Overview

Fig. 39: Timber-frame construction Fig. 40: Balloon frame construction, timber stud construction

Timber frame construction

This traditional method of building with timber, seldom used today, is based on a relatively small module with diagonal braces in the plane of the walls. We see the first signs of prefabrication in this form of construction. The loadbearing and separating functions are united in the same plane within the wall. Assembly of the individual pieces takes place on site storey by storey. The spacing between the individual vertical members depends on the loadbearing capacity of the timber sections which, prior to industrialisation, were cut to size with simple means (saws, axes). The individual connections are not highly stressed and can be in the form of true wood joints (e.g. tenons, halving joints, oblique dados). Vertical loads are transferred directly via the contact faces between the various timber members.

As the cross-sections of the members are often not derived from a structural analysis, in older timber-frame buildings they tend to be too large and hence uneconomic, or are an inevitable consequence of the usually considerable weakening of the cross-section at the joints. Today, mechanical fasteners are therefore preferred in order to achieve a more economic sizing of the sections.

The i nfill panels of historical timber-frame buildings are usually of cob, wattle and daub or clay bricks, with masonry and render in later buildings. Today, the infilling is usually insulating materials with a weatherproof cladding.

Balloon frame construction, timber stud construction

The balloon f rame system widespread in America consists of closely spaced squared sections of standard sizes based on a "2 x 8 inch" module (roughly 5 x 20 cm). When, as a result of a structural analysis, larger cross-sections are called for, these are made by simply nailing several smaller squared sections together. This timber stud construction is nailed together on site and usually extends over two or more storeys. Stability is assured by solid timber boarding or wood-based panels attached diagonally.

The simplicity of the system, in which additional members are often simply nailed to the main framework as required, enables rapid erection with unskilled labour, despite minimum prefabrication. The system is also characterised by a great degree of design freedom regarding plan layout, volume and positioning of openings. Indeed, openings can even be "cut out" subsequently because the construction is oversized. However, this oversizing is a disadvantage compared to newer systems because it leads to high material consumption.

In Europe timber stud construction is the equivalent of the American balloon frame. Timber stud construction also uses closely spaced squared sections of standard sizes extending over two or more storeys. However, there is less standardisation and the connections are not limited to nailing as in the balloon frame - tenons and halving joints are also used. Another aim is a more economic use of material.

Systems

Platform frame construction

Platform frame construction is a further development of timber stud construction. It is distinguished by a high degree of prefabrication and is therefore very popular these days. The loadbearing elements consist of storey-high pre-assembled frames of squared sections braced by flat cladding panels or diagonal boards. Platform f rame construction is based on a small module, although the spacing can be varied as required, e.g. depending on the thermal insulation used (mats or loose fill). The individual loadbearing ribs are assembled in the works and transported to the building site as self-contained elements. On site they are merely erected and clad if necessary. The tectonic structure of platform frame construction is based on the principle of stacking storeys one upon the other.

The advantage of this form of construction is its versatility because it can respond to many different design specifications. Platform frame construction is straightforward and economic because it uses identical timber sections wherever possible, which thanks to their small size are easy and cheap to produce. The simple nailed and screwed connections are another advantage of this system.

Panel construction

The latest development in panel construction is leading to a reversal of the principle of platform frame construction. The loadbearing element is now a slab, no longer a linear member. This slab must exhibit high strength and rigidity in order to achieve a structural plate action. One answer to such requirements is the solid timber panel, which consists of cross-banded plies of sawn timber strips. The addition of transverse ribs made from the same material increases the buckling resistance of such panels. Insulation is placed between the ribs. The planar, non-directional nature of this loadbearing slab results in structural and architectural characteristics hitherto unknown in timber construction. The traditional grid or spacing of loadbear-ing elements is no longer necessary. Openings can be cut almost at random.

The construction principle results in a rationalisation of the layered assembly. Single components can play a multifunctional role, which reduces the number of layers and hence the additive character of the layered assembly. The loadbearing solid timber panel, for example, needs no further surface finish internally, apart from a coat of paint. If the building is to be clad with a uniform outer leaf, this can be attached directly to the sheathing of the wall element.

Log construction

Traditional log construction is the only form of timber construction that also falls under the heading of "solid construction". The building envelope consists of a single leaf of timber members - stacked horizontally and joined by means of cogged joints - that performs the cladding, space-enclosing and loadbearing fractions simultaneously. Stability is achieved through the friction resistance in the bed joints, which leads to the solid timber wall acting as a plate, and through the cogged joints between the timber members at the corners. No mechanical fasteners are required. The possible spans depend on the timber members available, which do not usually exceed 4.5 m.

Log construction leads to substantial shrinkage and settlement movements because the timber members are loaded perpendicular to the grain. Settlement movements must be taken into account in the details, e.g. at window openings. The insulating value of a log building no longer meets modern requirements; contemporary log buildings must therefore be provided with extra i nsulation. This method of construction is only economic in places where the corresponding i nfrastructure (sawmill) and expertise (carpentry skills) are available.

Frame construction

This is the most delicate form of construction in timber. Vertical columns and horizontal j oist floors ("tie beams") or "plates" form the i oadbearing structure (similar to the column-and-slab principle of solid construction). The consistency of the materials used for the vertical and horizontal linear members (sawn timber or glued laminated timber) and the form of the joints determine the spans that can be achieved and the architectural appearance of the loadbearing construction. Besides solid timber, glued laminated timber and other glued structural elements are available these days. The joints usually employ mechanical fasteners such as gusset plates and dowels, the principle of which is similar to structural steelwork. True wood joints are hardly ever used in frame construction.

Stability is achieved through the inclusion of diagonal ties and struts, or wall plates, or solid cores that extend through all storeys.

Frame construction is distinguished from other forms of timber construction by the fact that the l oadbearing structure functions completely independently of the enclosing elements such as partitions or facades (glazing is conceivable). This specialisation of the elements is not very economic in terms of material consumption, but does lead to good flexibility in the internal layout and design of the facade, and enables longer spans.

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