Local Stability Design Method And Fatigue Property Of Buckling-restrained Braces With H-section Steel Core

The technology for manufacturing buckling-restrained braces(BRBs)with Hsection steel core restrained by assembled restraining members not only can be used to fabricate new BRBs but also can be used to reinforce the existing H-section member in structure.Because the H-section steel core is not well restrained by the restraining member,the mechanical stability and ultimate deformation capacity of the assembled BRBs with H-section steel core presented previously are not satisfactory.As one of the common failure modes,local buckling of BRBs has been widely studied in recent years.The problem is that most of researches were focused on BRBs with flat steel core,while researches on BRBs with section steel core were rather limited.The fatigue life of BRB is governed by the strain amplitude of inner core.Although it is relatively safe to estimate the fatigue life of BRB specimen with the strain amplitude of inner core obtained by adding the maximum bending strain to nominal strain amplitude,it should be noted that this method not only leads to that the fatigue property of BRB can not be fully utilized but also causes higher steel consumption and fabrication cost.In view of the problems mentioned above,an improved type of assembled BRB with H-section steel core named HBRB is presented in this dissertation,rational construction details for HBRB are investigated based on quasi-static tests,the local stability design method and the modified fatigue life estimation method of HBRB are proposed through theoretical and numerical analysis.The main research contents and conclusions of this dissertation are as follows:(1)An improved type of all steel BRB with an H-section steel core restrained by 2 U shape steels and 2 restraining plates assembled by high strength bolts named HBRB is proposed in this dissertation,based on quasi-static tests on 5 HBRB specimens,the effect of the arrangement of end ribs?the existence of stopper as well as the welding technology on the sismic performance of HBRBs were investigated.It is found that: When the web and flanges are well restrainted by the restraining members,the low order local buckling of the H-section steel core can be avoided,and thus the local stability performance as well as the ultimate deformation capacity of HBRBs can be significantly improved;Seting a stopper at the midspan of inner core?adopting multiple welds thchnology with the base weld using lower welding current and employing gradually changed cross section around the end of stiffeners can help to improve the cumulative deformation capacity of HBRBs.(2)Buckling mode development of H-section steel core is discussed based on buckling theory of plate and methods for calculating the minimum half-wavelength of high-order local buckling of web and flanges are put forward.Theoretical analysis and simulation results confirm that the classic buckling theory of plate should be adopted to obtain the minimum half-wavelength of local buckling for the web and flanges while the effect of lateral cantact force can be ignored;Theoretical analysis and test results verify that Lundquist's theory is more reliable for evaluating the half wavelength of higher-order local buckling for H-section steel core;(3)By introducing a reduction factor to consider the decrease of contact force caused by the mutual restraing effect among the web and flanges,the local stability design method of HBRB is proposed and vefied by test results in this dissertation.The effect of bolt spacing?clearance to thickness ratio and width to thickness ratio of flange on the reduction factor are investigeated numerically,it is observed that the value of the reduction factor linearly grows with the increase of width to thickness ratio of flange and with the decrase of clearance to thickness ratio,while the bolt spacing can hardly affect it;Test results show that the local stability design method proposed in this dissertation can effectively prevent the local buckling of HBRB.(4)A modified method for calculating the strain amplitude of H-section steel core is put forward theoretically and numerically laying a basis for the presentation of the fatigue life evaluation method of HBRB taking bending strain into account.Theoretical analysis indicates that the buckled doformation of inner core can hardly be straightened in elasto-plastic region for the existence of residual strain,hence,a reduction factor should be introduced for the maximum bending strain to obtain the total strain amplitude of H-section steel core,numerical simulation results show that the reduction factor linearly decreases with the increase of nominal strain amplitude,while the impact of clearance to thickness ratio as well as the width to thickness ratio of flange are rather limited.