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Specifier's choice: The Sentinel, Glasgow

Sutherland Lyall talks to project architect Reiner Nowak about gordon murray + alan dunlop's new class-A office development in Glasgow's nancial district

The Sentinel building is going up on the corner of Waterloo Street and Douglas Street in Glasgow's nancial-services district. The MoD's Wellesley House originally occupied the site, until it moved its offices across the road. Edinburgh-based developer Kenmore saw the opportunity and pulled it down to build around 1,000m 2 of class-A offices, with parking in a semi-basement. The architect is Glasgow practice gordon murray + alan dunlop (gm+ad).

The two most visible facades of the 10storey building, on Waterloo and Douglas Streets, are glazed in a pattern using 300mmdeep grey glass spandrel panels covering the oor-slab edges. The main glazing is a random arrangement of clear, fritted and illuminated fritted glass panels, all in a Schuco glazing system. Of the other two elevations, the more conventionally glazed south wall which faces over a narrow alley and the blank east wall, which rises above a seven-storey car park, are both in Storendered blockwork. These two walls, with attendant office space, give the impression of wings, a bit like the pages of a book, from whose centre is suspended an eight-level glass box that hangs over the transparent - which is to say structurally glazed - groundoor foyer. The impression of hanging or floating is aided by the fact that the only supporting column visible beneath this glass box, located where you might expect it near the front entrance, is disguised as part of a big, red, internally illuminated, square doughnut. This has no practical function but it will surely serve as a memorable marker for the building. The thickness of the 'book' pages is defined by vertical runs of slate cladding, on one side actually the blank side wall of the main staircase and on the other a narrow offset in the plan.

The open of ce plan potentially enables any sort of letting. Each of the roughly rectangular oors can be split across the middle, with access from a common lift lobby and with a perimeter re stair for the notional second half in addition to the main staircase. This latter is of a higher standard than the usual re stair (which it also is) because its potential function is to facilitate inter-floor communication, should a tenant take several floors.

The structure is a steel frame with composite concrete/steel tray floors.

Working offifice space has raised floors and false ceilings, the latter of which are set back around 400mm from the glazing to minimise the depth of the spandrel panel.

Project architect Reiner Nowak says: 'The main structural challenge was cantilevering the corner box.' The massive steel column supporting the 'cantilever' is concealed in the luminous red doughnut.

This was a May 1999 (revised January 2002) Scottish Building Contract with contractor's design portion and with quantities. It is much the same as JCT98 with quantities and a contractor's design portion. An important element built into the contract sum of just less than ú10 million was provision for building mock-ups of the cladding. You might think this unusual but it is increasingly common in building contracts. The counter argument is that architects should know, shouldn't they, what effect their choice of materials and forms will have. The short answer is no, they can't always be expected to, especially when specific and often subtle issues of transparency, reflection, colour and the impact on the design of local city environs are involved. Mock-ups also help to maintain the client's attention and to reassure them about how their building is going to look.

'It's simply a matter of building the mock-up into the contract, ' explains Nowak.

'It's vital for a building in such a location.

You have to do it by eye and on site.' The latter point came home to the design team when it had mock-ups built in the factory.

These were interesting and helpful but could not really give an idea of what, for example, would happen to the cladding half-way up the facade on a cold winter's afternoon.

'The purpose of the on-site mock-up is to judge the appearance and the colour, ' says Nowak. 'The factory mock-up didn't allow us to see the light and colour. You need to see it in the condition of Waterloo Street. What we are going to do is have the cladding attached to the existing scaffolding so we can be in the street and make a judgement.' This is one area where CAD visualisations don't seem to be able to help.

(For the record, the practice uses AutoCAD 2000 on around 25 networked workstations in the office.) Miller Construction won the bid for main contractor. The company has apparently taken the Egan report seriously and has established a set of partnering firms for the main packages in any big contract.

'Miller has a set of subcontractors which it uses all the time: it sends the tenders out to three or four of these partnering companies.

Of course, if it doesn't get a good price it goes outside, ' says Nowak. 'This has two effects. One is that you can get a bit more out of subcontractors because they are part of the main contractor's network. The second is that they are a useful resource because, for the same reason, you can get a quick price from them.'

Developing the skin Nowak says of the skin: 'Basically it's a Schuco facade, which we are individualising.

Systems Aluminium is responsible for the facade and this is who we are working with to develop the facade. The mock-up in the factory particularly focused on the corner 'box', for which we need some special extrusions.'

The system will be fully glazed by Pilkington with some of the 3.2m-high, low thermal-transmission double-glazing panels, some of which are 600mm wide and others 1,200mm. 'You get a good package from Pilkington, ' says Nowak. 'It's a very good option at the moment.' Some of the panels will have a full-height 40 per cent frit of 2mm white horizontal lines. The frit is on the cavity side of the rear panel. The ooredge spandrel panels are 600mm high, and their colour was changed from white to grey following the factory mock-up inspection.

Some of the full-height fritted glass panels have coloured LED arrays hidden in the perimeter ceiling recess, which will illuminate the frit from inside.

'The LED arrays allow you to work in different modes - purple, pink or green - and you can also have them changing constantly from one colour to another, ' says Nowak. 'The purpose of the frit was originally to give the corner facade a random, animated appearance.' But it turned out that the frit was needed anyway to reect the colours from the LED arrays.

The other facade element is the slate cladding to the edges of the 'book' pages. The 40mm-thick slate (from Burlington Slate) is fixed to the steel frame and blockwork structure with a 100mm cavity, half of which is insulation. The slate on the stair tower on the west facade has a closed joint with a DuPont Tyvek breather membrane behind it. The stepback on the other facade is a rainscreen and so is open-jointed and has a vertical DPM from Phoenix. Matching the dark grey colour of the slate is the mechanically fixed render from Sto, straight out of the catalogue.

Systems Aluminium is also doing the structural glazing on the ground floor, including a mock-up - and the more conventional windows on the alley facade.

The manufacturer of the automatic entrance door was changed from Geze to Rush. 'The Rush door was recommended by SAS Ceilings Systems. It was more pleasing, so we changed to it, ' explains Nowak. 'It is a new product, which we haven't used before, and it turns out to be an excellent door.

It's difficult with an important element such as the entrance door; you don't want to have any surprises, so you go and see it in the factory.'

Moving inside

The practice knows SAS Ceiling Systems because it has used the company before. On this project the architect worked with SAS to design a coffered ceiling for the ground oor using the standard metal panel and a 2.5m square pyramid rising to a light tting at the apex.

The timber internal doors are by Alastair Burnett, a local m and one of the Miller partners. 'Because this is a class-A office development, the quality of joinery was critical, ' says Nowak. 'Burnett has been working on a number of Miller developments in Glasgow, so we went to see the quality of its workmanship. As veneering is an important issue, Burnett is going to build a mock-up of a section. We are looking at cherry and the different grain and cuts possible with this wood. At the same time with mock-ups you discover what things can't be done, either because they are practically impossible or because of levels of workmanship. So working with mock-ups is quite important and it helps the client to understand what it is getting and what we are trying to do.'

As in almost all Specifier's Choice case studies, the architect has speci ed D Line architectural ironmongery from Allgood.

'We use lots of tiling suppliers but for the lavatories we were looking for a narrow tile with a 1:4 proportion, ' says Nowak. 'After some research we found Domus could supply us with 400 x 100mm tiles. For the lift lobbies we decided on 300 x 300mm tiles from Cosmo Tiles.'

The lifts themselves are by Schindler.

'Schindler is not as well established in Scotland as it is in England, ' explains Nowak.

'It is trying to break into the market here and was willing to offer a good price. This may actually be their rst big project in Scotland.'

The red square doughnut on the ground oor is being constructed by Powder Hall Bronze. Nowak had searched all over for someone who could make it, from large-scale resin manufacturers to artists, 'then we came across a small company here in Glasgow, Powder Hall Bronze'. He adds: 'The red resin will be illuminated from inside so it will glow - but not show what's inside.' What is inside includes one massive column, a lot of wiring and lamps, plus the weave of the reinforcing glass. Nowak thinks they have cracked it with almost invisible glass reinforcement, and access from the top of what are two mouldings, joined invisibly on site.

'We are trying to make this development a bit different, so with another focus, following research on resin for the red box, we came across Decorlux, ' Nowak says. 'We were thinking about the handwash basins and had some samples made out of single sheets of resin. But it wasn't satisfactory, so we tried out Decorlux translucent acrylic and developed a special shape, which we could have made up because we wanted 70 or so. The idea is that it is sitting on a black Corian worktop, and a light source changing from red to green shines up from the oor to the ceiling through the slightly tinted clear acrylic.'

From the top

'We were looking for a cradle system for cleaning the facades, ' says Nowak. 'But you would be able to see the crane from the street. So we went for an abseiling system which combines with the Access Group roof-based fall arrest system and has the least visual impact.'

The roofs have Ross ADT syphonic drainage. 'We've used syphonic drainage because it gives you the option of not having too many outlets, ' explains Nowak. 'So we only have two stacks in the building - and the little roofs to parts of the facades are drained normally.

'We chose a Sarnafil roof membrane because it's a well-established and proven product which we use all the time - not on every project but it's one of these standard things where if you have used it before and you are happy with it, you use it again. The same goes for insulation, ' he adds, which in this case is Rockwool.

He continues: 'When it comes to other things, it often happens that we specify a particular product and the main contractor asks us to change to a product which its subcontractors are more comfortable with.

There are perhaps three standard materials which you always use and it really depends on what the circumstances are. With a roof membrane there aren't too many alternatives.'

But with quality it's different. The alternatives suggested by subcontractors have to match up to the specified material.

At the moment Nowak's team has speci ed Burlington Slate and is looking into the suggested alternative. But as Nowak says, 'this is a class-A office development, it's in the premier league, so there is a focus on not making a compromise'. As a result, nishes are crucial. 'Of course, we have to compromise but not on too many occasions, ' he adds. And he points out that with materials sourced abroad there can be an ethical issue if there is uncertainty about, for example, the use of child labour.

CREDITS

ARCHITECT gordon murray + alan dunlop architects:Gordon Murray, Alan Dunlop

QUANTITY SURVEYOR Thomas + Adamson

STRUCTURAL ENGINEER Halcrow Group

SERVICES ENGINEER KJ Tait & Associates

FORM OF CONTRACT Scottish Building Contract, Contractors design portion with quantities, May 1999 Edition (January 2002 revision) PROJECT MANAGER CBA 3D VISUALISATION Pixelimage

GROSS EXTERNAL FLOOR AREA 1,000m 2

TOTAL COST ú9.8 million

START ON SITE August 2003

COMPLETION ON SITE December 2004

CAD PACKAGES USED AutoCAD 2000

SUBCONTRACTORS AND SUPPLIERS Structural steel Severfield Reeves; roof membrane Sarnafil; roof insulation Rockwool; fall arrest system Access Group; syphonic drainage Ross ADT; curtain walling Schuco; curtain walling subcontractor System Aluminium; toughened double glazing for curtain walling Pilkington; glazing subcontractor System Aluminium; stone cladding Burlington Slate;insulation Kingspan; Tyvek vapour control membrane DuPont; waterproofing membrane walls; Phoenix;waterproofing membrane for tanking Fosroc; render Sto; plasterboard Gyproc; automatic entrance door Rush; ironmongery D-line; timber doors Alastair Burnett; stairs and handrails Hunter Fisher; suspended ceiling SAS Ceiling Systems; ceramic tiles WC Domus Tiles; ceramic tiles, lift lobby Cosmo Tiles; sanitaryware Decorlux; lobby fl oor Concept Tiling; lightbox feature Powder Hall Bronze; lifts Schindler Lifts READER ENQUIRIES Access Group 1601 Alastair Burnett 1602 Allgood 1603 Burlington Slate 1604 Corian 1605 Cosmo Tiles 1606 Decorlux 1607 Domus 1608 DuPont 1609 Phoenix 1610 Pilkington 1611 Powder Hall Bronze 1612 Rockwool 1613 Ross ADT 1614 Rush 1615 Sarnafi l 1616 SAS Ceiling Systems 161 7 Schuco 1618 Sto 1619 Systems Aluminium 1620

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