# Seismic Behavior and Modeling of Ductile Composite Steel-Trussed Concrete Beam to Column Joints
## 1 Introduction  
Prefabrication in civil buildings represents a production and construction method now widely used in the realization of great modern construction. This approach can provide a number of benefits, including a reduction in the time/cost ratio, efficiency, reliability, quality control, and sustainability of solutions in both new construction and retrofitting. Prefabricated or partially prefabricated structural elements include composite sections of beams and/or columns. Generally, partially prefabricated beams consist of a bottom plate made of steel or precast concrete and a steel truss that provides stability and strength during installation. Beams are completed with concrete casting on site, giving the monolith an adequate connection grade and strength for operating loads. The composite steel-concrete section of columns includes concrete-filled tube CFT columns characterized by a small-thickness steel tube filled with concrete; the steel tube confines the concrete, while the concrete reduces the instability of the steel tube. Structures designed and built with reference to the latest seismic codes must also satisfy requirements aimed at ensuring adequate behavior of the structure during earthquakes. Energy-dissipative structures must provide stiffness, strength, and ductility at ultimate limit states in order to dissipate some of the input seismic energy by developing plasticization in specific areas of the structure in accordance with the principle of capacity design adopted by NTC18 [1], Eurocode 3 [2], and Eurocode 4 [3]. In this paper, a type of ductile beam-column connection, which is very common in pre-cast reinforced concrete structures involving composite precast truss beams, is considered. The connection element must ensure the continuity of beam and column reinforcement through the joint. If the beams are partially precast, the addition of rebar is required after the beams have been placed and before the concrete completion casting is carried out. Mechanically, the joints must have adequate resistance to shear stresses and must preserve the integrity of the column to allow the formation of plastic hinges in the beam sections near the joints. Experimental tests on full-scale structural elements represent a fundamental instrument for the correct characterization of the mechanical, static, and seismic behavior of structures. For years now, experimental tests on beam-column joints referring to different types of joints, materials, and construction technology have been widespread and easily obtainable in the literature. These can be classified by modality of the test (monotonic, cyclic) and load application (quasi-static or dynamic tests) and by the type of joint (internal, external) and layout of specimen restraints. Most of the solutions found in the literature, adopted for beam-column connections, involve the use of integrative trusses where plastic hinge formation occurs [4; 5; 6]. These connections allow for the standardization of the construction and reduce realization time and costs. This concept is in contradiction with the requirements of current Italian code NTC18 [1], for which composite trussed elements shall not be used in dissipative zones. To experimentally reproduce the effective stress distribution on the specimens, a series of constraints consisting of cylindrical hinges were applied. Studies conducted by [5; 6; 7; 8] demonstrated the ductile behavior of the connection with the localization of plastic hinges at the ends of the beams when the joint is stressed by bending. Experimental testing can be characterized by the different points of application of the external force. In the case of external joints, the specimens are simplified as they involve a single beam, a lower column, and an upper column properly constrained [7; 9]. Other authors in their experimental tests have adopted a scheme involving a cylindrical hinge at the base of the column, a connecting rod applied at the end of the beam, and an actuator that applies the force at the top of the column [10; 11; 12].