The use of small-diameter timber in construction has been
investigated. The aim of the work is to increase the use
of the wood harvested in forest thinning in construction
applications. The work has covered a wide range of
aspects, from availability of the material to design of
the structures. This publication summarizes the results
in following areas: availability, dimensions and quality
of conifers harvested in forest thinning, cost of
harvesting and woodworking, comparison of drying methods:
seasoning, warm-temperature and high-temperature
kiln-drying, improving durability, strength of round
small-diameter conifers, potential types of structures to
be built from round timber and new mechanical joints.
The tree species that were included in the study are
Scots pine, Norway spruce, Sitka spruce, Larch and
The main reasons why round timber is rarely used in
construction can be summarized as: the material is not
available via the normal commercial routes, the roundness
requires special methods and systems that are not known
by architects and carpenters, the strength values of
timber connections are not available for engineers, the
lack of standards and models.
This research aims to produce information needed in the
use of small roundwood in load-bearing structures in
order to remove the obstacles mentioned above.
Results concerning availability of construction-quality
round timber in the first commercial thinning reveal that
the resource itself is vast: millions of cubic meters in
Finland alone. The yield per hectare is, however, limited
and dependent on the dimensions required. When the
diameter of the final product is adequate at less than
100 mm, the first commercial thinning is also economic
for the harvesting of construction timber. The economics
of manual and mechanical harvesting have been compared.
When larger dimensions are needed, the second thinning is
more likely to produce the required material.
The cost of producting round timber is primarily
dependent on the surface quality needed: timber peeled
cylindrical is twice as expensive as material that is
only debarked. Both of these have their own market. The
cost of construction is dependent on labour costs, which
at the moment is higher for round timber than for sawn
timber because conventional systems are not suited for
the use of round timber. Additional costs may arise from
the deviations of cylindrical form.
Drying is a critical phase of production, which
determines how much checking is observed. In this
respect, high-temperature drying gives much better
quality than normal, commercial warm-temperature
kiln-drying or natural seasoning. Accordingly, the drying
method should be chosen based on the surface
requirements. End-cracking also affects the capacity of
The strength of small-diameter timber was observed to be
higher than expected. Characteristic values are presented
as well as a proposal for visual strength-grading. A
method for non-destructive mechanical strength-grading
based on X-ray is also proposed. A statistical analysis
is presented which indicates the dependence of strength
and stiffness on different factors such as density,
knots, moisture content, diameter and age.
New mechanical connections have been designed and tested.
For engineered structures, a round form enables the use
of steel lacing around the wood, which considerably
increases the load capacity of the joints.
The largest quantities of round timber are used, and can
be used, in non-structural applications and in small,
traditional-type buildings. Smaller in volume but
important for the image of roundwood is its application
in the architecture of medium-sized leisure industry
buildings in which the load-bearing structure is visible.
As part of the project, designs for a footbridge and a
watchtower have been made.
|Place of Publication||Espoo|
|Publisher||VTT Technical Research Centre of Finland|
|Number of pages||210|
|Publication status||Published - 1999|
|MoE publication type||D4 Published development or research report or study|
- structural timber
- wooden structures