Study On Ultimate Bearing Capacity Of Steel-concrete Composite Beam And Column Members At Low Temperature

This thesis studied axial compressive behaviours of steel tubular and concretefilled-steel-tubes(CFST)stub columns at low temperatures.Also,the research investigated flexural performance of steel-concrete-steel composite beams(DSCBs)with headed studs in cold regions such as arctic environment.Thus,the working mechanisms of these members can be identified and design reference will be provided for bridge piers in cold regions and ice resistance walls of polar oil and gas drilling platforms.The main contents and results are that:(1)For steel tubular stub columns,28 units subjected to axial compression at low temperature were tested.Then,the failure modes were analysed thoroughly as well as the effects of parameters,such as low temperature level and radius-to-thickness ratio(or width-to-thickness ratio),on ultimate bearing capacity,initial stiffness and ductility index.Meanwhile,the code formula was modified to account for the low-temperature effect.An important observation made was that,the low temperatures cause a modest increase,less than 10%,in the ultimate bearing capacity but a reduction in ductility.Moreover,the local bucking tends to occur earlier and the classification of section in modes is unsafe at low temperatures.(2)For CFST stub columns,besides length-to-width ratio,concrete strength grade and the types of cross section,other parameters of steel tubular stub columns such as low temperature at different levels,radius-to-thickness ratio(or width-to-thickness ratio)were also thoroughly investigated.Based on test results of 23 CFST columns under axial compression at low temperatures,the analysis on failure modes and effects of parameters was conducted.Research showed that,when temperatures reduced from 20℃to-80℃,the ultimate bearing capacity of circular and square CFST columns increased by 12% and 28%,respectively.In view of reliability and safety,modified EC4 was recommended to calculate the ultimate compressive bearing capacity CFST stub columns at low temperatures.(3)For DSCBs,12 members under four-point bending were well designed and tested with the different parameters including low temperature level,the thickness of steel faceplate,shear span ratio and spacing of headed stud.Apart from analysis on failure modes and effects of parameters on strength and stiffness,improved calculation methods of DSCBs were also proposed.It was found that DSCBs all failed in flexure no matter the shear span ratio is as small as 2.9 or the thickness of steel plate is as thick as 5.50 mm.Low temperature modified modes considering buckling of steel plate under compression can offer reasonable prediction.