Loads - Tensile Fabric Structures: Design, Analysis, and Construction - page 53

loads are transient and so careful attention to slackness or “loss” of prestress in the
membrane under wind is important to prevent detrimental flutter. The membrane
form and prestress will need to be revised in design if wind loading produces
excessive slack regions.
ASCE 7 addresses the climatic and exposure parameters for wind load determination.
However, development of surface loads from the standard can be quite difficult, as
the pressure coefficients provided do not cover the unique forms of tensile membrane
structures. The wind pressure coefficients in the standard and other codes and
references are derived from hundreds of wind-tunnel studies of various building
shapes almost all of which are rectilinear or are simple surfaces of revolution such as
barrel vaults and spherical surfaces. The pressure coefficients for these later surfaces
may be of value in establishing coefficients for some simple tension structure forms;
however, none of the standards provide real coefficient values for the complex double
curvature shapes common in tensioned fabric structure design. Similarly pressure
coefficients for freestanding roofs (monosloped, pitched, and troughed) in ASCE-7
are not directly applicable to most tension structure canopy forms. In summary there
is simply very little US code or standard data available for unusual and complex
shapes.
Ideally wind loads for unique forms would be determined by testing in a boundary
layer wind tunnel. See Appendix 2 for a discussion of wind tunnel studies. For
smaller structures wind tunnel modeling is often not possible due to the cost and time
required. Fortunately as the peak stresses in the tensile membrane and member
forces in many common tension structural systems are not particularly sensitive to
specific load distributions but rather overall loading, conservative bounding load
conditions can be established. For large structures where this might prove to be too
conservative and hence uneconomical or for systems that are sensitive to load
distributions, wind tunnel studies should be performed.
Where wind tunnel testing is not an option the following approach is of use:
1. Consider a uniform uplift case using a pressure developed for the exposure,
height and nature of the structure. The uniform uplift pressure should be
greater than the mean uplift that can be realistically expected over the
membrane surface. Keep in mind that this may not be a real load condition,
but it can provide useful demand information for design.
2. Consider the maximum positive pressure conditions that are likely to be
created by wind events. As with uplift, a uniform case may be useful.
3. Develop wind cases that produce the maximum “imbalance” likely to be
created by wind events for critical elements such as arches or guyed masts.
4. Develop wind cases that produce the maximum lateral load on the structure in
primary directions. In many instances, wind will produce the governing