Experimental and Numerical Study on the Collapsing Behavior of Structural Systems with Steel Members Arranged in Parallel Under Tension

Prediction of possible structural collapse scenarios requires the experimental characterization of post-peak behaviors of structural members. The post-peak behavior of one member in a redundant structural system will influence the behaviors of the other members and ultimately the overall structural response. In this work, an experimental and numerical study was conducted to explore the failure behavior of structural systems consisting of three members arranged in parallel under tension. Twelve cylindrical A36 steel specimens with diameter (D) of 7.62 mm (0.3 in.) and length (L = 10D) of 76.2 mm (3 in.) were made for the experiments. Three specimens were used in single member tests to determine the base material properties. Three tests of three-member systems (Test-1, Test-2, and Test-3) were conducted using a special fixture that could rotate around a central pin in response to unbalanced forces produced due to unsymmetrical failure evolution in the three-member systems. A 3D DIC (three-dimensional Digital Image Correlation) measurement device was used to capture the deformations along the specimen length. The experimental results showed that all single members failed at approximately the same strength but with distinctly different displacement fields. In the three-member system tests, one system failed at 10% higher load, 21% lower displacement, and 13 times the rotation, as compared with the other two systems due to the softening of one outer member substantially before the other two members. The numerical results of the three-member system showed an acceptable correlation with the experiments. It could be concluded from this experimental study that the inconsistency of softening response, due to the inevitable imperfection of geometry, material and/or loading, can trigger the softening of one member before the others and significantly impact lateral displacement or rotation in a collapsing system.