In this paper , the three level fortification criterion , two stage design method and the elastic response spectrum theory are adopted , and the program of nba ( the program of nonlinear - aseismic analysis for beam bridge ) is used to calculate . through gathering , analyzing and calculating a lot of bridge design materials , this paper induces a simple way to calculate the relative displacement of the beam and the pier , and then according to the current design method , it concludes a better design principle of the supporting length of the beams . at the same time the dynamics is adopted to put forward the design method of the carrying capacity and the lengthen of prestress wire ( steel bar ) falling - off prevention structures of highway bridges 本论文采用三水准设防、两阶段设计方法、弹性反应谱计算理论，运用桥梁结构非线性地震响应分析程序nba ( theprogramofnonlinear ? aseismicanalysisforbeambridge )进行计算；通过收集大量的实桥设计资料，经分析计算，归纳总结出计算上下部结构相对位移的简便计算方法；并综合国内外的经验值，提出梁的支承长度se的设计原则；同时运用动力学突加荷载的原理，推导分析了预应力钢绞线式(钢棒连接式)落梁防止装置设计承载力的计算方法和设计伸长量s _ f的取值；最后本文通过实桥计算，说明了落梁防止系统设计的方法。
