Figure 2.26 Small section of typical floor slab of WTC 7 with finer meshing

Figure 2.26 Small section of typical floor slab of WTC 7 with finer meshing.

In order to achieve accurate and reasonable results, we studied mesh refinement to achieve convergence. Figure 2.27 and Figure 2.28 show different floor slabs with different meshing. The meshing schemes are presented for comparative purposes. After conducting a convergence study to evaluate the appropriate mesh size, the model of the floor slabs was compared to the 5.5-inch-thick homogeneous material in SAP2000. The concrete used for WTC 7 was 3,500 psi in compressive strength and a 3,408,800 psi Young’s modulus, which was based on an assumed 145 pcf density. The steel used for the gauge 20 metal deck and welded wire fabric reinforcement was A36, with Young’s modulus of 29,000,000 psi and yield strength of 36,000 psi. The purpose of the analysis was to find an equivalent homogeneous material 5.5 inches thick that would behave in the same manner as a typical floor slab in WTC 7 for a given length and width and under the same loading conditions. Such material was needed to save computational time and resources so that the floor slabs could be modeled by using simple homogeneous shell elements instead of layers of materials consisting of frame, shell, and solid elements, with the exact and very complicated geometry representing the real floor slabs.