Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: precast prestressed bridge beam

[Subject Prev][Subject Next][Thread Prev][Thread Next]
		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.
	James L. Getaz, III, P.E.
	Winchester, Virginia

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at:
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*   Questions to seaint-ad(--nospam--at) Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: 
******* ****** ****** ****** ******* ****** ****** ********