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Glenn Howells Architects is creating an adventurous, unifying visitor centre for Savill Garden in Windsor Great Park. Sutherland Lyall talks to project architect Jamie Webb about the challenges of building with timber on the Crown Estate's land.

The Crown Estate wanted to create a landmark visitor centre at the entrance to Savill Garden in Windsor Great Park. Hitherto, there had only been a run-down shop and a car park. The new centre would bring together scattered park accommodation and include a ticket box, a shop and a self-service restaurant that could be hired out in the evening for formal occasions. There would also be seminar rooms, staff rest rooms, offices and a mini garden centre.

Glenn Howells Architects won the competition with an adventurous swooping timber gridshell covering the functioning spaces. The gridshell partly merged with landscaped beams so that the whole ensemble grew out of the landscape and had a low-lying cloud-like umbrella floating over it. There was a solid precedent in Cullinan's Weald and Downland Museum and there seemed to be a nice synergy in the fact that much of the timber could be sourced from the Crown Estate's own woodlands.

SIGNING UP The contract was the GC Works form - the public-sector version of JCT - in which everything in the specification has an 'or equivalent' qualification. As project architect Jamie Webb points out, this was the first time the practice had used NBS Building.

He says: 'This version of NBS is IT-integrated, which means that regulatory things like the British Standards references are built in.

It's really simple and you can't finish until you have filled in all the gaps. And it is updated on a regular basis. The beauty on a job like this is that the project architect is able to write the spec as he or she is going through the process of design.' The contract had no quantities. Webb says: 'We advised the client to go with quantities because a schedule of rates is never as good as a bill. But the quantity surveyors advised the client not to.' This view prevailed - and was influenced by the fact that there was to be a two-stage tender negotiated with the main contractor in order to get on site as early as possible. Webb says: 'At the end of the second stage we had to bring the project under cost control and so in the value engineering period we lost the advantage of a fast, two-stage contract.' Although time had been of the essence, the final reality was that budget won over time.

Value engineering was especially constrained by the roof's major cost - but Webb says: 'Although we re-specified some things which Green Oak [the timber specialist] could help us with, you couldn't exactly trim out half the timber.'

COLLABORATING CONSULTANTS The lead structural engineering consultant was London practice HRW, and Green Oak appointed Buro Happold as the roof engineer. The demarcation line between them was the tubularsteel edge beam supporting the timber roof structure. Webb says:

'It was an amazingly easy process between the two. And HRW are also engineers for the roof cladding and glazing. It has all been remarkably straightforward.' The services engineer was Atelier 10, whose 'environmental agenda' Webb appreciated. He says: 'They have been very good about strategies to do with natural ventilation and minimising the mechanical engineering so we looked at grey water and syphonic systems. They played an integral part in the design.'

DRAWING TOGETHER The Glenn Howells office is Mac based, so the CAD drawings were done in VectorWorks. Webb used to use MicroStation and AutoCAD but says he didn't find the conversion to VectorWorks all that difficult: 'There was a slow learning curve for the first couple of weeks and VectorWorks is cheaper than the others.' He adds: 'If you have a legacy of Macs the cost of changing over to a PC base is quite high. We do 3D modelling in the office for internal design massing studies - not for renderings. But this was a building that was extremely difficult to image. We had a student build it mostly in VectorWorks and the engineers did the final image in FORM-Z.' The engineers used AutoCAD. There were long discussions at the form-finding stage about the shape of the gridshell.

Webb says: 'Once we had settled on a form, the analysis, done in STAADpro, took the best part of a month - after it had been modelled in AutoCAD. This was an iterative process and eventually we came to the final design. We produced little 3D sketch models in the office about how the domes sat together and other initial ideas and had regular meetings with Happold and Green Oak.

At the same time, they were building up the technical detail for the grid-shell - things like the size and bendability of the timbers - and building little mock-ups in their yard. They were in discussions with the scaffolding company about how they would do it on site.

So we covered the issues of how it would look and work; whether it was possible in timber and how the connections would work. It was a massive learning curve for everybody and it took probably the best part of a year. Green Oak were in at the beginning and they brought in their scaffolders on an ad-hoc basis.

TIMBERING THE ROOF On top of the grid-shell structure is a Keybemo system with 160mm of insulation and a profiled standing-seam skin. It serves as the waterproof layer and support for the timber rain-screen.

Webb says: 'We had specified Kalzip and spent a lot of time checking roof radii so that [we could be sure that] the aluminium would cope. The main contractor preferred Keybemo, so we had to go over all the warranties and the guarantees in detail. Normally you wouldn't think twice, but a roof like this needed careful checks.' Initially the architects looked at a single-ply waterproofing skin but were worried about perforations. Webb says: 'We looked at alternatives and ended up with a series of standing-seam clips which support the timber rain-screen cladding:

you don't penetrate the waterproofing.'

The roof surface has 20mm-thick green oak from the Crown Estate woodlands, which was milled by Green Oak and screw-fixed at each end directly to the bearers. Webb says: 'Green Oak have an amazing knowledge of joints and weathering and were very helpful in the process of specifying and liaising with the Windsor Royal foresters.' The structure of the gridshell itself is much the same as Cullinan's Weald and Downland Museum - except that it is not rooted in the ground but on a 400mm-diameter tubular steel edge beam. Webb says: 'The work by steelworker SHS Structures on site is fantastic. The beam looked liked a roller-coaster track before they started fixing the timber roof. SHS had to plot coordinates off the 3D model and then create its own 3D model for fabrication.

It was more than you would expect from a steel contractor - and getting the edge beam on site meant that the roof was on time.' The sections of grass roof are by Erisco Bauder. Webb says:

'They have been very helpful and worked with the client. We were dead against sedum although it's low maintenance. But we wanted year-round consistency.' On the adjacent banks where it is not safe to have the public walking, it is proposed to plant juniper.

UNDER WRAPS In the beginning, the architect considered using a rammed-earth wall or dry stone but moved on to brickwork for the front and the return and flank walls. Webb says that the real requirement was that it wasn't an engineered brick: 'We wanted a bit of colour variation - an almost hand-thrown, garden-wall aesthetic. We were originally looking at a lime mortar, which would have eliminated movement joints. Still, the workmanship on site is very good.' Webb says the biggest issue for the curtain walling was the roof: 'It has quite a lot of movement and it moves independently of the glazing.' So an independent glazing system was required. The architect specified a capped aluminium system and it seemed that a secondary steel structure would also be needed. 'What we have ended up with, ' Webb says, 'is a bespoke system by Haran Glass with its own mullions and adapted Schüco T sections. It has been quite tricky. Haran Glass do bespoke glazing using other people's systems. They have a design team made up of people who have worked for Arup and are prepared to spend that little bit of money before they land a job. The end product is much better than using a standard glazing system.' Originally, the architect specified a ceramic-tile floor but the client was keen to use its own timber. So the specification was changed to a screed with underfloor heating. Webb says: 'We had to find a company which would mill and air- and kiln-dry the timber - and guarantee it. It was a six-month process, but eventually we found Weldon Flooring. But it all cost and took time.

Door handles are D Line from Allgood. Webb says: 'We have the real thing. We didn't want to be looking at imitations so we dug our heels in. It wasn't an issue in the end because there are so few.' The rest of the interior is still being worked on in preparation for the centre's scheduled opening in 2006.

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