Research On Mechanical Performances Of Timber Members Loaded Perpendicular To Grain By Bolted Connections

The modern timber structure is a kind of low carbon and green building in the whole life cycle.The load transferring and architecture modeling design of modern timber structures are inseparable from the use of connections with metal fasteners.The bolted connections play a decisive role in the overall behavior of modern timber structures.The working tensile stresses perpendicular to the wood grain are inevitable in the bolted connections.And the splitting failure of timber members loaded perpendicular to grain by bolted connections seriously affects the safety of modern timber structures.It is particularly important to optimize the design of connections in order to avoid brittle failure of bolted connections for timber structures.Therefore,the study on the mechanical behaviors of bolted connections for timber structures loaded perpendicular to grain will promote the updating and improvement of the design theory of bolted connections for timber structures,which has important theoretical significance and engineering practical value.Theoretical analysis,experimental research and numerical simulation were combined to study mechanical behaviors of bolted connections under loading perpendicular to grain in the dissertation.The corresponding strength,the elastic constants,and the fracture energy of Dahurian larch were firstly tested.Secondly,based on the slip line theory and the calculation model for local compression strength of timber members loaded perpendicular to the grain,the analytical model for calculating the timber embedding strength perpendicular-to-grain was studied.On the basis of the experimental study,digital image correlation and finite element numerical simulation,the analytical model for calculating the embedding strength perpendicular-to-grain of timber was modified and optimized.Considering the influence of the number of bolts,theoretical analysis and experimental study on the load-bearing capacity per shear plane of steel-to-timber connections were carried out in case of perpendicular-to-grain loading.Thirdly,the failure mechanism and calculation method of the splitting capacity for timber beam loaded perpendicular to grain by bolted connections were presented through the fracture mechanics theory and experimental research.Finally,according to the calculation method of European yield model load-bearing capacity and splitting strengthen,the design method of timber members loaded perpendicular to grain by bolted connections was studied.And the relations between theoretical results and test results were analyzed.The following conclusions can be drawn.1.Based on the calculation model for the local compression strength of timber members loaded perpendicular to the grain,a semi-empirical model for calculating the embedding strength perpendicular-to-grain of the timber was proposed,considering the transfer path and affected range of embedding stress as well as the local contact behaviors between the dowel and the timber.Based on the theory of European yield model,a calculation model of load-bearing capacity of bolted connections for timber structures loaded perpendicular to grain was presented.2.On the basis of experimental study of standard small specimens and simply supported beam specimens for embedment,the failure mechanism of the embedding perpendicular-to-grain of timber members with bolted connections is revealed.It is further confirmed that the method of calculating the embedment strength perpendicular-to-grain stipulated in relevant standard specifications is not suitable for timber beams loaded perpendicular to rain by bolted connections.The standard test method for the embedding strength perpendicular-to-grain of the timber was proposed.The reliability of the model for calculating the embedding strength perpendicular-to-grain was verified,and the model was further optimized.The average ratio of predicted embedding strength to mean experimental embedding strength is 1.05.By introducing the parameters related with the effective number of bolts,the model for calculating load-bearing capacity per shear plane of steel-to-timber connections under loading perpendicular to grain was optimized and verified.The average ratio of predicted bearing capacity per shear plane of steel-to-timber connections to mean experimental bearing capacity is 1.08.3.The results of DIC tests and ABAQUS finite element numerical simulation present the affected range and transfer path of embedding stress perpendicular to grain,which confirm the reliability of the semi-empirical model for calculating the embedding strength perpendicular to grain of the timber.And the DIC test results,simulated results using ABAQUS and experimental results are consistent with each other.4.According to the theory of linear elastic fracture mechanics and beam on elastic foundation,considering the impact of the number of connections,two semi-empirical models for calculating the splitting capacity of timber members loaded perpendicular to rain by bolted connections were proposed.According to the experimental study on splitting strengthen of timber beams loaded perpendicular to rain by bolted connections,the splitting failure mechanisms are revealed,and the influence rules of cross-sectional size,effective span height ratio,connections relative height,height and width of connections,the positions and the number of connections as well as the diameter and the number of bolts on the splitting capacity are obtained.The semi-empirical model based on the linear elastic fracture mechanics theory was selected as the model for calculating the splitting capacity of the timber members loaded perpendicular to rain by bolted connections.The average ratio of mean experimental splitting capacity to predicted splitting capacity is 0.95 for the sawn lumber beams,and the average ratio is 1.01 for glulam beams.And the range of the connections relative height was suggested to be 0.2???0.75.5.Through the experimental study on the load-bearing capacity of the simply supported glulam beams loaded perpendicular to rain by bolted connections,the semi empirical models for calculating the load-bearing capacity of timber members loaded perpendicular to grain by bolted connections with the European yield model and the splitting failure were verified.The average ratio of mean experimental load-bearing capacity to predicted bearing capacity is 1.06.By taking the smaller of the load-bearing capacity based on European yield model and splitting capacity of timber beams loaded perpendicular to grain by bolted connections,the splitting failure in practical engineering application can be avoided on the basis of the design optimization.The proposed semi-empirical model for calculating the load-bearing capacity of timber members loaded to grain by bolted connections provides a reference for the further revision of Chinese standard for design of timber structures GB 50005-2017.