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Perfect Plaza

As new hotels go they don't come anymore impressive than London's Park Plaza which is being constructed on the south bank of the Thames, close to Westminster Bridge.

The 16-storey hotel, which is due to open by 2010, will have 960 self-contained apartment style rooms with all of the modern facilities associated with a top city centre establishment. Below ground there will be four basement levels, one of which will house one of the capital's largest ballrooms with an area of 1,200m2.

The project is situated on the site of the old County Hall annex, once voted the capital's ugliest building. Currently in the middle of a busy roundabout, it will form an integral element of a larger regeneration scheme covering much of the adjacent river front. The plans include the part-pedestriainisation of the roundabout and realigning the road network to allow only buses and taxis to circulate around the hotel.

Befitting such a prestigious job the hotel structure will, in the words of the project managers, "be a modern contemporary masterpiece and a centrepiece for the entire regeneration scheme".

Creating this bold statement has led to a unique and interesting design that incorporates two steel mega structures within a larger concrete outer shell. A large steel truss structure will form the column-free basement ballroom's roof and house the uppermost basement level within its depth. The other large steel element consists of two giant Vierendeel trusses, from which all the upper hotel floors will be hung. The Vierendeels - which are centrally positioned, either side of a full height central atrium, also allow for a 6m cantilever which extends around the building's perimeter as well as a column-free lobby area.

Work on site began with the demolition of the unloved County Hall annex during the latter part of 2006. To speed up the construction programme, main contractor Gear has subsequently adopted a top-down building method, so that after basement excavation and piling was complete, the ballroom roof trusses could be installed. Once these giant steel elements were in place, the ground floor slab was cast on top allowing work to commence above and below ground level simultaneously.

"There are a number of key aspects to the job, such as the clear open spans required in the ballroom and the central atrium, so steel was the only option," explains Uri Mizrahi, director at structural engineer Taylor Whalley Spyra. "We are also working to a tight timescale and no other material could have formed the large column-free spaces in the allotted time."

Last autumn steelwork contractor RJD supplied and installed the six large 32m long, 4m deep fabricated trusses which form the ballroom's roof. These steel elements each weigh 55t and will allow the basement ballroom (level -2) to have clear spans of 32m. The trusses are so deep that they also house the mechanical floor (basement level -1) within their depth.

RJD Director Stuart Doleman, says each truss was delivered to site complete, but due to re-strictions on the movement of wide loads in central London, they had to arrive on-site before 7am.

This is RJD's biggest ever contract and it has more than a dozen of its engineers on site during the 12-month installation phase, which is due to be completed this August. As well as supplying, fabricating and installing steelwork, the company is also coordinating the erection of the Vierendeel trusses and their supporting columns. These elements are being fabricated and supplied by steelwork sub-contractor Cleveland Bridge.

Spanning all levels from second floor to 13th floor, the Vierandeel trusses will require an enormous amount of temporary works to support them. "Only when these trusses have reached the ninth floor will they be released from the temporary supports," explains Mizrahi.

The Vierendeel trusses are set at an angle and form a V-shaped central hotel atrium. These large structures will be encased in concrete and will be erected with studs to accept floors and diaphragm walls. The bottom level of the trusses (floor 2) will have hangers onto which the concrete first floor will be hung.

Because of the enormous tolerances involved in the truss construction, each section - com-prising either three or four floors of truss - is being trial erected prior to being delivered to site.

"Each truss is like a giant rectangular grille, but the trial erection ensures the tolerances and connections are exact," says Cleveland Bridge project designer Ken Hudson. "Load moments and shears are transferred to columns and this requires some substantial off-site welding."

Prefabricated elements are programmed to be ready in time RJD's erection programme. Each truss section will be transported to London on the back of an A-frame and then lifted and bolted into position on site.

The trusses are formed by plate girders up to 150mm thick, with some weighing in excess of 18t. Supporting each these two large steel elements are just two 1.2m x 300mm steel columns, positioned at either end of the structures.

There is an enormous amount of loading passing through four columns, 40,000kN in some cases," explains Mizrahi. "Nearly half of the building is supported by these large tapered columns, so they're directly connected to 25m deep steel plunge column piles at basement level."

Gear Construction director Neville Hall says the steel elements, although very complex, are key to the overall job. "Without the use of steel, we wouldn't have been able to work with a top-down method and consequently we'd find it difficult to meet our completion date which is set for 2010."

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