The Republic of Ireland is building a new motorway to link the capital city, Dublin, with Northern Ireland as part of the improvement to the country's infrastructure. Near the town of Drogheda, it passes through the Boyne Valley, an area of outstanding natural beauty, where an elegant cable-stay bridge that will span the River Boyne is currently being constructed by a joint venture between SIAC and Cleveland Bridge. Here, Adam Fixter, RMD Kwikform's Project Engineer for the scheme, describes how a unique formwork solution has been devised to meet all of the contractor's challenges.

A single, inverted Y-shaped pylon is being constructed on the south bank of the river to support the 212 metre span composite deck. The sloping pylon legs converge at a point 64 metres above sea level, and the hollow pylon stem then rises a further 31 metres. Within the hollow pylon stem, SIAC CLEVELAND designed a series of integral steel liners to form the inside face of the structure and house the tubes and anchorage units for the tendons. The plan area of the stem decreases as it rises, with the east and west faces inclined. All corners are radiused and there is a central feature on the north and south faces.

SIAC CLEVELAND faced several challenges when planning the construction of the stem. Construction would take place through the winter period when strong winds and bad weather conditions could reasonably be expected. Additionally, the programme time was critical, so the formwork system also needed to provide protection from the weather and ensure continuity of work for rebar fixers, carpenters and concrete finishers all working in unison. The tower crane servicing the pylon would be fully occupied servicing the rebar fixing and other operations, so the need for regular lifting of large formwork panels in high winds was not acceptable and could have led to considerable, and equally unacceptable, project delays.

RMD Kwikform's Autoclimb system was selected as the best option to meet all of the challenges involved in the construction of the pylon stem.

Autoclimb is a self-contained, hydraulically operated, bracket type self climbing formwork system designed to offer a high degree of versatility, allowing the use of a full range of either traditional, modular or purpose made formwork. Most significantly, the automatic lifting of the formwork eliminates the need for cranage,. Complete sections of formwork and platforms can be self lifted, allowing several site operations to function simultaneously. This drastically reduces the time required to prepare for concrete pours. Autoclimb's built-in adjustment ensures that formwork can be quickly and accurately aligned.

Autoclimb's robust design allows each bracket to safely support a load of 100kN while attached to the wall plates, and safely climb a load of 50kN up the masts. Additionally, the system will withstand extreme wind speeds, with gusts of up to 50 metres per second.

A trolley assembly, mounted on top of the bracket, allows the formwork panels to be retracted back by 750mm, enabling rapid and easy cleaning of the formwork face, fixing of rebar and fixing of features to the formwork face.

Essentially RMD Kwikform's Autoclimb solution comprises five major elements:

•	A wall plate anchored to the wall by pairs of anchor cones each with a buried plate and tie. The wall plates can be adjusted to accommodate construction tolerances.
•	A suspension shoe that slides over the wall plates to support the brackets and mast.
•	The climbing bracket, complete with the climbing heads and hydraulic cylinder.
•	A retracting trolley assembly that runs onto of the climbing bracket, and in turn supports the formwork.
•	A climbing mast, up which the climbing head assembly is hydraulically lifted.

RMD Kwikform engineers worked closely with SIAC CLEVELAND's management team to tailor the Autoclimb and formwork system to meet all of their requirements and provide a formwork solution with a construction cycle time of just 5.5 working days between concrete pours. RMD Kwikform also provided a complete design and supervision service, including assembly drawings for the brackets, access and formwork panels, along with design calculations, technical consultation, support and supervision during the erection and climbing process.

The pylon stem is divided into ten 3-metre high concrete pours. Twelve Autoclimb brackets are used; two on the east and west faces, and four on the north and south faces. Each face is climbed in one operation from a hydraulic power pack mounted on the lower platform. Four levels of platform are provided, each having handrails to shoulder height and complete weather protection, which engenders a sense of security and safety for the people working on the platforms.

The lower platform is 1.8-metres wide and houses the hydraulic control equipment, from where the climbing is operated. This platform is also used to fill tie holes and as a base from which to carry out any concrete finishing. Above this is the 2.7-metre wide main platform that is used primarily by the carpenters to retract the formwork, clean the formwork face, and make any modifications.

Above the main platform - at the top of the formwork - is a 1.6-metre wide concrete pouring and rebar fixing platform. From this location, concrete is placed and vibrated; the platform can also be used for access during rebar fixing.

Before climbing the brackets, concrete around the anchors has to achieve a strength of 10N/mm2. The concrete mix design and ambient temperatures at the Boyne Bridge mean that the initial gain in strength could be relatively slow. In order to overcome the potential for delays, an additional platform is provided, level with the top of the next pour, allowing advanced construction operations to be completed. This heavy duty rebar fixing platform is equipped with a hinged extension platform that can be extended when the bracket is retracted to give a 2.2-metre wide area designed to support a load of 6kN/m2, which allowed bundles of rebar to be stored on the platform prior to distribution.

Immediately after pouring concrete, rebar fixing can commence on the next pour, with the brackets on a particular face being raised when the rebar fixing is completed to that face. The formwork is then rolled into place and the concrete poured a few hours after the last face of brackets has been raised. Concrete curing is therefore removed from the critical path.

The pylon stem liners are fabricated in steel by Cleveland Bridge, and form the internal formwork to the core. These liners are lifted into position in approximately 1.5-metre high sections, with pre drilled formwork tie holes. An additional platform is provided on top of the stem liners, which is lifted on and off as more sections are added. The stem liner access platform is designed to support a load of 10kN/m2 and can be used to store rebar, tools and machinery. A trap door in the platforms gives access to a stairway running down through the pylon leg.

The five platform levels create an extensive working area that maximises the space available and allows a large number of different operations to be carried out simultaneously.

The pylon stem liners are also used to help restrain the formwork from the significant wind forces generated by the four levels of sheeted platform. Restraint turnbuckles tie into the stem liner just above the formwork at each bracket position.

These restraints are in position when the wind speed is above the working wind speed of 18 metres a second the speed up to which normal construction operations take place. Climbing of the formwork also takes place in wind speeds of up to the 18 metres a second. With the turnbuckles in position the system can safely resist the maximum basic gust wind speed of 46 metres a second for that area.

To gain the maximum benefit of using the Autoclimb system, an efficient formwork system needs to be utilised. RMD Kwikform provided a system based on its Super Slim Soldier, Alform Beam and plywood face, and the formwork formed an integral part of the Autoclimb solution. Concrete pressures were resisted by a series of 15mm tie bars, each with a 110kN allowable working load. When combined with the Super Slim Soldiers, this allows ties to be spaced up to 2-metres, minimising the number of ties and reducing the time involved with their placing.

The east and west faces of the formwork are inclined and specially fabricated radiused quadrants were attached at the corners. These are designed to slide inside the long north and south formwork faces, so the changing profile could be achieved without altering the formwork between pours. In the centre of the north and south faces, a feature is created using a specially fabricated section of formwork that is adjusted between pours by the use of turnbuckles.

Climbing the formwork up to the next pour level is a quick and simple operation. Wall plates are bolted to the pairs of anchor cones cast in the previous concrete pour and are levelled. Suspension shoes are slid onto the wall plates, whose design allows for plus or minus 65mm positioning tolerance to accommodate any anchor cones that are out of position.

The climbing mast is raised using the climbing heads and hydraulic cylinders. The climbing heads contain two sets of spring loaded triggers; one set of triggers climb the mast, while the opposite set raise the mast.

The hydraulic rams are retracted, the triggers in the bottom climbing head locate and bear on the face of the block and the mast climbs. At the end of each hydraulic stroke, the triggers on the top climbing head have engaged under the next block on the mast and support the mast in its new position. The hydraulics can now be extended allowing the triggers on the bottom climbing head to pass over and then latch under the next block down the mast.

As the mast rises into the suspension shoe, it locates and supports itself from the suspension shoe, subsequently allowing the climbing brackets to climb up the mast.

The mast and suspension shoes are designed to resist the combined bending and shear forces caused by the geometry of the brackets. The foot jack on the base of the brackets is then released, so the base reaction is taken off the concrete wall and onto the mast. At this stage the bracket is ready to climb.

To climb the mast, a second set of triggers in the climbing heads are engaged, and the hydraulic cylinder is retracted lifting the lower climbing head over and on to the mast block. Subsequent reversal of the hydraulics causes the bracket to climb the mast, supported from the lower climbing heads, until the upper climbing head engages the next mast block. This process is repeated climbing the bracket up the mast in 350mm increments.

Once the assembly reaches the next suspension shoe position, the suspension blade is inserted and the bracket lowered down onto the blade. The suspension shoe then restrains the bracket vertically and horizontally, and the hydraulic cylinder is turned off. The foot jack at the base of the bracket is now extended, taking the forces into the concrete wall and releasing the forces on the mast. The construction team on the Boyne Bridge are achieving a very efficient climbing time of 10 ñ 15 minutes to climb the platform 3.0m up the pylon.

To climb the platform, a team of three is used to move and attach the wall plates, raise the masts and then climb the platforms. The hydraulic power packs use a standard 4kVa three phase electrical supply, requiring only a single armoured cable that can be plugged into any of the four hydraulic units.

The operating simplicity and built in safety of the system enables the RMD Kwikform Site Technical Adviser to train SIAC CLEVELAND nominated personnel to operate the system effectively.

The RMD Kwikform Autoclimb system and customised formwork provided a safe working environment that catered for the needs of the different operations that are being carried out consecutively on the construction of the Boyne Bridge pylon. The design makes use of the steel stem liner to support a working platform, to form the inner formwork skin, and to restrain the formwork against the significant wind forces. By choosing Autoclimb, SIAC CLEVELAND were able to reduce their tower crane dependency and minimise the potential risks of lifting large areas of formwork in windy conditions.

From the early stages of the project, a team of engineers from RMD Kwikform Ireland and colleagues from the UK have worked in close collaboration with SIAC CLEVELAND's site team to tailor the system to meet very precise requirements. RMD Kwikform's Site Technical Advisers also worked alongside the contractor to build and install the platforms and formwork panels, and then train the site team in the use and operation of the systems.

FOR FURTHER PRESS INFORMATION PLEASE CONTACT
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