PSI - Issue 78

Agnese Natali et al. / Procedia Structural Integrity 78 (2026) 2015–2021

2020

4. Comparison between Concrete Infill and Composite Configurations As part of the European research project LASTTS (LASer cutting Technology for Tubular Structures), a series of experimental tests was carried out to investigate the seismic behaviour of CHS-to-I beam joints with through plates. Several configurations were tested, including Bare Steel joints, joints with Concrete Infill (CI) of the column and fully Composite (CU) joints featuring both internal concrete and a cast-in-place concrete slab. An additional configuration included a seismic protection strategy for the node. This chapter focuses on the comparison between the CI and CU configurations, aiming to assess the contribution of the concrete slab to the global performance of the joint. The Concrete Infill (CI) and Composite (CU) configurations both aim to enhance the seismic performance of CHS to-I beam joints by mitigating local instabilities in the joint region, but they do so through different mechanisms. The CI configuration, which involves filling the CHS column with concrete, provides a significant improvement over the Bare Steel setup. The infill effectively confines the CHS wall, reduces local wall distortions, and delays failure mechanisms such as punching and tearing at the plate-to-column interface. This results in a stiffer response, greater energy dissipation capacity, and enhanced overall strength, as previously demonstrated in both experimental and numerical investigations (Fabbri et al. (2025)). In contrast, the Composite configuration builds upon the CI setup by adding a cast-in-place concrete slab connected to the steel beams through shear studs. This additional component results in an approximate 10% increase in strength and stiffness compared to the CI specimens, primarily due to enhanced flexural capacity and improved moment redistribution throughout the slab. However, it also introduces more complex local interactions. While the internal concrete continues to mitigate punching effects at the column interface, the composite action shifts some stress toward the slab – column connection, resulting in extensive cracking in the slab — especially near the CHS face under hogging bending.

Fig. 8. Force-displacement comparison between Concrete Infill and Composite configurations.

Strain gauge measurements and visual inspections confirmed the emergence of a strut-and-tie mechanism within the slab in the CU specimens, along with a redistribution of forces between transverse and longitudinal reinforcement. While this redistribution aids in resisting lateral loads, it also intensifies cracking around the node at higher displacement levels. Numerical simulations supported these observations, showing that the internal concrete core in both CI and CU cases significantly reduces joint deformation compared to the Bare Steel configuration. However, the CU model captured more complex cracking behaviour in the slab and greater sensitivity to detailing at the slab-column interface.