Temporary Works: Slings, Wind, Battering and Haul Cycles

Temporary does not mean lightly engineered

Temporary works carry real loads, fail in real ways, and are responsible for a disproportionate share of construction incidents. What makes them different from permanent structures is not the standard of engineering but the CONDITIONS: a short design life, a structure that changes during its life, load cases that only exist during construction, and erection by people who did not design it.

The load cases in particular are their own. A frame that will eventually be clad and braced spends weeks unclad and partially braced; a member that will be propped at mid-span is unpropped while it is placed; a crane's worst moment is often a slew with a load at radius rather than the lift itself.

This is why temporary works are designed, checked and signed for like permanent ones, and why the calculations here are screening tools that feed that process rather than replacing it. Every page says so, and it matters more on this page than on most of the site.

Sling angle multiplies tension, and shallow is the trap

Two legs sharing a load do not each carry half of it unless they are vertical. As the legs spread, each one has to supply the same vertical component from a force that is increasingly horizontal, so the TENSION in the leg rises while the weight stays exactly the same.

The numbers are unforgiving. At sixty degrees to the horizontal each of two legs carries about fifty-eight per cent of the load rather than fifty. At forty-five degrees, about seventy-one per cent. At thirty degrees — which looks like a normal working spread — each leg carries the FULL weight of the load. Below thirty the rise is steep and most codes and manufacturers prohibit it.

The horizontal component is the other half of the problem and it goes somewhere. It squeezes the load inwards, which is how thin-walled items are crushed by their own lifting arrangement, and it loads the lifting points sideways when they were designed for a vertical pull.

Two further practical points. A three- or four-leg sling does not share equally in practice — rigid loads and unequal leg lengths mean two legs commonly take almost everything, so the conservative assumption is that only two are working. And the rated capacity on a sling tag is quoted for stated angles, so reading the vertical rating and then spreading the legs is using a number that no longer applies.

Wind: the square of speed, and the area you did not think you had

Wind force grows with the SQUARE of wind speed, so a gust twenty-five per cent above the working limit delivers over fifty per cent more force. That non-linearity is why in-service wind limits are set well below anything dramatic and why they are absolute rather than advisory.

The area is the term that changes most between a plan and a site. An open scaffold presents its members and little else; the same scaffold SHEETED or netted presents an almost solid face, and the wind load on it can be an order of magnitude higher. That is why sheeting a scaffold is a design change requiring more ties and often a check of the supporting structure, and why sheeting added by a contractor for weather protection is a recognised cause of collapse.

A crane has three separate wind conditions rather than one. In service, the limit is usually set by the load swinging and by the operator's control rather than by the crane's strength — and a large light load, a panel or a tank, reaches that limit at a far lower wind speed than its weight suggests, because what matters is its SAIL AREA. Out of service, a tower crane must be free to weathervane so it presents its minimum area, and a slew brake left applied in a storm is a known failure mode. And the erection condition is its own case again.

Temporary fencing and hoarding fail differently, by OVERTURNING rather than by breaking. The check is a moment balance about the toe: wind pressure times area times the height of its centre, against ballast weight times its lever arm. Height enters twice — a taller panel has more area and a higher centre of pressure — so raising a hoarding by half increases the overturning moment by considerably more. And because panels are coupled, the failure of one is a domino run rather than a single panel down.

Battering and setback: the load at the crest is the one that is forgotten

An unsupported excavation stands because the soil can form a slope. How steep a slope depends on the soil's type and condition, and the codes express it as a permitted ratio per soil class — which makes the width of ground consumed at the top a direct function of the depth.

That width matters more than it looks on a drawing. A deep excavation battered to a safe angle can consume several metres either side, and on a constrained site that is frequently the reason a trench box or a sheeted excavation is used instead: not because the batter would not work, but because there is nowhere to put it.

The commonest defect is a SURCHARGE at the crest. Spoil heaped beside the trench, a plant track, material stacked ready for installation, or an existing foundation nearby all add load to the ground above the slope, and the batter was calculated without them. Codes therefore specify a minimum setback for spoil and plant, and that setback is part of the excavation's design rather than a housekeeping preference.

Water changes the classification rather than the margin. Soil that stands at one angle dry does not at the same angle saturated — as the soil-state paper sets out, seepage roughly halves a slope's factor of safety — so a batter that stood through a dry week is not evidence about the same batter after rain, and re-classification after water enters is a requirement rather than a precaution.

Dropped and chuted material: the energy is the height

Material falling down a chute or a void arrives with kinetic energy proportional to the height it fell through. Doubling the drop doubles the energy and multiplies the arrival speed by about one and a half.

The force at the bottom is not that energy, though — it depends on the STOPPING DISTANCE. Material landing on a bare steel plate stops in millimetres and delivers an enormous peak force; the same material landing in a skip with a bed of debris already in it stops over a much greater distance and delivers far less. This is why chutes discharge into a container that is kept partly full rather than emptied to the floor, and why the first load into an empty skip is the worst one.

Chutes are also a containment problem as much as a structural one. A drop with no chute spreads material sideways as it falls and produces both an exclusion zone and a dust cloud, and the clearance needed at the discharge — for the skip, for the exchange vehicle, and for people to stay clear during discharge — is usually larger than the chute's own footprint.

Needle props and similar temporary supports are the opposite condition and carry an equally under-rated risk: they hold a load that is already there. Their failure is not an impact but a redistribution, and because a needle carries a tributary width of wall above it, the load it takes is a strip rather than a point — which is the calculation that is most often done by eye and should not be.

Haul cycles: adding trucks to a short haul achieves nothing

An earthmoving cycle is the sum of loading, hauling, dumping, returning and queueing. The haul and return vary with distance; the rest are broadly FIXED, and on a short haul those fixed elements dominate the total.

The consequence is a hard limit on throughput. Production is governed by whichever of the loader and the truck fleet saturates first, and on a short haul the loader saturates quickly. Once it does, adding trucks does not increase output — it only lengthens the queue, so each truck's cycle time rises by exactly what was gained. The fleet gets more expensive and the site moves the same volume.

The quantity that captures this is the MATCH FACTOR, the ratio of the fleet's demand for the loader to the loader's capacity to serve it. Near one, both are busy and neither waits much. Above one, trucks queue. Below one, the loader idles. Matching it is the whole of haulage optimisation, and it explains why a long haul supports a larger fleet than a short one carrying the same volume.

Two inputs move the answer more than the truck's payload does: the loader's bucket and cycle, which set how long loading takes, and the haul road, whose gradient and condition change speed far more than horsepower does. And every volume in the calculation has to be the same one — the bulking paper's distinction between bank, loose and compacted measure is the usual source of an error that makes a fleet look adequate when it is not.

Measurement conventions, and what a screening figure is for

Scaffold face area is a measurement convention before it is a quantity: the area of the elevation the scaffold serves, measured to rules that decide how returns, projections and open ends are treated. It is what the scaffold is priced by and what its wind area is derived from, and two people measuring it differently will price the same scaffold differently without either being wrong about the scaffold.

Toe boards, guard rails and their dimensions are likewise prescriptive rather than calculated — heights, gaps and the sphere or edge-protection criteria come from regulation, and the arithmetic on the page is a compliance check against those figures rather than a structural one.

All of which sets the boundary. These calculators estimate a tension, a force, a setback, an energy or a cycle time, and they are useful for deciding whether an arrangement is roughly right before a temporary works designer is engaged. A temporary works design is a checked and signed document under a coordinator, lifting operations are planned by a competent person, and an excavation is inspected before every shift. No figure here substitutes for any of those, and the pages say so explicitly rather than in a footnote.