concrete member with tension and bending 2

[357mag]

I have a concrete "beam" with tension and bending. An engineer has suggested that I check that the axial tension force and bending moment are inside the P-M curve.
I am confused as I thought a P-M curve was for Compression and Bending.
Am I missing something? Do you understand what He is saying??
Thanks for any comments

 

[bootlegend]

The P-M curve is for axial plus bending. P can be either positive or negative. Most books focus on only the positive P axis because that is the typical situation. You should end up with the same general shape for the tension area as for the compression side of the curve, but the magnitudes can be quite different based on reinforcement.

 

[AVak]

A typical P-M curve extends below the zero axis on the P-side. This covers the condition of tension combined with bending. There is minimal capacity for this condition as a concrete beam is tension controlled to begin with.

Adam Vakiener, P.E.

 

[357mag]

Thanks all. I guess I should not call it a "beam", as the axial tension force is much greater than the small bending moment. It would be more correct to call it a tension member with some small amount of bending moment.

 

[357mag]

The strength of the plain concrete section is more than enough to carry the actual tension load. The stress from the tension load across the concrete section is around 64 psi. The uncracked strength of the conc. is over 350 psi. If I assume its cracked, and let the rebar take all the tension, the rebar stress is also fairly low (around 19,000 psi). the bending adds another 1,500 psi max to the rebar so it's still well below 24000 psi allowable.
What might the P-M diagram show that my simple analysis wouldn't. I think I'm still not seeing something. Thank You kindly for any comments.

 

[357mag]

Any further comments appreciated Thanks

 

[ron9876]

It is your judgement if the forces/moments are neglible but a P-M diagram is the way to analyze.

Remember that when the reinf. is stressed that means that the concrete has cracked.

 

[ishvaaag]

It might also be that consideration of the tensile strength in concrete is not accepted or wanted. You might then resource to prestress or post-tension the member to meet the case, not quite dissimilar to prestressed bolts. A mere threaded bar or just a nut on the prestressed bar may do if not much tension.

 

[BAretired]

Is the bending moment large enough to put any part of the section in compression?

BA