Reinforcement yield strength 1

[aladdin76]

what is the effect of having two different yield strengths of bars in the same concrete section , what to do when designing the section ... if the smallest yield strenth to be considered, what about the elastic modulus

 

[Lion06]

The elastic modulus will be the same. Elastic, perfectly plastic behavior says both bars will reach their respective yield stress, just at different time.
Just use the summation of FyAs for the different bars and use that value as your tension force in the design/analysis equations.

 

[aladdin76]

thanks for your replay ,, but after the first bar reaches yeild point , further load would lead to failure before the other bar reaches yield

 

[Lion06]

No it wouldn't. It will continue to elongate, it just won't increase in stress. This is no different than a regular concrete beam in that the reinforcement yields long before the concrete reaches a strain of 0.003 (typically speaking), but the reinforcement just continues to yield. This is why we can make the assumption that the concrete has a strain of 0.003 and check the steel strain to make sure it's yielded.

 

[kslee1000]

Agree with EIT on this. See "Civil/Environ engineering Other Topics" for other opinions (in which you have posted the identical question with add'l description on nature of your problem).

 

[Lion06]

I'll take this one step further and say that's it not even all that uncommon. Think of a prestressed concrete member that gets mild steel added to it. The prestressing typically has a breaking stress (the prestressed equivalent of fy) of 250ksi - 260ksi (or even higher), while the mild steel has 60 ksi yield.

 

[Ron]

As noted in your other posting in Civil/Env, the differences in your yield strengths are not that great, so I wouldn't worry about it. It looks like you're looking at a mill certificate for two different heats of steel and reaching a conclusion that they are different classes of rebar. Based on the variation you gave of 297 to 380 MPa, that's the difference between about 43 ksi and 55 ksi....not necessarily an unexpected variation for rebar.

 

[hokie66]

I agree with Ron. If your design was based on Grade 250 steel, the steel probably complies. If it is based on Grade 400, you have a problem. We don't design based on the actual steel strength, but rather the grade of steel, including the appropriate ductility requirements.

 

[rapt]

structuralEIT,

The difference with the PT steel is that it is initially stressed to about 180-200ksi so there is still only about 50 - 70ksi increase available, and that is to fracture.

Everything said above is true as long as the modulus of the reinforcement and the ductility of the reinforcement is similar!

 

[Lion06]

When you're determining the flexural capacity of a PT or prestressed/precast beam, you use the fps value in the calcs, not fps-initial jacking stress.