basic rebar strength question

[a2mfk]

OK, forgive me I don't currently have access to any of my concrete references. Its also been a while since I have had to do this.

I want to use rebar to develop axial tension of a drag strut into a concrete wall. Development length calculation is straightforward.

Then I get to the actual rebar strength. Now for Grade 60, yield stress= 60ksi. Can I simply factor my load and then:
rebar strength= Area of rebar x yield stress x 0.9 (tension strength factor)

I have also seen several tables that simply use yield stress of 60 ksi x area, which would be an allowable stress approach, which is what I think I have done in the past for concrete and masonry.

I included a link to a Simpson table for an example.

 

[a2mfk]

Oh, I meant to add, for allowable stress would I use 0.66*Fy like you would in ASD for steel. I think this is valid, steel is steel at the end of the day. I know to not mix and match factors and stuff when I go into the concrete design...

 

[IDS]

I included a link to a Simpson table for an example.

The table is giving required embedment depth to develop the full bar strength, so in that case it would be unconservative to factor the bar strength down.

For your purposes you should do whatever the applicable design code tells you to do.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[peconsultant]

Maybe I don't understand your question, but all you have to do is design the rebar in the same manner you would design it for the tension in a bending member.

 

[SteveGregory]

Allowable Tensile stress using Working Stress Method (ASD) is 24 KSI for Grade 60 or higher.

 

[a2mfk]

Thanks Steve. Do you know if that has to do with strain compatibility between the rebar and the concrete? You are trying to limit elongation in the bar to ensure the bond between between the rebar and concrete is not compromised?

 

[SteveGregory]

The working stress method assumes elastic range stresses. 24 ksi is the allowable stress used for the rebar in a bending member such as a beam.

This method was called the Alternate Design Method and stuck in Appendix A of ACI 318-99. Before the Ultimate Strength Method, it was the "only" method in ACI 318.

 

[Lion06]

I'm not sure I'm following your comment about strain compatibility of a rebar in tension with concrete.

Every rebar in tension that you design in a beam is assumed to be stressed at yield. Why is this situation any different? The rebare doesn't know if it's in a member that is subject to bending or direct tension. All it knows is that it's in tension.

 

[a2mfk]

I agree tension is tension in rebar. Steve answered my question, just looking for the allowable stress of rebar in this situation. This is the same value used in ASD design in masonry. 60ksi/2.5= 24ksi, which differs a lot from a 0.66 factor that would be used in STEEL design. Also, the Simpson table I posted gave the full yield strength, which I knew you did not use that. I don't have access to my concrete references so I just needed a quick answer.

 

[JennyNakamura]

Not quite sure what you are trying to do exactly, but if "development length calculation..." is part of it as you state in your original post, then you are using the ACI code (assuming you are in the US) and the ACI code no longer allows working stress design. You cannot design your development length based on strength design then switch to allowable stress to analyze the rebar.

pe consultant is correct, as you are in your original post. You analyze the rebar using (phi)x(fy)x(area of steel) using factored loads.

 

[a2mfk]

I guess I was too wordy, my bad. Very simple question, how do I calc the tensile strength of rebar.

My original thought was confirmed:


rebar strength= Area of rebar x yield stress x 0.9 (tension strength factor)

 

[Bobber1]

I agree with your strength calculation. As this is based on LRFD, you will have to factor your actual loads per ACI 9.2

 

[SteveGregory]

a2, it looks like you can still use Working Stress Design.

See thread167-215126

 

[DaveAtkins]

I am not sure why allowable stress in reinforcing bars is limited to 24 ksi--but that has been the case for a long time.

DaveAtkins

 

[ajk1]

Limit states design has been the general design method for decades now, so I can discern no reason why you would go back to WSD. In Canada, the rebar factored strength is taken a phi x Fy x Area and that must equal the factored load (i.e. the actual load times the Code prescribed load factor for the particualr type of load). In Canada, phi is 0.85 (may be different on the U.S.)

Irrespective of what method you use, there is more to the design than you have indicated. For example, you must properly develope the bar in the concrete (depends on the concrete strength, whether there is >12" concrete below, and other factors) and so on.

 

[TLHS]

If you've got a tension only member you may have to watch out for other things. There are cases where you're required to use mechanical splices in the Canadian code for tension only members. So you might need to look and ensure there isn't a similar provision in whatever code you are using.