DOB Findings and Recommendations for WTC, Feb 2003

NYC 9/11 Public Portal Document

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The structure will be permitted to develop inelastic hinges when subjected to the removal of the vertical load-carrying member while supporting the factored loads. However, the extent of ductility that will be permitted will depend on the type and materials of constniction. The analysis should include the nonlinear geometric stiffening that results when membrane effects are considered as well as the inelastic stress hardening that results when materials are permitted to deform inelastically. These effects, based on the type of construction and materials, may be represented in equivalent force-displacement or moment­ rotation relations that can be used to develop the permissible limits of ductility and the corresponding demand capacity ratios. This approach may be used to establish the criteria, on a structure-by-structure basis, for evaluating the potential for progressive collapse. In addition, when performing an alternate path analysis, consideration should be given to the relative advantages and disadvantages of a strong diaphragm that ties the floor plate together as compared to a weak diaphragm that would allow the damaged portion of the structure to break away from the remaining structure. This is particularly important for relatively narrow structures that are incapable to resisting the large lateral loads that may be imposed by floor systems that rely on catenary action to span over a missing primary support. Structural analyses that consider the geometric stiffness effects, which accounts for the coupling of vertical and lateral forces as large displacements are developed, are required to determine the vulnerability of the lateral resisting system to these large diaphragm forces. Unless the structure can accept the loading patterns that result from an alternate path analysis, the damaged portion of the structure should be isolated from the adjacent bays. Also, because catenary action is ineffective at comer bays, except for wall panel construction, moment connections on both sides of the first inner column should be considered for comer bays. Specific Local Resistance , The analysis of a sfructure’s response to an extreme loading that may initiate damage mechanisms requires advanced analytical techniques. The specific local resistance method requires numerical simulation or empirical data to demonstrate a key structural element’s ability to withstand a design level threat. The successful simulation of structural response to extreme loading, as compared to the explosive testing of colunms, walls, beams and slabs have been achieved using analytical methods that account for the nonlinear dynamic behavior of the members. These methods are generally computationally intensive, I however, they need only be applied to an individual part of the sfructural system at one time and the i Page 38 WTC Building Code Task Force February 2003 Findings and Recommendations

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