The increasing prevalence of terrorist attacks has heightened the demand for structural systems capable of withstanding blast-induced loads, particularly in critical infrastructure such as bridges, stadiums, and defense facilities. Square hollow steel sections (HSS) are extensively utilized in construction due to their favorable strength-to-weight ratio, torsional rigidity, and suitability for dynamic loading conditions. However, their performance under explosive threats remains underexplored, necessitating advanced numerical investigations to enhance safety. This study aims to evaluate the dynamic behavior of T-shaped welded connections between square hollow steel beams and conventional steel or concrete-filled steel tubular (CFST) columns under blast loading. The primary objectives include assessing the load-carrying capacity of these connections, analyzing dissipated energy across varying blast intensities and standoff distances, and determining maximum von Mises stresses to guide the design of blast-resistant structures. The research addresses the strategic importance of these connections in high-stakes applications, given the rising incidence of terrorist activities. The methodology employs ABAQUS software with the Explicit solver to simulate blast effects on two connection models: TC100-B50 (hollow steel) and TC100G-B50 (CFST composite). Blast scenarios are designed per UFC 3-340-02 standards, incorporating TNT masses of 15, 30, and 45 kg at standoff distances of 2.5, 5, and 7.5 meters, representing threats such as handheld bombs and small vehicle-borne improvised explosive devices (IEDs). The CONWEP algorithm models the airburst load, accounting for high strain rates. Finite element modeling uses S4R shell elements for steel and C3D8R solid elements for concrete in the CFST model. Material properties are defined with a steel elastic modulus of 200 GPa, an initial yield stress of 375 MPa (increasing to 575 MPa under high strain rates), and concrete with a compressive strength of 30 MPa, modeled using the Concrete Damaged Plasticity (CDP) approach. The Cowper-Symonds model is implemented to capture strain rate effects, enhancing the realism of dynamic responses. The blast pressure is applied as a hemispherical distribution over the connection surfaces, with a time step of 0.01 seconds post-peak load to capture residual effects. Mesh size is approximately 10 mm, ensuring computational accuracy.
Results reveal significant variations in structural response based on explosive mass and distance. Increasing TNT mass from 15 to 45 kg markedly elevates von Mises stress and dissipated energy. For instance, in TC100-B50-W0.15R2.5, the beam experiences a maximum stress of 575 MPa with 120 J of dissipated energy, escalating to 2075 MPa and 918 J in TC100-B50-W0.45R2.5. Conversely, increasing the standoff distance to 7.5 m (e.g., TC100-B50-W0.15R7.5) reduces stresses to 158.4 MPa (beam) and 93.2 MPa (column), with dissipated energy dropping to zero. In TC100G-B50, the concrete infill at 2.5 m with 45 kg induces a stress of 520.3 MPa, but its influence diminishes at 7.5 m. Time-history analyses indicate that peak stress in TC100-B50-W0.15R2.5 reaches 575 MPa at 0.002 seconds, stabilizing at 550 MPa, reflecting plastic residual behavior, while at 7.5 m, it peaks at 158.4 MPa and rapidly declines. For TC100G-B50-W0.45R2.5, beam stress reaches 468.4 MPa, column stress 520.3 MPa, and concrete stress 520.3 MPa at 0.003 seconds, highlighting concrete’s energy absorption role, which fades at greater distances. Parametric studies show that higher TNT mass expands the plastic zone (e.g., 120 to 918 J in TC100-B50), while reducing distance from 7.5 to 2.5 m increases strain rates, shifting behavior from elastic (158.4 MPa) to severe plastic (575 MPa). TC100-B50 exhibits greater flexibility at close range, dissipating up to 918 J, whereas TC100G-B50 offers enhanced localized resistance due to concrete, though it underperforms at longer ranges (beam stress drops from 724.5 to 53.5 MPa). These findings suggest composite connections are optimal for near-field blasts (< 5 m), while steel connections suit greater distances. The elasto-plastic behavior dominates in steel connections, with visco-elastic properties in CFST connections mitigating damage at close range, providing valuable insights for optimizing blast-resistant designs.