This paper reviews and compares the results of ASTM Standard fabric tests and my Bi-Axial Rip Test (BART) to introduce a formula that can be further developed to provide for factors of safety (FOS) in designing tensile fabric structures that take into acount fabric tears and tear propagation.
Tensile ruptures of fabrics are rare. Actual failures occur at stress concentrations near fixed locations, along seams or are the result of tear propagation subsequent to a puncture or snag. In practice tensile fabric structures are typically designed using a single set of FOS against fabric tensile failure, regardless of fabric type, of four (4) for snow load, five (5) for wind load and eight (8) for prestress.
Using ASTM Standard test results, the calculated ratios of tensile strength per inch compared to the trapazoidal tear resistance (Sst/Rult) indicates that fiberglass fabrics are high (Sst/Rult varies from 6.5 to 12.5), low in polyethylene and ePTFE fabrics (2.0 to 2.8) and for polyester somewhere in between (2.8 to 6.5). The Trapezoid Tear Method ASTM D4857 (Old designation ASTM 5106) is typically conducted where a load is applied to a cut within the fabric plane at a test specified angle. A better test of the actual behavior of a fabric in a tensile fabric structure may be the mono-slit test in a uni-axially loaded cut fabric or even better a bi-axially loaded cut fabric. Comparing the mono-slit and bi-axial test results to the trap-tear tests may enable one to develop a formula for comparing fabrics when only given the strip tensile and trap-tear strengths of a fabric. This formula can then be used to chart the factor of safety required so that a tear of a certain length will not tend to propagate. Developing and using this formula should make for designing safer structures.
Refering to Table 1 (Mono-Slit Test), results indicate that to prevent an approximate four (4) inch slit from propagating requires a factor of safety with respect to tensile strength of approximately three (3) in Fabric A (polyethylene), while Fabric B (polyester) requires a factor of safety (FOS) of approximately five (5).
From the various test results it is possible that a formula can be developed for each specific manufactured fabric of each specific fabric type. In a tensile fabric structure what factor of safety should be used to prevent a given length cut? What cut length should be considered?
Four types of tests were performed in preparation of this paper.
Wayne Rendely, PE is a Licensed Professional Structural Engineer specializing in Fabric Structures. The Engineer of Record on more than 350 projects, he has also offered significant contributions on over 700 projects, both nationally and internationally. A former Director of the Fabric Structures Association, a division of the Industrial Fabrics Association International, Mr. Rendely has experience on numerous types of buildings including stadiums, amphitheaters, hangars, tents, pavilions, storage buildings, and temporary buildings.
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