Constitutive Model of Bolted Connections and Modeling Method of Bolt Connected Precast Concrete Wall Panel Structure
Objective The bolt-connected precast concrete wall panel structural system provides multiple advantages,including speedy construction,conveni-ent installation,trustworthy connections,disassembly,and repairability,which make it particularly suitable for low-rise rural residential buildings.In recent years,the engineering applications of this system have steadily increased.This research conducts studies on the bolt-connected precast concrete wall panel structure,including bolted connection node tests,wall panel tests,and structural system tests.The failure modes of the speci-mens are identified accordingly.The force transmission characteristics and seismic performance of the structure are systematically analyzed.In addition,calculation methods for the bearing capacities of nodes and wall panels are proposed.Numerical analysis software must be adopted to evaluate the integrity of the structural system and establish seismic design methods.However,existing solid element models or simplified calcula-tion models based on the Opensees platform are challenging to converge during calculations,requiring a high level of professionalism from de-signers.Therefore,a simplified model of wall panels and bolted connection nodes is developed in this research to adapt to current engineering design software,facilitating the engineering application of this structural system.Methods This study simplifies a single bolted connection node into three translational degrees of freedom(x,y,z)nonlinear springs to simulate the mechanical behavior of the connection node and establish a simplified calculation model based on the analysis of the force transmission mech-anism of bolted connection nodes.Formulas for calculating the tensile and shear bearing capacities of bolted connection nodes are proposed based on axial tensile and shear tests of bolted connection nodes.In addition,constitutive models for tension and shear of bolted connection nodes are established.Then,the effectiveness of the simplified model of bolted connection nodes is verified through quasi-static tests of bolted connection prefabricated wall panels.The simplified node model can be employed to investigate the seismic performance of the structural system.The cur-rent analysis and design software Midas Gen(2021 version)is utilized for system modeling.The specific modeling method involves using appro-priate"plate elements"for prefabricated wall panels and floor slabs,"nonlinear spring"elements for bolted connection nodes,and setting com-pression springs only at the corner of the wall toe to simulate potential concrete corner crushing.Taking a demonstration project as an example,with a seismic fortification intensity of 7 degrees in the project area,a design basic seismic acceleration of 0.10g,and the third seismic group,the project employs a bolt-connected precast concrete wall panel structure.First,the modeling method is adopted to establish the design model of the structure.Elastic analysis under small and medium seismic actions and elastic-plastic time history analysis under large seismic actions are conduc-ted to verify the inter-story drift angle of the structure.Elastic-plastic time history analysis is performed using El Centro,Kobe,and Artificially Generated Motion(AGM)waves,with the seismic response spectrum closely matching the design response spectrum specified in the code.The model can extract the internal forces of nodes under different conditions and calculate the bearing capacities of bolted connection nodes under the most unfavorable internal forces.Results and Discussions This study obtains force-displacement curves for the tensile and shear behavior of the bolted connection nodes in line with the conducted axial tensile and shear tests of bolted connection nodes.The mechanical behavior can be roughly divided into four stages:elastic,frictional slip,strengthening compression,and failure.Based on the constitutive models for the tensile and shear behavior of bolted con-nection nodes,a single bolted connection node is simplified into three translational degrees of freedom(x,y,z)nonlinear springs to simulate the mechanical behavior of the connection node and construct a simplified model of the bolted connection node.The simplified model of the bolted connection node is validated through quasi-static tests of bolted connection prefabricated wall panels.The validation results revealed that the peak bearing capacities simulated by the simplified model and the actual peak bearing capacity are 96.7 and 106.6 kN,respectively,with an error of-9.28%.Within a reasonable range of error,this indicates that the simplified model of bolted connection nodes can accurately simulate the bolted connection prefabricated concrete wall panel structure,predicting structural sliding deformation,peak bearing capacity,and lateral stiffness effect-ively.As illustrated by the calculations for the demonstration project,during the elastic analysis stage,the model's natural vibration period is 0.08 s(in the Y direction),with high lateral stiffness.Under the effects of seismic intensities of 7°,8°,and 8° fortification,the maximum inter-story drift of the structure is 1/9 999,1/8 852,and 1/3 281,respectively,which are less than the elastic inter-story drift angle limit of 1/1 200 specified in the code.This demonstrates that the structure is in the elastic state.The results of the elastic-plastic time history analysis under major seismic events showed that the maximum inter-story drift angles of the design model are 1/1 446,1/1 580,and 1/1 311,respectively,which are less than the required elastic-plastic inter-story drift angle limit of 1/120 specified in the code.These results indicate significant redundancy,meeting the seismic deformation requirements under rare seismic events.Taking an 8°(0.3g)fortification seismic event as an example,the verification of the bearing capacity of the bolted connection nodes showed that,at the most unfavorable location between the wall panel and the foundation,the max-imum tensile and shear forces on the horizontal seam bolted connection node are 45.3 and 44.78 kN,respectively.These values are less than the design values of 260.37 kN for tensile and 115.24 kN for shear,demonstrating the safety and reliability of the bolted connection nodes.Conclusions Based on the axial tensile and shear tests of bolted connection nodes,this study proposes a simplified calculation model for bolted connection nodes,using three degrees of freedom nonlinear springs to characterize the mechanical behavior of the nodes.In addition,the con-stitutive models for the tensile and shear behavior of the node springs are proposed to describe the elastic-plastic performance of the springs.A comparison of this simplified model with quasi-static tests of bolted connection prefabricated wall panels exhibits desirable consistency,demon-strating the accuracy of the simplified model for bolted connection nodes.A modeling method for the bolt-connected precast concrete wall panel structure system is proposed.Appropriate"plate elements"are used for prefabricated wall panels and floor slabs,"nonlinear spring"elements are utilized for nodes,and compression springs are placed at the corners of the horizontal seam to simulate possible corner concrete crushing.Using a demonstration project as an instance,seismic design is systematically conducted,illustrating that the inter-story drift angle of the structure met the code requirements while the bearing capacity of the bolted connection nodes satisfied the seismic requirements.This demonstrates that the bolt-connected precast concrete wall panel structure system has good seismic performance.
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