Investigation On Tension Behavior Of One-side Bolted T-stub Under Normal Temperature And In The Fire

Bolt holes with internal thread could be used as nuts to fix the high-strength bolts,which had been widely used in the mechanical industry.However,structural behaviors of beam-box column endplate connections using this kind of bolt under normal temperature and in the fire had no mature research results yet.Tests and finite element method were carried out to investigate tension behaviors of one-side bolted T-stub under normal temperature and fire situations.The one-side bolted T-stub was studied by experimental and theoretical methods.The failure modes,the deformation and the tension bearing capacity of the T-stub were presented.Based on three failure modes of the bolted T-stub described in the current Eurocode 3,considering the potential failure of the hole threads,two new failure modes and their corresponding design methods were proposed,which were:hole thread failure,and T-stub flange yielding with hole thread failure.Studied parameters including the bolt pre-tension force,the thickness of the T-stub flange,the existence of the standard nut and the screw depth of the bolt,the strengthening effects of the backing plate on the T-stub with thin flange were also studied.The bolt pre-tension force had little influence on the ultimate tension strength,but it could improve the initial stiffness of the connection.The ultimate tension strength and tension stiffness of the T-stub increased with the increase in flange thickness.The existence of the additional nuts did not improve the yield strength of the T-stub;however,it could prevent the separation of two T-stubs after flange yielding.Results also showed that adding backing plates could effectively improve the tension strength of the T-stub.However,for T-stubs with same screw depth,it was more efficient to increase the flange thickness directly.The finite element model of one-side bolted T-stub was established,and it was verified by the tested results.After verification,the deformation,the failure modes and the tension strength of the T-stub obtained from finite element model agreed well with the tested results.The failure mechanism of one-side bolted T-stub under tension was investigated by the verified finite element model,and parameter studies were conducted to study the T-stub connection.Results showed that the bolt diameter,the bolt spacing and the T-stub plate strength could affect failure mode and tension strength of the one-side bolted T-stub under tension,the undesired thread failure could be avoided by limiting the value of the bolt spacing.Tension behavior of one-side bolted T-stub in the fire was studied by the finite element model,totally three different thicknesses of T-stub flange and 8 temperatures were considered.Results showed that the high temperature did not affect failure mode of the T-stub connection.With the increase of temperature,the tension strength and the stiffness of the T-stub reduced,and the reduction degree was consistent with that the high temperature on the reduction of the steel properties.After being verified by the experimental and numerical results,the design method proposed in this paper could accurately predict failure mode of the one-side bolted T-stub under tension,and the design equations could be used to calculate tension strength of the connection on the safety side.