A high load capacity Megashor truss and prop solution, devised by formwork and falsework specialist, RMD Kwikform, is being used by joint venture contractor, CSM, for the construction of approach viaducts for the new M2 Medway Bridge, as part of the Cobham to Junction 4 widening project.

The contract involves the construction of 82-metre span in-situ concrete viaducts, supported on huge Megatrusses and inclined Megashor props. It also calls for the provision of wall, slab and cantilevered deck formwork for the post-tension twin box girder viaduct. In addition, the contract is utilising substantial quantities of RMD Kwikform's Kwikstage 75kN shoring system, Super Slim Soldier and Alform beam formwork, Alsec under-slab access and the Paraslim and Webtie modular composite bridge deck formwork systems.

Eight 6.3-metre deep, 128-metre long Megatrusses are used for each span, each having a bending moment capacity in excess of 13,000kNm. The Warren-type truss is constructed from standard modular Megashor sections that work in tandem with Super Slim Soldiers lacing and standard Rapid Tie and flat braces, to provide the required lateral stiffness and wind resistance. In the most highly loaded areas of the truss, standard 1,000kN Megashor sections are replaced by 1,400kN Megashor Plus that were designed especially for this contract. The overall weight of the truss assembly is approximately 900 tonnes.

Stool adjustment is incorporated on top of the trusses, again using standard Megashor components to accommodate the camber and cross fall geometry, as well as the deflection resulting from the post tensioning operation. The maximum design deflection due to the concrete load is 130mm.

The Megatruss falsework is assembled at ground level and is being raised into position by specialist contractor, Dorman Long. Climbing jacks are mounted on 100mm square climbing bars restrained to the viaduct piers. Once raised to working level, the inclined Megashor props are fixed to the pile caps at the base of the piers and winched up and fixed to the truss lower chords by the insertion of 100mm diameter high yield pins in special node connections. The base slab is then cast, followed by the walls and top slab.

The inclined props incorporate specially designed base units that enable the use of 300- tonne hydraulic jacks to measure and adjust prop loads and extensions continuously during sensitive stages of construction.
Dismantling is achieved by simultaneously unloading the props using these hydraulic jacks. The props are lowered to the ground and removed. Trusses are then brought down two metres, using the climbing jacks, before being secured in place to allow inspection of the base slab soffit. The assembly is then lowered to the ground where the trusses are split into three sections for advancing using multi wheeled transporters in true "Thunderbirds" style. Any length adjustment to accommodate the different lengths between piers is carried out, and the erection sequence is repeated for the next span.

In total, the contract calls for 16 trusses and 12 sets of four raking props. This compares with an earlier proposal that would have necessitated 24 trusses and 12 sets of six props. The new proposal was selected due to the significant reduction in material and transport costs, less weight and substantially reduced labour and programme requirements.
Erection of the formwork and falsework was started in January and is expected to be completed by the middle of 2002.

RMD Kwikform is part of an international network of RMD Kwikform companies located throughout Europe, the Middle East, Asia Pacific, Australasia and South America. It provides customer-focussed formwork and falsework solutions either on a hire or sale basis.

For several applications, Megashor and H-beams are combined to provide the solution, while elsewhere Megashor is being used to support specially-designed steel trusses.  These are used where it is essential to achieve longer spans, or where it is necessary to span over roads or other obstructions, without the need for central supporting towers, and while ensuring the minimum disturbance to traffic flows.   The H beams and the trusses support RMD Alform beams that, in turn, support plywood decking.
 
In-situ load testing of the RMD Kwikform trusses showed a deflection of less than 15mm over a clear span of more than 40 metres.  In many instances they also proved to be more cost effective than H-beam solutions due to their lightness and ease of transportation and installation.
 
Formwork for the box sections of the elevated highway deck is made up of RMD Super Slim Soldiers and Alform beams with internal Rapidshor falsework to support the upper deck.  The contracts have also called for the design of a number of special components.

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