0.0018*b*h rho for Very Thick Slabs

[swearingen]

As ACI states, the minimum temperature & shrinkage steel is 0.0018*b*h. The question is at what depth does the bottom steel not provide relief for cracking at the top surface? Clearly, the bottom steel of a 10' deep, doubly reinforced foundation would not help at all, and your top steel alone should prevent the cracking. Great, but what d should you use in that case?

Thanks in advance for your answers...


-5^2 = -25 ;-)

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[JAE]

ACI does not require "temperature" steel in footings. Section 10.5.4 (pre 318-14 codes) requires minimum FLEXURAL steel in footings and this AMOUNT of flexural steel is calculated at 0.0018Ag and placed in the tension zone of the footing in flexure.



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[swearingen]

OK, and so the limit for the flexural reinforcement would be when no reinforcement would be required, i.e., the concrete alone can take the flexure (such as my extreme 10' thickness case). Otherwise, the full h must be used to calculate this minimum amount. Thanks.


-5^2 = -25 ;-)

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[msquared48]

There is also the ACI provision that if you need less steel than rho minimum for flexure, you can use less than rho minimum if you provide 1/3rd more than what you need.

If this amount turns out to be greater than rho minimum, just use rho minimum.

Mike McCann, PE, SE (WA)