DOB Findings and Recommendations for WTC, Feb 2003

NYC 9/11 Public Portal Document

4 Findings and Recommendations I II

4.1 structural Strength

4.1.1 Progressive Collapse The current NYC code provisions for resistance to progressive collapse (see Appendix A) were k drafted in response to the collapse of a concrete panel and slab construction in Ronan Point England in 1968. The NYC Building Code progressive collapse provisions were adopted by rule into the current code on August 2, 1973. However, they are considered cumbersome to read and difficult to use by the engineering community. The Structural Strength Working Group developed a draft progressive collapse guideline to clarify these rules. Because it was unable to test these guidelines against actual buildings under design, these guidelines will be jsublished in this report for further development and testing. The draft provisions, which take much of their substance and methodology from the British Standard and Building Regulations Approved Document A, outline three methods of designing a structure for improved resistance to progressive collapse. The British Standard served as a model for this group because of its simplicity and nearly thirty years of satisfactory performance in various exceptional events. It should be noted that the performance of this standard is solely based on empirical evidence. Because the existing rules remain effective, these guidelines are meant to augment the. professional's ability to achieve a robust design for buildings at risk of extraordinary events. They are to be considered one possible approach. The Task Force recommends the development of coherent and rational standards through the analysis of economic impact and coordination with the NIST research and development effort, using review procedures similar to those that guided the development and adoption of the seismic code.

Recommendation J 1. Publish structural design guidelines for optional application to enhance 1

robustness and resistance to progressive collapse. )

4.1.2 Use of Lightweight Structural Members

Of major concern to the Structural Strength Working Group was the ability of fireproofing to adequately perform on lightweight structural members, specifically, open web bar trusses, the type of trusses used in the WTC towers. As the weight (mass) of the bar diminishes, typically so too does the surface area to which the fireproofing is applied. The adherence of spray on fireproofing, that most often used in conjunction with these trusses, has been called into question by initial investigations of the towers. To cover a lightweight element, one that has an already lower resistance to fire due to its size, with fireproofing of dubious performance is unwise at best. I A standard discussed by the group which uses a ratio of weight (mass) to surface area calculation to produce a minimum allowable size factor was not adopted because the group could not determine with the information available to it what would, ultimately constitute a "safe" minimum size.

i Page 18 WTC Building Code Task Force February 2003 Findings and Recommendations

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