Good form, fast work – Crane-independent solution ensures speedy progress Print this article

Soaring 81 metres high and sited directly on the bank of the River Lagan, the Obel Tower is easily the most spectacular build in progress in Belfast. Located in the heart of the city, the 28-floor apartment tower claims the status of highest building in Northern Ireland. Here we look at the innovative crane-independent formwork solution deployed on the project

O´Hare & McGovern Ltd.is lead contractor on the Obel Tower project. The contractor opted for a single-source formwork solution from Doka Ireland. Out of the ordinary in its architecture, the Obel Tower is also a challenging build in many ways because of the difficult boundary conditions singular to this project.

Space is at a premium on the site, which is bordered by a six-lane urban motorway, an elevated railway and the Lagan River itself, so only one slewing tower crane can be accommodated and the solution for formwork lifts has to be totally crane-independent. The project brief, moreover, calls for progress at the rate of one 600 m² floor fully cast per week of build time. This is the single most important prerequisite for compliance with the tight construction schedule.

Planning for the formwork on this build had to be detailed and trimmed for maximum efficiency, with every allowance made for the extremely limited crane capacity. Doka Ireland was commissioned to supply the complete formwork solution for all the CIP concreting.

"We are using automatic climbers for the CIP core and the table lifting system (TLS) for crane-independent lifts of the tables, so we can successfully deal with both those key challenges – limited craneage and the one-week cycle for the deck slabs,” says Ronan McHugh, project manager, explaining his confidence in this end-to-end concept.

Safety must be maintained at the highest possible level throughout every phase of the build, so the lead contractor has also opted to use the Xclimb 60 protection shield. As the building rises, the slab edges of the top four floors are constantly protected inside a full enclosure and everyone inside is shielded against the inclemency of the weather.

The shield also has to be climbed as rapidly as possible as soon as forming work is completed, so the formwork engineers allowed for a second platform level for the follow-on work on the CIP slabs.

Climbing 80 metres in a weekly cycle
The central CIP core of the building is being climbed with 22 of the high-end, high-capability Doka SKE 50 automatic climbing brackets. To keep setup times short and help minimise the complexity of on-site logistics, the working platforms for the climbers, and 250 m² of beam formwork, were delivered to site on a just-in-time (JIT) rota.

The potential of the automatic climbing formwork had to be exploited to the full right from the word go, so an experienced site foreman from Doka familiarised the site crew with the most efficient work routines and supervised final assembly.

“The assistance we had from the Doka site foreman was extremely important in terms of enabling us to meet schedule on the weekly cycle for the CIP core. The standard height is 2.8 metres, and we are able to form up, set the reinforcement, and cast the concrete within two or three days with only two men”, explains Ronan McHugh. The SKE 50 climbers have the adaptability to deal with changes in cross-section, so the two reductions in wall thickness between floors 10 and 20 were easily handled without time-consuming modifications. The shaft formwork has to be quick-lifted, so the Top 50 panels are combined with Framax stripping corner ‘I’ units to form lift-out formwork boxes.

The stripping corner ‘I’ units enable each of the beam-formwork panel assemblies to be backed off quickly from the concrete, so the shaft formwork can be lifted up into position for the next pour without any further disassembly. The formwork for the lift shafts is climbing with SKE 50 plus automatic climbers fitted up with a central climber section.

Seven SKE 50 automatic climbing formwork brackets are in use on the central stairwell shaft, carrying not only the wall formwork but also a concrete pump distributor up from floor to floor as progress on the high-rise continues.

TLS Table Lifting System
The floor slabs have a 600 m², sickle-shaped footprint and are being formed by the site crew with 150 identical Dokaflex tables. This approach ensures optimum adaptability of the floor-slab formwork to the unique floorplan with no need for the large adapter panels that would otherwise take a great deal of time to prepare.

Crane capacity is extremely limited and ideally, floor-slab forming should be crane-independent, so the tables are being lifted from level to level with two TLS table lifting systems. “It’s very much on account of the fact that we are using these two TLS units that we’ve been able to maintain this tight weekly cycle right through this phase of the build. The TLS units keep our shifting times short and allow us to reap the benefit of continuous and speedy workflows,” Ronan McHugh emphasises.

The table lifting system significantly reduces labour costs by comparison with craned lifts, according to Ronan. Lifting the floor-slab tables is a job easily handled by two labourers working on their own. One man with a Doka electro-hydraulic trolley removes the tables from their original positions and shifts them onto the TLS lifting platform.

He then hits a button to take the tables straight up to the next level in a matter of seconds. The other member of the two-man crew is waiting with a second trolley to take the tables off the lift and manoeuvre them straight into position. In terms of industrial safety, too, the TLS has a huge lead over crane lifts.

The table lifting system is permanently fixed to the structure, it incorporates a raft of active and passive safety features and can safely be used on high-rise builds when the wind is blowing strongly – in other words even after safety considerations have brought the site cranes to a halt.