Composite Column Design: Concrete-Filled and Encased Steel Sections
A steel column carries load efficiently per unit weight but buckles readily if unbraced, and its fire resistance is poor without added protection. A reinforced concrete column resists fire well and buckles far less easily for a given cross-section, but is heavier and takes longer to form and cure on site. A composite column, combining structural steel and concrete in the same cross-section, is an attempt to keep the advantages of both: the concrete restrains local buckling of the steel and adds fire resistance and stiffness, while the steel provides ductility, tensile capacity, and a rigid frame that can be erected quickly and loaded before the concrete has even cured.
Concrete-Filled Steel Tubes
In a concrete-filled tube (CFT) column, a round or rectangular hollow structural steel section acts as permanent formwork and is filled with concrete after erection. The steel tube confines the concrete core, restraining its lateral expansion under axial load, which increases the concrete's effective compressive strength beyond what an unconfined cylinder test would show − a round tube provides more uniform confinement than a rectangular one, since a circular shape can develop hoop tension uniformly around its perimeter while a rectangular tube's flat faces bow outward locally between corners.
The concrete core, in turn, braces the steel tube wall against local buckling from the inside, which is one of the main reasons CFT columns can use thinner-walled tubes than an equivalent hollow steel section would need to avoid local buckling under the same load. Design of a CFT column checks a combined nominal axial capacity built from a concrete contribution (with a confinement-enhanced strength for circular sections) and a steel contribution, along with a slenderness reduction using an effective stiffness that blends the steel and concrete moduli, since the composite section's resistance to overall column buckling depends on both materials acting together, not just the steel shell alone.
CFT columns also solve a construction sequencing problem on tall buildings: the empty steel tube can be erected many floors ahead of the concrete pumping crew, since it needs no falsework or formwork of its own, and the concrete fill can follow later without holding up the steel erection schedule the way a conventional reinforced concrete column would.
Steel-Encased (Composite) Columns
The other common composite arrangement embeds a structural steel shape − typically a wide-flange section − inside a reinforced concrete column, with conventional longitudinal and transverse rebar surrounding the steel core the same way it would in an ordinary reinforced concrete column. Here the concrete cover provides fire resistance to the embedded steel and restrains its local and overall buckling, while the steel core carries a large share of the axial load and adds ductility the plain concrete section would otherwise lack, particularly valuable in high seismic regions where ductile behavior under cyclic load reversal matters as much as peak strength.
Encased composite columns are common at the base of tall buildings and in seismic-resistant frames precisely because they combine high axial capacity in a compact footprint (steel does more of the work per unit of cross-sectional area than concrete alone) with the fire protection and stiffness concrete provides, letting architects hold column sizes down on lower floors where axial loads accumulate from all the floors above, without resorting to bare structural steel sections that would need separate fireproofing.
Combined Axial and Bending Interaction
Like any column, a composite section rarely carries pure axial load in practice; moment from lateral load, from beam end connections not perfectly pinned, or from construction misalignment always enters the picture, and design proceeds through an interaction diagram much like the one used for reinforced concrete, plotting the combinations of axial load and moment the section can resist. For composite sections this diagram is built from a strain-compatibility analysis across the full section − steel shape, concrete, and any additional rebar − assuming plane sections remain plane and applying each material's own stress-strain relationship at every fiber, essentially the same method used for reinforced concrete interaction diagrams but with an extra material (structural steel) contributing directly to the section's internal force and moment resultants rather than acting only as discrete reinforcing bars.
Load transfer between the two materials at connections is a detail unique to composite design: shear studs, tie bars through the steel shape, or direct bearing must be sized to transfer force between the concrete and steel wherever load enters or leaves the composite section, such as at a beam-to-column connection framing directly into an encased or filled composite column, a detail with no equivalent in either pure steel or pure concrete column design.