RETROFITTING COLLAPSE BEHAVIOR OF DOUBLE LAYER SPACE TRUSSES AGAINST PROGRESSIVE COLLAPSE BY FORCE LIMITING DEVICES

Previous collapses of Double Layer Space Trusses (DLSTs) show that they are vulnerable to progressive collapse phenomenon. Under certain circumstances, a local failure can propagate throughout the structure and lead to occurring a brittle failure in the structure. Therefore, it is crucial to identify and exploit proper retrofitting methods against progressive collapse in DLSTs. In the current study, the method of utilizing Force Limiting Devices (FLDs) to improve the collapse behavior against progressive collapse have been investigated for flat DLSTs with overall collapse and dynamic snap-through. The results show that introducing FLDs to the critical members of the top layer of a flat DLST with dynamic snap-through failure mode and a member imperfection of 0.005L provides 18.2 to 23.86% load bearing increase and ductility between 1.55 to 1.67. The results also show that when the DLSTs is made of members with geometric imperfections between 0.001 L and 0.004 L, the FLDs can convert the overall collapse of the DLSTs to a ductile collapse. For this member imperfection range, the obtained data show that the method can increase the load bearing capacity of the models from 18.35to 26.8% and provide ductility between 1.56 to 1.76. The provided ductility in models with smaller member imperfection is slightly greater than those provided in models with larger member imperfection. In the current study the activation level of FLDs are selected between 85 to 95 percent and the effect of FLD activation level is also investigated. In addition, the results showed that placing FLDs on critical members could provide a ductility significantly greater than the ductility provided by another method of retrofitting DLSTs called the method of over designing the members of compression layer and under designing the members of tension layer.