# Seismic Behavior and Modeling of Ductile Composite Steel-Trussed Concrete Beam to Column Joints
## 2 Experimental Tests  
### Experimental Specimens  
Two full-scale models of precast composite truss beam-column joints, extracted from an external joint of a three-dimensional residential prototype building (Figure 1a), were tested with different constraint conditions at the base of the column. The 2-storey prototype residential building has plan dimensions of 11.40 m \(\times\) 5.40 m with an inter-storey height of 3 m and consists of two main frames in the x-direction and three secondary frames in the y-direction. All truss beams have dimensions of 30 \(\times\) 25 cm, while all columns are made of reinforced concrete with a 40 \(\times\) 40 cm cross-section, in the first case, and of concrete-filled tube with a 30 \(\times\) 30 cm cross-section and a tube thickness of 8 mm. The prototype building was designed according to the Italian seismic code NTC2018 [1], considering gravity loads and seismic action using a linear dynamic modal analysis with a response spectrum. Gravitational loads due to structural and non-structural elements (\(G_{1}\) = 2.4 kN/m\({}^{2}\); \(G_{2}\) = 2.5 kN/m\({}^{2}\) and 3.0 kN/m\({}^{2}\)) and accidental loads (\(Q\) = 2.0 kN/m\({}^{2}\) and 1.5 kN/m\({}^{2}\)) were applied on the first and second floors, respectively. The seismic action at Life Safety Limit State LLS was defined assuming a return period \(V_{R}\) of 50 years, peak ground acceleration \(a_{g}\) = 2.44 m/s\({}^{2}\), magnification factor \(F_{0}\) = 2.41, reference period \(T_{C}\)* = 0.36 s, soil type B, and behavior factor \(q\) = 2 (related to dissipative structure in medium ductility class \"CDM\"). The LLS design response spectrum is shown in Figure 1b. The specimens are representative of joints in framed seismic structures, where MTR-A beams (Figure 2) are connected to an RC column cast in situ (specimen 1) and to a CFT column (specimen 2), a system patented by Metal-Ri S.r.l. [[https://www.metalri.it](https://www.metalri.it)]([https://www.metalri.it](https://www.metalri.it)) (accessed on 8 October 2023). Figure 1: (a) Prototype building and (b) design response spectrum (LLS). In particular, both specimens have the same composite precast truss beam, which consists of a 30 \(\times\) 25 cm rectangular cross section. In the central part of the composite beam, the steel truss ends immediately before the column element, and the two end parts consist of a simple reinforced concrete section in which six 24 mm steel rebars are placed at the ends of the beam and anchored in the joint panel, providing the required strength and ductility for the beam section. Additional masses constituted by steel plates were placed on the beams to simulate gravitational loads. Furthermore, external forces such as axial stress in the column and a cyclic horizontal force simulating an earthquake are applied to the model by hydraulic jacks. Additional masses, producing flexural and shear effects, allow the study of possible brittle mechanisms related to shear stresses. The specimens differ for column type, length, and base constraints. In specimen 1, the reinforced concrete column (Figure 3a) has a rectangular cross-section of 40 \(\times\) 40 cm, a length of 3.00 m, and is hinged at the base (Figure 4a). In specimen 2, the composite concrete-filled steel tube column (Figure 3b) has a rectangular cross-section of 30 \(\times\) 30 cm, a length of 4.50 m, and is fixed to the base (Figure 4b). Specim 1 was made up of C28/35 concrete, while specimen 2 was made up of C40/50 concrete. For both specimens, reinforcement rebars and steel elements are referred to as B450C and S355, respectively. The moment-rotation of the cross-sections of the ends of beams from the design is reported in Figure 4c. In the design, the resistant section in the proximity of the beam-to-column joint is considered to be made up only of concrete and the longitudinal reinforcement, neglecting the contribution provided by the upper chord and the bottom plate of the steel truss. The constitutive material behavior is a simplified bilinear behavior for steel and concrete.