Design Awards: 2004: Award

Selfridges Structural Frame, The Bullring, Birmingham

Selfridges Structural Frame, The Bullring, Birmingham

Architect

Future Systems

Structural Engineer

Arup

Steelwork Contractor

Severfield-Reeve Structures Ltd

Main Contractor

Sir Robert McAlpine Ltd

Client

The Birmingham Alliance

The structural frame of the Selfridges Building in Birmingham is like most other multi-storey building frames in many respects. It is designed to resist vertical and lateral loading (which in this case included reactions from the attached cable-stayed bridge) and to ensure acceptable dynamic performance and movements. However, there are two specific design criteria which differentiate this building from most conventional frames and which lead to an unusual and ambitious design. The first of these was the need to define the curved building shape and support the freeform sprayed-concrete facade system. Secondly was the desire to create retail floor-plates with minimum vertical structure and of maximum height. To achieve these goals the design takes advantage of CAD/CAM technology and mass customisation to allow the economic fabrication of an irregular framework. It also achieves a high degree of integration with services feeding the retail floors to maximise floor to ceiling heights. Neither of these strategies are ground-breaking in isolation, but in combination they create a truly holistic solution, an economic synergy of architecture and building engineering which could not have been achieved with a more conventional solution.

The starting point for the frame design was to derive a suitable column layout. The super-position of a standard cartesian column grid on the irregular plan shapes of the building seemed inappropriate and incompatible with the architectural layout. The chosen approach was to locate a string of columns around the building perimeter spaced approximately 12m apart and a separate necklace around the two internal atria at the same spacing. A handful of extra columns were required to limit primary and secondary beam spans to 12m and 16m respectively, the maximum spans that were considered as economically feasible. Again these additional columns were individually and strategically placed to suit both the structural and architectural requirements.

The plan shape of the building changes from floor to floor to match the curvature of the envelope in section. This requires secondary beams to cantilever from the perimeter column line by different distances around the slab edge and at each level. At the ‘waist’ of the building the columns sit tight against the inside face of the facade, where as at the ‘hips’ and ‘shoulders’ the floor cantilevers up to 4.5m deemed as a maximum practical limit, thus controlling the vertical curvature of the building. It was these relatively long spans and lack of regular grid resulting from this minimum column approach that drove the design towards a steel solution.

A desire for maximum floor to ceiling heights in retail areas lead the integration of structure and building services within the same 1500mm deep zone. This coordination exercise required a balance of practicality and flexibility, allowing the potential for future rearrangement and refitting of retail departments. The chosen strategy provides fixed routes for primary ductwork through standard notches at beam ends, with secondary ducts and pipe-work running through 650mm diameter holes in beam webs. These holes are not located specifically for the current services arrangement (indeed this arrangement was unknown until after the completion of the frame erection), rather they are designed to ensure that a reasonable level of variation in layout is possible. This standardisation of notch and hole sizes/spacing simplified the fabrication requirements and allowed a certain amount of repetition despite the large number of different beam lengths. The coordinated structure/services strategy also steered the structural design towards a deep but light beam solution with good stiffness characteristics and hence good dynamic performance. Asymmetric plate girders of a standard depth were chosen for the majority of beam sections working compositely with the 150mm deep concrete floor slab. The use of plate girders allowed greater control over the distribution of material than a solution using equivalent depth rolled sections, resulting in much lighter beams and less fabrication waste.

It became clear during design that the secondary floor beams made up over two-thirds of the total frame tonnage, and that small improvements in the design of these beam types would yield significant overall savings of weight and cost. The optimisation/rationalisation process was complicated by the large number of different beam lengths and support conditions resulting in a vast matrix of different demands. The resulting designs and number of different beam types are a balance of performance and practicality.

The choice of corrosion and particularly fire protection systems also formed an important part of the frame design and the potential for exposing the floor structure and services as a ‘technical ceiling’ was recognised early in the design process. A fire engineering study was carried out which resulted in a reduction of the fire resistance requirement of floor structure from two hours to 60 minutes, thus allowing the economic use of a site applied intumescent paint system. The result is a clean soffit appearance that has been left exposed in several areas of the current fit-out.

Traditionally the emphasis in multi-storey steel framed construction has been on regularity and repetition to create function and economy. In recent years the possibility for accurate fabrication of highly irregular frames has been demonstrated and such frames have been constructed, typically for the sole purpose of creating unusual architectural form. The Selfridges building is proof that these desires need not be mutually exclusive. As such Selfridges should not be considered as a ‘one-off’, and this ability to marry the requirements of function, economy and form helps to unleash the potential for a new generation of steel framed buildings.

Judges’ Comment

s:

A big retail name for Birmingham’s city centre, this is an architecturally challenging and exciting building, but with a common sense approach to its structural framing strategy. The rationalisation of steel sizes in the highly irregular multi-storey frame provides a sense of balance between the required flexibility of function and the practicalities demanded for construction

The Swansea Sail Bridge, River Tawe, Swansea

The Swansea Sail Bridge, River Tawe, Swansea

Architect

Wilkinson Eyre Architects

Structural Engineer

Flint and Neill Partnership

Steelwork Contractor

Rowecord Engineering Ltd

Main Contractor

Balfour Beatty Civil Engineering Ltd

Client

Welsh Development Agency

This project is part of a larger scheme comprising two bridges across the River Tawe with an additional opening span and associated Quayside walkways. The bridges connect the city centre with a publicly funded redevelopment project currently under construction to the east of the city on the site of the Swansea Docks. The requirement for units in the redevelopment to be pre-let at an early date, with necessary infrastructure visibly in place, resulted in an unusually compressed programme – from design inception through to constructed completion in less than 15 months.

The 140m north bridge, named the “Sail Bridge” by the WDA, is an iconic design explicitly required by the Client and City Council to form an emblem for the regeneration of the Port of Swansea. Though the structure of the North Bridge adopts a classic symmetric cable-stayed configuration, in cross section the deck is held along only one edge. The simplicity of the overall form is augmented by the apparent delicacy of the asymmetrically suspended walkway.

The 42m high mast, of varying cross section, is fabricated from a series of flat and rolled steel plates of decreasing thicknesses from base to tip. The cross section migrates from a filleted square at the base through to a kite shape at mid height, culminating in a triangular configuration at the tip. This developing form is achieved without the use of warped planes [all faces are ‘flat’] and yet the final form is visually complex. The plate thicknesses in the mast vary from a maximum of 45mm at the base to 10mm at the tip. The final craneage weight of the mast was 78 tonnes from an initial material procurement tonnage of 93 tonnes, and the mast was lifted in one piece using a 1200 ton crane on the west bank.

The project was procured using the NEC Target Cost Form with the specialist steelwork fabricator, Rowecord Engineering Ltd, appointed early on in the process. Very close co-operation between the design team and the specialist sub-contractor meant the highly modelled nature of the mast and the simplicity of the overall structural solution was retained through the design development to deliver an end product that is incredibly faithful to the original scheme design.

Furthermore, in terms of detailing, Rowecord were able to advise fabrication methodologies that surpassed the design team’s expectations in terms of formal clarity and final surface finish. One good example of this is the sharp arris on the back of the mast from mid height to apex. Because of the extremely acute angle between the side plates, the design team had envisaged a shadow gap detail at the junction between the plates to lessen the visual impact of the welding. Rowecord suggested an alternative detail where a kite shaped bead of solid steel, running the full height of the intersection, formed the junction between the two plates. This allowed for a sharp edge as plates met and greatly simplified the welding details.

Steel is also used for the deck box sections and the cantilever ribs supporting the pedestrian walkway. The deck is suspended on one side by 70mm diameter spiral strand stay cables connected to the mast. As a result of the eccentric cable support, the deck is a closed steel box in order to provide the necessary torsional stiffness. The 20mm deck plate is stiffened longitudinally, but the 15mm webs and 20-30mm thick bottom flange are unstiffened to simplify fabrication and box assembly. The stay anchorages are formed from simple intersecting tubes designed to facilitate proper alignment because of the complex 3-dimensional geometry. The whole bridge was modelled by Rowecord as a 3-dimensional plate model to ensure correct geometry and control of tolerances, with the result that there were very few problems in the alignment and assembly of the units. Following the installation of the mast, the deck was lifted into position in nine individual lifts of approximately 20 tonnes each. This allowed for off site fabrication of individual deck units, transportation by road and rapid erection onto temporary piled supports in the river using craneage from both banks. This method enabled the correct geometry to be achieved prior to welding up the splices and stressing the stays to lift the deck off its temporary supports.

The bridge parapets differ between the vertical parapet to the cycleway and the inclined tension wire downstream parapet. This counterpoint between sides of the curved deck is accentuated by the more visually solid parapet infill panels and bespoke lighting units on the upstream side, which provide effective functional lighting for bridge users as well as subtle coloured lighting elements to viewers further upstream. The bridge mast is uplit to enhance the night-time visual impact.

The bridge was opened to the public for the inaugural ‘Great West Wales Triathlon’ in June 2003, and has already received warm public acclaim, becoming a popular icon for the region and destination in its own right as well as an essential link between the new Port Tawe Innovation Village and the City Centre.

Judges’ Comment

s:

This bridge, providing an essential link in the regeneration of Swansea, is a culmination of collaborative expertise and teamwork. Its architectural form, design and fabrication have all paid great attention to detail, to give the people of Swansea an exciting, simplistic, but visually complex structure to use and look at.

More London Plot 1, Tower Bridge, London

More London Plot 1, Tower Bridge, London

Architect

Foster and Partners

Structural Engineer

Arup

Steelwork Contractor

Severfield-Reeve Structures Ltd

Main Contractor

McAlpine-Mace JV

Client

More London Development Ltd

More London Plot 1 is an £85M office development on the south bank of the River Thames in London adjacent to HMS Belfast, to the west of Tower Bridge. It consists of three 10-storey buildings with car parking, loading bays and some lettable space in a basement covering the entire site. Plantrooms are located on the roof, set back from the building perimeter and enclosed by a louvred screen. Two of the buildings, Plot 1A, are linked by a full height atrium with bridge links at every level. The third building, Plot 1B, is completely separate.

The superstructure consists of concrete/steel metal decking floors supported by a steel frame consisting of Fabsec beams spanning up to 22.5m and UC columns. Lateral stability for each building of Plot 1A is provided by a concrete box core located about mid-way along the length. Lateral stability of Plot 1B is provided by two concrete cores. The atrium structure consists of a glazed wall facing the river views and a glazed roof supported by a steel hollow section structure.

The scheme for Plot 1A was originally based on an 11.5m grid across the building with a central column on grid. During development of the scheme design the client asked what the consequences would be of eliminating the columns within the office floor plate, thus creating a 22.5m span, whilst maintaining the same overall floor depth. The benefits were added flexibility in the planning of the internal space, which would give the building an edge in the office letting market. The additional cost due to the increase in steel weight was evaluated and accepted by the client and the building was pre-let. This was crucial to the viability of the project in an over-supplied market.

Although the revised design met the brief upper limits for pedestrian-induced floor vibrations (Response Factor (RF) = 8) the client was advised that there was recent evidence of increased office worker sensitivity to floor vibrations. The introduction of flat computer screens mounted on arms projecting from office furniture had resulted in complaints from users who were experiencing difficulties reading text displayed on their screens. The problem was attributed to floor vibrations. In cases investigated by Arup the level of floor vibration was around RF=8, which is the generally accepted upper limit for normal offices.

It was decided to investigate the possibility of a cost-effective method of significantly reducing floor vibrations by providing additional damping. The idea of using a bituminous damping layer sandwiched between steel and concrete had been developed and used successfully on the long span spiral ramp structure in the atrium of the neighbouring City Hall building completed in 2002. In the case of City Hall the concrete was a non-structural topping covering the entire ramp and the damping layer similarly occupied the entire width. For More London Plot 1A the width of the damping layer is restricted to the width of the steel beams. A thin layer of bituminous material is sandwiched between two metal plates and placed between the top flange of the steel beam and the metal decking towards the ends of the beam. Composite action is maintained by providing shear studs over the central portion where the damping layer is absent. To test the effectiveness of the system, computer models of the concrete slab, shear studs, steel beam and damping layer, characterised by a ‘loss factor’, were analysed using the advanced finite element package NASTRAN and subject to transient dynamic loading. The results obtained were compared to similar analyses where the damping layer was omitted. The results showed that significant improvements in damping were feasible. Using this system a 22.5m span floor can achieve RF=6 or less without the need to increase depth or mass beyond that required for strength design. The idea was presented to the client and received favourably.

The solution adopted for the long spans is a 720mm deep Fabsec beam with 400mm diameter web holes, supporting a 130mm deep concrete floor slab. The 850 deep structure occupies the entire depth between the raised floor and false ceiling with the air-conditioning ductwork passing through the web holes.

Floor vibrations were assessed by calculating the vibration modes of the floor structure using a computer model consisting of steel beams offset vertically from a finite element model of the concrete floor. Response was then assessed by calculating the walking pace that would cause resonance for a particular vibration mode and then combining the response of all the vibration modes that could be excited by a person walking at this pace. Typically the lower frequency modes, around 3Hz, are excited by the first harmonic of the walking input, whilst the higher harmonics excite higher frequency modes.

Time was short since construction of the foundations and basement had commenced. A potential manufacturer of the product, Richard Lees Steel Decking (RLSD), was approached and registered enthusiastic interest. Two test panels, each consisting of two 12m long simply supported steel edge beams supporting a 3m width of concrete slab on metal decking were constructed by RLSD; one with the damping layer and one without. Vibration testing, carried out by Arup, consisted of attaching an accelerometer to the middle of the beams and measuring the natural frequency and damping due to a heel-drop at midspan, and the acceleration response due to a person walking along the middle of the slab, from end to end, parallel to the beams, at a walking pace chosen to excite the lowest natural frequency of the beams. The walker carried a hand-held metronome set to the desired pace. The results confirmed the analytical findings.

Following the testing and analysis of the results the decision was made to incorporate the damping system into the Plot 1A buildings. Safe erection methods for the decking in the absence of studs and the damping layer sandwich were developed with the decking supplier and main contractor and incorporated into the erection method statement.

Post-construction, further testing was carried out by attaching an accelerometer to the middle of the critical areas of the floor and measuring the real natural frequency and damping due to a heel-drop at the same location.The building is now fully occupied and favourable comments on the feel of the floors have been received. The cost of significantly reducing floor vibrations was approximately £2/m2 of floor area. The damping layer is now an RLSD product called “Resotec”.

Judges’ Comment

s:

As can be expected with this team, the development pushes the design process to the limit in meeting the client’s brief for large open office space that is essentially column-free and environmentally efficient and controlled, whilst taking full advantage of the superb views of its riverside location.It has achieved a simple, clear design that is logical in its master-planning, sophisticated in its detailing and uncompromising in its execution. The result is a classic of its type.