DOB Findings and Recommendations for WTC, Feb 2003

NYC 9/11 Public Portal Document

specified using a prescriptive indirect design approach or specified based on the results of a performance based alternate path analysis. The guidelines for general structural integrity are in the form of prescriptive requirements for minimum joint resistance, continuity and inter-member ties that will provide a robust, stable and economical design. The alternate path calculation, which explicitly determines the resistance to progressive collapse when a primary load-bearing member is removed, considers the most likely dead, live and wind load combinations along with the inelastic response (both material and geometric non-linearity) of the damaged structure. Both of these approaches will result in continuous tied reinforcement for concrete frame structures and stronger connections for steel frame structures to allow the structural elements to develop more of their capacity (either in flexure or membrane action) when subjected to abnormal loading and support conditions. Loads that were once supported by the damaged portions of the structure will be redistributed to undamaged elements. I1 While the alternate path approach is not associated with any specific threat that might cause the damaged state and therefore gives the appearance of being threat independent, it is limited in its applicability to abnormal loading conditions that would fail only one load-bearing member. Alternatively, the critical, elements may be identified and strengthened to be resistant to the identified threat, thereby averting the loss of the member. Strengthening (specific local resistance) may be preferable to alternate load paths in situations where more than one key element could be damaged by the considered hazard. In these cases, where loss of multiple key elements could overwhelm an alternate load path system intended to address loss of only one key element, the specific local resistance design approach provides more protection than a design based on the alternate path method. For most building systems, the Indirect Method require little more than providing a minimum amount of continuous tied reinforcement. At any location where these prescriptive requirements cannot be accommodated, the structure may be designed to satisfy the requirements of the Alternate Path Method. At those locations where the Alternate Path Method can not be accommodated, due to building irregularities and discontinuities, or where structural elements are exposed to a greater risk due to accidental loading or explosion, a combination of Indirect Method ties throughout the building and Specific Local Resistance at the most exposed and/or sensitive key elements is recommended. ' Recent experiences suggest that engineers should be alert to transfer girders and the columns supporting transfer girders which may be particularly vulnerable to abnormal loading, including the effects of fire. Transfer girders typically concentrate the load bearing system onto fewer structural elements. This system runs contrary to the concept of redundancy that protects structures from abnormal loading conditions. Typically, the transfer girder spans a large opening, such as a loading dock, or provides the means to shift i the location of column lines at a particular floor. Damage to the girder may leave several lines of columns, which terminate at the girder from above, totally unsupported. Similarly, the loss of a support column from below will create a much larger transfer span. Transfer girders therefore create critical sections whose loss may result in a progressive collapse. If a transfer girder is required and if this girder may be vulnerable to an abnormal load, then it is desirable that this girder be continuous over several supports. It is further recommended that there be substantial structure framing into the transfer girder to create a two-way redundancy and thereby an alternate load path in the event of a failure. The column connections, which support the transfer girders, are to provide sustained strength despite inelastic deformations. As such, these joints are to be designed as full moment connections. Indirect Method When using the indirect method it is recommended that both the connections are designed and detailed to develop the specified capacity of the members and the connection designs are consistent with the member deformations. The indirect approach corresponds to prescriptive requirements, e.g., ACI 318, Section 16.5. Alternate Path Method A structure’s response to redistributed loads following the sudden loss of a primary load-bearing member is dynamic and inelastic. Linear elastic analyses caimot account for the redistribution of forces, P-Delta instability, nonlinear material properties including rate effects, and the development of membrane modes of resistance. Therefore, the use of linear elastic analysis approaches requires engineering judgment and an independent check for P-Delta instability after the initial design is complete. The elastic analysis does not account for the greater capacity resulting from plastic hinge/yield line formation, membrane action or enhanced strength due to rate dependent material properties. 1 1

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