5.5-3 Use LRFD. A simply supported beam (Figure P5.5-3) is subjected to a unifor

Question

5.5-3 Use LRFD. A simply supported beam (Figure P5.5-3) is subjected to a uniform service dead load of 1.0 kips/ft (including the weight of the beam), a uniform service live load of 2.5 kips/ft, and a concentrated service dead load of 45 kips. The beam is 40 feet long, and the concentrated load is located 15 feet from the left end. The beam has contin uous lateral support, and A572 Grade 50 steel is used. Is a W30 × 1 16 adequate? Hints: 1. Calculate the design strength. 2. Calculate the required strength - the 25 "2 = 2.5 maximum bending moment due to the controlling load combination. W30 × 116 40 (1) The beam is prone to Lateral Torsional Buckling (LTB) (A)True (B) False. (2) The flange cannot buckle locally. (A) True (B) False. (3) The web can buckle locally. (B) False. (A) True (4) The design moment strength Md of the member is calculated most nearly as (A) 1575 ft-k(B) 1420 ft-k(C) 1371 ft-k (D) 1234 ft-k (5) The factored concentrated load Pu is calculated most nearly as (A) 45 k (B) 72 k (C) 63 (D) 54

Explanation / Answer

1) False. The reason is because the compression flange is continuosly braced and therefore lateral torsional buckling will not happen

2)True. The reason is because the the compression flange is continuosly braced

3)True. This is because there is nothing restraining the web from buckling

4)The plastic moment capacity of W30x116 per Table 3-2 of AISC is 1420 kip-ft. Correct option is (B)

5)The factored concentrated load is 1.2*45=54 kips .Therefore, correct option is (D)

6)Factored uniform load is (1.2*1)+(1.6*2.5)=5.2 kip/ft .Correct option is (A)

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