allowable stress in rebar ?? 2

[billn]

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. <br>
Thanks, <br>
Bill<br>

 

[concacc]

Most rebar in use today is manufactured in accordance with ASTM A 615, grade 60. That is, 60 ksi yield min and 90 ksi tensile min.<br>
There are other grades, grade 40, grade 75, etc.<br>
and other specifications, ASTM A 706, eg.<br>
I would assume ASTM A 615 grade 60 in most applications.<br>

 

[billn]

I called the supplier, which is a large local supplier and to my surprise, they told me that it was grade 40. Its in the slab as of yesterday, so its too late. One good thing though, I oversized by a lot. I used 3/4" bars 12" O.C.

 

[CoffeeMug]

20MM diameter rebars for slab on fill, must be really heavy loading.

 

[billn]

When I did the numbers acording to the ACI (american concrete institute)the span between control joints is figured it. My slab is 20 feet by 30 feet with water tubes imbedded for heating. Since it is difficuld to run piping through the joints, I eliminated them in the design which made the required steel greater.<br>The slab is 6 inches thick with an 8 inch haunch at the perimeter, admixes were fibers and super-P, 3500 psi mix. <br>I calculated the required steel 3 ways- thermal expansion method, subgrade drag, and loading, and all methods were somewhat consistent calling for 5/8&quot; bars 12&quot; oc. By accident, I ordered 3/4&quot; 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.

 

[crunch]

Need some design criteria information concerning a new steel that will soon become available for rebar applications. The steel yield strength is higher than 100ksi.
Is there a problem because it is too strong?

 

[Ron]

Not necessarily a problem since most concrete is designed as a cracked section in tension (meaning the steel takes the load); however, I have not seen a great deal in the literature about localized crushing, embedment length, and bond strength of the concrete/rebar interface for very high strength steels. Obviously to accompany the high strength steel, high strength concrete will be necessary. Current practice will routinely produce concrete in the 10ksi range, but above that is not being produced on a consistent basis (to my knowledge). [sig][/sig]

 

[Imagineer]

Bill,
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 [sig]<p> Imagineer<br><a href=mailto: > </a><br><a href= > </a><br> [/sig]

 

[Qshake]

Catastrophic failure? Slab on grade? Maybe some serviceability problems but catastrophic?

Yet a good point is set forth: Practice only in your profession. [sig][/sig]

 

[Imagineer]

You're right. In my haste I missed the &quot;on grade&quot; portion. I guess it was a preconception based on rarely reinforcing SOG's. It much more economical to just increase the thickness unless you have some high point loads.

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. [sig]<p> Imagineer<br><a href=mailto: > </a><br><a href= > </a><br> [/sig]

 

I was wondering if you have any specs on different grades of rebar

 

[Qshake]

For posting a new question you really shouldn't embed it in a long thread like this as it is likely to be over looked.

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.

 

Why is rebar deformed on the surface?

 

[Ron]

Put your question in a new thread and then we'll answer it. Read Qshake above.

 

[Taro]

Imagineer,

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.