Roof truss systems under blast loads
Authors:   , Bondok Doaa 
Published: April 2021

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Designing roof systems for blast loading is quite complex. Many uncertainties still exist in this vital research area. The typical single degree of freedom dynamic modeling approach that is used widely by the blast design community is based on idealization of the structural components. Limited information exists on the blast response of different roof systems and their blast design methodology is lacking. Moreover, the blast roof load is based on approximate methods to produce a blast wave equivalent to the actual propagated wave. This approximation needs to be evaluated to ensure the blast designs are sufficient. The uncertainty of blast loads and roof resistance can lead to either excessive costs or insufficient blast designs. Therefore, the research presented here aims to bridge the gap in the knowledge about the blast resistance of different roof systems; Open Web Steel Joist (OWSJ) systems and Cold-Formed Steel (CFS) roof systems as well as to assess the accuracy of the existing ASCE and UFC approximate roof blast loads. In this dissertation, dynamic analyses using the finite element method were performed on a roof component to compare the dynamic responses resulting from a propagated blast wave and the current equivalent blast load techniques. Blast field data were used to verify the dynamic finite element model. Results have shown that current methodologies should be corrected if used to design for blast loading. Previous experiments were used to verify advanced finite element models developed to predict the complete static resistance of OWSJs including the failure limits. The verified models were used to perform dynamic analyses to predict the system dynamic response under equivalent blast loads. Analyses and energy comparisons at superficial damage level showed that the current methodology, used to calculate OWSJ static resistance, predicted 27% and 88% higher energies than the experimental ones for 16K5 and 26K5 joists, respectively. While at moderate damage levels current methodology predicted 47% and 108% higher energies than the experimental ones for 16K5 and 26K5 joists. Evaluating the blast resistance of CFS roof systems is challenging. There is a lack of existing design guidelines and response criteria for CFS roof systems. The UFC manual provides information that is relevant to CFS panels only. The approach that was adopted in this dissertation started with an extensive testing program of different types of end connections used for CFS roof trusses to investigate their failure capacities in horizontal and vertical directions. Analyses of the experimental results showed that using Hilti PAFs are more favorable than using bolts for supporting CFS truss end-connections as it was indicated in their strength and toughness. Moreover, the experimental results were used to verify the deformable screw behavior and the finite element model developed to predict the progressive failure of the truss end-connections. Small-scale CFS roof truss specimens were tested to failure under quasi-static loading. The static resistance of these systems and the associated failure mechanisms were identified. Experimental results and energy comparisons show that the truss layout and the shape of loading significantly affect the performance of the truss and the failure mechanism. Three-dimensional numerical models were developed and verified against the experimental results. The advanced models predicted the static resistance to failure with a high level of accuracy. Numerical analyses were performed to enhance the static resistance of CFS roof systems for blast analysis. Experimental and numerical analyses have shown that the energy absorbed is improved significantly when the web members susceptible to buckling are strengthened. In addition, the numerical models were used to perform dynamic analyses on a flat CFS roof system subjected to different threat levels. Von Mises stress distributions were used to investigate and determine the damage level corresponding to each threat level. The research presented in this dissertation focused on investigating the equivalent roof blast load as well as the blast resistance of different roof truss systems. The static resistance required for SDOF analysis was evaluated and identified using physical experiments and verified advanced finite element models. Failure capacities of truss end-connections were identified to improve truss system performance against blast. Based on experimental and numerical analyses, recommendations are given to arrive at an enhanced blast resistance. Dynamic analyses on 3D truss numerical models were used to investigate the damage level under certain threats. It is recommended for future work to perform field tests to address the critical differences between the measured field roof wave and the UFC manual roof blast wave. The developed numerical models can be potentially used as an analysis tool to investigate the resistance of other truss profiles and to examine the failure mechanisms that may lead to the development of an analytical model for the static resistance of roof systems. It is recommended for future work to compare the dynamic analyses performed using the developed numerical models and the SDOF dynamic analysis to provide more insight into the idealization of this technique.