The opening of the Queen Elizabeth II Great Court, The British Museum
Engineering and constructing the glass and steel roof to the Queen Elizabeth II Great Court, The British Museum
The British Museum's Georgian buildings were designed in 1823 by Robert Smirke, originally consisted of four wings containing galleries set around a large rectangular courtyard. Since then, the courtyard became filled with extension buildings, including the circular Reading Room and adjoining bookstacks (1854-57). Smirke's stone clad museum quadrangle and iron framed Reading Room are now grade 1 listed buildings.
The historic Reading Room is now the centre piece of the largest covered courtyard in Europe. The 6700 sq meter (92 x 73m) courtyard -a little bigger than the size of a football pitch- is enclosed by a spectacular glass and steel roof, a hub at the centre of the museum complex providing new modern galleries, and education and visitor facilities including shops and a restaurant.
The famous circular Reading Room, designed by Robert Smirke's brother Sydney, is 42.7m in diameter. The iron frame of 20 ribs are clad externally with a brick drum pierced by large arched windows between the ribs. It rises 19m above the floor level to the Snow gallery, a 2m wide flat roof that encircles the copper clad dome. The dome is hemispherical, rising to a central lantern 12.2m in diameter 32.3m above the floor. In overall structural terms, the Reading Room is a braced shell in which the iron framing and the brickwork provide mutual restraint and support.
Having worked with the architectural team from Sir Norman Foster and Partners both on the winning competition entry and the subsequent scheme design, Buro Happold was appointed by the British Museum as the Structural and Building Services Engineers, Fire Engineers and Planning Supervisors for the Great Court Development project. MACE was appointed as Construction Manager.
The fact that the Great Court is contained within the heart of the museum complex, encompassed within the 30m high quadrangle of historically listed Georgian stone clad buildings, with one of Britain's oldest and historic iron framed domes in the centre, was the single most significant aspect of the design and construction of the project. The only access to the site were three small tunnels in the basements of the surrounding buildings. This meant that most of the demolition material and construction materials had to be lifted in and out of the site. These conditions controlled the size of equipment and prefabricated materials that could be used in the redevelopment work.
Geotechnical engineering
The fragile nature of the Reading Room and its foundations meant that the design of the basement levels immediately adjacent to this historic building was governed by the need to precisely control the ground movements. Initial assessment indicated the probable introduction of a major system of tie backs in conjunction with bored pile walls underpinning the Reading Room, before the proposed nine metre deep basement on the southern half of the Great Court could be built. However, because the proposed basement actually encroached locally under the Reading Room and the logistics of getting piling rigs in and out of the Great Court, an alternative scheme, installing piles formed using jet grouting techniques was developed. Following a programme of specialised field and laboratory testing of the soils, a series of finite element analyses were undertaken in order to determine the precise installation technique to control the behaviour of the ground and resulting building movements. The work was subsequently undertaken on site by Keller Ground Engineering.
During construction of the basement real time monitoring was employed to ensure actual movements remained within prescribed limits. These continuous observations allowed the contractor, with the design team, to make adjustments during the installation process, preventing movements exceeding the allowable displacements. On completion of the basement work the final measured movements of the Reading Room footings agreed with the predicted vertical settlement, and only deviated by 3mm from the predicted lateral displacement.
Structural engineering of the roof
The roof is a fine lattice shell structure spanning in three directions from the four sides of the quadrangle on to a ring of 20 columns surrounding the Reading Room. The Reading Room is not located at the centre of the courtyard, but some 5m towards the North facade. These columns carry the roof load down to the foundations, ensuring that no additional load is applied to the Reading Room. The tubular columns are filled with concrete to act compositely with the steel to provide the required fire rating and stiffness to span the 19m unrestrained from floor level to snow gallery, but is slender enough to hide behind the new stone cladding of the Reading Room. Designed in accordance with Eurocode 4, the columns were fabricated using tubular steel, with an outer 457mm diameter reinforced with an inner 250mm square, filled with concrete.
Around the Reading Room, the roof is prevented from spreading laterally by the Snow Gallery, a new concrete slab which acts as a stiff diaphragm balancing the thrusts from opposite sides of the roof. The roof is supported on sliding bearings around the outer perimeter, avoiding applying lateral load to the quadrangle buildings. The bearings allow the roof to spread laterally under load, normal to the relevant facade, and independent of the buildings. This freedom means that for the roof to hold its form, outer radial members near the perimeter quadrangle work in bending and compression. The size of the steel members are smallest adjacent to the Reading Room and increase in size towards the perimeter. The forces generated by the abrupt change in direction at the corners are large and the structure is further stiffened in these areas with the addition of external trusses reinforcing the corners.
The variable torodial geometry of the roof provides an easy transition from the circular form of the Reading Room to the quadrangle of the surrounding Museum buildings. It was generated originally as the 'natural' surface of an inflated soap film, then modified to create the required volume within the architectural constraints.
The roof's structural grid follows that of the glazing, supporting each panel along its edges, minimising the complexity and cost of the glass fixing. Maximum glass size available with the surface treatments to control solar gains within the occupied space of the Great Court set the final structural grid size. The roof works as a shell. Radial elements span between the Reading Room and the quadrangle buildings, inter-connected by two opposing spirals. As the roof has just one line of symmetry, there are 1826 individual structural nodes connecting six elements.
The roof shape curves to a tight radius of approximately 50m, acting much in the same way as a dome, imposing minimal loads onto surrounding structures. The curvature allowed Buro Happold to develop a lightweight construction relying on shell action in the three-way grid.
The roof's outer perimeter is supported at every other nodal point by a short steel column down to the new reinforced concrete parapet beam system around the top of the existing facades. The roof is laterally stabilised around the perimeter with cross bracing behind the porticoes working parallel to the facade. Behind the porticoes, the roof's lateral spreading movement is normal to the line of the facade.
The successful connection of around 6000 individual steel members was critical to the integrity of the roof structure. Grade D steel, more often used for marine, or petro-chemical applications, was used to limit the carbon content. This allows the steel to be more ductile, preventing the possibility of brittle failure of the connections, as the roof moves with diurnal temperature changes.
The architect's ambition for a roof with a light and clear appearance led to fabricated steel box beams, with sufficient selfweight to resist any wind induced uplift, and with enough strength to carry the roof and its cladding. The steel weight for the entire roof is approximately 420 tonnes, or 75 kg/sqm. The double glazed cladding system adds another 60 kg/sqm. As with all long span structures, it was fundamental that the roof was structurally efficient, and to this end it was essential that the dead load, or self weight, was minimised so that the structure was not working hard to simply support itself. To do thi, each of the structural box elements was fabricated from flats to firstly ensure that the steel making up the section was placed in the most beneficial configuration and secondly, to allow individual elements to taper between nodes so that the section sizes could be increased towards the perimeter without unsightly steps.
The roof was constructed in a series of prefabricated ladder beams erected off a crash deck that covered the entire court. The ladder beams were assembled using precise jigs in the steel fabricators workshop, then trucked to site when needed. They were craned over the museum buildings, which remain
Building Services engineering
Maintaining environmental conditions within the spaces was a key design task. Buro Happold Building Services engineers designed a range of mechanical and passive systems to ensure that the public areas are comfortable from the basement auditoriums to the restaurant on level six, directly under the glass roof. A massive 45 cubic metres of air is introduced every second into the Great Court, galleries, retail areas, education areas and Reading Room, and exhausted by the ingenious re-use of spiral stair shafts within the historic porticos. Systems range from tight control full air conditioning for the exhibition gallery areas, achieving tolerences of within 1 degree centigrade and 5% relative humidity, to the tempered environment of the Great Court itself.
The Great Court, the largest volume in the building, is covered with a carefully studies fritted double-glazing specified to optimise solar control with heat loss and appropriate daylight penetration. Peak summertime heat build up is limited by an integrated uniquely designed perimeter displacement ventilation system developed especially for the Great Court, natural ventilation and some mechanical extract. Chillers used to facilitate air conditioning are also used during off peak hours to actively pre-cool the concourse slab thus adding a significant cooling effect to the courtyard.
A destination restaurant is located high up in the volume of the Great Court, beneath the fritted glazing. Comfort conditions are maintained here by creating a “cool oasis” microclimate using chilled floor slabs, displacement ventilation and an internal shade canopy to reduce the sun's direct radiant heat and glare.
To provide the different conditioning requirements, four plant rooms were integrated into the basement of existing quadrangle buildings, the services fed into respective areas of the Great Court through a network of floor trenches. The arrangement of plant rooms and services infrastructure is designed to provide an extremely robust supply of environmental control to all critical gallery areas even in the event of unexpected power failure.
As well as the high tech approach to the computer modeling of the environment and the innovative design of the engineering systems the project includes a pair of historic lanterns that hang within the front entrance hall. These lanterns are exact replicas of the original arc lamps which first adorned the entrance in the 19th century.
Fire strategy
Buro Happold FEDRA was responsible for the Fire Strategy for the Great Court, and co-ordination of this strategy with that for the existing museum spaces. The fire strategy provides an appropriate level of safety and property protection, with minimum intervention to the existing, Grade 1 listed, structure. An optimum fire strategy was obtained by careful co ordination with the Client and Design team, to enable the Great Court to efficiently satisfy all Client, Legislative, Architectural and Engineering objectives. Fire engineering also provided an escape strategy for disabled people, developing the optimum blend between structural provisions (including evacuation lifts, refuge areas, ramps and means of raising alarm) and management procedures for assisting escape.
Construction management
MACE carried out the Planning and preparation for the demolition and construction works during the fifteen month pre-construction period. A complex sequence of overlapping and phased activities was developed to achieve the overall programme. The principal activities, methods, programmes and design were developed with a number of trade contractors during pre-construction.
The programme and sequence was refined to reduce and remove activities from the critical path. Co-ordination meetings were held with British Museum departments to engineer methods and programmes for works at live interfaces with the Museum, with the aim of reducing disruption.
In conjunction with the British Museum, MACE held briefing sessions and presentations to local businesses and neighbours. The aim was to advise on the actual construction activities and any potential impact in the immediate area, such as road closures or increased vehicle movements.
A programme of fabric and structural site investigations was carried out during pre-construction. This enabled more detailed development of methods and design of potential risk areas. Noise and vibration analyses, which simulated the proposed works, were carried out around the building, and were monitored in galleries and other spaces.
Access into the Great Court was planned around two tower cranes, three existing ground level passageways and an additional route, which was broken through the north façade of the Museum.
Demolition, which started in March 1997, was initially slow and fiddly since the courtyard was completely filled with buildings. Once sufficient space was created, plant of up to 35 tonnes was lifted in, in pieces, and then re-assembled in the courtyard. Skips of demolished material was lifted out of the Great Court over the roof of the Museum, or taken through the passages using dumper trucks and skid loaders.
The demolition was sequenced to allow the earliest start possible for the underpinning of the Reading Room and the new Portico. The most significant unknown item to occur during demolition was extensive foundations to the south west building, which had been bombed during World War II and subsequently re-built.
In October 1997 Keller Foundations started underpinning using the jet grout technique described earlier, whilst demolition continued in the north half of the site. As underpinning was completed, excavation and concreting carried out by J.Doyle followed on.
The sequence and method of concrete construction was designed to enable the earliest erection of a scaffold working platform, to enable roof construction by Wagner Biro to commence. By March 1998 the basements and ground floor slab were completed in the north of the courtyard, allowing the scaffolding to begin to take shape by SGB, sub-contracted to Wagner Biro.
During the latter stages of the demolition, the restoration and re-construction of the facades began, carried out by St Blaise. Extensive damage had been inflicted on the facades and porticoes through the previous construction of additional buildings within the Great Court. The facades were cleaned using a low pressure JOS spray.
The work to the Reading Room ceiling started during the Great Court building demolition stage. The entire volume of the Reading Room was filled with a bird cage scaffold, allowing access to the whole of the beautiful domed ceiling. Extensive paint scrapes and investigations took place to determine the original decorative scheme of the ceiling. The final result, which can now be seen, was agreed with English Heritage and other bodies, before restoration of the dome was completed by Hare and Humphries.
The scaffold working platform, built to allow construction of the steel and glass roof, was engineered to allow numerous other activities to continue in parallel sequence. The working platform was made up of bird cage scaffolding, towers and steel beams topped by a weatherproof platform. The new roof, a shell structure, needed to be fully supported while in its temporary condition. Roof loads were transferred through the platform structure to specific points in the concrete structure below. The working platform, which eventually filled the whole of the Great Court, was designed to maintain access and circulation at ground level, while also allowing façade works, stone cladding, concreting and portico construction.
Below the ground floor slab, services and fitting out works started. The Reading Room has been clad with limestone, concealing complex service risers routing mechanical and electrical systems from the basements to roof level. This was built while the roof platform was in place, using an additional extensive, independent scaffold that surrounded the Reading Room.
The steelwork for the new roof was erected progressively from the north, in a combination of pre-fabricated sections and single members. The position of each roof lattice node was known from the structural design and this was used to position the temporary props supporting selected nodes. Constant monitoring and adjustment of the supports and the welded joints enabled the roof steelwork to fit and close to 3mm. When sufficient steelwork had been completed, the glazing process started and began to follow on behind steelwork erection. Once the steelwork was complete, and loaded with the glazing, removal of the support props began.
Removal of the working platform allowed the final activities of the finishing trades to start on the ground floor slab and restaurant area. Principle amongst these activities was the paving and cladding carried out by Grants. As areas were completed, access was given to the British Museum for gallery fit out and familiarisation.
