In the United States, previous studies on composite frames have mainly focused on the joints between reinforced concrete columns and steel beams. In a typical structural system composed of reinforced concrete columns and steel beams, small steel sections are embedded in the column, primarily for construction convenience rather than for load transfer. Other research has demonstrated that innovative structural approaches using post-tensioning and bolting can provide adequate strength and ductility for pure steel or encased concrete bending frames. However, research on the seismic performance of steel-reinforced concrete column-to-steel beam joints remains limited <1,2>. Under a project funded by the National Science Foundation of the U.S., Uang and Chou conducted tests on two full-scale steel-concrete columns and steel beam assemblies to evaluate the seismic behavior of their joint systems <3,4>. The test revealed that a joint with a weakened steel beam section and a stiffened plate showed promising seismic resistance. Nevertheless, the node design proposed by Uang et al. is still complex due to the need for field welding between steel beams and columns.
The proposed connection method for the steel-reinforced concrete column-to-steel beam bending frame aims to enhance constructability while maintaining excellent seismic performance. The main researchers in this paper have developed several innovative combined connection designs. A new hybrid steel-bending frame system is introduced, which uses end-plate bolting instead of field welding. Three specific connection methods are presented: first, a non-energized end-plate bolt connection between the steel-reinforced concrete column and the steel beam, where the steel beam is equipped with a factory-welded end plate that is bolted to the column’s flange. The bending moment is transferred through high-strength tension bolts, stirrups, and concrete support between the end plates and the built-in steel columns. Both stiffeners and continuous plates can be used, and stiffeners may also be added if necessary. The construction process involves: 1) placing the steel column, 2) bolting the steel beam end plate to the column, 3) tying the column reinforcement, and 4) casting the column (and plate) concrete.
Regarding the suggested design method, the selection of test specimens is recommended based on the capacity design approach. This method includes the following steps: 1) identifying a plastic hinge mechanism that occurs at the end of the steel beam and the base of the first column; 2) designing a concrete or steel-reinforced concrete column with sufficient flexural and shear strength to ensure the activation of this mechanism; 3) designing the end plate and selecting appropriate bolts; and 4) verifying the shear capacity of the beam-column joint according to two criteria: (1) comparing the principal tensile stress in the joint with the tensile strength of the concrete to determine if cracking will occur; and (2) if cracking is expected, further analysis of bracing and ferrule details to enhance the joint's shear resistance.
In conclusion, the steel beam-concrete composite bending frame system with an innovative joint structure is the central focus of this research plan. The proposed joints are pre-welded to the end plates and columns of the steel beams using bolting. These columns can include reinforced concrete, prefabricated, or cast-in-place concrete columns.
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