You first typed that it didn't have to be explosives, it could have been through some other means. You changed this again.
You're backing off explosives, so that's out.
The girder fell off the seat because the seat bent or sheared.
The girder sagged more than the calculated 2 inches or so because first it buckled through axial expansion trapped by the two columns and by the eccentric push of the top placed expanding beams. Once the girder buckles it loses its geometry and strength and the deflection is much greater than ~2". This places the load at the heated unstiffened col 79 seat tip, this fails and the girder slides off.
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Detail A below shows the initial conditions as drawn.
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Detail B below shows the girder displacement as calculated by TS without the column 76 girder push. The girder load is at the tip of the cantilevered seat.
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Detail B1 below shows the girder displacement with the 1” column 76 girder push. The girder load is at the tip of the 4 7/16” cantilevered seat.
“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.” (NCSTAR 1-9 p527).
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Detail C shows the sagging girder with a 20” deflection, similar to the beams deflection modeled by NIST. Any girder deflection would place the girder load at the tip of the cantilevered seat.
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Detail D shows the condition at 13th floor Column 79 seat. The 83.7k load was calculated at 100 psf for 837 sf of area at the tip of the 4 7/16” cantilevered seat. It looks like torque would fail the seat in bending or shear and the girder would slide off, but I can’t do the math.
NIST didn’t consider the vertical failure of the seat and its model included a seat stiffener at Col 79 not shown in the plans.
“Since vertical failure of the seat was not considered (Section 11.2.5), the connections at Columns 79 and 81 were both modeled as stiffened seats.” (NCSTAR 1-9 p.558) See
“Figure 12-25 Seat connection in global model Column 79. (p. 559)
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The conclusions that the columns, beams and girders failed by fire were the result of the Fire Dynamics Simulator (FDS), FEA - ANSYS and LS-DYNA models over time.
NCSTAR 1-9 p 536
“ The ANSYS model included nonlinear effects, such as: nonlinear temperature-dependent material properties including thermal expansion, plasticity and creep; nonlinear geometry; and user defined elements that captured the details of temperature dependent connection failures.”
Other forces unaccounted for in the above details leading to failure of this girder or buckling of Column 79:
- Twisting and failure of the girder at the Critical Twist Angle:
NCSTA 1-9 p487
" When lateral support of the top (compression) flange was lost, floor beams and girders could laterally displace and buckle in a lateral-torsional mode. ...If a beam or girder twisted half of its flange width laterally, it would not be able to support its gravity loads. ….Figure 11-19. Critical Twist Angle in Beams/Girders." The beams framing onto the girders were eccentrically located at the top section of the girder.
- Lagging differential expansion of the concrete due to the fires on the 12th and 13th floors bearing and laterally pushing on the girder.
- At Column 79 Floors 6-13 failure of the bolts or welds for the two girder fin supports. Loss of east-west horizontal support of the columns over 8 floors shown. (NCSTAR 1-9A Figure 4-17 p. 81)
The opinion that fires had nothing to do with the collapse of WTC7 is political and false.
