Because frequency constraint is the key constraint for dynamic topology optimization of truss , in this pa-per, a new method for extremum calculation of natural frequency for truss is put forward ---an optimization method based on virtual material.It is optimized in two levels with sensitivity analysis; there are nodal design variables and cross-section area variables of truss elements, the bars related to the removed nodes being assigned new material properties; then the extremum of natural frequency is calculated with steepest descent method (SDM).The results of a numerical example and their analysis show preliminarily that the proposed method is correct and effective .% 基于固频约束时结构动力学拓扑优化的关键约束，提出了一种求解桁架结构固频极值的新方法---基于虚拟材料法的优化计算方法，从节点设计变量和杆件截面积两个层次进行灵敏度分析以进一步优化计算，将拟删除节点相对应的杆件材料特性进行重新赋值，通过极值求解算法以迭代计算得到固频极值。数值算例结果表明所提方法正确、有效。
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