You asked what could cause enough heat for the beams to fail - the building
contents supplemented by 10,000 gal diesel fuel from the backup generator
Most items and furnishings today are made of synthetic materials aka
plastics (remember the film "THE GRADUATE"??)
Plastics are derived mostly from petroleum and burn with much great heat
than organic material (wood, paper, cloth). Plastics produce from 12,000
to 16,000 btu/lb vs 8,000 for organic type materials. A modern office is
almost entirely made of plastics . Chairs - urethane padding, cubicle
dividers - styrofoam, urethane, desks - particle board aka sawdust held
togather with resin (plastic binder), Even the computers are almost all
plastics these days. FEMA set up test where typical office workstations
were ignited and heat output measured - small cluster of workstations
generated 12 mega watts thermal energy. Add to that diesel fuel from
broken pipe caused when top of WTC 1 gouged out corner of WTC7 have
makings of nice big bonfire.
Also the spray on fire proofing common in many buildings has tendancy to
crack and flake off over time - assuming its applied right. Tests found
applying on painted steel reduces adhesion by factor of 2 to 3 (more likely
to peel off)
Add to this that FDNY did not fight the fire - had no water pressure from
collapse of towers which cut mains which with lack of equipment (FF on
scene when initially told to go into WTC7 had to scrounge for air packs,
hose and tools from destroyed/damaged apparatus). Fire chiefs considered
structural damage to building (read accounts from FF and chiefs on scene
in FIREHOUSE magazine
WWW.FIREHOUSE.COM in their 9/11 archives) and
realized lacked resources to adequately fight the fire without risking more
death/injury to personnel - 343 were already dead and many more injured.
WTC 7 was abandoned early afternoon after building was searched and
nobody remained. Collapse can 5:20pm some 7 hours after being damaged
by debris shower from collapsing towers.