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Engineer22

Structural System for Lateral Resistance

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Engineer22    1

Hey guys, I am truly stuck on this question, and am torn between these two configurations (see attached). Thank you again to the members of this forum for any help you can provide...

Which structural system is the best for lateral resistance? Given the attached, the solid bold line is shear wall, the circles are columns.

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damascus    1

I have a educated guess, and I have a hunch it has to do with torsion. I am guessing if you said all the walls have same rigidity, center of mass, and center of gravity then you would get a different J (torsional rigidity). The higher the J is the lower the shear due to torsion is for each configuration. Go through this problem like that and I think you will find the answer you are looking for. Unless there is more to the problem statement that is how I assume you can get the best configuration solely based on torsion. 

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OHBridgeGuy    3

I agree with damascus generally.  The only issue is that the second option is irregular and angle of attack would have more impact, but yes it would have more torsional capacity due to J.  From a design perspective, you would likely have the same L condition as in 2, but with walls on each of the 4 corners - that would be the best 

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Engineer22    1

Thanks for your replies! Yes, I believe torsion is the culprit here.

 

Could you explain 'J?' My understanding is that to minimize torsion, you must be: a) symmetrical and 2) AWAY from the center. That is why I leaned towards the second. But, Choice (1) has shear walls in the center and I've always understood this to be rigid.

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damascus    1

J = Summation of all walls being considered  ((distance of wall to the center of rigidity)^2 X rigidity of wall) 

J = Summation: r^2 x R

Your example all the distances are 5'  and all four walls have same rigidity = 1. J = 4(2.5^2 x 1) or 4(7.5^2 x 1)

They are both symmetrical with 4 walls with the same rigidity, so the center of rigidity is i the same spot because of this. Which makes it easy to do the math. For the exam this is a pretty standard type of question with the rigidity being different  the walls being anywhere and solving where the center of rigidity is and then the torsional shear or just the torsion. 

Remember that your diaphragm and collectors tie the system together. So although the walls are not connected (not next to each other) the other elements should be designed to tie them together.

A simple thought exercise would be if you spread your feet apart, is it harder to knock you down if each of your feet are farther away or side by side? 

Anyone please correct me if I am wrong, I make mistakes all the time. 

 

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Engineer22    1

Thanks Damascus. I am just looking for the conceptual reasoning here. THe choice on the right would have variations in perimeter  stiffness, thus contributing to weaker lateral resistance, right? 

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I think the one on the right is better for a somewhat shorter building without a "core", for the reasons listed above.  Additionally, the diaphragm doesn't cantilever out to the outer columns and you would have to make more robust connections of the diaphragm to the shear walls.  For taller buildings, the left scenario is better, but you would need moment frames around the perimeter. That is what you end up with taller buildings because you want glass around the perimeter and usually have an elevator/stair core that can be used as shear walls. 

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Engineer22    1

This is helpful, thank you so much! If there wasn't a tall or short building specified, which one would be best... I know this is arbitrary but this is the question I've been presented with

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