文章以一次结构力学课程考试为切入点，对其中考查学生对位移法掌握情况的试题，按解题步骤进行得分统计分析，认为此题得分率较低的原因是大部分学生不能准确计算位移法基本方程的刚度系数和自由项。文章提出，对于有两个及两个以上基本未知量的位移法计算题，只要确保在整个解题过程中附加约束单位位移的参考方向不改变，每个附加约束上约束力的参考方向设置一致且不改变，就能够避免将刚度系数和自由项计算错误。

After a course examination on structural mechanics, a step-by-step score analysis was carried out on a test question aiming at examining the students’ displacement method application. The result of the analysis showed that the main reason for the failure in solving the problem was that most of them can not obtain the stiffness coefficients and free terms of the essential equation based on the displacement method correctly. This paper presents a symbolic rule to eliminate the drawbacks of mistaking the stiffness coefficients and free terms of basic equations that have 2 or more basic unknown quantities. The rule requires that the reference direction for the displacements and constraint forces taken placed at the additional constraints unchanged, meanwhile all constraint forces generated at the same additional constraint should have the same reference direction.

武汉工程大学教学研究项目（X2011025）