Study On Seismic Damage Endurance Assessment And Improvement Measures For Long-span Single-layer Spherical Lattice Shells

Most large-scale public buildings are supposed to act as post-earthquake disaster relief centers and great importance has been attached to them.Thus,those buildings cannot collapse or suffer severe damage during earthquakes and expect to be safe for a significant possibility of having subsequent aftershocks in practical.Long-span single-layer spherical lattice shells have been widely adopted in these buildings as roof and enclosed systems due to their good spanning capacity and seismic performance.Thus,the post-earthquake damage level and residual seismic capacity of this type of structure often determine whether the whole building can continue to be used.Especially during the emergency shelter period,if the structure does not have enough residual seismic capacity under the damage state,once the subsequent earthquake occurs local or progressive collapse,it will cause unpredictable casualties.Developing a reasonable post-earthquake performance assessment method to measure the structural endurance to different levels of seismic damage and then analyzing the attenuation law of seismic capacity of damaged shells to put forward some specific improvement measures have a significant positive impact on the seismic performance analysis and design of space lattice shells.This paper focuses on two subjects,i.e.,seismic damage endurance assessment method and corresponding improvement measures.A comprehensive post-earthquake performance evaluation method is developed for lattice shells,which combine the global seismic damage model and seismic capacity indicator.An appropriate method to improve the evaluation index is proposed based on computational efficiency.A framework for evaluating the structural damage endurance performance is put forward combining with the classification of seismic fortification in the design code.Some equations for identifying the vulnerable areas of lattice shells are derived using the equivalent continuum analogy.Finally,the optimization and reinforcement for improving damage-endurance of rigid shells with a series of resilient structures are studied.Some research has been conducted and conclusions are as follows,(1)A comprehensive post-earthquake performance assessment method,which combines seismic damage and earthquake resistance,is put forward to evaluate the damage endurance of lattice shells.The structural capacity reserve(SCR)index is presented based on the residual seismic capacity(RSC)ratio.Through the study of seismic capacity degradation of typical single-layer shells,it was verified that the curves of the RSC ratio during the overall damage interval can reflect the post-earthquake performance level under different damage states in combination with failure modes.The impact of the arrangement of the components on the structural deformability,failure mode,plasticity and damage development as well as the RSC and SCR are clarified for the specific case study.The investigation indicates that the ability of lattice shells to retain their post-earthquake resistance reserves is found to depend primarily on the locations where damage develops.Besides,normalized RSC ratio curves can reflect the design rationality of structural layouts and that they can capture the dynamic instability of lattice shells.(2)A new RSC assessment model is developed based on the deliberately selected multiple response-based indicators for long-span single-layer lattice shells.The modified RSC ratio is defined as the normalized relative distance with respect to the collapse-critical state,with two state difference vectors used to identify the distances measured from the current damage state to the intact state and collapse-critical state,respectively.The modified RSC ratio can benefit the calculation efficiency of damage endurance evaluation,which is suitable for large-scale complicated shells.Further,referring to the damage performance design,an RSC performance level matrix was put forward to achieve the performance level classification based on the structural function and importance.The objective values of the RSC ratio corresponding to different fortification categories were determined.And an evaluation framework of seismic damage endurance has been established according to the current design codes(3)An approach for quickly predicting the vulnerable areas of long-span spherical lattice shells under vertical earthquakes using the equivalent continuum analogy.According to the moment theory and plastic limit analysis theory,the equations for locating the position of maximum nodal displacement(MND)are deduced in the elastic and plastic state,respectively.A defined relative stiffness index calculated by integrating the curvature variation over the corresponding node-bearing area can be used for identifying whether the design scheme of a shell is reasonable in the preliminary design stage.The case study indicates that the vulnerable area in a long-span single-layer lattice shell can be predicted and controlled based on the relative stiffness index values to provide help for the subsequent design stages.Furthermore,the desired seismic damage-endurance capacity can be obtained by adjusting the relative stiffness index of each ring to ensure sufficient residual seismic capacity under a certain damage degree as well as under an intact state.(4)The effects of prestressed elements on the RSC of rigid lattice shells after earthquakes are analyzed and design procedures for damage-endurance improvement are put forward.Prestressed optimization shells that aim at improving the internal plasticity distribution and counteracting the external unfavorable load are selected.i.e.,a cable-stiffened lattice shell(CLS)and suspen-dome structure(SDS)are used as the research objects.The investigation indicates that the introduction of the cable-stiffened system can significantly improve the earthquake resistance of a damaged CLS by changing the plasticity concentration and force transmission paths in the shell.Instead,the desirable seismic damage-endurance of the SDS depends more on the earthquake resistance reserve provided by the cable-strut system due to self-centering property.Furthermore,based on the performance levels and objectives,some key design procedures for damage-endurance improvement using prestressed elements are presented to establish a preliminary framework of damage endurance-based seismic design.(5)Comparative studies on the performance improvement of seismic damage endurance of long-span single-layer lattice shells were carried out based on the concept of energy dissipation.The effects of different types of energy dissipation control methods and devices on the improvement of the damage endurance of the structure were investigated to give design suggestions.According to the case results,significant differences were found when improving damage endurance through different control measures.The method of adjusting cross-sectional sizes of a shell to divide the main and secondary parts was more suitable for a structure with separation design of the main and secondary regions.Both fuse components and friction nodes adopted in local areas to achieve damage orientation and control.When the replaceable method can be further combined,the seismic capacity of a shell under the same damage state can be significantly improved.The differences of the placement and number of damping energy dissipating members may shift final failure modes of the original shell,which would lead to reducing the structural damage endurance levels under the same damage state.Three-dimensional isolation bearings can significantly reduce the proportion of damaged components in a shell and protect the overall structure,thus ensuring the subsequent seismic capacity.