Abstract: “Strong column and weak beam” is the current international mainstream seismic design concept of engineering structures. The investigation of existing earthquake disasters have shown that due to the complexity of earthquake action mechanism and the lack of understanding of the ultimate earthquake resistance of engineering structures, frame structures after strong earthquakes will not only appear “strong column and weak beam” damage caused by beam hinges, but also “strong beam and weak column” damage caused by the overall collapse, column hinges and local floor collapse.
In order to reasonably understand the various failure forms, the author first subdivided the traditional concept of plastic hinge into “compression hinge” and “tension hinge”, and pointed out that “tension hinge” is likely to cause the overall loss of the structure; then taking the steel-concrete composite frame structure as the object, the refined finite element seismic calculation model of composite frame structure based on solid element and shell element was established and used to carry out the ultimate seismic resistance of composite structure, and preliminarily discussed the effects of various horizontal seismic wave conditions on the displacement, stress, axial compression ratio and other time history responses of composite frame structure, as well as the distribution mechanism of plastic energy dissipation, the formation mode of plastic hinge and failure mechanism of frame beam column were preliminarily discussed.
The analysis results show that: 1) The tie bars stiffened column end reduce the slip between the steel tube and the concrete, thus increase the stiffness of the column and the frame, reduce the strain level of the steel tube and concrete, and increase the stress level of the steel beam. Under the action of seismic waves with intensity of 620 cm/s² and above, the “strong column” construction of the column end tie bars can significantly reduce the maximum inter story displacement angle of the composite frame structure. The column end tie bars yield when the horizontal seismic wave is close to the ultimate strength, and the frame beam end longitudinal reinforcement of concrete slab is generally not easy to yield; 2) The “strong beam and weak column” composite frame is shown as “constrained beam” and “energy dissipating column”, at this time, the frame beams strongly restrain the frame column, and the frame mainly consumes energy from the frame column. The beam ends only form “compression hinges”, at this time, the energy dissipation capacity of the frame depends on the frame columns. The “strong column and weak beam” composite frame appears as “energy dissipating beams” and “load-bearing columns”, at this point, the frame beam is weakly constrained to the frame column, the frame is based on the energy consumption of the frame beams so that the beam ends form a “compression hinge”. When the energy consumption of the beam ends reaches the limit, a “tension hinge” is formed, which causes the frame column slenderness ratio to increase, and then leads to accelerated failure of the frame, which is not conducive to the use of frame columns energy dissipation potential; 3) The “strong column” construction of the tie bars stiffened column end technology will improve the stiffness, plastic energy dissipation, and anti-collapse ability of the composite structure, in particular, the seismic capability of the 6-storey frame with energy dissipation column.