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Structural Concrete - Hassoun

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538 Chapter 15 Design for Torsion<br />

Solution<br />

1. Section 1<br />

a. Cracking moment, φT cr , can be calculated from Eq. 15.19:<br />

φT cr = φ4λ √ ( )<br />

A<br />

2<br />

f c<br />

′ cp<br />

P cp<br />

For this section, A cp = x 0 y 0 , the gross area of the section, where x 0 = 16 in. and y 0 = 24 in.<br />

A cp = 16(24) =384in. 2<br />

P cp = perimeter of gross section<br />

= 2(x 0 + y 0 )=2(16 + 24) =80in.<br />

φT cr = 0.75(4)(1)√ 4000(384) 2<br />

80<br />

= 349.7K⋅ in.<br />

b. The allowable φT a that can be applied without using torsional reinforcement is computed from<br />

Eq. 15.20:<br />

T a = φT cr<br />

= 349.7 = 87.4K⋅ in.<br />

4 4<br />

2. Section 2<br />

a. First, calculate A cp and P cp for this section and apply Eq. 15.19 to calculate φT cr . Assuming<br />

flanges are confined with closed stirrups, the effective flange part to be used on each side of the<br />

web is equal to four times the flange thickness, or 4(4) = 16 in. = h w = 16 in.<br />

A cp = web area(b w h)+area of effective flanges<br />

=(14 × 20)+2(16 × 4) =408in. 2<br />

P cp = 2(b + h) =2(14 + 2 × 16 + 20) =132in. 2<br />

φT cr = 0.75(4)(1)√ (4000)(408) 2<br />

= 239.3K⋅ in.<br />

132<br />

Note: If the flanges are neglected and the torsional reinforcement is confined in the web only,<br />

then<br />

A cp = 14(20) =280in. 2 P cp = 2(14 + 20) =68in. φT cr = 219K ⋅ in.<br />

b. The allowable φT n that can be applied without using torsional reinforcement is:<br />

φT n = φT cr<br />

4<br />

= 239.3<br />

4<br />

= 59.8K⋅ in.<br />

3. Section 3<br />

a. Assuming flange is confined with closed stirrups, effective flange width is equal to b w = 15 in.<br />

< 4 × 6 = 24 in.<br />

A cp =(14 × 21)+(15 × 6) =384in. 2<br />

P cp = 2(b + h) =2(14 + 15 + 21) =100in.<br />

φT cr = 0.75(4)(1)√ (4000)(384) 2<br />

= 279.8K⋅ in.<br />

100<br />

Note: If the flanges are neglected, then A cp = 294 in. 2 , P cp = 70 in., and φT cr = 234.3K⋅ in.<br />

b. The allowable φT n = φT cr<br />

= 279.8 = 70K ⋅ in.<br />

4 4

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