Buckling Behaviour Of Welded H-Section And Box Section Members With High-Strength Steel Under Axial Compression

High strength steel(HSS)has a wide application prospect in high-rise and long-span structures because of its high strength-to-density ratio.The high strength-to-density ratio of HSS makes the member and cross-section more slender.With it,the stability problem of HSS member is more prominent.However,the Chinese specifications for steel structural design is not applicable to HSS,and foreign specifications simply apply the specification of conventional mild steel(CMS)to HSS.At present,there is a lack of comprehensive and systematic research on the buckling behavior of HSS welded section members under axial compression.Relying on the National Key R&D Program of China[grant number 2018YFC0705501],the buckling behaviour of welded H-section and box section members with HSS under axial compression is studied.The welding residual stresses of eight H-section members and seven box section members made of HSS were measured by sectioning method.The influence of the crosssection size,yield stress,weld size and weld pass number on welding residual stresses was derived.Then,a three dimensional thermal elastic-plastic finite element model(FEM)was established by ANSYS to simulate the welding process of the specimen.To meet requirement of subsequent parameter analysis of buckling member,a parametric analysis was conducted for the width-thickness ratio of flange of H-section and the widththickness ratio of wall of box section using the verified FEM.The corresponding unified distribution models of residual stress of welded H-section and box section of Q420—Q960 steel were established respectively.The models were in good agreement with the measured residual stress results.Taking the yeild strength and slenderness ratio as parameters,twelve H-shaped compact section and ten box compact section long columns were designed.The overall geometric initial imperfections of the specimens were measured.The overall buckling tests were carried out,and the failure mode,axial deformation and axial strain of the specimen were analyzed.It was found that with the increase of steel strength and slenderness ratio,the ductility of the specimen became worse.The influence of steel strength was less than slenderness ratio.The greater the steel strength and slenderness ratio,the smaller the section entering the plastic stage under the overall buckling bearing capacity.The section of specimen Q960-OB-70 didn’t enter the plastic stage at all,resulting in the rebound of horizontal deformation from 116 mm to only 20 mm after unloading.The overall buckling factor of the specimen decreased with the increase of steel strength and slenderness ratio.The higher the steel strength was,the more conservative the specification was,and the influence of slenderness ratio was very small.A nonlinear FEM considering overall geometric initial imperfections and welding residual stress was established by ANSYS.The parametric analysis was conducted for steel strength and slenderness ratio using the verified FEM.Based on the results of parameter analysis,in the existing buckling curves of European,Chinese and American codes,the applicable curves for welded H-section and box section members with Q420—Q960 steel were recommended.And the modified formulas of overall buckling bearing capacity were put forward,when the specified buckling curves in specifications were used.Taking the yeild strength and width-thickness ratio of plate as parameters,twelve Hshaped slender section and twelve box slender section stub columns were designed.The local geometric initial imperfections of the specimens were measured.The local buckling tests were carried out,and the failure mode,axial deformation and axial strain of the specimen were analyzed.It was found that the ductility of the specimen increased with the increase of yeild strength and width-thickness ratio of plate,and the influence of width-thickness ratio was more obvious.The local buckling of all plates of the specimens occured at the same time.The "curve inflection point method" was used to determine the local buckling bearing capacity.It was compared with the ultimate bearing capacity.It was concluded that with the increase of yeild strength and width-thickness ratio of plate,the steel strength utilization rate of the specimen under local buckling decreased,while the post buckling strength increased.The steel strength utilization rate at failure also decreased with the increase of steel strength and width-thickness ratio of plate.A nonlinear FEM considering local geometric initial imperfections and welding residual stress was established by ANSYS.The parameter analysis of steel strength and widththickness ratio was carried out.The limit values of width-thickness ratio of the plate of the welded H-section and box section members with Q420—Q960 steel under axial compression were derived,and they were smaller than the values of the specifications.And the modify factor formulas of the width-thickness ratio limit value of the plate were determined.At last,the modify factor formulas for calculating the ultimate bearing capacity using the European,Chinese and American specificaions were determined.Taking the yeild strength and width-thickness ratio of plate as parameters,twelve Hshaped slender section and twelve box slender section long columns were designed.The overall and local geometric initial imperfections of the specimens were measured,and the interaction buckling tests were carried out.With the help of digital image correlation,the failure process,axial deformation and the coupling relationship between overall deformation and local deformation of all specimens were analyzed.It was found that the out of plane bulging deformation continuously distributed along the longitudinal direction of specimens appeared in the later stage of loading,which was similar with the first-order local buckling mode.At failure,only one out of plane bulging developed into the final interaction buckling failure under the coupling action of overall bending.With the increase of width-thickness ratio of plate,the local buckling occurred earlier than the overall buckling,and the influence of the yeild strength was very small.The steel strength utilization rate decreased with the increase of width-thickness ratio and yeild strength.A nonlinear FEM considering welding residual stress,overall and local geometric initial imperfections was established by ANSYS.The parameter analysis of yeild strength,slenderness ratio and width-thickness ratio of plate was carried out.It was concluded that for the calculation of interaction buckling capacity of welded H-section and box section members made of Q420—Q960 steel,the European and Chinese specificaions were too conservative.The American specificaion was suitable for H-section members,but the test value of box section member was close to the calculated value of American specificaion.Finally,the calculation methods of interaction buckling capacity were proposed based on the direct strength method.