The mechanics of materials method is applied to derive the accurate solutions to bending normal stress and deflection of equal strength beam considering its weight. The solutions show that the design error is caused by ignoring the weight of the beam. Such error problems as bending normal stress and deflection of equal strength beam were studied. The affecting factors and the variation law of the error were analyzed via an example. The re-sults show that absolute errors of maximum bending normal stress and deflection are independent of the load size, but are related to the position of cross section. Their relative error is associated with the load size and the position of cross section, but with a different variation law.%利用材料力学方法推导出考虑自重影响的等强度梁弯曲正应力及挠度的精确解，得到忽略梁自重引起的设计误差。研究等强度梁应力及挠度的误差问题，通过算例分析误差的影响因素及其变化规律。结果表明，最大弯曲正应力及挠度的绝对误差均与荷载大小无关，但与横截面位置有关；两者的相对误差与荷载大小与横截面位置有关，但变化规律不同。
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