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