Design Awards: 2009

Wimbledon Centre Court Redevelopment London

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Architect

POPULUS

Lead Concept Designer Including Structural Concept Design

BIANCHI MORLEY LTD

Structural Designer

CAPITA SYMONDS

Detail Structural Coordinator

EDGE STRUCTURES

Steelwork Contractor

WATSON STEEL STRUCTURES LTD

Main Contractor

GALLIFORD TRY

Client

ALL ENGLAND LAWN TENNIS CLUB (AELTC)

The new roof, incorporating the retractable roof and air conditioning plant, has been designed to function not only during the Championships when the retractable roof can be deployed in the event of rain, but also throughout the rest of the year as part of a maintenance programme. It is designed to withstand inclement weather conditions including snow and high winds statistically for a 1 in 100 year event – normal design standards are for a 1 in 50 year event. The main limitations on this are that the roof should not be operated or deployed in wind speeds exceeding 55mph.

The fixed part of the new roof has a similar structural form to the old roof in that it consists of four main trusses spanning between four ‘supercolumns’ located at the corners of the stadium, with a series of cantilever trusses supported at the perimeter of the stadium and extending inwards towards the centre of the court. The main difference between the old roof and the new is that the new roof not only has to span further over the extended seating, but also has to support the new retractable roof and the air conditioning plant and ducting housed within.

The retractable roof alone weighs 1,100 tonnes and the air conditioning plant adds another 400 tonnes. This means that the four main trusses spanning between the columns have to be very substantial not only in terms of strength, but also in terms of stiffness to limit deflections when the retractable roof is deployed. The total weight of structural steelwork in the fixed part of the roof is approximately 1,700 tonnes.

With the total all up weight including cladding, plant and the retractable roof, the maximum load transmitted to the two northern columns is just over 1,200 tonnes. However, the ‘supercolumns’ in the south elevation take a lot less load than those in the north since they share the weight of the southern truss with two neighbouring columns on the front elevation which supported the old roof. This means that the two exposed columns on the south elevation take maximum loads in the order of 700 tonnes and are thus designed to be more slender and elegant.

The moving roof is divided into two sections with 4 bays in one section and 5 in the other. Both sections of the roof are to be normally parked to the north. One section is fixed to the northern end of the fixed roof and one will normally be parked to the north but is able to be relocated to the south in preparation for the roof closure, from where it will extend out as required. When the roof is deployed and each section is extended over the court, they actually join in a type of overlapping seam over the centre of the court. For the majority of time, and when the court is not in use, both sections will be parked in a ‘concertina’ (parked fashion) to the north which allows the maximum of sunlight onto the court (as opposed to leaving one section to the south for long periods).

Steel trusses span approximately 77m across the court and rise to a maximum of nearly 6m above the fixed roof eaves line. The trusses are supported at each end by a wheel set (bogies) consisting of four wheels (for stability) which move on tracks connected to the fixed roof. The bogies are driven by electric motors and, when the roof is required to be deployed (after one section has been moved to the south), the trusses unfold the fabric by being moved primarily by the use of these driven bogies but also by synchronised electro-mechanical actuators, which help open and stabilise the hinged ‘arms’ at the ends of the trusses. At the same time as the fabric is being unfolded by the hinged arms there are restraint arms in four lines on top of each truss and fabric bay, and these help to push and keep the trusses apart and, again, these arms are also operated by electro-mechanical actuators.

Between the trusses the structural fabric is stressed by a valley cable to control its movement under fluctuating loads, in particular those from wind. To ensure free drainage, and to provide the necessary curvature in the fabric and valley cable system, the tops of the trusses have been designed in an arch shape whilst the bottom of the truss runs nearly straight. This arch shape (peaking at the centre of the court) assists in sustaining loads as well as maintaining clearance for high ball flight.

Moving the southern section of the roof from its parked position in the north to its position in the south will take approximately 20 minutes. Deploying the roof over the court from this position will take under 10 minutes, the actual time being dependent on detailed considerations including safety.

In its fully parked and fully deployed positions the roof is designed to sustain the maximum design wind and snow loads which are approximately equivalent to two people per square metre on the roof. The roof is designed to be operated (parked or deployed) in wind speeds up to 25m per second, roughly 55 mph.

Judges’ Comment

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Overcoming the challenges of design, programming and logistics over a 3-year period, has ensured minimal disruption to the Championships. The main 1100 tonne retractable roof uses structural steel to its full advantages, with skilful marriage of heavy precision engineering and state-of-the-art technology, to achieve the all-weather operations so long desired.

This project exemplifies quality, integrated team working.

Xstrata Aerial Walkway Kew

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Architect

MARKS BARFIELD ARCHITECTS

Structural Engineer

JANE WERNICK ASSOCIATES LTD

Steelwork Contractor

W S BRITLAND & CO LTD

Main Contractor

W S BRITLAND & CO LTD

Client

ROYAL BOTANIC GARDENS, KEW

The Client wanted a walkway to enable visitors of all ages and physical abilities to experience the tree canopy in an arboretum originally laid out by Capability Brown. The walkway is an example of the complete integration of architectural idea with an elegant and efficient structure. The use of weathering steel was suggested as a material whose colour would blend well with nature, yet would look man-made and would need no future painting.

The walkway is approached via a Rhizotron and then visitors ascend to the tree canopy, via stairs or a lift. It was important that the walkway should not compete with the trees. By using the balustrades as trusses spanning between slender pylons, a simple yet elegant solution was found. The choice of weathering steel was crucial; all the elements are made from pieces of plate welded together.

Each pylon is a cantilever, supported by a group of four piles that resist the overturning moments. The triangular section is an efficient closed section, and looks smaller than an equivalent square. It splits into three tapering branches that support the circular node platform. A larger pylon supports the stair and lift. Halfway round, another larger pylon carries a “classroom” platform.

Reinforced concrete piles were acceptable for the foundation design, provided a survey was done to locate the main radial roots that provide stability, so these would not be damaged.

Pile caps were problematic since they would occupy the top metre of soil, where the fibrous roots occur. It was decided that the piles should be connected to each other and to the pylon base via a customised galvanized steel grillage, composed of stiffened, welded 356mm deep UC sections, installed above the roots close to existing ground level. A steel CHS plunged into the concrete pile and welded to the grillage acts as the principal pile reinforcement. The grillages are separated from the baseplates by neoprene isolators.

The spacing of the intersection of the diagonals with the top and bottom booms follows a Fibonacci sequence, with the closest spacing at the supports. The arrangement looks fairly irregular, has a maximum of three diagonals meeting at any point, and allows each truss half-span to be identical, but handed, to allow standardisation. Vertical “U” frames at each end, and at mid-span, reduce the effective buckling length of the handrail boom. The trusses also provide a safe balustrade, and support the walking surface. The steel mesh surface allows rain to fall through and is non-slip. The galvanized mesh is supported on secondary plates. Fixings are stainless steel, with neoprene strip isolators. The “ring beam” around the node is designed to carry the walkway loads applied at any point.

To reduce fabrication costs, most elements are single plates in standard thicknesses of 20mm, 25mm and 50mm. The node platform “ring beams” are T-sections composed of a 50mm thick flange and a 100mm thick web to take torsion. The pylons and their branches are equilateral sections that taper from 1300mm side at the base to 550mm at the branch point. The stair and classroom platforms, carrying higher loads, are bigger at 1500mm tapering to 700mm and 890mm respectively.

The lift runs between the pair of guide pylons via rails and brackets welded to them. For emergency access or evacuation, the classroom platform incorporates a gate to allow escape via a cherry picker or similar. The standard pylons complete with branches, the node platforms, and the walkways were fabricated off-site and brought to site as single components, to be connected on site via articulated pinned connections. For the stair and lift pylons and the two large platforms site welding was required. Welds between weathering steel elements used matching electrodes for capping passes.

Wind specialists were engaged to assess both the quasi-static wind loads for structural analysis and the dynamic effects. Dynamic studies focused on the accelerations at walkway level from wind effects, and their effects as perceived by visitors. This is a serviceability issue, with acceptability levels dependent on frequency of occurrence. As a result it was recommended that the walkway be closed to the public when wind speeds reach Beaufort force 5. Above force 6 the walkway is closed to all, including site staff. These speeds are expected to be exceeded on average 57 hours and 1.4 hours per year respectively.

It was anticipated from the outset that the structure would have some “liveliness”. Natural frequency analyses gave a first frequency of approximately 1Hz. Synchronous lateral vibration does not occur on the walkway. Its non-symmetrical, closed-loop nature, and its use by visitors who generally walk only short distances before pausing, also mitigate against resonance effects. In practice, small vibrations of the structure as people walk around it and on the stairs can be felt at walkway level as tiny shivers, entirely consistent with the design intent.

The walkway was opened on 24 May 2008 to over 9,000 visitors and continues to be a great success.

Judges’ Comment

s:

This dramatic walkway provides an elevated view of the green canopy by a series of platform nodes and connecting decks on steel “trees”. The weathering steel plated structure is precisely located to avoid the trees and their roots, enabling close proximity without damage.

This brilliantly harmonises with its arboreal setting.

Oxford University Biochemistry Building

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Architect

HAWKINS BROWN

Structural Engineer

PETER BRETT ASSOCIATES

Steelwork Contractor

WILLIAM HARE LTD

Main Contractor

LAING O’ROURKE

Client

OXFORD UNIVERSITY ESTATES DIRECTORATE

The client’s brief called for an iconic building to house a state-of-the-art biochemistry facility, providing functional laboratory space alongside a high quality working environment. A flexible and adaptable building had to include interactive circulation space to promote transfer of ideas, good quality daylight and public art to set the standard for subsequent development.

The new Biochemistry building (phase 1 of 2) for Oxford University is a striking example of contemporary design co-existing with historic buildings. The choice of cladding provides a stunning yet complementary contrast to the Cotswold stone surroundings, giving the required iconic feel.

The laboratories surround a large central atrium which includes secluded study alcoves, footbridges for cross linkage and communal areas ensuring an interactive, functional yet personal atmosphere. The atrium space also contributes to the sustainable strategy for the building by drawing in cool air at basement level and venting warm air at roof level, providing large quantities of natural daylight and including photovoltaic panels on the atrium roof feeding into the energy requirements of the building. A green roof area provides relaxation space for building users and rainwater harvesting for toilet flushing.

A highly serviced building with significant services zones presented a challenge with regard to staying within strict height limits. This was overcome by integrating the services and structural zone using a two level hybrid ‘parallel beam approach’, with structure at each level running alongside the services in both directions.

Future flexibility was paramount due to the phase 2 extension of the building. Studies into the building grid concluded that a 9 x 6.6m grid provided the optimum layout for laboratory modules, economy of components based on an industry standard 600mm grid (cladding, ceiling and floor tiles) and would allow ease of future extension. A slight over-run in the basement footprint was also provided to facilitate the extension of the two level basement.

Particular constraints to construction included a congested site in central Oxford, surrounded by operational buildings and adjacent Grade 1 listed historic buildings. The use of top-down construction for the basement with the steel frame connected at basement slab level provided high speed installation of the permanent steel structure propping the secant walling. CO2 audits of various structural framing options for the building showed that a steel composite frame produced the least CO2 emissions for structural materials as well as the lowest haulage emissions, helping the client and team to decide that steel framing was beneficial on a number of measures.

The building required numerous cantilever areas such as atrium balconies and walkways along perimeter corridors where columns were set back from the elevation.

Ventilation requirements to laboratory areas and a high density of ancillary services also required a structure that facilitated services to an unusual degree. For these reasons, a hybrid ‘parallel beam approach’ was developed, where steel framing was split into two orthogonal levels, each level of steelwork running parallel to the services in that direction.

This system provided the shallowest and most economical floor zone, whilst ensuring that future refit of services is made easier by the continuous dedicated service zones in two directions. Other structural advantages of this system included a grillage of continuous beams which enhanced structural efficiency, provided a natural ability to form the required cantilevers, and a faster and safer erection process due to simplicity of connections including many ‘land on’ arrangements and fewer beams.

Basement construction was a major influence on the design development. Top-down construction of the two level steel-framed basement placed severe constraints on the installation of steel columns within plunge piles. However, the piling and steelwork contractors produced 18m long plunge columns with a typical plan tolerance at the top of the cantilever of +/-10mm.

Future extension of the basement steered the design to integrate a section of steel sheet piled wall into the typical secant piles in order to ease removal of the wall in the next phase, allowing an uninterrupted atrium for the entire length of the future building. Temporary propping of the secant piled retaining wall was performed by the permanent steel beams prior to the slab pour, providing some challenging connection details for the steelwork contractor in areas of high axial loads and changes in level. However, dealing with these basement details, as well as very neat countersunk column splice details to minimise column casing, showed that the steelwork contractor was equal to the task.

Geotechnical modelling of the piled wall movements during and after construction economised on the steel tonnage within the basement, and provision of a drainage layer below the basement slab reduced hydrostatic pressures and slab thickness substantially, helping to produce a more sustainable and economical design.

The steelwork design evolved with the architectural, services and construction requirements, and provided a solution which was proven to reduce embodied CO2 emissions. The Biochemistry building delivers a design which can be regarded as truly integrated, flexible and sustainable.

Judges’ Comment

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A surprisingly airy building with laboratories around an atrium. Internal bridges and generous circulation spaces encourage academic interaction. A steelwork solution was optimal for restricted height and future services flexibility. Steel “plunge” piles and sheet piling support steelwork on split levels, with parallel services runs, behind a colourful external elevation.

A very effective steel solution to a complex and adaptable building.

Cabot Circus Roof Broadmead Bristol

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Architects

CHAPMAN TAYLOR
BENOY

Artist For The Roof Forms

NAYAN KULKARNI

Structural Engineer

SKM ANTHONY HUNTS

Steelwork Contractor

S H STRUCTURES LTD

Main Contractor

SIR ROBERT MCALPINE LTD

Client

THE BRISTOL ALLIANCE

Cabot Circus is a 92,900m2 scheme incorporating free-form vaulted and shell shaped glass and steel roofs with a combined area of 5,850m2. The roofs are designed as a series of 10 interlocking undulating glazed panels, spanning up to 50m and ‘floating’ above the scheme. In total more than 2,800 individually sized 17.52mm thick laminated glass panes are set in elegant steel grillages.

As the building design progressed the proposed roofs became divorced from the intended building elevations. The structural design was complex and involved re-aligning the roof geometry with the building development without compromising the artistic intent, and also tracking the architect’s elevations and achieving support from adjacent structures. Structural optimisation of the central atrium by form finding resulted in geometry that was reasonably spherical. Rationalising the grid to fit a pure geometric surface such as a sphere or torus offered a number of advantages. The grid has a degree of repetition that provided fabrication benefits, particularly in controlling fabrication and erection tolerances. The glazing has a similar repetition to benefit the ordering and future ordering of glazing components. The toroidal grid that was chosen provides an aesthetic, practical and structurally economic solution. The other nine street roofs that form part of the scheme use barrel vaulted geometry.

The atrium roof is carried by five buildings and has the potential for differential movements across its supports. The atrium is also the most slender of the roof shells and has a maximum span of 55m and an average span of 40m.

The buckling capacity of the structure was carefully assessed and the significance of all second-order effects was considered.

The design solution lay in a rigorous study considering imperfect geometric models and full non-linear analysis to determine the critical buckling loads and appropriate design forces. The atrium structure has a minimum buckling factor of 2.95 times the worst case of dead and imposed ULS loading, which is not surprising given a grid of 80mm wide x 120mm deep RHS structural sections that span more than 40m.

Another significant consideration was the support conditions. Each of the roofs is supported by independent structures at each side of the street in the case of the barrel vaults and by five discreet buildings around the perimeter of the atrium. The glazed structures are designed to accommodate movements of the supporting buildings. Elastic supports were determined and then used in the calculation of all subsequent design forces, global capacities and support reactions. Each support is pin jointed using a sliding bearing to avoid transferring differential temperature loads and shearing forces between the buildings and roofs. The centre of each bearing has a spherical component which allows a degree of angular tolerance between the roof canopies and the buildings; this ensures that the bearings cannot bind with differential movements.

The expertise of the steelwork contractor determined the most practical way to fabricate and erect the structure to the necessary tight tolerances. The engineering design required the connections to be welded. The atrium roof was fabricated by cutting the tubular steel with square ends and then welding to precision machined solid nodes. The roofs were divided into transportable ‘ladder’ frames that were two bays wide and approximately nine bays in length. The fabrication of the toroidal geometry was closely controlled through precise jigs constructed off-site by the steelwork contractor. The tolerance of the fabrication and construction of a frame of this form was essential to the success and quality of the roof.

The fabricated frames were craned onto a full birdcage scaffold covering the entire 2,000m2 of the atrium. On completion of the internal grid, the 355mm diameter circular edge frame, incorporating a 450mm wide walk-in gutter profile was positioned and securely supported on the scaffold. The perimeter members were also welded connecting the internal grid and the boundary elements. A number of surveys were carried out throughout the assembly of the roofs to ensure the roof nodes were kept within specified +/- 20mm tolerance for any node position.

Each roof was modelled using cutting-edge 3D software and the lotting and nesting of the materials was carried out using this 3D model which significantly reduced the waste material onsite. Waste was recycled to make jigs for future projects or used to manufacture any new sections of the structure.

On completion of the steelwork, a full survey was carried out before glazing began. The glass is a single glazed system comprising a laminate of twoheat strengthened 8mm panes. Each unit is fabricated as a 1.5m2 (approximately) panel edge supported over a silicone gasket with stainless steel discs clamping the glass at each node against wind uplift. The glass is a low-iron composition exhibiting the highest possible visual transparency.

Cabot Circus was opened to the public in September 2008, as programmed. Since the opening, Cabot Circus has received an ‘Excellent’ BREEAM rating for its lowenergy features including LED lighting to the streets and natural ventilation of the unconditioned streetscape.

Judges’ Comment

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The mixed styles of this retail development are unified by a series of spectacular glazed roofs in various shell and vaulted forms. The structural design is an intelligent re-engineering of the initial concept. By rationalising the geometry and detailing for maximum repetition the fabrication, erection and glazing were all simplified.

Design and execution of very high quality.

Unilever House, London

Unilever House, London

Architect

KOHN PEDERSEN FOX ASSOCIATES

Structural Engineer

ARUP

Steelwork Contractor

WILLIAM HARE LTD

Main Contractors

BOVIS LEND LEASE

Client

UNILEVER PLC

One of the major challenges on the redevelopment of Unilever’s main UK headquarters, which overlooks the Thames at London’s Victoria Embankment, was to retain the grade II listed façade originally built in 1931. Inside this façade the architect, KPF and the structural engineer, Arup, designed a modern 8-storey headquarters and an adjacent 5-storey gatehouse building, with a structural form consisting of a combination of reinforced concrete stability cores and steelwork bracing. This equated to a total of approximately 2000 tonnes of steelwork and decking.

The focal point of the main building is the central atrium structure that boasts four levels of high-quantity suspended structures that are accurately described as the “Flying Carpets” due to the nature of their support system, which is series of spiralling rod supports hanging from the diagrid roof level. These platforms provide a communal meeting place for the people working in the surrounding offices, and are illuminated by spotlights attached to the CHS support columns that also incorporate the necessary electrical wiring.

Looking upward from the ground floor each of the four “Flying Carpets” differs in shape and orientation, and each was pre-fabricated in William Hare’s workshops and transported to site in two pieces to be site welded after alignment. The hanger systems are fabricated from duplex rods more commonly used in the yacht-building industry, with each containing a bulletshaped node end formed from a steel casting.

TDue to the complexity of the hanger system, the design of the rods necessitated precise calculations to determine the tension in each such that the final positioning of the platforms was achieved. Close monitoring of their installation was specified and controlled by Arup to ensure that the design criteria were met.

The “Flying Carpets” are accessed by a series of opaque glass walkways that are formed from box sections and fabricated tees and taper into the adjacent floors. These were also prefabricated in the shop and transported and installed in single lifts. In addition to the usual benefits of steelwork construction, the concept of preassembled units such as the “Flying Carpets” helped to reduce the overall programme duration to achieve effective completion in 25 weeks.

Significant temporary works were necessary to enable the construction of the atrium steelwork, and so to minimise any disruption at ground level an innovative propping platform was first assembled at level 5 from which temporary props and access platforms were sprung. The temporary works included a number of vertical and horizontal jacks with which to accurately position and hold the platforms during the site welding procedure. On completion of all levels, the temporary works were dismantled from the top-down and at each level the loading in the rod hangers was measured and checked against the calculated loadings. The records show that the correlation between the calculated and actual rod loadings was extremely accurate, thus very little adjustment was necessary during the de-propping process.

Other features of the building include a series of lozenge columns, which were formed by the welding of two continuous side plates between two CHS sections. These columns support the highly architectural lift guide arms, which are fabricated from plate and hollow sections. The tolerance requirements for these lift guides required them to be site welded after the erection and plumbing of all support columns.

The fire protection to the new steelwork frame above ground floor is provided by shop applied intumescent paint with a siteapplied topcoat on the visible members mainly within the atrium. Applying the fire protection off site greatly reduced the build time and ensured good quality and a minimum life to first maintenance period of up to 12 years. As is now relatively common in office construction, cellular beams were specified to integrate the services within the structural floor depth. The latest Fabsec software, Fbeam, was used to optimise the beam mass with the most efficient paint thickness, and this was one of the many value-engineering exercises incorporated by the design team.

The building is an excellent example of sustainable project delivery that surpasses BCO specification. The project achieved a BREEAM “excellent” rating and carbon emissions were reduced by 25%, which is far greater than regulations currently require. From the start the project team was tasked with delivering the most sustainable project possible and the targets set were more than achieved. Detailed lists of all elements of the existing building identified what could be re-used or recycled.

The existing structure was modelled using StruCad 3D software, which proved invaluable to cope with the constant updates as more and more site survey data became available.

Judges’ Comment

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Steel and glass have been used thoughtfully and skilfully in this fine refurbishment of a London landmark. The new open-plan office environment is stunning, behind the magnificent retained façade. The suspended “flying carpet” circulation areas float dramatically in the soaring atrium.

The dramatic effects result from complex engineering in the steel suspension and retention structures. A wonderful example of what can be achieved in a landmark restoration.

Castleford Footbridge

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Architect

MCDOWELL + BENEDETTI

Concept Designer

ALAN BAXTER & ASSOCIATES LLP

Structural Engineer

TONY GEE AND PARTNERS LLP

Steelwork Contractor

ROWECORD ENGINEERING LTD

Main Contractor

COSTAIN LTD

Client

WAKEFIELD METROPOLITAN BOROUGH COUNCIL

The £4.8 million bridge has been funded by Wakefield Council, Yorkshire Forward and English Partnerships. It creates a safe new pedestrian route uniting the north and south of Castleford’s riverside community connecting Aire Street to Mill Lane.

Building the bridge’s three piers in the deep and often fast flowing river was one of the main challenges. A cluster of piles beneath each pier was initially considered, but constructing a cofferdam would have been tricky – not least because the ground conditions consisted of cracked and weathered mudstone. Instead, the main contractor decided to support the piers on single, cased piles measuring 1800mm in diameter. This immediately reduced the construction footprint and minimised the duration of the piling. The pile casings provided a mini cofferdam within which preparation works for installing a precast pile cap could be carried out safely in the dry.

With the piles in place, a prefabricated box known as a ‘limpet’ was clamped onto the pile cap before installation. Once the pile cap and limpet were in position on top of the pile, the area inside the limpet could be pumped dry to create a watertight working area for the connection of the pier bases below water level. The pile cap, pier base and pier are held down by four high tensile bars, positioned 4m deep within the pile.

All three foundations were completed in time for the arrival of the steel legs, despite the site team having to contend with two major floods which swelled the river to record levels and swamped the site for over three weeks, making work in the river impossible. Of primary concern was keeping hold of the crane mounted jack-up barge due to the velocity of the river. The jack-up barge was a vital aid to all lifting and piling operations during the floods.

The 131m span is made up of seven spans, four 26m main spans and three 9m spans over the support piers. A twin steel box girder forms the spine of the structure. The girder varies in depth around the outer curve in order to counter the span differential between the boxes. This variation in depth rises out of the deck to provide a bench for those using the bridge. The top flanges of these double height girders curve in two directions providing a natural cross fall to the flange from the abrication process. The curved geometry of the bridge presented great challenges with respect to setting out and providing precamber information. Circular hollow sections, each topped by a tapering section terminated in a solid steel machined forkhead, support the box girder deck.

The curved nature of the bridge presented a complex articulation to overcome as the support legs are inclined in two planes and the line of the deck varies over the connection. This connection challenge was overcome by utilising a complex arrangement of spherical bearings and bimetallic isolation. The development of a connection with this level of multi-axial flexibility facilitated a swift installation of the deck.

Cantilever brackets at approximately 1m centres along the bridge provide the bridge width. The cantilevers support the complex timber arrangement and the handrail posts. The hardwood decking runs along the line of the bridge and is supported at regular intervals by timber bearers that sit within the cantilever brackets. The boards are fixed to the bearers with a bespoke hidden fixing that locks into a groove in the lower half of the board.

The connections and support to the timber decking are consistent throughout the bridge in the main deck area and the bench. The risk of bimetallic corrosion had to be considered in many connections. The bridge is a combination of S355 J2 steel and grade 1.4401 stainless steel – with the majority of the latter being used in the architectural features. Isolation could be achieved in most locations with nylon pads and top-hat washers. However, the pin arrangement required a harder wearing compound to carry the 100 tonnes design load and isolate the duplex stainless steel pin from the mild steel machined head which was achieved by inserting a fibre-wound bushing into the connection.

The steel v-shaped piers are each prestressed through a pre-cast concrete pile cap and onto a 1.8m diameter concrete monopile that is socketed into the mudstone at each pier. This prestressing process was carried out inside a temporary steel limpet structure bolted to the concrete pile cap to provide a dry operation which was critical to establishing the geometry of the bridge piers.

The narrow streets beside the river were filled by some of the biggest mobile cranes in the country – the clearance between the houses being so tight that the cranes’ wing mirrors had to be removed. But even this tested the cranes to their limits to lift the carefully assembled segments of bridge into place.

The result is a truly breathtaking bridge.

Judges’ Comment

s:

A robust and exciting link between parts of Castleford, across the River Aire. High quality engineering design and construction techniques have led to an outstanding result, which in turn has transformed peoples’ regard for themselves and their town.

A triumphal demonstration of infrastructure improving the quality of life.

New Academic Building London School Of Economics & Political Scene

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Architect

GRIMSHAW

Structural Engineer

ALAN BAXTER & ASSOCIATES

Steelwork Contractor

BOURNE STEEL LTD

Main Contractor

OSBORNE

Client

LONDON SCHOOL OF ECONOMICS & POLITICAL SCIENCE

The LSE’s New Academic Building is an extensive remodelling of an Edwardian government office block. A range of strategies was employed to help achieve the aim of an ‘Excellent’ BREEAM rating, with over half the building’s original fabric being reused.

The atrium is the central hub and contains the main circulation cores. Its timber floor folds down to provide access to the lower ground floor before sweeping three storeys upwards to the roof, framing the two cantilevered mezzanine gallery levels.

There are eight storeys above ground and two below. Teaching and larger social spaces are centred round the atrium circulation toward the lower levels of the building. Departmental space is arranged in a U-shape around the central lightwell on the upper levels.

The most dramatic feature of the new structure is the suspended floor system. This was key to providing extensive column free spaces and opening up the core of the building. All major teaching and social spaces are located on the lower floors providing a clear division between teaching and departmental space. To achieve this, all floors above ground level are suspended from continuous steel hangers connected to an 18m long storey-deep truss which forms the backdrop to the rooftop pavilion. Separate hangers suspend the second and first floors which are stepped back from the atrium to allow more daylight to enter. To support the 15 tonne truss two 600mm diameter CHS columns were erected on new piled foundations. These extend through the atrium and straddle the lecture theatre beneath.

The 10-storey high columns were installed in three sections and restrained at each floor level from third upwards via an edge beam. Primary structure and condeck were installed to provide work decks on levels 1-3. By not pouring the slabs to these floors their weight was kept down so the level 1 and 2 hangers could be used as temporary props. The roof truss was installed to the top of the columns using a 700 tonne crane. The exposed hangers to levels 3-8, each made up of two 100mm x 50mm solid steel bars, were also craned into site in 16m lengths. The floor plates were then progressively completed up the building.

Another 18m long 6 tonne truss was installed at ground level to provide support for eight 18m long sculpted pre-cast concrete ribs. These ribs form the lecture theatre roof and express the curvature of the atrium floor above. The installation of these major prefabricated elements presented one of the biggest challenges of the project. The operation’s scale required the work to be scheduled over the weekend, with road closures arranged in advance.

The project benefitted from the use of Corus’ Bi-Steel core system for two of the lift shafts. These were completed twice as quickly as an equivalent concrete jump-formed version providing major programme benefits.

These structural interventions have provided the LSE with a 400 seat lecture theatre and an expansive atrium at its centre. The atrium is capped by a glazed roof with delicate bowstring trusses spanning 13m, allowing maximum light to enter.

This impressive 12,700m2 building meets all the client’s aspirations and was completed to programme and within budget.

Judges’ Comment

s:

Steel and glass skilfully combined to transform an Edwardian office block into light, airy accommodation. To provide column-free spaces in the old cellular offices, the elegant solution is to suspend the new floor system from a deep roof-level truss.

An excellent example of updating an existing building.

201 Bishopsgate And The Broadgate Tower London

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Architect

SKIDMORE OWINGS & MERRILL

Structural Engineer

SKIDMORE OWINGS & MERRILL

Steelwork Contractor

WILLIAM HARE LTD

Main Contractor

BOVIS LEND LEASE LTD

Client

BRITISH LAND PLC

The development consists of a 12-storey office building known as 201 Bishopsgate and the 35-storey Broadgate Tower, which reaches a height of 165m above ground level. Between the two buildings is a Galleria area complete with a glazed roof which is supported as a cantilevered structure from the Broadgate Tower structure above level 5.

The development, which is located directly above the railway route at Liverpool Street station, was only structurally possible after significant enabling works were completed to create a raft structure over the live railway. Due to design changes the original 6,000 tonne raft structure needed extensive modification from the design constructed in the mid 1990’s, and resulted in an additional 1,000 tonnes of steelwork being incorporated.

In order to limit deflections and settlement it was necessary to distribute the resulting vertical forces from the two buildings evenly across the raft support structure, and thus the option of a heavy centralised RC core was not a viable structural solution for either building. Hence, the vertical load-path for the tower includes a series of raking A-frame legs that effectively prop the structure at level 5 and transfer the loads to strategically located transfer beams and columns within the raft structure.

The specification of 5-storey high raking struts provided a challenge during the construction of the frame, and an extensive arrangement of temporary props and trestles was required in order to build the structure up to level 5 before installation of the A-frame legs could begin. This included a sophisticated load transfer operation using a jacking system to transfer the loads from the temporary supports onto the permanent main raking columns. These raking columns and their associated bracing members were delivered as individual components of up to 26m in length.

The structural form of the Broadgate Tower is an elongated rhombus shape and the lateral stability is provided by a perimeter diagrid bracing system to all four sides of the structure located just inside the external cladding. The diagrid nodes occur at 6-storey increments, with plan bracing and RC diaphragm floor plates distributing the horizontal loadings into the external bracing lines.

Intermediate floor stability is provided by a secondary internal bracing system that spans between each 6-storey module.

The main service cores of the structure are constructed in steel and house the lift voids, staircases and service risers. The floor plates are RC on steel decking supported by long span cellular beams in the E-W direction.

In order to simplify the transport, handling and construction of the main perimeter members, the structure was constructed in 3- storey increments with the central welded splice being carried out on site to form the continuous 6-storey elements.

Wherever possible the main diagrid column and bracing connections were designed as a bolted joint but in certain locations, due to the resulting forces, it was necessary to site weld the joints. This detail required a robust temporary connection to be installed during erection in order that construction could continue above the splice location, and this methodology assisted in allowing control of the tower verticality during construction.

The building façade is designed to mimic the structural form so that the diagonal shape of the perimeter bracing system is visible from the outside. This gives the structure a unique feature that differentiates it from other buildings in and around the development.

Judges’ Comment

s:

These landmark offices are prominent and prestigious. The steel-framed Tower is sited above the Liverpool Street railway, above a bridging raft which had been constructed for an earlier, smaller scheme. The East façade has therefore been carried on large raking steel shores across to the raft abutment.

An heroic solution creating interesting spaces as a bonus.

Cabot Circus Footbridge Broadmead bristol

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Architect

WILKINSON EYRE

Concept Designer

WATERMAN CIVILS

Structural Engineer

DAVID DEXTER ASSOCIATES

Steelwork Contractor

S H STRUCTURES LTD

Main Contractor

SIR ROBERT MCALPINE LTD

Client

THE BRISTOL ALLIANCE

Cabot Circus Footbridge fulfils a utilitarian brief to provide the seamless linking of ‘satellite’ parking with new retail development. The bridge was planned as a continuation of a long sweeping pedestrian boulevard that cuts longitudinally through the new 2,500 space car park. Its long curving form is clearly related to the curving plan of the car park and can be read as having been ‘extruded’ out of the void created within the building.

Externally the bridge is viewed almost exclusively from Bond Street South below, and generally from the confines of vehicles passing beneath. The so-called ‘third’ elevation of a bridge – its underside – takes on particular significance in this context, and the design of the wide bridge soffit is critical in defining the visual signature of the structure. The careful use of concealed deck splices along the deck also provides the required continuity on plan.

The deck structure comprises a closed steel torsion box which is essentially triangular in section and provides a smooth soffit plane. The section varies along the bridge length such that the soffit seam meanders from side to side along the structure providing a fluid threedimensional form. Deck splices were bolted with TCB bolts which were accessed through hatches in the deck.

The deck is supported at mid span and at each end on tapered cantilevering raking steel columns, orientated in alternating directions such that the deck section at each support is identical but handed. These support arms from the main car park columns also house an arrangement of pot bearing, to accommodate differential movements on plan, while providing local vertical support to the bridge.

Internally, the changing deck geometry exerts an influence on the enclosure, defining the internal space of the bridge. The side plates of the deck are variable in inclination, and this variance is translated through to the steel portals, which incrementally ‘rotate’ relative to each other along the length of the bridge. These portal frames support wall and roof panels, which form warped planes of glass to provide a dynamic spatial experience for the bridge user.

Since the external and internal geometries of the bridge are visually complex the detailing of all elements is minimal to ensure a legible and ‘clean’ aesthetic. Services are subtly integrated, with lighting strips set flush into the portals internally and placed at varying heights to further accentuate the warping geometry.

The steel frames are portalised laterally to provide the principal axis of stability while, out of plane, the frames work compositely to antilever from the main deck. This stability system, with the aid of variable stiffness sealants, can accommodate the vertical and lateral deflections of the bridge, while maintaining stability of the glass panes required in an overhead condition.

The limited access and restricted number of road closures available during the construction phase meant that employing a structural steel solution enabled the bridge to be fabricated in large sections, which were trial assembled for fit and alignment before being delivered to site for installation.

Judges’ Comment

s:

The bridge provides a striking pedestrian approach to this major centre. The sweeping form of the box deck, sculpturally curved and twisted, supports the glass enclosure. The careful detailing, masking of the deck splices, the integration of the services and the high quality fabrication, all ensure an impressively clean appearance.

High standard of work has ensured an equally high quality result.

Ryanair Maintenance And Training Hangar Stansted

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Architect

CAPITA ARCHITECTURE

Structural Engineer

BARRETT STEEL BUILDINGS LTD

Steelwork Contractor

BARRETT STEEL BUILDINGS LTD

Main Contractor

KIER EASTERN

Client

EAST MIDLANDS TRAINING (RYANAIR)

Not far from the main runway and within sight of the passenger terminal at Stansted Airport, a new hangar capable accommodating up to five Boeing 737s is up and running. A series of engineering and logistical challenges had to be overcome to lift four 125m long steel trusses on a design & build contract for the cavernous interior of the new maintenance hangar at the airport.

The design was progressed around the need to be able to lift, on a specific day, the major roof girders. The box girder had to be lifted and made secure in one day. It was recognised on day one that if the girders were erected out of tolerance it would be impossible to bring them back to true.

The design solution was to provide a 10m deep x 7.2m wide x 125m long box girder, pre-cambered for self weight and dead deflections. This would form the spine to the structure. Two further 125m trusses could then be erected in three 41.5m sections, and braced back to the spine box girder. The stiff spine would force the vertical trusses into position. Final adjustment of the door guides would take place after roof and wall cladding was complete. To do this a system of vertical and horizontal adjustment to the door guide support brackets was devised, which could be locked off once before handing over.

The erection method statement called for the girders to be transported “piece small” and built up on the ground in temporary stillages. The main contractor invested in the ground providing a high quality level erection platform capable of carrying the stillages with minimal settlement. The stillages were staged to allow a natural positive pre-camber to be built in. The completed box girder was checked and then locked by the application of TCB High Strength Friction Grip bolts.

On 16 April 2008 the box girder was hoisted into place using two 300t all-terrain mobile cranes. The girder was lifted between two pairs of 25m high lattice columns positioned 125m apart. The physical lift took 30 minutes, but all day to complete, from early morning briefings and safety checks to late in the evening tying back and making fast. Two further 50 tonne plain trusses were lifted end of April and braced back to the rigid box girder. The three 41.5m sections were then lifted individually by one of three 100t capacity mobile cranes and connected together and to the lattice columns while suspended in the air. Whilst the three sections of the truss were held at the right level and location, a fourth 25t capacity mobile crane traversed between the truss and box girder erecting tying in steelwork.

Only when the doors were installed and successfully commissioned did the project team celebrate a design well executed.

Judges’ Comment

s:

A fine example of an optimised long-span structure, at minimum cost with fast erection. The design & build criteria were limited to the volume contained, loadings and time of erection. Therefore the structural details were simplified to achieve economy, ease of transport and predictability of erection.

A text-book example of this direct approach.