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Re: precast prestressed bridge beam

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	Greetings,
		ACI 18.4 limits bottom tension stresses due to service loads
to 6 x square root f'c and 12 x square root f'c for bilinear analysis of
cracked transformed sections, except for two-way slabs which are not common
in precast concrete. ACI 318-02 drops this limit in favor of reinforcement
spacing requirements for "Class C" (cracked) concrete. A beam with bottom
compression under service loads may be limited by its bottom compression at
prestress transfer to 0.60 x f'ci (18.4.1(a)). Most precasters in the US
violate this requirement and use a limit of 0.70 x f'ci, calculated
compression + 1400 psi with a minimum of 3500 psi, or something like one of
these, and see no deleterious effects.
		I am not aware of a restriction of the change of sign of a
stress, though the new 18.4.4 mentions fatigue. Fatigue is not defined in
Chapter 2, nor is it discussed further in 18.4.
	Regards,
	James L. Getaz, III, P.E.
	Winchester, Virginia
	jgetaz(--nospam--at)shockeyprecast.com

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