Introduction

Beam-column joints are critical components in reinforced concrete (RC) frame structures, especially in seismic regions where they are subjected to complex loading conditions such as bending, shear, and torsion. The integrity and ductility of these joints significantly influence the overall seismic performance and structural resilience of buildings. Conventional concrete often struggles to meet the demands of high-performance applications, especially in joint zones where stress concentrations are high.

Ultra-High Performance Concrete (UHPC), characterized by its superior compressive strength (>150 MPa), high tensile ductility, low permeability, and enhanced durability, presents an innovative alternative to traditional concrete for such demanding applications. When properly utilized, UHPC allows for more compact joint designs, reduced congestion of reinforcement, and improved long-term performance.

However, even with UHPC’s advanced properties, reinforcing the beam-column joint region is crucial to enhance strength, stiffness, and energy dissipation capacity. The use of steel angles and bolts as external reinforcement offers a promising hybrid solution. This method provides a practical means of retrofitting or enhancing the mechanical performance of joints by introducing steel elements that can directly resist tension, shear, and flexure forces.

Explanation of the Analysis

The analysis of UHPC beam-column joints reinforced with steel angles and bolts involves a detailed investigation of the mechanical behavior under various loading scenarios. The steel angles and bolts function as external reinforcement mechanisms that:

1. Increase Shear Resistance: The steel angles, typically installed along the joint edges, act as shear keys and directly carry part of the joint shear force, thereby reducing demand on the UHPC core.
2. Enhance Confinement: The bolts and angles provide external confinement to the joint region, which increases the ductility and prevents brittle failure modes.
3. Improve Load Transfer: Through proper anchorage and bolted connections, the load transfer mechanism between the beam and column is improved, ensuring better load path continuity.
4. Delay Crack Propagation: The external steel assembly helps in distributing the stresses more evenly, which reduces localized stress concentrations and delays the onset and propagation of cracks.

Analytical and Experimental Considerations

The analysis typically includes:

• Finite Element Modeling (FEM) of the joint, incorporating nonlinear material behavior of UHPC, contact interface between steel and concrete, and bolt pretension if applicable.
• Experimental Testing under monotonic or cyclic loading to simulate earthquake effects and to validate numerical models.
• Parametric Studies to investigate the effect of variables such as angle size, bolt spacing, UHPC strength, and joint geometry on performance indicators like ultimate load capacity, energy dissipation, and joint rotation.
• Failure Mode Identification, such as bolt slippage, angle yielding, UHPC crushing, or interface debonding.

Applications and Significance

This hybrid reinforcement strategy is highly relevant for:

• Seismic retrofitting of aging infrastructure.
• Accelerated bridge construction (ABC) techniques.
• Modular precast construction, where mechanical connections using bolts are advantageous.
• High-rise and offshore structures, where UHPC’s durability and compactness are needed.

By integrating steel angles and bolts with UHPC, engineers can harness the strengths of both materials, concrete’s compressive capacity and steel’s tensile robustness, to develop joints that are stronger, safer, and more resilient under extreme loads.