Building close to and over live railway lines is always a challenge - especially when the trains that run on those lines have no drivers. NCE reports on two unusual projects that have a particularly intimate relationship.
Almost hidden among the forest of 13 towers under construction on and around Canary Wharf in London's Docklands, a small five storey air rights structure straddling the Docklands Light Railway (DLR) at Heron Quays is taking shape.
Its size belies the challenges it set Canary Wharf project managers and structural engineers.
When complete, the structure will cantilever from the much bigger, 154m high, 32 storey building to the east of the DLR, known as HQ2. It is designed by structural engineer Yolles, with architect Cesar Pelli and Adamson Associates.
Getting the air rights structure into place was a tough challenge for steel contractor Victor Buyck and its temporary works consultant Tony Gee & Partners. The Heron Quays site is extremely cramped, with building sites for HQ2 and its sister HQ1 (see box) just to the west of the DLR taking up much of the space around the railway's viaduct as it runs through the site. Under and next to the viaduct, contractors are also building an increased capacity Heron Quays station.
To the north and south, the site boundaries are deep water filled docks, adding further space and logisitical constraints.
In another complication, developer Canary Wharf's in-house construction manager Canary Wharf Contractors is also building an underground retail tunnel linking Heron Quays to Canary Wharf Jubilee Line station, just to the north, The live DLR presented the biggest problems to the HQ2 construction team. The line has to be kept open for 21 hours a day, as it provides a vital transport link between Lewisham, Docklands and the City. As a result, lifting structural components into place above the track was restricted to engineering hours between 1.30am and 4.30am.
DLR civil engineering manager Drew Bradley claims that this combination of constraints makes the site among the most potentially dangerous in Europe.
'You've got high rise buildings on either side and in front is a retail tunnel excavation, ' he says. Safety was paramount and work on the air rights building and the new station below it involved a great deal of consultation and co-ordination between Canary Wharf's construction teams, the DLR and its private operator Serco.
Space constraints on the site also meant that it was only possible to erect a temporary works platform above the DLR at the south end of the site. This and excavations for the retail tunnel on the north bank of Heron Quay meant that it would be difficult to position by crane the eight, 28m span, 4m deep trusses which bridge the DLR and support the more conventional five stories of steelwork above.
'Tony Gee & Partners came up with a hanging truss design, ' says Canary Wharf Contractors project manager Patrick Shaw.
The consultant's solution was to roll the eight trusses into place from the southern working platform along parallel running beams on either side of the DLR viaduct.
The trusses were shipped from the Netherlands to the south side of Heron Quays by boat in November. 'We put in the temporary platform during a weekend line closure, ' says Canary Wharf Contractors project executive Winston HuthWallis. To save time, the trusses, weighing between 30t and 40t, were all lifted into place on to the beams over one weekend and parked above the temporary works platform.
The first truss was then slowly jacked along the running beams at the rate of about 1m every five minutes using two 15t jacks.
Jacking stopped to allow it to be connected to its neighbour with structural steelwork and interconnecting bracing. Decking was then added to create a completed bay module and the process repeated until all the trusses were in place, spread equally along the 90m.
A concrete floor slab was then cast, creating the launch deck for the upper steel frame. Jacks then lifted the launch deck off the running beams, allowing them to be removed, before the whole deck was lowered down to the columns already waiting to receive it.
Construction of the first bay was further complicated by a design requirement for a cut out section of the floor plate at the northern end of the air rights structure. As a result, the loads from the truss at the north end of the building were transferred on to two raked steel legs before the ends of the truss were dismantled.
With the launch deck in place, erection of the air rights structure was unconventional in that floor slabs will not be installed until after the frame is finished. This is because a bridging truss strong enough to support the entire structure would have been too deep, and would have been unable to give adequate clearance for the DLR trains. 'We would have lost a floor or we would have had to go higher, ' says Yolles associate David Cuckow.
Instead, a much larger series of trusses will be erected at the top of the building, cantilevering off HQ2. When this is complete, loads will be transferred from the truss at the bottom, leaving it with a large degree of redundancy.
Work on the air rights frame is well under way and it is expected the whole of HQ2 will be structurally complete next spring.
Britain s most complex slipform Slipforming a complex seven celled high rise building core, including heavily reinforced shear walls and using a single 53m long rig, would normally be a sufficient challenge for any specialist contractor. To fix 2,000 steel plates of varying shapes into the core during the slide - more than 70 to be fitted every four hour floor cycle - adds spice to the challenge.
To then place the entire 260t, four level rig just 8m from - and up to 80m above - London's busy Docklands Light Railway (DLR), programming the slide without track possessions, turns that challenge into a major logistical operation.
But to further be asked to incorporate into the slipform a couple of tall concrete columns, packed with T40 reinforcement;
nothing at all to do with your building, yet located less than 500mm from your rig, turned just such an operation in London's Docklands into one of the most complex slipforms yet attempted in Britain.
'It caused a few sleepless nights, ' recalls Glennan Blackmore, contracts manager for slipforming specialist Bierrum. 'But time was crucial and slipforming was over three times quicker than a conventional jump shutter.'
This 14 storey office block in the latest cluster of Heron Quays' multi-million pound commercial developments is destined to become one of client Canary Wharf Contractors' most prestigious. By this time next year, the building known as HQ1 will have metamorphosed into the headquarters of merchant banker Morgan Stanley located adjacent to - and integral with - a new DLR station.
The need for Bierrum's fast track, five week slipform was because the multi-core superstructure had to be several storeys high before foundations contractor Byrne Brothers could start the equally complex three level basement. Steel floor beams are being installed between the completed cores, but beam connecting plates had to be embedded during the slipform.
More than 2,000 plates averaging 1m long - the maximum number physically practical to fit during a continuous slide - were placed to 25mm accuracy. This in itself proved a significant challenge.
'If just one went wrong, breaking it out and resetting afterwards using abseilers would prove expensive, ' says Blackmore, adding with a smile only possible after the event:
'We did not need any abseilers.'
The original plan was for follow-on steelwork contractor Cleveland Bridge to weld connecting fins to the plates later, as its steel frame rose around the cores. But advance co-operation between contractors allowed Bierrum to offer a valuable helping hand.
'By hanging a fourth lower tier beneath our planned three level rig, Cleveland Bridge welders rode up with us, fixing fins into the just embedded plates, ' explains Blackmore.
Similar contractor co-operation with Byrne Brothers on the adjacent office block HQ2 just across the railway, benefited both projects.
To allow HQ2 to span over the new DLR station, eight massive steel trusses were needed. Their western row of 26m high, 1,200mm square concrete support columns stand just 450mm from the Morgan Stanley building.
The plan had been for all these columns to be constructed using conventional shuttering in advance of the adjacent slipform. But two had not been built by the time Bierrum's rig was ready to rise. 'So we incorporated their construction into our slipform, ' Blackmore explains. 'To say this made our rig a bit more complex would be an understatement.'
Special add on cantilevered extensions had to thread through the maze of rig shuttering, while steel for the heavily reinforced columns - designed for conventional construction - was reanalysed.
Instead of the planned pre-assembled 7m long rebar cages spliced together on site, numerous individual T40 bars were fixed insitu with steel density ruling out any splicing.
Each bar needed couplers, staggered across the column, all inserted against tight deadlines as the unstoppable rig, with its 42 strong crew, completed an average 1.5m of building every 12 hour shift.
Last but not least Heron Quays is the last of the stations on the original DLR network to be modernised since it opened in 1987. The original structure comprised two platforms at viaduct level linked to the ground via long, steep staircases. It is being replaced by a £10M cat's cradle type structure designed by WS Atkins with architect Will Alsop and dispenses with the stairs in favour of two pairs of escalators.
Unlike its predecessor, the new station is structurally independent of the viaduct and is suspended from nine pairs of raked steel masts. These are given vertical stability by pretensioned tie bars stretching across and above the tracks, and down to anchor points on either side. Pairs of steel beams running along the outer edge of the platform give the masts their lateral stability.
The platforms themselves form part of a steel framed 'hull' supported by the masts and straddling the underside of the railway viaduct.
The sprayed concrete cladding is designed to absorb noise generated by the trains - an important consideration given the station will have offices above it and on either side.
The station redevelopment cost is being split between Canary Wharf and DLR, as both stand to benefit from the work.