ARC322 Quiz 6 2008

ARC322 Quiz 6 2008 - conflict with moment Why Yes there is...

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ARC322 Quiz 6 Name ______ KEY _______________________ Calculate the require section modulus for a simple beam that spans 28 ft and carries a uniform load do 2200 lb/ft. No sketch required. M = w L 2 /8 = (2200 lb/ft)(28 ft) 2 /8 = 215,600 ft-lb S = M/ f = (215,600 ft-lb x 12 in/ft)/24,000 lb/in 2 = 107.8 in 3 Select the most economical steel member from the S x Table shown on the screen. Based on the information you have presented or calculated. The most economical section that meets the requirements is a W24x55 based on the S x Table provided. If you had selected the member with the closest section modulus from the S x table, is there any
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Unformatted text preview: conflict with moment? Why? Yes, there is a conflict. The closest S in the S x table is 108 in 3 for a W18x60 with a listed moment of 214,000 ft-lb. BUT, the calculated resisting moment is 215,600 ft-lb in the actual member. Why is this? Because AISC uses M R (resisting moment) as 0.66 F y = 23,760 lb/in 2 which they round to 23,800 rather than 24,000 lb/in 2 as we do. But, if you check the S x using the 23,800 lb/in 2 you get a required S of 108.7 in 3 which STILL leaves the AISC table in conflict. There are very few silver bullets where everything in life is perfectly solved....
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