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A novel solution

A new Cambridge library is a showcase for timber engineering.Dave Parker reports.

When it was agreed that the new Faculty of Education building at Cambridge University would have two separate but indivisible functions (see box), two different types of structure seemed an obvious response. So says structural engineer Whitbybird director Simon Smith, adding: 'The four storey teaching block would be densely populated, with high heat gains. A concrete frame exploiting concrete's thermal mass for heating and cooling seemed the best solution here. For the library area, however, architect BDP had the vision of a 'galleon', and asked us if we could come up with an exposed timber structure with a one hour fire resistance.'

The library was conceived initially as a laminated veneer lumber (LVL) design, not least because of the curving roof trusses. But it had to avoid the 'bouncy, squeaky' reality of many earlier timber structures.

Smith explains that Whitbybird worked up quite a detailed indicative design before Amec won the £7.25M design and build contract in late 2003.

'Then we held discussions with leading timber engineering specialists. The contract went to German company Merk who produced the detail design and suggested switching to glulam for most of the main structural elements - basically because LVL couldn't cope with the curved sections of the frame.'

Resin-rich larch rather than spruce was the chosen timber, selected for its greater durability and colour stability. Source of the larch was certified plantations in the Czech Republic. Using it exposed without visible fire protection in a building that had to have a one hour fire resistance was a complex challenge for the design team.

'The current British Standard is ambiguous in many areas when it comes to fire performance, ' says Smith. 'Designing the columns was particularly complex.

'Obviously they're much larger than needed for purely structural reasons, but calculating the exact size, depth of charring and so on wasn't easy.'

Intumescent varnishes were never seriously considered due to their effect on the finished appearance of the wood.

But, despite the protestations of the design team, Building Control insisted the structural elements be impregnated with surface spread of flame treatment.

'This made for complicated logistics, ' points out Whitbybird associate James Greatorex.

'The larch went from the Czech Republic to Germany for lamination, then over here for the impregnation treatment.

'This has to be done before final machining as it causes the glulam to swell. Then the elements had to go back to Germany, be machined, and then come back to the UK again.'

Straight sections of columns and beams have laminations 45mm thick. These are reduced to 15mm at the curved section of the frames. Maximum sections are 600mm x190mm for the 12 main 'giraffe' frames, 400mm by 160mm for the paired floor beams, which span up to 6m and carry heavy library loading.

Frame centres are nominally 4m;

height to the neck of the 'giraffe' is 16.5m. Width across its legs is 14.5m.

Metal connections are needed, of course, but these are usually well protected by the surrounding glulam.

Bolts are set 40mm below the timber surface and hidden by dowels.

Floor panels have exposed soffits, and are made up of 45mm thick LVL topped with a 30mm thick rockwool layer and 25mm of standard plywood to provide the extra fire resistance.

They support conventional access flooring. All windows and curtain walling are also larch framed, maintaining the ambiance as well as contributing to the sustainability of the building.

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