宁波保国寺大殿北倾原因浅析

宁波保国寺是浙东现存最古老的宋代木结构建筑，自清乾隆时期起，大殿结构发生北倾，后对4号转角檐柱进行修缮并加固至今，通过跟踪测试没有发现新的倾斜趋势。然而，探讨大殿曾经北倾原因以及经修缮后现今大殿的可靠性无疑仍是必要的。古木结构的特点是梁柱的榫卯连接具有半刚性性质。采用有限单元法对大殿结构进行力学分析时，榫卯连接可采用空间二节点虚拟弹簧元进行模拟，单元刚度矩阵中相应的刚度特性系数可由接触有限单元法近似求得。这样，大殿结构可离散为杆、梁、壳、体元和上述虚拟弹簧元等的组合，在屋面荷载、雪荷载和风荷载作用下采用有限元法可求得各节点的位移和相应应力。通过计算表明，不计4号柱时产生的向北位移比考虑该柱参加受力时要大得多，可见北倾原因确系4号柱年久失效而引起。计算结果也表明，现今大殿结构由受力来看是安全的。同时预示了可能出现的最危险部位。保国寺大殿结构的力学分析结果，为物保护工作提供了科学依据。

Study on the north-inclination of the main hall of Ningbo Baoguo Temple

The study of structural mechanics on the Chinese historic timber - structured architecture is helpful to their preservation. The great and magnificent main Hall of Ningbo Baoguo Temple is one of the oldest wood - structured buildings in Song Dynasty in the east of Zhejiang Province. Since the Qing Dynasty (Qian Long) , the Hall's structure has inclined gradually to the north. Later, the fourth column in the Hall was repaired and the structure was reinforced. Since then, no sloping has happened. However, it is still necessary to search the reason of north - inclination and to reevaluate the reliability of the improved Hall. The feature of the ancient timber structures is the connection of beam and column by use of mortise and tenon. The connection is semi - stiff. If the finite element method (FEM) is used for structural analysis, the connection can be assumed as a virtual spring element. Its stiffness coefficients in the stiffness matrix can be solved by the use of contact FEM analysis. Thus the structure of the Hall can be solved by the use of contact FEM analysis. The structure of the Hall can be discreted as a combination of elements of bar, beam, sheU, 3D and the virtual spring mentioned above. Under roof weight, snow load and wind load, the nodal displacements and corresponding stresses in the structure can be obtained by the use of FEM. The results have shown that if the fourth column had lost its efficacy the displacements to the north would have been more than that if the fourth column functions. Losing efficacy of the fourth column would have been the main cause of north - inclination. The results also present that the improved Hall is reliable, and meanwhile, some possible danger places can be predicted. The structural analysis of the Hall has provided scientific basis for the preservation of cultural relics.