RMD Kwikform innovation rises to the challenge of the Country Way Viaduct, Hounslow
Withstanding the pressure of holding up the equivalent of three fully loaded jumbo jets using a single prop, is enough to give anyone a headache! But for engineers from formwork and falsework specialist RMD Kwikform, that is exactly the challenge they faced when asked to design a propping solution that could withstand a massive load, for the replacement of more than 100 bearings on the A316 Country Way Viaduct in Hounslow, London.
Having served as a main tributary road for commuters accessing London from the M3 for decades, the multi million pound project to prolong the life of the 768m long viaduct, which was built in 1975 has been funded by Transport for London. The 8 month long scheme, due for completion early 2009 allows for the replacement of all of the bearings on the piers supporting the viaduct, as well as numerous other improvements, including the installation of new safety barriers and the waterproofing and resurfacing of the viaduct itself.
With work aimed at minimizing disruption to commuter traffic, it was essential that the bearing replacement could take place without the closure of the viaduct. This is why leading support services and construction company Carillion appointed experienced international bearing replacement and renovation works specialist, Freyssinet to undertake this important part of the refurbishment programme. Having worked with RMD Kwikform on a number of projects including the recent support and bearing replacement to Thorpe Bridge on the A1260 Nene Parkway in Peterborough, Freyssinet approached the RMD Kwikform engineering team to provide a solution to support the considerable loads exerted on the piers by the combination of the viaduct’s deck and the weight of traffic.
Awarded the contract in June 2008, RMD Kwikform was tasked with provision of a packaged propping solution that included the provision of equipment, onsite customer service support and the design of each of the 108 individual props required to complete the project.
Early schemes involved a more conventional method of securing props into place and transferring horizontal loads into the pier tops using heavy fabricated steel wrap-around yokes. Rapidly rising steel costs and a desire by the Freyssinet team to standardize prop construction and minimize strip and reposition cycle times and labour cost however demanded a more radical approach. Faced with these multiple constraints, RMD Kwikform UK’s Engineering Director, Ian Fryer pulled together a specialist team to design a new type of support using the minimum number of specially designed components and the company’s heavy duty Megashor shoring product.
Ian: “The challenge we faced was three fold, firstly we had to design a system that was flexible enough to cope with the gradual increase in propping height along the viaduct, then we had to accommodate the various pier sizes and numbers at each support location as the viaduct widens along its length, increasing the number of piers from two to four to six. Finally we had to make sure that the props were strong enough to withstand the enormous concentrated loads, larger than any we had previously undertaken anywhere in the world, yet make them mainly from standard equipment in an arrangement that the site team could quickly erect and secure.”
“The largest vertical load was 8724kN or 890 tonnes, however in any viaduct or bridge propping of this nature, you also have to cater for transverse and longitudinal horizontal loads. For this project we had to cope with accidental side loads that could result from vehicles hitting the viaduct safety barrier which resulted in a transverse load of up to 300kN being applied by the deck and via the temporary bearings into the tops of the props. Finally, thermal expansion of the bridge deck causes it to slide over the temporary bearings and friction between the sliding surfaces results in longitudinal loads being transferred into the tops of the props.
In order to meet all of the required criteria and avoid any potential damage to the viaduct whilst the bearings were replaced, RMD Kwikform and Freyssinet’s engineering teams worked together to develop a design based around nine standard Megashor shafts in a close-spaced 3×3 configuration. A specially made prop head unit and a newly developed batten plate played a key role in the overall design, connecting together and bracing the RMD Kwikform’s Megashor components to create each composite ‘Megaprop’.
Ian: “We had to come up with a way of forcing the nine Megashor shafts to act as a single composite unit in their weak axis so that these props would not need any lateral restraint to prevent them from buckling for the maximum 9.5m prop height and we knew that the site team would want to arrange the prop so that once assembled, prop lengths corresponding to standard 1.8, 2.7 and 5.4m Megashor shaft modules could be bolted end to end in a different order without having to take the whole packs of equipment apart to start from scratch. With the Megashor shafts arranged in a 3×3 square arrangement we quickly settled on using batten plates laser cut from 20mm thick high yield steel plate and bolted in the two spaces between the three rows of Megashor legs. With these plates used at 1800mm centres along the prop there was never the need for them to straddle the end to end connections of the Megashor shafts which meant that the assembled packs could stay together even when the overall makeup of the Megaprop needed to be changed.
With the batten plates at 1800mm centres however, the Megashor shafts are unbraced for around 1200mm in between each batten, an arrangement more akin to systems such as Alshor Plus aluminium shoring used in building falsework than heavy refurbishment propping. Our team modeled entire Megaprop assemblies using second order (p-delta) structural analysis software, complete with appropriate initial member and whole assembly bow imperfections and axial loads combined with the side load bending moments to ensure that the arrangement could carry the loads in safety.
The other area that took a lot of time was the development of the Megaprop head unit. This large unit, weighing nearly 600kg, was required to perform a number of tasks, it had to connect together and portalise the top of the nine Megashor legs, thus restraining them, provide a stable platform for the hydraulic jacks to sit on and interface with the adjacent concrete pier. This interface was particularly challenging as the prop head had to be restrained to the existing pier in both directions horizontally without the use of bolts to enable the transfer of the applied horizontal loads but allow vertical movement to permit the prop to shorten elastically under load.
Ian continued: “This tricky problem went around and around in my head for a whole weekend. In the end we came up with a serrated vertical surface on the rear of the head unit where it sits next to the pier. Before the props were positioned the pier surface local to the prop head was roughened with a needle gun. With the Megaprop erected and temporarily restrained, the gap between the serrated prop head and the roughened concrete was filled with cementitious grout. Once the grout hardened the head units of the two Megaprops on either side of the pier were connected together using 20mm RMD Kwikform Rapid Tie Bars which were stressed to 215kN. The ties prevent the props from moving away from the pier in the transverse direction and the large contact pressure generated between the pier, grout and serrated surface were enough to prevent the props from moving in the longitudinal direction. Coating of the serrated surface with grease prior to prop erection ensured that the grout didn’t stick to this surface and permitted the prop head to slide up and down the serrated grout profile under varying vertical load. In effect working we created a very economical vertical axis sliding bearing.”
Fine length adjustment of the Megaprops was provided by adding or subtracting 90,270 and 450mm Megashor legs to the makeup. Ian: “This introduction of the smaller sized leg sections meant that we were able to do away with the traditional use of Megashor screw jacks at the base of the prop. With nine Megashor legs required, it would have been impossible to adjust a jack in the middle of the cluster. Length adjustment finer than 90mm was made by grout at the prop base and by adding and subtracting 40mm thick Megashor Jack Plates at the head.
Between the bridge soffit and the top of the Megaprops, Freyssinet designed and supplied temporary bearings above an arrangement of four hydraulic jacks, which enabled the props to be pre-loaded so that the viaduct could physically be lifted by 3mm off its existing bearings so that they could be replaced. At this time the entire weight of the viaduct, including all of the traffic running across the top comes down through the Megaprops.
Commenting on the unique design Tom Jones, Freyssinet project manager for the A316 said: “The original scheme involved carrying the longitudinal horizontal loads into the permanent works by using some substantial steel yokes that wrapped around the tops of the piers. Faced with a tight programme time and the additional cost of hiring more equipment and paying for specially fabricated parts for the different sized yokes to suit the various piers, we asked RMD Kwikform to work to reduce our costs from both a labour and materials perspective.
“We also wanted to see whether RMD Kwikform could develop a new innovative and more efficient way of propping that we could exploit for other potential jobs in the future. What they came up with was completely different to any other propping solution we have previously worked with. From an innovation perspective it was the lack of additional parts and the simplicity of design that really impressed us. But more importantly it was the practical help and customer service support from RMD Kwikform’s on-site Customer Service Engineer, Anthony Wood that helped us to actually get ahead of our scheduled programme by over two weeks, a great achievement considering how tight the initial project timing was.”
Armed with this new family of system components, RMD Kwikform engineers needed to design three variations of the propping system in order to support the viaduct. The design for the first and narrowest section of the viaduct involved providing a Megaprop on either side of two very large piers 3.3m by 1.5m in size. In this configuration, with each Megaprop could easily be connected back to back with the Rapid Tie Bars.
Further along the bridge where the structure widens slightly, the permanent works change from the use of two very wide piers to four slightly smaller piers. Here RMD Kwikform engineers had to adjust the design to cope with the considerable increase in vertical load.
Ian: The four piers were arranged in two pairs and we placed a Megaprop in-board of each. We could no longer tie the head units back to back and so a combined system using horizontal Superslim Push Pull Props and Rapid Tie was developed. Three close-spaced push pulls were used to strut apart the two head units and the Rapid Tie bars were now positioned over the outside of the two piers and tensioned thus placing the push pulls in compression. With such large stiff piers, in order that we could guarantee the contact pressure at the head interfaces, the push pulls were initially pre-loaded by torque control methods. The Engineer didn’t want to lock in any bending moments in the piers and we thus had to vary the push pull prop pre-load and Rapid Tie Bar load depending on the various pier heights and cross sections so that when the bearings were out, the pier tops were in their ‘at rest’ position.”
Finally the viaduct widens still further to be supported by six piers in two groups of three. RMD Kwikform engineers extended the arrangement used in the four pier section so that the Rapid Tie Bars passed over the outside of three piers with the gaps between the three Megaprops strutted apart each with 3 pre-loaded Superslim Push Pull Props.