Mr Doyle writes:
"I think you are saying the lack of electricity caused by a chain of blackouts stemming from the impact in the north Tower caused a lack of normal control in the generator room that led to the explosions and fires. Diesel is hard to ignite accidentally. Propane, not so.
If I'm grasping this you are saying the cantilevered truss got hot or was sabotaged and pulled inward off the center of the vertical columns it supported and caused an instantaneous total failure of the truss and therefore all the structure above it. And this was due to the inherently unstable nature of the flying truss bridge design of WTC7 used to carry a tall building over a power plant located beneath it."
Not exactly.... I am suggesting that there were probably:
explosions of electrical equipment / transformers and switch gear in both the Con Ed sub station AND on the mech fls 6&7.. 5 was electrical sub station for the B7 tenants...
fires resulting from the explosions of electrical equipment on the mech floors 6&7 where the back up power generators were located along with diesel tanks to run them.. several of them.
diesel IS hard to ignite... but exploding transformers will do the trick
There were several off structural elements on floors 1-8.. the first 7 floors
transfer trusses
These were effectively bridges like a bridge span supported on two columns. The span supported columns above such as column 79. If the truss fails.. the brdge drops and the column it supports mid span drops as well.
cantilever girders
these structures supported 8 columns at the end of the cantilver. Those 8 columns could not continue down to bedrock because the Con Ed equipment below... The girders were supported by 2 columns... one of the was located at the north side of the core. If either of the two columns supporting the cantilever fails.. the girder comes down and the columns at the end of the cantilever will have no support
The key here is that columns (vertical) were supported by BEAMS (horizontal) not the usual means where beams are supported by columns. So when these horizontal members failed the columns they supported had no support. If one beam fails in a typical structural design... ONLY what it supports collapses.. say.. a section of one floor. In this case if one of the cantilever girders fails.. ALL the columns above it.. and all the areas of floor adjacent to THAT column line have lost support.
This is why those girders and trusses had to be very strong because they could collapse the building if they went... in a sort of putting all your structural eggs in one basket. THIS is the bone headed nature of the design of WTC 7. An analogy... suppose a design requires 25 columns. But owner says he wants to have a theater on the first floor and can't have all those columns in the way. So engineer designs with FEWER larger columns and long strong (trusses) beams to span between them to support the columns above where the office space is. No problem. But if the long truss beams fails the tower above collapses into the theater... or if one of the fewer columns fails it's HUGE load has to be carried by the other columns to keep the building up.
I can't address HOW exactly those structural members failed... if it was from heat or placed devices. I am quite certain that this is what failed. Engineer agrees. I think heat from diesel and gas fires for 7 hrs MIGHT explain the weakening and then collapse. It should be noted that this does not have to destroy ALL the structure or ALL the columns for 8 floors. It is a STRAW THAT BREAKS THE CAMEL'S BACK situation... like the rusting pin which failed the
I95 Mianus River bridge. One rusted pin (1 or 2" on diameter took the bridge down).
"The bridge had a 100-foot (30.5 m) section of its deck of its northbound
span collapse on June 28, 1983. Three people were killed when two cars and two tractor-trailers [SUP]
[1][/SUP] fell with the bridge into the Mianus River 70 feet (21.3 m) below; three were seriously injured.[SUP]
[2][/SUP]
Casualties from the collapse were few because the disaster occurred at 1:30 a.m., when traffic was low on the often crowded highway.[SUP]
[3][/SUP]
Causes
The collapse was caused by the failure of two pin and hanger assemblies that held the deck in place on the outer side of the bridge. The hanger on the inside part of the
expansion joint at the southeast corner was forced from the pin that was holding it, and the load was shifted to the only other pin in the joint.
The problem was caused by rust formation within the bearing on the pin, exerting a force on the hanger which was beyond design limits. The extra load on the remaining pin started a fatigue crack at a sharp corner on the pin. When it failed catastrophically, the deck was supported at just three corners. When two heavy trucks and a car entered the section, the remaining expansion joint failed, and the deck crashed into the river below.
The ensuing investigation cited corrosion from water buildup due to inadequate drainage as a cause. During road mending some 10 years before, the highway drains had been deliberately blocked and the crew failed to unblock them when the road work was completed. Rainwater leaked down through the pin bearings, causing them to rust.
The outer bearings were fracture-critical and non-redundant, a design flaw of this particular type of structure. The bearings were difficult to inspect close-up, although traces of rust could be seen near the affected bearings. "
Nothing INHERENTLY unstable about the design... but it is the sort of design which LACKS REDUNDANCY and therefore is subject to catastrophic failure... as we saw.
