## SS NCARB EXAM GUIDE Q21

Structural Layout Vignette and Multiple Choice

### SS NCARB EXAM GUIDE Q21

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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jianglimeng1231

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Joined: Sat Mar 26, 2016 11:42 am

### Re: SS NCARB EXAM GUIDE Q21

Think tributary area.

Coach

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### Re: SS NCARB EXAM GUIDE Q21

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
lali

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Joined: Thu Sep 01, 2016 10:21 am

### Re: SS NCARB EXAM GUIDE Q21

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.

Coach

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### Re: SS NCARB EXAM GUIDE Q21

Thanks coach and Lali!!

jianglimeng1231

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Joined: Sat Mar 26, 2016 11:42 am

### Re: SS NCARB EXAM GUIDE Q21

Coach, where can I find an explanation of this problem?
steveds31

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Joined: Sun Nov 30, 2014 9:25 am

### Re: SS NCARB EXAM GUIDE Q21

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.

Coach

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Joined: Wed May 23, 2012 2:08 am

### Re: SS NCARB EXAM GUIDE Q21

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.
jdunks

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