Old Rebar Yield Stress (1940's) 1

[ugandabob]

I'm checking the capacity of a reinforced concrete slab from 1944 in Canada. Allowable stress design was used. The steel is specified as "hard grade billet" with a working stress of 20 ksi. f'c is specified as 2500 psi.

I'm trying to figure out the yield strength of the rebar. I've done some research (including this website) and it seems that hard grade billet steel has a yield stress of 50 ksi. But that would mean the allowable stress was limited to 40% fy. I'm under the impression that working stress is usually 60% fy, which gives a yield stress of only 33 ksi.

If fy= 33 ksi the slab does not appear to work for the existing loads (9" slab, 20ft bays, Live Load is 250 psf), let alone the additional loads for which I'm checking.

Any advice on the yield strength I should use?

Thank you.

 

[bridgebuster]

The AASHTO Manual for Condtion Evaluation says:

Grade 50 rail or hard, Fa = 20 ksi; Fy = 50 ksi. However, for unknown grades prior to 1954 it recommends 18 & 33.

Hopefully it helps.

 

[JedClampett]

Can you remove some bars from a unstressed area and test them? It doesn't seem to be a good idea to guess at this, especially if you're going to increase the loads.
Are you using the two way slab analysis of that era? It would be unusual for an old time design to not be conservative.

 

[JAE]

Here is the CRSI table for historical bars (in the US I think)

 

[BAretired]

Jed,

It would be unusual for an old time design to not be conservative.

They may have been conservative in the allowable stress but as I recall, the old time design allowed for only 72% of statics in each direction of a flat slab.

BA

 

[hokie66]

JAE is correct, although I didn't recall any percentage. Anyway, some designs didn't allow for full bending in both directions. As well, many of the early flat slab designs were seriously deficient in punching shear.

 

[BAretired]

The following link is to a paper written by Sozen and Seiss of the University of Illinois:

http://www.google.ca/url?sa=t&source=web&cd=1&ved=0CBkQFjAA&url=http://ecn.

http://www.ecn.purdue.edu/~frosch/C...xi_A51xRg&sig2=wOmzvZ7ITXib83CzLxl6LA&cad=rja

Fig.4 of that paper is a bar graph indicating the variation in weight of steel used by various design methods, C.A.P. Turner's design being the lightest. There was a huge amount of controversy within the engineering community about the statics involved but Turner's design seemed to withstand load tests.

ACI 318-63, Equation 21-6 gives:

M_o = 0.09 WLF(1 - 2c/3L)²

where F = 1.15 - c/L but not less than 1.

So even as late as 1963, only 72% of statics was considered adequate for design purposes. In 1944, the amount of steel in a flat slab in Canada was likely in accordance with the ACI Joint Committee recommendations of 1941 which are mentioned in the above paper.

BA