Hello Ziggi, welcome to the forum.
Actually, and just to nitpick, they don't. They say that, but for the beams, not for the girder, though they mention girders failing "in this manner" (which may be interpreted as the failure of the flange as described for beams, but it may also refer generically to a displacement by a distance of half the seat width). But since you concede anyway that that distance is enough, let's not focus on that.
NIST said in p.482 of NCSTAR 1-9 volume 2 (last sentence in the page) that
"eam walk off in the lateral direction was monitored during the analysis". That sentence appears in the description of the analytical model, which also mentions that walk-off in the axial direction was monitored through a control element (I presume that they meant the COMBIN37 one in the illustration).
Then in section 11.3, titled "ANALYSIS RESULTS", they give more details on that monitoring:
The analysis of the structural response of WTC 7 to the effects of elevated temperatures produced a large volume of output data. Results for Floors 2 to 7 and Floors 15 and 16 are not presented as they were subject only to gravity loads and were not subjected to thermal loads. The analysis results for Floors 8 to 14 were first examined graphically for a selected response, such as vertical displacement or strains. Areas of interest were then examined in more detail by listing the results of interest.
(p.489).
Then in the same section, in subsection 11.3.2 (Case B temperatures), p.504, it says: "The following beam and girder failures occurred: [...] On Floor 13 (Figure 11-35), all four of the north-south girders attached to Columns 79, 80, and 81 had failed, due to either buckling or girder walk off of the bearing seat at Columns 79 and 81".
Note that they are not mentioning here the precise mechanism that caused the failure; in the context of a section that reports the results of the ANSYS analysis, the only possible meaning of their words is that they are saying that the FEA produced that result. Nothing more, nothing less. This necessarily means that by that time, the distance between the girder's center and the seat's center at the point where the girder and the seat were when the failure occurred, exceeded a certain predetermined distance, as that's the criterion for determining if that girder walks off the seat or not. And that distance was monitored from ANSYS output, as I've already established above.
At no point in the report do they say that any beam expanded by 5.5" or 6.25". That claim doesn't exist in the report. That distance is indicated as "the distance for walk-off", not "the distance by which the beams expanded".
What they do mention, however, is that "[o]n Floors 10, 11, and 12 (Figure 11-32, Figure 11-33, and Figure 11-34), the girder between Columns 76 and 79 failed due to a tensile weld failure in the knife connection on the west side of Column 79. Temperatures in this region were less than 100 °C on these floors. The tensile force in the connection was due to an eastward lateral displacement of Column 79, which was primarily caused by thermal expansion of the girder between Column 76 and Column 79 at Floor 13". This NECESSARILY indicates a displacement of the seat to the east in ANSYS. No way around it. The girder between 76 and 79 was next to the seat.
So, that's primarily what NIST is saying here: that the simulation showed the girder walking off the seat. They don't point to exact reasons.
They proceed to interpret the results in the next section, 11.4: "DISCUSSION OF RESULTS". They repeat this (p.525, top): "By 4.0 h of heating, there was substantially more damage in the WTC 7 structural system, particularly the loss of lateral support to Column 79 after the failure of girder connections at Floors 10, 11, 12, and 13. [...] The girder between Columns 44 and 79 had walked off the bearing seat at Column 79 on Floor 13, and all 4 bolts had failed on Floor 14 and two to three bolts had failed on Floor 12 at this seated connection".
Later in this section, in page 527, they say: "Walk off occurred when beams that framed into the girders from one side thermally expanded and the resulting compressive forces in the beams pushed laterally on the girder from one side, sheared the bolts at the seated connection, and then continued to push the girder laterally until it walked off the bearing seat". Note that it doesn't say what happened to the seat. That sentence is followed by: "A girder was considered to have lost vertical support when its web was no longer supported by the bearing seat. The bearing seat at Column 79 was 11 in. wide. Thus, when the girder end at Column 79 had been pushed laterally at least 5.5 in., it was no longer supported by the bearing seat". That distance was later corrected to 6.25 in.
Note that:
1. They don't say that the beams expanded that much. They only say that when that situation happened, the girder would no longer be supported.
2. This is an interpretation of the results, not the results themselves.
3. For discussion purposes, it's reasonable to consider that since the column displaced east, they are talking about the main factor that displaced the girder from the seat and not each and every factor.
The mention to the displacement of the column comes in the paragraph next to it: The temperature of the girder between Columns 76 and 79 on Floor 13 was sufficient to displace Column 76 to the west and Column 79 to the east. The forced displacements at Floors 10, 11, and 12 created a tensile load in the girder knife connections to the columns, and failed the connection fillet weld to the column". Note how they mention column 79 displacing east.
So, are they guilty of not being specific enough about the pushing of the column to the east, as they said happened in the simulation, in the fragment of the discussion related to the girder walk-off? Yes, definitely. That doesn't invalidate the report, however. That's focusing too much in the primary cause and forgetting about the secondary ones, that certainly happened. But they are not lying or misrepresenting the primary cause of the failure. That also does not invalidate the initiating event experienced in their FEA in any possible way.
Hope I have made the point clear enough to you.
