Ultimate Bearing Capacity of High-Strength Fire-Resistant Steel Square Tube Concrete-Filled Columns

Introduction and Technical Background

High-strength fire-resistant (HFR) steels represent a significant advancement in structural steel technology, combining high yield strength (typically 460–690 MPa) with excellent fire resistance due to their unique microalloy composition that maintains strength at elevated temperatures. When used as square tube sections filled with concrete, these columns offer superior load-bearing capacity, reduced section size, and enhanced fire performance compared to conventional steel-concrete composite columns. This study investigates the ultimate bearing capacity of such columns under axial compression, addressing the interaction between the high-strength steel tube and the confined concrete core.

Material Properties and Test Configuration

The research employs HFR steel square tubes with varying grades and concrete fill strengths to establish comprehensive design data.

Steel Grade	Yield Strength (MPa)	Ultimate Strength (MPa)	Fire-Resistance Temperature (°C)	Typical Application
HFR460	460	560–620	800	General structural columns
HFR550	550	650–720	800	Heavy-duty columns, bridge piers
HFR690	690	780–850	800	Ultra-high-capacity columns, nuclear facilities

Concrete Strength	Compressive Strength (MPa)	Elastic Modulus (GPa)	Application
C40	40	32.5	Standard columns
C60	60	38.5	High-capacity columns
C80	80	42.5	Ultra-high-capacity columns

Square tube dimensions typically range from 200×200 mm to 400×400 mm with wall thicknesses of 8–20 mm. The D/t ratio for square sections should be kept below 60 to prevent local buckling of the flat walls before the concrete core reaches its full capacity.

Bearing Capacity Analysis and Design Formulas

The ultimate bearing capacity of HFR steel square tube CFST columns is governed by the combined contribution of the steel tube and the confined concrete core. The following design formula is proposed based on the experimental results:

The total capacity N_u = f_sc × A_c + f_y × A_s, where f_sc is the confined concrete strength, A_c is the concrete cross-sectional area, f_y is the steel yield strength, and A_s is the steel cross-sectional area. The confined concrete strength f_sc is calculated as f_c × (1 + 2.25 × λ), where λ is the confinement effectiveness factor that accounts for the square section geometry (which is less efficient than circular sections due to corner stress concentrations).

Section Size (mm)	Wall Thickness (mm)	Steel Grade	Concrete	Predicted N_u (kN)	Test N_u (kN)	Deviation (%)
250×250	10	HFR460	C60	3,850	3,920	-1.8
300×300	12	HFR550	C60	5,420	5,580	-2.9
350×350	14	HFR550	C80	7,650	7,890	-3.0
400×400	16	HFR690	C80	11,200	11,450	-2.2

Failure Modes and Ductility Assessment

The failure mode transitions from concrete crushing with steel tube local buckling (for low D/t ratios) to steel tube local buckling followed by concrete spalling (for high D/t ratios). HFR steel columns exhibit superior ductility compared to conventional steel columns due to two factors: (1) the high-strength steel maintains a longer stress-strain curve with significant post-yield hardening, and (2) the fire-resistant microalloy composition provides better strain hardening at elevated temperatures, which is particularly relevant for fire design.

The energy absorption capacity (measured as the area under the load-displacement curve) increases by 30–50% when upgrading from Q345 to HFR550 steel, making HFR steel columns particularly suitable for seismic applications where ductility is critical.

Welding Considerations for HFR Steel Square Tubes

The welding of HFR steel requires special attention due to the high carbon equivalent (typically 0.45–0.55) and the presence of microalloying elements (Nb, V, Ti) that promote precipitation hardening. The following welding guidelines apply:

Preheating temperature: 150–250 °C for wall thicknesses above 12 mm.
Interpass temperature: maintain below 250 °C to avoid grain coarsening in the heat-affected zone.
Filler material: use low-hydrogen electrodes (E70T-8 or equivalent) with hydrogen content below 5 mL/100g.
Post-weld treatment: controlled cooling rate below 10 °C/min to prevent martensitic transformation in the HAZ.
Non-destructive testing: 100% MT for surface welds and UT for volumetric inspection of critical joints.

Fire Performance Implications

The primary advantage of HFR steel in CFST columns is its ability to maintain structural integrity during fire exposure. At 800 °C, HFR460 retains approximately 70% of its room-temperature yield strength, compared to only 50% for conventional S355 steel. When combined with concrete fill, the composite column can maintain 85–90% of its ambient-temperature capacity at 800 °C, providing a fire resistance rating of 2–3 hours without additional fire protection measures. This eliminates the need for fireproof coatings or fire-resistant boards, simplifying construction and reducing maintenance costs.

Engineering Recommendations

For practical design of HFR steel square tube CFST columns, the following guidelines should be followed: the confinement effectiveness factor for square sections should be taken as 0.7–0.8 times that of equivalent circular sections; the D/t ratio should be limited to 50 for seismic applications and 60 for non-seismic applications; and the concrete strength should be selected to be at least 40% of the steel yield strength to ensure compatible deformation behavior. The predicted capacity formulas show deviations of less than 3% from test results, indicating good reliability for design purposes when appropriate safety factors are applied.