72

Allowable deflection = span/180 =-= 4 . 8 in. > 2 .1 in. , OK

8.8.5 Cracking Behavior and Crack Control in Prestressed Beams

If A/s is the net stress in the prestressed tendon or the magnitude of the tensile stress in the normal steel at any crack width load level in which the decompression load (decompression here means fc = 0 at the level of the reinforcing steel) is taken as the reference point, then for the prestressed tendon

where fnt is the stress in the prestressing steel at any load level beyond the decompression load and fd is the stress in the prestressing steel corresponding to the decompression load.

The unit strain es = Af/Es. Because it is logical to disregard as insignificant the unit strains in the concrete due to the effects of temperature, shrinkage, and elastic shortening, the maximum crack width can be defined as

where k and a are constants to be established by tests, acs is the spacing of cracks, and es is the strain in the reinforcement.

For pretensioned beams, the maximum crack width can be evaluated from the maximum crack width at the reinforcing steel level

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