黑洞社区

Peoplebuilding in Hemel Hempstead is the first speculative office building to be constructed in the UK since the toughening up of Part L of the 黑洞社区 Regulation, which covers energy efficiency. The problem is that the regs set a limit to the amount of mechanical ventilation that can be included in a building. On the other hand, any self-respecting high-tech office block has large expanses of glass, and is therefore vulnerable to solar gain. The louvres are part of an attempt to resolve this contradiction.

Stanhope, the developer behind the Peoplebuilding, hopes the group of six blocks on the brownfield site will be a refinement of its earlier development at Chiswick Park. Designed by architect Richard Rogers, Chiswick Park consisted of a series of fully-glazed rectangular offices with brises-soleil 鈥� horizontal louvres fixed to each floor plate that helped to keep solar gain to a minimum. However, the solution would not have complied with the regulations at Hemel Hempstead. In addition to the brises-soleil, the Peoplebuilding would have had to have been clad in heavily coated glass to make sure that there was enough solar shading to comply with Part L. Stanhope wanted to avoid heavily coated glass for two reasons: cost, and because transparent glass provides tenants with a light and airy environment. The solution was to install large vertical brises-soleil on the south, west and east facades of each building.

Specifying the facade and motorised louvres presented the design team with a complex challenge. Consultant Arup had to liaise closely with architect Fletcher Priest and the three contractors involved in supplying the facade's brises-soleil, aluminium louvres and curtain walling. What's more, Stanhope was looking at ways to value-engineer cost out of the design. Keith Priest of Fletcher Priest called the facade budget "staggeringly low", and says he couldn't believe the figure when he first saw it.

The biggest challenge for Merlin was to specify an inexpensive motorised system that would keep the louvre in line and moving at the same time (see "Now we're motoring", page 10). Merlin calculated that it could use a 24 V rather than 240 V motor, which meant that it didn't have to spend so much insulating the cable against the steel brise-soleil. A 240 V motor would have required thick cable insulation to protect against the risk of the steel becoming live. Merlin specified a motor from Italian manufacturer Magnetic Mingardi because it had used similar models before and had had no problems; Haughton calls it a "fix and forget" motor.

Merlin was able to cut costs by utilising the building management system (BMS) provided by Eton Associates to control the louvres, rather than providing a stand-alone system. The motors are controlled by a master control box situated in the rooftop plant room.

The windspeed and design-load parameters set by Arup meant that Merlin couldn't use its standard link-arm design to connect the louvres to the motor. The aluminium specified for the louvres, link bar and motor brackets came from Alcoa, a regular supplier to Merlin. Skanska Technology, Merlin's project engineer, concluded that the aluminium would tear unless the metal was strengthened at the pivot points, so Merlin had to get the manufacturer to produce a more robust version.

One of the worries for the design team was that workers would feel imprisoned by the louvres when they were in their closed position. Merlin's solution was to perforate them so tenants could see beyond the brise-soleil to the surrounding countryside. Arup worked out that there would still be adequate shading if 32% of each louvre was perforated.

Merlin was chosen because it had worked on the fixed louvres of the brise-soleil at Chiswick Park, and its contract was negotiated in open discussions with Stanhope at an early stage. This meant, crucially, that the contractor was able to offer input at the design stage.

For Stanhope, these early discussions were intended to crystallise the specification, risk apportionment, budget and design at the start of the project. "We try to avoid the hidden traps," explains project executive Matthew Lusty. "We were constantly reviewing the specification to eliminate anything that could cause us problems later in the build process." Haughton said that Merlin's role was to make sure that "Fletcher Priest's cladding concept was buildable and achievable".

The team was presented with another challenge: Merlin's aluminium couldn't be used for the 13.5 m high louvre-supporting structure because the sections required would have been too large, and impossible to extrude. "We thought about fixing the aluminium structure to each floor slab, but Fletcher Priest wanted a gap between the inner window wall and the louvres to enable the windows to be cleaned; so a secondary steel support was used to hold the aluminium louvres," says Haughton. The structure was fixed to the roof to keep the 4.5 m gap between the curtain walling and the brise-soleil clear.

This is where steel subcontractor Westbury Tubular Structures came in. The most difficult challenge for Westbury was working within tight 5 mm tolerances between the louvres and their support brackets, which were welded to the steel support. The firm's problem was that the tolerance of the steel tubes supplied by British Steel was 15 mm (in accordance with the National Steelwork Specification). Westbury got round the problem by physically altering the tubework. "In some instances we had to straighten the tubes by heating and rerolling them," says Westbury project manager Steve Langan.

It wasn't only the louvre contractor that was under pressure to cut costs. Stanhope demanded that curtain walling manufacturer Gartner came up with a cheaper design for the curtain walling than it did at Chiswick Park. Gartner's solution was relatively simple: the triangular mullions and transoms used at Chiswick were replaced with squarer elements at Hemel. "The new shapes lend themselves to a stronger, more refined structure, which made it a very economic extrusion run," says project manager Gary Cornwell.

After extensive testing with Fletcher Priest and Arup, Gartner also managed to reduce the number of glass panes needed in a single cladding panel. At Chiswick Park there were three vision areas; at Hemel only two. The altered design means that less aluminium is required and, as a result, tenants will get a better view from the building.

There are also fewer panel shapes at Hemel because, once Fletcher Priest decided to fix the brise-soleil to the roof instead of the floor slabs, there were fewer penetrations in the facade. "At Chiswick, there were walkways and external staircases piercing the facade," says Cornwell. "At Hemel we don't have to worry about other contractors' structures lining up with our holes."

There are also only two types of cladding at the Peoplebuilding 鈥� a corner and straight unit 鈥� and only two physical interfaces with other manufacturers' products. The Schuco doors specified by Gartner were chosen because they worked out cheaper than its own; there was no problem connecting the cladding to the doors because they had a similar connection. The other interface point was with the entrance screens supplied by Rush Entrances. Fortunately, Rush had worked with Gartner at Chiswick Park, so any problems of compatibility had already been eliminated.

Another member of the cladding group trying to engineer cost out of the glazing was consultant Arup.

It studied which size of pane would be easiest to install and produce, and would require the least wastage and transportation. It also helped Fletcher Priest to specify Part L-compliant glass for the panels (see "Complying with Part L", right).

All the contractors met regularly with Fletcher Priest, Stanhope and construction manager Mace to review the cost and design of the cladding and set benchmarks for its installation. "We were permanently wrestling with the cost plans, looking for ways of saving money," says Priest. "Key team members were quantity surveyors Davis Langdon & Everest and Mott Green & Wall, which permanently monitored the budgets to an intense degree."

The contractors involved in the first Peoplebuilding office are hoping they will be retained by Stanhope for the future units. All the parties are keeping log books to note how improvements could be made. For example, in the first Peoplebuilding, the tower crane and external staircase had different bases, when in fact they could have shared the same one. "These are things that weren't apparent at the time. They help save money that can be spent elsewhere," says Priest.

Stanhope is hoping that the interrelationships between the specifiers and other team members will enable it to continue to evolve its Peoplebuilding brand. "We have a well-tuned model, but we must improve elements where we can, and examine why other areas haven't worked," says Lusty.