SS NCARB EXAM GUIDE Q21

Structural Layout Vignette and Multiple Choice

SS NCARB EXAM GUIDE Q21

Postby jianglimeng1231 » Thu Feb 22, 2018 9:38 pm

I cannot figure it out how to solve the Q21 and if there is any horizontal shear and column 1. The answer is 2.5k
Any assistance will be greatly appreciated.
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Re: SS NCARB EXAM GUIDE Q21

Postby Coach » Thu Feb 22, 2018 9:57 pm

Think tributary area.
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Re: SS NCARB EXAM GUIDE Q21

Postby lali » Fri Feb 23, 2018 10:25 am

To elaborate a bit on the coach answer.
Rigid frames (unlike pinned connections and flexible diaphragms) distribute the load based on tributary areas. so column 1 gets 1.75K column 2 gets 2.5K and column 3 gets 1.75K
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Re: SS NCARB EXAM GUIDE Q21

Postby Coach » Fri Feb 23, 2018 7:36 pm

lali wrote:so column 1 gets 1.75K column 2 gets 2.5K and column 3 gets 1.75K

Actually, it's 1.25--2.5--1.25.
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Re: SS NCARB EXAM GUIDE Q21

Postby jianglimeng1231 » Fri Feb 23, 2018 9:52 pm

Thanks coach and Lali!!

Really helpful!
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Re: SS NCARB EXAM GUIDE Q21

Postby steveds31 » Fri Mar 02, 2018 5:24 pm

Coach, where can I find an explanation of this problem?
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Re: SS NCARB EXAM GUIDE Q21

Postby Coach » Fri Mar 02, 2018 7:28 pm

If you approach this like an SE you'll get into the weeds real fast and probably get it wrong. So, keep it simple. To get the approximation asked for, assume base shear is proportional to gravity loads.
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Re: SS NCARB EXAM GUIDE Q21

Postby jdunks » Thu Mar 08, 2018 5:15 pm

Here's how it was explained to me. Total the shear force, 1k+2k+2k=5k. Distribute the 5k evenly among the bays. In this case each bay gets 2.5 kips. Each leg of the frame then gets 1/2 of the bay's load, in this case 1/2 x 2.5k = 1.25k per leg. This ends up giving middle legs 1.25k twice, once for their role in the left bay and once for their role in the right bay. That's why the middle leg is higher than the other two. You'll find that no matter how many bays are in the moment frame, it ends up being that middle or inner legs get twice as much as outermost legs. This works for moment frames (as Coach points out, due to trib load), but shear forces are distributed differently in shear walls and braced frames. And of course post and beam systems don't distribute shear forces, they just wrack and fail.

Credit for this goes to Thaddeus. Can't recommend him enough - I think his course is well worth the price. He and this forum are real life savers.
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