TYPES OF CONSTRUCTION

Section 602 Construction Classification

Section 603 Combustible Material in Type I and II Construction

Key Points

Section 602 Construction Classification

Since early in the last century, the fire protection required for the various types of construction has been based on hourly fire-endurance ratings as established by the American Society for Testing and Materials. Prior to this time, fire-resistance requirements were developed by specifying the type and thickness of materials used.

Many of the concepts in previous building codes that have carried over to today were developed from the reports issued by the committee known as the Department of Commerce Building Code Committee, which was appointed by Herbert Hoover, then Secretary of Commerce. The committee was also dubbed the Little Hoover Commission and was appointed to investigate building codes. This was an outgrowth of the findings of the Senate Committee on Reconstruction and Production, which was appointed in 1920 to study the various factors entering into the recovery of our economy from the depression of the early 1920s. Although the committee studied a wide-ranging set of those institutions and groups affected by the economy, it was especially interested in construction. During its tenure, the committee held numerous hearings and expressed the following sentiment at their conclusion: “The building codes of this country have not been developed on scientific data, but rather on compromise; they are not uniform in practice and in many instances involve an additional cost of construction without assuring more useful or more durable buildings.” Thus, the stage was set for improvement in building regulations, and the timing was especially favorable for the model codes to take advantage of the reports of the Department of Commerce Building Code Committee.

The IBC classifies construction into five basic categories, listed in a somewhat descending order from the most fire resistant to the least fire resistant. These five types are based on two main groupings, noncombustible (required) construction (Types I and II) and combustible (permitted) construction (Types III, IV, and V). The various types of construction within the five categories are further subdivided based on fire protection and are represented as follows:

1.   Noncombustible, protected—Types IA, IB, and IIA

2.   Noncombustible, unprotected—Type IIB

3.   Combustible and/or noncombustible, protected—Types IIIA, IV, and VA

4.   Combustible and/or noncombustible, unprotected—Types IIIB and VB

Although Types III, IV, and V are commonly considered combustible construction, the use of noncombustible materials, either in part or throughout the building, is certainly acceptable. The reference to combustible construction more simply indicates that such construction is acceptable in Types III, IV, and V but not mandated. A perusal of Tables 504.3, 504.4, and 506.2 will show the reader that the IBC considers Type II, III, and IV buildings to be of comparable protection. For example, Types IIA, IV, and, to some degree, IIIA are permitted the same approximate areas and heights for most occupancy classifications. The same is also true for Types IIB and IIIB.

Differing from the concept of mixed-occupancy buildings, the code does not permit a building to be considered to have more than one type of construction. In simple terms, classification of a building for construction type is based on the weakest link concept. If a building does not fully conform to the provisions of Chapter 6 for type of construction classification, it must be classified into a lower type into which it does conform. Unless specifically permitted elsewhere by the code, the presence of any combustible elements regulated by Table 601 prohibits its classification as Type I or II construction. Similarly, the lack of required fire resistance in any element required by Table 601 to be protected will result in a fully nonrated building.

Table 601 identifies the required fire-resistance ratings of building elements based on the specified type of construction. Exterior walls are further regulated by Table 602 based on the building’s location in relation to adjoining lot lines and public ways. Reference is made to Section 703.2 for those building elements required to have a fire-resistance rating by Table 601. Section 703.2 establishes the appropriate test procedures for building elements, components, and assemblies that are required to have a fire-resistance rating.

The provisions of Chapter 6 in regard to fire resistance are intended to address the structural integrity of the building elements under fire conditions. Unlike those fire-resistance-rated assemblies, such as fire walls and fire barriers, whose intent is to safeguard against the spread of fire, the protection afforded by the provisions of Chapter 6 is solely that of structural integrity. As such, the protection of door and window openings, ducts, and air transfer openings is not required for building elements required to be fire-resistance rated by Table 601 unless mandated by other provisions of the IBC.

The IBC intends that the provisions of the code are minimum standards. Thus, Section 602.1.1 directs that buildings not be required to conform to the requirements for a type of construction higher than the type that meets the minimum requirements of the IBC based on occupancy. A fairly common case in this regard is where a developer may construct an industrial building that complies in most respects to the requirements of the code for a Type IIIB building, but the occupancy provisions are such that a Type VB building would meet the requirements of the code. In this latter case, it would be clearly inappropriate and, in fact, a violation of the code for the building official to require full compliance with requirements for a Type IIIB building. However, where the building does comply in all respects to Type IIIB, the building official may so classify it.

602.2   Types I and II.   Buildings classified as Type I and II are to be constructed of noncombustible materials unless otherwise modified by the code. The various building elements in these noncombustible buildings are regulated by Table 601. Although Type I and II buildings are defined as noncombustible, it is evidenced by Section 603 that combustible materials are permitted in limited quantities. Wood doors and frames, trim, and wall finish are permitted, as well as combustible flooring, insulation, and roofing materials. Where these combustibles are properly controlled, they have proven, over the years, to not add significantly to the fire hazard.

Furthermore, Type I buildings are to be of the highest levels of fire-resistance-rated construction. The fire-resistance ratings required for Type I buildings historically have provided about the same protection over the years and, thus, have proved to be satisfactory for occupancies with low to moderate fire loadings, such as office buildings, hotels, and retail stores. Type IB construction is very similar to Type IA construction except for a reduction of 1 hour in the required ratings for interior and exterior bearing walls, and the structural frame, while providing a ½-hour reduction for roof construction. Thus, and particularly because of the reduction in the fire-resistance rating required for the structural frame, the Type IB building does not enjoy all of the unlimited height and areas that accrue to the Type IA building. It will be noted from Tables 504.3, 504.4, and 506.2 that Type IB construction typically has height and, to some degree, area limits placed on it.

Buildings of Type II construction, although noncombustible, may be of either protected (Type IIA) or unprotected (Type IIB) construction. The building elements of a Type IIA building are typically required to be protected to a minimum fire-resistance rating of 1 hour. Such elements in a Type IIB structure may be nonrated.

602.3   Type III.   The Type III building grew out of the necessity to prevent conflagrations in heavily built-up areas where buildings were erected side by side in congested downtown business districts. After the severe conflagrations of years past in Chicago and Baltimore, it became apparent that some control must be made to prevent the spread of fire from one building to another. As a result, the Type III building was defined. The Type III building is, in essence, a wood-frame building (Type V) with fire-resistance-rated noncombustible exterior walls.

Around the turn of the 20th century, and prior to the promulgation of modern building codes, Type III buildings were known as ordinary construction. They later became known in some circles as ordinary masonry construction. However, as stated previously, the intent behind the creation of this type of construction was to prevent the spread of fire from one combustible building to another. Thus, the early requirements for these buildings were for a certain thickness of masonry walls, such as 13 inches (330 mm) of brick for one-story and 17 inches (432 mm) for two-story buildings of bearing-wall construction. Later, the required fire endurance was specified in hours. Thus, any approved noncombustible construction that would successfully pass the standard fire test for the prescribed number of hours was permitted.

In spite of the requirement for noncombustible exterior walls, Type III buildings are considered combustible structures and are either protected (Type IIIA) or unprotected (Type IIIB). Interior building elements are permitted to be either combustible or noncombustible. There is an allowance for the use of fire-retardant-treated wood as a portion of the exterior wall assembly, provided such wall assemblies have a fire-resistance rating of 2 hours or less.

602.4   Type IV.   Type IV buildings are designated as heavy-timber buildings. In the eastern United States during the 1800s, a type of construction evolved that was known as mill construction. Mill construction was developed by insurance companies to reduce the heavy losses they were facing in the heavy industrialized areas of the Northeast.

This type of construction has also been known as slow burning. Wood under the action of fire loses its surface moisture, and when the surface temperature reaches about 400°F (204°C), flaming and charring begin. Under a continued application of the heat, charring continues, but at an increasingly slower rate, as the charred wood insulates the inner portion of the wood member. There is quite often enough sound wood remaining during and after a fire to prevent sudden structural collapse. In recognition of these characteristics, the insurance interests reasoned that replacement of light-wood framing on the interior of factory buildings with heavy-timber construction would substantially decrease their fire losses.

The Type IV building is essentially a Type III building with a heavy-timber interior. It is of interest to see how the 1943 edition of the National Building Code, developed by the National Board of Fire Underwriters, defined heavy-timber construction:

“Heavy-timber construction,” as applied to buildings, means that in which walls are of approved masonry or reinforced concrete; and in which the interior structural elements, including columns, floors and roof construction, consist of heavy timbers with smooth, flat surfaces assembled to avoid thin sections, sharp projections and concealed or inaccessible spaces; and in which all structural members which support masonry walls shall have a fire-resistance rating of not less than 3 hours; and other structural members of steel or reinforced concrete, if used in lieu of timber construction, shall have a fire-resistance rating of not less than 1 hour.

From this definition, it can be seen that in the early development of heavy-timber construction, not only did the heavy-timber members have large cross sections to achieve the slow-burning characteristic, but, furthermore, surfaces were required to be smooth and flat. Sharp projections were to be avoided, as well as concealed and inaccessible spaces. Thus, the intent of the concept is to provide open structural framing without concealed spaces and without sharp projections or rough surfaces, which are more easily ignitable. In this case, flame spread along the surface of heavy-timber members is reduced, and without concealed blind spaces, there is no opportunity for fire to smolder and spread undetected.

In accordance with Table 601 and Section 602.4, modern-day heavy-timber construction can be a mixture of heavy-timber floor and roof construction and 1-hour fire-resistance-rated bearing walls and partitions. Although heavy-timber construction is not generally recognized as equivalent to 1-hour fire-resistance-rated construction, the code considers heavy timber to provide equivalent protection in Type IV buildings.

In keeping with the concept of slow-burning construction by means of wood members with large cross sections, the IBC specifies minimum nominal dimensions for wood members used in heavy-timber construction. As the code specifies the size of members as nominal sizes, the actual net surfaced sizes may be used. For example, an 8-inch by 8-inch (203-mm by 203-mm) member nominally will actually be a net size of 7¼ inches by 7¼ inches (185 mm by 185 mm). Therefore, even though the code calls for a nominal 8-inch by 8-inch (203-mm by 203-mm) member, the net 7¼-inch by 7¼-inch (185-mm by 185-mm) member meets the intent of the code. As indicated earlier, the minimum sizes for heavy-timber construction as listed in this section are based on experience and the good behavior in fire of heavy-timber construction.

Wherever framing lumber or sawn timber is specified, structural glued-laminated timber or structural composite lumber may also be used, as all have the same inherent fire-resistive capability. However, because solid sawn wood members, glued-laminated timbers, and structural composite lumber are manufactured with different methods and procedures, they do not have the same dimensions. Table 602.4 compares the solid sawn sizes with those of glued-laminated members and structural composite lumber to indicate equivalency in regard to compliance with the Type IV construction criteria.

Section 602.4.8 specifies that partitions shall be of either solid-wood construction or 1-hour fire-resistance-rated construction. However, various provisions of the code address the use of fire partitions and fire barriers. In these cases, the fire-resistant-rated fire partitions or fire barriers in heavy-timber buildings should be constructed as required by the code for the required rating. For example, where there is a requirement for fire-resistance-rated corridors in heavy-timber buildings, 1-hour fire-resistance-rated construction must be used rather than solid-wood construction for the partitions.

It is highly unusual for any building designed and constructed today to be considered compliant as a Type IV structure. As previously addressed, in order for a building to be properly classified, all portions must be in conformance with the established criteria. Many buildings may have some heavy-timber elements that qualify as Type IV; however, the floor construction and/or roof construction does not fully comply with the prescriptive requirements of Sections 602.4.6 and 602.4.7, respectively. In such cases, the building cannot be classified as Type IV. Such buildings are most likely Type III or V construction. However, even if the building as a whole is not considered a Type IV structure, the recognition of individual heavy-timber elements is very important. For example, the provisions of Section 705.2.3 recognize Type IV heavy-timber projections for use in locations where unprotected combustible construction is not permitted. For this and other reasons, the requirements for Type IV heavy timber must be fully understood.

602.5   Type V.   Type V buildings are essentially construction systems that will not fit into any of the other higher types of construction and may be constructed of any materials permitted by the code. The usual example of Type V construction is the light wood-frame building consisting of walls and partitions of 2-inch by 4-inch (51-mm by 102-mm) or 2-inch by 6-inch (51-mm by 152-mm) wood studs. The floor and ceiling framing are usually of light wood joists of 2-inch by 6-inch (51-mm by 152-mm) size or deeper. Roofs may also be framed with light wood rafters of 2-inch by 4-inch (51-mm by 102-mm) size or deeper cross sections or, as is now quite prevalent, framed with pre-engineered wood trusses of light-frame construction. Wood-frame Type V buildings may be constructed with larger framing members than just described, and these members may actually conform to heavy-timber sizes. Such structures sometimes have a limited number of noncombustible building elements. However, unless the building complies in all respects to one of the other four basic types of construction, it is still a Type V building.

Type V construction is divided into two subtypes:

1.   Type VA. This is protected construction and required to be of 1-hour fire-resistance-rated construction throughout.

2.   Type VB. This type of construction has no general requirements for fire resistance and may be of unprotected construction, except where Section 602.1 and Table 602 require exterior wall protection because of proximity to a lot line.

Table 601—Fire-Resistance Rating Requirements for Building Elements.   This table provides the basic fire-resistance rating requirements for the various types of construction. It also delineates those fire-resistance ratings required to qualify for a particular type of construction. As previously discussed, even though a building may have some features that conform to a higher type of construction, the building shall not be required to conform to that higher type of construction as long as a lower type will meet the minimum requirements of the code based on occupancy. Nevertheless, any building must comply with all the basic fire-resistance requirements in this table if it is indeed the intent to classify it for that particular type of construction. For example, in order for a building to be classified as Type IIA noncombustible construction, a minimum 1-hour fire-resistance rating is required for any and all structural frame members, bearing walls, floor construction, and roof construction. In addition, and with limited exceptions, all of these building elements must be recognized as noncombustible.

Footnote a.   Limited to buildings of Type I construction, the fire-resistance ratings of primary structural frame elements and interior bearing walls supporting only a roof may be reduced by 1 hour. In other words, primary structural-frame members or interior bearing walls providing only roof support shall have a minimum fire-resistance rating of 2 hours in Type IA buildings and 1 hour in Type IB construction. Additional provisions addressing the protection of certain primary structural frame members are found in Section 704.

Footnote b.   This footnote, an exception to the general rule for roof construction, addresses those situations where the roof and its components are 20 feet (6,096 mm) or more above any floor immediately below. Under these circumstances, the roof and its components, including roof framing and decking, may be of unprotected construction. The reduction of the fire-resistance rating would apply to buildings of Type IA, IB, IIA, IIIA, and VA construction. The footnote mandates that all portions of the roof construction must be located at or above the 20-foot (6,096-mm) height requirement. For example, in a sloped roof condition, it is not acceptable to merely protect those portions below the 20-foot (6,096-mm) point and leave the remainder unprotected. See Figure 601-1. It is important to note that the elimination of any required fire-resistance rating is not applicable to elements of the roof construction considered as primary structural frame members.

Figure 601-1   Fire-resistive ratings for roof construction.

The reduction in rating applies to all occupancies other than Groups F-1, H, M, and S-1, where fire loading is typically higher. In all occupancies other than those just listed, the relaxation of the requirements is based on the fact that where the roof is at least 20 feet (6,096 mm) above the nearest floor, the temperatures at this elevation during most fire incidents are quite low. As a result, fire protection of the roof and its members, including the structural frame, is not necessary. For those occupancies where the fire loading and the consequent potential fire severity is relatively high, such as factory-industrial, hazardous, mercantile, or storage uses, the code does not permit a reduction in roof protection. It is also quite common in these occupancies for combustible or hazardous materials to be located in close proximity to the roof structure, as in the case of high-piled storage.

Footnote c.   Applicable to Types IB, IIA, IIB, IIIA, and VA construction, the code permits heavy-timber members complying with Section 602.4 to be used in the roof construction without any fire-resistance rating as required by the table. It is assumed that roof members sized and constructed in compliance with the details of heavy-timber construction are equivalent to roof construction having a 1-hour fire-resistance rating. In addition, heavy-timber members are permitted to be utilized in the roof construction of an otherwise noncombustible Type IB, IIA, or IIB building.

Footnote d. In addition to any required fire-resistance rating based on the type of construction of the building per Table 601, it is also necessary that such rating requirements for exterior walls, both bearing and nonbearing, be in compliance with Table 602. The table regulates the hourly fire-resistance ratings for exterior walls based on fire separation distance. This footnote specifically indicates that exterior bearing walls have a fire-resistance rating based on Table 601 or 602, whichever provides for the highest hourly rating. Exterior nonbearing walls are totally regulated by the rating requirements found in Table 602. The provisions of Section 704.10 must also be consulted where load-bearing structural members are located within the exterior walls or on the outside of the building. See Application Example 601-1 for the appropriate use of these provisions. In addition, applicable provisions in Section 603 regulating combustible material in Type I and Type II construction may apply to exterior walls.

Table 602—Fire-Resistance Rating Requirements for Exterior Walls Based on Fire Separation Distances.   The IBC, as far as exterior wall protection is concerned, operates on the philosophy that an owner can have no control over what occurs on an adjacent lot and, therefore, the location of buildings on the owner’s lot must be regulated relative to the lot line. In fact, the location of all buildings and structures on a given piece of property is addressed in relation to the real lot lines as well as any assumed or imaginary lines between buildings on the same lot. The assumption of imaginary lines is discussed with other exterior wall provisions in Section 705.

Application Example 601-1

The lot-line concept provides a convenient means of protecting one building from another insofar as exposure is concerned. Exposure is the potential for heat to be transmitted from one building to another under conditions in the exposing building. Radiation is the primary means of heat transfer.

The code specifically provides that the fire separation distance be measured to the center line of a street, alley, or public way. As the code refers to public way, this would also be applicable to appropriate open spaces other than streets or alleys that the building official may determine are reasonably likely to remain unobstructed through the years.

The regulations for exterior wall protection based on proximity to the lot line are contained in Table 602. The IBC indicates that the distances are measured at right angles to the face of the exterior wall (see definition of “Fire separation distance” in Section 202), which would result in the fire-resistive requirements for exterior walls not applying to walls that are at right angles to the lot line. See Figure 602-1.

Figure 602-1   Fire separation distance.

In order to properly utilize Table 602, it is necessary to identify the fire separation distance, the occupancies involved, and the building’s type of construction. As the fire separation distance increases, the fire-resistance rating requirements are reduced, based on the occupancy group under consideration. Figure 602-2 illustrates the application of exterior wall protection where the exterior walls of the building are parallel and perpendicular to the lot line. In this case, the illustration assumes that the building is one story of Type VB construction and used for offices (Group B). Referring to Table 602, it is noted that exterior walls less than 10 feet (3,048 mm) from the lot line must be of minimum 1-hour fire-resistance-rated construction. Figure 602-3 depicts a similar building located such that the exterior walls are not parallel and perpendicular to the lot line, but are at some angle other than 90 degrees (1.57 rad). The regulation of openings in exterior walls is set forth in Section 704.8. Several footnotes to the table address modifications to the general requirements. Footnote a repeats a previous requirement that load-bearing exterior walls must comply with both Tables 601 and 602.

Figure 602-2   Exterior wall rating.

Figure 602-3   Exterior wall rating.

Although Table 602 requires a Group S-2 occupancy of Type I, II, or IV construction to have a minimum 1-hour exterior wall where the fire separation distance is less than 30 feet (9,144 mm), Footnote c reduces that distance significantly where it is a complying open parking garage. Under such conditions, a minimum 1-hour fire-resistance-rated exterior wall is required only where the fire separation distance is less than 10 feet. Footnote d indicates that each story of the building is regulated independently for the fire separation distance provisions, as shown in Figure 602-4.

Figure 602-4   Fire separation distance measurement.

Table 705.8 allows for an unlimited amount of unprotected openings in exterior walls of a sprinklered building that has a fire separation distance of at least 20 feet (6,096 mm). However, in certain buildings, Table 602 requires those same exterior walls to be fire-resistance rated for a minimum of 1 hour. Footnote g recognizes that any nonbearing exterior wall permitted to be entirely open due to the unlimited unprotected opening allowances of Table 705.8 need not be required to have a fire-resistance rating due to fire separation distance.

There are only a small percentage of buildings where this footnote is applicable. It has no effect on:

•   Exterior bearing walls

•   Group H-1, H-2, and H-3 occupancies

•   Nonsprinklered buildings

•   Buildings of Type IIB and VB construction, other than Groups H-4 and H-5

•   Exterior walls with a fire separation distance of less than 20 feet (6,096 mm)

•   Exterior walls with a fire separation distance of 30 feet or more (9,144 mm)

Section 603 Combustible Material in Type I and II Construction

Buildings of Type I and II construction are considered noncombustible structures. As such, all of the building elements, including walls, floors, and roofs, are to be constructed of noncombustible materials. There are, however, a variety of exceptions to the general rule that allow a limited amount of combustibles to be used in the building’s construction. It has been determined that the level of combustibles permitted by Section 603.1, as well as their control, does not adversely impact the fire-severity potential caused by the materials of construction.

The following listing provides an overview of some of those combustible materials permitted in Type I and II buildings:

1.   Fire-retardant-treated (FRT) wood may be used in the construction of interior nonbearing partitions where the required fire-resistance rating of the partitions does not exceed 2 hours. In nonbearing exterior walls, FRT wood is permitted provided no fire rating of the exterior walls is mandated. Roofs constructed of FRT wood are also acceptable in most buildings. This would include roof girders, trusses, beams, joists, or decking, as well as blocking, nailers, or similar components that may be a part of the roof system. Where the building is classified as other than Type IA construction, the use of FRT wood roof elements is permitted in all cases, regardless of building height. The same allowance is permitted in one- and two-story buildings of Type IA construction. For Type IA buildings exceeding two stories in height, the use of FRT wood in the roof construction is only allowed if the uppermost story has a height of at least 20 feet (6,096 mm). Logically, the 20-foot measurement would be taken in a manner consistent with that described in Footnote b of Table 601, from the floor to the lowest point of the roof construction above.

The allowances provided in Section 603.1, Item 1.3, do not reduce any required level of fire resistance mandated for wall or roof construction as established by Table 601. Rather, they simply allow the use of FRT wood in the locations listed where noncombustible construction is otherwise required. In reviewing the permitted use of FRT wood, there are two obvious building elements where such materials are not permitted in Type I or II construction. In Type I or II buildings, FRT wood is not permitted to be used in the floor construction and any bearing wall assemblies.

2.   Combustible insulation used for thermal or acoustical purposes is acceptable, provided the flame-spread index is limited. Additional regulations addressing the use of thermal- and sound-insulating materials within buildings are found in Section 720.

3.   Foam plastics installed under the limitations of Chapter 26 are permitted, as are roof coverings having an A, B, or C classification as specified in Section 1505.

4.   Wood doors, door frames, window sashes and frames, trim, and other combustible millwork and interior surface finishes are acceptable, as is blocking for handrails, grab bars, cabinets, window and door frames, wall-mounted fixtures, and similar items. Combustible stages and platforms are also permitted when complying with Section 410, and wood-finish flooring may be used when applied directly to the floor slab or installed over wood sleepers and fireblocked in accordance with Section 805.1.2.

Another allowable use of combustible elements in noncombustible buildings, detailed in Item 11, addresses the situation where nonbearing partitions divide portions of stores, offices, or similar spaces occupied by one tenant only. The key words in this item are “occupied by one tenant only.” It is the intent of the IBC that this expression applies to an area or building that is under the complete control of one person, organization, or other occupant. This would be contrasted to multitenant occupancies, where the various tenant spaces in the building would be under the control of two or more individuals, companies, or occupants. In such a multitenant space, the walls common to the public areas and to other tenants would not be regulated under this allowance. However, within each of the tenant spaces, those nonbearing walls and partitions not common with other tenants or public areas could utilize the optional construction methods of Item 11.

Reference is also made under Item 24 to Section 718.5 for the allowance of specific combustible elements within concealed spaces. The allowance for combustible items in concealed spaces is limited because of the increased potential for fire spread. Therefore, the flame spread index and smoke-developed index of the permitted items are often highly regulated. Combustible piping is permitted to be installed within partitions, shaft enclosures, and concealed ceiling spaces of noncombustible buildings. Various combustible materials are also permitted in plenums of Type I and II buildings, including wiring, fire-sprinkler piping, pneumatic tubing, and foam plastic insulation under the limitations imposed by Section 602 of the International Mechanical Code® (IMC®).