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Re: Effect of Two Grade of Concrete on Flexural Reinforcement

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While I've never accepted a lower strength concrete when my beams were designed using a higher strength, I have used one strength of concrete in my slab and a  higher strength of concrete in my girder.  Back when I used to design bridges, we would design our prestressed I girders for an f'c=6000 psi and the conventionally reinforced slab for an f'c=4500 psi.  The composite section was analyzed by reducing the width of the slab based on the n value, also known as the ratio of the modulus of elasticity of the slab to the modulus of elasticity of the girder.  This is the same method that I was taught for the design of steel composite beams (back in the ASD days).

I'd still be a little leary of accepting the lower strength concrete, but if you can get your beams to work with it, it should be OK.  Remember that the lower strength means a lower E value, which means higher deflections.

Kipp A. Martin, P.E., S.E.
Carollo Engineers
503-227-1747 (fax)

>>> the_best_always(--nospam--at) 5/23/2006 12:09 am >>>
Hi all!
  Has anyone of you accepted heterogeneous beam construction using two grades of concrete?  Part of the beam (about 2/3 of its height) is poured with C60 and the rest of the beam (about 1/3 of its height) is poured with the slab using C45.  This construction method is suggested to speed up construction of a 50-storey building.  The beams were designed using C60.

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