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Bending the building

TECHNICAL

The Timber Frame 2000 building is six storeys, of platform-frame, large-panel construction. Its rectangular plan could accommodate four two-bed flats per floor. The 1991 changes in Building Regulations for England and Wales permitted up to eight-storey timber building (20m to top floor) using 60-minute fire- resistant construction. But straightforward design guidelines do not exist - one of the planned outcomes of this project. (Fire requirements are more restrictive in Scotland, despite its much higher use of timber framing. Results may influence future Scottish Building Standards.)

Key issues being investigated are:

disproportionate collapse - various panels will be removed at ground level. Work will also be done on stabilising the outer brick skin during this experiment

fire performance of staircases and compartmentation

differential movement of masonry and timber. Among measures being taken, cross-grain timber shrinkage is being controlled by precision kiln drying to 12 per cent for horizontal timbers. Making sure the right timber is in the right place is controlled by the factory-based prefabrication. Building shrinkage is to be monitored

whole-building lateral stiffness. Wind and thus racking loads increase with height, which may be countered by upgrading of sheathing and/or nailing

thermal and acoustic performance in selected rooms.

A forthcoming article1 will give details of the structural issues.

The construction process has also been investigated, starting with a survey of what clients want from timber framing. Today their critical concerns are:

speed of construction

cost

better management of the design process (lead time)

quality of construction

buildability of the frame.

And for the future:

greater prefabrication, standardisation and automation (this prefabricated building incorporates a range of composite timber elements and joists, trusses and floor systems)

lower embodied energy (most timber is C16 softwood from the uk)

greater certainty of quality

increased safety during construction

increased overall structural efficiency

less skilled labour, fewer site restrictions.

These led to setting performance targets for construction at Cardington:

lead time five weeks

frame value (cost) £60/m2

erection value (cost) £10/m2

productivity 900m2/week.

This is one of the tallest timber-frame buildings in the world. A few are similar in the us; lower multi-storey timber apartment buildings are common there. The project team hopes to push large timber buildings in the uk, helped by growing interest in renewable materials and the suitability of lightweight timber construction for building on brownfield sites.

The concrete frame

The seven-storey concrete frame is trying out several ideas architects will find useful:

uninterrupted, thinner flat slabs - no thickening at column heads or slab edges. Proprietary reinforcement at column heads helps deal with reinforcement congestion there

uniform column size, achieved by using high-strength concrete (C85) for the lowest three floors

lateral stability by bracing with crossed steel flats connected to columns in the core area. This speeds construction and can make service routing easier than with solid concrete shear walls

concrete 'welds' using very high-strength concrete (200MPa) incorporating steel fibres and Densit, a proprietary Danish cementitious glue. There are two applications:

segmented concrete dog-leg stairs are welded together and to the frame; smaller continuity reinforcement is needed

for the top floor, permanent formwork (Omnia planks) is welded together, then a structural topping applied, to test this as a new form of two-way spanning flat slab construction.

The concrete sector feels design codes are unnecessarily conservative, so there will be a variety of whole-building tests to demonstrate actual performance, as well as tests of localised fire and explosion.

The sector has been feeling the pressure from steel in terms of frame contracts being let and in the range of information available to designers. Just published is a National Concrete Frame Specification2, tested in this building and, it is hoped, to be taken up by nbs. Also recently published is Economic Concrete Frame Elements3, a guide to rapid selection and sizing of concrete frames.

The concrete frame is also being used for work on buildability - formwork, the sourcing, prefabrication and placing of reinforcement, concrete placing and fast striking of formwork. The project may go on to attach cladding to the frame, and to study concrete repair and demolition.

In the longer term there are plans for European joint ventures (another strength of the steel sector) to produce at Cardington:

a five-storey, innovative precast frame with two-way spanning floors

a five-storey hybrid composite in-situ and precast frame

perhaps another precast frame.

You can watch the frame as it nears completion with an image updated every 30 minutes, plus related information, on the project web site, at http://www.bre.co.uk/bre/construct/ecbp/index.html

REFERENCES

1 Timber Frame Construction for Medium-rise Buildings. C J Mettem. In Progress in Structural Engineering and Materials. 1988. Vol 1. No 3. pp253- 262. From crc, tel: 0171 505 6622.

2 National Concrete Frame Specification for Building Construction. First Edition, 1998. From crc, tel: 0171 505 6622. £19.50 plus £1.95 p&p.

3 Economic Concrete Frame Elements. C H Goodchild. Reinforced Concrete Council. 1977. From British Cement Association, tel: 01344 762676. 128pp. £14.40 plus £1 p&p.

CONTACTS FOR TIMBER

Vahik Enjily, BRE, tel: 01923 664392

Simon Palmer, trada Technology, tel: 01494 563091

CONTACTS FOR CONCRETE

Procurement and process research - David Moore, bre, tel: 01923 664578

Performance testing and research - Chris Judge, bre, tel: 01923 664577

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