2.2 Solid Works Modeling

can happen without any medium between different physical systems. The principle of heat transfer analysis is a key element in analyzing the collapse of WTC 7 because it is crucial to determine the temperature distribution of the structural elements — such as the girders and the concrete slabs — by analyzing the heat transfer from the fire to the structural elements.

2.1.3.4 Material Properties Utilized A Portland cement mix was used for the concrete deck of WTC 7 to give a 3,500-psi concrete. The aggregate type was dolomite. The density was 145 pcf. The thermal expansion coefficient for the concrete deck was assumed to be 10.6 microstrain/°C. Aggregate type has some effect on the coefficient of linear thermal expansion of concrete. However, the w/c (water–cement) ratio and cement content did not show any effect on the coefficient of thermal expansion (CTE) (Alungbe et al., 1992). Naik et al. (2011) proved that the source of dolomite aggregates had an insignificant effect on the CTE of concrete, with the CTE varying between 10.4 and 10.8 microstrain/°C (from 5.8 to 6.0 microstrain/°F) and an overall average of 10.6 microstrain/°C (5.9 microstrain/°F).

The steel used for all beams and interior columns in WTC 7 was ASTM A572 Grade 50, and the steel used for exterior moment frames was ASTM A36. The thermal conductivity was 35 BTU / (hr-ft-F). The thermal expansion is reported to be the same for all typical structural steels (Rahman et al., 2004). The CTE of steel is assumed to be constant, so that the thermal expansion strain is given by 14 microstrain/°C (7.78 microstrain/°F) (Wang, 2002).

2.2 Solid Works Modeling

Prior to conducting our analyses in SAP2000 and ABAQUS, we used SolidWorks to model a simulation of the thermal properties of the structural elements of WTC 7. As with our SAP2000 modeling, in order to obtain equivalent homogeneous shell elements representing the floor slabs and to save computational time and resources, effort was given to simulating the thermal equivalent material of the floor slabs. Figure 2.7, Figure 2.8, and Figure 2.9 show a small section of concrete block with voids for reinforcing, welded wire fabric for the reinforcement, and gauge 20 metal deck. The three models were then assembled into a unit section of the floor slab and assigned two different surface temperatures, both at the top and at the bottom. The