huangyhg

temporary stress increase for concrete
is there an allowable stress increase for concrete under temporary load? if so, where is this stated, what is the duration for temporary stress increase?

no, not that i know of. temporary stress increases apply for wood, which is more "ductile" or basically bendable. concrete is relatively brittle, so it isn't going to have a temporary stress increase allowance.
when you push on a wood board, you expect it to bend a little and pop back into place.
when you push on a concrete beam, it will crack and stay how it is.
you have lower load factors for wind, seismic- is that what you mean?
unless the beam is prestressed, and then it will regain the camber and cracks close, but it is still considered a cracked section.
for prestressed you can even exceed 12sqrt(fc')now so there really is not a limit so long as you meet other criteria for design and detailing.
unless your specification documents or exposed condition requires you to keep the stresses below 5 to 7.5 sqrt(fc`) you can go as high as you need to so long as your crack width is within limits and your deflection also meets aci. you should properly detail your section for crack control also.
see aci 318-02:
9.5,10.6, & 18.4
beyond that there is not a temporary stress increase you can use if you have specific limits specked out. even though it is a temporary load as aggieyank points out it has lasting affects on the concrete
all "increses" are handled through the load combinations in chapter 9 (or the appendix). concrete is not designed by allowable stress methods so an allowable stress increase doesn't really make sense.

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