CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL ...

of moisture and creep are added, a normal design procedure for structures
with a long life expectancy, more expensive material combinations or
connections will probably make the design much too conservative.
When a steel sheet for example is rigidly connected with timber beams,
it would perhaps be reasonable to use smaller reduction values of the
modulus of elasticity (kcreep) due to creep for these beams than the values
obtained from the codes. Another solution could be to use some sort of reduction
factor for the calculated deflection of built-up structures in Serviceability
Limit State Design which takes account of various material
combinations and durable connections.
In order to promote the use of built-up or composite wood-based structures
with more rigid and durable connections (which are often more expensive)
in future, we should find a way of producing designs on the conservative
side and of taking account of various material combinations and
various connections at the same time.
26-9-1 S Thelandersson, J Nordh, T Nordh, S Sandahl
Long term deformations in wood based panels under natural climate
conditions. a comparative study
Introduction
Most long term studies of the behaviour of wood based panel products has
been performed under controlled moisture conditions, mainly with constant
relative humidity. In practice, the relative humidity is always more or
less variable. For this reason, the relative ranking in design of the materials
with respect to creep factors and moisture sensitivity might not reflect the
performance in practice in an adequate way. The objective of the investigation
reported here was to study the relative performance of some wood
based materials under rather humid and variable conditions. To this end,
comparative long term tests were performed for a number of panel products
exposed to the same natural conditions. The design codes considered
in the analysis of results given in this paper are Eurocode 5 and the Swedish
building code.
Summary and conclusions
The following main conclusions can be drawn from the investigation so
far:
1. The relative ranking between the tested materials used in Eurocode 5
and in the Swedish building code is reasonably correct with regard to
creep factors.
2. The creep factors specified in the codes seem to be somewhat underestimated
for all structurally classified materials considered in the investigation.
3. The rate of creep deflection for materials with a high degree of processing
such as hardboard, MDF and particleboard is markedly higher
during wet periods than under dry periods.
4. For wood the rate of creep is largest during dry (or drying) periods.
During wetting periods the wood beams exhibit a spring back i. e. the
beams rise against the load.
5. Plywood and OSB exhibit similar behaviour as wood, but the rate of
creep is less dependent on humidity changes.
The tests will continue for at least another one year period. This may give
further experience, which could modify the above preliminary conclusions.
28-9-1 R Gupta, R Shen
Evaluation of creep behavior of structural lumber in natural environment
Abstract
In order to describe creep behavior of structural lumber in natural environment,
a bending test with twenty Douglas-Fir beams subjected to a
constant load was conducted under an open shed in the Forest Research
Laboratory at Oregon State University. Deflections of the beams were
measured along with daily fluctuations in temperature and relative humidity.
An existing five-element creep model was used to fit the experimental
data. The five-element model did not describe creep behavior of structural
lumber in natural environment. The general observations show that stiffness
of the beams has strong influence on magnitude of creep strain, and
the creep strain closely follows the fluctuations of air temperature. A fourelement
model, including the stiffness and air temperature effects, has
been developed. The model fits the experimental data very well.
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.64