The material properties of architectural fabrics are an important consideration prior to their use in high profile tensile structures. The mechanical characteristics of these fabrics play a fundamental role in the overall engineering of a structure, helping achieve safe, efficient designs. Biaxial testing is an excellent method to accurately determine the fabric’s material properties.
Biaxial testing allows the fabric’s performance to be investigated under different load conditions in warp and weft directions simultaneously in response to the stress conditions. Data from this testing can be used to analyse the initial behaviour of the fabric at installation, pre-stress behaviour and long-term behaviour. This information will also help determine the compensation of the panels of fabric, together with the erection and pre-tensioning processes.
Fabrics that have been biaxially tested and found stable will provide more even force distribution and pre-stress loads, resulting in easier installation and a more efficient structure, which better maintains its shape and performance over its service life.
There are currently limited standards for biaxial testing of fabrics used in building fabric structures. International standards for architectural fabrics have historically focussed on the uniaxial behaviour or performance in a single direction of a fabric. This is commonly examined using tensile testing equipment, which concentrates on comparing the tensile strength, breaking strength and maximum elongation in one direction. However, this data alone cannot be used to produce a complex loading analysis as fabric forces during a loading event affect both the warp and weft directions simultaneously. When examining the material beyond its elastic properties, uniaxial results become unusable. Only a biaxial test can determine the fabric’s bi-directional mechanical characteristics under all types of loads.
A biaxial rig is a testing machine that can pull both warp and weft directions simultaneously. The test rig holds the sample and allows elongation in both directions to create a central area, characterised by a uniform stress state to measure the elongation in the warp and weft directions.
In the absence of Australian, European or American standards for the biaxial testing of fabrics, the only relevant standard is the Japanese code MSAJ M-02-1995, which is often regarded as a guide for testing fabrics. Since the analysis can differ between laboratories, fabrics should be tested in the same laboratory under the same conditions for comparison.
When designing a lightweight tension membrane structure, a fabric that exhibits similar structural properties in both warp and weft directions has the advantage of more even distribution of forces into supporting structures; more equal pre-stress loads in both warp and weft directions resulting in easier patterning and installation; and better sharing of loads between warp and weft fibres resulting in less elongation of fibres over time and enabling the sail to retain its original design shape and avoid the fabric sagging.
Tensile structures designed with dimensionally stable fabrics will last for a long time. If the fibres in the fabric have elongated excessively it will cause the fabric to sag and billow in the wind, resulting in a shade structure that no longer performs or has visual appeal, leading to maintenance or replacement. Billowing can also cause excessive stress on the fabric, the structure and connection points, increasing the possibility of a failure.
To avoid unnecessary maintenance and replacement of the membrane component of a structure, you will need to select a fabric with superior biaxial stability. Commercial Heavy 430 from Gale Pacific, featuring the patented intertwining knit pattern is designed to extend the service life and lifespan of lightweight tension membrane structures.
Gale Pacific is committed to the development of fabrics that have superior dimensional stability when compared to alternatives.