The calculation of membrane structures as roofing is a relative young discipline of civil engineering where, in contrast to many other structural engineering tasks, the final shape is not known at the beginning of the design but must be identified through prescription of certain boundary conditions, such as pretension and geometric constraints.
The fact that the membrane structures can absorb tension forces only is the major characteristic for the load bearing behaviour of membrane constructions while compression and shear forces can not be transmitted.
The materials used are fabrics (polyester or glass fibre) or foils. Regarding fabrics, the textile is responsible for load carrying capacity while the coatings protect against environmental influence like rain and chemical attacks. The different mechanical behaviour in warp and weft direction results in an anisotropic structure. In contrast, the load transmission as well as the resistance to environmental influences must be performed by a single material when foils are applied. Usually thermoplastic materials are used then. A big advantage of foils is the air impermeability. Thus they are typically the first choice when faced with air-inflated structures.
Along the process of dimensioning, the shape of the membrane structure must be found on the one hand in non-stressed initial state of the single fabric-segments (pattern/ cutting) and on the other hand in installed, stressed condition of the entire assembled structure (formfinding). After being sewed and prestressed the textile must have the aspired form with constant tension forces in every direction of its surface. This property is very important to prevent the membrane from wrinkling and fluttering in case of strong wind.
This means that the tension forces, as result of the prestressing, must maintain throughout the life-cycle and not get lost due to external forces such as snow or wind. Ensuring the operation of the membrane in case of strong wind, diverse strength classes (I to V) can be chosen, which allow different grades of pretension.
There are only few normative specifications for wind load calculation regarding irregular geometries. Beside our knowledge from technical expert literature our results rely on the experience in computational fluid dynamic calculations (CFD) to model and prescribe the wind pressure on membrane structures as realistic as possible.
Membrane construction is a very complex and demanding field of civil engineering which requires much experience in the area of finite elements. Here the research on finite plate elements of Mr. Wittmann during his time at lecturer at the Technical University of Munich at the Faculty of Civil Engineering is of great advantage.
B&W Engineering often uses membrane structures in combination with tents. We will be pleased to assist you with the calculation, formfinding and pattern of your membrane structure as well as with the dimensioning of all components such as edge cables and connecting elements. Contact us.