Scaffolding ties and loads on existing structures

A scaffold looks independent. It is not.

Beyond a modest height, a scaffold relies on being tied to the building for stability, and every tie is a load applied to that building at a point somebody chose.

On a new concrete frame that is trivial. On an older masonry building, a heritage facade or a lightweight structure, it is a real engineering question.

Why scaffolds need ties

A tall, narrow, open structure is not stable against wind on its own. The ties transfer horizontal load into the building, which is stiff and heavy.

The number and spacing of ties depends on the scaffold height, the width, whether it is sheeted or netted, and the wind exposure of the site.

Sheeting matters enormously. A scaffold wrapped in shade cloth or plastic becomes a sail, and the tie loads increase substantially. A scaffold designed unsheeted and then wrapped is a common and serious problem.

What the ties do to the building

Each tie applies a point load, in tension and compression, into whatever it is fixed to.

Into masonry, that is a load into a wall, usually via an anchor into brickwork or into a mortar joint. Old lime mortar has limited capacity. Weathered brick has less than it looks. A tie that pulls out takes a piece of the wall with it.

Through a window opening, using a transom across the reveal. This spreads the load into the jambs, which is often better than an anchor, and it depends on the reveal being sound.

Into a concrete frame, which is generally straightforward provided the anchor is correctly specified and installed into structural concrete rather than into a topping or a facade panel.

Where the loads go that nobody checks

Facade panels and cladding. A tie into a non-structural panel loads an element that was designed to carry its own weight and wind pressure, not a point load from a scaffold.

Parapets. Frequently used because they are convenient and frequently the weakest part of the building. A masonry parapet is a cantilever with limited capacity and it is often in poor condition.

Awnings and verandah roofs. Scaffold base loads landing on a footpath awning rather than on the ground.

The footpath and the ground. Scaffold base loads are concentrated at the standards. On a footpath over a basement, a cellar, a service trench or soft ground, those loads have to be checked and usually spread with sole plates or spreader beams.

The heritage case

Heritage facades are the highest risk version of this.

The fabric is old, often in variable condition, and the whole point of the project is usually to preserve it. Drilling anchors into significant fabric may be restricted by the approval conditions.

The answer is usually a combination of window transoms, ties into locations agreed with the heritage consultant, and in some cases an independently supported or partially free-standing scaffold with its own ballast, which is more expensive and does not touch the facade.

What an assessment covers

The scaffold design loads, including the sheeting condition, which has to be stated rather than assumed.

The capacity of the existing structure at each proposed tie location, based on inspection rather than on the age of the building.

Alternative tie arrangements where the preferred locations are not capable.

Base loads and the capacity of whatever the scaffold is standing on.

Any temporary strengthening required.

Why it is worth doing

Scaffold failures are rare and they are severe when they occur, and the more common outcome is less dramatic: ties pulling out of masonry, damage to a facade that then has to be repaired, or a scaffold that has to be redesigned after erection because the building could not take it where it was planned.

We assess existing structures for scaffold tie and base loads under structural engineering, including heritage facades and buildings over basements and services.

Doing it before the scaffold is designed costs a fraction of doing it after it is up.