15 entries
Structural engineering & failure
How steel structures carry load and how they fail: buckling, connection failure, uplift, fatigue, corrosion and settlement.
Entries
Steel column bucklingA 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.Failure mechanismBeam bending and deflectionA 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.Failure mechanismInstability of steel trusses out of planeA truss is efficient in its own plane and close to helpless outside it. Instability comes from a compression chord or a compression diagonal moving laterally, twisting, or buckling between the panel points where it was supposed to have been restrained. Trusses therefore fail during erection, and during maintenance work that removes purlins or bracing, far more often than in a complete and fully restrained roof.Failure mechanismFatigue failure in welded steel structuresFatigue is damage accumulated by repeated stress, and what drives it is the stress range — the difference between maximum and minimum stress in a cycle — together with the number of cycles, not the peak stress on its own. A detail can be safe under a static load that never varies and fail under a much smaller load that repeats a few million times. Welded details are the usual origin, because a weld toe is a built-in stress concentration and a built-in crack starter rather than a smooth surface.Failure mechanismCorrosion and structural deterioration of steelCorrosion removes section, and a reduced section carries less load. Whether the loss is uniform wastage or localised pitting, crevice or galvanic attack decides whether it can be measured and assessed or whether it stays hidden under a coating that still looks sound. The controlling variable in service is seldom the paint brand: it is the surface preparation beneath it and the corrosivity of the environment the system was selected for.Failure mechanismBolted connection failureA bolted connection fails in whichever of its several modes is weakest, and the bolt is frequently not the weak one: the plate tears out to a free edge, the net section ruptures between holes, an unstiffened end plate bends and pries the bolts, or a slip-critical joint slips because it was never pretensioned. The engineering question is not how many bolts are in the group but whether the joint that was installed is the joint that was analysed, and that is where most of the trouble sits.Failure mechanismWeld failureA weld rarely fails because the steel was too weak. It fails because the joint that was made is not the joint that was analysed: the throat is thinner, the effective length is shorter, the fusion is intermittent, or there is a crack in it that opened the night it cooled. Capacity is a product of geometry, so a millimetre of leg lost, or a bead that never reached the root, can remove a large part of the strength of the connection while leaving a weld that looks finished.Failure mechanismAnchor bolt pull-outAn anchor bolt in tension usually fails in the concrete before it fails in steel. The concrete breaks out in a cone that starts at the embedded end of the anchor and runs up to the free surface, so the capacity depends on how deep the anchor actually went in and how much concrete there was around it, not on the diameter of the bar. A bolt specified as a certain size is really a concrete cone of a certain size, and every millimetre of lost embedment or lost edge distance comes straight off the load it can carry.Failure mechanismChemical anchor bond failureA bonded anchor holds by adhesion rather than by expansion: the adhesive has to bond to the steel, bond to the concrete, and the concrete around the hole has to stay intact. It is the most installation-sensitive anchor type in common use, because the bond is made against dust, moisture and temperature rather than across a mechanical interface. A hole that was drilled and set in a few minutes can look perfect and carry very little.Failure mechanismBracing system failure in steel framesBracing is the part of a steel frame that resists lateral load, and without it a set of pinned columns and rafters has no dependable stiffness against sway. A bracing system fails when it is under-designed, incomplete in one direction, over-slender, or removed before the frame can stand without it. The outcome is not usually a broken member but a mechanism, and a mechanism moves suddenly and by a large amount.Failure mechanismRoof uplift and fastener failureWind flowing over a low-pitch roof pulls it upward rather than pressing it down, and the suction is strongest at the edges, the ridge and the corners — precisely where the sheeting is most often fixed at the same spacing as the middle of the roof. The load enters the structure through the fasteners, the smallest and least considered parts of the roof. When they let go the sheet leaves, the purlin loses the restraint the roof was giving it, and the bay next to it is weaker than it was.Failure mechanismFoundation settlementSettlement is the movement of a foundation into the ground under load, and it is checked separately from whether the ground can carry that load at all. A footing can hold a wide margin against bearing failure and still settle enough to distort everything above it. Total settlement is rarely the problem; differential settlement between footings is, because steel frames, masonry and sheeted roofs all distort when their supports move by different amounts.Failure mechanismCollapse of temporary support, propping and falseworkProps, staging and falsework are designed structures carrying a stated load at a stated spacing, and they fail as soon as either of those changes. Collapse usually follows a base that punched into the ground, a system that was never braced laterally, or a prop struck before the concrete above it could carry itself. A prop failure is sudden and total, so the warning is almost always in the arrangement rather than in the individual member.Failure mechanismCrane and lifting failure during erectionA crane fails a lift when the load is heavier, further out or less favourably rigged than the load chart allows, or when the ground under it cannot carry the reaction. Most lifting failures are decided before the load leaves the ground, at a radius measured too early, with the rigging weight left out of the sum, or with an outrigger standing on fill. Once the load is in the air there is very little margin left to recover the error.Failure mechanismDamage to adjacent structures from excavationAn excavation changes the stresses in the ground around it, and everything founded in that ground responds to the change. Damage appears when a footing is undermined, when the soil beneath it settles as the water table is lowered, or when the retained face moves — and the structure that suffers is normally the neighbouring one, not the one being built. The 45-degree load spread is a starting point for the geometry of that influence, not a substitute for assessing it.Failure mechanism
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