Situated close to the Thames and right beside the main tourist route from St Paul's Cathedral to the Millennium Bridge, the linked office developments at 99 and 101 Queen Victoria Street have a prime location. But in the City of London prime locations usually come with a high price tag, a catalogue of obstacles and restrictions that pose serious challenges for the structural engineers responsible.
This site is no different. Like all its neighbours it has to steer clear of the sight lines to St Paul's (see box). Under Queen Victoria Street to the north, District & Circle Line trains rumble incessantly just outside the basement wall. And, says main design and build contractor Bowmer and Kirkland project leader Marcus Fuller, a preconstruction study predicted significant archaeological remains in the area.
'In Roman times the bank of the Thames ran along the southern end of the site, ' he says. 'We found lots of timber piles, but nothing really spectacular.'
For several decades the Salvation Army had its headquarters in a 1960s building on the site, but when this reached the end of its natural life the Army decided it needed only one third of the original floor space. A deal with developer Hines resulted in a plan for two side by side office blocks, number 99 being a straightforward commercial development with twice the floor area of number 101, which would be the new Salvation Army HQ.
Originally there were plans for two basement levels, but a further significant obstruction caused a rethink.
'There was a major British Telecom duct in a very awkward location, ' Fuller reports. 'They wanted £2.5M to move it - it just wasn't worth it, so we settled on one basement level.'
Under the £37M design and build contract, structural engineer Arup had the responsibility of producing an efficient, cost effective and buildable solution to all these problems - plus the usual city centre bugbears of logistics and on-site storage.
There was another unusual design challenge as well.
'The two buildings are designed to look like one, ' explains Arup project director Tim McCaul. 'But we had to build in the potential for number 99 to be demolished and rebuilt without disturbing the headquarters building.'
Number 99 is a relatively straightforward seven floor structure with columns on a 6m by 7.5m grid. The Salvation Army HQ, as might be expected, is somewhat more distinctive architecturally, with complex raking struts making a strong visual statement at the front elevation. It has a slightly roomier grid at 6m by 9m.
McCaul says the choice of structural frame was a logical response to all these constraints.
'Insitu concrete construction simplified the logistics - it would have been very difficult to get large steel sections into the site, and there was little storage space available.
'We also needed to restrict floor slab thickness to 250mm to keep below the sight lines to St Paul's. Normally reinforced slabs would have worked over most of the project - but not where the spans were 9m.'
Post-tensioned flat slab construction throughout, the final choice, offered a number of advantages. Steel content would be lower, an important factor when steel prices are rocketing.
Flat soffits encouraged large scale prefabrication of the services. The result was a 'skinny' structure with a floor to floor height of only 3.675m.
Arup worked on the design in collaboration with concrete frame contractor PC Harrington and its specialist subcontractor Structural Systems (UK). The basic concept is well proven:
shallow rectangular galvanised steel ducts laid on a parabolic profile are cast into the slab.
Through each duct five 12.7mm diameter strands are passed, tensioned and grouted up. Concrete in the floor was C32/40.
'Shallow ducts maximise the eccentricity of each strand, and the overall efficiency of the design, ' explains Structural Systems (UK) technical manager Kevin Bennett.
'In practice each floor of 101 - which measured around 20m by 35m - was a single pour which we stressed in one half day operation.
'On 99 each floor was done in two separate pours. All pours were taken up to 25% of final stress the day after to minimise cracking. Then, when cube tests indicated, the concrete had passed the 25Mpa mark, we applied the rest of the load.'
Even the roof is a flat concrete slab. 'Among other advantages this cuts down on the usual mixture of tradesmen working at the same time, ' says Fuller. He adds: 'This is my first experience of post tensioned flat slab construction and it is certainly the right answer for this project.'
At first glance the two buildings appear to share a large single central service core with a 'party wall' at its heart. Closer inspection reveals an ingenious answer to the challenge of allowing number 99 to be remodelled or demolished without disturbing the Salvation Army.
'In fact there are two cores with a 75mm cavity separating them, ' McCaul explains. 'We slipformed both cores in a single operation, with a 'cassette' former across the middle to create the cavity.'
Architectural precast concrete was the only possible choice for the raking 'X-struts' in number 99. The tapering ovoid sections that make up the struts are up to 7m long and 9t in weight.