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This paper provides the background to the design of two current projects by Foster and Partners. The British Museum Great Court in London and the Music Centre at Gateshead have a single major architectural feature in common both are covered by lightweight, largespan roof structures. The complex geometries of the roofs have required cuttingedge computer models and parametric modelling software to assist in the design process. Technically, the two roof structures are amongst the most advanced of their kind. But the two projects share themes that have more farreaching cultural and philosophical ramifications than the technical virtuosity of their structures. Both create new democratic urban spaces public spaces that serve not only as circulation areas for their respective buildings but also as internal piazzas for their respective cities at large. Each project involves repairing its site the British Museum Great Court reclaims a space that has been lost to the public for more than 150 years and the Gateshead Music Centre makes a major contribution to the cultural redevelopment of the derelict south bank of the River Tyne. The Great Court will be completed in November 2000, and the Music Centre at Gateshead, which is still in design development, will be completed in late 2002.
276 THE DESIGN OF THE ROOFS OF THE BRITISH MUSEUM GREAT COURT AND THE MUSIC CENTRE AT GATESHEAD Spencer de Grey Architect Foster and Partners, Architects and Designers ABSTRACT This paper provides the background to the design of two current projects by Foster and Partners. The British Museum Great Court in London and the Music Centre at Gateshead have a single major architectural feature in common - both are covered by lightweight, large-span roof structures. The complex geometries of the roofs have required cutting-edge computer models and parametric modelling software to assist in the design process. Technically, the two roof structures are amongst the most advanced of their kind. But the two projects share themes that have more far- reaching cultural and philosophical ramifications than the technical virtuosity of their structures. Both create new democratic urban spaces - public spaces that serve not only as circulation areas for their respective buildings but also as internal piazzas for their respective cities at large. Each project involves 'repairing' its site - the British Museum Great Court reclaims a space that has been lost to the public for more than 150 years and the Gateshead Music Centre makes a major contribution to the cultural redevelopment of the derelict south bank of the River Tyne. The Great Court will be completed in November 2000, and the Music Centre at Gateshead, which is still in design development, will be completed in late 2002. Figl I will begin by outlining the historical background to each of the projects, focusing on the ways that each of the buildings deals with broader issues of urban planning. This will be followed by a description of their sites and the functional requirements that led to the development of wide-span structures. I will conclude with a brief technical description of the structures and their energy strategies. Fig 2 277 HISTORICAL BACKGROUND TO THE PROJECTS The Music Centre at Gateshead is a central part of the regeneration of the Tyne riverside. Other key projects include the new Baltic Centre for Contemporary Art and a new pedestrian bridge by Chris Wilkinson. The area will be further enlivened by shops, a hotel and leisure facilities. The Music Centre complex will provide accommodation for three auditoria and the Regional Music School. Each of the auditoria has been designed to provide acoustic excellence in relation to the number of seats required. As such, the exact forms of the three spaces have essentially generated themselves along functional lines. The largest hall will seat 1650 people. The second hall is intended for folk, jazz and blues concerts, including those by the resident Folkworks, and will have an informal and flexible seating arrangement with a maximum capacity of 400 seats. The third hall will be used as a rehearsal space for the Northern Sinfonia and a major performance space for the music school. The three auditoria are conceived as separate enclosures placed alongside each other on the riverbank. Fig 4 It would have been possible to leave the auditoria as three discrete buildings, housing their own foyers and auxiliary spaces. However, the specific characteristics of the site and the future development of the quayside suggested an alternative solution - a large roof structure enveloping the auditoria. Firstly, the windswept nature of the site required some kind of common shelter for the three buildings. Secondly, the building complex needed to supply its own access routes and infrastructure on what was a totally derelict site. Both issues suggested the need for a concourse, in the form of a covered 'street' on the riverfront, beneath a large roof structure. Below the concourse is the music school. This concourse becomes a major public space - a shared foyer for the three auditoria, a common room for the music school, and a sheltered environment from which to enjoy the river. It symbolises the ethos of cultural fusion inherent in the establishment of the Music Centre - a complex shared by musicians and audiences of a range of different music, and a meeting point for students, professional performers and the public. This integration has been encouraged by reducing the back-of-house hospitality areas for performers, so that visiting musicians will meet with students and their audience in the concourse bars. Lastly, the roof gives visual cohesion to the project, and provides the waterfront with a landmark structure that formally echoes the great arch of the neighbouring Tyne Bridge. Fig 5 The Great Court project at the British Museum is, at one level, a solution to the problems of welcoming visitors to one of the world's busiest museums and providing a clear primary circulation route from which they can visit the many galleries. But it also rescues from obscurity one of the most impressive public spaces in the capital - a courtyard the size of the football pitch at Wembley Stadium. 278 The present British Museum building was designed by Sir Robert Smirke to house the King's Library and act as a permanent home for the collections of the museum founded in 1753. Completed in 1847, Smirke's design was conceived as four wings of galleries arranged around a central quadrangle. Measuring 96 x 72m, this courtyard was to be used as a breathing space at the heart of the museum - a space to perambulate, relax, talk and think about the museum's extraordinary collections. Unfortunately this dramatic space existed for no more than five years. Almost as soon as the building was completed, it became clear that there was insufficient space for the museum's growing collections. The solution, conceived by the then Keeper of Printed Books, Antonio Panizzi, was to construct the great circular Reading Room within the central courtyard. Sydney Smirke, who had succeeded his brother as the museum's architect, began construction of the Reading Room in 1852. Completed in 1857, it is undoubtedly one of the most impressive and beautiful interiors in London. Fig 7 The remaining space between the facades of the museum quadrangle and the drum of the Reading Room was gradually filled in with buildings to house the ever- growing collection of books that now constitutes the British Library. These book stacks were extended between the wars, partly damaged in the Second World War and subsequently re-built. As a result, not only was the central courtyard lost to the public for over 150 years, but the museum was robbed of a primary circulation route. This problem became more acute as the museum's popularity grew. Today it has a worldwide reputation for the scope, quality and rarity of its collections and for its role as a centre of education and scholarship. Every year the museum attracts 5.4 million visitors compared to the Louvre's 5.7 million and the New York Metropolitan Museum's 5.2 million. The museum's entrance hall is a magnificent space but its plan dimensions are small and contained. It now has to accommodate seventy times more people than allowed for by the original design, so that it is constantly packed with visitors and is a frustrating and disorienting space from which to move on to the galleries. The removal of the British Library to a dedicated building at St Pancras has left the accommodation in the courtyard empty, freeing approximately 40 per cent of the museum's area. This has provided the perfect opportunity to establish the Great Court as the museum's central orientation space. The undistinguished post-war buildings that served as bookstacks have been demolished to recreate the courtyard at the heart of the Museum. The Reading Room is retained, serving as a reference library and a multi-media information centre about the museum's collections. Upon completion of the project, the Reading Room will be open to the general public for the first time in its history. Fig 8 In order that the Great Court can be used by visitors all year round, it is being covered with a lightweight roof that spans the space between the facades of Sir Robert Smirke's original quadrangle and the drum of Sydney Smirke's Reading Room. The lightweight roof is designed to let in light and keep out rainwater. This creates an indoor piazza - the largest of its kind in Europe - that will be open outside normal museum hours, providing London with a dramatic space for evening events. The courtyard links the main museum 279 entrance on Great Russell Street, via the new gallery in the North Library, to the rear entrance on Montague Place, establishing a public thoroughfare directly through the centre of the museum. In this respect, the project can be seen in a much wider context. It offers the opportunity of establishing a new diagonal route - a cultural route - across London, perpendicular to the River Thames. This route starts in the north with the new British Library and the three major railway stations - Euston, St Pancras and King's Cross. It continues through Russell Square, leading to the extensive area occupied by London University. Opposite Senate House, the British Museum and Great Court with its through-route and covered public space is a focal point. The route then moves south to Covent Garden, which attracts in excess of 10 million visitors each year. The improved pedestrian walkways on Hungerford Bridge, currently under construction, link Covent Garden with the revitalised South Bank and the international terminal at Waterloo Station. Fig 9 Fig 11 280 THE DESIGN OF THE ROOFS The Gateshead Roof The starting point for the Gateshead roof was to design a structure that would shelter the auditoria, the concourse and the music school beneath the concourse in the most efficient manner, closely hugging the buildings, and generating a form that would unify the complex. Initially a tensile structure was considered, but was abandoned in favour of a more permanent solution. Three adjacent shell-forms were generated. Initial these were entirely free-form shapes - not governed by any geometry. However, it was clear that it would be necessary to make the roof conform to geometric rules in order to rationalise the setting out and the manufacture and construction of the building components. Parametric modelling was employed to do this. GcwMion of COM Section erntntion of Spiral AA*H Enclosure Geometry Fig 12 In long section, east to west, the roof is a series of arcs that meet tangentially. These arcs are rotated longitudinally to create a toroidal geometry. The parametric model allowed the architects to alter the radii of any of the arcs and immediately generate a new roof form. This meant that recalculating the information each time a change was made, which would have taken hours or days if done conventionally, could be done within seconds. In response to structural, financial and aesthetic issues, the design team generated more than 100 alternative roof designs, sometimes mocking up 4 or 5 schemes per day. Such a degree of responsiveness would have been impossible without the parametric model. The roof has an area of 10,200 m2, spanning a distance of 100 metres north to south and 115 metres east to west. The three shell forms are cut at the rear and cantilever at the east and west edges to provide entry canopies. As it swoops down to the riverfront a portion of the roof is glazed. At the mid-point, the roof varies in height from 22 to 37 metres. The majority of the roof is clad in 2mm rain-screen stainless steel. This sits 600mm above a waterproof membrane. The glazed area on the riverside is 1,700 m2, with a 20m2 free area of high-level opening glass. The roofing system ensures that all panels, whether solid, glazed or louvered are interchangeable. The faceted roofing panels vary in length, but have been rationalised to only twelve different widths. Fig 14 The steel structure consists of four primary arches running north to south, which are 838mm universal beams. These are supported by sixteen props, which are 457mm circular hollow sections. There are an additional four props, which are 323 circular hollow sections, for each of the two cantilevered entrance canopies at the eastern and western edges. The props are set out radially. The secondary arches run east to west and are 406mm universal beams. The tertiary members running north to south are 168mm circular hollow sections. This integrated structural system is further braced with diagonal rods of 32mm diameter. The three main sets of structural elements are fixed with bolted connections, while the diagonal bracing is pinned. The whole forms a continuous shell structure. Fig 13 281 The Great Court Roof The key element of the design for the Great Court is the glazed roof. The underlying strategy is to produce a canopy that is delicate and unobtrusive, avoiding the need for columns within the court, which would obscure the handsome internal facades of Smirke's building. Geometrically the roof has to negotiate the space between the Reading Room and the surrounding facades and is constrained by planning requirements, which limit its height relative to existing structures. The roof had to be constructed of components that would be small enough to be lifted into position by crane, there being no other access to the construction site. This has resulted in a geometrical form, generated by a complex mathematical model, in which, despite its apparent simplicity, every single triangular glazing panel is unique. Fig 15 The roof is 6100m2 and comprises 3312 triangular glass panels. Only the north-south axis represents a line of symmetry for the roof because the Reading Room is off- centre within the Great Court by 5m towards the north facade. The structure spans lengths varying between 14 and 40 metres. The varying lengths result in the mid- point heights of the roof varying from 3 to 7 metres in relationship to the horizontal boundaries. The maximum distance from the floor level of the Great Court to the highest point of the roof is approximately 26m. The triangular glass panels vary in size from 800mm x 1500mm to 2200mm x 3300mm; the average area of the glass panels is approximately 1.85m2. Fig 16 The double-glazed units are assembled with an outer 'monolithic' lOmm-thick, toughened-glass panel; a 16mm air-filled cavity and an inner laminate glass, comprising two panes of clear-float glass and two clear PVB interlayers. The total thickness of the glazing unit is 38.76mm. The roof allows daylight to filter through and illuminate the court, passing into the Reading Room and, in very controlled quantities, into the surrounding galleries. In order to reduce solar heat gain the glazing units combine body-tinted glass with a white dot-matrix fritting pattern- over 75% of the sun's heat is prevented from entering the court - while a high proportion of the visible spectrum is transmitted. The glazing panels are supported on a fine lattice made up of 5162 purpose-made steel box beams that intersect at 1826 structural six-way nodes, each totally unique in its x, y and z co-ordinates and rotation angles. The 80mm-wide roof members are both the primary structure and the supporting frame for the triangular glazing units. The structure consists of 10 km of steel. Fig 17 An extruded silicone gasket provides the interface between the supporting steel frame and the glass panels. This 15mm-high gasket is not only shaped to cater for the angles at which each of the panels meet - varying between nearly 0° and 30° - but also to respond to the combined system's tolerances. As the steel roof members and nodes are fabricated through computer- controlled machining, precise tolerances can be achieved in the steelwork fabrication. 282 The glazing panels are mechanically restrained by means of stainless steel bolts and cleats, fixed to the steelwork at approximately 500mm centres around the double glazing units' perimeters. The double glazing units are manufactured with stepped edges, which provide the beaming surface for the fixing cleat. At its junction with the Reading Room the roof is supported on a ring of 20 composite steel and concrete columns which align with the structural form of the original cast-iron frame of the Reading Room. These columns will be concealed by a new skin of limestone surrounding the entire drum of the Reading Room, the exterior of which was not designed to be seen from within the museum. This skin also provides space for vertical services. At its perimeter the roof is supported by Smirke's original load-bearing masonry walls. It is connected to the walls by a sliding bearing carried by a concrete ring beam surmounting the existing walls. The roof's glazing system has been designed to be walked on for cleaning and maintenance. To ensure operatives' safety 200 harness attachment points, linked by continuous cables, have been provided in strategic locations across the roof. Both glass and steel have been designed, fabricated and installed with fully tried and tested technology and rigorously tested before assembly. Fig 18 Heating and Ventilation Stategies With both projects we have attempted to rely as much as possible on passive systems of cooling. The aerodynamic form of the Gateshead roof assists in a system of natural ventilation. The south-west wind is drawn over the roof, creating an area of low pressure at the building's riverside facade. This encourages air to be drawn in through low-level opening vents. A natural stack effect is created and air is exhausted through high- level opening glazed panels. This system is augmented with mechanical ventilation that supplies air and warmed air as necessary. Heating to the concourse is provided by an under-floor system using hot-water pipes. The auditoria are fully air-conditioned. At the British museum it was important to integrate modern services with minimal alteration to the building's historical structure. Having sealed the Great Court in order to keep the weather out, it is necessary to bring fresh air into the new spaces and the Reading Room at a rate of 45m3/ second. This is achieved by the construction of four new primary plant rooms in the basement of the existing buildings to the north-east, south-east, south-west and north-west of the court. These perform the initial filtering of the incoming air before it is passed to four secondary plant rooms beneath the court. Within these, full conditioning of the air takes place before it is distributed to the education centre, gallery spaces and the restored Reading Room. In the Reading Room the new systems follow, in broad principle, the original strategy of Smirke's design by using the existing 'spider' - a series of brick air ducts to carry insulated ductwork beneath the floor to supply air through the reading desks. The extract system will also use the original routes in the structure of the dome. The first level of environmental control is provided by passive, natural ventilation. Air is drawn in through high- level openable louvres around the perimeter of the Great Court. These, combined with a direct fresh-air feed to the floor-recessed displacement louvres, produce a large stack effect and wind effect to self-ventilate any internal heat gains. The passive system can also be used to 'purge' the entire volume at night when outside air is much cooler. This prevents the 'heat soak' from which many large structures can suffer if they are not allowed to 'breathe' at night. During the winter months, the Great Court is heated by an underfloor heating system with a network of pipes in the screed. To enhance cooling in the galleries and auditoria during the summer, the same pipes are used for a chilled water system. At night, when the galleries are closed, the central chiller plant is redundant. It is therefore possible to run this plant outside of occupied periods, using off-peak electricity to feed cold water to the slab. This then pre-cools the large floor area to approximately 18°C in preparation for the following day. The chilled slab encourages fresh air to remain at floor level rather than being drawn into the higher, unoccupied volume of the space. The resultant scheme allows the Great Court to be maintained between a minimum temperature of 18°C in winter and a maximum temperature of 25°C in the summer. 283 Engineering the British Museum Great Court Roof Stephen Brown Parner, Buro Happold The British Museum is one of England's most popular venues, visited by millions of tourists, students and academic researchers every year. To create more space for the Museum's continuing expansion and modernisation of its visitor facilities, it is witnessing change on a scale never before experienced on this tightly populated site in Bloomsbury. THE DESIGN The architectural scheme proposes spanning the Great Court, and encircling the grade one listed Reading Room, with a graceful streamlined glass roof enclosing the court below, providing a sunlit, comfortable space for visitors and museum staff. To meet the requirements of planning consent, the height of the new roof construction is restricted and the support of the outer perimeter on the quadrangle buildings does not visually intrude on, or structurally disturbing the classical Georgian facades that face into the Great Court. 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 that will surround the Reading Room. The Reading Room is actually 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. They will be of structural steel composite construction to achieve the required fire rating and stiffness to span from floor level to the snow gallery while remaining slender enough to be hidden behind a new stone cladding of the Reading Room. The columns designed in accordance with Eurocode 4 will be fabricated using tubular steel, an outer 457mm diameter reinforced with an inner 250mm square and filled with concrete. Around the Reading Room, of the roof will be prevented from spreading laterally by the Snow Gallery, which acts as stiff diaphragm balancing the thrusts from opposite sides of the roof. To achieve this the existing brick arched snow gallery will be demolished and replaced with a new reinforced concrete construction which will also house the main extract fans. On the other hand, around the outer perimeter of the roof, to avoid applying any lateral load to the quadrangle buildings, the roof is supported on sliding bearings. These bearings allow the roof to spread laterally under load , normal to the relevant facade, independent of the buildings. This freedom means that for the roof to hold its form, the outer radial members near the perimeter quadrangle must work in bending and 284 Roof Plan colours show how the stress corres[ponding element size varies. The torodail framing of the roof has been generated to provide an easy transition from the circular form of the Reading Room to the quadrangle of the surrounding Museum buildings. The geometry has been defined using customised form generating computer programme resolving both the architectural and structural requirements. Forming a smooth flowing roof that adheres to the height restrictions while curving over the stone porticoes in the centre of each of the quadrangle facades. The high points in the roof are located such that the lateral forces exerted on the Snow Gallery from opposing sides of the roof are generally balanced, minimising the risk of any nett force being applied to the Reading Rooms iron frame. As a further precaution the new reinforced concrete snow gallery will be supported on sliding bearings, so that the stiff ring floats above the historic frame. THE STRUCTURAL GRID The roofs structural grid follows that of the glazing supporting each panel along its edges minimising the complexity of the glass fixing. Therefore, the maximum size of glass available set the final structural grid size. The grid is formed by radial elements spanning between the Reading Room and the quadrangle buildings, that are inter-connected by two opposing spirals so that the roof works as a shell. While rectangular fabricated hollow sections are the preferred structural solution for the structural elements, a alternative slightly finer option using solid sections has been prepared. For both options the elements taper to smoothly accommodate their increasing depth towards the Quadrangle buildings. This reflects the architecture maintaining the sharp flowing lines of the structural elements dividing the individual glass panels. With the roof having only one line of symmetry, there are individual 1826 structural nodes where six elements are connected. All connections must fixed to transfer the forces and bending moments between the structural elements. • TYPICAL SECTION NEAR READING ROOM: TYPICAL SECTION NEAR QUADRANGLE: Section sizes increase from 80 x 80m around the reading Room to 80 x 180mm deep at the extremeties of the Perimeter. compression. These effects must pass through the joints in all directions. The size of the steel members therefore are smallest adjacent to the Reading Room and increase in size towards the perimeter, being largest at the corners. The forces generated by the abrupt change in direction at the corners are large and the structure is further stiffened in these areas with a tension cable across each corner. Design of the roof evolved using a three way lattice of steel members which add in plane stiffness, creating a very efficient form. The roof shape itself is curved to a tight radius of approximately 50m, which means it can act much in the same way as a dome, while imposing minimal loads onto the existing surrounding structures. The curvature of the roof has allowed Buro Happold to develop a light weight construction relying on arch compressions. The curvatures of a perfect torodial are usually steep so that it acts in an arching fashion, converting vertical loads into compression in radial members. In this project, the great Court roof is restricted in height and the outer perimeter is unrestrained laterally. Wind tunnel tests carried out by Bristol University provided information on the external and internal pressures which will influence internal ventilation and air movement of the great Court once it has been covered over. The results showed that wind flow separates at the outer perimeter of the museum, and does not re-attach over the new steel and glass roof in the great Court. This means 285 that the wind pressures on the roof will be small and consistently negative (uplift). On this basis, the net once in fifty year uplift force does not exceed 0.3kN/m2. This is well below the total dead weight of the roof with double glazed cladding. 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 situated behind each of the porticoes working parallel to the relevant facade. At the centre of each side of the roof, behind the porticoes, the lateral spreading movement of the roof is one directional, normal to the line of the facade. At these locations the roof can be laterally restrained parallel to the facades sitting the stub columns on one directional sliding bearings without inducing awkward secondary effects. A wide range of materials was considered for the construction of the structural support for the roof grill before steel was selected as the most appropriate. Steel is commonly selected for long span structures for many reasons, particularly because it provides high strength and stiffness at low cost. It is easily connected by bolting, or welding, and with a surface coating, has excellent weathering characteristics. By suitable selection of different components to form the whole cross section of the beam elements, the amount of fabrication can be kept to a minimum and the efficiency of the section can be maximised. The successful connection of the some 6000 individual members is critical to the integrity of the roof structure. The high stresses and slenderness of the steel elements lends itself to welded connections. To minimise the risks of weld failure, Grade D steel, more often used for marine, or petro-chemical applications rather than construction, is to be used. With such a precise project, it was felt that the impurities present in lower grade steel may allow too much margin welding error. Buro Happold sought the advice of The Welding Institute (TWI) when preparing the structural welding specifications to ensure that the welded joints will have sufficient ductility to prevent brittle failure. The specification included a stringent testing program to ensure that the quality of the steel and welding will allow the structure to behave as predicted. The architects are keen that from the ground, the double glazed roof has as light and clear an appearance as possible. This has led to the use of 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 will add another 60 kg/sqm. This light weight form of roof minimises additional loads imposed onto the existing facades. [...]...286 the ladder sections will be craned over the museum buildings, that will remain open to the public throughout the construction process, on to a precise system of temporary props Adjacent ladders will then be stitched together using on site welding techniques The installation of the glazing will follow the steel erection Only when the structural lattice is complete and vast majority of the glass... will the temporary props be systematically removed During this process the roof will be carefully monitored to ensure that it is behaving as predicted to achieve the defined final shape Installation of the steel roof structure finished at the Museum in early 2000, with the project due for completion this Autumn CONSTRUCTION It is proposed that the roof will be constructed in a series of prefabricated... ladder beams erected off a crash deck that will cover the entire court As there will be only a very limited degree of repetition in the node types, the use of steel castings to form the nodes would be uneconomic As a result, the star shaped nodes, some 200mm deep, may be cut from single thickness of plate, the points of each star shape set at the appropriate relative angles for each and every node Members... will be made as a series of straight elements meeting at the nodes .The prefabricated ladder beams will be assembled using precise jigs in the steel fabricators workshop Because of the cramped congested site there is no area for storage, the ladder beams will be trucked to site to meet immediate requirements On site Stephen Brown BE (Civil) CEng MIStructE is Group Director at Buro Happold CREDITS Architect:... site to meet immediate requirements On site Stephen Brown BE (Civil) CEng MIStructE is Group Director at Buro Happold CREDITS Architect: Sir Norman Foster & Partners Structural and Building Services Engineers, Fire Engineers and PlanningSupervisors: Buro Happold Construction Managers: Mace