Designing Architectural Fabric Structures

It's basically steel, fabric and cables, but it's not as easy as it looks!

Architectural fabric structures come in a variety of shapes and sizes. They can be inside, outside, permanent, temporary, big, small, air-supported, tensioned or air-inflated.  Also referred to as tensioned membrane structures, membrane structures or textile buildings, these unusual forms have played a major role in architecture for the last 40 years.

Designing with Fabric

Designing even simple fabric structures to perform under a variety of conditions can be a complex task.  Each component is both visible and structural and relies on all other components to function properly.

The first step in designing a fabric structure is to create a form with sufficient pre-stress or tension. Fabric structures must be clamped to a frame or be pre-stressed in order to avoid “fluttering” like a flag or sail. It is best when designing lightweight structures with minimal surfaces to have what is referred to as “double curvature” to enable the fabric to transmit lateral loads.  The degree of clamping or pre-tensioning and curvature are dependent upon the type and weave of the fabric as well as the loads to be supported.

The second step is to determine the boundaries of the tensioned fabric. Boundaries include frames, walls, beams, columns and cables. The fabric is either continuously clamped to frames, walls or beams or attached to columns with membrane plates with adjustable hardware.  In most cases the fabric forms a curved edge or “catenary” between connection points requiring a cable, webbing belt or rope to carry loads to the major structural points. Catenaries are usually curved inward from 10 to 15% of the total length of the span.  The shallower the curve in the perimeter, the more tension there is in the cable and ultimately in the overall structure and foundation.  Very high tension loads can increase the cost of a fabric structure substantially.

Once the primary points have been determined, the next step is referred to as “form finding”.  Form finding is the art of discovering the most efficient structure which can be fabricated with as little waste as possible and can be transported and installed with ease, installation and fabrication There are two methods of form finding: physical modeling and computer aided design.

Fabric structures are best visualized with physical models or full scale prototypes depending on the complexity of the design.  Models are created by stretching nylon over wire frames. Working with physical models or prototypes enables one to view the structure from any angle.

Today, most fabric structures are created using sophisticated computer technology.  Computer software not only helps create a 3D model, which can be viewed in various materials and angles, it can be customized to provide information to facilitate fabrication and installation. It can determine the amount of fabric to be used, how it should be cut, the dimension of each piece, the size and length of structural members, the size and length of cables and the hardware the project will need.  The designer is also able to modify the shape more easily than with physical models.

Structural Analysis

The next step in the design process is analyzing the structure’s response to loads, including dead loads and live loads (snow, wind, etc.).  An experienced specialty firm should carry out a structural analysis of the proposed structure.  Presently, there are few consultants and fabricators devoted entirely to this unique field.  The majority of consultants are engineering or architectural firms with a “lightweight structures division” or “form finding capabilities”, while fabricators range from tent and awning companies to air structure and “pre-engineered” building firms.