Experimental Research On The Seismic Performance Of Beam- To-column Connections In High Strength Steel Frame

Compared with normal steel structure, high strength steel structure also possesses the advantage of homogeneous material property, large stiffness, good plasticity and toughness, and convenient assembly etc. The employment of high strength steel in steel structure may diminish the section dimensions of members, thus reduce structural weight and decrease earthquake loading, which diminution of weld size resulting from may also improve the fatigue performance of steel structure. Hence high strength steel is gradually utilized in building structures and bridge structures around the world. On account of high ratio of yield strength to tensile strength and poor ductility of high strength steel, the seismic performance of structure is bound to take on a new feature. Hereby this paper conducts the research on seismic behavior of high strength steel base metal, welded joint and beam-to-column connections so as to provide the foundation for design of high strength steel structures in seismic fortification area.In terms of theoretical analysis, specifications on verification of panel zone shear capacity and limited ratio of width to thickness of panel zone in main nations’code are thoroughly comprehended. On the basis of four-spring model, design formula on ultimate shear capacity of panel zone in American standard is derived in detail, which entails to be validated in the subsequent experiment on connections.In respect of material property, firstly experimental study on mechanical property of domestic Q690GJ high strength steel and corresponding weld connection under monotonic loading is conducted. The results indicate that stress-strain curves of base metal and weld connection under monotonic loading have no evident yielding plateau and both compressive stresses are higher than tensile stresses. The nominal yield strengths of weld connection under tension and compression are far lower than base metal, while tensile strength of weld connection is merely slightly lower than base metal. Besides, ultimate tension strain of weld connection is far higher than base metal, which proves that ductility of welded joint is not really poor. Secondly experimental study on mechanical property of Q690GJ steel and weld connection under cyclic loading is launched. Results show that Bauschinger effect occure to all specimens and hysteretic behavior of base metal and weld connection under various loading systems are favorable, but both ductility decline compared with monotonic loading. The increase of steel strength due to cyclic hardening is not obvious and stress of skeleton curve dramaticly decline after strain amplitude reaches 1.75%. However, the increase of weld connection stress on account of cyclic hardening is considerable and cyclic softening only occurs when strain amplitude is relatively large.As far as structural members are concerned, two sets of beam-to-column connections in high strength steel frame are designed, and then experimental research on them under reversed cyclic loading is launched to reveal diverse yield mechanisms and failure modes. For exterior connections, the impact of steel strength and flange plate on their seismic behavior is investigated, with regard to interior connections, the influence of doubler plate and diagonal stiffener welded in the panel zone on their seismic behavior is inquired into. Results indicate that for exterior connections, bottom flange welds of normal welded-flange-bolted-web connections fracture early since mechanical properties of base metal and weld don’t match, which triggers that hysteretic curves obtained in test are not plump. But plastic hinge of welded-flange-plate connection appears at the end of the flange plate avoiding the brittle fracture of welds, hence bearing capacity and hysteretic behavior are greatly promoted. For interior connections, hysteretic curves attained are fusiform with the combined advantage of coverplate and reduced-beam-section. Shear plastic hinge occurs in the panel zone for unreinforced connections, while flexural hinge occurs in the deepest reduced beam section for connections with strengthened panel zone. The inference can be made from connection capacity that the reinforcing effects of doubler plate and diagonal stiffener are fairly similar.On the basis of experimental results, finite element analyses of all the connections under reversed cyclic loading are carried out by software ANSYS. Von Mises stress nephogram extracted from numerical simulation results can precisely reflect the formation process of connection plastic hinge under different failure modes. The calculating beam tip load-displacement hysteretic curves and corresponding skeleton curves agree well with experimental curves. The calculating hysteretic curves of exterior connections show that the higher steel strength is, the earlier stiffness softening occurs as displacement amplitude increases. The calculating moment-rotation hysteretic curves of interior connections manifest that the largest rotations of strengthened panel zones are merely 0.01-0.015rad, that is, ultimate rotational capacity of connection with reinforced panel zone sharply descend.In respect of the seismic performance of connections, stiffness deterioration of connection is not relevant to steel strength. Comparing theoretical values with numerical calculating values in elastic stiffness of interior connection panel zone and deducing effective thickness of the panel zone, the conclusion can be drawn that participation rate of doubler plate is 54% and participation rate of diagonal stiffener is 38%. In the light of experimental results and numerical simulation results of all the connections, all displacement ductility coefficients are less than 3 and equivalent viscous damping coefficients are less than 0.4. In contrast with normal strength connections, ultimate deformation capacity and energy dissipation capacity of high strength steel beam-to-column connections distinctly decline. Further analyzing numerical calculating energy dissipation indexes of all the connections, the effect of steel strength on energy dissipation is tiny and energy dissipation capacity of reinforced panel zone in interior connection drasticly drops with the transfer of primary energy dissipation parts.Comparing theoretical results with experimental results, specification on ultimate bearing capacity of beam-to-column moment connection in domestic code for seismic design of building is not applicable to high strength steel normal welded-flange-bolted-web connection, while specification on shear capacity of panel zone in American code AISC is comparatively conservative for high strength steel.