BS EN 17117-2:2021 pdf free download – Rubber- or plastics-coated fabrics一Mechanical test methods under biaxial stress states Part 2: Determination of the pattern compensation values.
A.3 Load profiles – example II
Example II load profile represents the load history of a structure subjected to predominant wind loads, that is the loads represent characteristic values of alternating wind pressure and suction events.
The load profile shown in Figure A.2 consists of the representation of nominal prestress and overstressing during installation at the beginning of the test. Subsequently the membrane stress due to external loads carried in the warp (respectively wale) and fill (respective course) directions are simulated alternating. Five load cycles are applied each, followed by periods over which nominal prestress is held constant. At the end of these periods, selected strain values after warp and fill (respective wale and course) loading are recorded. Additionally, selected strain values are recorded after unloading and after average loading in warp and fill (respective wale and course), see No 7 in Figure A.2. Moreover, a security check is included by recording selected strain values as theoretical maximum boundary values after maximum loading up to 25 % of UTS and subsequently after unloading. As the interpretation of compensation values is basically the individual balancing of various partly contrary influences (for example high compensation for avoiding prestress lower than nominal prestress vs. low compensation for enabling installation), the idea of the additional selected strain values is a full coverage of the material response to limit events like complete unloading and extreme high loads. This is beneficial for rather stiff coated fabrics where small differences in the compensation
and thus the stretch during installation lead to high differences in the resulting stress.
This annex provides an example evaluation of project specific compensation values. The procedure is illustrated for two different imaginary fabrics.
The example structure is a plane vertical facade frame of 8 m x 7 m, covered with PVC-coated polyester woven fabric. The span of 7 m is aligned with the fabric warp direction, the span of 8 m is aligned with the fabric fill direction. The fabric shall be biaxially prestressed in the plane, with a nominal prestress of 1 kN/m in either fabric principal direction warp and fill. To ensure that the prestress does not fall significantly below 1 kN/m during the service life time, the compensation planning is performed such that the fabric reaches an initial prestress of 2 kN/m after installation.
In this example, a comparison of two different fabrics A and B is performed using comparative extensibility measures according to Annex C, see example values for imaginary batches of fabric A and B in Table B.1. The comparative extensibility measures are assumed to be provided by the material producers for every batch.