2008机制2材料力学试卷A答案

厦 门 理 工学 院 机 械 工 程 系2008机制材料力学试卷A一、（20）5.83 The working stresses for the manufactured wood beam are 10 MPa in bending and 1.0 MPa in shear. Determine the largest allowable value of the load intensity w0.Solution 5(a)Design for shearThe maximum shear force is the first moment of the cross-sectional area above the neutral axis is 3the moment of inertia of the cross-sectional above the neutral axis is 3 -P163 4 (b) Design for bendingThe normal stress 4So the largest allowable value of the load intensity 1二、（20）The overhanging beam ABC carries the uniformly distributed load between its supports and the concentrated load P at end C. Determine the displacement of the beam at the C, by superposition. 5According to deflection formulas for beams, we know， 5， 5 4 1三、（20）8.27 The cross sections of the members of the pin-jointed structure are 200-mm square. Find the maximum compressive stress in member BDE. 15SolutionThe maximum compressive stress in member BDE 5四、（20）8.110 Determine the smallest allowable diameter of the shaft that carries the three pulleys. The working stresses are w= 18 ksi and w = 8 ksi. Neglect the weights of the Pulleys and the shaft as well as the stress due to the transverse shear force.Solution According the support, we know there is the largest bending moment occurs at C, the largest torque occurs in segment BC. Show in the figure.， FBY=938.33(lb)， FDY=1511.67(lb)C点弯矩和扭矩：， D点弯矩和扭矩：， B点弯矩和扭矩：， 7At D section, we have，Therefore, the stress at the bottom of the section are 4 4Draw the Mohrs circleAccording to Mohrs circle, we haved=2.119(in.) 4Settle it, we getd=2.397(in.) 五、（20）For the truss shown in the figure (a), the cross section of bar CD is show in Fig. (b), using AISC column specifications with E = 200 GPa and yp = 250 MPa., determine the maximum safe value of P. Assume that cross-bracing prevents the joints from deflecting perpendicular to the plane of the truss. SolutionDetermine the force Fcx Determine the moment of inertia of the cross-sectional area about the z-axis the moment of inertia Area ，The slenderness ratio of a 1600mm x 2400mm rectangleThe least radius of gyration with z-axis 2 4 The slenderness ratio 4For the slenderness ratio is less than , so that the concrete column is of intermediate length. These equations yield the factor of