Abstract:Lingzhao Veranda (also called Crystal Palace) is located in the eastern part of the Palace Museum (the Forbidden City), which was planned for the amusement of the royal family in 20th century.However, construction of the building lasted only three years, and the building has lain idle until the present.The building is composed of a center bearing frame and four surrounding white marble verandas.The frame is composed of I-section steel beams and a ring of iron columns.In addition, on top of the bearing frame and verandas, there are five iron booths.Because the building has been idle for nearly 100 years, structural problems have appeared on the frame due to a number of factors, such as shortage of bolts, slack in joint connections, and cracks in the iron columns.The National Center for Quality Supervision and Testing of Building Engineering has conducted on-site inspections of the steel structure, and concluded that the beams and columns are structurally sound.However, the joints need to be strengthened.As a historic building, the Lingzhao Veranda is worth protection for its artistic, historical, cultural, and architectural values.To effectively protect this historic building, it is necessary to assess its current structural safety.Such an assessment may provide a direction for future maintenance and restoration of the building.The status of the structure can be determined through simulation methods.For the purpose of this study, the ANSYS program was used to study the aseismic performance of the steel structure.Beams and columns of the frame are connected by steel bolts, which form a type of semi-rigid joint to release part of moment of the joint.Three spring elements of the ANSYS program are used to simulate rotation stiffness values of the beam-column joints.The degradation of rotation stiffness in the joints is also considered.In addition, because some steel beams are embedded in the white marble walls, the embedded locations of the beams are considered as a fixed boundary.Accordingly, the finite element model of the steel structure is developed.Using modal analysis, the structure's basic frequencies and primary modes are determined.Using response spectrum analysis, the distribution of its deformation as well as internal forces under Ⅷ-degree intensity of frequently occurred earthquakes are analyzed.Using time history analysis, the structure's anti-collapse performance under Ⅷ-degree intensity of rare earthquake events is estimated. Results of modal analysis show that the basic frequency of the steel structure is 5.31 Hz.The structure's primary modes focus on mode 5 in x direction and mode 1 in y direction.Both modes behave as a local vibration of the steel booths in level directions, which relates closely to degradation of the stiffness in its beam-column joints.The response spectrum analysis shows that, under Ⅷ-degree intensity of frequently occurred earthquakes, values for deformation as well as stress in the steel structure are within permissible ranges.This is attributable to many factors, including several beams embedded into walls that serve as additional supports for the center frame, and the high strength of the steel (iron) material.In addition, the locations of the peak deformation and stress are near the top of the columns, reflecting the ease with which the slack in the joints can easily cause lateral deformation or failure of the columns.The results of time history analysis show that, under the Ⅷ-degree intensity of rare occurred earthquakes, displacement response curves of the typical nodes in the steel structure reflect a nearly even fluctuation based on balance locations.In turn, this means that the steel structure maintains a stable vibration status.Because the peak deformation value of the weak layer of the structure is still within permissible ranges, the structure is not expected to collapse.
