Failure mechanism
Instability of steel trusses out of plane
A 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.
How this failure works
Within its own plane a truss is a triangulated assembly in which every joint is a two-force node, members carry axial force only, and the depth of the truss delivers bending stiffness for very little steel. Out of that plane there is no triangulation at all. The truss becomes a flat frame with negligible transverse stiffness, and its resistance to lateral movement rests entirely on secondary members, bracing and the deck. The two are different kinds of check: the in-plane design is a set of member forces, and the out-of-plane design is a stability problem in which the restraint condition, rather than the member size, usually governs.
The compression chord of a loaded truss is a strut. Its buckling length is not the span, it is the distance between the points at which it is genuinely held against lateral movement — normally the panel points, where purlins or secondary members cross it and are fixed to it. A chord restrained only at its ends has an effective length several times longer and a capacity a fraction of the designed value. Where purlins run across the chord and are bolted down, they restrain it. Where the purlin line exists on the drawing but the purlins are stacked at the side of the roof, or are resting on the chord without their cleats, it does not. The same argument applies to compression diagonals and verticals, and it applies with more force there because their slenderness is higher to begin with.
Even with both chords held laterally, a truss can twist. The chords are separated by the depth of the truss, so load applied to one chord, or restraint provided to only one of them, produces a torsional response that appears as differential lateral movement of the two chords and as force in the plane of the roof. In a slender roof truss with light top chord restraint this is a real mode rather than a theoretical one, and it is why the bracing in the plane of the top chord, together with the vertical or diagonal bracing between adjacent trusses, belongs to the roof design and not to the fitter who happens to have the rods on the truck.
Erection inverts the whole arrangement. A truss arrives as a plane, is lifted as a plane, sits temporarily on two bearings at its ends, and is restrained by nothing until the purlins and bracing between it and its neighbours are made good. That is the state in which the compression chord has its longest unrestrained length, in which wind acts on the full profile of an unclad member with no sheeting to shelter it, and in which the only resistance to falling sideways is whatever temporary guys the erection plan provided. A truss released before its purlin line is fixed is a plane standing on edge, and a plane standing on edge has a very small reserve before it becomes a plane lying down.
Illustration
Conceptual illustration of a roof truss line with the compression chord bowing between panel points, and the purlin restraint it should have had shown against it.
Typical causes
- Compression chord left unrestrained between panel points because secondary members were removed, or had not yet been fixed.
- Erection proceeding with temporary guys only, and those guys released before the permanent purlin line was made good.
- Top chord plane bracing incomplete, or omitted in bays left open for a monitor, a vent or an access hatch.
- Load applied to one chord alone — a concentrated services load or a purlin line bearing on the top chord with nothing to balance the resulting torsion.
- Trusses handled, transported or stacked without restraint against twisting, so the member is deformed before it is ever loaded.
- A chord or diagonal replaced or reinforced with a section that changed its slenderness or the fit at the node.
- Construction or snow load applied to a partly clad roof, with the truss restrained less than the finished condition assumes.
Warning signs
- A chord bowing laterally along the line of the truss, visible under load or in the heat of the afternoon.
- Purlins resting on the chord without cleats, or a run of fixings simply left out along a truss line.
- Trusses out of plumb relative to one another, or a whole bay leaning as a unit rather than a single member.
- Racking or creaking in the roof plane as wind or a crane load passes, with the movement recovering afterwards.
- Local distortion at a node — a bent gusset, a buckled leg of an angle, or a kink in a chord at a panel point.
- Diagonal bracing members that are visibly slack, bowed, or disconnected at one end.
Inspection methods
- 01Check the restraint condition rather than only the section: for each compression chord, establish what actually holds it laterally and at what spacing, and record the spacing you measured.
- 02Confirm that every purlin or secondary member shown as restraint is fixed and not merely bearing, before erection tackle or temporary guys are released.
- 03Survey the trusses for plumb and the chords for straightness along their length, at erection and at intervals afterwards.
- 04Inspect nodes for local deformation and gusset condition, including the welds and every bolt group at the chord-to-node connection.
- 05Review the installed roof bracing against the layout bay by bay, in both the top and the bottom chord planes where both are braced.
- 06Before removing purlins, sheeting or bracing for maintenance or a modification, confirm what the truss is relying on and what will replace it.
Preventive measures
- Design the out-of-plane restraint explicitly: state what holds the compression chord, at what spacing, and design the secondary members and their fixings for that restraint force.
- Do not release a truss until the purlins and bracing that stabilise it are made good, or until designed temporary restraint is in place and signed for.
- Include the erection condition in the design, with the chord restraint that exists at each stage rather than only at completion.
- Provide top chord plane bracing and vertical bracing between trusses as one designed system rather than as separate details.
- Handle, lift and stack trusses with restraint, so they are not twisted or bent before installation.
- Prohibit the removal of purlins or bracing without a check of the temporary condition, and reinstate before leaving the roof.
- Keep services and concentrated loads off a single chord unless the truss was designed for the torsion they introduce.
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
- IS 875 (Part 3)
- IS 2062
- AISC 303
- EN 1993
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.