1960 Concrete Design 1

[bookowski]

I'm evaluating a concrete building from 1961 where we will be increasing the loads. I have original drawings. I'm trying to understand the original design and looking for any tips from others with experience in this era. I am consistently finding the original reinf to be much more than appears required.

As an example - the slab in question is 9.75" thick lightweight (f'c = 3500) with 4" drops (13.75" total) spanning 31ft, the loading is listed on the drawings as 75psf LL and 10psf super dead. The drawings specify (38)-#7 column strip top bars. The applicable code is likely '56 ACI. '56 makes mention of ultimate design but otherwise references working stress. For ultimate they specify load factors of 1.2D + 2.4LL. For working stress they list max steel stress = 20ksi and concrete compression as 1575psi. I have tried to check this with lrfd using the original load factors and wsd and in both cases it appears overdesigned. We are adding load to these floors so overdesign would be great, but I am suspicious that I am missing something in the original design. If anyone with experience in this era of concrete design has any tips or flaws in my logic the input would be appreciated.

 

[bugbus]

Is the reinforcement cold-worked or just hot-rolled? I think the former would be in the order of 60 ksi grade, the latter ~35 ksi.

For that era, I think the stronger cold-worked bars would have been used but I could be wrong.

 

[hokie66]

I don't remember reinforcement in the States to be cold worked. I think the bars would have had a yield of 40 ksi.

This was before Ultimate Strength Design, which first became common with the 1963 ACI Code.

 

[bookowski]

The drawings specify “medium grade billet with an allowable stress of 20ksi”

‘56 ACI has lrfd as an appendix, but since the are otherwise wsd. I checked it both ways and get much less steel required than what’s on the drawings. I’m wondering if there’s something I’m missing that someone familiar with this era would spot.

 

[JStephen]

Would deflection normally be considered there?

 

[dik]

Grade 40 was common back then and some Grade 50 or 60. Stirrups were nearly all Grade 40. Can you do some metallurgical work. You may be in luck because there is a considerable strength gain (flexure and compression, but not for shear) in reviewing it as ultimate strength as opposed to working stess. Good luck...It's a long span for a slab with those proportions...

I always made drop panels much deeper than the minimum; there is a lot to be gained in reducing the overall slab thickness. I used to use the actual depth of dimensioned lumber and add 3/4" for plywood thickness... like 5-1/2+3/4 for an 6-1/4" drop, or whatever... odd sizes, and occasionally was queried about it... an old RJC habit.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[bookowski]

I'm not asking about the steel grade, I've got that sorted out.

I was hoping for someone with experience evaluating structures from around this period to opine on if there would be a reason that a design of that era would appear substantially overdesigned (a design method, code provision, standard practice, etc that I am missing). I'm doubtful that it was just overdesigned for simplicity as it's a big building (about 400k sq. ft.) with very detailed drawings. So I assume that I am missing something that was part of the design. One thing that I haven't been able to rule out is the possibility that they added in lateral loads, assuming some slab frame action. There is nothing on the drawings that would tell me either way.

31ft (center to center) is a decent span, especially given that it's lightweight 3500 concrete, but it seems ballpark reasonable for that slab thickness. But the 38-#7 top bars at columns is a lot more than calcs out using either lrfd or wsd. It may remain a mystery but if anyone has come across this let me know.

 

[SlideRuleEra]

In 1961, chances are 99%+ design was with working stress.

With increased loading, shear cracking/failure is the most likely problem. WSD was not very good for shear. Verify with modern design methods.

 

[JAE]

The only thing I can think of is:
1. Using 20,000 psi allowable for rebar suggests to me a 40 ksi bar.
2. I have the ACI 318-47, quite earlier than 1960, but has two-way flat slab provisions determining column strip/middle strip moments, etc. They may not have changed that much between 1947 and 1960. I could somehow send that to you for review. Is there anyway to get me an email address or something?

 

[bookowski]

Thanks SRE. Yes, so far based on modern analysis I am finding more flexural steel than required but deficient shear capacity. Deflections based on analysis are terrible but that is not such a concern at this point since most of it is baked in.

 

[SlideRuleEra]

If analysis won't resolve the issue and considering size of the project (400,000 sq. ft. building) an instrumented, full scale ACI load test is one way to get a definite answer. Not "cheap", but sometimes the only way... we had to do one. Proceed carefully because of potential shear failure without warning.

 

[WARose]

I was hoping for someone with experience evaluating structures from around this period to opine on if there would be a reason that a design of that era would appear substantially overdesigned (a design method, code provision, standard practice, etc that I am missing).

It's very common to find reinforced concrete (that was originally done by working stress design) is overreinforced by current ultimate code design. If you have any of Jack McCormac's texts on reinforced concrete, he presents a method for evaluating overreinforced concrete by current code. (At least he did in the 3rd ed of 'Design of Reinforced Concrete'.)

 

[haynewp]

It may help to look at the CRSI manual from that era to get a general idea of what was being used back then. But I don’t know why that many bars were used on your bldg. Lateral would also be my guess, but then some of the old buildings I’ve come across in my area didn’t explicitly consider lateral it seems.

https://archive.org/details/ConcreteReinforcingSteelInstitute/page/n206/mode/1up