Resistance Of Bolted Connections Under The Combined Effect Of Tension And Shear In Fire

Connections are the critical components of a structure because they transfer theinternal forces from beams to columns, and their failure can lead to progressive collapseof the structure. So it is very important in structural fire safety design to study the actualbehavior of the connections. At ambient temperature, connections are mainly loaded bysimple and constant loads such as shear forces and moments under small deformations,and there have been a lot of research on the connection behavior under shear andmoment actions. In fire situation, steel beams will develop large deflection and underextreme occasions, develop catenary forces along the beam axis to help resist theapplied load. As a consequence, the connection should have sufficient rotationalcapacity to develop compatible deformation with the beam. Also, as the beam endinternal forces change significantly, the connections must be able to resist all the loadscoming from the beam end, including the tying force. Therefore, connections aregenerally loaded to failure by combinations of tension and shear forces. Currentresearch on connection fire behavior generally followed the method used for ambienttemperature study, with the main purpose to obtain the moment-rotation-temperaturecurves. And for that purpose, experimental study or finite element simulation of isolatedconnections were generally performed, without considering the actual interactionsbetween the connections and other structural members in fire. Considering this situation,it is proposed to study the behavior of connections at elevated temperature consideringtheir actual loading state near failure, which means to consider the effect of combinedmoment, shear and tension. This thesis addresses the interaction between the shear andtension resistance for common bolted connections at elevated temperature. The maincontents of this dissertation are:(1) Transient fire tests were conducted on flush endplate connection and doubleweb cleat connection to study their resistances in fire when subjected to differentcombinations of tension and shear. The furnace temperatures were set to increaseaccording to a specified bilinear curve. With the test, the temperature distributions,failure modes, connection resistances and connection deformations were studied. (2) The three-dimensional finite element models were established for flushendplate connections using general finite element program. Both geometricalnonlinearity and material nonlinearity were considered in this model as well as thecontacts between various components of the bolted connections. The analysis resultsshowed a good agreement with the experimental results. With the finite elementanalysis results, the connection failure procedures and failure mechanisms wereobtained. Parametric study was performed to obtain the fire resistances for an extensiverange of load combinations of tension and shear using the finite element model.Reasons for the interactions between shear and tension in the fire resistance ofconnections were discussed and conclusions were drawn regarding the tension and shearinteractive relationship at different temperatures.(3) Simplified analytical model of flush endplate connection was established basedon the component based method. The simplified model was validated against the testand the three-dimensional finite element analysis results. Bolt tensile strength reductionwas applied to the model to consider the effect of shear force, and based on this modelwe can obtain the comprehensive performance of connection when subject to tension,shear and moment conveniently. The effects of shear and tension forces on connectionfire behavior was further studied, and design recommendations for steel structure in firewere presented.