Failure mechanism
Steel column buckling
A steel column under axial compression does not usually fail by crushing: it fails by losing its straightness, at a load below the yield strength of the section. Once the buckling load is reached, a small lateral deflection grows and the member collapses without carrying any more load. The load at which that happens is set by the effective length, the second moment of area and the restraint actually provided, which is why a column can be substantial in section and weak in position.
How this failure works
Two capacities govern an axially loaded member and the smaller one wins. The squash load is the full section area multiplied by the yield stress, and it is what a stub column carries. The elastic critical load is the Euler load, proportional to the flexural rigidity EI divided by the square of the effective length, and it is what a slender column carries. Between those two limits sits the range that most real columns occupy, where the member yields before it buckles elastically, and the design strength there is read from a column curve indexed on the slenderness ratio and on an imperfection class rather than from either limit alone. The curve is what accounts for the fact that no real member is straight, stress-free and perfectly loaded.
The effective length is not the distance between floor levels. It is the distance between the points the member actually buckles between, and it is fixed by restraint: how the base is held, whether the frame can sway, whether the girts and cladding are genuinely connected to the column, and where a brace terminates. A nominally pinned base grouted over a large plate is partly fixed; a nominally fixed base on four bolts over an ungrouted gap behaves close to pinned. Both change the buckling load and neither change is small. Sway is the assumption that costs the most, because a frame that sways has an effective length greater than its storey height and the check is often run as if it did not.
For a rolled I-section in axial compression the minor axis governs almost always, because the radius of gyration about the weak axis is a fraction of the strong axis value. That is why the restraint that matters is restraint to the weak axis: girts at a defined spacing, fly bracing to the roof, or the diaphragm action of the sheeting. Restraint shown on the drawing but not achieved on site moves the effective length back towards the unrestrained value and takes most of the capacity with it — a purlin cleat with slotted holes and one bolt, a sag rod never tensioned, a girt fixed at alternate columns instead of every column.
Global buckling is not the only mode. Slender flanges and webs buckle locally at a stress below yield, which is why sections are classified and why a slender element is not used as a compression member without an allowance for it. Real members also carry initial out-of-straightness, residual stress from rolling and welding, and a load that arrives slightly eccentric through a bracket or a connection whose bolt group is not concentric with the member axis. Each of these brings the load at which deflection starts to run closer to the working load, and the reserve after buckling begins is small: the failure is sudden and the visible warning period is short.
Illustration
Conceptual illustration: a pin-ended column buckling about its weak axis under axial load, with the deflected shape exaggerated and the intermediate restraint that would have shortened its effective length shown removed.
Typical causes
- Effective length taken as the member length, with frame sway or the absence of real restraint at the ends ignored.
- Bracing and restraint that exist on the drawing but not on the structure: slotted cleats left loose, sag rods untensioned, girts fixed at alternate columns.
- Minor-axis buckling never checked, because the section looked substantial about its strong axis.
- An axial load that arrives eccentric, through a bracket, a crane bracket or a bolt group whose centroid sits off the member axis.
- A slender section, or a slender element within a compact section, used in compression where local buckling reduces capacity below the squash load.
- A member supplied out of straightness, bent in transport or distorted by hot work, so the initial imperfection is worse than the one the column curve assumes.
- Load added after the design: a mezzanine, a piece of plant or a new duct support, without rechecking the column that ends up carrying it.
Warning signs
- A column out of plumb beyond the erection tolerance, or losing plumb after cladding and equipment went on.
- Cracked or flaking paint along the flange-to-web junction, which is where local buckling first shows as a wave in the plate.
- Rust staining or fretting at the base plate, with grout that has cracked or lifted away from the plate.
- Roller shutters, sliding doors or fixed panels that begin to bind, because the frame around them has moved.
- Girts, rails or bracing members visibly bowed, or sag rods and turnbuckles hanging slack.
- A frame that creaks or shifts as load changes across the day, or a lean that shows against a plumb line held to the column face.
Inspection methods
- 01Plumb the column in two directions against the erection tolerance and record the readings, not just the conclusion.
- 02Compare the restraint actually present against the design: count the girts fixed, check the bracing is complete and tensioned, and record where restraint is missing.
- 03Look along the flange and the web for waviness, and read the paint condition at the plate junctions for local buckling.
- 04Check the base plate, holding-down bolts and grout, and establish what the base is really acting as before relying on it.
- 05Verify the section and grade as supplied against the drawing, using the mill certificates and measured section dimensions.
- 06Re-derive the load on the column from what is now supported and compare it with the design load, rather than assuming the building is as drawn.
Preventive measures
- Take the effective length from the frame action and the real restraint, and write the assumption on the drawing so it can be checked on site.
- Design the weak axis with restraint at a spacing that can actually be built, and show the bracing on the erection drawing.
- Keep fabricated and erected straightness within tolerance and reject members bowed beyond it before they are lifted.
- Do not remove temporary bracing or temporary props until the permanent stability system is complete and has been inspected.
- Allow for connection eccentricity and any bracket load in the axial path, not only in the connection check.
- Classify the section, and do not use a slender element in compression without an allowance for local buckling.
- Recheck the column whenever a mezzanine, item of plant or additional load is introduced above it.
Relevant standards
The codes and specifications a design is normally judged against. Naming them is not a claim that AKEN certifies, inspects or approves against them — the design authority for a project does that.
- IS 800 — general steel construction: compression member design, effective length and buckling classes.
- IS 808 — dimensions and properties of hot rolled steel sections, from which the radii of gyration are taken.
- IS 2062 — structural steel, including the grades and weldability the design assumes.
- AISC 360 — an alternative compression design basis, used where a client specifies it.
- Eurocode 3 (EN 1993-1-1) — buckling curves and section classification in the European basis.
Related AKEN services
Scope note
This article explains a failure mechanism for engineering education. It is general guidance, not a design check, an inspection report or a certification for any particular structure. Assessments of an actual structure must be carried out by the responsible design authority against the project’s own specifications.