Pseudo-Dynamic Testing of Hollow Sandwich Steel Tube Concrete Composite Frame

Literature Overview

This 2022 study published in Journal of Vibration Engineering by researchers from Hefei University of Technology investigates the seismic performance of a prefabricated composite frame system consisting of hollow sandwich steel tube concrete (HSSC) columns connected to steel-concrete composite beams through high-strength single-sided bolt connections. The study employs pseudo-dynamic testing methodology, combining numerical simulation with OpenSees software and physical testing with MTS hydraulic servo loading equipment. The bottom two floors of a multi-story frame are designated as the test substructure, where damage is expected to concentrate under rare earthquake conditions.

Structural System Description

The composite frame system under investigation features several distinctive design elements:

Component	Material/Type	Key Characteristics
Column	Hollow Sandwich Steel Tube Concrete (HSSC)	Outer steel tube + inner concrete core + sandwich layer; provides high axial and lateral stiffness
Beam	Steel-Concrete Composite (SC)	Steel section encased in or topped with concrete; provides bending capacity and mass
Connection	High-Strength Single-Sided Bolt	Semi-rigid connection; allows controlled rotation and energy dissipation
Connection Type	Prefabricated	Field-assembled using bolted connections for rapid construction

The hollow sandwich steel tube concrete column is a hybrid structural element that combines the advantages of steel tube confinement (enhanced concrete ductility) with the efficiency of hollow section design (reduced self-weight while maintaining lateral stiffness). The single-sided bolt connection is a distinctive feature that creates a semi-rigid joint, which is neither fully pinned nor fully rigid.

Pseudo-Dynamic Testing Methodology

The pseudo-dynamic testing approach was selected to study the seismic response of the full multi-story frame while physically testing only the damage-prone bottom two floors:

Substructure identification: The bottom two floors were identified as the test substructure based on preliminary numerical analysis showing that plastic hinging and damage would concentrate in these regions under rare earthquake ground motions.
Numerical model development: A complete finite element model of the entire frame was developed in OpenSees, with the test substructure (bottom two floors) modeled with detailed component-level elements and the remaining structure modeled with simplified elements.
Loading protocol: The MTS hydraulic servo system applied displacements to the test substructure based on real-time numerical predictions of inter-story displacements, with the boundary conditions at the top of the test substructure representing the effect of the remaining structure.
Axial force simulation: A key technical challenge was simulating the axial force in the HSSC columns, which varies with lateral loading due to P-Δ effects and load redistribution. Two boundary conditions were compared: one with simplified constant axial force and one with horizontal-following approximate axial force.

Key Experimental Results

The pseudo-dynamic tests yielded several important findings regarding the seismic behavior of the composite frame:

Observation	Finding	Engineering Implication
Failure sequence	Beam-column joints yielded first, followed by beam ends	Joint flexibility provides energy dissipation without beam plastic hinge formation
Connection behavior	High-strength single-sided bolt connections behave as semi-rigid joints	Joint rotation capacity is finite; connection design must ensure adequate rotation capacity
Inter-story drift	Slightly exceeds the 2% limit specified in seismic code	Semi-rigid connections increase frame flexibility; design adjustments may be needed
Axial force effect	Axial force slightly increases lateral stiffness of HSSC columns	Simplified boundary without axial force is acceptable for testing
Energy dissipation	Joint rotation is the primary energy dissipation mechanism	Connection detailing is critical for seismic performance

Technical Analysis of Connection Behavior

The semi-rigid behavior of the high-strength single-sided bolt connection is a critical aspect of the frame's seismic performance. The connection moment-rotation relationship can be characterized as follows:

Initial stiffness: The connection exhibits moderate initial stiffness, lower than a fully rigid moment connection but higher than a pinned connection.
Yielding rotation: The connection yields at a rotation of approximately 0.01–0.02 radians, depending on bolt preload and bearing plate configuration.
Post-yield behavior: After yielding, the connection exhibits strain-hardening behavior with gradual stiffness degradation.
Cyclic performance: The connection demonstrates acceptable hysteresis loop characteristics, with limited pinching and moderate energy dissipation per cycle.

The fact that the joint yields before the beam end is a favorable failure mode from a seismic design perspective, as it ensures that damage is concentrated at repairable locations rather than at beam ends where plastic hinge formation would compromise structural integrity.

Comparison of Axial Force Boundary Conditions

The study's comparison of two boundary conditions for axial force simulation provides valuable methodological insights:

Boundary Condition	Description	Impact on Test Results	Acceptability
Simplified (no axial force)	Axial force in columns is neglected	Slightly underestimates lateral stiffness; overestimates inter-story drift	Acceptable for preliminary testing
Approximate (horizontal-following)	Axial force varies with lateral displacement	More accurately captures P-Δ effects and stiffness variation	Preferred for detailed testing

The finding that the simplified boundary condition is acceptable for testing purposes is practically significant, as it reduces the complexity and cost of pseudo-dynamic test setups. However, for detailed research on the precise seismic capacity of the system, the approximate boundary condition should be employed.

Engineering Practice Implications

Several practical implications emerge from this study for the design and implementation of prefabricated composite frame structures:

Connection design: The high-strength single-sided bolt connection provides a practical solution for prefabricated construction, but its semi-rigid behavior must be accounted for in structural analysis and design. Design software should model the connection with appropriate moment-rotation curves rather than assuming either pinned or fully rigid behavior.
Drift control: The slightly excessive inter-story drift (exceeding 2%) under rare earthquake conditions suggests that either the connection stiffness should be increased, the lateral system should be supplemented with bracing or shear walls, or the seismic design category should be adjusted.
Column design: The HSSC column's hollow sandwich configuration provides efficient use of materials, but the interaction between the outer steel tube and inner concrete core must be carefully designed to ensure composite action under cyclic loading.
Prefabrication advantages: The bolted connection system enables rapid field assembly, which is particularly advantageous for earthquake-prone regions where rapid reconstruction is critical.

Summary

This pseudo-dynamic study provides valuable experimental evidence on the seismic performance of prefabricated composite frame systems with hollow sandwich steel tube concrete columns and semi-rigid bolted connections. The findings demonstrate that the system achieves favorable failure modes with joint-first yielding and effective energy dissipation, while also identifying the need for drift control measures. The methodological contributions regarding axial force boundary conditions offer practical guidance for future pseudo-dynamic testing of similar structural systems. The study bridges the gap between theoretical seismic design and practical prefabricated construction, providing engineers with confidence in the seismic adequacy of this innovative structural system when properly detailed and designed.