huangyhg

release stresses (bridge girder design conspan)
precast-top values in conspan - is it correct that as long as the release stresses are less than 0.75 f'c 1996 aashto 9.15.1 then i am ok even tho my conspan program red flags these values
first the 1996 edition of aashto has been out dated for some time.
second, for most girders the release stress concerns the top flange since the cip decking or precast panels/cip slab hasn't been added to the system yet thereby counteracting the upward camber resulting from prestressing.
the potential result is cracking in the top flange. you can check this rather quickly from conspan stress output to the newer code and learn what's going to happen.
regards,
qshake
eng-tips forums:real solutions for real problems really quick.
engineerall,
do you have your numbers right here. do you really mean to say the allowable tension stress in the top at release is .75f'c?? thos sounds awfully high. are you sure you are reading it properly? the tensile strength of concrete is about 7 root f'c (in american units). the allowable limit should be significantly less than this. your value is an order of magnitude higher.
our local state agency limits our residual tension to zero psi at the dl + p/s stage and 3 root f'c at service load conditions (dl + p/s + (ll+i).
first, the original post meant 0.75 f's.
second, did qshake read the post correctly? the limit on fs is the allowable stress of the cable.
third, steel and concrete are pretty much the same, yet the code changes continually.
would someone please straighten this out. the thread is confusing i can't make heads or tails.

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