Subject: Collapse of twin towers

Sent to: engineering@byu.edu

 

Hello,

    I am curious if you can help me with a couple questions. I have already sent this to a physics expert, but I believe two opinions are better than one. Besides, your specialty is engineering...all the better. I will be happy to pay you for your time. Please provide an estimate to figure out the following:

 

The north tower was struck between the 93rd and 98th floor. Common sense would imply the bottom layers of construction would need to be far stronger / heavier than the upper floors ( as the 10th floor would be required to support the weight of 100 floors above it, the 90th floor would only need to support the weight of 20 floors.)

Under no circumstances could the upper 17 floors (93 - 110) represent more than 15% of the building's total weight. However, if my hypothesis is true (these huge buildings are built progressively lighter as they go higher) it is likely they represented less than 10%. (If this is NOT correct, please advise.) 

The real question is: Could such a small mass of falling debris (10 - 15% of the buildings total mass) which only initially fell approximately 5 stories (damage from 93rd to 98th floor) CRUSH without any resistance whatsoever all of the intact structure below? (Even if the intact floors would have provided one quarter of a second resistance before failing, the collapse would have taken approximately 23 seconds. Instead, the top 12 floors (98th - 110th) fell to earth as quickly as they would have if they had fallen down on nothing. ...at free-fall acceleration. 

My understanding is that these buildings were built to hold five times their weight. (if this is incorrect, please advise.) How much in excess of the rated capacity would the falling debris achieve from its approximate 5 story fall?  Then (to go further) can we figure out how much downward force would have been necessary to crush all of the enormous supporting columns into oblivion over a distance of 930+ feet without any slowing whatsoever?

I know the much stronger argument for controlled demolition rests in WTC 7, but I believe physics can also reveal a problem or two with the towers collapse. Other arguments have been made, but I've yet to see this one studied. I look forward to your thoughts and my offer stands to pay you for your time (if you're willing to help me perform these calculations.)

Best Regards,
Joe Plummer