3, and 4 hours are selectively required based on the occupancies involved. In some cases, however, no fire separation is mandated. The intent of the table is to provide for relative separation requirements based primarily on dissimilar risk. The fire-resistance ratings, including the lack of such required ratings in many circumstances, appropriately recognize the degree of dissimilarity between the various occupancies.

In a general sense, the following logic was utilized in formulating the table. High-hazard (Group H) occupancies are required to be separated from each other and from all other occupancies. Ordinary or moderate-hazard commercial/industrial (Groups B, F-1, M, and S-1) occupancies require no separation from each other; however, they are required to be separated from all other occupancies. People-intensive (Groups A and E) occupancies also require no occupancy separation between each other but must be separated from all other occupancies except for fully sprinklered Group F-2 and S-2 occupancies. Group R occupancies require no separation from other Group R occupancies, but such separations are mandated between all other occupancy classifications. Similar criteria apply to the Group I occupancies with a modification for Group I-2. The philosophy of the provisions set forth in the table dictates increased fire-resistance ratings on the basis of greater inherent dissimilar risk. Where Table 508.4 mandates some degree of occupancy separation, an occupancy shall be physically separated from the other occupancy through the use of fire barriers, horizontal assemblies, or a combination of both vertical and horizontal fire-resistance-rated assemblies.

In some cases, the rationale behind the use of fire-resistance-rated separations between incompatible occupancies concerns itself with the amount of combustibles encompassed in the adjoining occupancies and is termed in fire-protection circles as fire loading. Thus, if the amount of combustibles or fire loading in one occupancy is quite high while there is a limited fire load anticipated in the other occupancy, some degree of fire separation between the two distinct occupancies is necessary. However, the relationship of fire loads is not the only factor in determining an appropriate occupancy separation. In some cases, the separation is specified to be of 1 hour or more in duration mainly because of what the code implies to be incompatibility between the activities that occur within the two occupancies. For example, the code requires a separation of 1 hour between a Group I-2 occupancy and a Group S-2 occupancy in a fully sprinklered building. The limited amount of combustibles in either occupancy does not justify a fire-resistive separation; however, because of the presumed incompatibility between the two occupancies, the 1-hour separation is considered to be justified.

As a general rule, a reduction of the fire-resistance ratings in Table 508.4 by 1 hour is permitted in buildings equipped throughout with an automatic sprinkler system. The potential of a sizable fire spreading throughout a building is minimal under fully sprinklered conditions.

The fire-resistance ratings set forth by the table are further modified by Footnote b. The required separation for storage occupancies may be reduced by 1 hour where the storage is limited to the parking of private or pleasure vehicles, but may never be less than 1 hour.

It is necessary to again emphasize that there is no relationship between the separated occupancy provisions of Section 508.4 and the fire area concept utilized in Section 903.2 for automatic sprinkler systems. Compliance with Table 508.4 does not relieve the responsibility to comply with Section 901.7 and Table 707.3.10 regarding the proper separation of fire areas. An example is shown in Figure 508-3.

Section 509 Incidental Uses

There are times where the hazards associated with a particular use do not rise to the level of requiring a different occupancy classification; however, such hazards must still be addressed because of their impact on the remainder of the building. These functions are identified as incidental uses, which are regulated independently of the mixed-occupancy provisions. Incidental uses are uniquely addressed through the use of fire-resistance-rated separations or automatic sprinkler system protection.

What is an incidental use? There are occasionally one or more rooms or areas in a building that pose risks not typically addressed by the provisions for the general occupancy group under which the building is classified. However, such rooms or areas may functionally be an extension of the primary use. These types of spaces are considered in the IBC to be incidental uses and are regulated according to their hazard level. These areas are not ever intended to be considered different occupancies, creating a mixed-use condition, but rather are classified in accordance with the main occupancy of the portion of the building where the incidental use is located. There is no specific definition for an incidental use, as it is simply described as any of those rooms or areas listed in Table 509. If it is not listed, it is not considered an incidental use for code purposes.

The designation and regulation of incidental uses do not apply to those areas within and serving a dwelling unit. Otherwise, the special hazards that may be found in buildings of various uses and occupancies are addressed through the construction of a fire barrier and/or horizontal assembly separating the incidental use from the remainder of the building, the installation of an automatic sprinkler system in the incidental use space, or, in special cases, both the fire separation and automatic sprinkler system.

Incidental uses are listed in Table 509. Most of the rooms or areas identified in the table are regulated where located in any of the occupancy groups established by the code, other than dwelling units as previously noted. A few of the incidental uses are to be regulated only where located within a specific occupancy or a limited number of occupancies.

It is common for many of the listed incidental uses to be unoccupied for extended periods of time, creating the potential for a fire to grow unnoticed. Oftentimes, combustible or hazardous materials are present in such areas. Because of the potentially high fuel load and lack of constant supervision, spaces such as furnace rooms, machinery rooms, laundry rooms, and waste collection rooms are selectively considered incidental uses. Other uses, such as paint shops, laboratories, and vocational shops, may cause concern to the point where they too must be protected or separated from other areas of the building. A significant number of incidental uses are located in Group I-2 occupancies and ambulatory care facilities due to the concerns related to occupants incapable of self-preservation.

How are incidental uses regulated? The fire-resistance-rated separations required by Table 509 are to be fire barriers and/or horizontal assemblies, typically having a minimum fire-resistance rating of 1 hour. For incinerator rooms and paint shops, the minimum required rating is greater. Where an automatic sprinkler system provides the necessary protection, it need only be installed within the incidental use under consideration. Examples of the requirements are illustrated in Figure 509-1.

Figure 509-1   Incidental use.

There is a variety of combinations regarding the separation and protection methods identified in Table 509. It should be noted that where an automatic sprinkler system is utilized without a fire barrier as the protective element, the incidental use must still be separated from the remainder of the building. However, this separation need only consist of construction capable of resisting the passage of smoke. Although not required to have a fire-resistance rating, partitions must either extend to the underside of the floor or roof deck above, or to the underside of a fire-resistance-rated floor/ceiling or roof/ceiling assembly. Doors are to be self-closing or automatic-closing upon detection of smoke, with no air-transfer openings or excessive undercuts. See Figure 509-2. The room must be tightly enclosed, providing for the containment of smoke while assisting in the heat increase necessary to activate the automatic sprinkler system.

Figure 509-2   Incidental use smoke separations.

Table 509 Incidental Uses.   Incidental uses are limited to those rooms or spaces listed in this table. The listed rooms have been selected for inclusion because of the increased hazard they present to the other areas of the building. However, it is recognized that the degree of hazard is such that a separate occupancy classification is not warranted. In fact, such a classification may be overly restrictive. The intent of the fire separation and fire protection requirements is to provide safeguards because of the increased hazard level presented by the incidental use.

Furnace rooms and boiler rooms. The hazard potential for fuel-fired heating equipment is addressed once the thresholds established in the table have been exceeded. It should be noted that the limitations are based on individual pieces of equipment, rather than the aggregate amounts from all equipment within the space. For example, the requirements of Table 509 are not applicable where there are two furnaces within the furnace room, each with an input rating of 300,000 Btu per hour (87,900 watts). Because no furnace exceeds the 400,000 Btu/hr (117,200 watts) threshold, there is no fire barrier separation or automatic sprinkler system protection required.

The regulated furnace or boiler is anticipated to be located within a room isolated from the remainder of the building. More specifically, the furnace room or boiler room must either be separated from other portions of the building with a complying fire barrier and/or horizontal assembly, or isolated by construction capable of resisting the passage of smoke and provided with a fire-extinguishing system. In either case, it is not permissible to eliminate the equipment enclosure. Because the intent of the requirement is to address the hazards associated with specified boilers and furnaces, the prescribed degree of separation and/or protection must always be provided.

Hydrogen fuel gas rooms. Hydrogen fuel gas rooms are defined in Chapter 2 as rooms or spaces that are intended exclusively to house a gaseous hydrogen system. Special requirements applicable to such rooms are set forth in Section 421. Where the quantities of materials would cause a hydrogen cut-off room to be classified as a Group H occupancy, the separation requirements of Section 508.4 for separated occupancies will apply rather than those of Table 509.

The reference in the table exempting those rooms classified as Group H is not strictly limited to hydrogen cut-off rooms. In fact, where the quantities of hazardous materials in any of the rooms or areas designated by Table 509 exceed those permitted by Section 307.1 causing classification as a Group H occupancy, the use of the table is not appropriate. In a mixed-occupancy building, all spaces with a Group H classification must be separated from the remainder of the building in accordance with Section 508.4 for separated occupancies.

Paint shops. The provisions of Section 416 control the construction, installation, and use of rooms for spraying paints, varnishes, or other flammable materials used for painting, varnishing, staining, or similar purposes. The paint shops regulated by Table 509 are those rooms where the same types of spraying operations occur. As a general rule, a minimum 1-hour fire-resistive enclosure is mandated by Section 416.2 to isolate the spraying operations from the remainder of the building. However, in all but Group F occupancies, Table 509 mandates a higher degree of protection.

Laboratories and vocational shops. In educational buildings, particularly secondary schools, it is common to find multiple laboratories and vocational shops that are an extension of the educational function. Because of the presence of some quantities of hazardous materials in such laboratories, as well as in those labs associated with Group I-2 occupancies, the code mandates some degree of separation and/or protection. Where the quantities of hazardous materials warrant a Group H classification, the use of this table is inappropriate. Vocational shops in schools also pose a hazard to the remainder of the building that is due to the hazardous processes and combustible materials involved. Where no such hazards exist, such as in a computer lab or design lab, the table is not intended to apply.

Section 510 Special Provisions

The provisions of this section allow for modifications or exceptions to the general provisions for building heights and areas as regulated by Chapter 5 of the IBC. These special provisions are viewed as specific in nature and, based on Section 102.1, take precedence over any general provisions that may apply. Because this section permits, rather than requires, the use of the special conditions established in Section 510, the provisions are optional. Much like the application of the mezzanine provisions of Section 505, only where the designer elects to utilize the special allowances does this section apply.

It is evident that several of the provisions overlap in their scope. For example, Sections 510.2, 510.4, and 510.7 all address a potential condition where an open parking garage is located below a Group R occupancy. It is the choice of the designer which of the three methods to use where such a condition exists, based on the benefits and consequences of each method. Or, as stated above, none of the methods need to be applied. The requirements could simply be based on the general provisions of Chapter 5 for building height and area.

510.2   Horizontal building separation allowance.   This section is one of several that contain provisions that might be considered the only exceptions to the principle that a fire wall is strictly a vertical element without horizontal offsets. In Item 1, the code makes provisions for the use of a minimum 3-hour fire-resistance-rated horizontal assembly as an equivalent construction feature, in certain aspects, to a fire wall. This methodology is often referred to as “podium” or “pedestal” buildings.

The provisions create, in effect, an exception that allows those stories below the horizontal fire separation to be considered portions of a separate building for specific purposes, provided no Group H occupancy is involved. The stories above the horizontal separation must only house Group A, B, M, R, and/or S occupancies. As these occupancies are quite common in Type V construction, the typical application is to grant the maximum number of stories for Type V construction without the penalty of sacrificing one or more stories for a parking garage or other permitted uses located on the level below the 3-hour horizontal assembly. Distinct buildings are also created for area limitations and fire wall continuity.

It is fairly common in terrain that has a rolling or hillside character to erect apartment houses and small office/retail buildings with a garage below. Because of the slope of the ground surface, the lowest level is usually partially within the ground; therefore, the walls are normally designed as reinforced concrete or reinforced masonry-retaining walls. The construction of the lowest level is thereby easily able to conform to the code construction requirements for a Type IA building.

If the lower levels are classified as stories above grade plane, they would typically be included in the number of total stories permitted by the code. However, as depicted in Figure 510-1, those stories would not be included where the building below the 3-hour horizontal separation is of Type IA construction.

Figure 510-1   Horizontal building separation.

The code lists six conditions that must be met in order to take advantage of these provisions. The first condition regulates the fire-resistance rating of the horizontal assembly. Second, the minimum type of construction for portions below the horizontal assembly is established. The third condition addresses the methods for protection of openings that will occur in the fire-resistance-rated horizontal assembly. Fourth, only Group A occupancies having an occupant load of less than 300, and Groups B, M, R, and S are permitted to be located above the horizontal assembly. The fifth condition limits the use of the building below the horizontal assembly to those that are not classified as Group H. An often-forgotten provision is the sixth condition, requiring that the overall height in feet (mm) of both buildings not exceed the height limits set forth in Section 504.3 for the least type of construction that occurs in the building.

510.3   Group S-2 enclosed parking garage with Group S-2 open parking garage above.   The provisions of this section are similar in nature to those of Section 510.2, insofar as the two different parking uses in a single structure, one located above the other, may be considered two separate and distinct buildings for the purpose of determining the type of construction. Five specific conditions must be met in order for an open parking garage, located above an enclosed parking garage, to be regulated on its own for construction type. Details of this special situation are shown in Figure 510-2.

Figure 510-2   Open/enclosed parking.

510.4   Parking beneath Group R.   Consistent with the concepts expressed by Sections 510.2 and 510.3, the provisions of this section address residential uses located above a first story used as a parking garage. Where parking is limited to the first story, the number of stories used in the determination of the minimum type of construction may be measured from the floor above the garage. The construction type of the parking garage and the floor assembly between the residential area and the garage are further regulated. See Figure 510-3.

Figure 510-3   Parking beneath Group R.

510.7   Open parking garage beneath Groups A, I, B, M, and R.   The excellent fire-safety record for open parking garages is the basis for this modification in the general provisions for allowable floor area and allowable height. Where located below assembly, institutional, business, mercantile, or residential occupancies, an open parking garage is regulated for height and area by Section 406.5. Those permitted occupancies located above the parking garage are independently regulated by Section 503.1 for height and area. The only exception requires that the height of the portion of the building above the open parking garage, both in feet (mm) and stories, be measured from the grade plane.

The details of construction type are applicable to each of the occupancies involved; however, the structural-frame members shall be of fire-resistance-rated construction according to the most restrictive fire-resistive assemblies of the occupancy groups involved. Egress from the areas above the parking garage shall be isolated from the garage, with the level of protection at least 2 hours.

Because the provisions of Section 510.2 can also address open parking garages below similar occupancies, the application of this section may be limited. There are several minor differences between the two provisions; however, the general concept remains consistent.

510.8   Group B or M with Group S-2 open parking garage.   A desirable feature in high-density areas is to have offices and/or retail stores on the lower levels of open parking structures. This provision allows for the type of construction for the portion of the structure below the parking garage to be evaluated separately from that of the open parking garage above, provided the uses are properly separated and the means of egress from the garage is independent from that of the first floor (and basement if applicable). This provision reverses the conditions addressed by other provisions of Section 510 where the parking garage is located below other occupancies. The resulting benefit provides for a potential reduction in the type of construction by permitting the evaluation of allowable floor areas independently for the open parking garage and the Group B and/or M occupancies.

510.9   Multiple buildings above a horizontal assembly.   Where the varying provisions of Section 510 are utilized to create separate buildings above and below a complying horizontal separation, it is acceptable for two or more buildings to be located above the separation while only one building (a parking garage) is located below. For example, a condominium building is permitted to be regulated as a separate building from an adjacent office building even though both are located above a single parking facility designed under the special provisions of Sections 510.2, 510.3, or 510.8, as applicable. An example is shown in Figure 510-4.

Figure 510-4   Separate buildings above horizontal separation.