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RE: Effect of Two Grade of Concrete on Flexural Reinforcement

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Dear Best!
 

I think Kipp explained it the best way.

Considering the "Composite Section" analysis, here are some other points:

-     The Positive Flexural capacity & D+L Deflection of beam will be based on C60 (Top portion) Concrete (evaluation of Mpr, Vpr per ACI)

-     The Negative capacity of beam will be based on C45 (Bottom portion) Concrete

-     The lap length of bottom reinforcing is based on  C45 (Bottom portion) Concrete

-     The lap length of top reinforcing is based on  C45 (Top portion) Concrete

 

Regards

Casey (Khashayar) Hemmatyar, PE

California


-----Original Message-----
From: Kipp Martin [mailto:kmartin(--nospam--at)carollo.com]
Sent: Tuesday, May 23, 2006 7:52 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Effect of Two Grade of Concrete on Flexural Reinforcement

 

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

503-227-1885

503-227-1747 (fax)

kmartin(--nospam--at)carollo.com