Cold-formed steel roof truss systems response under static loading
Authors:   , Elsawi Mahmoud Mohamed Othman Mohamed 
Published: September 2024

The exterior envelope is the most vulnerable component of a building to an external explosion hazard because it is part of the building closest to the source of the hazard. As part of the envelope, the roof system will require a special design to resist blast loads. Coldformed steel (CFS) members are lightweight structural elements having superior stiffnessto- self weight ratio that are widely used for roof truss systems. Currently, their static resistance to failure is unknown and design guidance for CFS roof truss systems under blast does not exist. It is necessary to fully understand the entire response of such light-weight systems to static loads to be able to develop engineering analysis methodologies to design protective CFS roof systems. Therefore, this research developed 3D finite element (FE) models capable of predicting the entire response of full-scale CFS roof truss systems under large deformation up to failure. To validate these developed models, this study investigated experimentally the structural response up to failure of 32 full-scale CFS roof truss systems under uniform static loading. A new of its kind, pulleys-and-cables system is used to apply equal forces on the truss joints throughout the testing. Unlike hot-rolled steel sections, there are no standardized cross-sections for CFS members currently in use by the industry. Therefore, the effect of the variations of the shape of chord cross-section, flange size, and thickness were studied experimentally. Also, the experimental investigation included varying the depth of the truss and the end-bearing configurations. The validated numerical models were used to perform a numerical parametric study extending the range of experimental variations to assess comprehensively the effect of various parameters on the global response of the truss systems. Since connections, which are generally fabricated using self-drilling screws, have a significant role in the global response of the truss, their entire static response up to failure was experimentally evaluated in this research. A total of 81 truss and end-connection configurations were tested. In addition, an experimental parametric study was performed on the effect of the numbers and arrangement of screws and the thickness of the connected parts. It was found that the six screws are the optimum number of screws in terms of energy absorption (EA). Results of the connection testing were incorporated into the CFS truss FE simulation. Based on the experimental results, it was observed that the symmetric configurations outperformed the asymmetric ones. In terms of EA, a key measure for blast-resistant design, the wide-flange chords outperformed the narrow ones for asymmetric cross-section designs unlike the symmetric cross-section designs. Also, increasing the truss depth in the thinner chords was more beneficial than the thicker chords for both EA and ductility. The numerical models showed that increasing the chord thickness is more effective compared to the web members thickness in terms of EA. Also, increasing the chord-flange size improved the ductility and the EA capability of the CFS truss. The truss depth exhibited mixed effects on the improvement of the EA. The numerical and experimental results demonstrated that the end-bearing conditions significantly enhanced the tension membrane capability, ductility, and the EA of the CFS truss. The connection and full-scale testing and numerical modeling of CFS truss systems, along with parametric studies, are anticipated to enhance engineering analysis and predictive modeling. These improvements will facilitate the development of engineering-level dynamic response models, ultimately contributing to blast-resistant designs.