Construction of the Emirates Stadium, which is to be Arsenal FC’s new home from August 2006, is now approaching the halfway stage. Having initially been appointed as the project’s structural engineer Buro Happold has since been engaged to provide multi-disciplinary services including environmental systems design, fire engineering, access design, bridge and civil engineering.
Buro Happold project principal, Paul Westbury, points to the practice’s work on this major project as an excellent example of integrated multi-disciplinary design. He
acknowledges that the success of this major project, which involves so many external organisations, is due to the close collaboration established between the design and construction teams.
Structural engineering overview
The overall form and structural configuration of the Emirates Stadium was derived by striving for a balance between a wide range of sometimes conflicting factors. The main design drivers included:
· The aim for high quality viewing standards
· The desire for an awe inspiring atmosphere within the spectator arena
· The desire to maximise the quality of the pitch microclimate
· The desire for the stadium form to be iconic and instantly recognisable as the home of Arsenal Football Club
· The limitations imposed by the tight site footprint and the proximity of the site to existing Network Rail and London Underground lines
· The height restrictions embodied within local planning legislation
· Restrictions on budget and construction period
The resulting configuration is an approximately elliptical plan-form, featuring five main levels of accommodation, and four seating tiers, yet is contained within a compact site footprint and also respects the height restriction imposed by local planners.
The roof form features a constant eaves line around the entire perimeter, and then slopes downwards towards the pitch. This creates a naturally dished form that will enhance the pitch-microclimate and the atmosphere within the arena, and will ensure the stadium is a landmark icon.
Main frame and substructure
The primary elements of the main superstructure are:
· Insitu reinforced concrete (RC) floors and frame from Level S1 to 3.
· RC rakers that support the lower tier, club tier and box tier
· Structural steelwork rakers and the frame which supports the upper tier and Level 4.
· Precast concrete seating and associated elements supported by the rakers of each tier.
This configuration enabled compact floor and frame thicknesses to be achieved, allowing services zones to be maximised while still resulting in an overall compact form of stadium. The chosen configuration also resulted in a very flexible form of construction for the lower levels, which could be moulded to suit the varying grid that is inherent with an elliptical building. This meant that the lower levels could be constructed very quickly, with rapid response times for ongoing development, at the same time that off-site manufacture and assembly of the steelwork and precast concrete elements was being carried out.
Just inside the elliptical perimeter of the envelope are eight primary cores. These provide the primary stability to the stadium and also house many of the vertical circulation routes and services distribution routes. All of the superstructure floor-plates are split into eight zones by a system of movement joints. Each zone is attached to its adjacent zones so that it is stabilised by two cores.
The seating tiers at all levels were extensively analysed to tune their dynamic response to ensure spectator safety and comfort.
All of the loads are transferred into the ground by means of a system of 600mm diameter bored-cast-insitu RC piles that extend up to 18m into the underlying London Clay. Piled transfer-structures transfer the superstructure loads across the 10 m exclusion zone around the existing storm-relief sewer.
Iconic roof design
The iconic main roof shelters all spectators and covers a plan area of approximately 26,000 m2.
The main elements of the primary roof structure comprise a system of 3-dimensional primary, secondary, and tertiary trusses, connected together around the perimeter of the roof by a prismatic perimeter-truss. The primary trusses span approximately 204m between tripods attached to the tops of opposing pairs of cores at the north and south ends of the stadium. All of the trusses are triangular in cross-section, and made from circular-hollow-section members. The prismatic form of the trusses means they are inherently stable against lateral-torsional buckling effects. Their inter-connectivity ensures their overall lateral and torsional stability.
Vertical loads are transferred into the frame by 8 tripods [one on top of each core], and by a total of 64 articulated props that transfer loads from the perimeter truss down to the rear of the rakers of the upper-tier. Overall stability of the roof is provided by the 8 tripods which then feed the lateral loads into the substantial cores that support them.
Much of the roof covering is in the form of two skins sandwiching the purlins that are suspended beneath the primary steelwork by a system of hangers. The upper of the two skins provides weatherproofing while the lower-skin is in the form of aluminium trays, or panels, spanning between the bottom flanges of the purlins. A 10 metre wide band of polycarbonate single-skin roofing is set into the roof just behind the gutter zone around the pitch-side leading edge. The aim of this is to soften the shadows that will arise during events held on sunny days.
Site constraints
The Ashburton Grove site presented Buro Happold with a series of significant engineering design
challenges.
Its triangular shape occupies 70,000 m2 of brownfield land which over the years has been used for a
variety of purposes. Most recently it has been home to a number of industrial units including a waste
transfer station.
The new stadium has been positioned as far to the apex of the triangular site as possible so that the
full development potential of land lying to the south can be realised.
The west side of the site is bounded by a Network Rail 6m high brickwork retaining wall, behind which
lies the East Coast main railway line. Lying next to the east side of the site is a railway cutting within
which run four local rail lines.
Approximately 7m beneath the western wall are two London Underground tunnels for the Piccadilly
line. On the eastern site boundary are two further London Underground tunnels serving the Victoria
line at a depth of 8m.
Three Thames Water main sewers run beneath the site. Two of these pass through the site from north
to south, one at a depth of 3m and the other at 8m. The deeper of these forms part of a storm relief
system for the area which discharges into a primary storm relief sewer crossing the site from east
to west at a depth of 17m.
The sub-surface strata generally consists of made-ground overlying alluvial deposits overlying London Clay. The London Clay generally starts at between 3 to 6 metres below ground level ends to a depth of approximately 25m below ground level.
Due to the variety of industrial uses some of the land was contaminated at upper strata level.
Bridge engineering
Buro Happold designed two new bridges (the North and South Bridge), connecting the stadium to Drayton Park and the Arsenal underground station. They have been designed to provide a rapid evacuation route for spectators – over 16,000 would need to escape across each bridge within an eight-minute period. The bridges are thus 14m and 22m in width respectively and the larger (South) bridge has been designed to accommodate emergency vehicles.
In designing the bridges, Buro Happold had to work within some challenging site constraints. Existing transport and utilities infrastructure severely restricted the available positions for permanent and temporary supports associated with bridge construction.
Due to the difficult site, it was important to consider the method of construction of each bridge from an early stage in the engineering design. It was possible to split the smaller North Bridge into two sections that could be lifted into position using a large crane. However, with a steel weight of 1,000 tonnes, the South Bridge was too heavy to erect in this manner. This bridge was therefore fully assembled alongside the railway lines and launched across by sliding over temporary supports during a railway closure. The timing of this bridge launch was critical to the overall programme for stadium construction as the bridge assembly occupied space that was needed for construction of part of the new stands.
Background information for editors:
Press office and practice information at www.burohappold.com
Buro Happold is a multi-disciplinary international practice of consulting engineers established in 1976 offering civil and structural engineering, mechanical and electrical engineering, quantity surveying, building services and environmental engineering, health and safety management, infrastructure and traffic engineering, ground engineering, façade engineering, fire engineering, computational fluid dynamics analysis, disability design consultancy, project management, urban design and a range of specialist CAD services.
Elspeth Wales
Telephone 01225 320 600
Fax 0870 787 4148
Email Elspeth.Wales@burohappold.com
