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Old pan joist system

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John,

CRSI's Engineering Data Report No. 11, "Evaluation of Reinforcing Steel in 
Old Reinforced Concrete Structures," is probably what you are looking for.

Working Stress Design was what was used, of course.  Allowable stresses 
(1924) for steel were:

   16,000 psi for structural and rail steel grades
   18,000 psi for intermediate and hard grade bars, all cold-drawn
               wire, and twisted bars.

Concrete strength probably was specified as 1,500 psi at 28 days, and the 
allowable stress would have been 0.40f'(c) in positive bending and 
0.45f'(c) in negative bending.

All bars in 1924 were round, with the exception of 1/2" and 1", which were 
provided in both round and square, and 1-1/8" and 1-1/4" which were provided 
in square only.

HTH

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

John Riley wrote:

>>I am checking out an existing roof for installation of an HVAC unit.  Making
some conservative assumptions, I'd like to be able to show in my
calculations that the system is capable of carrying the additional load.
So, unless the numbers scare me, no testing will be done to ascertain
concrete strength, etcetera.

The HVAC unit will impart a line load of 160 plf on a single joist for a
distance of 7', nominally at center of span.

Concrete pan joist system:
        Design year:  1923
        10" x 2 1/2" joists  x 5" wide @ 25" oc
        Reinforcing:  1- 7/8" Dia BT  +  1- 1" Dia ST
        Span:  23' nominally

Immediate questions:
(1) What strength can I assume for the reinforcing steel?
(2) Would the bars have been deformed in 1923?

Any input is appreciated.
__________________
John P. Riley, PE, SE<<