BAM Construct takes on Hadid’s tunnel vision

When BAM Construct took on Zaha Hadid Architects’ Softbridge at Oxford University there was heated talk of ‘destroying the vision’ and architects on site with tape measures. Stephen Cousins reports. Photographs by Peter Morgan.

Dame Zaha Hadid has consistently rewritten the rules of modern architecture and pushed contractors to come up with ever more ingenious structural solutions.

Her latest project, an £11m extension to the Middle East Centre at St Anthony’s College, Oxford University, is one of her most challenging to date. It is designed to resemble a polished steel “tunnel” and, floating just metres above the ground, will span two existing historic buildings.

Currently around half way through construction, the building will provide the college with an extra 1,200 sq m of floorspace for a new library, reading room and archive on the first floor, a spacious reception area on the ground floor, and a 125-seat auditorium in the basement.

The building, also known as the Softbridge, may be modest in size, but its tight location and extreme geometry have tested main contractor BAM Construct’s ability to develop a structural solution that would also match Hadid’s vision.

Under a tricky value engineering process, the BAM attempted to slash £4.5m from the original cost estimate and simultaneously accommodate the architect’s bold forms and high-spec palette of materials. The stainless steel walls of the tunnel at first floor level will curve in three dimensions and appear almost seamless with no visible joins, requiring an unusually precise pre-fabricated solution from Italian cladding specialist Frener & Reifer.

Meanwhile, proximity to the historic buildings it connects to – a grade II-listed Victorian building at 68 Woodbridge Road to the north, and an Edwardian building at number 66 to the south – meant sheet piles were driven just centimetres away from their delicate foundations.

The site’s location near historic buildings demanded extreme care

Working with Zaha Hadid Architects has been a tough but exhilarating experience, explains Anthony Nagle, construction manager at BAM Construct: “Managing the architects’ expectations during the value engineering process was a challenge and I was in various meetings where I was accused of destroying their vision, but my approach was always to say ‘you can have your vision on a piece of paper, or you can have a significant part of your vision standing as a real building in Oxford, you’re not going to get both within the budget’. In the end we arrived at a solution everyone was happy with.”

Rising student numbers

Founded in 1957, the Middle East Centre is the University of Oxford’s facility for research and teaching on the Arab world, Iran, Israel and Turkey. Since 1978, the centre has been at number 68 Woodbridge Road, a former church refectory built in 1887, but rising student numbers created the need for a new facility with state-of-the-art resources to preserve and manage the collections.

Construction began in January 2013 and on completion in summer 2014, the new building will become known more formally as The Investcorp Building, after the international alternative asset management firm that funded its construction.

Under a two-stage tender process BAM won the design & build contract, which includes a novation agreement allowing the architect, structural engineer AKT II, and services engineer Max Fordham, to help carry the design through to completion.

Structurally, the Softbridge will comprise a cast in-situ concrete frame, the first floor slab of which will cantilever out, partially supported on a large V-shaped column on the west side to give the impression that the stainless steel tunnel above is floating. The tunnel itself will be formed around a ribcage-like structure of 34 thick curved glulam beams positioned on top of the first floor slab and clad in the reflective stainless steel.

The tunnel fans outwards and upwards towards the south, creating a double height window and space for a second floor office. The 7m deep concrete basement level, which runs east to west across the site, accounts for about half of the building’s overall volume, but will remain invisible from ground level, covered by a layer of grass.

A focal feature of the scheme is a preserved sequoia tree, which the college considered so important that the Softbridge has been designed to wrap partially around it. The redwood tree, normally native to California, is one of a pair in the college grounds and, given its proximity to construction works, BAM had to take extreme precautions to protect it.

 “The preservation order meant hand-digging the area around the tree using air spades to prevent damage to the roots, special piling had to be installed to prevent any chemicals leeching from the concrete into the surrounding soil, and a 200mm temporary concrete slab was positioned over the roots to reduce loads from machinery,” explains Nagle.

A sensitive approach was also required when excavating the basement, where sheet piles had to be driven less than 100mm away from the foundations of the historic buildings. To prevent vibration and enable piling to take place directly next to the buildings, a zero sheet piling machine was used, which, like a caterpillar, ran across the tops of piles, using previously driven piles as an anchor from which to lever in the next pile.

BAM has had to take great care in working near a protected redwood tree. The finished building will arch around it

Around 70 sensors have been attached to the buildings to record any movement, but the delicate piling work came under threat when the team hit what was thought to be an underground stream around three metres down. “Although we knew we were going to hit water, we weren’t prepared for how much, I’ve never come across water running so fast through an excavation,” says Nagle. “My biggest concern was that the flow of water would loosen the soil and damage the stability of the existing buildings, but thankfully it was running clear and increased monitoring of the buildings showed there was no movement.”

Developing the model

The complex geometry of the Softbridge, which curves in three dimensions, was not possible to build using conventional structural drawings, so the entire scheme was modelled using the NURBS-based 3D software package Rhino. The model was developed over an unusually long 28-week period by Zaha Hadid Architects and AKT II, in collaboration with the glulam contractor, the cladding contractor and the glass fibre reinforced gypsum (GRG) contractor to create a highly-detailed model from which the various elements could be prefabricated to fine tolerances.

The sequencing of the stainless steel rainscreen cladding will be key to the construction programme as every panel is unique and any damaged or ill-fitting sections could delay work. The panels will be installed over a layer of insulation and a waterproof membrane fixed to the glulams, and be delivered by lorry in sizes of around 1.5m wide and up to 4m tall.

“We wanted to maintain the illusion of a homogenous external skin with as little panellisation as possible, which becomes more important when using reflective materials where joins can be much more obvious,” says ZHA project architect Johannes Hoffmann. “As a result we tried to push contractor Frener & Reifer to create panels as large as possible, which was a challenge for their machinery as many panels have double curved surfaces.”

The curving panels are treated in an electro-polishing bath. To ensure a near-continuous surface, the contractor has developed a secret fix solution that includes welded studs screwed into the subframe and a mechanical adjustment system able to achieve installation tolerances between horizontal joints of virtually zero. “The vertical joints are essentially butt joints, which is highly unusual because of the size of panel,” says Hoffman.

The roof, which forms a continuation of the cladding, will include a series of 21 teardrop-shaped rooflights, designed to provide the main source of light for the library, to be installed in two large panels.

Attention to detail will be evident across the whole project. Manufacturing the formwork for the V-shaped cast in-situ structural column, which had to curve in plan with no visible joins, took nine weeks and cost £27,000. The column will work in combination with large amounts of reinforcement at the rear of the first floor slab to stabilise the partial cantilever.

The glass curtain wall that will hang between the cladding and the ground will use Frener & Reifer’s Minimo glazing system, currently the narrowest transom-mullion system in the world with a face width of just 30mm. The glazing will be completely unsupported and held in place by stainless steel angles on the underside of the slab and on the ground.

Where the Softbridge connects with the existing historic buildings at either end, their walls will be exposed within the building, revealing historical features that juxtapose with its futuristic minimal lines.

First floor floorplan: 1. Library reading room 2. Library rolling stack book storage 3. Library IT room

Section view: 1. Library reading room 2. Archive reading room 3. Library rolling stack book storage 4. Archive rolling stack book storage 5. Lobby/cafe/gallery space

Melting meringue

The Softbridge’s centrepiece is a dramatic white curving staircase with timber treads and a glass balustrade, located at the building’s entrance, which links the above ground floors with the basement. The staircase will be clad in GRG and will fold back in on itself and merge into the floors, creating impression of a melting meringue or a twanging rubber band in motion.

GRG panels will also form the interior walls and ceiling of the library and reading room and use of the material was one of the compromises Hadid was compelled to accept as part of the value engineering.

“In the original scheme, the architects wanted Corian, but if that was used the cladding on the staircase alone would have cost around three quarters of a million pounds,” says Nagle. “GRG will give them the solid white effect they want but not the same feel.”

The staircase will be clad in GRG and will fold back in on itself and merge into the floors

Other aspects of the scheme that were value engineered included the auditorium’s curving timber walls and ceiling, which have been pared back to a simpler flat design. Hard landscaping, including steps and a small entrance plaza on the Woodbridge Road, was also scrapped in favour of moving the entrance to the east of the building.

As part of the process, BAM sought advice from its supply chain to see how it could achieve a cheaper alternative to what the architect wanted. “We tried to ringfence the showpiece elements of the building to ensure the client and the investor still got a very high quality building,” says Hoffmann. “On the whole we are very happy with the outcome and the fact we didn’t have to compromise the external appearance and materiality, or the quality of workmanship in the reception areas. Even in auditorium, where the complex geometry was removed, we still ended up with a very high-end specification.”

Meanwhile, dealing with the demands of one of the world’s most prestigious architects imposed other restrictions, says Nagle. “Managing Zaha’s team and their aspirations under a novation agreement has been a challenge,” he says. “Any alterations to the design have been checked five or six times by the team, with Hadid reportedly giving the ultimate sign off. The architects also walk around site with tape measures to check the as-built dimensions, which I’ve never come across on a job before.

“However, we have to be sensitive to what they are trying to achieve here because this is a one off building that people will visit and enjoy for many years to come,” he concludes.