| The progressive collapse is referring to the case where the local weakening or failure of structural members results in the collapse of adjacent members and eventually disproportionate collapses.When accidental or occasional loads strike building structures,the progressive collapse may be initiated.For instance,once a column fails because of an earthquake,vehicular impact,fires,or other natural or human-made hazards,the own weight of the structure shifts to adjacent columns of structure.If columns are not adequately reinforced or designed to redistribute or resist the extra load,the part which is adjacent to the removed column fails.The vertical load-carrying components of the structure continue to fail until the resulted extra loading is stabilized.To date,numerous achievements have been made towards understanding the failure mechanisms to prevent progressive collapse.But the effect of multicolumn removal scenarios on the progressive collapse of RC structures is not fully understood.Considering the possibility that two supporting elements of building can be removed simultaneously,especially with a planned terror attack,this research aims to provide a comparative study of structural collapse performance between single and multi-column removal scenarios and to examine which columns should be given priority when designing against progressive collapse.Firstly,review of existing design codes,timeline with major progressive collapse events,design guidelines and mitigation strategies of progressive collapse are presented.Then,a series of numerical investigations on the structural behavior of an eight-storey RC building subjected to progressive collapse under ten column removal scenarios are conducted for the first and second storey,three of which are multi-column removal scenarios for each storey.To this end,finite element model is developed in SAP2000 for the building considering its mechanical and geometrical properties.Gravity loads and design load combinations were given and applied as per ACI 318.For the progressive collapse analysis,the linear analysis procedure following the GSA guidelines is carried out by applying the specified GSA load combinations,from which the forces and the Demand Capacity Ratio(DCR)are calculated for all members of the two frames intersecting the removed columns.The member is considered failed if its DCR value exceeds the acceptance criteria as per GSA guidelines.The finite element model is validated by comparing the results with literature.By comparing moment diagrams and DCRs for the members adjacent to the removed column for the different cases,it is observed that most of the DCR values have exceeded 2 which suggests that the structure is vulnerable to progressive collapse for all the 5 cases of each storey.It is shown that removing the column from a ground storey makes the structure more susceptible to progressive collapse than other stories.The corner case is most critical for the single column removal cases because of the observed higher DCR’s and according to literature it is most critical because of the limited development of catenary action,and that is due to the absence of adequate restraints against rotation and horizontal movements in the corner column loss scenario.However,by observing DCR values for multi column removal cases,the most critical case was removing exterior and interior column.It is suggested that for the designing of multi storey building against progressive collapse under single or multi column loss cases,the corner columns should be designed and reinforced properly and where possible,tie bars should be placed to provide the horizontal restraints and thus to develop the catenary action.The above finding will provide information for frame structures design in addressing the progressive collapse. |
