In all type of structures, but mainly in lightweight construction, we can apply the concept of redundancy. Redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the form of simple duplicity or by adding some components that will be useful only in emergency cases. Membrane structures are a paradigmatic case of such type of structures. Generally the biggest element of the structure is a textile membrane, which is very resistant to tensile forces, but not so much to tear forces. For example a typical membrane with a tensile strength of 800 daN/5cm (16 kN/m) has tear strength of 120 daN (1,2 kN). This fact is the responsible of the complete failure of a membrane structure in case the structural membrane tears apart for any reason.
In this situation there is an added problem: primary structure (mainly masts, arches…) may collapse and cause further problems, even fatal dangers for people underneath. For that reason it is compulsory (in terms of engineering design) to check the behavior of the structure once the membrane has disappeared. Sometimes, as in Fig. 1, the remaining structure will keep more or less standing. Perhaps in this position no one would be affected. But in most of the cases, the primary structure could fall to the ground, being an important danger that has to be prevented. WinTess allows us to easily analyse the remaining structure. Just removing the membrane through menu Membrane | Remove_membrane and analyzing the rest of the structure under self weight. Let’s suppose we have this hypar:
With menu Membrane | Remove_membrane we leave only metal structure:
Now we analyse this structure and get this final position:
It is obvious that this structure is safe, because in case of serious failure of membrane it will remain standing. Is this enough? No. We have to check forces on the remaining elements: tension on cables, compression or tension in tubes, etc. With WinTess we could check it through “ratio” button.
According to this diagram, most of elements are under-stressed. That’s true because self weight is a very low load, but what is really important here is the impact force. When the membrane disappears, remaining metal elements fall giving a strong pull to the cables and other elements as well. Analyzing properly this pull means using energy analysis which is neither easy nor simple. However, I can say that it is advisable to increase appropriately the safety factors of cables and tubes (masts, arches,..) to consider the impact energy in case of a total membrane failure. In conclusion, we can say that always we must apply a redundancy analysis to textile structures. It is very important to avoid serious danger for people and objects below.
Prof. Dr. Ramon Sastre