| Timber is an ideal renewable building material.However,the inherent limited design strength of timber,especially in tenson,limits its use in long span structures as well as in designing tall timber structures that require heavy bearing capacities.In order to expand the application scope of wood structure,give full play to the performance advantages of the material,timber can be combined with other materials to improve its bearing capacity.Steel structures are characterized by low self-weight,high bearing capacity,rapid construction progress,and good seismic behaviour,but these structures are sometimes susceptible to instability due to the use of thin-walled members.In current study,superior strength and ductility offered by steel is used to complement bulky yet light-weight timber sections to develop a new type of steel-timber composite(STC)member by connecting a steel H section with glulam using shear connectors.In this proposed composite member,steel serves as the major load-bearing member,whilst timber renders lateral stiffness for steel,increase the sectional height of the member,and improves its bearing capacity.In comparison to bare steel members,STC members have improved stability and bearing capacity,high degree of assembly and low cost of construction and maintenance,which eventually have a relatively positive impact on the environment.Experimental study,theoretical analysis and numerical simulation were carried out on the material properties,connections bearing capacity and bending performance of various STC members.Based on test results and analysis,the following conclusions can be drawn:(1)The physical and mechanical properties of Larix gmelinii,steel and shear connectors were tested,the basic mechanical strength and elastic constant of the materials were obtained by statistical analysis.Phenol resorcinol formaldehyde(PRF)adhesive has good water resistance and aging resistance,which can meet the requirements of timber bridge environment.Compared with one component polyurethane(PUR)adhesive,it is more suitable for glulam exposed to outdoor environment.(2)The compressive specimens showed obvious brittleness when they were damaged.The embedding yield strength of self-drilling screw(SDS)compressive specimens was much smaller than that of bolt specimens,it could be seen that the pin diameter should not be taken as the calculation parameter,but also the influence of pin type on the embedding yield strength should be considered during calculating the theoretical value.The embedding yield strength decreased with the increase of bolt diameter,which was basically in linear proportion and tended to be stable at 8mm diameter specimen.The calculation results of the related literatures and national standards were compared with the test results,although the calculation results of Eurocode 5 were conservative,the correlation was the highest.Based on the comparison of the relevant literatures and standards with test results in this study,the calculation model of the embedding yield strength in Eurocode 5 was revised.The revised formula had a stronger ability to predict the embedding yield strength of Larix gmelinii.(3)For all considered specimens of push-out tests,“two-hinge” yield mode was observed but the failures of joints were characterized by the simultaneous occurrence of embedding strength failure of glulam flanges and bending failure of shear connectors.The ultimate load of connections was directly proportional to the bolt diameter but was inversely proportional to the bolt spacing.The timber flange produced the highest ultimate load for a thickness of 40 mm.With the increase in glulam flange thickness,the yield load initially increased,at a wood thickness of 50 mm,the yield load reached its peak.SDS connected joints performed better than the bolt connected joints both in terms of stiffness and ductility.Experimentally obtained key design parameters were compared against those predicted using GB/T 50005-2017,NDS-2018 and Eurocode 5.It is worth noting that the code predictions are not suitable for design application due to significant conservatism or over-prediction of connection response;this shows a clear gap in the existing knowledge on STC connections.Foschi formula was used to predict the nonlinear load-slip behaviour of considered specimens and analytical predictions showed reasonable agreement with test results.(4)In the process of four-point bending,timber slabs and steel beams showed good composite action,and the STC beams failed showing enough ductility.Increasing the thickness and width of the timber slabs can effectively limit the lateral deformation of the specimens,improve the bearing capacity of the specimens,and provides a secant stiffness to the STC beams.It was observed that for the whole cross section of STC beams,the plane section assumption is not applicable,but the strains on timber and steel seemed to satisfy the plane section assumption individually.According to the formula of the effective bending stiffness and deflection,the deflection value of STC beams in elastic stage can be calculated according to the elastic theory.Compared with other methods,?(Gamma)method has been observed to better reflect the deformation capacity of STC beams.The theoretical derivation of the elastic bending capacity of STC beams shows that the calculated value of elastic theory is in good agreement with the experimental value,and the error is between 6.4and 13.5%,which meets the needs of practical engineering application.(5)The established finite element(FE)model,selected material parameters and contact element model could better simulate the bending performance of STC beams,which has certain guiding significance for the prediction of bending capacity of STC beams.The connection of bolt connector to steel-timber interface is a kind of flexible connection,resulting in partial composite action of structure,and its mechanical properties are between full and no shear connection.With the increase of the thickness and width of timber slabs,the composite efficiency decreases gradually.It was determined that the distribution density of the shear connectors and the mechanical characteristics of the connection can affect the overall mechanical behaviour of STC beams.To improve the bearing performance of STC beams,shear connectors with a larger stiffness can be selected.In addition,the spacing of connectors is inversely proportional to the bearing capacity of STC beams.In practical application,the material utilization rate should also be considered to make the layout of connectors more economical.(6)Based on obtained test results and subsequent analysis it is obvious that hygrothermal aging,artificial simulated acid rain and seawater aging,and freeze-thaw cycles aging treatment could significantly weaken the mechanical properties of glulam.Therefore,it is suggested that use of glulam must be carefully and thoroughly assessed prior to using for construction.For hygrothermal and freeze-thaw cycles aging,it is suggested that the shear strength of the adhesive layer can be taken as an important index to determine the durability of wood,whereas for artificial simulated acid rain and seawater aging,the static bending strength can be used as a reliable index. |
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