This paper reviews the design and engineering of fabric membrane clad buildings. As these structures become more prevalent, larger and used in structures of higher importance, it is essential that engineers consider the unique forces imposed onto the structural framing by the membrane cladding.
Topics include the classification of the buildings according to building codes, code loads to be used to design the building, frame spacing, bracing and details to consider as well as the membrane forces onto the frame and how they are determined and how they affect the design of the building.
The geometry of the structure will determine the environmental loads imposed as well as the internal forces that determine the size, details, and the residual forces (opposite of reactions) imposed upon the foundations. For this paper a building using typical 2D truss frames spaced with purlins that, along with longitudinal and sway bracing, brace the frames, is considered.
- Span: Span is the distance between supports. and a "clearspan" would have no interior support(s). The span is a major factor in determining the magnitude of forces in the structure.
- Ridge Height: In a symmetrical building the center highest point of the structure. Local codes may restrict the height and wind loads increase with a higher roof.
- Eave Height: A consideration for internal clearances and also the magnitude of wind forces on the side wall. A higher eave may require a lower slope roof which will affect the magnitude of both the roof snow and wind loads. A larger radius will reduce clearance and reduce the magnitude of the inner chord axial force due to snow loads.
- Roof Slope: A low roof slope will require higher roof snow than a greater sloped roof and a high slope will need to be designed for windward roof pressure.
- Truss Depth: A deeper truss will lower the axial forces on the chords but the increased length of the webbing (diagonals) may affect the webbing design and size. Internal clearances, fabrication techniques and shipping may limit the depth.
- Webbing Spacing: Webbing members (diagonals) spaced to minimize their number will increase their size, internal forces and possibly the connection design to the chord member. Too few diagonals and upper chord local bending governs.
- Frame Spacing: To minimize the number of frames, foundations, purlins, fabric panels, connections and other items the frames are typically spaced as far apart as practical so as to optimize the cost. The frame spacing is a major factor in the design of the purlins. The purlins are horizontal and as compression members their self weight and slenderness will be a major factor in their design in large frame spacing. Greater frame spacing will also increase the horizontal membrane forces on the outer chord of the end frame and thus the purlins.
- Purlin Spacing: The spacing and design of the purlins is determined by a number of factors. Each purlin braces the truss chords and also resists the tension force in the fabric. The top chords of the end frame have fabric forces perpendicular to the truss plane and will experience out of plane bending. The purlin spacing will be a major factor in the design of the outer chord member. The purlins are also part of the longitudinal bracing system and an increase in the number of braced bays will lower the force onto the purlins.
- Braced Bays: (Longitudinal Bracing) Large span or tall structures will need a greater number of braced bays, bays with X-cables, so as to minimize the forces on the internal members and also the foundations.
- End Wall Wind Posts: These vertical tubes or trusses transfer the wind load onto the fabric directly to the ground and to the lower chord of the end frame. The horizontal force at the top of the post is transferred into the purlin, then cable bracing of the braced bays down to the frame anchors.
- Enclosure Classification: Internal pressure coefficients will vary based on the classification of exposure of: Open, Partially Enclosed, or Enclosed. The sum of external suction and internal pressure of a partially enclosed structure may be twice that of an open building.
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Wayne Rendely, PE is a Licensed Professional Structural Engineer specializing in Fabric Structures. The Engineer of Record on more than 350 projects, he has also offered significant contributions on over 700 projects, both nationally and internationally. A former Director of the Fabric Structures Association, a division of the Industrial Fabrics Association International, Mr. Rendely has experience on numerous types of buildings including stadiums, amphitheaters, hangars, tents, pavilions, storage buildings, and temporary buildings.
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