OM Engineering
PO Box 56
Miami QLD 4220
Australia
Phone: +61 7 5575 1432.
Fax:       +61 7 5575 1432.

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OM Engineering specialises in the design and supply of custom-designed formwork systems, ranging from building, bridge and industrial tank jump forms to major temporary works projects.

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Bridge Design and Engineering Magazine Third Quarter Issue 2008

By Bridge Design and Engineering Magazine
October 29, 2009
 
Brisbane, Australia --- Saving time and improving safety were the two main aims of the design of the formwork system for the second Gateway Bridge in Brisbane, Australia. The new bridge is being built by Leighton Abigroup joint venture, and will provide a near-duplicate of the existing structure (see article page 42).

The formwork for the construction of the piers and pier top units on the main span of the bridge has been specially designed by OM Engineering. Two systems are being supplied; crane-lifted forms for the approach piers, which are rectangular in cross-section with two internal voids, and self-climbing forms for the river piers. The forms for the river piers are designed so that the same system can be used for building the twin-bladed piers, the pier top units and the deck section above the pier head.

\r\nAs OM Engineering director Oliver Mork explains, the whole idea was to minimise the changes required in transition from the pier blades to the pier heads and then the wings, both for safety and to save time. “There are some modifications to the system that are done at each transition stage,” says Mork, “but these have been minimised and the whole system travels up the piers unchanged.”

Following initial meetings with the site team, a few key insights emerged which led to the development of this system. Firstly, the pier blades themselves only required ten lifts of 4.4m height, which was hardly enough to justify the use of a full-blown jump form system. Secondly, it was made clear from the start that the rebar for the pier blades would be fully fixed before installation, which eliminated the use of a conventional type of self-climbing jump form system. Traditional self-climbing systems have overhead steel beams that support the inside forms and these would have clashed with the rebar. Thirdly, the contractor required a large working area. This is what led to the development a self-climbing bottom frame enclosing both pier blades with forms that roll back on the upper deck.

“I could also see that the construction of the pier heads and concrete deck wings were going to be much more difficult and time consuming for the contractor than the pier blades, so I decided to focus on solving these problems,” he adds. “When I first presented this proposal, it was well received by the contractor,” says Mork. “But the bidding system required me to provide a complying bid that would allow comparison with those suppliers who had submitted jump form bids for the pier blades.” In response, he explained that he was selling them a pier table system that just happened to be assembled at ground level and required the construction of the piers in order to reach its final level. In that respect, the pier blades were an aside and would be built for no extra cost.

In addition to the cost savings, Mork points out three other advantages of his custom-designed system. Firstly, he says, there are major time savings and safety implications in not having to install more than one system on the main piers, particularly at height.

But one of the major benefits, he says, is in terms of savings in administration time and effort. For the contractor, each subcontract entails major administration – writing up contracts, monitoring progress, independent checking, compliance, quality control and so on. By designing a single system for the entire pier head, this element of the works is simplified enormously for the contractor.

One complaint about jump form systems that is commonly heard from contractors is that the working space they provide is too limited. On the Gateway Bridge, says Mork, his design goes to the opposite extreme. The upper working deck of each of the main pier jump forms is around 600m2, providing a huge working deck. “We also allowed the contractor to store up to 40t of equipment,” he adds. Having this working area allowed the contractor to have an almost totally self-sufficient work space at height, with obvious advantages for safety and productivity.

As well as the system for the main river piers, OM Engineering has designed and supplied the crane-lifted forms for the piers on the approaches – five piers on the south side and ten on the north side. All the piers have a cross-section of 9.2m by 2.3m which is constant over the full height of the pier, with two internal voids each measuring 3.6m by 1.4m.

It was always clear that this was to be a crane-picked form, Mork says, so although no specific weight limitations were given, this was a consideration in the design of the different elements.

His design philosophy for column formwork is to minimise the use of form ties as much as possible. “Form ties are difficult to install, reduce durability, look messy and the voids have to be patched once the concrete has been completed,” he says, “so in this case I decided to eliminate them altogether.” Instead they are tied using external PT bars at the ends of the forms, tensioned with hollow jacks to ensure that all the PT bars are effective and to stop the form opening up during concreting. The absence of through ties means that these forms have to span the full 9.2m width of the piers. Deflection of the form is related to the concrete pressure, which is variable, and in order to control deflection, this form has a feature which allows the contractor to make adjustments for actual site conditions. This was done after the first concrete pour.

Another factor is that the internal cells are very small and working space is limited, so it has an internal box with special corners that allow the box to collapse in on itself.

All equipment was fabricated in China, so a key design restraint was that every element had to fit into a 40t container. Elements are designed to be as large as possible, subject to this particular restraint, in order to minimise assembly on site
Bridge Design and Engineering Magazine Third Quarter Issue 2008
 
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