Demolition sequencing and structural stability

Demolition looks like the simple end of construction and it is one of the higher risk activities on a site.

The reason is that a building is only stable because all of its parts are present. Remove them in the wrong sequence and the structure that remains may not be capable of standing on its own.

Stability is a system property

A wall is held upright by the floor at its base, the floor or roof at its top, and the walls returning at each end. Remove the roof and that wall has lost its top restraint. It is now a free-standing wall in the wind, and free-standing masonry is not stable at any real height.

A frame is braced by its floors acting as diaphragms. Remove a floor and the frame loses its bracing at that level.

None of this is visible while the building is intact. It becomes apparent only as the demolition proceeds, which is why the sequence has to be designed rather than improvised.

Partial demolition is the higher risk case

Full demolition of a detached building is comparatively simple. The risk concentrates in partial demolition, and that is most of what happens in Sydney.

Removing the rear of a terrace while the front and the party walls stay. Taking out a floor to create a void. Removing part of a building attached to a neighbour. Stripping a building back to its frame for a refurbishment.

In every one of these, something is being kept, and what is kept was relying on what is going.

Party walls and attached buildings

A shared wall between two terraces is doing structural work for both. Demolishing one side removes lateral restraint from that wall, exposes it to weather it has never seen, and can leave the neighbour's building relying on a wall that is no longer supported.

This needs assessment before anything starts, temporary support designed and installed, and usually a dilapidation record of the neighbouring property.

What a demolition engineering assessment produces

A sequence. The order in which elements are removed, so that at no point is the remaining structure unstable.

Temporary works. Propping, shoring, bracing and needling designed for the loads that exist at each stage, installed before the element it replaces is removed.

Load paths at each stage. What is carrying what, at every point in the sequence, not only at the start and the end.

Identification of what must not be touched. Elements that look removable and are carrying something. This is where most incidents originate.

Plant and access. Where machinery can operate, what it can stand on, and what the remaining structure can tolerate in terms of vibration and impact.

The hazards that are not structural

Asbestos. Buildings of a certain era contain it in sheeting, insulation, flooring and elsewhere. It has to be identified before demolition begins and removed under a licensed process, not discovered during.

Services. Live electrical, gas, water and communications, including services running through the building to somewhere else.

Adjoining property. Vibration, dust, noise, and the physical support the neighbour was receiving.

Excavation. Where demolition includes removing footings or basement structures, the ground support to adjacent buildings changes.

Where it goes wrong

The most common failure is not a collapse. It is discovering mid-demolition that something that was scheduled for removal is holding up something that is staying, and having to stop and design a solution with the building half open.

That is expensive, it exposes the site to weather, and it is entirely preventable with an assessment beforehand.

What we provide

A structural assessment before demolition, the sequence, the temporary works design, and the certification the certifier and the council require.

We handle this under structural engineering and project management, including the dilapidation records for adjoining properties that protect you if a neighbour makes a claim afterwards.

The assessment is a small proportion of a demolition cost and it is the part that stops the job becoming an incident.