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Highrises, camber, superelevation, compression, and scotch

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(@scott-zelenak)
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I have spent all day discussing the theoretical aspects of compression in highrise construction. Imagine, if you will, a building composed of a steel perimeter and a concrete core. We have rules of thumb for the anticipated compression of steel and concrete. And, of course, often those rules do not apply. So, here we have one of those situations in which the computations are built upon a theoretical model which violates our rules of thumb. So that you can have an idea of the anatomy about which I shall discourse herein I append a rough sketch.


A corner quadrant.

Now, I seek no guidance or advice, I just feel a need to vent the theoretics.
Steel and concrete will not compress at the same rate and, of course, variables beyond our control also enter into the discussion. More particularly, as relates to concrete. Hence differential compression rates per tier will result in superelevation of the steel beams (as opposed to steel columns). The superelevation for each member, obviously depends, not only upon its length and orientation, but proximity to the perimeter or core. Each beam will also have a camber. Primarily based on length and member size. And our superelevated and cambered beams may have to tie into other superelevated and cambered beams. Consider, as well, rates will vary in structural components, based on loading. Our perimter will not compress at an equal rate to our interior columns until all dead loads are applied. So even our steel compresses at different rates based upon distance from the perimeter. Now, were our theoretical highrise to have elevated floors, we might not worry so much. Adjustments could be made.
If just one of our theoretical engineering computations were to fail, well, you have a fair idea, I suppose, of the resultant chaos.
So now, these considerations apply to each floor and become compounded, obviously, by the number of floors stacked up.
Suppose, we enter in some theoretical numbers then to get an idea.
Let's say, for arguments sake, we have one hundred floors (we do not, but say we do).
And suppose, from experience, reinforced concrete compresses at a rate of 1/8 of an inch per floor. While steel compresses at a rate of 1/16 of an inch per floor.
Now do the math for the bottom and top floors.
Now, just for giggles, assume someone (I would love to use a different noun, but hey, my first amendment rights only go so far) ran a bench in from the street to set elevations.

Now, thank God for a fine glass of 12 year old Jameson.

 
Posted : September 1, 2010 3:58 pm
(@gunter-chain)
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Won't need any scotch to be stumblin' round and falling down in there?

 
Posted : September 1, 2010 4:10 pm
(@scott-zelenak)
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The corner quadrant is, of course, in plan view.

 
Posted : September 1, 2010 4:28 pm
(@butch)
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you're dead wrong by the way...

Jameson is Irish Whiskey, not scotch! 😛

 
Posted : September 1, 2010 5:52 pm
 ddsm
(@ddsm)
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> you're dead wrong by the way...
>
> Jameson is Irish Whiskey, not scotch! 😛

Butch,
I sent this problem to my son, the Systems Engineer (NASA Intern)...and he said the same thing...

DDSM
(what does this old man know?...a beerleg?)...:beer:

 
Posted : September 1, 2010 5:59 pm
(@mike-falk)
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Scott

Great post. Good IRISH WHISKEY.

Here in Chicagoland the SEs ask just for the raw survey data. They want to apply all the loading information themselves.

 
Posted : September 1, 2010 6:29 pm
(@daryl-moistner)
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That is a great post...it got me thinking about the compression of the ground floors of the Burj Dubai. Then I started thinking about this incredible mass sitting on a relatively tiny footprint... its just not something that occurs in nature. Asides from flying aircraft into them I always wonder really how stable they are...with wind/weather loads on one side only..or earth movements and such...

 
Posted : September 1, 2010 9:59 pm
 RADU
(@radu)
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I will settle for a light beer.

Reminds me of an engineering job where the engineer computed the deflection in a runway when large jet ran over a tunnel carrying a large water main crossing below the main run way.

Their calculations came up with centimetres of deflection.

Solution was to close main runway at mid night and park said largest jet for half an hour on top of tunnel. Deflection in ceiling ........zero!

Some times theoretical calculations by engineers.....

RADU

 
Posted : September 2, 2010 2:36 am
(@andy-j)
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that pic makes me queasy.

 
Posted : September 2, 2010 3:59 am
(@holy-cow)
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Although construction starts at the bottom and works up, design loads start at the top and work down.

 
Posted : September 2, 2010 4:33 am
(@pdop-10)
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There is a difference between static loads ( such as a planes wheel parked above the tunnel) and dynamic loads or falling loads such as the same wheel touching down on the surface in a landing at 120knots.

 
Posted : September 2, 2010 9:32 am