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

to draw an approximate value for same from a graph (diagram). The residual
load-bearing capacity of existing, structures or components can be
checked and verified by means of the timber cross sections reduced by d.
In the draft of the new GDR Code "Timber Construction - Loadbearing
Systems" adopting the method of limit states, for timber the most
significant agents have been assigned to ranges (grades) of aggressivity,
and stress degrees have been defined accordingly. Subject to the stress degree
and to the cross-sectional dimension of the timber component concerned,
adaptation factors for the influence of aggressive media have been
fixed.
29-11-1 P Galimard, P Morlier
Load duration effect on structural beams under varying climate influence
of size and shape
Abstract
The Duration of Load effect has been of increasing interest for both the
understanding of wood or wood products in long term behaviour and for
the determination of safety factors for engineering purposes. Many kinds
of testing, mainly in bending, have been made but the large amount of
configurations has always been reduced to few moisture contents and load
histories. However, it is still not easy to compare all the results for the protocol
of testing and presenting the results is not yet a standard.
Different attempts of evaluating the stress level have been done and
their accuracy depends on what is wanted to be shown. The methods are
applied on 2 meter LVL beams subjected to bending in natural environment.
Forty beams are stepwise loaded in an open shed in the Laboratoire
de Rhéologie du Bois de Bordeaux in order to have a failure of 80% of the
beams within two months. The step levels are from 50% to 90% of the average
short term strength. Two beam depths (100 & 150 mm) are tested.
The beams can be sealed or unsealed. The curvature, the time-to-failure
and the climatic conditions are monitored.
A ranking method is used to show the main trends of the duration of
load results: the time-to-failure are not affected by daily or short time variations
of hygrothermal conditions, but by the two month mean variation of
the moisture content.
The long term strength reductions are evaluated and finally compared
to standard (EC5) ones. This work is a part of a European AIR-Project
which has been set up to get experimental data on DOL of LVL and glulam
straight, notched and curved sized beams.
Conclusion
– We have shown, on straight LVL beams that opposite conclusions can
appear from the same data depending on the way of presenting the results.
– However, the influence of the general changing of the climate was
more important than the absolute moisture content.
– The strength reduction estimation has roughly given the same results
from the two tested methods; so #50% beams can be accurately used.
– The short term strength has been influencing the results in a large scale,
because of the moisture content reference and of the definition of the
stress level.
– The duration of load effect is fairly depending on the tested shape: it is
more important on straight beams than on notched beams.
– The EC5 krnod factor, for LVL beams, seems to be optimistic for EC5
Service Class 2 and conservative for EC5 Service Class 3.
A protocol of Duration of Load test and analysis is still needed because of
the numerous testing configurations, the variability of the result analysis,
and the influence of the analysis on the conclusions that can be contradictory.
30-11-1 R H Leicester, C H Wang, M N Nguyen, G C Foliente
Probabilistic design models for the durability of timber constructions
Introduction
The theory of structural engineering for timber construction is now sufficiently
well defined that computational design procedures can be developed
and even codified. By contrast, design for durability is still very
much of an art, with design solutions varying from person to person. More
usually, design for durability is omitted and the control of performance is
undertaken by complying with 'good building practice'.
The key to developing a design procedure is first to have predictive
models for durability. Because of the uncertainties associated with these
models, they will of necessity have to be probabilistic models. Once such
models are available computational design procedures can be developed
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.24