Wood can be incorporated into outer walls in many ways. A very old but still current construction type is that of the log cabin', with tree trunks stacked one on top of the other; providing a load-bearing, watertight and insulating structure. However; this construction is no longer common, due to the high cost of materials and the limited thermal insulation offered. Today, wood is more commonly used to protect against rain in the form of boards, sheets or tiles which are affixed to a wooden, metal, concrete or masonry supporting structure. It is also used for door frames and window frames, and in curtain walls, possibly with wooden (sandwich) panels.
This construction method can be seen in many historic buildings in Europe, Asia (Russia and Japan), and North America. Although still in use, it is generally only applied to buildings which do not have high requirements in terms of energy efficiency and comfort.The relatively high thermal insulation value of wood, combined with the extreme thickness of the walls renders those walls reasonably efficient insulators. Nevertheless, a thickness of 330 mm would be required to meet the current European standards of 2.5 m2 K/w The use of secondary walls (interior or exterior) and some form of insulation material is possible, but the swelling and shrinkage of wood as moisture content varies must be taken into account. Any structural supports attached to or within the stacked log wall must allow for the vertical movement of the wall.The moisture content of wooden structures exposed to the elements can vary between I 3 to 25%. A 2.5 m high wall of white deal (Picea abies) would therefore be subject to shrinkage of between 50 and 55 mm when dried down from a moisture content of 25% to I 3%. If no allowances are made for this, there can be major problems Involving the connections with other structural elements like interior walls, which are far less susceptible to movement. The uprights of door frames and columns will account for particular problems, since wood does not expand greatly in the linear direction. The joint between the topmost lintel of the frame and the wall above it must therefore allow adequate space for swelling and shrinkage of the wall. If the wall also supports a floor or roof, it is not possible to incorporate columns into the load-bearing structural system. Although the problem is limited to the initial shrinkage of logs, this effect has to be taken in account in all detailing. Despite this constructional disadvantage, this form of insulation is now common in log buildings.
Tree trunks used in this form of construction can be subject to various degrees of preparation, ranging from merely removing the bark and making a notch on one side, to complete planing, squaring-off and incorporation of tongue-and-groove joints. Walls are stable by virtue of the bracing of the cross-walls, forming corners and junctions.The connections between the walls must be such as to prevent the collapse of one wall when under stress, either externally applied or that within the wood itself. One of the methods applied results in the typical structure that we associate with the stacked log cabin, with the logs crossing each other and protruding at the corners (Figure 8.1 I). If this appearance is undesirable, other measures must be included to ensure stability. Such measures may involve a different design of the connections. The beams are often grooved along the underside to control splitting. When wood dries, it is the outer faces which shrink first. If the outer diameter becomes smaller while the inner core cannot shrink to compensate, there will be splits along the length of the trunk. Some form of profiling can be used to improve the seal between the beams and will also enhance lateral stability.
8.1 I Typical corner details in stacked log construction.
Stress caused by wind is transferred by the logs or beams to the transverse walls. Where the exterior walls are supporting a floor or roof construction, they will be more stable.
Conradin Clavuot designed his School Hall in St Peter; Switzerland, as a stacked log construction (Figure 8.1 2). The logs form walls of up to 36 m in length and 7.5 m in height, supporting both the floor and the roof. A cladding system of horizontal boards has been applied to the exterior and a thermal insulation layer has been put into the cavity.The posts supporting the cladding system are attached to the main inner wall by means of sliding connections (Figure 8.13). Because the interior conditions don't change very much during the use of the building, the wall is not very susceptible to movement. Nevertheless, it was considered necessary to incorporate this system, as the wood lost a considerable quantity of moisture immediately following the construction phase and therefore underwent marked shrinkage. Space has also been allowed for movement above the doors and window frames (Figure 8.14). Because the roof sinks as the load-bearing walls shrink, a malleable seal has been included between the roof and the uppermost part of the exterior cladding.The cladding is of larch sections. The joints between them remain more or less open
8.12 School Hall, Conradin Clavuot, St Peter; Switzerland throughout the year; depending on the weather conditions.
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