CIVL6264 Chap.7 Composite Columns and Stability
Composite Columns and Stability
Composite columns combine steel and concrete in filled, fully encased or partly encased forms, but added material does not remove stability checks. This chapter separates short-section resistance from local plate buckling, global member buckling, long-term stiffness and second-order effects.
It uses axial-moment interaction to prevent separate capacity checks from double-counting resistance and connects section choice to erection, fire and joint demands. Concrete-filled tubes provide permanent formwork and can restrain inward steel-wall deformation, while encased sections add fire protection and stiffness. Those benefits depend on section proportions, concrete placement, reinforcement and connection transfer.
Cross-section resistance is only the first layer. Plate slenderness determines whether the steel can develop the assumed stress without premature local buckling. Effective length, end restraint and composite stiffness control global buckling, and concrete creep and shrinkage can reduce long-term stiffness. Imperfections and axial compression amplify lateral displacement through second-order response.
Combined compression and bending are checked with an interaction envelope because full axial and full moment resistance cannot be used simultaneously. The demand point must reflect member effects and the relevant bending axis before it is compared with resistance.
The chapter uses the inverse-square elastic relation as a sensitivity tool, not as a complete design formula, to make the strong influence of effective length visible. It also treats filling quality and force transfer into both materials as verification items rather than assumptions hidden behind a section label.
What this chapter covers
- 01
Filled and encased sections
- 02
Squash resistance
- 03
Local plate slenderness
- 04
Effective stiffness
- 05
Global buckling
- 06
Axial-moment interaction
Worked example · free
Compare buckling lengths
- +1Cancel the common stiffness and pi-squared terms.
- +1Write the inverse-square length ratio.
- +1Substitute 3.0 squared over 4.5 squared.
- +1Calculate the ratio as 0.444.
- +1State that full design resistance still needs imperfections and interaction.
Key terms
- Filled tube
- A hollow steel section filled with concrete so the shell also acts as formwork.
- Encased section
- A steel section partly or fully surrounded by reinforced concrete.
- Local buckling
- Instability of an individual steel plate element within the cross-section.
- Global buckling
- Member instability involving lateral displacement over its effective length.
- Interaction envelope
- A boundary of compatible axial and moment resistance combinations.
Composite Columns and Stability FAQ
Why is squash resistance not enough for a column?
Squash resistance describes a short cross-section under compression. Real members have effective length, imperfections, local plate slenderness, reduced stiffness and second-order moments. These effects can lower usable resistance well before the material areas reach their simple squash values.
What does concrete filling do for a steel tube?
The tube provides permanent formwork and external reinforcement, while concrete adds compression resistance and can restrain inward local deformation. Confinement benefits are conditional and reduce with increasing slenderness, eccentricity and wall slenderness, so they must follow the applicable design rules.
Why use an axial-moment envelope?
A column cannot mobilise full compression and full bending resistance simultaneously. The envelope records compatible pairs from one stress state. A demand point can fail interaction even when its axial and moment components each sit below their separate endpoint capacities.
Exam move
Build a four-line column checklist: cross-section resistance, plate slenderness, member stability and axial-moment interaction. Redraw the N-M envelope and place trial points near each end. Recompute the inverse-square length ratio, then explain which design change targets axial, moment-dominated or instability-controlled demand. Prepare one comparison sheet for filled, fully encased and partly encased columns.
Include erection role, fire, local plate behaviour, concrete placement, reinforcement, connection and inspection. For each trial member, complete four checks in order: short-section resistance, local slenderness, global stability and axial-moment interaction. Redraw the interaction envelope and place demand points near the compression and bending ends, then choose a design change that targets the actual controlling region.
Recompute critical-load ratios for several length changes and predict the result before arithmetic. Add a site verification section for fill continuity, concrete strength and the joint zone where force enters both materials. This prevents a material sum from masquerading as a finished column design. Do not let the interaction diagram become a decorative curve.
Label the pure compression and pure bending endpoints, then place a trial demand and describe how a longer effective length or larger second-order moment moves its utilisation. Rotate the section and repeat for the other principal axis. For a filled tube, add a construction note about concrete placement and a connection note about transferring force into the core.
For an encased section, add cover, reinforcement and fire considerations. This combined sketch makes clear why member form, stability and buildability cannot be reviewed in isolation. After selecting a section, state which axis governs and why. Connection eccentricity, frame restraint and section geometry can make the weak-axis result decisive even when the strong-axis interaction plot appears comfortable.