Rebar/welded wire allowable strength 1

[canwesteng]

I'm working on a retrofit of a structure from '67, in the general notes it calls for rebar with allowable stress of 20ksi. Digging around the forum, I've found this thread

http://www.eng-tips.com/viewthread.cfm?qid=258580

with a post by hokie stating 20ksi allowable meant 40ksi actual. I'd like to use 40ksi but I know I'm going to have to justify it to my boss and can't use a forum post for that. Can anyone point in the right direction to substantiate this assumption? The general notes reference the 1954 CSA G30.1 and G30.6, both of which have been long since replaced.

 

[hokie66]

You need to understand the differences between Working Stress Design and Ultimate Strength Design, and ACI318-63 is the place to start, as it was the first code to contain both. Instead of "40 ksi actual", you should say "40 ksi yield". If you are using the current code to check an existing structure, you will be using the yield stress of the reinforcement. How old is your boss? He will probably accept 40 ksi as the yield strength without question based on experience. Now if you have reason to believe that Grade 60 bars were actually used, then you would have to investigate further by testing. As 1967 was in the transition period from Grade 40 to Grade 60, the higher strength reinforcement is possible.

 

[BAretired]

There were three grades of steel in 1956, namely Structural, Intermediate and Hard Grade with minimum yield point of 33, 40 and 50 ksi respectively. The ultimate strength was 55 to 75, 70 to 90 and 80 ksi respectively.

The allowable steel stress in ACI 318-56 was 18,000 psi for Structural Grade and 20,000 psi for Intermediate and Hard Grade.

Intermediate Grade was the usual grade in Toronto, Ontario. I don't remember anyone using Structural Grade in those days.

BA

 

[KootK]

These are two handy documents to have if you're going to be messing with elder-concrete.

Link
Link

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[fegenbush]

I cannot disagree with the wisdom of the above posts. I can, however, point you to a direct code reference for the 20 ksi limit. A.3.2 of ACI 318-99 (the latest version to contain the specifics of the working stress design method, Appendix A) specifies f_s, the allowable tensile stress in reinforcement, shall not exceed 20 ksi for Grade 40 or Grade 50 reinforcement.

I hope this helps. Please keep us updated as to the status of this project.