Structural round timber for buildings
An alternative to log construction is to form structures using machine-debarked or machine rounded timbers as conventional structural elements such as columns, beams, rafters etc. Assemblies such as trusses and portal frames are also used.
In construction the terms “round timber” or “pole” commonly denote machine debarked timbers of round cross section which retain the natural taper of the original tree. On the other hand, term “machine rounded timber” denotes mechanically shaped cylindrical round timbers of constant cross-section. The mechanical removal of bark reduces the strength of poles in comparison with manual debarking which limits damage to the natural structural arrangements of the wood fibres, eg swelling around knots. Logically therefore, machine rounding also affects strength, and it increases material wastage. Using machine rounded timbers can however facilitate detailing, jointing and construction and, depending on end use, improve the appearance of the structure. Material specification must also consider drying and durability aspects in relation to the service conditions of structural timber elements. Finally the term “structural round timber” denotes strength graded round timber.
A number of UK grown timber both hardwood and softwood timbers are available that will meet designer's specifications - eg sweet chestnut, European oak, Douglas fir, Sitka spruce, European larch, European whitewood and Scots pine.
Service conditions affect the structural performance of timbers and durability throughout the service life. The heartwood of some timber species feature good natural durability without preservative treatment (eg oak, sweet chestnut, larch) in external uses isolated from the ground. Sapwood of all timber species, as well as heartwood of less durable timber species, requires preservative treatment when fully or partially (ie covered) exposed to weather.
For building applications, round timber structures may be sorted into two categories. The first, "conventional structures" includes post and beam forms, portal frames, and propped frames mainly for agricultural uses and semi-rural domestic and office uses. The second, "alternative and experimental applications" comprise space frames, specialist roofs as well as towers, domes, timber-fabric structures and prestressed pole structures (light pre-stressed arches and bent poles). Typical structural forms, spans, heights and uses are illustrated below.
Round timber buildings such as pole barns are popular for low-cost self-built developments as well as rural buildings, particularly open or uninsulated buildings.
Very simple structures can be achieved using timber poles. Figures 5 and 6 give good examples of timber pole farm buildings in North Wales. The hay barn shown in Figure 5 utilises Sitka spruce columns, rafters and purlins used in the round while the struts and ties of the trusses are halved. It is worth noticing the half round pressure treated Sitka spruce cladding boards in the lean-to shed.
The use of round timber is increasing in semi-rural commercial buildings and visitor centres. Commonly, round elements are used as columns and bracing as well as roof members. Sawn timbers may then be used as principal beams, floor joists and for cladding.
At the Anglo Saxon Village in West Stow, the post and beam structure of the museum and cafeteria building features exposed 200 mm diameter machine rounded British larch columns together with sawn timber beams, floor joists, roof truss elements and cladding boards.
Figure 7. Anglo Saxon Village museum and cafeteria, West Stow, UK. Architectural and structural timber engineering: TRADA Technology; Timber contractor: Carpenter Oak and Woodland. 1998.
Space frames allow larger spans using short lengths of round timbers and sophisticated nodal connections involving flitch steel plates, dowel type fasteners and even reinforcement systems to enhance connection capacity. A few proprietary systems are available, and one prototype farm building was built in the UK in 1987 using a jointing system developed in the Netherlands by Dr. P. Huybers of the Delft University of Technology. More recently, Feilden Clegg Bradley Architects and Carpenter Oak and Woodlands Timber Framers followed the same principles to build the Solar Canopy of the Earth Centre in Conisborough.
Figure 8. Space frame. Solar Canopy, The Earth Centre, Conisborough. Feilden Clegg Bradley Architects. Timber contractor: Carpenter Oak and Woodlands Timber Framers. Mechanical testing of timbers: TRADA Technology. 1999.
