421_Mechanics SolutionInstructors_Sol.Manual-Mechanics_Materials_7e.book_Gere_light.1

421_Mechanics SolutionInstructors_Sol.Manual-Mechanics_Materials_7e.book_Gere_light.1

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Unformatted text preview: 05Ch05.qxd 9/24/08 4:59 AM Page 415 SECTION 5.6 Design of Beams 415 Solution 5.6-5 NUMERICAL DATA sa P 17000 psi L q 2200 lb 28 ft 425 lb ft FIND REACTIONS (EQUAL DUE TO SYMMETRY) THEN MAX. RECOMPUTE REACTIONS AND MAX. MOMENT THEN CHECK lb MAX. STRESS w 25.4 ft RA P L L +q +w 2 4 2 MOMENT AT CENTER OF BEAM RA L 2 RA qL L 1L a+ b 44 24 4.431 * 103 lb L 1L a b 2 22 Mmax P L +q 2 4 Mmax 2.83 * 104 ft-lb smax Mmax (12) Sact smax 13,806 psi less than allowable so OK 4.075 * 103 lb RA qL L 1L a+ b 44 24 L 2 RA Mmax RA Mmax 2.581 * 104 ft-lb w Compute Sreqd & then select S shape Sreqd Mmax (12) sa select S 10 * 25.4 (Sact 3 24.6 in. , w Sreqd 18.221 in.3 ; 25.4 lb/ft) Problem 5.6-6 A pontoon bridge (see figure) is constructed of two longitudinal wood beams, known an balks, that span between adjacent pontoons and support the transverse floor beams, which are called chesses. For purposes of design, assume that a uniform floor load of 8.0 kPa acts over the chesses. (This load includes an allowance for the weights of the chesses and balks.) Also, assume that the chesses are 2.0 m long and that the balks are simply supported with a span of 3.0 m. The allowable bending stress in the wood is 16 MPa. If the balks have a square cross section, what is their minimum required width bmin? Solution 5.6-6 Chess Pontoon Balk Pontoon bridge FLOOR LOAD: W 8.0 kPa ALLOWABLE STRESS: sallow Lc length of chesses 2.0 m Lb 16 MPa length of balks 3.0 m ...
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