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MADE TO MEASURE

Designing London's Silken Hotel is all about pure, non conformist structural engineering, Ruby Kitching discovers.

Some construction projects aim to make best use of repeating modular elements, identical floor plates and a simple, easy to erect structure. That's fine if the building is on a greenfield site without stringent planning restrictions and easy access. Not so, for a tight corner plot at the junction of the Strand and Aldwych in London.

This is the location for the five star Silken Hotel and apartment building which is close to being topped out. Engineers from consultant Buro Happold have been exercising all the fundamentals of structural engineering to meet the needs of this Foster + Partners designed 10 storey building for Spanish client Grupo Urvasco.

In short, the £80 to £90M project involves designing a 173 bedroom hotel plus 79 apartments in an adjacent block, retaining a façade on one side of the triangular plot, excavating a three storey basement, constructing a full height conical atrium – think "church spire" – and creating transfer structures on the second floor so that the ground floor can be a virtually column-free.

So non-standard is the building that Buro Happold project leader Julian McFarland insisted that NCE walk around the site before sitting down and talking through the intricacies of the project. A casual perusal of floor plans and elevations just wouldn't be enough.

"There are structural gymnastics on every floor," points out McFarland. "And every floor is slightly different."

The site was previously occupied by the landmark Marconi Building built in 1903 and the more recent 10 storey Citibank offices. These were demolished in 2005 and what occupies the site now is a steel framed building with precast concrete plank flooring and a concrete core the same height as that which originally stood on the site. The hotel will occupy the Citibank plot, while the apartments will sit in the Marconi footprint with a retained facade.

Buro Happold was tasked with minimising the structural floor depth so that the maximum number of floors could be accommodated in the building.

A Slimflor system has been adopted with doubled up beams each side of the supporting columns on every floor. The system consists of 203mm Universal Column sections with a flat plate welded to the underside to receive 150mm deep precast concrete planks. The doubled up beams leave room for services to rise up through the building. Precast planks have a 65mm thick insitu concrete-topping to give the floor diaphragm sufficient strength. The top of the slab is also flush with the top of the steel beams.

One of the main features of the hotel is the central atrium. It is triangular in plan (due to the shape of the building plot) and tapers as it rises up the building.

This is because the planning restrictions imposed on the building required that the upper storeys step in. Since this would decrease the hotel room width, the atrium area has been reduced to compensate.Raking columns have, therefore, been designed to frame the atrium. The braced atrium steelwork also supports adjacent floor beams and also form a key part of the lateral stability system.

"These inclined columns introduce huge horizontal forces into the floor plate, which posed quite an interesting, but rewarding challenge," says McFarland.

Computer modelling enabled Buro Happold to assess the structure for stability. The main stability elements are the concrete core, a curved concrete wall on the corner, or "nose "of the building and the steel frame itself. The inclined atrium columns continue down to basement level where they are encased in concrete. Concrete subcontractor is Knight Build."We've kept an inclined element at the nose of the atrium through to the foundation, the alternative would be to have a vertical support here but we'd have struggled to deal with the horizontal forces as this corner has the shallowest angle," says McFarland. This is true for the other two corner columns of the triangular plan atrium, but at the first basement level the columns underneath are straightened with significant anchoring in the basement concrete slab to transfer the horizontal forces.

This was done to allow sufficient car parking space at this level. Another innovation can be observed on the ground floor where the span of the atrium beams was too great to keep the slim floor depth, so an intermediate support was required. Amalgamating this support with the corner atrium column required the installation of a 7t lump of steel to resolve the three "chord" columns meeting at
one point.

But the atrium isn't the only element affecting column design. Self supporting Portland stone will clad the building over seven floors (the top three floors will be part of the mansard roof), supported only at the ground floor. The stone will be tied back to the structure at each floor to provide nominal restraint. This means that the building's stability system will have to be stiffer to prevent excessive horizontal movement which might cause the stonework to crack.

"The stability system is three times stiffer than for a normal building of similar height," says McFarland. Unusually, the atrium steelwork will be clad in stone, with a few windows pierced through at each level.

Natural light will enter the atrium via the sliding glazed roof. This will mean that steelwork fabricator Rowen Structure's fancy inclined columns and details will be concealed when the building is finished. However, for now, all that is kooky about them, the trussed transfer structures and cranked beams can be admired.

As with most modern hotels, the ground floor lobby area is column free. This is achieved by a series of storey high transfer structures situated within partition walls between the first and third floors.The concept is that all vertical load is either transferred to the perimeter of the building or to the centre. A total of 23 steelwork transfer trusses are required, along with 16 different types of trusses.

Steel truss transfer structures are also peppered throughout the building to hang floors in some instances and create column free spaces below.

Other "bespoke" aspects of this building include cranked beams across differing floor levels between the hotel block and the apartment building and transfer structures at the perimeter to make sure the retained façade window openings line up between the ground and first floor.

1. STEEL WORK
A total of 23 steelwork transfer trusses are required, along with 16 different types of trusses
2. FACADE
Self supporting Portland stone will clad the building over seven floors, supporting only the ground floor
3. COLUMNS
Ranking columns have been designed to frame the atrium

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