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ARC322 HW#1C 2008 KEY

ARC322 HW#1C 2008 KEY - ARC321 Homework 20 KEY 32 POINTS 1...

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ARC321 Homework 20 KEY 32 POINTS 1. A 6x12 spans 18 ft and carries a concentrated load of 17000 lb at its center. Is it OK in deflection? This is a Case 1 condition where D = PL 3 /48EI I have given you E as 1,900,000 lb/in 2 . Later in the quarter, you will have to select the appropriate E. I for a 6x12 can be either looked up or calculated. Just for exercise, I’ll calculate it here I = bd 3 /12 = (5.5 in)(11.5 in) 3 /12 = 697.07in 4 Now, we can do the deflection calculation: D = PL 3 /48EI = (17,000 LB)(18 ft x 12 in/ft) 3 /(48)(1,900,000 lb/in 2 )(697.07 in 4 ) D = 2.69 in 1 Point Test d = L/240 = (18 ft x 12 in/ft)/240 = 0.9 in < 2.69 in … NOT OK 1 Point Why isn’t it OK? Should we have known that it was not likely to be Ok just by examination of the problem? How could you easily tell that it is not likely to be OK? You should be able to answer these questions. 2. A 4x14 is cantilevered 8 ft with a concentrated load at its end of 6000 lb. Is it OK in deflection? This is Case 4 where D = PL 3 /3EI I = 678.47 in 4 D = (6000 LB x (8 ft x 12 in/ft) 3 )/(3 x 1,900,000 lb/in 2 x 678.47 in 4 ) D = 1.37 in 1 Point Test d = L/240 = 96in/240 = 0.4 in < 1.37 in … NOT OK 1 Point 18 ft 17,000 LB Note: If you didn’t do the drawing, you would lose points. This is the only drawing I’m going to do.

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ARC322 HW#1C 2008 KEY - ARC321 Homework 20 KEY 32 POINTS 1...

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