Failure mechanism
Collapse of temporary support, propping and falsework
Props, 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.
How this failure works
A prop is a column. It has a buckling capacity, an effective length, a bearing pressure at each end and a lateral restraint requirement, and it was designed for a stated load at a stated spacing — which means that altering the spacing alters the design. The load it carries is not the weight of the slab alone: it is wet concrete, formwork, workers and their equipment, and the impact of placing, and where props stand in more than one tier it also includes load from the tier above. In a designed system none of that is guesswork. Where the props are whatever was in the yard and the spacing is whatever the deck allowed, all of it is guesswork and none of it is written down.
The base of a prop concentrates its load onto a small area of whatever is underneath. On prepared ground the bearing capacity has to be adequate beneath a sole plate sized for the job. On a suspended slab the prop load has to be checked against the slab, and against the slab below through the backprops — this is punching shear at the base, and it is the failure mode that removes a whole support line at once rather than one prop at a time. Ground that is firm when it is dry carries a great deal less when water reaches it, and a prop standing in a puddle is already a different support from the one that was designed.
Falsework is braced laterally in both directions, to the ground or to the permanent structure, and that bracing is what turns a grid of props into a structure rather than a field of independent columns. Without it the system has no stiffness against horizontal forces that are always present: an out-of-plumb prop, a concrete hose swung across the deck, wind on the exposed face of the formwork, a vehicle nudging a standard. Failure is then by sway, and the load at which sway becomes collapse is far below the axial capacity of the props themselves — which is why a falsework collapse looks like the whole system going over rather than one prop giving way.
Striking is a structural operation, not a finishing one. The sequence, the age or measured strength at which each element is released, and the order in which the props come out are all part of the design, because releasing props redistributes load into the permanent structure and that structure has to be strong enough and connected enough to take it. Striking a prop early drops load onto concrete that has not reached its design strength, and the deflection that results is permanent. Striking out of sequence — one end of a span before the other, or one tier before the tier beneath — produces forces the structure was never analysed for. A prop left in place beyond its time has the opposite problem: it holds load the permanent structure is supposed to be carrying, so that structure is never proven under its own load.
Illustration
Conceptual illustration of a propped slab with an unbraced prop grid off plumb, bearing failure at one base, and the load path continuing down to the slab below.
Typical causes
- Falsework assembled by the crew rather than designed, with props at whatever spacing the deck and the available props allowed.
- Total load underestimated, omitting the weight of formwork, construction live load, placing impact, or the second tier of props.
- Bearing at the base inadequate — props on unprepared ground, on soft fill, or on a slab never checked for the concentrated load.
- No lateral bracing, or bracing in one direction only, or bracing removed to let people and barrows through.
- Props struck out of sequence, or before the concrete had reached the strength the design required.
- Props over-extended, of mixed type, or bearing on too few threads of the extension pin in one support line.
- Load concentrated at a point during placing, with concrete heaped before spreading, rather than distributed as assumed.
Warning signs
- Props visibly out of plumb, or a line of props all leaning the same way.
- Sole plates bedding into the ground, ground cracking alongside, or water standing at the base of a prop.
- Bearing timbers crushed, split or compressed at the head or the base of a prop.
- Formwork deflecting as concrete goes down, and the deflection not recovering once the placing moves on.
- A falsework deck that moves when someone walks across it, or props that wobble when leaned against.
- Missing bracing tubes, loose couplers, or standards sitting on the ground without a base plate.
Inspection methods
- 01Check the falsework against an approved design before the pour: spacing, member size, extension, bracing in both directions, and bearing under every base.
- 02Check the ground or slab beneath each prop for bearing, and confirm sole plates or bearers are present and properly bedded.
- 03Plumb-check a sample of props in both directions and record the readings against the tolerance in the design.
- 04Verify concrete strength at striking by test rather than by elapsed time, and confirm the striking sequence against the design before any prop is released.
- 05Watch the placing operation itself — rate, sequence and where the concrete is being dropped — against the method that was agreed.
- 06Inspect props and accessories for straightness, damage and extension limits before each use rather than at the first use only.
Preventive measures
- Design falsework and staging as an engineered system with a stated load, spacing, bracing arrangement and striking sequence.
- Prepare and check the bearing under every prop, with sole plates sized for the load rather than for convenience.
- Brace the system laterally in both directions and keep that bracing in place until the falsework is struck.
- Agree the concrete placing sequence and rate in advance, brief them, and forbid heaping concrete at a point.
- Strike only on measured strength and only in the sequence the design states.
- Keep props within their rated load and extension, inspect them before use, and do not mix systems in a single support line.
- Hold a pre-pour inspection with a signed check sheet, and stop the pour where the arrangement does not match the drawing.
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 2750
- IS 3696
- IS 456
- IS 800
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.