Feature

Going Up In The World

Tony Whitehead finds out how skilful repair and renovation by Orms has given a tired council building on London’s Euston Road new life as an ultra-stylish hang-out.

The mid-seventies fashion for combining precast concrete with tinted glazing did not age well. Many examples were demolished as early as the 1990s and, by 2014, it seemed a bleak future loomed for one of the survivors: the eight-storey concrete annexe to Camden Town Hall.

Situated across the road from St Pancras Station, the annexe did not compare well with the restored masterpiece opposite. It was woefully inefficient, covered in grime, and boasted some 500 windows of a truly hideous tint. Unsurprisingly, all schemes submitted for the site’s redevelopment proposed to flatten it.

All, that is, except the one devised by architect Orms with Crosstree Real Estate. “With all our projects we look to see what can be reused,” explains Orms associate Simon Whittaker. “It’s just a more sustainable approach. We saw past the grime. There was architectural merit in the building and its concrete frame was still in good condition.”

Having looked at repurposing the building as offices or residential space, it became clear to Orms and Crosstree that the floorplan and location were ideal for a hotel. Whittaker believes the sustainability argument helped to swing Camden council behind the idea, “though they might also have been a little sentimental about their old HQ”.

The building now houses a 266-room hotel, The Standard, the latest in a high-end, ultra-fashionable global chain designed to appeal to the well-heeled hip. So how to transform unloved council offices into a place where the beautiful people stay?

Most obviously, the building needed cleaning. “The facade is structural, and comprised glazed precast panels that were actually in pretty good condition,” says Whittaker. “Just a few mastic joints needed reinstating and there was the odd small crack which was easily repaired. After cleaning with a mild abrasive system (see box, below) it came up much brighter than we had hoped. The concrete has a large whitish aggregate in it, and with the dirt removed you can appreciate its true colour.”

Needless to say, the single-glazed tinted windows have been replaced with double-glazed clear units with various coatings to improve efficiency and, on south-facing elevations, limit solar gain. Changing the glazing and cleaning the facade has effected a dramatic transformation from “tired” to “fresh”. The building also benefits greatly from the removal of a stair core and roof-level plant room that ran up the right-hand side of the building and connected it, via a covered walkway, to the neo-classical town hall next door.

With the core removed, the sculptural mass of the building is clearly expressed as a standalone structure for the first time. Its removal did have consequences, however. “It was a key contributor to lateral stability,” Whittaker says. “Without it the building would sway from side to side. The solution was to introduce two new concrete stability cores.”

One of these is a new lift shaft, constructed from in-situ concrete, which has been stitched into the original reinforced concrete structure and runs from basement to roof. The second is a C-shaped core, also in in-situ concrete, that rises from the basement to the fifth floor. This encloses a shaft for dumb waiters at basement level, but on the upper floors is simply a C-shaped shear wall enclosing bedrooms. The construction of both new cores was sequenced early in the programme to stabilise the building prior to the removal of the old stair core.

“The annexe has an interesting structure which made it amenable to reuse,” adds Whittaker. “The concrete slabs span from four large 750mm2 central columns out to the precast facade which is structural and supports the slabs at the perimeter. This gives spans of over 10m in all directions around the columns – so lots of clear space to reconfigure as we like.”

Andrew Middlebrook, associate with structural engineers Heyne Tillett Steel (HTS), also sees merit in the structure. “Having a structural facade means you don’t need columns near the perimeter and that produces a clearer floorplan. It also saves doubling up on structure and cladding. The floors are made from waffle slabs which have rather gone out of fashion today, but they can be a very efficient use of material.”

The waffle slabs at The Standard are just 106mm thick, but with 300mm downstands. The 406mm total allows long spans to be achieved with relatively little concrete. “An equivalent solid slab would have to be maybe 350mm thick, so the waffles have saved a lot of weight and material.”

As well as boasting admirably reconfigurable floorplates, the existing concrete frame also proved robust enough to take three further storeys (see box, below). This added 1,914m2 to the building’s existing 15,360m2 and was vital for the commercial viability of the scheme. To save weight, the rooftop extension was constructed largely from steel, with composite metal deck slabs with a 175mm depth of concrete.

The 10th floor, featuring a restaurant and entertainment area, is acoustically separated from bedroom accommodation below by an extra 150mm slab above a 100mm air gap. “This is supported above the 175mm ceiling slab by sprung jacks to absorb noise and vibration,” says Middlebrook.

The new floors are set back from the facade of the original building, as supporting their full weight off the facade was impracticable. Instead, HTS developed an ingenious solution which involved threading hollow steel section columns through the existing structure.

The weight problem

Adding three extra floors to the top of the existing annexe was always crucial to the future viability of the building. The question was: to what extent could the existing structure support the additional loading?

Structural engineer Heyne Tillett Steel’s first move was to acquire a set of original drawings from the archives of Pell Frischmann, the engineer that designed the structure back in the early 1970s. HTS associate Andrew Middlebrook describes the existence of these as “an absolute lifesaver” as, without them, a number of “safety first” assumptions would have been made early in the design, with the risk of over-engineering the solution to the building’s extra weight.

“As it was, we still had to carry out intrusive investigations on site to verify the drawings,” says Middlebrook. “We dug trial pits to a depth of 4m around four of the piles that would experience the biggest increase in loads and confirmed the diameters shown in the original drawings.”

He adds that the piles were under-reamed – designed with wider sections or “bulbs” at depth to help stability. “These needed checking too, so we dug bore holes all around the existing piles to establish the depth and width of the under-reams.”

Once HTS had confidence in exactly what lay below ground, the true capacity of each retained pile was recalculated by geotechnical consultant RSK Group. “This is a useful calculation as, after 45 years, the foundations are unlikely to settle any further,” says Middlebrook. By comparing the existing versus proposed loads for each pile, HTS found that the increased loading produced by the weight of the extra floors was justified for all but two of the piles.

“These were strengthened by sinking smaller piles around them,” explains Middlebrook. “We put three around one, in a triangle configuration, and five around the other. We didn’t want to break out the basement slab so we piled right through it, sinking the new piles around the existing piles beneath. For each of the two original piles we then cast a very strong, 1m-thick pile cap to connect it to its new piles. This was an obtrusive structure, upstanding from the slab, but being in the basement it was buried in back-of-house space.”

“These sections are 150mm by 250mm and are threaded through the waffle slabs down to the first floor,” says Middlebrook. The first-floor slab is 1m thick and supported by columns set in from the facade above. “The slab is very strong and cantilevers out to support the facade at the perimeter and provide a covered area at pavement level. We positioned the steel columns to land on the slab directly above this inner set of concrete columns.”

Middlebrook says that most of the steel columns missed the structurally important ribs of the waffle slabs, but even where ribs were pierced, the inherent strength of the concrete building came to the rescue: “Most of the pierced waffle slabs were strong enough to cope. We only had to mend and strengthen four of them.”

As the new steel columns are hidden within partitions, the hotel’s glamorous interior betrays little of this structural ingenuity. Inside the bedrooms, the precast window panels have been insulated and replastered and the only concrete visible is the exposed waffle slab.

There is a pleasing irony here as, in 1974, the slab was covered with ceiling tiles. Now, The Standard’s interiors specialists have made it integral to a stunning and up-to-the-minute design.

Busting 45 years of grime

Over the years, Restore London has cleaned the brick and stone facades of some of the capital’s most impressive classical buildings, including the Royal Academy in Piccadilly and the Royal Hospital Chelsea. Cleaning brutalist concrete, however, is becoming a more common occurrence. “It’s coming back into fashion,” says managing director John Rushworth. “But each job is different and we have to proceed carefully to ensure we clean thoroughly without doing any damage.”

To deal with the precast-concrete facade of the former Camden town hall annexe, this involved trialling various methods to remove a film of dirt built up over 45 years of exposure to the pollution of the Euston Road. “We usually try a plain water-pressure wash first,” says Rushworth, “or maybe heated water, and then move up through various levels of abrasive cleaning until we get the right result.”

For The Standard, Restore London used a Jos (pronounced “Yoss”) type system originally developed in Germany. This uses a selected abrasive particle carried by a flow of compressed air to a nozzle, at which point water is added. The mixture exits in a vortex and is then manually directed at the substrate to be cleaned. The process is akin to sand-blasting, but much gentler and more finely controlled.

“The trick is to use minimum force,“ says Rushworth. “For The Standard, we used a fine and relatively soft particle known as Olivine 80 [a magnesium/iron silicate]. We can then adjust the pressure of the jet, and the distance from the nozzle to the facade, to make sure we use just enough force to get it clean.” Troughs are placed at each floor as cleaning progresses, and the water is collected and filtered to remove the particles before being either reused or drained away.

For one particular precast panel, extra care was required. “The building has a small Banksy original on it,” explains Rushworth. “Obviously we had to be careful not to clean that away, so as a trial we did our own drawing on the facade – not quite as good – and practised cleaning right up close with an extra-fine pencil nozzle. Only when we were sure we could clean around a drawing accurately did we go anywhere near the original.”

PROJECT TEAM
Architect Orms
Structural engineer Heyne Tillett Steel
Main contractor McLaren
Concrete cleaning Restore London

Photos Tim Soar, Chris Mosier