Adding a lift to an existing building

Adding a lift to an existing building

Retrofitting a lift into an existing building comes up constantly in Sydney, driven by accessibility upgrades, apartment conversions and owners wanting to stay in a home as they age.

The lift itself is a supplier item. What the engineering covers is the hole it goes in, and that hole runs from below the footings to above the roof.

Cutting the shaft removes structure

A lift shaft is a vertical opening through every floor. Each of those floors was designed as a continuous element, and cutting a rectangle out of it interrupts the load path that used to pass through that area.

The slab around the opening now has to span differently. Where the opening cuts a beam, the beam has to be replaced or the load rerouted. Where it cuts a load-bearing wall, everything that wall was carrying needs a new path.

That is manageable. It just has to be designed floor by floor rather than treated as one detail repeated.

The shaft usually becomes a structural element itself

In most buildings the practical answer is to make the shaft do work. A concrete or masonry shaft is a stiff vertical box, and once it is tied into each floor it can carry the loads that the removed structure used to carry, plus contribute to the lateral stability of the building.

That is often the cheapest solution, because it turns the problem into the fix. It does mean the shaft has to be designed as structure from the start rather than as an enclosure the lift sits in.

The pit is the part that surprises people

Almost every lift needs a pit below the lowest served level, and in an existing building that pit is being excavated next to, and often below, existing footings.

Excavating below the founding level of an adjacent footing removes the support under it. That requires either underpinning the footing first, or designing the pit walls to retain the ground and carry the adjacent load, which is a retaining structure in a confined space with no room to work.

If the water table is high, the pit also has to be tanked and designed against uplift, because an empty concrete box below the water table will try to float.

This is where a geotechnical assessment earns its cost. Ground conditions and water level decide how difficult the pit is, and they are the first things to establish rather than the last.

Overrun and headroom

The shaft has to continue above the highest served level by an amount the lift manufacturer specifies, which frequently means going through the roof.

On a heritage building or one with a strict height control that is a planning problem as much as a structural one. Machine-room-less lifts reduce the requirement but do not remove it, and the option chosen affects both the overrun and the loads at the top of the shaft.

Loads at the top and bottom

The lift applies concentrated loads where the guide rails are fixed and where the machine and the buffers bear. Those are supplier-specified and they are significant, particularly the buffer loads at the base under an emergency stop.

Those loads have to be carried by the shaft and taken to the ground, and they need the supplier's actual load data rather than an assumption. Designing before that data exists means redesigning.

Fire and separation

A shaft passing through floors is a penetration through every fire-separating element in the building, and it has to be constructed to maintain that separation. In an apartment building that is a substantial part of the specification.

The order that works

Establish the ground and the water level. Get the supplier's load and dimension requirements. Then design the shaft as a structural element, the openings floor by floor, and the pit with the adjacent footings assessed.

We handle this as structural engineering alongside the geotechnical work, and the projects that run smoothly are the ones where the pit was investigated before the lift was ordered.