Pole timber roof structures
• Unprocessed roundwood is cheaper and more easily available than sawn timber, and is mainly used for frame structures, ie skeleton wall and roof structures, trusses and the like.
• The advantages of using pole timber from young trees (5 - 7 years old) as compared to those of using sawn timber are numerous. The main ones are:
• The cost and wastage of sawing are eliminated.
• 100 % of the harvested timber's strength is utilized, while the immense original strength of large tree trunks is forfeited by sub-division or lost in the sawing wastes.
• A timber pole is stronger than sawn timber of equal cross-sectional area, because the fibres flow smoothly around natural defects and do not end as sloping grain at cut surfaces.
• Poles have large tension growth stresses around their perimeters and this assists in increasing the strength of the compression face of a pole in bending.
• Sawn timber is a product of trees that have grown for several decades. Since their replacement takes so long, excessive felling can cause serious environmental problems.
• Hence, from the points of view of economy, strength characteristics and environmental acceptability, the use of pole timber (eg from mangrove swamps, thinnings from eucalyptus or softwood plantations, etc.) can be far more appropriate for a range of building constructions than the use of sawn timber.
Scrap Metal Plate Connections (Bibl. 00.39)
• This simple and cheap technique, developed at the Intermediate Technology Workshop in Cradley Heath, U.K., uses thin sheet metal, cut to the required size and shape, which is wrapped around the joints and firmly nailed onto the timber.
• The most suitable application of this method is in the prefabrication of pole timber trusses. To ensure uniform dimensions, the trusses are made with the help of a template laid on the ground and held in place by wooden or steel pegs. The poles are placed as accurately as possible on the template, then cut to size and joined together as described above.
Steel Flitch Plate Connections (Bibl. 14.10)
• The nailed flitch plate connection, developed at the Building Research Establishment, Garston, U.K., consists of mild steel sheets inserted into longitudinal saw cuts in the timber poles and connected to them by nails driven through the timber and the steel at right angles to the plate.
• Mild steel sheets up to 1 mm thickness can be easily penetrated by normal round wire steel nails without pre-drilling. Thicker sheets require drilling or the use of hard steel nails. Tests have shown that for most applications and timber species two 1 mm plates provide sufficient strength of the connections. (Considerations of cost suggest that it is better to increase the number of 1 mm plates rather than their thickness.) Stronger timbers may require flitch plates of larger areas to achieve appropriate design stresses.
• The ability of the nailed flitch plate connection to sustain loads after initial failure is a characteristic which could prove valuable in areas where buildings may be subjected to earthquakes and high winds.
Timber Jointing with Dowels (Bibl. 14.02)
• Nails and toothed plate connectors are quite often impossible to use on harder timber species. When used on softwoods, they tend to loosen when the timber shrinks.
• A more appropriate alternative, developed at the University of Nairobi, Kenya, is the use of dowels, which are fitted into predrilled holes. Where structural considerations permit, these are preferably wooden dowels, as they are cheaper and do not corrode. They should, however, be prevented from slipping out by means of nails or pegs, inserted at different angles.
• Alternatively, holes can be drilled into the ends of the wooden dowels, into which hardwood wedges can be fitted to keep the dowel in place. Thus the hole into which the dowel is inserted can be slightly oversized to facilitate and speed up work.
• Where strong connections are vital, steel bolts and nuts are most suitable, but also very expensive, costing three to four times that of the mild steel rods from which they are made. Using the rods straight away as dowels is cheaper and equally effective. To prevent them from slipping out of the timber, 10- 12mm deep holes should be drilled into the ends of the dowels, as described above in the case of wooden dowels. With a cross saw cut, the end pieces can be bent back like flower petals, holding down a steel washer.
Space Frame Connections (Bibl. 23.10)
• A method of using short length, local pole timber to construct space frames for large covered areas (such as meeting halls, workshops, markets, etc.) was developed in Sweden by Habitropic. The system is based on special space frame connectors, comprising a cros-scomponent of welded steel, and tail end connectors with screws, washers and nuts.
• The poles are all cut to the same length, say 1.5 m, and cut lengthwise at both ends with a saw. Holes for bolts are drilled at each end, the steel tail-end connectors inserted in the saw cut and fixed with bolt, washer and nut. After prefabricating all the required pores, they are assembled on the ground, directly below their final position and lifted into place by a pulley system.
• With pole thicknesses of 5 - 6 cm the weight per m2 is 20 kg, and the consumption of material per m2 is approximately 3.5 poles and 1.1 space frame connectors.
Hogan Roof Construction (Bibl. 23.16)
• The North American Navajo Indians traditionally build their homes (hogans) with this simple method. A hogan is usually an octagonal house covered by several layers of timber poles, which are laid across the corners of the layer below, thus reducing the void with each new layer. The same system can be used to cover triangular, square or other polygonal structures, without the need for supports other than at the periphery of the roof.
• A well designed roof with accurately cut and assembled poles should in theory be stable with only a few bolt or dowel connections at certain strategic points. However, it is advisable to fix each pole firmly to the one below to avoid excessive lateral movement, especially in earthquake or hurricane prone regions.
• Traditionally, the hogan roof is covered with earth to provide a high thermal capacity, which is advantageous in climates with large diurnal temperature fluctuations. Lighter roofs with low thermal capacity are also possible by merely constructing a framework and bridging the gaps with a waterproof membrane and light roof cover (eg wooden lathing and shingles, mats, thatch).
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