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Re: Class C of prestressed concrete member (ACI318-05)

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                        Engineering judgment is definitely required. The old days of 12 sqrt f’c are past, though some owners did not accept that, anyway.

For your example, ponding is the first issue to check. Sag of ½” over 60’ with any normal end-to-end slope should not be a problem, but it must be confirmed. I assume the analysis that shows the sag has at least a bilinear stiffness curve, so the sag is a result of the reduced section due to cracking.

The expected crack width can be reviewed to see if that could be an issue. For example, is the project near the ocean, and salt air could get in the crack? Another thing to consider is what the service loads are: if they are not likely to be realized frequently, then 15 sqrt f’c may be more easily accepted than if it is a chronic loading, or a load that is imposed and removed frequently. The project may have a wealth of other particular conditions that define the rules of deciding what stress is acceptable. Keep in mind that mild-steel reinforced concrete is designed to crack. However, my experience is that owners do not like cracks.


            Jim Getaz

            Precast Concrete Engineer

            Winchester, Virginia