3. TYPICAL JOINTS AND EDGE DETAILS 3.1. Joints Within a textile membrane, the high-strength fibres are the loadbearing elements. Joining two strips therefore means to create a connection which is able to transfer the forces from fibre to fibre. Three groups of joints may be distinguished: Welding, especially high-frequency (HF) welding, is the type of joint mostly used. Within welded joints, the forces are transferred from one fabric via the fused coatings to the other one. The strength of the detail therefore is determined by the strength of the coating itself and by the adhesion of the coating to the fibres of the fabric. Fig. 7Today, high quality HF-welds of PVC/Polyester membranes show a strength up to 95 % at 23' C (70 % at 70' C) of the strength of the fabric itself. The light qualities of PVC-coated Aramidfibre-fabrics can reach the same range. For heavy qualities of this material, the combination of welding and sewing is inevitable. Welding ofPTFE/GJassfibre membranes is done in a high-temperature contact-welding process. The strength of these joints is about 80-90 % (23' C and 70' C) of the strength of the fabric. The second group of joints are the sewed connections. Sewing, the oldest technique in joining fabrics, is of reduced importance today. There are several reasons to that: Today's welded joints usually are of such good quality that they produce a strength which is even higher than that possible with sewed seams. In addition, sewed seams are not completely waterproof and the sewing threads may be attacked by environmental effects. Additional measures therefore must be taken in order to protect the joint, Fig. 8. Sewing therefore may become non-economic if applied to the very long (standard) joints of single strips. However, for a lot of details, especially ifforces have to be introduced into the membrane locally, sewing, also in combination with welding, yields optimal results. The strength of the sewed seams of the PVC/Polyester membranes is about 70% at23'C (50% at 70' C) of the strength of the membrane itself. For the PVCcoated Aramidfibre qualities, sewing is used in combination with HF-welding. An ultimate strength of
Fig. 8: Sewed joint with protecting nlm.
about 80 % at 23' C of the strength of the membrane itself can be reached. For the PTFE-coated Glassfibre materials, sewing is not possible.
The third group of joints may be named as «Mechanical Joints». It is typical for this group that the forces are transferred via clamping, bolting or ropes passing through eyes in the membrane. Figs. 9, 10, 11. The joints belonging to this group are used for the assembly of large membrane elements on site usually, with those according to Figs. 10, 11 for low strength or temporary connections only.
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