Design Awards: 2011

The Hauser Forum, Cambridge

hauser_forum_01

Architect

Wilkinson Eyre Architects

Structural Engineer

Mott MacDonald

Main Contractor

Willmott Dixon

Client

Turnstone Estates Ltd

The Hauser Forum is a new, mixed-use development at the University of Cambridge’s West Cambridge site. The development comprises the Cambridge Enterprise Building (CEB) and the Broers Building.

The CEB will be the new home for Cambridge Enterprise, a University owned company which assists with the commercialisation of research, and also incorporates a new campus café. The most striking feature is the double height café space which cantilevers 11m from the southern façade of the building.

The Broers Building provides 4,000m2 of high quality commercial, research and development and ancillary space to be let to private organisations. Office space is provided over four storeys, with the steel framed superstructure also boasting a cantilever to the south.

Steel was chosen for both buildings to enable them to meet architectural aspirations and achieve a tight programme. The project retains a large amount of exposed architectural steelwork in canopies and edge detailing. Services and structural zones were combined through the use of cellular beams to limit the floor to floor heights whilst maximizing the available floor to ceiling height in the 18m wide floor plates.

The two buildings are connected visually by a high level steel canopy which covers the new landscaped forum space between the buildings. The canopy also serves a practical purpose in providing solar shading to the buildings.

The principal challenge for the structural design was the double height fully glazed café which cantilevers a total of 11m from the CEB over a reflecting pool under, 3.5m of which forms a terrace extending beyond the building enclosure. Achieving this, while meeting vibration limits for occupant comfort and deflection limitations for the proposed curtain walling system, led to significant analysis and development.

The concept adopted was to use large trusses in the elevations to form the cantilever, which could only be practically achieved in steel. This scheme was developed using Macalloy tension rods as the diagonals in conventional N-trusses. An additional pair of tie bars was added in the southern façade to limit serviceability deflections of the 13.5m single span fabricated box floor beam to the limits required by the curtain walling. The cantilever was erected on temporary props with a precamber of 35mm to counter overall dead load movements and to leave it with an aesthetically pleasing slight upward camber. Once the floor slab had been poured, the Macalloy rods in each elevation were stressed to lift the structure off its temporary seating and the props removed.

The project incorporates numerous design features that enabled both buildings to achieve BREEAM ratings of ‘Very Good’ and EPC ratings of ‘B’. Both buildings are provided with ground source heat pumps installed integrally with the structural piles, which provide up to 117kW of cooling and 188kW of heating. This is approximately 7% of the buildings’ energy demand and delivers a calculated annual carbon saving equivalent to 26,000kg of CO2.

Additional energy savings on heating and cooling were made via the inclusion of innovative thermal labyrinths in the basement of each building. These consist of a series of closely spaced dense concrete block walls arranged in a zig-zag formation through which air is drawn before reaching the building air handling units. This has the effect of lowering the air temperature during the peak summer months by up to 5ºC from ambient, and providing a more stable temperature output throughout the day, reducing the amount of energy expended on conditioning the air. The installed labyrinths were calculated to achieve an equivalent energy output of approximately 91MWh per year, realising a carbon saving equivalent to 10,572kg of CO2.

Judges’ Comment

These two buildings are joined by a covered public space, and the steelwork frames are key to the whole project. Whilst one building is air-conditioned, the other makes effective use of greater mass for natural cooling. The link block has a striking large cantilever over the adjoining lake.

A good example of practical steelwork design integrated into an intelligent solution.

The Rose Bowl, West End, Southampton

the_rose_bowl_01

Architects

The Miller Partnership
EPR Architects

Structural Engineer

Ian Black Consulting Ltd

Steelwork Contractor

Rowecord Engineering Ltd

Main Contractor

Andrew Scott Ltd

Client

The Rose Bowl

The addition of two new stands at The Rose Bowl cricket ground in Hampshire has increased capacity from 20,000 to 25,000, making it one of the largest grounds in the country. The number of permanent seats increased to 15,000.

Keen to ensure that the new stands fitted the original visual concept for the ground, a sickle structure was developed visually separating the roof to the permanent seating from the back of house structure, and expressed on the rear elevation by a large visually independent column supporting a timber louvered façade to marry with the surrounding landscape. The stands follow the curve of existing terracing to maintain the bowl concept.

The steel structure which provides the framing for the envelope consists of main frames radially placed at approximately 8.250m centres. Each frame comprises the central accommodation structure, consisting of two floors plus roof. This is 12.2m wide and formed of a regular grid of steel beams acting compositely with a 130mm in-situ slab on a profiled metal deck. A conventional built up system provides the roof to this structure.

Springing from the second floor, the terrace rakers slope down to meet the back of the existing terracing. The rakers are split into two spans by a mid-support which follows the line of the back wall of the new welfare facilities.

Above the two storey structure supported on the front column is the sickle rafter, which forms the main structural member to the canopy roof. This rafter curves round the back corner of the two storey structure forming an independent column which slopes back into the building. This column is discretely connected back to the two storey structure at roof and floor levels. The sickle rafter is formed of standard UB sections with the curved portion formed in plates.

The structure in its final form is a simple braced structure with the floor slabs acting as diaphragms. The main (canopy) roof is braced for the full length to ensure effective transfer of lateral loads. Provision of vertical bracing was only possible longitudinally in the line of columns mid-span of the terrace rakers, and between the sickle columns to the rear of the building. No vertical bracing was possible radially or in the walls of the ‘back of house structure’ due to the numerous openings and glazed areas.

The sickle columns are discretely connected at each floor level and at roof to the accommodation block, taking the form of a radial prop between the columns on each line. At two locations along the length of each stand diagonal members spring from the 914 x 305 UB sickle columns to third points in the adjacent floor edge beams. By this means the sickle columns are discretely connected to the ‘building’ structure, maintaining the required architectural illusion of independence. Thus the vertical bracing between the sickle columns provides the lateral stability to the rear of the building.

The terrace rakers provide a natural brace up to second floor level, the column simply cantilevering from second floor to roof. Again the connectivity to the sickle columns provides a degree of additional stiffness, due to the sheer size of these feature columns.

A cost-effective structural solution was delivered maintaining a ‘simple’ frame, avoiding the need to introduce significant moment connections into the structure, whilst meeting all the architectural requirements in terms of unhindered elevations within the accommodation building, and the main visual impact of an independent roof and rear feature wall.

Judges’ Comment

These two new stands flank the existing pavilion. Simpler than the first building, the design reflects the need for an economic and low-maintenance solution, with clarity of structural steelwork, cladding and the resulting space beneath the stands.

A good example of ‘less is more’.

ExCeL Phase 2, Royal Victoria Dock, London

prologis_01

Architect

Grimshaw Architects LLP

Structural Engineer

McAlpine Design Group

Steelwork Contractor

Severfield-Reeve Structures Ltd (Severfield-Rowen PLC)

Main Contractor

Sir Robert McAlpine Ltd

Client

Excel London

ExCeL Phase 2 creates new spaces that harmonise with and extend ExCeL’s existing offer in London’s Royal Victoria Docks. Phase 2’s key objective was to provide flexibility to respond to the changing requirements of exhibitors, conference organisers and other users. The Phase 2 developments take ExCeL to a total capacity of 1,000,000 sq ft of flexible space, complemented by the new bespoke conference facility, the 15m high hall, a ‘grand entrance’ and a less crowded, easier-to-navigate arrivals experience for all visitors.

An inhabited spiral accommodates foyer and boulevard facilities and brings the east elevation to life. Elements of the spiral act as the foyer floor, the mezzanine floor accommodating casual dining space, and high-level foyer floor serving the conference hub. It stands independently of walls to dropped exhibition halls so that it can be read autonomously on the east elevation and increase the sense of arrival.

The super structure was fully erected in just over four months and a fully covered working enclosure delivered six weeks later. With the Phase 2 venue pre-booked for the day after practical completion for X Factor auditions, the building was fully constructed in just 22 months.

The clear span halls of 87m are structured through the use of a mast assisted steel truss. The structural masts also pick up the grid of suspended demountable hall partitions. This structural solution was adopted from the existing Phase 1 building which had been proven to be the most cost effective solution, but was distilled and improved upon such that the masts were placed back to back centrally rather than to the perimeter. With this modification it was possible to:

  • Remove the requirement for tie down members and increase structural efficiencies through a balanced structural system.
  • Reveal the structural solution to the visitors occupying the central circulation boulevard.
  • Support the roof lights that line the central boulevard.
  • Avoid any conflict with City Airport take-off and landing zones to the south, along with a clash with the electricity pylon to the north.
  • Construct the building in a logical fashion from east to west by erecting the structurally stable masts, then the trusses.

All environmental solutions were required to be cost neutral in terms of construction costs. Phase 2 includes the provision of a grey water storage tank, waterless urinals and low flow fittings throughout. A significant aspect of energy use for the existing ExCeL venue was through its lighting. Simple effective measures were introduced including PIR occupant sensors and low energy fittings. The main improvement was in the introduction of expansive evenly distributed roof lights to the internalised boulevard and also the upper level breakout space. The Phase 2 building demonstrates a reduction of almost 60% in light energy consumption compared to the equivalent Phase 1 environments.

The use of steel ensured a minimal construction period (just 22 months), a lightweight cost-effective solution, and lent itself to the architectural requirement for numerous cantilevers and large column free spans. Steel really was the only viable structural solution and allowed the design team to express the structural and engineering solutions of the building.

Judges’ Comment

Doubling ExCeL to a million square feet of high column-free space has been admirably achieved by this large steelwork structure. Following the earlier outline, this phase is adapted for the constraints of City Airport and local infrastructure, whilst improving the circulation areas. The cost-effective solution continues the 87m spans, also now with some 15m high bays.

The client is delighted, and exhibitors’ enthusiasm is testament to steelwork’s success.

The St Botolph Building, London EC3

st_botolph_building_03

Architect

Grimshaw Architects LLP

Structural Engineer

Ove Arup & Partners

Steelwork Contractors

Severfield-Reeve Structures Ltd (Severfield-Rowen PLC)
CMF Ltd

Main Contractor

Skanska Construction UK Ltd

Client

Minerva PLC

TThe St Botolph Building is an exceptional new office building offering 560,000 sq ft of lettable space distributed over 13 floors. Planned as a multi-tenanted building, maximum flexibility for letting was crucial.

The design places the cores at the edges of the floorplate which, combined with a stepped internal atrium, creates space suitable for the broadest possible range of occupiers. At the lowest levels, the atrium is offset from the centre of the floor to provide open spaces suitable for small-scale trading. Above this the atrium widens in a series of steps to ultimately occupy the whole centre of the floor. The variety of floorplates created between these two configurations enables a broad range of needs to be met.

The steel frame allowed large structural spans and uninterrupted floorplates. The steel atrium bridges, linking the passenger lifts to each side of the 18m wide atrium, allow the floorplates to be divided into multiple tenancies.

The external form of the building is articulated along each elevation by the expression of perimeter service cores framed in steel with the stairs, lifts and boiler flues on display through steel- framed glazing. The steelwork of these elements is clearly articulated internally and externally. The perimeter stairs were designed as prefabricated steel assemblies, capable of spanning between framing members and bracing the perimeter core structures. This eliminated temporary works and early completion of the stairs provided safe access for trade contractors during construction. Careful coordination greatly reduced the number of site processes required, eg site drilling of steelwork was almost entirely omitted.

A large proportion of the steelwork in the building is visible, clearly displaying its structural function. In the central passenger lift core very tight construction and positional tolerances have been achieved. This was necessary to achieve coordination between the four separate contracts that form the sculptural centrepiece of the building. A comprehensively coordinated floor construction provides the shallow floor/ceiling zone necessary to achieve generous floor to ceiling heights within the maximum permitted building height. Close collaboration within the team resulted in a consistent 690mm deep fabricated beam being used throughout, with flange and web thicknesses tuned to suit loads and spans up to 16.5m.

The team produced a fire engineered solution to minimize the extent of intumescent paint. Fire protection of the perimeter cores was achieved through adjacent slip-formed concrete, omitting the requirement for fire protection to the steelwork of the stair and lift shafts.

A key feature of the building is the innovative passenger lift core. The lifts are situated in a carefully coordinated steel and glass structure visible as a kinetic sculptural centrepiece throughout the building. At higher levels of the structure four atrium bridges brace the steel frame and provide access to the floor plates. The adjacent structures have been designed for future installation of additional bridges, preserving future flexibility.

Temporary steel assemblies built on level 8 transferred the load of four tower cranes directly into the superstructure, allowing the lower crane mast sections to be removed so that completion and fit-out of the floors below could commence early.

Photovoltaic panels contribute electricity directly into the tenants’ small power switchboard. During periods of low demand, the escalators run at a slower, energy saving speed and the lift selection panels in the lobbies switch to an energy saving mode. Car lighting systems switch off automatically when the lifts are not in use.

This impressive building is an excellent example of steel enabling the creation of a dramatic yet functional commercial building.

Judges’ Comment

This large steel framed office is of significant architectural merit.

A major feature is the atrium where banks of twin lifts operate simultaneously in the same open shafts. The steelwork is crisply detailed throughout the floors, glazed atrium bridges, and even to the expressed lift supports.

An exceptionally stylish commercial office building, showing steelwork to great effect.

Exposure, Lelystad

exposure_01

Architect

Antony Gormley Studio

Structural Engineer

Haskoning Nederland BV

Steelwork Contractor

Had-Fab Ltd

Main Contractor

Had-Fab Ltd

Client

Municipality of Lelystad

Exposure is based on Antony Gormley’s own body and stands 25.6m high. The initial shape was created when the artist was covered in plaster of Paris and removed from the shell when the plaster had set. This shell was surveyed by Cambridge University who produced an electronic 3D wire model. This model was then used by the engineers for the project to refine member locations to Gormley’s instructions. The design was then imported directly onto the steelwork contractor’s detailing package. The process of the transfer from design software to the detailing package was breaking new grounds as it had never been done before with such a complex project.

With such a detailed and precisely engineered project a web viewer tool was used to provide the engineer with 3D details of the nodes so that he could confirm that the construction met his design requirements. The detailing process took approximately 12 months to complete.

The web viewer tool was also essential in the fabrication process to gain the true angles of members as well as rotational and other important dimensional checks. It is safe to say that this project would not have been possible just 10 years ago, however with the advancement of modern CAD technology and direct links with CNC fabrication machinery the project was executed using detailed CAD information by interrogating the model using the laptop on the shop floor.

The sculpture is constructed entirely from steel angle profiles. Every angle member is a different length and produced from steel sections ranging from 60mm x 60mm to 200mm x 200mm. Once the angles were punched and sheared to length then the highly skilled and labour intensive shaping of the sculpture began.

Full size paper templates were used to achieve the extremely complex cuts and shaping of the member ends. The welded nodes were produced from randomly rotated angles all joining together on the neutral axes of each member and, more importantly, having full member cross sectional area connection into the node.

The sculpture is made up of 547 nodes, the largest of which are the heart node, weighing 280kg and the brain node, weighing 56kg. The heart node has 29 individual members joining together in the centre. Producing these 2.5m diameter nodes was extremely time-consuming and complex. The nodes meet together to form locating points around the structure which were bolted in position.

Conventional 2D drawings were not suitable for communicating the fabrication information needed by the shop floor team. If a true view is made in 2D then there are no other members on the same plane that could be dimensioned on that true view. It is truly a 3D project that very unusually had to be manufactured on the shop floor in 3D (it would be more usual to produce pieces in the shop and then produce the 3D structure on site when it is erected).

The main key to achieving the project was the node building jig that was developed by the steelwork contractor. This is a 3m diameter ring pivoted top and bottom with graduations at every degree on the base and a clamping system around the ring. This allowed for a main member to be held in a fixed position to the centre of the ring and then members could be accurately added at any position within a complete sphere (anywhere in the world).

A total of 32,000 holes were punched or drilled in the angle profiles and the total weight of the structure was 60 tonnes.

The galvanizing of the structure, particularly the nodes, also posed some challenges. Venting of the nodes was impossible at the detailing stage as the expertise of how to hang the nodes to gain the best coating was impossible to visualise. Whichever way the node was dipped air pockets formed restricting the coating integrity. The galvanizers used an ingenious way of dipping each node into the bath on two wires and then releasing one wire while the node was in the bath, which then allowed the node to rotate and vent the air pockets, resulting in a fully galvanized node.

This is a one-off structure which epitomises the way software, machine technology and manual fabrication techniques can work together. The challenges of the design, detailing, equipment performance and quality fabrication have been extensive.

Judges’ Comment

A remarkable structural steelwork sculpture inspired by the artist’s own crouching body.

The immensely complex arrangement of hundreds of galvanized angle sections, precisely as required by the artist and structural designer, was fabricated in modules for transport and erection. The challenge facing the steelwork contractor was enormous.

This exciting work became a ‘labour of love’ for the whole team, who have achieved a high profile tour-de-force.

American Express Community Stadium, Brighton

amex_stadium_01

Architect

KSS

Structural Engineer

SKM (Europe) Ltd

Steelwork Contractor

Watson Steel Structures Ltd (Severfield-Rower PLC)

Main Contractor

Buckingham Group Contracting Ltd

Client

Brighton & Hove Albion Football Club

Brighton & Hove Albion FC kick off the 2011/12 football season with a new 22,500 seat stadium whose signature feature is a spectacular steel arched roof, engineered from the outset to sympathetically mirror the undulations of the surrounding South Downs.

The stadium comprises 4,200 tonnes of structural steelwork which was constructed between July 2009 and July 2010. The form and shape of the stadium evolved out of a direct response to the site and its topography. The curve and tilt of the roof, together with its arches, effectively replaces a mound of chalk that was scooped out to form the pitch and surrounding stands.

Efficiency and economy of operation of the stadium is key to its long term viability. To this end the stadium has been designed to respect its environment and reduce running costs by achieving a BREEAM ‘very good’ rating. This was met by minimizing construction traffic disruption by relocating spoil locally for the remediation and improvement of the adjacent arable fields, minimal environmental impact of the operational green travel plan, minimizing CO2 emissions by reducing the quantity of construction materials, optimizing insulation standards and minimizing energy consumption and waste.

The challenge to achieve an exceptionally low profile, flat arched roof to meet the visual impact needs and conceptual design ideas required entire world-class long span engineering expertise and unparalleled fabrication and construction experience.

The design trades a relatively complex, but short term, erection strategy for a long term elegance and considerable weight saving per m2. The entire structure works together and every part plays a role so that the roof only became completely self-supporting when all the pieces were in place. This minimized any redundancy and gives a structural form of breathtaking beauty and uncanny lightness of appearance for the spans and roof covering involved.

The 43m wide east roof and the 55m wide west roof are supported by 170m long arched and leaning ‘toblerone’ roof trusses, each weighing in the order of 350 tonnes. The smaller north and south roofs have more conventional cantilever roofs. All four sides of the roof are interconnected and incorporate over 1,000 sliding bolted connections to enable the roofs to continually ‘flex’ during the life of the structure.

Each roof truss is leaning outwards and thus has a natural tendency to deflect outwards and downwards effectively displacing the primary rafters in the same directions. The lateral movement of the rafters is restrained by a catenary member in the plane of the roof running between the rafters outside the line of the arched truss and transfers the tension back to bracing and foundations. To prevent the lateral loads being transferred into the terrace the rafters are supported by double pinned tubular struts.

The 20MN thrust at the ends of each roof truss is transferred to the permanent concrete thrust walls by means of bearings, each of which weighs seven tonnes and is also engineered to cater for rotation of the roof trusses as they ‘flex’ in-service. The construction method was developed alongside the design. The objective was to pre-assemble as much as possible on the ground and minimize temporary works whilst, at the same time, ensuring that the temporary stresses induced within the lightweight roof did not exceed permissible limits.

The 170m long arched roof trusses were delivered to site ‘piece small’, apart from the heavily loaded tapering end sections which were delivered as a plated fabricated section.

The individual members of each truss were first assembled into three 15m deep x 60m long sub-sections, each weighing 120 tonnes. These three sub-sections were then lifted onto a pair of 20m high temporary trestles located at ‘third points’ on the concrete terracing. The ends of the arch are supported by heavily reinforced concrete thrust walls.

Only when the entire roof structure was completed could the temporary trestles be de-jacked and removed from below the west and east roofs of the structure allowing the thrust blocks to take up the load. This process was carefully controlled with each iteration of the de-propping carefully monitored against predicted loads and movements.

Initially the weight of the entire roof was transferred onto sets of jacks installed on each of the four temporary trestles, and the ‘starting’ weight in each of those jacks recorded and compared to predictions. Thereafter the roofs were lowered in approximately 20mm increments at each of the four temporary trestles in turn. Extreme caution was required to ensure that the loads remained equally distributed between the trestles, and that temporary forces being induced into the truss members above each trestle did not approach the safe capacity.

One complication in the de-propping process was that as the roof lowered the truss deflected outwards, thus deflecting the top of the temporary tower which had to be reset to vertical using a set of horizontal jacks installed at the top of each trestle. Eventually the reactions at all four temporary trestles reached zero – at which stage the roof was free-standing and the temporary towers could be removed.

The cutting edge design has achieved a roof weight of just 101Kg/m2 which is a new benchmark for the industry for a long span roof of this form.

Judges’ Comment

This provides a long-awaited home for Brighton & Hove Albion, with community facilities.

A large structure, with curving roof and main trusses, sits comfortably into the rolling South Downs. The blue translucent roof shows off the well executed roof steelwork.

A well conceived and executed steelwork project, demonstrating close and effective cooperation in the team, bringing success for client and public.

Marks & Spencer Distribution Facility, Prologis Park, Bradford

prologis_01

Architect

Stephen George & Partners

Structural Engineer

BWB Consulting

Steelwork Contractor

Barret Steel Buildings Ltd

Main Contractor

Winvic Construction

Client

Prologis Development Ltd

Measuring 512m x 176m this is said to be one of the largest distribution centres ever built in the UK. Occupying the 90-acre ProLogis Park Bradford site, the 1.1m ft2 distribution centre represents the new logistics model for Marks & Spencer. It was officially opened by Marks & Spencer Chairman Stuart Rose in July 2010.

The design and build steelwork package consisted of steelwork for the main building, attached office blocks, stair towers and link bridge.

The 176m span of the roof is achieved through a single curved roof of 1.5km radius and is formed through a series of seven monopitch portal rafters. CA Group River Therm was rolled on site at the eaves in 178m long sheets, the longest single roof sheet ever produced. The programme for the roof installation was set at just 12 weeks. In order for this programme to be achieved it was vital that the highest quality working platform be provided.

It was quickly identified that the combination of a very shallow curved roof over a huge span allowed a unique set of design issues to be addressed by the design team:

  • The cold rolled purlins had to run straight, avoid rotation between supports, be pitched at differing amounts along the rafter, and be set vertically at exactly the correct level for 512m.
  • Each purlin would need to be set slightly different from its adjacent neighbour.
  • The roof had to remain positive under all loading conditions to avoid ponding.
  • The span is formed in seven straight monopitch rafters, each preset from the eaves to reduce dead and service load deflections.
  • The curve is formed by stooling up individual purlin cleats from the rafter backs to the line of the liner tray. This resulted in over 50 differing purlin cleat designs, some differing by only a few millimetres.

At the early stages of design the risk of getting purlin cleats in the wrong location was identified as being high, as was the potential severity of the on-site remedial work and resulting disruption to follow- on trades.

The solution was in two parts:

  1. To scribe each purlin cleat with a unique reference mark
  2. To employ unique scribe technology to not only locate each purlin cleat along the rafter, but also to scribe the corresponding purlin cleat reference mark alongside the cleat scribe location.

Innovative software enabled the close coupled drill/saw to mill unique reference and identification marks onto the steel for identification purposes.

This is taken further by corresponding scribing of the outline of the fittings for location purposes. Each rafter was delivered to the steelwork contractor’s production line with its corresponding set of purlin cleats allowing the plater/welders to easily and efficiently fabricate the rafters, enabling up to 300 tonnes of error free fabrication to be delivered to site each week.

This combination of design risk assessment and fabrication technology allowed the erection of over 7,000 purlin cleats and over 56km of roof purlins to be provided without a single instance of rectification, thus achieving a very high quality finish to the agreed programme.

The client wanted to maximize internal space and to that end no internal cross bracing was allowed. A complex system of roof bracings was designed which allowed the wind loads to be distributed to a series of side bracings, strategically positioned to miss doors, offices and windows. To cope with any possible thermal expansion the design also includes a thermal movement joint which has been positioned half way along the length of the building.

Erection of this 3,000 tonnes shed started in July 2009 and the main shed was erected in only 12 weeks. Erection at this pace demands hour by hour planning with no room for error or delay. This was achieved by detailed planning of the erection and production process. Close liaison with the client and erection sub- contractor was also key. Off site quality control ensured that virtually zero site remedial works was achieved, thereby keeping to a tight cost plan and minimizing disruption to following trades.

The frame has been designed to accept an additional three levels of mezzanine to allow future expansion by Marks & Spencer.

The building achieved ‘Excellent’ BREEAM ratings and the shell is carbon neutral thanks to the use of various organic reductions and the purchase of carbon credits.

Judges’ Comment

This state-of-the-art warehouse and distribution centre, meticulously planned and designed, was erected in 12 weeks.

The exterior conceals the sophistication of the engineering design, fabrication and erection for economy and speed. The extremely slender columns, at wide spacing, soar upwards to support a very light curved roof, avoiding valley drainage. Rooflights allow daylight to fill the cathedral-like space.

The steelwork is outstandingly light, and may well become a benchmark for such projects.

Cannon Place, Cannon Street Station, London

cannon_place_01

Architect

Foggo Associates

Structural Engineer

Foggo Associates

Steelwork Contractor

Watson Steel Structures Ltd (Severfield-Rowen PLC)

Main Contractor

Laing O’Rouke

Client

Hines

Cannon Place is a 37,000m2 ‘air-rights’ office development in the City of London above Cannon Street mainline and underground stations. The site is broadly rectangular measuring 67.5m x 87m. The brief called for a building which would appeal to financial, legal and corporate tenants, be capable of multiple sub division and optimize the quantum of lettable space on the site.

The site sits in the foreground of protected views of St Paul’s Cathedral, which has the effect of ‘capping’ the height of the scheme at 51.3m AOD. A minimum height of 5.1m above the running tracks of the mainline station had to be maintained. As a result, a height of just 32m existed within which to plan the eight floors of office space needed for commercial viability.

The site was further encumbered by:

  • the inability to found vertical structure in the London Undergound tunnels on the north elevation
  • significant existing foundations under the previous Cannon Street station facing slab block
  • the limitations on column setting out imposed by the mainline railway
  • the requirement to minimize foundations in the archaeologically sensitive Scheduled Ancient Monument.

A structural solution was devised which balances a cantilevered 21m deep ‘strip’ of office space to the north with the equivalent accommodation in the south through the use of façade deep transfer structures. This obviates the need for columns in impossible areas and transfer structures, by virtue of their location, do not eat into the development zone and reduce floor to ceiling heights.

The striking nature of the structure has been expressed by placing the fully glazed curtain wall inside the structural frame allowing the engineering to become the ‘architecture’ of the design. A plan form was devised dividing each of the floorplates into five ‘strips’ of accommodation – three of 21m deep space, separated by two of 12m. At each end of each 12m strip sits a 12m sq fire escape and service core. In the centre of each 12m zone is an atrium which allows daylight to penetrate deep into the heart of the floor plan, each atrium also contains six scenic passenger lifts.

Only columns in these 12m strips continue through the station to foundations. However, in the south atrium the number of columns passing through the station is reduced with transfer structures, a further eight columns terminate above the station acting as ‘link columns’ between the office floors above improving the floors’ dynamic performance.

Floor structures comprise 21m secondary steel beams spanning between 6m or 12m primary beams. All beams are 725 deep Fabsec beams which act compositely with a 130 lightweight concrete slab on re- entrant metal deck. The deep long span cellular beams allow the building services to pass through them maximizing floor to ceiling heights.

The north and south façade structures comprise 67.5m long deep trusses, with horizontal and vertical CHSs and diagonal ties that pick up the 21m secondary beams supporting the north and south office fingers. These trusses are supported by cantilever fabricated box section ‘X frames’ along the east and west façades, which are supported at the eastern and western edges of the two founding zones on four 12m x 14m x 1.3m wide composite steel and concrete thick-plate structures, ‘hour- glassed’ on three edges to distribute the load evenly to the foundations.

The completed structure had to be within tolerance of a pre-determined line and level to allow installation of the cladding and to align internal elements. Elastic deformations, fabrication and erection tolerances and semi-random variables were assessed and pre-set, deformed shapes were defined – a camber for the north and south façades and a trapezium for the four ‘X- frame’ cantilevers.

The huge exposed cross frame structures consist of plated box sections with plate thicknesses up to 100mm engineered to be site bolted with exposed pinned connections with just 2mm clearance holes. The accurate fabrication required was achieved with the use of 3D laser equipment and specialized jigs and machining operations. The welding processes were carefully controlled to minimize distortion and each frame was trial assembled during the fabrication process to ensure the pinned fit-up was achieved.

It would have been impossible to build the cantilever ‘X frames’ without some form of temporary support as any dead load deflection during erection would have prevented fit-up of the large diameter pins. However due to restrictions, it was not possible to provide temporary towers under the ends of the cantilever. Instead a method was developed whereby the bottom boom of the façade trusses to each elevation was supported by two sets of 12 sets of cables and strand jacks tied back diagonally to the completed central core area. As the cantilever structure was progressively erected the geometry had to be continually monitored and the strand jacks adjusted to maintain the theoretical shape. Once the frame was fully erected and all the pins installed, the strand jacks were released to allow the structure to cantilever and the floor slabs were cast.

In order to ensure the final load distribution from the ‘hour glass’ structures to the foundations was within the assumed parameters, a controlled minimum load applied through flat jacks was introduced at the base of one end of each “hour glass” structure. The jacks were then filled with resin and the bases were grouted.

The design and construction team have successfully overcome huge design and logistical challenges which would have been impossible without the use of steel

Judges’ Comment

This City site has a busy rail terminus, an Underground station and extensive archaeology!

The structural design has only four principal locations for foundations and supports. The façade structure is loadbearing, contributing to the support of the upper floors, onto four massive trunnion columns expressed externally.

The structural concept was heroic, the appearance is striking, and the site constraints were remarkable. A huge challenge, outstandingly well answered with steelwork.