Code considerations in fabric structure design

May 1st, 2010 / By: / Continuing Education

Elegance, airiness and sculptural possibilities may motivate an architect to incorporate a fabric structure in a building design. Implications of this choice are not limited to building form. Design of a fabric structure as part of a large-scale commercial project may require an architect to collaborate with a manufacturer to properly complete design and construction. Similarly, achieving code compliance may involve a code consultant. As owners, architects and others involved in building design and construction adopt an Integrated Design Process (IDP), such collaborative efforts will facilitate better integration of fabric structures in a complex building design.

The International Building Code (IBC) includes provisions for fabric structures in Chapter 31-Special Construction. In this way, they are addressed separately from conventional building systems covered in Chapter 6-Types of Construction. In Chapter 6, structural frame, walls, floors and roofs are assigned minimum fire-resistance ratings for each of the five basic construction types. Fabric structures are not mentioned. Chapter 31 refers to Chapter 6-Types, classifying noncombustible membrane structures as Type IIB construction. This means membrane structures, even where noncombustible, cannot achieve Type I classification, which entails the highest level of fire-resistance. Membrane structures other than the noncombustible variety are classified as either Type IV or Type V.

IBC Chapter 31 contains a brief section for “membrane structures” and another for “awnings and canopies.” The membrane structure section applies to spaces sheltering large numbers of occupants-assembly occupancies-for example. Assembly occupancies trigger requirements for high-level fire-resistance such as Type I construction and automatic sprinkler systems, both of which present challenges to incorporating a fabric structure.

Two projects illustrate complexity of code and structural issues triggered where fabric structures are designed for large-scale facilities. Each project is an example of how collaboration has optimized fabric structure design and achieved code compliance.

Case study: Fabric entrance canopy

University of Nevada at Las Vegas Student Union Building

Architects Tate, Snyder & Kimsey designed a fabric entrance canopy as part of a 135,000 sq. ft. student center, completed in 2007 for the University of Nevada, Los Vegas (UNLV). This facility includes office, lab, conference, food service, theater and ballroom-a mix of assembly and other occupancies enclosed with conventional construction.

The fabric structure is separated from the building proper and is limited to shading an outdoor plaza-amphitheatre. Nineteen steel columns support a two-level horizontal grid, incorporating what IBC Chapter 31 refers to as a “membrane-covered frame structure.” The fabric membrane is tensioned to support itself (resisting wind loads) and to function as a weather barrier, shading the plaza below.

Separating the canopy from the main building and elevating the fabric grid 40 ft. above plaza level complied with fire-safety provisions of the building code; however, there were structural issues to resolve. The original design envisioned rectangular fabric panels oriented horizontally, which raised concerns identified by the canopy fabricator, Span Systems, over long-term structural performance of the fabric. After completing 3-D modeling and shade studies, the fabricator proposed tensioned saddle-shape fabric panels spanning between the two parallel grids. Curved panel design tensions the fabric surface more uniformly and complements the rectilinear grid. Shading coverage is evident in the photograph of the finished canopy.

The canopy fabricator assumed responsibility for generating revised panel geometry and for internal structural design of the canopy system. This procedure is similar to a floor and roof truss manufacturer assuming responsibility for engineering the design of trusses. Building permit submittals to code officials include canopy structural design documents certified by the canopy fabricator’s engineer. In such situations, it is necessary to clearly define the line (or plane) separating responsibilities of the canopy engineer from those of the building structural engineer. For this project, the building structural engineer was responsible for canopy foundations, the canopy engineer for the superstructure.

Case study: Fabric Tensile Roof System

Sun Valley Pavilion, Idaho

In 2007 architects Ruscitto/Latham/Blanton Architectura designed the Sun Valley Music Pavilion within the ski resort of the same name. The building code for this snowy region demands that a roof be designed to carry a minimum snow load of 100 lbs./sq. ft.; consequently, it was structurally and visually desirable to minimize the extent of a permanent roof. The outdoor concert facility is roofed by a permanent structure of steel cable net and wood and a seasonal roof of lightweight fabric membrane that can extend in the summer. The roofed area seats 1,500 and an adjacent lawn seats an additional 2,000. Such high occupant loads trigger stringent code requirements for fire safety and emergency exiting.

Behind the stage are extensive ancillary spaces including loading docks and storage rooms, as well as restrooms, music practice rooms, dressing areas and other support spaces. This mix of occupancies extending over an area exceeding 50,000 sq. ft. presents a challenge to the architect analyzing code requirements and devising a strategy for compliance.

Rolf Jensen and Associates (RJA) was retained by the architect to provide code consulting services. A central issue taken on by RJA was the requirement for an automatic sprinkler system to cover the entire building complex, including the area covered by the seasonal roof membrane. The lack of a permanent solid frame at the membrane roof made it impractical to provide a sprinkler system over the seating, and constructing a fire separation wall between sprinklered and unsprinklered areas also was impractical.

Using a weighted average approach, RJA showed that the sum of ratios of actual to allowable areas for sprinklered and non-sprinklered portions of the complex was less than one. In other words, actual areas were shown to be relatively small compared to allowable areas, yielding small fractional values-indicating a margin of safety. This approach was approved by local building officials under the Alternative Protection provision in IBC Section 903-Automatic Sprinkler Systems.

RJA also performed detailed evaluation of likely fire scenarios to deduce that areas along the path of egress where the roof membrane is less than 20 ft. above the floor surface do not pose an increased threat to occupants. In addition, RJA calculated egress widths as though the entire building were without a sprinkler system.

The ability of RJA to analyze complex code issues and successfully pursue approvals from local building officials confirms the wisdom of the architects’ decision to retain a code consultant.

James A. Strapko, RA, LEED AP, is a principal in Strapko Associates, and a regular contributor to Fabric Architecture.

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