Factor of safety in concrete shear walls

Structural Layout Vignette and Multiple Choice

Factor of safety in concrete shear walls

Postby Transit650 » Tue Feb 10, 2015 5:42 pm

Can someone shed some light on question 31 in the ARE Exam Guide sample MC questions?
"What is the factor of safety against overturning for the concrete shear wall shown if resisted only by gravity forces?" The answer is 4.2 but I don't know how it is derived. Does the foundation need to be considered in resisting the overturning? Or is this just a code question?

Thanks!
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Re: Factor of safety in concrete shear walls

Postby Design intent » Wed Feb 11, 2015 12:50 pm

Mike's Notes on the NCARB problems (found in the first post in this forum) discusses this question in depth.
Essentially this is 3part problem:
1) Calculate the Overturning Moment (OTM)
2)Calculate the Stabilizing Moment (SM)

3)Divide the SM/OTM= factor of safety.
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Re: Factor of safety in concrete shear walls

Postby Transit650 » Wed Feb 11, 2015 1:44 pm

Thank you! I will look into this.
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Re: Factor of safety in concrete shear walls

Postby Transit650 » Wed Feb 11, 2015 7:37 pm

I read through the notes and I understood how to get to the saftey factor. However, in this particular question, where should one assume the pivot point?
Is it at the end of the foundation or at the corner of the shear wall where it meets the foundation? Also, is the depth of the shear wall accounted for in the DL? I still don't get to the factor of 4.2.
Your help is very much appreciated.

Thanks!
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Re: Factor of safety in concrete shear walls

Postby Design intent » Thu Feb 12, 2015 8:34 am

Transit650 wrote:I read through the notes and I understood how to get to the saftey factor. However, in this particular question, where should one assume the pivot point?
Is it at the end of the foundation or at the corner of the shear wall where it meets the foundation? Also, is the depth of the shear wall accounted for in the DL? I still don't get to the factor of 4.2.
Your help is very much appreciated.

Thanks!


Mike has a topic on pivot point. If you go back to this question in his notes, he directs you to it. Essentially, the pivot point is the point about which the whole item would turn.

Example: A pinwheel has a center pivot point. If you blow on it, the pinwheel will rotate about it's center pivot point. If you push on a wall from the left, it will rotate clockwise about the pivot point
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Re: Factor of safety in concrete shear walls

Postby Transit650 » Thu Feb 12, 2015 10:55 am

I am clear about the pivot point concept but in this case, I don't quite know whether the pivot point is at the bottom right corner of the foundation (because shear wall and foundation are connected with a moment connection over the entire length) or where the shear wall meets the foundation, creating a lever around that point in the other direction from the outer point of the foundation. The same could be said for the left side, where the foundation is sticking out and would create a moment due to the soil but the question says to not take the soil pressure into account.
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Re: Factor of safety in concrete shear walls

Postby gascap » Thu Feb 12, 2015 11:58 am

The pivot is the lower right point of the footing.
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Re: Factor of safety in concrete shear walls

Postby Transit650 » Thu Feb 12, 2015 12:37 pm

Thanks for the clarification.
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Re: Factor of safety in concrete shear walls

Postby pbiermanlytle » Sun Dec 09, 2018 4:17 pm

when calculating Stabilizing Moment (SM) in a shear wall, I see the use of 0.15 kips/cuft used to multiply W x H x D of wall. What does 0.15 kips/cuft refer to? Is this the SM factor always?
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Re: Factor of safety in concrete shear walls

Postby Coach » Mon Dec 10, 2018 7:15 am

pbiermanlytle wrote:when calculating Stabilizing Moment (SM) in a shear wall, I see the use of 0.15 kips/cuft used to multiply W x H x D of wall. What does 0.15 kips/cuft refer to? Is this the SM factor always?

It's weight/cuft of concrete.
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