Download pdf - CIB-W18

4.9 LOAD DURATION
22-7-4 A J M Leijten
The effect of load on strength of timber joints at high working load level
Introduction
In many publications load duration has received a considerable amount of
attention. Tests have revealed that the strength of timber is time- and load
level dependent. For timber, damage accumulation models have been developed
to describe this phenomenon. A point of discussion is still, whether
the strength decreases in time, independently of the load level, or that
damage occurs after a certain load history. Some theoretical models backup
this last approach. Although the attention is mainly focused on the load
duration properties of timber, tests on timber joints seem to behave in the
same way. In present timber codes long duration factors are used for timber
and timber joints which range from 0,5 to 0,8
The prime objective of the present long duration tests on timber joints
at the Stevin Laboratory is to get more insight at what load level and at
what time damage occur. Although the research program will last another
5 years (started in 1983) already some results have become available and
will be presented in this paper. The results indicate that there is indeed reason
to believe that below the long duration threshold value no damage occurs
and that therefore present long duration values might be to conservative.
Acceptation of the idea that the strength will not be affected by loads
below this threshold level would in its ultimate consequence lead to a total
neglect of the load duration effect.
Tentative Results
In 1986 a number of joints loaded to 50 % were unloaded and tested to reveal
any damage. No significant damage was recorded. In 1988 again a
number of joints which had been loaded for 4,5 years were unloaded and
SSD*-tested. During the first two years these joints were loaded to 40 %
after which the load was raised to 50 % and continued for another 2,5 year.
Prior to the SSD*-tests, the joints have been left unloaded for a 5 months
period to record the creep recovery.
The most important conclusion is that at the present stage of the research
program no strength loss is detected.
*SSD = Standard-Short-Duration
25-7-9 J W G van der Kuilen
Determination of kdef for nailed joints
Summary
In 1962 a comprehensive test program was started at the Stevin laboratory
at Delft University of Technology to study load duration effects in timber
joints. Nailed joints, tooth-plate and split-ring joints were incorporated in
the program. The load levels chosen were 60, 65, 70, 75, 80, 85 and 90 %
of the short duration strength. Meanwhile all specimens failed except the
nailed joints at the 60 and 65 % load levels.
In 1983 the original test program was extended with specimens of the
same type but at lower load levels. The load levels were chosen at 30, 40
and 50 %. The 30 % load level is considered as the level of a service load.
In the meantime a general creep and damage model has been developed
based on reaction equations of plastic deformation in the molecular structure
and on the transformation of stresses to surrounding elastic material.
This model is used to determine parameters and deformation factors.
Conclusions from the study
Creep measurements of nailed joints have been analyzed using the creep
and damage model of Van der Put
The creep results are analyzed as two processes. Both may be described
by a simple In(t) formula with two parameters. The first parameter represents
the steepness of the creep line on a logarithmic time scale. The second
parameter represents the bending point in the creep line approximation.
In the evaluation the sudden increases in deformation which occur in
the first years of the tests are not considered. After longer periods this effect
seems no longer noticeable. It seems clear, however, that the second
process is induced by climatical changes. The changes occur during a distinct
change in relative humidity. This change introduces a change in the
microstructure i.e. the number of flow units or the number of load-bearing
molecular bonds.
The start of the second process depends on the season in which the
creep test was started. Hence, to predict the deformation of a joint over a
number of years may be done with an average delay time. Based on the
above mentioned creep tests, this average delay time may be taken as half
a year or 180 days. The accuracy of the prediction highly depends on the
time of loading.
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.66