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Two into one does go

14 Oct 14 Two shoring systems, one erected inside the other, have helped contractors speed up a demanding concrete pour in Oman.

The central formwork structure will support the mosque’s massive concrete dome
The central formwork structure will support the mosque’s massive concrete dome

Big concrete pours require big formwork structures for which there are numerous systems available. It’s not often that the complexities of a concreting job exceed the capabilities of existing equipment.

However, Carillion Alawi, main contractor on the new US$64m (£38m) Sultan Qaboos mosque in the Omani city of Nizwa, needed to find a solution quickly when faced with potentially costly over-runs on the project. Changes in the construction programme meant that in order to stay on schedule, concrete for both beams and slabs would have to be cast at the same time.

Formwork supplier RMD Kwikform had already supplied and erected formwork towers using its Alshor Plus system. These birdcage structures were among the tallest Kwikform towers ever erected and were designed to support the casting of the roof-dome, concrete slabs and beams in sequence.

But, despite permissible loadings of up to 120kN/leg, the structure was not strong enough to support the loads imposed by the huge volumes of liquid concrete resulting from casting the slabs and beams at the same time. Alawi Carillion needed some way of reinforcing the existing formwork and naturally turned to RMD Kwikform for a solution. Mohammed Tariq, project manager for Carillion Alawi says that having worked with RMD Kwikform on numerous other projects in Oman, including the Wave retail centre in Muscat, he was familiar with the capabilities of the Alshor Plus system.

“So for the initial sequencing for the project we opted for this solution, with RMD Kwikform site staff supporting us with the system design and erection training,” says Tariq.

“When we decided to change the sequencing as part of a fast-track programme, we asked RMD Kwikform to help us design a solution that did not involve us dismantling the already erected Alshor Plus towers.”

Working with the contractor, RMD Kwikform analysed the additional loadings and suggested constructing towers using its Rapidshore system inside the Alshor Plus towers, then bracing the two together to create a combined shoring tower structure.

By combining the two systems, Carillion Alawi would be able to cope with the increased loading of the slab and beams construction sequencing and speed up the construction cycle.

Both Alshor Plus and Rapidshor are modular systems, so the Carillion Alawi site teams were able to erect the tower structures quickly, bringing forward the concreting element of the programme to meet the needs of the overall accelerated programme.

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Another advantage of erecting Rapidshore towers inside the Alshor Plus towers was that the additional equipment did not create an obstruction or restrict access.

“With the changes made part way through the project, and having calculated all of the loadings, we knew we could not release the loads in the Alshor Plus to strike these additional slabs,” says RMD Kwikform regional sales manager, Bellphine Campbell. “So we introduced a 900mm-wide run of Rapidshor shoring that sat inside the Alshor Plus birdcage support to distribute the loads and keep both systems within their loading capacities.

“In doing so, we also recognised the need to increase the size of the working platform, so we designed additional bays using Alshor Plus. To reach the 43m top height of the dome structure, we literally put shoring on top of shoring, on top of shoring,” explains Campbell. The first Alshor Plus birdcage tower arrangement was between 12.5m and 15.8m tall. To save time and money on dismantling the Alshor Plus at this lower level, the next set of shoring combined Alshor Plus with Rapidshor, reaching from 18m to 30m to cast the beams and slabs at this height.

“With different segments of the structure requiring different shoring setups, like the two level slabs next to the prayer hall, the shoring puzzle required a great deal of engineering and site support,” says Campbell. But it was the dome that was the ultimate challenge. Cast at a height above ground of almost 50m, the concrete dome is a massive 90m in diameter. Rapidshor shoring, reaching a height of 17m from the base to the top of the dome, was arranged at varying heights and adjusted using the built-in jacks to support the curved structure.

The shoring was supported by the 25m-high Alshor Plus birdcage erected from the main 5.5m slab. In total this created a 43m-tall tower – a record height for any RMD tower. To enable the project to stay on track, even with the changes to the construction programme, RMD Kwikform had to source additional equipment and deliver the new design solution without impacting on the programme time. In total RMD Kwikform supplied a staggering 1,200 tonnes of equipment for this job.

To support the project, an on-site team of RMD Kwikform customer service representatives was formed to provide further product training to the site operatives on the assembly of the Rapidshor within the existing Alshor Plus tables. When completed in 2015, the new Sultan Qaboos Mosque will be one of the largest in Oman.

This article first appeared in the September 2014 issue of The Construction Index magazine. To read the full magazine online, click here.

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