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1.5.2 Approach 2: Evaluation of NIST’s Collapse Initiation Hypothesis
a. WTC 7 was not symmetrical. Therefore, during collapse it would naturally sway towards the mass center for the floors. The mass center for the floors were initially evaluated and then used to examine the building’s response during collapse. b. The lack of combustibles was critically examined, as was the idea that primary damage by fires could occur on floors where financial centers were located. The questions we addressed were: Would this type of business have paper lying around or would privacy be most important and therefore paper stock locked in a fireproof safe?
Why was a fire in this building so significant? c. We simulated fires in the building. The heat transfer characteristics for the steel framing with and without fireproofing was examined. We also studied how fire may have interacted with the concrete floors (in the flutes) and between flutes. We examined the steel frame response where the drop-down ceilings (acoustical tile) were hung below the floor system. We also studied the thermal conductivity of the concrete floors and their interaction with the steel beams and girders that made up the floor systems. The worst case for structural response to fire was to consider steel members without fireproofing and this was subsequently used to evaluate structural response. d. We prepared a detailed simulation of the floor slab connection to the beams and floor slab connection to the girders. These connections included non-composite (no shear connectors), fully composite case 1 (shear connectors between the floor slab and the girders only), fully composite case 2 (shear connectors between the concrete floor slab and both beams and girders), and a partially composite condition. We also simulated the connections between beams and girders, the connections between girders and columns, and the connections between beams and columns. These connection details are important as the connectivity significantly affects the structural response during fire and the deformation response due to self-weight and dynamic forces that occur during collapse.
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1.5.2 Approach 2: Evaluation of NIST’s Collapse Initiation Hypothesis
Second, we supplemented our own simulation by evaluating the validity of NIST’s collapse initiation hypothesis and probable collapse sequence. In particular, we focused on
