KẾT CẤU MỚI THE DESIGN OF THE ROOFS OF THE BRITISH MUSEUM AND THE MUSIC CENTRE AT GATESHEAD

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.

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

A B S T R A C T

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

F i g l

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

H I S T O R I C A L B A C K G R O U N D T O T H E

P R O J E C T S

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

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

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

T H E D E S I G N O F T H E R O O F S

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

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 7 5 % 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

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

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

T H E D E S I G N

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

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

T H E S T R U C T U R A L G R I D

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 S E C T I O N 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

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