Composite HSS columns remain one of the most underutilized structural systems in North American practice. The concept is straightforward: fill an HSS column with concrete and the two materials work together to resist load far more efficiently than either can alone. The most common application is columns supporting conventional steel framing (including composite beam-and-deck floors). 

What holds many engineers back is not skepticism about the concept; it is unfamiliarity with the governing provisions and a perception that composite design adds complexity without proportionate benefit. The sections below address that directly, with specific reference to AISC 360-22 Chapter I, CIDECT Design Guide 5, AISC Design Guide 26, AISC 341, and ASTM E119.

How Does Composite HSS Increase Axial Load Capacity?

Filling an HSS with concrete can increase axial capacity by up to 100%, depending on wall thickness, section size, and concrete strength, with lighter-walled columns tending to see larger percentage gains. That increase in capacity allows for a reduction in steel tonnage, reducing not only material cost (the concrete fill is comparatively inexpensive), but potentially also allowing use of a smaller crane for erection.  

AISC 360-22 captures composite strength using the plastic stress distribution method for composite HSS columns not subject to global buckling. The method assumes the steel walls reach yield stress Fy in tension or compression simultaneously with the concrete reaching 0.85f‘c in compression. For round HSS sections, AISC 360-22 allows an increase to 0.95f‘c to account for the confinement effect: the circular steel tube prevents the concrete core from dilating outward under load, generating a triaxial stress state that increases the effective compressive strength of the concrete beyond what the unconfined cylinder strength would predict. 

Rectangular HSS sections do not generate the same confinement pressure, because the flat walls deflect outward rather than constraining the concrete, so the 0.85f‘c value applies.  

For columns subject to global buckling (the vast majority of columns in practice), the presence of concrete also increases the moment of inertia, (EI)eff, resulting in increased compression capacity compared to an equivalent unfilled section.  

AISC 360-22 Section I2.2a requires that the steel cross-sectional area be at least 1% of the total composite cross section. Every standard domestically produced HSS section from Atlas Tube satisfies this limit, a detail worth confirming once and setting aside. 

CIDECT Design Guide 5 provides design guidance and test data relevant to Composite HSS columns.

Source: Steel Tube Institute

How Does Concrete Fill Improve Structural Performance and Ductility?

When a composite HSS resists bending, a compression block forms in the concrete. This shifts the neutral axis towards the compression side and increases the moment capacity contributed by the steel on the tension side. This is the same principle that lends efficiency to framing supporting composite slabs on metal deck, albeit with a smaller compression block. However, CFTs add a second advantage: the concrete core braces the steel walls against inward buckling under compression. 

Local Buckling Limits 

Bare HSS walls are subject to local buckling when the width-to-thickness ratio B/t (for rectangular) or diameter-to-thickness ratio D/t (for round) exceeds the compact section limit. AISC 360-22 Tables I1.1a (compression members) and I1.1b (flexural members) specify relaxed compactness limits for composite filled sections because the concrete core braces the inner face of the steel wall. 

Compare those to the bare-steel limits in AISC 360-22 Table B4.1a, and the difference is significant. More HSS sections qualify as compact (meaning the full plastic stress distribution is available) when they are concrete filled than when they stand alone. In practical terms, this allows for a reduction in tonnage since larger, thinner-walled sections of a given weight will generally have more capacity than smaller, thicker-walled sections if buckling does not control the design. 

Seismic and Cyclic Performance

The ductility advantage of CFTs becomes most pronounced under cyclic loading. Experimental studies on CFT moment-frame connections, including Ricles, Peng, and Lu (2004), support the ability of properly detailed square CFT column connections to sustain substantial inelastic drift under cyclic seismic loading. The concrete core and steel tube work together to promote a more stable inelastic response, which can improve cyclic deformation capacity and energy dissipation. Together, these mechanisms improve the capacity of the column to absorb seismic energy through repeated cycles of inelastic deformation without sudden strength loss, also called energy dissipation. 

For projects in higher seismic categories, that cyclic deformation capacity can make CFT columns a credible option within appropriately detailed composite seismic systems. However, system selection, drift, and member sizing still depend on the overall frame configuration and the governing code requirements. Engineers designing in high-seismic zones should review the specific CFT requirements in AISC 341, which addresses both special and intermediate composite moment frames using CFT columns. 

How Does Composite HSS Reduce Construction Cost?

The HSS acts as permanent formwork for the concrete fill. That fact drives four distinct cost reductions: 

Reduced tonnage. As noted above, the contribution of the concrete fill to compressive and bending capacity, as well as the elimination of inward local buckling of the HSS wall, both allow for the use of lighter, larger, thinner-walled members. This can reduce steel tonnage when compared to wide flange members or unfilled HSS. 

Labor savings over cast-in-place concrete. Concrete columns require rebar cage fabrication and erection, followed by formwork placement and stripping. This is a labor-intensive sequence that adds both direct cost and schedule float. Concrete-filled HSS eliminates that sequence entirely. The steel tube replaces both the rebar and formwork and remains in place. For a commercial building, the eliminated formwork labor and material cost can be substantial. 

Faster construction. Steel erection and concrete placement can be sequenced to keep pace with floor framing above. Compared with cast-in-place reinforced concrete columns, CFT systems can reduce some sequencing constraints because the steel shell is erected first and does not require separate column formwork. 

Crane Requirements. Crane rental can contribute a significant portion of erection costs for a project, with crane cost and capacity increasing in tandem. In many buildings, the heaviest members are the columns at the lower levels. Use of HSS columns generally, and concrete-filled HSS columns specifically, can reduce the weight of the columns, thus reducing the required crane capacity and cost for the project. 

Connection simplicity. HSS columns, both hollow and concrete-filled, eliminate web connections, which tend to require more material, larger welds, and more bolts. By allowing for “flange” connections on all four sides, fabrication and erection costs are reduced. 

How Does Concrete Fill Provide Fire Resistance for HSS Columns?

As the temperature of structural steel increases, its strength and stiffness decrease. At 1,100°F, the modulus of elasticity and yield strength is reduced to approximately half compared to room temperature. While building fires cannot generally create temperatures required to melt steel, the reduction in strength and stiffness can lead to conditions where the steel members are unable to support the loads they were designed to resist. 

Concrete fill changes the thermal response of an HSS column by adding thermal mass, thus reducing the rate at which steel temperature rises in a fire event and preserving composite load-carrying capacity. 

The actual fire rating of a composite HSS column may be obtained using the calculation procedure in AISC Design Guide 19, provided the member satisfies the applicable limits for geometry, concrete, effective length, loading, and other parameters.  Supplemental longitudinal reinforcement or steel-fiber-reinforced concrete can increase the member’s fire rating. Steel-fiber reinforcement is generally more cost-effective than rebar cages because it requires less fabrication and erection labor. Results depend on the section dimensions, concrete properties and aggregate, reinforcement, effective length, applied load, and fire exposure.  

In some applications, concrete-filled HSS columns may eliminate the need for intumescent paint or sprayed-on fire-resistive material (SFRM) on column surfaces. CIDECT Design Guide 4 and the AISC Engineering Journal article Response of Concrete-Filled HSS Columns in Real Fires (Kodur, 2009) provide additional useful information.

Why Are Composite HSS Columns Effective for Blast-Resistant Design?

Blast loading can impose highly directional and rapidly varying demands, making member behavior about multiple axes important. This can disadvantage wide flange members and other sections with substantially unequal stiffness about their principal axes.  

Square HSS have identical geometric properties about their two principal axes, while round HSS have identical properties about every centroidal axis. When filled with concrete, these sections retain their geometric symmetry while gaining additional axial and flexural strength, stiffness, mass, and resistance to local buckling. These characteristics can make concrete-filled HSS columns attractive for resisting blast loads. The actual benefit depends on the section geometry, material properties, axial-load ratio, boundary conditions, and blast demand. 

AISC Design Guide 26, Design of Blast Resistant Structures, provides general guidance on blast-resistant design and progressive-collapse mitigation. It discusses blast loading, structural response, ductility, energy absorption, residual capacity, and alternate load paths. It also identifies concrete-filled HSS columns as a potential means of mitigating local buckling in blast- or progressive-collapse-resistant systems. 

Concrete fill can improve blast performance in several ways. The walls of the HSS member confine the concrete core, reduce or restrain outward spalling, and protect the core from some localized damage. The concrete can delay local buckling of the steel tube and contribute compressive strength, stiffness, mass, and energy absorption. The steel shell can provide ductile tensile and flexural resistance after portions of the concrete have cracked or crushed. These benefits are not automatic: local tube buckling, tearing, shear failure, connection failure, concrete crushing, debonding, and global instability must be checked for the specific blast scenario. 

Frequently Asked Questions About Composite HSS 

What concrete strength should engineers specify for concrete-filled HSS columns?

Per AISC 360-22, normal-weight concrete with f‘c between 3 ksi and 10 ksi is permitted for determining nominal strength. One of the main benefits of concrete fill is allowing the use of HSS sections with thinner walls than would ordinarily be practical by raising the B/t and D/t limits.  

Do engineers need shear connectors inside a composite HSS column?

Filled composite columns generally rely on bond and friction at the steel-to-concrete interface rather than mechanical shear connectors. For most gravity column applications, this is sufficient. Connection regions where significant force transfers directly into the concrete core, such as column base plates or beam-to-column moment connections, require explicit evaluation. 

Can composite HSS columns be used in seismic force-resisting systems?

Yes. AISC 341 recognizes composite moment frames (C-SMF, C-IMF, and C-OMF) using CFT columns and addresses their seismic detailing requirements explicitly.  

How is concrete placed in an HSS column on a multi-story project?

Concrete is typically placed from the top of the HSS using standard field placement methods appropriate for the project, with procedures coordinated to ensure complete filling and avoid trapped air or voids. Designers and contractors should consult current composite-column construction guidance for project-specific recommendations on placement sequence, venting, inspection, and quality control. 

Are composite HSS columns cost-effective compared to wide-flange sections encased in concrete?

For axial-load-dominated columns, composite HSS typically has a cost advantage over encased wide-flange sections because the HSS tube acts as its own formwork, the compact geometry simplifies finishing, and fire resistance can often be achieved without additional coating.

Conclusion

Composite HSS columns, or concrete-filled tubes, are a mature, code-governed structural system that routinely delivers more axial capacity than bare HSS, while simultaneously delivering benefits to fire resistance and blast resistance. The construction economics, particularly the formwork elimination, make the comparison to reinforced concrete columns favorable in many applications. 

Engineers who have not yet incorporated composite HSS columns into standard column selection will find AISC Design Guide 6 (2nd Ed., 2025) on composite column design a practical starting point. Atlas Tube provides design resources and technical support for engineers specifying HSS, including the HSS Connections Hub™.