1950's Rebar Yield Strength

[Hokie93]

I am working on the renovation of a cast-in-place concrete structure that was built in 1953. I have partial original construction drawings but, unfortunately, the drawings I have do not indicate the yield strength of the reinforcing steel. Based on research I have conducted, including reviewing ACI 318 in effect at that time, it seems likely the reinforcing steel is ASTM A15. This is helpful but ASTM A15 was available in 33 ksi, 40 ksi, and 50 ksi strengths. Does anyone know what reinforcing yield strength was typically specified in the early 1950's? Using fy = 33 ksi, some of the existing members do not 'work' so I suspect the actual yield strength is greater than 33 ksi.

 

[structSU10]

don't know how much help this will be, but the 3 different grades were referred to as structural (33ksi) intermediate (40ksi) and hard (50 ksi). I have seen both the structural and intermediate grades defined on some 1950's buildings I have evaluated.

 

[manstrom]

You can cut out a piece and have a tensile strength test done. It is pretty straightforward.



When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[SlideRuleEra]

In the 1950s it was common to design assuming 33ksi rebar. A higher strength may have been used for construction. As manstrom suggested, it is a good idea to do some testing. Also, in the 1950's the design would have been made using working stress theory, not strength design. Those working stress calculations may have allowed less rebar than would be required under todays code. Watch out for that. The Concrete Reinforcing Steel Institute (CRSI) document "Evaluation of Reinforcing Bars in Old Concrete Structures" may be of help. See the attached file:

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[msquared48]

Are you absolutely positive you have not misread the beam reinforcing diagrams and omitted some steel? Sometimes they can be very confusing as different symbols and techniques were often used from my experience.

Also, have you employed Live Load reduction in your calculations?

Mike McCann, PE, SE (WA)

 

[jfmann]

In general, allowable ("working") stress design should result in greater amount of flexural reinforcing compared to ultimate strength design. However, original design might very easily have been based on lower design loads than you are considering now.

Also...if you have 2-way slab or other condition that is more complex that simple one-way bending, all kinds of "assumptions" about behavior could have been used for design that might or might not model actual behavior reasonably.......or, more to the point, might not match whatever analysis model you are using now.



John F Mann, PE

http://www.structural101.com

 

[jfmann]

As for yield strength of reinforcing bars..........similar to conditions in 1960s and 1970s when Grade 40 bar was common......it would have been quite unusual for designer in 1953 to have specified rebar with yield greater than 33 ksi..........which is what you should use for analysis unless proven otherwise by testing.

How much overload are you calculating using Fy = 33ksi?......if "small" percentage (say, 15% or so) I would not be particularly concerned, especially if reality of full design live load is rather remote.

Consider also that as-built locations of flexural bars within depth of section may not be all that close to position specified in original plans....or your assumptions. Check sensitivity of analysis to variations in "d" dimension.



John F Mann, PE

http://www.structural101.com

 

[mtu1972]

ACI 318-56 gives the Allowable Tensile Stress as 20,000 psi for intermediate and hard grades and 18,000 psi for structural grade bars. It is the earliest that I have access to. Hope this is of some help.


PS: Price was $1.00 and it was 75 pages long (6" x 8").

gjc

 

[racookpe1978]

And, even as good as they were back then, they were using slide rules and approximations and hand-punch adding machines - if those.

Check very, very carefully, but don't miss the simple possibility of an error when fully analyzed with today's tools. That is, is IS slightly understrength to today's requirements A factor of safety is just that = The actual day-to-day loads may not have ever been high enough (after all assumptions are made) to "break" what seems too lightweight.