Frame-supported fabric structures: codes and loads defined

It is always prudent to research and develop a thorough understanding of the design and code criteria of any project.

Produced by the Fabric Structures Association

The origins of frame-supported fabric structures predate historical records. They are arguably among the oldest of man-made structures and provide many benefits, including low cost, ease and speed of erection, and light weight with efficient use of resources.

The fundamental principles of frame-supported fabric structures have not changed, but they continue to evolve through new designs, materials and the technologies used to make them. For this reason, fabric structures are increasingly recognized as some of the most sophisticated structures on the planet, yet they remain foreign to many people in the construction industry.

Fabric structures are not “conventional” buildings, and knowing this means certain questions should be posed by construction professionals when considering their use:

• What design criteria differ from that of a conventional building?
• Does the engineer use the tensioning effects of the fabric membrane to stiffen the building?
• Does the engineer allow for snow and roof live-load reduction due to the nature of the fabric material?
• How is the loading from the fabric membrane distributed to the structural frame?
• Is the frame designed to meet current design codes?
• Is the fabric tensioned to eliminate fluttering effects?

Without fully understanding the interaction between the fabric membrane and the building frame, design guidelines will not properly be interpreted to define the structure. This renders it virtually impossible for construction professionals and code officials to properly enforce codes. For this reason, skilled engineering design professionals should be consulted when questions of standards that govern design are present.

Fabric as a structural element

Fabric membrane is unique in that it is flexible. How this affects the structural frame is not clearly defined in the International Building Code (IBC) or American Society of Civil Engineers (ASCE) 7. More specific design parameters are addressed in ASCE 55-10 standard, “Tensile Membrane Structures.” ASCE 55-10 has been formally adopted as a reference standard by the IBC.

In conventional building construction, rigid cladding (OSB, metal decking, reinforced masonry) is used as a structural element that is mechanically fastened to the primary and secondary structural components. The mechanical fastening of the individual members may allow the components to function as one composite member. In such conventional structure applications, the rigid cladding is said to function as a “diaphragm,” and structural elements are assumed to be braced by the cladding at intervals dependent upon the stiffness and fastening intervals of the cladding.

In ideal conditions, the fabric membrane may be used to add stiffness to a building. However, engineers must anticipate conditions that are not ideal. In tensioned fabric structures, the fabric is designated to a certain tensioned state, however, over time, this tension load capacity will decrease and diminish the building’s initial stiffness. For that reason, it is not a recommended practice to rely on the stiffness of the fabric to increase the rigidity of the frame.

Another concern with relying on the tensioned fabric membrane to brace structural elements is that the fabric can be damaged by fire, flying debris and other causes that could dramatically reduce its load-carrying ability. ASCE 55-10 section 4.3 states that where the membrane is relied upon to provide stability to building components, the designer must ensure that a local failure of the tension membrane structure does not cause a collapse of the entire structure by either a loss in capacity of any individual member of the support structure or by excess movement of the structure when the membrane becomes compromised. Other codes, like Canadian Standards Association, simply state that the practice of using the fabric to brace the structure is not permitted.

Load reductions

Whether to allow snow- and roof live-load reductions to values below what is stated in the building code is often a topic of discussion. At a slope of 25 degrees or more, one could argue that the low coefficient of friction of fabric justifies reductions for snow loading to loads less than those already prescribed by the code for slippery, unobstructed roof surfaces. Snow shall be determined in accordance with ASCE 7 or applicable building codes. ASCE 55-10 states, “Design snow loads shall not be reduced by implementation of snow melting or removal methods except on temporary structures if approved by the authority having jurisdiction.” In a temporary structure design, this reduction is permitted only if the building will not exist during the winter season.

Snow load often governs design. When the reduced sloped- or flat-roof snow load pressure is less than 12 psf, roof live-load pressures need to be considered. Roof live loads primarily account for loads that would be applied to a roof during construction or repair.

Current codes address roof live-load reduction for conventional building design, but do not address frame-supported fabric structures directly. The typical conventional building live roof load is currently listed at 20 psf. This load is reducible with a lower minimum limit of 12 psf. In designing a fabric building, it can be argued that the structure is less likely to take on the construction loads of conventional construction practices.

Another argument to support a reduction in live loads below the code minimum of 12 psf is to note that ASCE 7-05 (Table 4-1) allows roof live loads of 5 psf for “Awnings and Canopies—Fabric construction supported by a lightweight rigid skeleton structure.” However, codes typically define awnings and canopies separately and distinctly from enclosed structures.

Wind loads

Wind design on fabric structures is not unlike that used for conventional structures. The fabric flexibility and anisotropic properties make it plausible for the design engineer to distribute wind pressures in unorthodox manners. However, simplified analytical methods used in conventional wind force distribution should be imposed in accordance with ASCE design criteria unless the method of force distribution can be proven with the use of finite element modeling.

To understand how wind design loads impact building design, discussions of the following parameters are advised:

Building height. Design criteria for buildings less than 60 feet differ from those needed for buildings 60 feet and taller. The 60-foot dimensional criterion is taken from ground elevation. This means stem walls that sit above the ground are included in the overall height of the building.

Building shape. The values given within the ASCE design guide have been derived from wind tunnel tests on structures with certain shapes. The most common of these shapes are flat, gable, hip, saw tooth, monoslope, stepped, mansard and dome-roofed buildings. Buildings that are outside the shape spectrum given within the ASCE provisions should have wind tunnel tests performed.

Building exposure and geographical features. Building location plays a large role in wind design. Geographical features, such as water bodies, open fields, mountains and other structures, can add to the intensity of wind pressures. Because of the wide variance in these features, ASCE general guidelines have been established to categorize exposure values and topographic factors. The following general guidelines are what define and govern the exposures categories:

Exposure B: urban and suburban areas, wooded areas and other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. This exposure is limited to areas where the terrain has obstructions that surround the structure in all directions for a distance of at least 2,630 feet or 10 times the height of the structure, whichever is greater.

Exposure C: open terrain with scattered obstructions having heights less than 30 feet. This includes flat, open country, grasslands and shorelines in hurricane-prone regions.
Exposure D: flat, unobstructed shorelines exposed to wind flowing over open water (excluding shorelines in hurricane-prone regions) for a distance of at least 1 mile. The exposure extends inland a distance of 660 feet or 10 times the height of the structure, whichever is greater.

Wind speed. Wind speed for each region is prescribed by the ASCE. The noted wind speeds were compiled through regional weather services based on 3-second wind gusts. In special wind regions, call the local code enforcement office to obtain the design wind speed.

Enclosure classification. The amount of openings contained within the building will have a profound effect on pressure distribution during a windstorm. Different opening locations within the building envelope define whether a building is classified as “enclosed,” “partially enclosed” or “open.” Open building designs may reflect the least amount of pressure, while partially open designs must allow for the highest level of pressure. Equal distribution of openings around the perimeter of the building, along with limiting the amount of openings, will assist in reducing design pressures.

Mean reoccurrence values. In determining wind speed and correlating design pressures, researchers have determined the probability of these design pressures being exceeded in relation to time. The probability is based on a 50-year period. Code permits the design of buildings with shorter life spans, outside of hurricane regions, to use a mean reoccurrence factor that is less than the prescribed 50-year (1.0) factor.

For instance, a temporary building that will be used for maintenance work and will be erected for two years can be designed for a 5-year reoccurrence. This factor would decrease wind speeds by 22% and thereby decrease design pressures. When designing permanent structures, using reduced factors is not advisable.

Seismic

Due to the light weight of fabric structures, seismic design is typically eclipsed by wind design. Nonetheless, there are times when the seismic design region and the force-resisting system will necessitate a demand for the detailing of the structure to meet seismic requirements. In cases where seismic response modification coefficient exceeds a value of 3, detailing of connections and material should include ductility in the design.

Material design properties

Another area of concern among steel designers is the use of higher yield strengths than those listed in the AISC manual as the “specified minimum yield stress” for structural materials. In the United States, this practice is not expressly prohibited. However, when an engineer chooses to use a higher value than that documented in the AISC standards, justification of the properties should be provided in the form of:

Certified Mill Test Reports (CMTR). This requires traceability on the project, proper representation of the member being analyzed and assurance that the values are based on a proper statistical analysis. Make sure the CMTR used is representative of the individual member being analyzed.

Coupon Testing. Most CMTRs represent mean values but not standard deviation values. Standard deviation values are important in providing confidence in the CMTR. Without standard deviation values, the engineer takes on a level of liability in accounting for chemical and mechanical properties that may vary along the length and depth of the material.

Other localities/jurisdictions may not use the same design philosophy that the AISC has adopted on this matter. For instance, the Canadian code (CSA A660 and CAN/CSA S16) prohibits the use of higher yield strengths than those listed in the design standards, mandating that the specified minimum yield stress documented in the AISC manual be used in all steel designs. The design professional and the code enforcement agency have the responsibility to address this topic in the course of project approval.

Conclusion

To reduce the uncertainties involved with any project, it is always prudent to research and develop a thorough understanding of the design and code criteria.

It is the desire of the Fabric Structures Association to openly discuss relevant design information related to frame-supported fabric structures. When concerns arise about a design, it is always advisable to contact the local code enforcement agency, and design content should be reviewed by both the responsible engineer and code official to ensure that minimum design practices are met or exceeded.

The Fabric Structures Association, a division of the Industrial Fabrics Association International, promotes the use and growth of fabric structures and represents the interests and concerns of the fabric structures industry in the Americas.