TEXTILES.ORG
Carrying the load: Framed awnings and tensile awnings must meet different engineering and code standards.
Erik Jarvie, structural engineer and sales manager for TensionStructures (a division of Eide Industries Inc., Cerritos, Calif.) lays it out straight on the curvy business of tensile awnings:
“Tensile awnings typically have fabric that is under a significant amount of tension so that it remains taut with minimal displacement, even under load conditions of wind and/or snow. Insufficient tension may cause repeated fabric displacement, which can result in wear and tear of the fabric premature to the life expectancy of the fabric. Manufacturers’ warranties typically only cover claims of issues due to UV degradation, but often will specifically exclude any guarantees against improper installations. It is the responsibility of the design-build manufacturer to produce an awning that will withstand the test of time and site-specific load conditions.
“A framed awning can use very little tension and simply use a frame component to support the membrane. With a tensile awning, the membrane may span the entire length of a bay with very little framing. We designed a series of storefront tensile awnings in Manhattan Beach with catenary awnings performing all the work to tension the fabric. The arms protruding from the building were not attached to the fabric, but more of a cantilever for the cable working points.
“Catenary cables have an inherent pre-tension in order to keep the fabric taut. Any wind loads on top of that will add to the loads they induce on the framing. Since the direction of the canopy loads is lateral, this creates a moment force on the arms causing torsion loading at the building. By placing a compression member between the arms, this absorbs the opposite reactions of the cables and eliminates the lateral moment on the building.
“Since the reaction loads on the building need to be coordinated with the Engineer of Record (EOR) for the building, it’s a good idea to be prepared to design a canopy with minimal moment forces. Most existing buildings are not designed for the tens of thousands of pounds that a tension structure can produce. Compression members are the easiest way to eliminate unacceptable loads. If the compression members are not satisfactory to the client, then tie-back cables can be used as an alternative. These cables run back to the building in the opposite direction of the awning and the catenary cables, so additional real estate on the wall is required for this option.
“Counter-balancing the forces of the catenary cable will also reduce the stress and bending in the arm. A cantilevered beam with an unbalanced load at the end of its projection will be much larger than one that is balanced. The engineering code calls out maximum deflection allowances. If the projection arm deflects under load, it will not only be unsightly, but it won’t pass engineering standards. The fix is a larger member size to mitigate deflection, but large frame members are often not tolerable to owners and architects when they are expecting a ‘lightweight structure.’ Again, sometimes the best fix is the compression member, so that all framing components can remain as slim as possible and the structure becomes well balanced (structurally as well as aesthetically).”
Bruce N. Wright, AIA, a licensed architect, is a media consultant to architects, engineers and designers, and writes frequently about fabric-based design.