00习题 要求：
1． 将题目转化为Microsoft Word文档；
2． 解题过程用Microsoft Word文档，公式用公式编辑器，只交电子文档作业；
3． 期末考试前必须作对所有所给题目，否则不能参加期末考试，请于指定时间前交作业。
第一题（2010年3月5日前交该题作业）星期五
1.21 Determine the smallest allowable cross-sectional areas of members BD, BE, and CE of the truss shown. The working stresses are 20 000 psi in tension and 12 000 psi in compression. (A reduced stress in compression is specified to reduce the danger of buckling.)
Solution
The free-body diagram of homogeneous BC in Fig.(b). The equilibrium equation are
?MF?0,16?36?24PAy?0, P=24(kips)=24000(lb)
The free-body diagram of truss in Fig.(c). The equilibrium equation are
?ME?0,16PAy?3?(8??MB464?16?8?864?16)PBD?8?36?0,
PBD=－8.944(kips) (Compression)
?0,8PAy?8PCE?0 PCE=24(kips) (Tension )
?Fy?0,PAy?0.707PBE?36?464?16PBD?0
PBE=-11.32(kN) (Compression)
The normal stress of a member CE, DE and DF is
?BD?PBD8944lb??12000(lb/in.2) (Compression) ABDABD ABD=0.745(in.2)
?BE?PBE11320lb??12000(lb/in.2) (Compression) ABEABE ABE=0.943(in.2)
?CE?PCE24000lb??20000(psi) (Tension ) ACEACE ABE=1.2(in.2)
1 .37 Compute the maximum force P that can be applied to the foot pedal. The l/4-in.-diameter pin at B is in single shear, and its working shear stress is 4000 psi. The cable attached at C has a diameter of 1/8 in. and a working normal stress of 20 000 psi.
Solution
The free-body diagram of bracket in Fig.(b). The equilibrium equation are
?MB?0,?Fx?0,?Fy6P?2Tsin10??0, P=0.05788T (a) PBx?Tcos10??0, PBx=-0.9848T PBy?P?Tsin10??0, PBy=0.2315T
?0,According to the normal stresses formula, we have ??4T?20000(lb/in.2) T=245.4(lb) 2??0.125in.According to the shear stresses formula, we have
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