The cross members required to support the load of 20 stories falling at
9.8 meters per second would require exponentially stronger members below
to support 40 stories falling at 9.8 meters, etc.  Otherwise someone
could pick the weak spot and hit it.

This would actually add tremendously to the cost of the building.

Furthermore, the building held up long enough that they could have
gotten everyone who was not hurt by the initial blast, including the
firefighters if they understood how long they had, and had better plans
in place.

As has been said so many times before, we are learning some terrible,
but necessary lessons because of this.

The building did what it was supposed to do.  It is not necessary to
beef every other or every future high rise up to withstand the truly
extreme forces the WTC withstood.  You can't account for everything,
and, like actuaries, we have to find a point where the safety systems in
place meet the costs associated with them.

Of the 50,000 people who were inside the building at the time fo the
attack, probably less than 10% will have died.  Please don't
misunderstand - it is an unacceptable loss for us - but financial
backers and the engineers who must cut costs will ultimately design the
building for the stresses placed on it 99.999% of the time, and not well
enought for that extra one thousandth of a percent which would have
allowed some 10k pound of jet fuel to burn on several floors for several
hours without collapsing.  They will then say that in the one in a
million chance that the building had undergo those stresses, and 90% of
the population of the building escaped largely unscathed, then it would
be good enough.

I would be surprised to find many people on this list who design a GFI
into their circuitry for the one in a million chance that it would be
dropped into the tub, when it is not used anywhere near one.

-Adam

Dan Michaels wrote:

>Chris Pringle wrote:
>
>>I was quite interested to hear from one of our structural engineers earlier
>>today. Apparantly it wasn't the aircraft that caused the buildings to
>>collapse. The buildings were designed to take the hit of an aircraft.
>>
>>What brought the buildings down was the fire. The buildings hadn't been
>>designed to survive a fire. When the structure became warm, the steel
>>supports were no longer strong enough to support the weight of the building
>>and so it gave way.
>>
>
>
>While watching the towers collapse [again and again, ugh], I was
>struck by the way the entire structures simply pancaked vertically
>[had they fallen "over", they would have wiped out half of Manhattan],
>picking up speed as they went, and wondered how on earth do you
>build a building that does just that.
>
>I heard several interviews with structural engineers, and not one
>had the balls to address this particular issue. They all said, "we
>designed for hurricanes and wind loading and [small] fires ......,
>but not for anything like this .....".
>
>At any rate, turns out the reason for the collapse dynamics is
>because they build these things as a tube within a tube, with an
>inner tube bearing most of the weight, and the outer tube a series
>of girders up the outside wall - but there are essentially "no"
>structural elements connecting the two. This is what the paper
>said today.
>
>Take a close look at pictures of the 1st tower collapsing, and
>one of the "obvious" things you are similar length girders flying
>off in all directions. These are about the length of one story
>height or so.
>
>Small wonder they came down the way they did. One would think
>that adding heavy cross members every 20 floors or so might
>prevent the entire structure from pancaking like that, and
>might add only minimally to the overall cost.
>
>--
>http://www.piclist.com#nomail Going offline? Don't AutoReply us!
>email listserv@mitvma.mit.edu with SET PICList DIGEST in the body
>
>
>
>
>

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu