Experimental And Theoretical Research On Fire Resistance Of Steel-to-laminated Bamboo Bolted Connections With Slotted-in Steel Plates

Laminated bamboo is a typical engineered bamboo with many special advantages.The lightweight,high strength,low glue content,and stable material properties guarantee it a good prospect in engineered structures.But due to the lack of fire resistance theories and design method in modern engineered bamboo and timber structures,people are skeptical about its fire safety performance,especially the bolted connections.The fire resistance design of modern engineered bamboo and timber structures has always been an obstacle.The behavior of steelto-laminated bamboo bolted connections with slotted-in steel plates have been studied in the paper,including the material properties of laminated bamboo at elevated temperatures,the dowel-bearing strength,the load-carrying capacity,and fire resistance of the connections.The results could complement and improve the fire resistance design of laminated bamboo structures and have important practical significance to promote the green building and the new building industrialization in China.The specific research work and results in the paper are as follows:(1)The steady-state test of laminated bamboo at elevated temperatures employed the TGA-DSC and Hot Disk method to determine the mechanical and thermal properties and the constitutive relation of laminated bamboo.The reduction factors for the tensile strength,compressive strength,and elastic modulus with temperature are established.Compared with timbers,the specific heat of laminated bamboo has two peak points,lower than that of timbers in the temperature range of 100～120℃ given in Eurocode 5.The density ratio of laminated bamboo decreases with temperature increasing from 20℃ to 500℃,and reaches a definitive value above 500℃.It shares the similar tendency with the bamboo scrimber and timber.The difference of conductivity of laminated bamboo for different grain directions is obvious and the conductivity ratio is 1.93～4.11,which is between the values for conductivity parallel and perpendicular to the grain direction.It is larger than that of timber in the temperature range of20～100℃ but the difference becomes less obvious with the temperature increase.(2)The laminated bamboo exposed to one-side and multi-side fire is conducted to study the charring property and compared with laminated bamboo members under different stress states.Based on the three common statistical functions,the charring rates of laminated bamboo perpendicular to the grain in the edgewise section and flatwise section are provided with 96%reliability.The temperature of the charring front is appropriately 280°C,and the function between temperature and distance is established in the temperature range of 20～280°C in the decomposition layer and the values of zero-strength layer thickness in tension and compression are suggested in the paper.(3)The bending performance of bolts,dowel-bearing strength of laminated bamboo at normal and elevated temperatures,and the plate specimen test are conducted to determine the key parameters of bolted connections.The dowel-bearing strength of laminated bamboo under different temperatures and grain directions are determined and calculated as bolt diameter and compressive strength,which provides the data for the bolted connections at normal and elevated temperatures.(4)The load-carrying capacity of steel-to-laminated bamboo bolted connections with slotted-in steel plates parallel to grain and perpendicular to grain are investigated through test specimen thickness,bolt diameter,the spacing of row and column,and the number of bolts.Compared with the two calculated models,the rigid-plasticity model has better applicability.By introducing the check coefficient,the calculation method is provided for the connections parallel to the grain.The effect of loaded edge distance and end distance of the bolts on the load-carrying capacity is studied.And the three types of connections are provided,brittleness,low ductility,and moderate ductility based on ductility ratio interval.Through the comparison between different theoretical models,it is found that the Van der Put model has the advantage of good accuracy and simple form.(5)The elastic-plastic model and cohesive element model are established to simulate the load-carrying capacity of bolted connections and development of cracks by conducting the finite element simulation of bolted connections at normal and fire conditions.The simulation results are close to the test results,which verified the accuracy of the model.By comparing the temperature distribution in the same position considering the effect of the bolt or not,with the exposure time increase,the effect of the bolt on the temperature transfer of surrounding becomes more significant.With the load ratio increases,the fire resistance of bolted connections decreases gradually.Therefore,it can improve the fire resistance of the connections by decreasing the load ratio appropriately.(6)For the steel-to-laminated bamboo bolted connections with slotted-in steel plates in the fire,the temperature distribution in different exposure time and position is determined.Combined with the reduction factor for dowel-bearing strength with temperature and Johanson yield theory,the load-carrying capacity of bolted connections can be calculated.And the fire resistance of bolted connections can be determined by the reverse calculation of load-carrying capacity through repeated iteration.In the paper,based on the study of basic properties of laminated bamboo at elevated temperatures,the behavior of bolted connections is investigated to determine the load-carrying capacity and fire resistance of bolted connections.Based on the fire test,model modification and theoretical analysis,the results play an important role in the extension and application of laminated bamboo structure,and further promotes the development of green building in the prefabricated building.The innovation points of this paper are as follows:(1)By steady-state test method,the constitutive relation model of laminated bamboo under different grain directions and temperatures are established and the relationship between thermophysical parameters,e.g.specific heat and conductivity,and the temperature is proposed,which can provide the parameter basis for the fire resistance of laminated bamboo members and the finite element simulation analysis.(2)Based on the standard fire test method and typical probability distribution function,the charring model of laminated bamboo perpendicular to the grain in the edgewise section and flatwise section are provided.And the value of zero-strength layer thickness is suggested according to the static equivalent principle,which could provide important parameters for calculating the residual section of laminated bamboo members in fire-resistant design.(3)The load-carrying capacity of steel-to-laminated bamboo bolted connections with slotted-in steel plates parallel to grain and perpendicular to grain are investigated,and the calculation method of fire resistance of bolted connections is established based on the temperature distribution and reduction factor for dowel-bearing strength.(4)By comparing the results of the three different rotation centers and the calculation methods corresponding to the two failure modes,the rotation center is determined to take the intersection point of the beam end and the bottom bolt as the center.The lower limit of the loadcarrying capacity of bolted connections is calculated according to the splitting failure,and the upper limit of the load-carrying capacity is calculated according to the ductility failure.