Temporary Bracing Of Precast Panels

A precast concrete panel weighs several tonnes, is tall, thin, and has almost no out-of-plane stability on its own.

In the completed building it is held by the floors, the roof and the panels either side. Between being craned in and being permanently connected, the only thing holding it upright is temporary bracing, and that bracing is one of the highest consequence temporary structures on a site.

What the bracing resists

Wind on a large flat surface. A panel is an efficient sail. Wind load on a tilt-up or precast panel during erection is the governing case, and it is assessed against the wind speed that can reasonably occur during the erection period rather than the fifty year design event.

Construction loads, meaning anything leaning on, hanging from or bumping the panel.

Out of plumb effects, because a panel erected slightly off vertical has a permanent overturning component from its own weight.

Impact, which is why the braces are sized with a margin rather than to the calculated minimum.

The system

Props from the panel to a founded point, usually a floor slab or a dedicated footing, at an angle that gives both compression and tension capacity.

A connection at the panel, cast in at manufacture. Retrofitted anchors into a finished panel are a different and weaker proposition.

A connection at the base, which is where most of the failures originate. A brace anchored into a slab that is too thin, too young, or too close to an edge has the capacity of its weakest link, and that is usually the concrete rather than the steel.

Two braces per panel as a minimum, positioned so the panel is stable about both axes.

Knee braces on tall panels, adding an intermediate restraint so the panel is not spanning its full height between the base and a single brace point.

Where it fails

The brace footing. Anchors into a young slab, into a slab thinner than the anchor needs, near a slab edge or a construction joint, or into a slab that is itself propped. The anchor capacity depends on the concrete strength at the time of installation, not at twenty-eight days.

Braces removed too early, before the permanent connections are complete. This is the single most common cause of precast collapse, and it usually happens because a following trade needed the floor space.

One brace removed to get access and not reinstated.

Weather, where a wind event arrives while panels are braced but not permanently tied, and the braces were sized for a lower speed than occurred. The erection sequence needs a wind management plan, not a hope.

Panels erected ahead of the floor that was supposed to provide the permanent restraint, leaving a long period on temporary bracing that the plan assumed would be short.

Lifting points used as brace points, which they are not designed for.

The sequencing question

The permanent stability of a precast structure comes from the diaphragm action of the floors and the connections between panels. Until those exist, every panel is a cantilever on braces.

Which means the erection sequence is a structural design problem: which panels go up, in what order, what provides restraint at each stage, and at what point each panel can be released. That sequence is the deliverable, not an afterthought, and it belongs with the [temporary works](/blog/backpropping-and-temporary-works) package rather than with the crane schedule.

It also interacts with [demolition and construction sequencing](/blog/demolition-sequencing-and-structural-stability) thinking: a structure part-built is a different structure from the one that was designed, and it has to be stable at every stage rather than only at the end.

Connections and tolerance

The permanent connection is what releases the bracing, and it can only be made if the panel is within tolerance. A panel out of position cannot be connected without modification, the bracing stays on longer, and the risk window extends.

Which makes erection tolerance a safety matter and not only a finishing one, and it is the subject of [precast connections and site tolerances](/blog/precast-connections-and-site-tolerances).

Who designs it

Normally the erector or the contractor, because it depends on their plant, their props and their sequence. It still needs an engineered design, a drawing showing brace positions and anchor details, confirmation of the concrete strength the anchors require, and a documented hold point before any brace comes off.

The building's structural engineer has an interest in it, because the permanent structure is being used as the anchorage for the temporary one. Where that is not coordinated, the result is anchors drilled into elements that were never checked for the load, which is the usual path to [who is responsible when a design changes on site](/blog/who-is-responsible-when-a-design-changes-on-site).

ACSES provides structural engineering and temporary works review across Sydney. Talk to us about a project.