DOB Findings and Recommendations for WTC, Feb 2003

NYC 9/11 Public Portal Document

resulting models are relatively small and efficiently analyzed. To increase the overall perfonmance of the structure, the Specific Local Resistance Method should be supplemented with ductile redundant detailing associated with the prescriptive Indirect Method. For concrete structures, this detailing entails the use of continuous bottom reinforcement over supports, confinement at joints, adequate ties to allow for load transfer, peripheral ties at the spandrels, internal ties through floor slabs and beams, horizontal ties to columns and walls, vertical ties along perimeter structure and tension ties for precast concrete construction. For steel structures, this detailing may sometimes entail the use of a moment resisting frame at perimeter of the building and either moment resisting splices at first floor perimeter columns or establishing column lengths to avoid first-floor splices. Either appropriate calculations or relevant empirical data may be used to demonstrate the adequacy of members to resist the specified threats. Where no specific threats are identified, the key elements should be designed so that the full resistance of the key element against unanticipated load can be developed without failing the connections or supporting members framing to it. The capacity of the key elements and the associated connection forces may be most accurately determined through the use the advanced analytical techniques described above. However, where no specific threats are identified, these capacities may be estimated using conventional design and analysis methods considering the ultimate material strengths (e.g. LRFD approach with 'phi' = 1) and at a minimum, load bearing walls must be capable of resisting a uniform static pressure of 720-psf. This balanced approach activates the full resistance available in the key members, maximizing their ability to deal with unforeseen hazards without having to redistribute loads. As a result, this balanced approach precludes a progressive collapse that might be triggered by the limiting capacity of an individual key element.

February 2003 WTC Building Code Task Force Page 39 Findings and Recommendations

NYC-WTC_000166525