7th December 1999
Engineering the glass and steel roof to the British Museum Great Court Project
By Stephen Brown BE(Civil) CEng MIStructE
The British Museum, the most visited museum of the kind in the world, 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 British Museum, the most visited museum of the kind in the world, 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 Museum's Georgian buildings were designed by Robert Smirke, who in 1823, as Attached Architect to the Office of Works, presented to the then trustees plans for the new British Museum. These plans originally consisted of four wings containing galleries set around a large rectangular courtyard. As work progressed, it became evident that additional storage was needed. This led to the construction in the courtyard of the circular Reading Room and adjoining bookstacks (1854-57) housing the library collections. Since then, almost all the of the open space within the courtyard has been filled with extensions added to Smirke's original buildings. Smirke's stone clad museum buildings forming the quadrangle and iron framed dome of the Reading Room are now grade 1 listed.
Now that the British Library has moved in to its new premises, the Museum is redeveloping the courtyard, demolishing the Bookstack structures, leaving the historic Reading Room as the centre piece of what will be the largest covered courtyard in Europe. The 6700 sq meter (92 x 73m) courtyard -a little bigger than the size of a football pitch- will be enclosed by a spectacular glass and steel roof. The new space will provide a hub at the centre of the museum complex together with new modern galleries, 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.
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 disturb 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, the roof will be prevented from spreading laterally by the Snow Gallery, which acts as a 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, and 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 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.
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 a 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 Room's 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 roof's 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, an 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.
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 for 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.
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 type
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 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 has been installed 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 commences at the Museum in the Summer of 1999, with the project due for completion in the year 2000.
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