Study On Seismic Response And Failure Mechanism Of Air Cooled Condenser Support Platform Structure Based On Primary-Secondary Coupled Effect

The nonstructural members and equipment systems are commonly simplified approached in the traditional seismic design concept and methods, but for some special or important building structures, it may lead to error or insecure results when the dynamic interaction between secondary systems and primary systems is ignored.Air cooled technology is widely applied in the thermal power plants where coal rich and water deficient for increasing requirements of energy conservation and environmental protection in recent years. The direct air cooled condenser support platform structure is a new structure, whose primary function is to support the large air cooled condenser equipments, and it appears with the application of air cooled technology. The structural system and loads are complex. The equipment systems have significant dynamic feedback effect on the structure system, hence forming a typical structure-equipment coupled system. The seismic behavior and the key seismic calculation problems of direct air cooled condenser support platform structure are comprehensively researched based on the primary-secondary coupled effect, consequently it can provide theoretical basis for the seismic design of the structure.The dynamic characteristic of direct air cooled condenser support platform structure is the basis of seismic calculation and earthquake resistant experiment. The vibration behavior of direct air cooled condenser support platform structure in service is measured in site. Also, the dynamic property of a model is tested, and the vibration period, frequency, vibration shape and damping ratio are obtained. Modal analysis of prototype structure and model structure are performed with SAP2000~?, and the calculation results show agreement with the results of field measurement and model test. The study shows that it is necessary to consider the primary-secondary systems coupled effect for the dynamic calculation of direct air cooled condenser support platform structure, and the effective damping ratio of coupled system 3.5% is suggested. It is also found that torsion is the main vibration shape of the coupled system, so the couple of vibration shapes must be considered.Selecting 1/4 practical structure as the substructure, imitating the boundary condition, a 1/8 scale model is constructed. Dynamic properties of the model are measured, and then a pseudo-dynamic test and a pseudo-static test are carried out to study the seismic behavior of direct air cooled condenser support platform structure. In the experiment the model is subjected to EL-Centro wave. The failure process and dynamic response of the structure under earthquake action are obtained in the test. The variation of natural frequency of vibration and damping ratio, the deformation performance, the hysteretic behavior and the failure mode of the model are analyzed. The skeleton curve of the model and its characteristic points are also presented. A formula to calculate the seismic bearing capacity of the structure is presented. The study shows that the direct air cooled condenser support platform structure is a single-prevention system. It is also found that the equipment system is in elastic working state when the structure system yields under seldom-occurred earthquake action and the structure fails in the column hinge mechanism.The direct air cooled condenser support platform structure is a vertical hybrid structure consisting of reinforced concrete pipe columns and steel truss. The equipment and structure have dynamic interaction, so forming a coupled system; hence the direct air cooled condenser support platform structure is a non-classical damping system. The coupled damping matrix of structure-equipment system for direct air cooled condenser support platform structure is presented in the paper. Three methods for disposing the coupled damping matrix are suggested, including effective damping ratio method, forced decoupled method and substructure method, which is convenient for the theoretical derivation and numerical calculation.The key problems of ground motion input in seismic calculation of direct air cooled condenser support platform structure are researched. It is suggested that the earthquake action of the structure should calculated based on the response spectrum and time history analysis as a complementary method. According to the vibration behavior of direct air cooled condenser support platform structure, the horizontal earthquake action, the vertical earthquake action and torsional action under bi-direction earthquake action should applied on the structure. Kanai-Tajimi model of earthquake ground motion is chosen. The parameters of the model are studied based on Chinese seismic code ( GB50011—2001) and random extreme value theory. The model parameters under frequent-occurred earthquake action are suggested and the ones under seldom-occurred earthquake action are studied preliminarily. Random seismic response spectrum is presented based on the random vibration theory of SDOF linear system. Spectral value and its probability distribution are deduced. Elastic-plastic random seismic response spectrum is also researched preliminarily. Power spectra input mechanism of equipment system is presented in the paper, including double filtering floor power spectra which is suitable for light non-tuning systems and floor power spectra with primary-secondary coupled effect which is applicable to large mass systems.The dynamic calculation model of direct air cooled condenser support platform structure is presented, which considering secondary system and the torsional effect caused by eccentricity of secondary system. Lateral-torsional coupling seismic response under bi-directional earthquake action is theoretical deduced based on primary-secondary coupled effect. Assuming the ground motion is stationary stochastic process, the seismic response of direct air cooled condenser support platform structure under random ground motion is also derived.Imitating the equipment system using appropriate elements, a space calculation model is established with SAP2000~?, which includes primary system, secondary system and their connection. The response spectrum analysis, linear and nonlinear time history analysis and power spectra density analysis are carried out. The earthquake response of the whole structure and members are obtained through the calculation. The calculation shows that the earthquake response of corner column is greater than the middle column, cantilever end greater than middle part. The equipment on the edge is more easily subjected to fail, which is due to the torsional vibration behavior of structure system.Failure criteria and failure mechanism of structure-equipment coupled system are analyzed with substructure method. Seismic performance levels of the coupled system are analyzed and seismic objectives of direct air cooled condenser support platform structure are suggested. Calculation models for equipment systems are concluded and their failure criteria are presented. The seismic behavior of reinforced concrete pipe columns with large size and thin wall are experimental studied. Mechanical performance and failure mode of space steel truss are theoretical analyzed. It is found that the temperature deformation and earthquake displacement have contradiction, and structural measures are suggested to solve the problem.Based on the above researches, the seismic performance, seismic calculation methods and seismic design indexes are analyzed at last. The new system for direct air cooled condenser support platform structure is expected in the paper.