Bring on the big guns

‘Forewarned is forearmed’ might have started as a truism of military strategy, but the maxim bears up well to scrutiny in any field. And it was duly heeded by Foster + Partners in its design approach for London’s Imperial War Museum – part of the ongoing phased refurbishment of its 19th century home. The £40m Phase 1 opened this summer, part of a bigger masterplan to spatially reinvent the former mental hospital to cope with burgeoning visitor numbers – expected to increase by 30% this year alone – and to maintain it as a world-class museum to conflict. The firm’s dramatic new atrium space has undergone a wholesale remodelling in time for the anniversary of the start of World War I.

Part of that strategy, says Fosters’ senior partner Michael Jones, was not just about having a long-term idea for what the museum could be, but making sure that the future curation of the gallery spaces was considered at design stage. This concept drove the design of the atrium space from the outset. ‘Because the exhibits here are tanks and planes, their size, scale and weight were key considerations,’ says Jones. ‘Getting them in meant close collaboration between us, the client, engineers and exhibition designer Casson Mann – there was a real desire for a spatial narrative that ordered historical conflicts by floor and made sense of exhibits that previously felt randomly placed.’

Visitors previously entered at first floor level into a top-lit, exhibit-crammed space with dark gallery levels either side, so the result of Fosters’ intervention on this experience has been dramatic. The whole first floor slab was removed, exposing  ground floor spaces and opening the building up to its full 22m height. All the gallery level floor plates meanwhile have been pulled out and into the 12m wide atrium, attached to concrete structural fins that widen as they go up. The overall effect is vertiginous and ravine-like. Jones explains that there was real pressure on exhibition space, so although they have lost a floor, pulling in the gallery levels has offset this, yielding a net increase of 2000m2. ‘The new structure is stitched into the old. Spacing of the fins is now half of the previous structural grid, going from 6.8m to 3.4m,’ notes Jones, ‘But their raking out to the structural line of Arup’s barrel vault roof, also creates the “vitrines” for exhibits, part of the new curation strategy.’ Thus, flanked by white pre-cast concrete raking fins, exhibits now jut out into the main atrium, relating to this space and also the themes of any particular floor, increasing the intelligibility of the museum.

The choice of ‘U’-shaped white RC concrete columns rather than slicker steel was part of a deliberate approach of using self-finished materials. In this way, Jones explains, the concrete and composite floors were finished in screed, and the main stair that runs up through the atrium is sheet steel with a hot zinc applied coating. It gives, says Jones, ‘a robust feeling to the building using a palette in keeping with the objects on display.’

This materials strategy extends to the new floor that has been inserted directly beneath Arup’s 1980s barrel vault, sitting on raked hollow steel trusses that span the space to the fins on either side. These chunky trusses, from 400mm to 1100mm deep, are designed not only to accommodate 500 people mulling over the new ‘contemporary conflicts’ exhibition and events space that will occupy this upper floor, but to allow a Harrier jet from the Kosovo conflict to hang almost invisibly from the structure on steel cables.

The raked beams themselves are hidden by triangulated, polished aluminium panels that tessellate together to form a continuous, faceted soffit. Angling away from the sides at their edges, this new floor is also a key aspect of the museum’s lighting control and M&E strategy, limiting direct light into a central space that was previously flooded with it. ‘The museum used to be a light-drenched atrium with “black box” galleries all around. For the first time light levels are right to display things like textiles and other delicate exhibits in the main gallery areas so that everything is visually open to everything else,’ says Jones, explaining the curatorial changes that have occurred as a result.

Lower light levels have also meant a reduction in the cooling load. Buro Happold, which was in charge of the M&E as well as structural engineering, faced a difficult balancing act. The air handling strategy meant supplying  conditioned air to a larger exhibition space, but planning conditions forbade any increase in height to the roof plant zones on the building’s east and west sides.

‘We were working with existing constraints, which meant holistic approaches,’ explains Damian Wines, M&E associate at Buro Happold, which co-ordinated with Fosters on the issue. ‘We didn’t want to increase air volumes as that would have a major effect on energy performance, so we had to look at everywhere we could to cut energy demand.’ The reduced atrium daylight levels played a major part, but this was augmented with an LED strategy for space and exhibit lighting, significantly cutting the electrical load. Both power and data runs were designed so that they would use the spare space capacity in the ‘U’-shaped concrete columns.

Most importantly however, the decision was made to allow temperature levels to fluctuate outside the narrow range usual for museum buildings, ranging here from 16-25°C and 40-60% relative humidity. ‘We came to a decision with the museum that at peak demand and at times of low occupancy we would allow temperatures and humidity to swing over a greater range than would normally be considered suitable, meaning a significant cut in the building’s carbon emissions,’ explains Wines. ‘At peak load the air handling plant is designed to work at full capacity throughout the museum, with delicate exhibits singled out for special air handling treatment within dedicated enclosed areas.’ The new floor beneath the barrel vault – which is proposed to be changed from polycarbonate to ETFE – has had underfloor cooling and heating installed within its composite floor to deal with its more demanding heating and cooling load.

Further phases of the museum masterplan have also been considered in this first phase, with a new service trench dug into the ground floor for potential services distribution. Funding permitting, future phases are to see a whole new entrance lobby and ticketing area inserted beneath the current stone access steps on the south elevation, to allow for direct and step-free access into the newly revealed ‘ground’ level of the museum – obviating the need for the staircase from the old lobby and truly opening out the ground floor level to its park landscape. Future plans will also sees the formal south rooms of the 19th century building incorporated into the main museum, creating grand gallery spaces to display the IWM’s sizeable collection of war art, all accessible from the atrium space and exhibit floors. These additions will provide both a fitting sense of formal welcome and a reflective component to a museum whose structure, services and curation will, by then, be state of the art. •