SPECIAL CONSTRUCTION

Section 3102 Membrane Structures

Section 3104 Pedestrian Walkways and Tunnels

Section 3106 Marquees

Key Points

Section 3102 Membrane Structures

3102.1   General.   Because membrane structures have several unique characteristics that set them apart from other buildings, they are regulated in Chapter 31 under the provisions for special construction. The regulations cover all such structures, including air-supported, air-inflated, cable-supported, frame-supported, and tensile membrane structures. The intent of the provisions is that, except for the unique features of membrane structures, they otherwise comply with the code as far as occupancy requirements, allowable area, and other regulations are concerned. Membrane structures are limited to one story in height insofar as there is insufficient experience to justify multilevel structures enclosed with a membrane.

The membrane structures regulated by the IBC are deemed to be permanent in nature, erected for a period of at least 180 days. Membrane structures in place for shorter periods of time, such as temporary tents, are to be regulated by the International Fire Code® (IFC®). Where a membrane structure is erected as a part of a permanent structure, such as a covering for a building, balcony, or deck, it must comply with the provisions of Section 3102 for any time period.

Because of the limited hazards present in structures not used for human occupancy, such as water-storage facilities, water clarifiers, sewage-treatment plants, and greenhouses, only a few provisions are applicable where membrane structures cover these types of facilities. Limitations on the membrane and interior liner material, as well as the structural design, are the only criteria in the IBC that apply to membrane structures covering facilities not typically used for human occupancy.

3102.3   Type of construction.   In general, membrane structures are considered to be of Type V construction, except where the membrane structure is shown to be noncombustible. In this case, the membrane structure should be classified as Type IIB construction. Membrane structures supported by heavy-timber framing members are to be considered Type IV construction. The code permits the use of nonflame-resistant plastic material for the membrane of a greenhouse structure that is not available to the general public.

3102.6   Mixed construction.   This section permits the use of a noncombustible membrane on a structure that would otherwise comply as Type IA, IB, or IIA where the membrane is used exclusively as a roof or skylight and is located at least 20 feet (6,096 mm) above any floor, balcony, or gallery. This exception is similar to Footnote b of Table 601. This exception will permit nonrated noncombustible membranes to be constructed as roof systems for sports stadiums and similar buildings as well as for atriums. In other types of construction under the same conditions, the membrane need only be flame resistant.

3102.8   Inflation systems.   Where membrane structures are air-supported or air-inflated, this section addresses the regulations for equipment, standby power, and support. The primary inflation system shall consist of one or more blowers, designed in such a manner that over pressurization is prevented. Air-supported or air-inflated structures exceeding 1,500 square feet (140 m²) in floor area must also be provided with an ancillary inflation system. This backup system, connected to an approved standby power-generating system, shall operate automatically to maintain the inflation of the structure if the primary system fails.

Additional support for the membrane must also be provided where covering structures that have occupant loads of more than 50 and where covering swimming pools.

Section 3104 Pedestrian Walkways and Tunnels

This section regulates connecting elements between buildings, such as tunnels or pedestrian walkways, that are utilized for occupant circulation. The provisions of this section are only applicable to such tunnels and walkways designed primarily as circulation elements, typically for weather-protection purposes. A covered walkway or bridge connecting two buildings is the most common example of a pedestrian walkway. These elements may be located below, at, or above grade. When in compliance with this section, pedestrian walkways and tunnels are not considered to contribute to the floor area or height of the connected buildings. In addition, those buildings connected by the pedestrian walkway or tunnel are permitted to be considered separate structures. These allowances establish the primary reason for the use of this section. Where multiple structures are attached, they would generally be considered by the code to be a single building and regulated as such. These provisions not only allow each building to be regulated independently, but also limit the requirements of the tunnel or pedestrian walkway to those provisions of this section. A common use of pedestrian walkways is the connection of buildings that are on separate lots, including situations where the buildings are on opposite sides of a public way. Often referenced as skyways in northern climates, pedestrian walkways allow for a method to connect buildings across lot lines without the normally mandated fire-resistance-rated exterior walls and opening prohibition associated with a fire separation distance of zero. The pedestrian walkway is treated as a nonbuilding and therefore is not regulated where it crosses the lot line. Instead, this approach allows for a level of fire protection at the connection between the buildings and the pedestrian walkway.

It is important to understand that this section is essentially voluntary in application, and is only utilized where the design professional chooses not to consider the walkway and connected buildings a single structure. There is always the option of regulating the entire structure as a single building, in which case this section would have no application. See Figure 3104-1.

Figure 3104-1   Pedestrian walkways.

This section establishes specific requirements for the protection of walls and openings between the connected buildings and the pedestrian walkway or tunnel, specifies minimum and maximum widths, and limits the length of exit access travel within a pedestrian walkway or tunnel. Section 3104.3 further requires that pedestrian walkways be constructed of non-combustible materials or of fire-retardant-treated wood unless all connected buildings are of combustible construction.

As previously mentioned, where pedestrian walkways and tunnels are designed and constructed in accordance with the provisions of this section, the code intends that they not be consid-ered part of the connected buildings. Furthermore, they need not be considered in the determination of the allowable area or height of either of the connected buildings. In effect, this section is an exception to the general construction requirements, and its use is optional. Otherwise, the designer would consider the connected buildings to be a single building on the same lot, including the pedestrian walkway portion.

Section 3106 Marquees

A marquee is defined in Section 202 as a canopy with a flat, or relatively flat, roof surface located in close proximity to operable building openings. A canopy is defined as a permanent roof structure or architectural projection that is structurally independent or attached to and supported by a building. A classic example of a marquee is the theater marquee or the entrance marquee at a hotel. The restrictions placed on the construction, projection, and clearances for marquees are intended to prevent interference with:

1.   The free movement of pedestrians.

2.   Trucks and other tall vehicles using the public street.

3.   The fire department in its fire-fighting operations at a building.

4.   Utilities.

Thus, the height, construction, and location of the marquee are regulated to meet the intent of the code. Figure 3106-1 depicts the permissible projection and clearances required for marquees.

Figure 3106-1   Marquees.