Pre-stretching

The tendency to stretch under load is inherent in strand and wire rope. This tendency is due to two factors:

• The elasticity of the product. Elastic stretch is fully recoverable upon release of the load. If the elastic limit is exceeded, the result is plastic deformation, which should be avoided.
• The non-elastic constructional (permanent) stretch, which is a variable quantity, depending upon diameter, construction and lay-length.

For most wire rope uses, constructional stretch presents no problem. However, for most structural applications, strand and wire rope must provide predictable, uniform elasticity. In a suspension bridge, for example, the elongation of the main cables under load must be uniform and predictable so the mid-span sag is uniform.

To obtain uniform elastic behavior and the close tolerances required for assembly applications, the inherent constructional stretch of the structural strand or wire rope must be removed. This is accomplished by pre-stretching.

Pre-stretching is the repeated application of a predetermined load to a finished structural strand or wire rope, and is required for the following reasons:

• To make the strand and rope more elastic by removing the constructional stretch inherent in the product as it comes from the stranding and closing machines. This is essential for most suspended or guyed structures, since it ensures the stretch parameters used by the designer in predicting the elastic behavior of the cables after installation.
• To permit measuring and marking at prescribed loads of the strand and rope assemblies.

Assuming the applied tension does not exceed the elastic limit, pre-stretched structural strand and wire rope return to their original length once tension is released. Constructional stretch, on the other hand, results in a permanent set or increase in length.

With most of the constructional stretch eliminated, the predetermined measuring tension can be applied and overall length measured, and any reference marks can be located and marked within precise tolerance. For example, in the case of suspension bridge cables, locations of all cable-band and tower centers can be accurately measured and marked after pre-stretching, while under measuring tension.

The amount of constructional stretch in strand and rope can be significantly reduced, but it cannot be eliminated entirely. If a project requires minimized constructional stretch, the strand or wire rope needs to be pre-stretched. Once the product is pre-stretched and measured under load, a small amount of constructional stretch may creep back into the assemblies, shortening the length slightly. This usually happens during handling, shipping and installation. The constructional stretch which crept back is again removed after installation, with tensioning and brief exposure to service loads. Note that field measurement of assemblies is impractical and of little value since accurate measuring practices used in fabrication cannot be reproduced in the field.