الفهرس 
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Abstract
Cold-Formed Steel (CFS) roof truss members are generally over-designed and consequently inefficient, due to an overall uncertainty in the industry especially when related to performance and strength. Usually During the experimental phase, in order to investigate the behavior of cold formed steel trusses, the ultimate capacity of the tested truss is not previously known; even the first member that will fail before doing the test is still unknown. This research aims to investigate the behavior of cold-formed steel roof truss systems in residential constructions and in longer spans’ systems such as commercial constructions.
Preliminary nonlinear analysis using ABAQUS software were first done on truss tested in (Dawe et al, 2010), and experience gained from this analysis used to examine the behavior of twelve cold formed steel roof trusses. Trusses examined with different configurations under the same load and boundary conditions to study the effect of span, truss geometry, the truss web pattern and the truss members on the ultimate strength of the truss. Demand to capacity (D/C) ratios using Direct Strength Method (DSM) (AISI‐S100 2012, Appendix 1) were calculated to predict the failure location in the truss assembly and it’s cause.
The trusses failure modes (top chord buckling, webs buckling, and connections
failure) were examined using FEM and compared with (D/C) ratios. These results revealed that ABAQUSE is able to successfully simulate the realistic collapse of CFS roof trusses, and that Newton‐Raphson with artificial damping method was the only method that successfully predicts the peak load and the post peak response. Imperfection sensitivity analysis on truss tested in (Dawe et al, 2010) showed that higher buckling mode provided the most critical imperfections. Comparison between ABAQUS models failure mode and the maximum component’s D/C, showed that the member with the maximum D/C ratio will be the first member to fail in ABAQUS and cause of failure was the same in ABAQUS and design.
So designer engineer can use either D/C components’ ratios or ABAQUS model to predict the behavior of CFS roof trusses, the first member that will fail, and the cause of failure. For longer spans, Fink truss configuration is better than Howe truss because in Fink truss longer diagonals experienced tension rather than compression in case of Howe truss. D/C ratio results showed that each truss was designed with high efficient critical components and with low and highly variable components’ D/C ratios which increase the probability of producing an alternate load path to the loads previously carried by the failed truss members and this phenomenon came from preferring cost savings from using consistent member sizes and ease of constructability over component-by-component optimization.