Abstract: Scientific and effective safety assessment of existing civil air defense structures has become a key issue to ensure urban underground safety. In order to overcome the shortcomings of the existing safety evaluation methods, a safety evaluation method based on load structure method and strength reduction method is proposed. First, the load structure method is used to convert the impact of surrounding soils on the tunnel lining into load and foundation spring. The concrete damage plastic (CDP) model is used to simulate the damage of lining structure, and the value of safety factor (K1) of lining is obtained by load increasing method and stiffness reduction method. Meanwhile, the finite element modeling analysis of soil lining is carried out by using the strength reduction method, and the safety factor value (K2) of the structure or soil mass at failure is obtained. The overall safety factor (K) of the structure is the smaller value of both. Finally, the influence of different factors such as crack condition and water level change on the structural safety factor is analyzed by using the above methods. The results show that the load structure method and strength reduction method can be used for evaluating the safety of civil air defense structures. The safety factor of the structure is mainly affected by cracks and water level. The deeper the crack is, the more unfavorable it is to the structure, and the lower the safety level of the structure is. When the underground water level is above the structure bottom and there is water seepage inside the structure, the higher the water level is, the safer the structure is. When there is no water seepage inside the structure, the higher the water level is, the lower the safety factor of the structure is. The quantitative evaluation of structures is conducive to the subsequent disposal, and the research results provide a new way for the safety assessment of underground structures.

Key words: civil air defense structure, safety assessment, load structure method, strength reduction method