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Allowable Steel Stress in Foundation Hairpin 3

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azogr

azogr

Structural
Feb 21, 2007
59
I am looking through a hairpin design example in Alexander Newmans book "Metal Building Systems" and I noticed that he is using an allowable steel yield stress of 24 ksi for rebar. Is this because he is using working loads instead of factored loads or is he trying to limit the cracking in the slab to which the hairpin is connecting? If I am working with factored loads can I not just use 0.9 x 60 ksi x Area of the hairpin to determine the tensile capacity of my hairpin? Asssuming of course that it is properly developed and lapped into the slab reinforcing.

I would greatly appreciate any clarification on this matter.

Thanks!
 
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Small rebar sizes are assumed to be 40 ksi,(I do not think any mill is still rolling 40 ksi but there may be some in inventory). This is 0.66 yield for working stress. LRFD would be 0.9 x 40 or 36ksi which is similar working stress with an average load factor of 1.5.
 
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Thank you for the quick reply! I tried to give you a star but the feature was not working for some reason.

All my bar is 60 ksi and since I am using factored loads I have been using the 0.9 x 60 ksi. I guess because the entire book is on metal buildings he is trying to keep things consistent by using allowable stress for the rebar? I am just not used to seeing that since I have only ever worked with the newer ACI codes.

P.S. I'll keep trying on that star.
 
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Trying to keep things consistant, maybe, but he may also just be old. I think ASD design was in the appendix of ACI 318 until the 02 edition. I think the same thing will eventually happen to steel. Since I learned LRFD in school, that is what I will always use. Most old timers still use ASD. Some day somebody will be calling me an old timer, because I am going to design wood and masonry with ASD as long as I can.



 
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Your rebar capacity is Area x [Φ]Fy against factored loads.

The 24 ksi is an allowable rebar stress for Allowable Stress Design.

 
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Typical allowable working stresses for grade 60 bars is 24 ksi, 20 ksi for grade 40.

It looks like what you're doing with LRFDS is correct. I wouldn't allow the stress at service to exceed 0.5fy, but that's not in a code anywhere so it isn't mandatory.
 
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It sounds like your engineering reasoning is 100% valid.....as UcfSE said, 50% might be a substantial understress, but it is also fairly cheap insurance as well.

As I get more experience, the thing I feel is the most important to me is where I can place the redundancies without a large subsequent charge....this seems to be one of them....but definitely your call...good luck
 
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You're absolutely right. This is a component that can be given a healthy factor of safety and have very little impact on the cost of the project. Especially given the critical role that it plays it makes sense to build a little fat into it.
 
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