Axial Compression Performance and Bearing Capacity Calculation of Tie-Rod Confined Square Steel Tube Ultra-High Strength Concrete Columns

Overview of the Study

This research investigates the axial compression behavior of square steel tube (SST) columns filled with ultra-high strength concrete (UHSC, typically ≥120MPa) and externally confined by high-strength tie rods. The combination of UHSC and tie-rod confinement represents a novel approach to achieving extremely high load-carrying capacity while maintaining ductility through the steel tube and tie-rod system. Ultra-high strength concrete is typically produced using supplementary cementitious materials such as silica fume, fly ash, and slag, along with optimized aggregate grading and high-pressure compaction.

Core Technical Content

The UHSC used in such studies typically achieves compressive strengths of 120MPa to 180MPa, with elastic moduli of 55GPa to 70GPa. The tie rods are made from high-strength steel bars with yield strengths of 690MPa to 1000MPa, arranged in a grid pattern around the steel tube perimeter with spacing of 200mm to 400mm.

Key findings include:

The axial compression capacity of tie-rod confined SST-UHSC columns is 1.5 to 2.2 times that of unconfined UHSC cylinders of equivalent cross-section.
The steel tube provides primary confinement, while the tie rods provide secondary confinement and prevent outward buckling of the tube wall.
The failure mode transitions from concrete crushing to steel tube local buckling as the axial load increases, with the tie rods delaying the buckling onset by 30% to 50%.
The ductility factor is significantly improved compared to unconfined UHSC, increasing from 1.2 to 3.5 due to the combined confinement system.

Technical Parameter Analysis

Parameter	Value	Unit	Notes
UHSC compressive strength	120-180	MPa	28-day cube strength
UHSC elastic modulus	55-70	GPa	Secant modulus
Steel tube grade	Q460	-	Yield strength 460MPa
Steel tube section	300×300×12	mm	Square hollow section
Tie rod grade	HRB1000	-	Yield strength 1000MPa
Tie rod diameter	16-20	mm	High-strength rebar
Tie rod spacing	200-400	mm	Along column length
Steel ratio (tube)	8-10	%	Tube area / total area
Tie rod ratio	0.5-1.0	%	Tie rod area / total area

Standards and Design Methodology

The design of tie-rod confined SST-UHSC columns involves several analytical approaches:

Design Approach	Formula	Key Parameter	Limitation
Empirical model	N_u = A_c × f_cu + A_s × f_y	f_cu = confined UHSC strength	Requires calibration for UHSC
Confinement model	f_cu = f_c0 × (1 + k × f_l / f_c0)	k = confinement coefficient	k varies with UHSC grade
Interaction model	N_u = η × (f_c × A_c + f_y × A_s)	η = interaction factor	η > 1.0 for UHSC
Finite element	Nonlinear FE analysis	Material nonlinearity	Computationally intensive

The confinement coefficient k for UHSC is typically lower than for normal-strength concrete, ranging from 1.5 to 2.5 compared to 2.5 to 4.0 for normal-strength concrete. This is because UHSC has a lower Poisson's ratio and exhibits more brittle behavior, reducing the effectiveness of lateral confinement.

Process and Quality Control

The fabrication of tie-rod confined SST-UHSC columns involves several critical steps:

Steel tube fabrication and inspection per GB/T 14976, with wall thickness tolerance of ±0.3mm and straightness of ≤1/1000 of length.
Tie rod fabrication with precise cutting and bending, ensuring that the tie rod ends are properly anchored into the end plates or embedded in the column end concrete.
UHSC mixing and placement requiring high-pressure vibration or pressure compaction to achieve full densification within the steel tube. The slump of UHSC is typically very low (20mm to 50mm), requiring specialized pumping equipment.
Welding of tie rod anchor plates to the steel tube end plates using full-penetration butt welds, inspected by UT per GB/T 11345 with Level I acceptance criteria.
Post-construction inspection including ultrasonic testing of the concrete fill to detect voids or honeycombing, with acceptance criteria of ≥95% sound area.

Engineering Practice Integration

The application of tie-rod confined SST-UHSC columns is most relevant in applications requiring extremely high load capacity within limited cross-sectional areas, such as:

Tall building core columns in seismic zones
Bridge piers and abutments with high axial loads
Nuclear containment structures requiring high compressive strength
Marine platform columns subject to high axial loads and corrosion

From a manufacturing perspective, several challenges must be addressed:

The UHSC mix design requires precise control of water-cement ratio (typically 0.20 to 0.25) and supplementary cementitious material content (silica fume 10% to 20% of cement weight). Any deviation can significantly affect the final strength and durability.
The low workability of UHSC demands specialized pumping and vibration equipment. Air-lift pumps or pressure pumps with pressures of 10MPa to 20MPa are typically required for vertical tube filling.
The tie rod installation must be coordinated with the steel tube fabrication to ensure proper alignment and anchorage. Field welding of tie rod anchors is generally not recommended due to the difficulty of achieving full penetration in tight spaces.
Quality control of UHSC within steel tubes is challenging due to limited access for testing. Non-destructive testing methods such as ultrasonic pulse velocity and impact-echo methods should be employed during and after placement.

Key Questions and Reflections

A fundamental question is whether the combination of UHSC and tie-rod confinement provides a cost-effective solution compared to alternative approaches such as larger sections of normal-strength CFST columns. The analysis suggests that while the tie-rod confined SST-UHSC approach achieves higher capacity per unit cross-sectional area, the material and fabrication costs are significantly higher, making it suitable only for applications where space constraints are critical.

Another important consideration is the long-term durability of UHSC within steel tubes. While UHSC has excellent resistance to chemical attack and low permeability, the interface between the UHSC and the steel tube wall may be vulnerable to corrosion if the concrete cover thickness is insufficient or if the steel tube has coating defects. The tie rod system may also be susceptible to corrosion if not properly protected, particularly in aggressive environments.

Study Insights and Implications

This study demonstrates that tie-rod confinement significantly enhances the axial compression performance of square steel tube UHSC columns, achieving capacity increases of 50% to 120% over unconfined UHSC. The key implication for steel pipe manufacturers is that the steel tube wall thickness must be carefully designed to accommodate the high confinement pressures generated by UHSC, which can reach 15MPa to 25MPa at peak load. This may require thicker walls or additional internal stiffening rings to prevent local buckling. The study also highlights the importance of proper tie rod detailing and anchorage, as the effectiveness of the confinement system depends critically on the integrity of the tie rod connections. Future research should focus on the cyclic loading behavior of tie-rod confined SST-UHSC columns to evaluate their seismic performance and fatigue resistance.