Building codes fixed after 9/11?

I was wondering if the design flaws of the twin towers have ever been completely discovered and incorporated into new building codes for high-rise buildings.

For example, has it been decided exactly what caused the concrete to be pulverized in mid-air while at the same time, this field of airborne debris, most of it being trusted outwards, still had enough concentrated downward force to collapse and pulverize every level of the building below it?

What changes have been made in construction techniques to ensure that this same sort of anti-physics effects do not happen again?

 

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"Concentrated downward force"? It's called gravity.

And to answer the OP: I believe they have changed some of the regulations about sprayed on fireproofing.

 

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Not sure this can be called a design flaw. The forces involved in resisting the entire collapse of a floor are tremendous. A building designed to resist this kind of failure would need to be built like a nuclear bunker.

 

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Not really.

The towers did have a design flaw (IMHO).

While the engineers complied with building codes, the codes did not anticipate this kind of design.

The key to the tower's failures can be seen in this image.

http://www.boulder.nist.gov/div853/Program6_infra_files/image017.jpg

Those projections from the column trees that the floor is bolted to are called Truss Seats.

There are similar ones on the inside mounted to the core columns.

They are the ONLY thing that holds a floor up. (to be clear you can a number of these seats in the picture but only the one in the middle is a truss seat that held up the floor, there was ~one truss seat for every two vertical external columns. The top most ones are for those lateral straps to be welded to, but provide no vertical support, and the bottom one is for the elastic dampers to be connected to, to reduce the sway in the towers, but again, do not provide any vertical support to the floor)

I'll repeat this so it is clear. The core and external columns have attachment points for the Truss Seats, but ONLY the truss seats hold up the actual floors. Floors which are nearly an acre in size and weigh several million pounds each.

It was a real selling point of the towers, as they had far more rentable space per sq ft of tower than any previous design, and just as important, the tenents could divide up that space and load it in almost any fashion that they desired.

BUT there was a flaw.

The lightweight floor trusses (which you can see how lightweight they are in the pics) were NOT considered to be structural elements, because they only supported that one floor.

Because they weren't considered to be structural elements they did not need the 3 hour fireproofing that the columns required.

This is the loophole in the building codes.

(ps for all the 9/11 conspiricists that go on and on about how hot jet fuel can burn, two things, if a normal office fire was not hot enough to destroy a steel building they wouldn't require fire proofing. But it is. And in the WTC towers, the towers themselves, had FAR MORE fuel in them from paper and furniture and carpets than the planes had as fuel. NIST estimates that almost all the fuel burned up within a relatively short time. It was the office contents that were burning which caused the extensive damage)

This fireproofing was applied to the trusses in the form of a spray on foam that hardened. For best adhesion, it was to be sprayed onto unprimed metal, but because of the long time from fabrication to installation, the trusses in the towers were primed.

So when the planes hit, one of the most serious effects was that on the major fire floors, a lot of this spray on foam would have been knocked off.

As it turns out, the towers had floors with long sides and short sides, and the long trusses (spanning 60 feet) of both towers sagged from the damage, the heat of the fire and the extra weight of the planes contents.

Which is where the structural angle comes in. Though the floors were not considered structural, they were absolutely needed to brace the external columns, because the load carrying ability of those columns was directly related to their unbraced length.

THIS is where the building codes failed. Though the trusses weren't structural elements their failure would lead to the structural elements not having the load carrying capacity that they should have had, so the reality is they should have had 3 hour protection. Or there should have been some beams in the flooring, not just trusses.

Given the impact of the plane severed so many core columns, the heat from the long burning fire caused the center columns to undergo creep (shortening), thus transfering loads to the external columns which because at least three floors on the long side sagged, the external columns lost most of their side bracing and worse, the sagging floors caused the columns to be pulled inward, further lowering their load carrying ability until finally (on both towers) the long wall failed, and then global collapse ensued. It was a steady progression of failure entirely unlike any kind of demolition. No explosive or thermite demolition technique would cause the progression of damage that was seen in the long walls of the towers just prior to collapse

As to the fall of the towers, when the upper floors hit the lower floors, a set of undamaged truss seats could only handle about 6 floors worth in a dynamic impact scenario, thus the intact floors were quickly overwhelmed by the top floors coming down, causing them to fail on to the floor below them which caused the lower part of the tower to collapse internally in a rapid pancake fashion.

Why the floor concrete pulverized was because that's what it does when it's that thin (4") and mixed with fly ash (the floors were all lightweight concrete ouside the core and mechanical floors), of course a great deal of the dust seen that day was also all the smashed wall board that made up the partitions in the core and the tennent spaces.

The NIST report contains details to support all of this if anyone cares to take the time to actually read it.

Arthur

 

Inside Jerb!

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To design a building that can take an impact of a million pound jet, fully loaded with over 100,000 gallons of fuel going over 300 MPH would cost so much money it wouldn't be practacle to build it. Anything hit with that kind of force wouldn't hold up at all.

 

Well to put those numbers in perspective:

The jets that hit the WTC were quite large, but fully loaded they only weighed about 280,000 lbs and only carried about 10,000 gallons of fuel.

They were going quite a bit quicker though. AA 11 which hit WTC 1 was going about 440 MPH and UAL 175 that hit WTC 2 was going about 540 MPH.

Since impact energy goes up with the sqaure of the speed, it's really a very important variable.

Still, the NIST modeling showed that had the insulation not been dislodged, the WTC towers, even as constructed, would most likely have stood.

I'm convinced that if they just had a few beams instead of all trusses on the long sides of the buildings, the towers would have stood.

Would they ever have been able to be occupied again is an entirely different
issue.

Arthur

 

>The towers did have a design flaw (IMHO).

No shit, really? Those silly engineers didn't count on bad/intentional piloting of jumbo aircrafts??

No building prior to 9/11 was engineered that way...And your post above doesn't state anything to the contrary..

 

A 777 jet airliner, as an example, has a take off weight load of 766,800 lb(347,800 kg) so that is a possible aircraft, which you must consider when designing any

structure, that could impact a building.

 

Really?
You've examined every set of building plans prior to the mid 60s?

The fact is every building is different, and when they built the tallest building in the world they used techniques not ever used before.

But, because the structural integrity of the trusses were responsible for the load bearing capacity of the main structural elements of the tower, they should have been considered structural themselves and built to have the same fire resistance of the columns.

They did not.

You can argue against that, but had that been done it's highly likely that the towers would have stood, because it was the sagging floors which provided the pull in force that caused the heated exterior columns on the long side of both buildings to become overloaded and collapse due to their lack of longitudinal bracing.

By the way, I do this mindful of the fact that hindsight is 20/20, and all this is something, in pre-computer days the engineers could not have explored like was done on the NIST fire and structural models.

I don't believe the engineers ever considered such a massive amount of structural damage to the towers back then, even though they did consider what would happen if a large jet hit them. I don't think they considered that the impact speeds would be over 400 mph and that at those speeds the vibrations and debris would knock off/scour multiple floors of the fireproofing critical to maintaining the building's structural integrity in a long burning fire.

Which, as NIST pointed out, was the critical difference between the tower's standing or falling.

Arthur

 

sacrasm is fine, but not to the point of being rude. most especially since this 9/11 is a sensitive subject for most americans. please be more mindful of your posts.