Table 2.3 Comparison of UAF and NIST Modeling Steps Taken Floor framing – steel connections (springs) Exterior steel framing connections included (springs) Girder to column stiffener plates at Column 79 Floors (composite with beams, not major girders) Floors (composite with beams and girders) Thermal expansion of the concrete deck Thermal conductivity and expansion of material properties
UAF Yes
NIST Partial
Yes Yes
No No
Yes Yes Yes
Yes No No
Yes
No
As Table 2.3 illustrates, we took several steps in our modeling of WTC 7 that NIST did not take. These steps enabled us to more accurately simulate the building’s behavior and to consider conflicting assumptions regarding the as-built condition of the building. In summary, based upon our analyses, we found the following: 1. During our nonlinear connection study, we discovered that NIST over-estimated the rigidity of the outside frame by not modeling its connections, essentially treating the exterior steel framing as thermally fixed, which caused all thermally-induced floor expansion to move away from the exterior. The exterior steel framing was actually flexible, while the stiffest area resistant to thermal movements, i.e., the point of zero thermal movement, was near the center of the building. 2. Therefore, during our analysis of WTC 7’s response to fire loading, we found the overall thermal movement at the A2001 base plate support at Column 79 was not sufficient to displace girder A2001 to the point that it walked off its seat. Whereas NIST asserted that the differential westward displacement of girder A2001 relative to Column 79 was 5.5 inches and later revised its calculation to 6.25 inches, we found that the westward displacement of girder A2001 relative to Column 79 would have been less than 1 inch under the fire conditions reported by NIST (Figure 2.66). The little relative displacement is justified as the building was constructed as a composite-beam structure where the concrete slabs and the steel elements were connected with nelson studs. Therefore, even if the connections between some steel elements were broken, the steel elements were still held together by the studs to the concrete slabs.
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