Papers and Abstracts page

The Use of Timber as a Construction Material -
The Only Chance to Save
the Forests of the World

J. Natterer

The chances of timber engineering and of the use of timber as load-bearing material depend on the quality criteria. The most important factor is the economical use of timber in order to increase its utilization in construction and make it competitive in comparison to other constructional materials. Timber as a material for supporting systems is an indicative orientation for the use of wood in further constructions. Therefore, all kinds of wood and timber, from roundwood to squared timber through composite sections made of boards and squared timber, as well as glue-laminated timber, plywood, etc., especially when combined with other materials, must be developed and employed.

Timber Quality Wood Sampling

The modulus of rupture of massive wood, like round wood, squared timber, block boards, as well as gluelaminated timber, plywood and
other wood materials have different dispersions. The influences on the resistances mostly depend upon the raw density, the nodosity, and further upon the section form, fibre obliqueness, etc., as well as upon other manufacturing criteria like moist, or the use of finger joints to prevent fissuring, etc.

The reliability of optical sampling methods does not correspond to the degree of accuracy presupposed by the engineer's calculations or calculation models. The assumptions of modulus of elasticity and rupture, respectively the admitted stresses, are dispersing between 100 and 200 per cent, while an exceeding stress of 3 per cent measured in a static test already leads to a conflict with the expert. The five-percent fractile given by the statistics, i.e. the five weakest from one hundred which the carpenter is allowed to build in at the most solicited points of the construction, leads to an uneconomical exploitation of available qualities in the very large general dispersion of timber, when compared with other construction materials.

In order to take advantage of better qualities, one needs only to build in the best pieces of timber in the most solicited construction elements. This requires the admission of non-destructive testing equipments which can, like the Sylvatest® ultrasound method, determine single resistances much more precisely; this goes for the elasticity modulus as well as for the modulus of rupture. This testing equipment can be used for roundwood as well as for squared or glued timber. It is also very useful for reconstructions and renovations of old buildings. The nondestructive testing method with ultrasound is very suitable for the latter, as it proves the resistance slope given to the aging of old constructions.

Material Quality - Material Selection

The manifold material values can also be expanded with the material selection. The selection between different materials like roundwood, squared wood, sawnwood or gluedlaminated timber which has been improved through
industrial methods, or different sorts of plywood, provides different resistance qualities
with economical and competitive construction possibilities.

Material Quality - Profiles

Wood sections are a further quality to take into account beside resistance criteria when using round- or sawnwood. The treatment preceding the drying and the considering of the extension of the year rings in different forms of sections, as well as the profiles of the wood sections are most important for an economical formation of detail in the constructive use of timber.

Assembling Techniques
- Assembled Sections

The many types of material and section forms must be used with new assembling techniques in order to manufacture larger sections. There are many examples of an economical use of roundwood, sawn roundwood, squared timber, profiled sections of squared timber as well as assembled joists, and they will open further economical utilization of raw wood in the future.

Techniques and Means of Assembling

New highly efficient means of assembling, i.e. connections with lower section weakenings and needs of steal, have to be developed for a highest possible degree of pre-fabrication in the workshop and in order to reduce the working time on the site as much as possible.

The use of new connecting systems like nailed tinplates, screws, lag bolts as well as connectors with wood contact allow a much higher quality of more filigree supporting systems when linked with deterministic non-destructive
testing methods in order to avoid sporadic problems, which arise in highly stressed construction elements. Connectors combining fiberglass and mechanical fasteners allow also a noticeable increase in the load capacity,
as shown in a recent study on fiberglass reinforced timber joints.

In order to give timber a new chance as a construction material, the different research development and marketing programs should not aim at the quantity of material used, but at the manifold quality of material steadiness, section variability, material diversity as well as facilitated construction control and quick usage of the new techniques in timber engineering construction.

Composite Systems

In the history of timber construction, there have always been composite constructions timber frameworks with glue or mortar, walls of stone and bricks - the most lasting were in timber architecture. Examples from China and Japan, to Frank and Alsacian framework construction are well known. Essential criteria are the better behaviour of the whole construction during a fire, as well as acoustics and vibration properties.

Today, quality criteria - fire, acoustics, vibration - are easily fulfilled through new shape applications, i.e. massive nail laminated floors and wood-concrete composite systems for widespan and load supporting structures.

Nail laminated decks and wood-concrete decks including a load-bearing concrete slab present new advantages, especially for houses, schools and public buildings. With these techniques the steadiness and bending properties of structures with minor dead loads can be fulfilled economically. Fire resistance times of F30, F60 or F90 AB, as well as phonic insulation criteria up to 60 dB for walls and decks can be reached.

Planification Criteria for Timber as LoadBearing Material

Timber as load-bearing material has only a chance if the conception of the construction can show a quality which is not only functional, technical or architectural, but which can also justify its economy. This presupposes, how-ever, a more important planification and a better cooperation between architect and engineer, in order to make the most of the diversity of forms, structures and techniques applicable to timber.

It is essential to define clearly the quality criteria of a timber construction and to aim at reading easily the force and load fluctuations, and reducing the material through load- and detailplanification with an optimally functional adaptation to technique and construction.

Building Shape

The first planification criterion of a timber construction is its shape. The many design possibilities given by the easy manipulation and the low dead loads of timber, especially for houses, halls and roof shapes, are well known. The links between building shape, construction support, energy requirements and costs of maintenance are simple, but do not go without planification.

Building Shape and Maintenance

General knowledge of timber constructions being superficially planed in terms of material quality and use of material, and badly executed, leads to an image of high maintenance costs concerning timber constructions. That this is not the way it has to be is proven by timber structures in roofs, bridges and frameworks dating from the Middle Ages.

Nevertheless and in spite of the good experiences made with historical constructions, planification of new timber constructions must not lead to nostalgic, history or museum oriented building or roof shapes.

Building Shapes and Energy Requirements

In comparison with other construction materials, using timber is already energy saving.
Further energy saving measures like the creation of mid-temperate zones in timber-glass
constructions have an important impact on the shape and allow a diversity of forms. Integration of active solar techniques can be fulfilled in a satisfactory way only with special planification.

Further criteria concerning energy saving are lighting, ventilation and heating. They can be estimated in different ways, but must be considered first.

Supporting Structures

In timber construction, each building shape can be constructed economically when the supporting structure is developed adequately in the section, whereas the structure of the inner space as well as the direction of load transmission, and main and secondary support systems have to be considered in the design. Optimization of the supporting structure through reducing of flexion for the benefit of normal forces needs a special planification.

Further reduction of the material needed in timber construction can be achieved by using compressed frameworks, with the advantages of simple contact connectors. Frameworks and bracing structures can be designed in many ways, and are filling the inner space at the same time. When left visible they are part of the inner space arrangement together with the supporting structure.

A further step towards optimization, respectively minimization of the material used, are the statistically undetermined systems. New developments are made into this direction, whereby the dispersions of material are balanced through static uncertainties and the deformation behaviour becomes decisive for the dimensioning. This way, the construction of orthotrope systems made of roundwood, squared timber, beams or glue-laminated timber in association with concrete becomes economical.

Why Timber Constructions?

Without forest economy, the costs of ecological challenges cannot be coped with. The use of timber as construction material is the only chance to save the world's forests. The use of timber is directly linked to forest conservation and the planting of new trees.

The resistance of forests against wind, snow, frost, drought, insects, and fungus, and of course pollution through emissions, cannot be maintained or improved by only financial support and environmental protection actions.

Furthermore, the different functions of forests, like protection against avalanches, landslide erosion, flood, fire, etc., or the role of forests as life space for plants and animals, as well as relaxing areas for forest visitors and tourists - especially when in urban neighbourhood - cannot be covered with the sole incomes obtained through the sale of fuelwood. Lacking maintenance reduces the forest's health and increases the damages caused to forests.

Two hundred years ago, forestry was financed through the use of timber, not only in rural zones, but especially in the cities where all houses - up to 10 storeys - were built with timber joist floors and wooden roofs.

The role the forests of the future will have to play for mankind and environment cannot be assured only through environmental protection - as little as the role of future cities can be granted through the sole protection of
monuments.

Therefore, given the constant decrease in reserves of fossil energy and raw material, the importance of the role of forests as suppliers of timber, respectively raw material, will grow significantly in the future.

New Techniques in Timber Construction

The increasing use of timber in the construction depends on engineering developments of timber as a load-bearing material, in order to raise the modest portion of the total construction volume from about 1 per cent to perhaps 2 or 3 per cent.

The criteria of development are: better evaluation of the timber quality, increase of the diversity and better treatment of material varieties, development of new time-sparing assembling techniques which allow the highest possible degree of pre-fabrication.

Quantity related techniques for floors, walls and roofs of the dense housing and public buildings linked with other massive construction materials, as well as quality related high-tech systems, which play a significant role in the modern architecture of roof, hall and bridge construction, should correct the image of timber and offer a competitive alternative to other materials used in construction.

The material selection is no proof for "good architecture". It is, however, an important contribution to the environmental conservation, even if it needs more concentration on the planing phase.

References

Natterer, J., Herzog, Th., Volz, M., 1991: Holzbauatlas Zwei. Hrsg. Arbeitsgemeinschaft Holz e.V. und Inst. f. int. Architektur-Dokumentation. D-München.

OE Nov 21, 1996