Failure mechanism
Beam bending and deflection
A beam is checked twice and the two checks are not interchangeable: once for the bending stress it can resist, and once for how far it deflects in service. A beam can pass the first comfortably and still fail the second, cracking the finishes above it, ponding water on the roof it supports, or moving enough that nobody trusts the calculation behind it. Deflection depends on stiffness, which is why a change of steel grade moves one check and leaves the other exactly where it was.
How this failure works
Bending resistance and deflection use different properties of the same section. Resistance comes from the section modulus and the yield stress, so the usable moment is a stress limit on the extreme fibre, reduced by lateral torsional buckling where the compression flange is not restrained. Deflection comes from the second moment of area acting over the whole span, evaluated at service loads with no load factors, and it is indifferent to how strong the steel is. Substituting a higher grade to resolve a stress overrun raises the resistance and leaves the deflection unchanged, which is why so many deflection problems arrive with a section that was upgraded rather than enlarged.
The compression flange is the part of the beam that wants to move sideways, and the restraint along it decides whether the beam reaches its full bending resistance or some fraction of it. Deck fixed to every purlin, a concrete slab acting through shear connectors, or a sag rod at a defined spacing restrains that flange; a purlin cleat with slotted holes and a single bolt does not. The case most often missed is the one where the compression flange is the bottom flange: a cantilever, a beam with a heavy tip load, or a roof where wind uplift reverses the bending and the restraint that was there for gravity load is suddenly on the wrong side.
The deflection limit is set by what the beam supports, not by the beam. A ratio in the region of span/360 is inherited from brittle finishes — plaster, block partitions, tiling — which crack at strains a bare steel member would never notice, while a roof carrying only sheeting is commonly allowed a limit nearer span/180. Applying the roof limit to a floor with a false ceiling, or the finish limit to a bare structure, substitutes one limit state for another. Where a brittle element spans across several beams, the check is against the differential movement between adjacent beams, not against the absolute sag of either.
Two effects make deflection more than a cosmetic matter. On a low-pitch roof, deflection reduces the fall to the outlets, water collects, the extra water deflects the beam a little further, and the process runs away; ponding is a stability problem wearing the clothes of a drainage problem. In a member carrying axial load as well as bending, deflection multiplies the moment, so the capacity of a beam-column is less than the sum of its parts and the magnification itself depends on the deflection — a second-order effect that no first-order analysis will find. Camber is not a substitute for either check; it changes where the member sits, not how far it moves.
Illustration
Conceptual illustration: a simply supported beam sagging between its supports under service load, with the deflected profile exaggerated and the unrestrained compression flange twisting over the span.
Typical causes
- Section sized on bending stress with the deflection check left until the end, by which time the section was already fixed in the drawings.
- A higher steel grade substituted for a deeper section to clear a stress overrun, which does nothing at all for deflection.
- Compression flange restraint assumed from sheeting fixed at alternate purlins, or through cleats whose slotted holes never bear.
- Collateral load omitted from the design: ducts, cable trays, sprinkler pipe, a false ceiling and a solar array added afterwards.
- Roof fall too shallow, outlets undersized or blocked, so ponding adds load and deflection in a repeating cycle.
- Web openings, notches or a bracket cut after the design was completed, removing stiffness where the moment is highest.
- Uplift or a cantilever tip load that puts an unrestrained bottom flange into compression, a case the gravity check never sees.
Warning signs
- Standing water at midspan or in a low corner of a roof, or staining that shows where water has stood before.
- Cracks in plaster or partitions running parallel to the beam soffit, or opening at the head of a partition.
- Ceiling or cladding joints that have opened on one side of a room, or a fascia line that is visibly not straight.
- Roller shutters, sliding doors and windows that bind under live load and free themselves when it goes.
- Paint cracking along the flange-to-web junction, or a section that looks twisted when sighted along the flange.
- Sag rods hanging slack, purlins twisted on their cleats, or fixings that have backed out of the sheeting.
Inspection methods
- 01Level survey along the soffit or the top flange, compared with the design camber and the expected deflection, read at more than one load state.
- 02Count the fixings actually restraining the compression flange along each purlin line, and establish whether the cleats bear.
- 03Verify the section, grade and span as built against the drawing, including any splice, section change or added plate.
- 04Look for openings, notches and welds added after fabrication, and for any load the member is carrying that is not on the drawing.
- 05Check the roof falls, outlet condition and evidence of past ponding on a low-pitch roof.
- 06Look for lateral displacement or twist of the compression flange, and for cracked paint at the flange-web junction near the supports.
Preventive measures
- Size for deflection with the serviceability load case, independently of the strength check, and state both limits on the drawing.
- Choose depth rather than grade when stiffness is the issue, and record the reason for the section in the design notes.
- Detail restraint that can be inspected: deck fixed at every purlin, sag rods at the designed spacing, cleats that actually bear.
- Carry collateral loads in the design, and control later additions through a change check rather than an assurance that it is only a small duct.
- Provide an adequate fall to sized outlets so ponding cannot start, and check the roof after the first rains.
- Camber long spans against the drainage fall rather than against a straight line, so the camber does not flatten the roof.
- Check the compression flange for the reversed load case, and restrain the bottom flange wherever uplift puts it into compression.
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: bending resistance, lateral torsional buckling and deflection limits for the structure.
- IS 875 — imposed and wind loads, which set both the strength case and the serviceability case.
- IS 808 — dimensions and properties of hot rolled sections, from which the section modulus and second moment of area are taken.
- AISC 360 — an alternative bending and serviceability basis, used where a client specifies it.
- Eurocode 3 (EN 1993-1-1) — bending resistance and lateral torsional buckling 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.