Info

Therefore fps = 28500 x 0.00824 (See Fig. 7.48)

= 234.7 kips T = 4 x 0.153 x 234.7 = 143.7 T = 143.7 + 74.4 = 218.1 kips

This is practically the same as the value we assumed in the second trial. Therefore T -218 kips may be used to compute the flexural strength of the beam.

Figure 7.48d. Example 1: force diagram.

Flexural Strength. The nominal moment strength is obtained by summing the moments of T1 and T2 about the C.G. of compressive force C (see Fig. 7.48d).

Mn = 74.4 x (20 -2.14) + 143.7 (22 - 2.14) = 4183 kip-in = 348.6 kip-ft

Usable capacity of the beam = fMn

Given. Same data as for example 1, except 3 - 12" f strands are used instead of 4 -12" f strands. This example illustrates the calculation of stress in the strand in the nonelastic range of the stress strain curve shown in Fig. 7.48. Required. Ultimate flexural capacity of the beam. Solution. As before, we use a trial-and-error procedure. First Trial. Assume stress in the strands = 240 ksi.

= 3 x 0.0153 x 240 + 4 x 0.31 x 60 = 110.16 + 74.4 = 184.56 kips

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