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GTS finite element software. This indicated that the foundation barrettes did not need to extend to the deep clay and could be shortened from 20 m to 17.5 m.
It also showed that a pinned, rather than a fixed, connection between the raft and the barrettes was better in terms of bending moments, although this created the need to "beef up" the steel in the raft locally. The analysis also showed that, perhaps counter-intuitively, the load is actually carried by the barrettes, resulting is very little strain at raft level. Leszczynski feels Arap's design has certainly led to a better understanding of the ground and foundation behaviour.
Foundation work was carried out by contractor Inkom, which installed the 17.5 m long, 0.8 m by 2.8 m barrettes through slots cut in the foundation raft. Leszczynski was concerned that creating these slots and exposing the underside of the original raft could lead to a softening below it. As a precaution, Inkom injected grout at low pressure below the raft around the slot locations. In addition it prestressed the barrettes by base-grouting them, although this was primarily to mitigate against potential construction errors, rather than a necessary part of the design.
Foundation work was complete when NCEI visited the site and the structure was three storeys above ground. Provided Poland's residential market rides through the credit crunch, general contractor Besix should be completing the project by autumn next year.
(New Civil Engineer International, January, 2009)
^TEXT7
DOWNTOWN by Adrian Greeman
Birmingham's latest development includes a 44 storey-tower with a five storey basement. Bachy Soletanche is just finishing the foundations.
Birmingham, England has long had a reputation as a windswept concrete jungle, the result of road focused re-development in the
1960s. But a wave of new development is modernising the city centre with friendlier mixed use schemes. One of the biggest is transforming a bleak space close to the Snow Hill station, the city's second central railway station. For years the area has been mainly rough ground, used for car parking alongside a main road, with railway lines nearby and assorted 1960s concrete multistorey car parks.
Now steel frame blocks are rising on a three part site being developed by Ballymore for mixed use, with offices, retail space and hotel floors above. Largest will be a development with two towers on the square site at the end, one of these a future landmark with 44 floors, the city's highest building. Landscaped space will also be over a five-storey basement car park filling the whole 96,000 m2 space. To create this large volume and tower foundations, groundwork specialist Bachy Soletanche has been installing a deep contiguous piled wall around the site this summer. In recent weeks, as the large excavation inside got underway inside, it has been back on site to install a line of ground anchors in the wall.
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"These are for temporary support of the wall during the basement construction," explains contracts manager Steve Mallinson. "Once the concreted base slab and floors are in place they will provide all the structural support needed and the anchors will be cut through." The tendons will remain in the ground afterwards. "We also had to do ten plunge columns for the site approach ramp within the main wall," says Mallinson. These hefty steel H-section columns, surrounded by pea gravel inside their pile casings, are gradually being exposed again as the site excavation proceeds.
Contractor PC Harrington is doing the excavation and base concreting at present. But until recently Bachy has unusually had the site to itself. "We were effectively a main contractor," says Mallinson, "installing security and site welfare, arranging spoil disposal and concrete deliveries."
It was a change, he says, not having to interleave between other work, though with two support cranes, two Bauer BG 22 piling rigs, spoil heaps, reinforcement deliveries and site accommodation to deal with, the site became full enough. As the 241 piles in the perimeter wall were installed he even had to block off two of three site entrances, which meant some careful logistics were needed.
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For the 220 m length of the main wall, project design consultant WSP had opted for contiguous piles, "which is the right choice," says Mallinson, "because the ground is dry and you don't need any interlocking." Piles are 750 mm diameter.
The wall will hold back the ground which comprises a few metres of fill, then a 3—5 m thick sand layer which becomes weathered sandstone further down and gradually more competent rock. "The bedrock layer slopes from 2 m to 14 m down across the site and the piles must be up to 17.5 m deep," says site engineer Mathew Brown, "though they average out a little less."
To get through this fairly soft ground should be relatively straightforward. Bachy hoped to work with continuous flight piling mainly, which is quick and economical. But there is always a but. On this site it was obstacles in the ground, remnants of the 1960s, including various road underpasses and subways. "A lot of it was grubbed out in a preparatory contract," says Brown. "But there was some left where it would have caused undermining of the highway."
The obstacles were mainly several metres down and up to 3 m thick. To get through meant using the full strength of the Bauer rigs in straight boring mode — the dual purpose rigs could be converted for such work in about 24 hours and then drove through the hard material with tungsten carbide boring heads. "We had site investigation data but did further probe piles at various locations around the perimeter to work out what we could do with the CFA and what would take the harder cased bored work," says Mallinson. In the end about 30% bored piling was needed, somewhat less than Bachy had estimated, which meant it came out ahead.
But there is often another but. The sandstone and sand caused difficulties with both types of piling "because porous ground tends to suck the moisture out of the concrete," says Mallinson. "That made it stiffer and harder to get the pre-made reinforcement cages in after the augur was withdrawn." Bachy switched to a more fluid mix and a highly disciplined pile procedure where cages were positioned within a minute of the augur being withdrawn.
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For the ten top-down piles Bachy installed a basic bored pile with casing and then used its special plunge column rig to achieve the 5 mm accuracy needed for positioning the I section steel columns.
A steel frame sitting on the casings had three sets of hydraulic rams for precision adjustment of the central steel while it was fixed with around 5 m of concrete at the pile base. Pea gravel fills the casing. The 12 weeks' schedule met, Bachy retired for a month while the excavation began, returning in late October to begin anchoring. Some 70 anchors go in, a row of one every three piles. Each is 15 m long and 178 mm diameter, driven by a Casagrande M6 articulating rig.
Five strand reinforcement bundles from Diwidag are grouted into the bottom 6 m or so of the anchor which runs at a 45° incline into the sandstone. That too has gone to schedule and the site is now almost ready for the main works by contractor Altius. The Snow Hill development as it will look. Snow Hill development includes 56,000 m2 of office space, a five-star hotel and 332 luxury apartments in a 44-storey tower, five major new public spaces which — it is hoped — will create a new core to Birmingham's commercial heartland Kier Group is the main contractor with Amp heading up the mechanical and electrical engineering contract, while Alan Baxter Associates is the structures and highways consultant. Ballymore Properties is the developer of the Snow Hill project. It has worked on 22 city centre projects in Liverpool, Luton, Bristol and London. In London's Docklands, current schemes include Pan Peninsula, Ontario Tower and Leamouth Peninsula.
(New Civil Engineer International, February, 2008)
^TEXT8
OPEN PLAN SURGERY by Andrew Mylius
Opening up the basement of St Pancras Station's Midland Grand Hotel has called for radical re-engineering of its foundations.
Getting miners, their excavation equipment and construction materials into the tight spaces beneath the Midland Grand Hotel fronting London's St Pancras Station was like playing sardines, says Claire Carr. She is overseeing a surgical operation to remove
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walls in the old hotel basement to create a direct link between King's Cross St Pancras Underground station in front of the Midland Grand Hotel, and St Pancras International railway terminus, which is immediately behind it.
"Eurostar will start using the station in November. We're creating easy through-access for passengers moving between the London Underground and high speed trains," says Carr. She is section manager for CORBER, a joint venture between Costain, Laing O'Rourke, Bachy Soletanche and Emcor Rail, which is carrying out the rejuvenation of St Pancras station. It is doing the work for London & Continental Railways, owner and operator of High Speed 1, formerly known as the Channel Tunnel Rail Link.
Carr says that to support the hotel's seven storeys of neo-gothic brickwork, walls in the basement chambers were up to 1.5 m thick, carrying point loads of 500 kN. The space was divided into four rooms, roughly 7 m square, two either side of a 3 m wide corridor. Doors giving access to the corridor, and from the corridor into each of the rooms, were between 800 mm and 900 mm wide. "Space within the chambers was limited, and the doorways formed extremely tight bottlenecks on movements of people and materials," Carr says. Opening the basement up to create space for free-flowing passenger movement follows a 60-point method statement. "We've arrived at the point where we've got a large open plan area dotted with columns — we've come a long way," Carr summarises.
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Alongside working in confined spaces, one of CORBER's key challenges was to limit settlement. "Our work strategy has been governed by the requirement to keep settlement to under 5 mm," says Carr. Instrumentation has been installed on the upper floors of the hotel to keep tabs on the building response to changes being carried out to its footings.
Work started 17 months ago with the excavation of 3 m by 2 m pits to locate the hotel corbelled brick foundations. These were found 6 m down, bearing onto London Clay. A team of miners employed by Costain carried out the excavation work, using timber props and shoring to support the sides of the holes. "Because of the
conditions in which we're working, we've gone back to very traditional methods and materials," Carr notes. "Timber's far easier to use than steel in tight spaces like this."
With footing levels established, ground was taken down to the same level throughout the basement area. Powered wheelbarrows and a small conveyor were used to remove spoil as two mini-diggers toiled away. Next, lmwide, 4.5 m deep reinforced concrete strip foundations were cast either side of the walls to take temporary works loading. "We needed very substantial foundations to take propping forces when it came to opening up the walls," Carr explains. Opening up the walls involved taking cores at high level, where they met the edges of vaults making up the basement jack arch ceiling. Subcontractor Shepley inserted I-section needles through these holes supporting them on propped I-beams running flush with and either side of the walls.
"We were strictly prohibited from opening up more than 25% of the wall at once, so we had to install the needle using a hit one, miss three, hit one pattern. Once we'd been around all the walls once, we went back and did the same again and again." Grout was used to fill cavities in the brickwork of the topmost section of wall, sandwiched between the longitudinal I-beams. The grout also flooded the void between the wall and the web and inner flanges of the I-beams, creating a composite steel-masonry-steel sandwich. Only when the grout had achieved full design strength were props supporting the I-beams jacked imperceptibly, relieving the walls of load. This enabled slots to be cut in the walls. Reinforced concrete saddles were cast, bridging between the strip foundations, on which new cast iron columns were positioned. With all of the columns in place it was finally possible to cut out the remaining brickwork. Floor level between the strip foundations was raised to the same height by placing mass concrete.
(New Civil Engineer International, February, 2007)
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^ТЕХТ 9
BASEMENT BUILD UP by Jessica Rowson
Chicago's horizon is skyscraper heaven, but soon one building will stand head and shoulders above the rest. Jessica Rowson reports from the Windy City.
When built, the 150-storey Chicago Spire will be 610 m tall. Compared to the proposed 54 m tall Freedom Tower on Ground Zero in New York and the current tallest building in the United States — the 110-storey Sears tower in Chicago which stands 442 m tall — this will be a real skyscraper among tall buildings. It is designed by Spanish engineer architect Satiago Calatrava. Not only will the height make it stand out from the crowd, but it also has a rather unusual shape as it twists into a spiral which soars skywards. The floor plate is based on a circle but the edge is pinched into cantilever points at even spaces around the outside, giving it the appearance of a wide toothed cog. The cantilevers will be rotated to give the facade the appearance of a very elegant helter skelter.
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"It looks complicated, but there's high repetition which means less cost," says D. McLean vice president of the Spire's structural consultant Thornton Tomasetti. "The [concrete] core remains in the same position, but the floor plates appear to slowly rotate in plan with each change in floor elevation. However the columns and the inner floor plates are repeated at each level and the edges of the floor slab rotate." All great things must start somewhere and this project begins with some heavy duty ground work. Twenty, 3 m diameter rock caisson piles will support the building central circular core and there are seven outer columns at ground level with a further pair of the 3 m diameter rock caissons beneath each of these.
The rock caisson piles are large diameter concrete piles installed with a permanent casing typically used when very high loads need to be supported. These huge piles pass through the eight-storey basement and socket into the bedrock to an average depth of 3 m. Workers have already installed all of the 3 m caisson piles as well as
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smaller intermediate bell or under-reamed piles. These under reamed piles support the columns that will in turn support floor slabs in the eight-storey basement.
There are two types of basement column, both are formed by driving 25 m long, 910 mm diameter tubular casings into the ground. The bottom of these casings forms piles below lowest basement slab. Most are filled with reinforced concrete up to ground level. Where the loads are greatest the casings are only filled with reinforced concrete up to basement slab level. Steel drop-in columns are then placed on top of them to form supports for the basement floors. The casings are later removed before basement excavation work begins. "To construct the steel drop-in columns we terminate the concrete caisson at the bottom basement level and leave an empty shaft," says McLean.
A steel column is hung into the shaft just above the installed caisson. The base plate area of the steel column is concreted in place and left to cure. Later the shaft is filled with sand or weak slurry to prevent the clay collapsing when the steel casing is removed." It is important to prevent the ground around the steel columns collapsing after the casings are removed, because there is a risk that underground voids could make the areas around the columns dangerous to work in.
The basement will be constructed top down from the ground floor. The finished ground floor slab will brace the walls. The outriggers usually run through the core walls leaving space for lifts, stairs and services. But the Spire's circular shape and circular core meant that the designers had to find a different solution. "Normally cores are rectangular and the main structural elements can be installed through the core," says McLean. "With a circular core one cannot do that as the diagonal elements intersect at the core centre making it very difficult to fit anything in the triangular spaces."
"We decided on a ring system around the core walls connected at two floor levels, which would not interfere with the inner core layout. The ring elements are horizontal steel trasses which encircle the core wall." In addition, it was decided to include the outer column transfers within these outrigger systems. These outrigger/ transfer levels were situated at levels 35 to 40, 72 to 74,109 to 111 and 142 to 144 and designated as space for the plant rooms.
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The building floors are framed in composite steel. "We needed concrete for a robust core, but an all concrete construction would have resulted in very large columns in the lower part of the building," says McLean. "With steel we can use smaller columns." McLean says the one very important issue in composite construction is the differential movement between the concrete core and the steel columns. Concrete shrinks and creeps over time whereas steel shortens under loading, but at a different rate. This is not such a problem with low rise buildings or buildings which are all steel or all concrete, as the columns and cores shrink at the same rate. The core of the Spire is being constructed above its final elevation so that there is a slight fall in the level of the slab from the core outwards. As the concrete core shrinks, the floor level should approach a flat surface The main basement work has yet to start but tenders for a general contractor have been invited. When one is chosen the Spire will really start to motor.
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(New Civil Engineer International, November, 2008)
^TEXT 10
BIRD'S NEST SUPERSTRUCTURE by Andrew Mylius
As it nears completion China's bird's nest Olympic stadium is shouting for attention.
Beijing's architecturally flamboyant Olympic stadium is not universally admired. "The stadium bird's nest motif is an insult to birds," spluttered German structural engineering luminary Jorg Schlaich at a symposium in Beijing last month, organised by the International Association for Shell & Spatial Structures.
"It uses huge quantities of steel," Schlaich complained, "far more than necessary to enclose 100,000 seats. It is a grossly inefficient structure." Warming to his theme, Schlaich advised: "Good engineering should be about solving a problem as economically as possible, using the least possible materials. It should never be subjugated to art whatever art is." That is a debate, of course, that Schlaich should have had with client for the stadium,
the Beijing Organising Committee for the 2008 Olympic Games (BOCOG), before it decided to build the structure now nearing completion. The stadium has consumed a jaw dropping 45,0001 of steel but to stunning visual effect.
Welding of the final structural members is taking place; the precast concrete supports for the seating bowl have been installed, with seating and mechanical/electrical fit out following close behind. Beijing's Olympic stadium is due to open for business this time next year. Questions of "good engineering" aside, design and construction of the stadium have been remarkable challenges. Consultant Arap had the task of turning an architectural concept by Swiss architect Herzog & De Meuron into something buildable.
"Herzog & De Meuron put a lot of effort into making the structure unconventional," said head of Arup Sport Jay Parrish in 2004. "Columns are skewed off the vertical. There's not a concourse with a straight edge. The whole effort has concentrated on creating an exciting space." The stadium measures 230 m wide by 330 m long and 55 m high. Structurally it is composed of two independent parts — the bowl, which is a fairly conventional precast reinforced concrete structure, and the steel exterior facade and roof.
The roof has a "Pringle-like" geometry, derived by taking a small patch from the inside face of a vast toroid. At its edges, the roof flows into smooth corners, creating a seamless transition into the facade, which slopes inwards towards its base at 14° from the vertical. Herzog & De Meuron sketched the facade and roof as a random scribble, from which Arup had to establish a clear structural order. This was achieved by disguising primary structural members amid a web of secondary steelwork. All members share the same section of 1.5 m2. This has been so effective that is difficult to see that there is a structural system that has been repeated around the stadium perimeter.
Roof and facade are interconnected. The stadium performs like a collection of giant portal frames. Twenty-four perimeter columns consist of two outer chords and a single inner chord — all primary members. These rise from a single point, diverging as they go and wrap around the corner between facade and roof to continue across the roof. At the centre of the roof there is an elliptical opening
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