By Sam Armijos, AIA
The first step in designing a fabric structure is to create a form with sufficient pre-stress or tension to prevent it from fluttering like a flag or sail. The three basic forms associated with tensioned fabric structures are the hypar (hyperbolic paraboloid), the cone, and the barrel vault. The hypar—or simple saddle—is often a square or rectangular form in plane, and in elevation a series of high and low points. Mast- and point-supported structures are conical forms. Arch- and frame-supported structures, where the membrane is supported by a compression member, are similar to barrel vaults.
The second step of the design process is to determine the boundaries of the tensioned fabric. Boundaries include frames, walls, beams, columns and anchor points. The fabric is either continuously clamped to frames, walls or beams or attached to columns and anchor points with membrane plates with adjustable tensioning hardware. Membrane plates are custom designed plates used to link the membrane and edge cables to the structural supports. In most cases, the fabric forms a curved edge or catenary between connection points and requires a cable to carry loads to the major structural points.
Once the primary points have been determined, the next step is formfinding, the art and engineering of designing the most structurally efficient structure. Fabric structures today are modeled with computer software programs specifically designed for these types of structures. These programs allow the designer to create a three-dimensional perspective model that can be viewed at various angles and that provides information for facilitating fabrication and installation. The programs can calculate the surface area of the membrane, the layout of seams based on the material being used, the length and tension of cables, and the necessary “links” for hardware.
The last step in the design process is analysis of the structure’s response to loads, including dead loads and live loads such as snow and wind. Structural analysis identifies areas of possible ponding and shows where high stresses are located on the structure. The analysis enables the designer to collect reaction loads at specific locations, size structural members and cables, determine the appropriate fabric, and create computer-generated cutting patterns.