huangyhg

old reinforcing material
i have some old drawings that i am analyzing the existing 2- way flat slab for an occupancy load increase. the drawings are over 40 years old and list the following:
vertical reinforcing in columns (intermediate grade)..fs=16,000psi
welded wire mesh....fs=24,000 psi
all other steel (intermediate grade)...fs=20,000 psi
all concrete to have a compressive strength of f'c=3,000 psi
i think the reinforcing is being given as the allowable design stresses using a factor of 0.4. in other words, the "all other steel" actually has fy=20/0.4=50 ksi reinforcing steel material, and the column steel is 16/0.4=40ksi material.
is this correct for 40 year ago aci practice?

check out our whitepaper library.
if it helps- in app. a of the 1989 aci-318, they give allowable tensile stress as 20,000 for grade 40 or grade 50. columns were required to be designed per the strength design method, with allowable stress as 40% of the result using that method. the commentary states that this was a change from the 1963 and previous codes.
i don't think you can assume anything better than grade 40 rebar without doind some testing
i agree with bjb. for a building done in the 1960's i would use 40 ksi steel. see the attached paper (specifically the underlined portion on page 2).
here is another paper (the first page anyway). notice at the bottom right hand corner of the page, the allowable stresses are called out as 16 ksi for compression bars (what your column bars are called out as), and 20 ksi for tension bars (presumably your "other" bars). both of these are for 40 ksi steel.
i am not sure why they listed fs on the drawings rather than fy, but in the late 60's "intermediate grade" meant 40 ksi, and was gradually being replaced by 60 ksi as the most prevalent grade for everything but ligatures. so i don't think there was any 50 ksi reinforcing steel available, and your bars are most likely 40 ksi, but depending on the date and location, it is possible that 60 ksi was substituted.
for wsd, the 1983 code restricted reinforcement in spiral columns to .4fy, and tied columns to .85 x .4fy.

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