here's some more reading for you....
"Under contract to NIST, Leslie E. Robertson Associates (LERA) constructed a global reference model of
each tower using the SAP2000, version 8, software. SAP2000 is a software package for performing finite
element calculations for the analysis and design of building structures. These global, three-dimensional
models encompassed the 110 stories above grade and the 6 subterranean levels. The models included
primary structural components in the towers, resulting in tens of thousands of computational elements.
The data for these elements came from the original structural drawing books for the towers. These had
been updated through the completion of the buildings and also included most of the subsequent,
significant alterations by both tenants and The Port Authority. LERA also developed reference models of
a truss-framed floor, typical of those in the tenant spaces of the impact and fire regions of the buildings,
and of a beam-framed floor, typical of the mechanical floors.
LERA's work was reviewed by independent experts in light of the firm's earlier involvement in the WTC
design. It was that earlier work, in fact, that made LERA the only source that had the detailed knowledge
of the design, construction, and intended behavior of the towers over their entire 38-year life span. The
accuracy of the four models was checked in two ways:
• The two global models were checked by Skidmore, Owings and Merrill (SOM), also under
contract to NIST, and by NIST staff. This entailed ensuring consistency of the models with
the design documents, and testing the models, e.g., to ensure that the response of the models
to gravity and wind loads was as intended and that the calculated stresses and deformations
under these loads were reasonable.
• The global model of WTC 1 was used to calculate the natural vibration periods of the tower.
These values were then compared to measurements from the tower on eight dates of winds
NIST NCSTAR 1, WTC Investigation "
