125

a Pretopped.

a Pretopped.

8.6.2 Example 3: Flexural Design of Prestressed Beams at Service Load Level

Design an I-section for a beam having a 65-ft (19.8m) span to satisfy the following section modulus values:

Required St = 3,570 in.3 (58,535 cm3) Required Sb = 3,780 in.3 (61,940 cm3)

Given

WSD = 100 plf Wl = 1,100 plf fc' = 5,000 psi (34.5 MPa) fi at transfer = 75% of cylinder strength fpu = 270,000 psi (1,862 MPa)

Solution

Since the section moduli at the top and bottom fibers are almost equal, a symmetrical section is adequate. Next, analyze the section in Figure 8.3 chosen by trial and adjustment.

Analysis of Stresses at Transfer fci — fti — ct ( fi — fci.

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