1920s concrete beam analysis - working stress design 2

[BubbaJ]

When analyzing an existing structure for new loads, where the original design was "working stress" with the allowable working stress of the concrete reinforcing steel at 16ksi, how would you convert that to a value that could be used with strength design and factored loads?

Or would the more appropriate thing to do be to analyze it using the working stress method?

 

[jayrod12]

The second option.

 

[msquared48]

Personally, I would use 33 ksi for the Fy of the steel of the time (per an undated PCA article I have), and use the current load factors to use USD design. Done it on a lot of older buildings and it works just fine.

However, that being said, you still have to be concerned with bond and check it as many of the bars of the time were not the deformed shapes we use today.

Mike McCann
MMC Engineering

 

[BubbaJ]

Thanks to you both. I had a feeling that both arguments would be valid.

Msquared48, do you have the number/name on that PCA article? I'd like to read it.

 

[msquared48]

No. I'll have to scan and post it.

Mike McCann
MMC Engineering

 

[boo1]

SRE had a pdf from PCA

 

[boo1]

CRSI has this doc

 

[hokie66]

Concrete beams from that period often are deficient in shear capacity. The concrete typically wasn't very good. Have you determined the concrete strength?

 

[BubbaJ]

Thank you, boo1!

 

[JAE]

I would use either ASD or LRFD under the current applicable code. Don't just arbitrarily use 16 ksi as an allowable as that was based on an earlier code and level of safety....neither of which apply anymore.

The info from boo1 is great and hokie's point about shear brought up another issue to consider as well.

Of the many past old beam designs I have checked - besides shear giving me headaches I also found that the longitudinal flexural rebar rarely was detailed to properly develop moments like we do today.

Generally top steel was insufficiently developed past inflection points, or were turned down (the old "crank" bars) too soon.

Makes for some dicey interpretation of how much capacity you actually have....even though the beams have stood for decades.

 

[BubbaJ]

Plan notes indicate all reinforcing bars to be "hard grade deformed steel bars..." and notes the concrete is to be a "1:2:4 mix", but I don't see any strength associated with that.

 

[BubbaJ]

An article I just found indicates that in historical structures a 1:2:4 mix is equivalent to 3000psi. Granted that could vary widely on the condition of the structure.

 

[CELinOttawa]

Bubba: I make a habit of collecting everything I can find about the expected strength of historical mixed... Mind posting a scan of the article that had the 1:2:4 = 3000psi suggestion?

 

[BubbaJ]

2 months late, but I think this is the link:

http://www.concrete.org.uk/fingertips_nuggets.asp?cmd=display&id=737

There is also a book by J. Stanley Rabun "Structural Analysis of Historic Buildings", that you can preview on google books. He gives 3700 psi for a 1:2:4 mix at 6 months.

 

[CELinOttawa]

Hi Bubba: I own Rabun's book. It is pretty good, occassionally delving too deeply into historical designs of little use, even in review... However it did start as his academic thesis, so this can be understood.

A useful book!

 

[TXStructural]

If you need to get into details of old reinforced concrete, and can wait a couple of weeks, this is the new book on old reinforcement and reinforcing systems. I had a chance to look it over pre-production, and it is truly outstanding. Not a sales pitch, despite my job, it is really worth reading.

http://www.crsi.org/index.cfm/vintagesteel

 

[CELinOttawa]

Price? I would likely have most or all of the info already, just scattered... Always like to have the latest resource, however, as clients love seeing old and new both in the references. Definitely looks like a good text!