allowable stress in rebar ??
allowable stress in rebar ??
(OP)
I am designing a slab on grade and need a design number for allowable stress in steel rebar. I used 30,000 PSI and the amount of steel the equasion came up with seems too high. If I allow more stress, I can reduce the steel a bit.
Thanks,
Bill
Thanks,
Bill





RE: allowable stress in rebar ??
There are other grades, grade 40, grade 75, etc.
and other specifications, ASTM A 706, eg.
I would assume ASTM A 615 grade 60 in most applications.
RE: allowable stress in rebar ??
RE: allowable stress in rebar ??
RE: allowable stress in rebar ??
The slab is 6 inches thick with an 8 inch haunch at the perimeter, admixes were fibers and super-P, 3500 psi mix.
I calculated the required steel 3 ways- thermal expansion method, subgrade drag, and loading, and all methods were somewhat consistent calling for 5/8" bars 12" oc. By accident, I ordered 3/4" bars of what I thought was grade 60 steel. I later found out that it was grade 40 steel. The cylinders that I had tested @ 30 days broke at only 1500 PSI. The cement company's predictable response was that the testing was handled improperly. The good news is that my slabs have NO (0) cracking at all, for almost 1 year.
RE: allowable stress in rebar ??
Is there a problem because it is too strong?
RE: allowable stress in rebar ??
RE: allowable stress in rebar ??
I noticed that you are an aeronautical engineer. Do you know that the steel in concrete is not designed using the Allowable Stress method? Because of the differences in strength and failure modes, concrete design is predicated on the steel yielding. By code, the steel is limited, effectively, to 75% of the strength of the concrete in a section.
I can go more into the details but be aware that if you use the Allowable Stress then you are risking a catastrophic failure. It appears that you've already installed this slab. If you used Allowable Stress, I would not load your slab more than 50% of AS design strength.
Imagineer
Imagineer
RE: allowable stress in rebar ??
Yet a good point is set forth: Practice only in your profession.
RE: allowable stress in rebar ??
But the warning still stands. I hadn't realized how much discipline specific information/tecnique there was until I switched from designing ship's structure to building structure. At times it was like going back to school to learn a new subject.
Imagineer
RE: allowable stress in rebar ??
RE: allowable stress in rebar ??
Specifications for reinforcing can be found using ASTM, ASTM 615. This should cover Grade 60 and 40 which are the most common although 40 is rarely used anymore.
RE: allowable stress in rebar ??
RE: allowable stress in rebar ??
RE: allowable stress in rebar ??
I take exception to your claim that reinforced concrete design by the Allowable Stress method automatically puts a structure at risk for catastrophic failure. In fact, reinforced concrete was designed exclusively by this method for many decades and the method remains a viable design option that is included in an ACI 318 appendix today.
Also, reinforced concrete slabs typically have quite a lot of reserve capacity above that predicted by either the Allowable Stress method or the Strength Design method. Analysis by the Yield Line method or other collapse mechanism methods explains how some slabs built a century ago have stood the test of time in spite of deficient design methods such as the Turner slab system.