Figure 2.3 Approach used to model buckled beams in LS-DYNA model (NIST, 2008, NCSTAR 1-9, Vol. 2

In the NIST investigation, ANSYS was used to model local failures and LS-DYNA was used to model large-scale collapse. When column buckling appeared to be imminent in ANSYS, the analyses were continued in the LS-DYNA 47-story model (NIST, 2008, NCSTAR 1-9, Vol. 2). The damage was applied in LS-DYNA as a sudden removal of damaged or failed elements as calculated in the ANSYS analysis. Buckled beams were modeled by removing flange and web elements, such that the beam would lose its axial and flexural strength, but the weight of the beam would remain in the calculation (see Figure 2.3). The connection damage data from ANSYS was transferred to the LS-DYNA global model using a damage index. Horizontal support and vertical support were specified separately (see Figure 2.4 and Figure 2.5).

Figure 2.3 Approach used to model buckled beams in LS-DYNA model (NIST, 2008, NCSTAR 1-9, Vol. 2).

Figure 2.4 Shear connection damage; LS-DYNA model (NIST, 2008, NCSTAR 1-9, Vol. 2).

2.1.3.2 WTC 7 Nonlinear Connection Study Seven types of shear connections were used for interior floor-framing connections in WTC 7: fin, header, knife, seated top plate, seated top clip, seated web clip and a seated moment connection (NIST, 2008, NCSTAR 1A). The exterior wind girders to exterior columns were connected by using moment connections. Tied contacts are defined in the NIST Report as between the concrete deck and frames. No failure was allowed in the tied contact (NIST, 2008, NCSTAR 1-9A). In the NIST investigation, the failures of the floor-framing connections and the shear studs were modeled with break elements on Floors 8 to 14. Outside the selected area in Figure 2.5 shown below, structural damage — such as buckling of the steel frame and crushing and