Can you define "key bits" in regard to the NIST report so i can try to find it?
Executive summary.
Isn't it unlikely that anything from the load bearding columns would be significantly pushed when they are the main supports for the building,
Nope.
the main force from thermal expansion of trusses I would have thought would push outwards from the main support columns, using their structural strength to expand into more external areas with less structural strength.
Trusses are irrelevant. Learn the definition of terms so that you make sense.
Beams & girders matter. Look up the difference between the two. (Hint: what ties into them defines the difference.)
Here is the over-riding principle: direction of forces matter.
If I gave you a football helmet & some shoulder pads, I could hit you with a pretty large downward force on your shoulders, and would be very unlikely to break one of your leg bones. They are designed to take loads in the vertical direction.
But if I were to take a Louisville Slugger and hit your tibia horizontally, there is a fair chance that the bone will snap.
Direction of forces matter.
The tall, thin columns were well designed to take vertical forces. They also take some pretty high horizontal forces … AS LONG AS those forces are balanced from each side and sum to approximately zero. As they do, for horizontal thermal expansion forces, in most buildings & most parts of WTC7.
But there was a previously unrecognized flaw in the design. With fire & thermal expansion, there was the possibility that the horizontal forces from thermal expansion did NOT laterally cancel out for just a couple of main girders in WTC7.
Read the executive summary. Then look at the beam & girder floor plan.
It becomes immediately obvious where the problem lies.
tk