An interesting construction challenge we often get asked about is - how do contractors get the crane off a tower's roof when the build is finished? To answer this, we've run through some key points and used a tricky CBD tower project as a case study.
Site Challenge
During construction, tower cranes are placed external to the building, with the base at the ground level
Crane ties are installed on every few floors up the building to stabilize the crane
These ties create gaps in the facade which need to be finished by removing the ties and putting in the final facade panels and finishing internal works
A crane is needed on top of the roof to finish the facade and dismantle the external tower cranes which creates an interesting challenge - how do you remove this roof crane once the other cranes have been dismantled?
Tight logistical and engineering requirements always make this process more difficult than it might seem
The cranes and slabs need to have the right reach and capacity to dismantle each other and there needs to be enough room to manoeuvre the cranes and then pull them apart, piece by piece
Risks
If planning, coordinating and scheduling isn’t done perfectly the roof crane could end up being stuck on top of the building - Not good for the site teams' reputation at the pub
If the engineering is incorrect the crane could collapse destroy both the supporting slabs and possibly falling off the building - Not good for the site teams employment
If the right exclusion zones aren’t used and the teams on-site don’t follow the right safety procedures someone could get seriously injured - Not good, full stop.
The solution - 3 stage recovery crane process
To remove an isolated crane from a high-rise roof, you need to build another slightly smaller crane on the roof next to it, to remove the original larger crane. This process then continues progressively until the last crane installed is small enough to be dismantled on the roof by workers using winches, and then for all components to individually fit down the goods lift;
A key components of planning a crane recovery is:
Planning must be done in advance of the structure reaching the top of the roof, as a lot of additional structural strengthening work is often required to put into the slabs and core walls etc;
Design items to consider include;
Picking the right position of the crane - this needs to allow sufficient room for the 2’nd crane to reach all key lifting points as well as needing to be positioned near columns and core walls where there is the greater load capacity;
Often, the slabs will need to be further reinforced and have temporary propping support below to accommodate the extra load from the crane;
Testing of all cast-ins and pull points is required before installing the cranes. You don’t want to find a weak point after the crane is installed!;
If the crane components overhand the building, clear exclusion zones, and barriers are required, as well as informing road closures to council well in advance.
The final crane is often a Derek crane, which can be built manually and the components are small enough to take up and down in the Goods lift. This now means everything can be taken off the roof, and the tenants don’t have a random crane sculpture on their roof.
