Mention Sweden to an engineer and currently the Malmo Ring Road and the Øresund Link are likely spring to mind, such is the international interest in the projects. However, Sweden is also a land of rivers and lakes. So when a new huge arch suspension rail bridge was planned to span the 126 metre wide River Staksund at Staket near Stockholm, RMD Kwikform's bridge falsework reputation in Sweden and its experience on the Malmo and Øresund projects made the company a logical choice. The project is due for completion late this Summer and RMD Kwikform's Export Manager, Colin Neill, describes the project, which was a true partnership between engineers, contractors and the falsework supplier.

INTRODUCTION.
The city of Stockholm is only in part located on the Swedish mainland, much of the urban area is spread over 20 or so of the larger surrounding islands that stretch out into the Baltic Sea. The entire region is a labyrinth of rivers and a maze of lakes and estuaries.

The new rail bridge near the small towns of Staket and Jarfeller will span the River Staksundet. When completed in a few months time it will replace an existing steel rail bridge a few hundred metres upriver in the valley, easing the busy flow of river traffic. For, unlike the existing swing bridge, which can be opened only when rail traffic is not crossing, the clear span and clear height of the impressive new in-situ concrete bridge will allow both commercial and leisure craft to pass beneath unhindered.

Ultimately, the Staket Bridge will have a clear span over the Staksundet River of approximately 120 metres, and provide between 18 metres and 20 metres head clearance for river traffic. The overall height to the top of the twin tapering hollow concrete arches, from which the 2.2 metre thick main railway deck will be suspended, is approximately 39 meters above water level, and the arches themselves taper from 4.0 metres at the heavy mass concrete arch springer positions to 2.5 metres at their apex.

THE FALSEWORK CHALLENGES.
Among the major challenges facing the Stockholm-based main contractor, Skanska Anlaggning AB, was the need to construct the ELU Konsult AB's designed bridge without inhibiting the free passage of river traffic. This meant that, throughout virtually the entire construction period, a channel of sufficient width and height had to be available, through which boats could pass day and night.

So, when devising a practical, safe and cost effective solution, RMD Kwikform's specialist falsework engineers had to overcome a number of problems. In addition to the falsework having to support a very considerable weight, it would additionally have to accommodate some extremely complex structural geometry. From the outset it was clear that the solution would call for a substantial stock of falsework equipment to be readily available in Sweden, and that only falsework systems that combined high load retention with adaptability would meet the designer's and contractor's key objectives.

At an early stage in the project's life, RMD KWIKFORM engineers, many with personal experience designing and erecting the formwork and falsework for a number of the bridges on the Malmo Ring Road, travelled to Sweden to meet with the consultant and main contractor to plan the work. Working closely in partnership with the construction team and the specialist formwork and falsework contractor, Skanska Maskin AB of Malmo – the plant division of Skanska Konstruction - RMD KWIKFORM developed a number of workable concepts. The selected option was an ingenious solution that utilised to the full the unique characteristics of two of the company's leading systems, Megashor and Rapidshor.

THE RMD KWIKFORM SOLUTION.
The suitability of both RMD KWIKFORM systems – and particularly Rapidshor - had already been demonstrated on the Malmo Ring Road project, where both were used extensively for in-situ concrete bridge construction. Equally important, the company had proved its ability to perform in terms of meeting construction targets, equipment availability and solution reliability. In all, RMD KWIKFORM supplied between 250 tonnes and 300 tonnes of equipment from its Malmo and Stockholm depots for the Staket Bridge project, and this was supplemented by Megashor and Rapidshor components supplied from the UK especially for the contract.

For the construction of the Staket Bridge, RMD KWIKFORM Megashor towers were used on both banks of the river to support the four arch springers. Two mid-river Megashor towers supported the steel beams that provided temporary clear access to passing vessels, supported the formwork for the main in-situ-cast, post-tensioned concrete box section rail deck, and supported the Rapidshor falsework supporting the upper central sections of the two arches.

Megashor was chosen because of its 1,000kN leg capacity and its high degree of flexibility, which makes it suitable for a host of high-load propping, jacking, strutting, support tower and spanning-truss applications. The system is built around 286mm by 280mm steel Megashor sections that are available in eight standard lengths between 15mm and 5.4 metres. These options allow virtually any beam length to be assembled using standard, off-the-shelf components. They can be assembled with header beams, fixed and adjustable bracing, screw jacks, hydraulic units, tilt heads, bracing units and node assemblies.

Support for the bridge arches and outer sections of the horizontal rail deck was provided using RMD Kwikform's lightweight 80kN leg capacity Rapidshor modular support system. Significantly, Rapidshor requires fewer components and less equipment tonnage than comparable systems on the market and, as was demonstrated on the Malmo Ring Road, is faster to erect. This meant that there was a lower total weight of equipment to lift, position, assemble and dismantle.

The Rapidshor system comprises 60mm diameter high-grade galvanised steel vertical members that are typically spaced 1.2 metres, 1.5 metres, 1.8 metres or 2.4 metres apart and each upright incorporates 8-point ledger fixing disks at 500mm centres. 48mm diameter ledgers are spaced vertically at a maximum of 1.5 metres. Connection between upright members and ledgers is a simple wedge assembly and the ends of the ledgers incorporate unique self-locking mechanisms into which the bracing members are an easily achieved push fit. Hence, the call for only low skill levels to assemble Rapidshor further reduced the overall labour cost element on the project.

FALSEWORK ERECTION SEQUENCE.
Before work started in November 1998 on the falsework support structure for the arched bridge, the ground around the arch springer concrete supports had to be coffer damned and the water drained to a solid bed level. This meant that the bases of the support system, and the construction team, were two metres below the surrounding water level. To provide support for the two mid-river support towers, a number of steel piles were driven into the riverbed and temporary concrete rafts were cast above water level.

Most of the RMD KWIKFORM equipment was delivered to the north shore of the river. That destined to be used to form the mid-river towers was then transferred by barge to a large pontoon anchored close to the temporary concrete rafts, where the towers were pre-assembled. Access between the river banks and the mid-river towers was provided by walkways linking the coffer-damned areas with the temporary rafts.

Construction work was divided into two phases – the construction of the arch - which is now nearly complete - and the second phase, the construction of the rail deck, which is yet to be started.

The first part of the first phase was the construction of the huge mass concrete springers for the arched beams, from which the in-situ-cast hollow arch beams span the river. The Megashor supports for these four arch springers were positioned on steel beams located on specially-cast concrete bases. Each of these Megashor supports comprised three sections, each of which was raked at a different angle. Extensive RMD KWIKFORM Super Slim Soldier bracing was incorporated and Rapidshor was used to add extra support and provide lateral rigidity to the Megashor raking towers.

The largest, innermost Megashor sections were almost vertical, and were designed to carry the total weight of the mass concrete arch springers, allowing the safe removal of the two smaller sections after the mass concrete had cured. Megashor hydraulic jack units were incorporated at the base of each of these larger sections to enable the concrete arches to be jacked up, compensating for any deflection following the removal of the outer sections.

The mid-river towers were assembled incorporating large sections of Megashor façade bracing in addition to the standard flat Megashor braces and Super Slim Soldiers. This extra bracing was included in the design to assist with the transfer of wind forces from the Rapidshor erected at high level, back to the concrete raft base, that were the result of wind speeds that, on occasions, reached 36 metres a second during the Winter of 1999.

These mid-river towers supported a dozen deep clear-span steel beams that spanned the central section of the river and provided the necessary head height for the passing river traffic. H33 beams spanned between the mid-river Megashor towers and Megashor towers at the outermost edge of the coffer-damned areas at each bank. Additional support was provided by steel beams laid on top of the H33 beams and the central clear-span steel beams, and Rapidshor falsework was progressively assembled off this deck to support the timber formwork used in the construction of the two arches.

However, the central solid steel beams were located at the rail deck level; taking up the depth that would ultimately be required for the rail deck. So the second phase – once the main arches have been completed and the arch-supporting Rapidshor falsework has been removed – will be to lower the steel beams to allow the slabs to be cast at the appropriate level. This will be achieved by locally reducing the height of special Megashor support members. Naturally though, this will reduce the available head height for passing craft for a short period of time.

Once the concrete rail deck has cured, its weight will de supported by steel hangers suspended from the concrete arches at intervals across the river. The final sections of the remaining bridge falsework, including the mid-river Megashor towers, will then be removed and the temporary rafts demolished. Rapidshor will then be used to form the falsework for the extensions of the rail deck at either end of the bridge to tunnels that are being blasted in the hillsides.

CONCLUSION.
The Staket Bridge is an excellent example of what can be achieved when designers, contractors and the falsework designer and supplier work closely together and share a common goal. It also clearly demonstrates the creativity that can be achieved when falsework systems combine high load capacity with simplicity and adaptability. The result at the Staket Bridge was a trouble-free solution that met all of the time, cost and access objectives. Indeed, all parties agree that the only possible alternative to the RMD KWIKFORM solution would have been to resort to expensive, delaying and ultimately wasteful structural steel.

A significant proportion of the Megashor and Super Slim components, along with a considerable amount of Rapidshor was purchased for the contract or had been previously purchased for use during the construction of the Malmo bridges. The remaining equipment was hired from RMD KWIKFORM that, in addition to providing the falsework systems and designing the solution for the Staket Bridge, also supervised the erection.

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