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Structural Concrete - Hassoun

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750 Chapter 19 Introduction to Prestressed <strong>Concrete</strong><br />

K b =<br />

K t =<br />

= 66, 862 in. 4<br />

I 66, 862<br />

=<br />

Ay t 372 × 19.2 = 9.4in.<br />

I 66, 862<br />

=<br />

Ay b 372 × 20 = 8.6in.<br />

(a)<br />

(b)<br />

(c)<br />

Precast, prestressed concrete sections: (a) single T-section, (b) double T-section, and (c) U-section.<br />

2. Estimate prestress losses, given F 0 = 175 ksi.<br />

a. Assume elastic loss is 4%, or 0.04 × 175 = 7ksi.<br />

b. Loss due to shrinkage is 0.0003 E s = 0.0003 × 29, 000 = 8.7 ksi.<br />

c. Loss due to creep of concrete: A good first estimate of creep loss is 1.67 times the elastic loss:<br />

d. Loss due to relaxation of steel is 4%:<br />

1.67 × 7 = 11.7ksi<br />

0.04 × 175 = 7ksi<br />

Time-dependent losses are 8.7 + 11.7 + 7 = 27.4 ksi.<br />

Percentage = 27.4<br />

175 = 15.7%

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