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

site that the test material sampled is representative for that population.
In the field of timber engineering various questions can arise, that presume
different test material selection procedures. Among these are e.g.:
– Evaluation of characteristic mechanical properties related to a population
defined in space and time. This requires sampling of specimens
that are representative for the defined population.
– Evaluation of the effect of specific treatments on characteristic mechanical
properties.This may require so-called matched sub samples,
that are selected from a given sample so that they have the same distribution
of mechanical properties.
– Evaluation of mechanical properties of timber structures and of joints
made with mechanical fasteners.
In general this requires selecting timber whose growth characteristics vary
only in predetermined narrow limits.
In the following, sampling or selection procedures are described for the
aforementioned points under discussion.
These sampling procedures are generally applicable. They do not depend
upon the planned sample size nor on whether all elements of the
sample or, as in the so-called In-Grade Testing Programs, only a part of
the sample will be destructively tested.
Decisions on sample size and number of elements tested destructively
are governed by the financial scope of the program, the chosen degree of
precision with which the properties are to be estimated and the chosen test
and data analysis procedures. Some relations between sample size and the
precision of the statistical inference are shown as general information.
19-17-1 R H Leicester
Load factors for proof and prototype testing
Summary
Load factors for proof and prototype testing are derived by calibration
with existing structural design codes. This calibration is done on an element
by element basis, even though the testing is in general applied to
multiple member structures.
The load factors take into account uncertainties of loads and strength,
duration of load effects and the test configuration.
Concluding comment
The procedure developed herein is based on selecting a test load in the
format given by equation (1).
To do this, each structural element in turn is examined and the appropriate
load factors kc, kD and kU derived; the largest multiple factor kC, kD,
kU obtained is the appropriate one to be used in equation (1). For structural
acceptance, the test load must be carried without distress by a proof tested
structure or by every tested structure in a sample of prototypes.
It is outside the scope of this paper to discuss the use of proof and prototype
testing as a basis for acceptance. However it may be worth mentioning
that in general the use of a pure prototype testing procedure is inefficient
when applied to timber structures. This is because the high variability
of some timber elements, together with the small sample sizes tested
means that a large margin of safety must be used to allow for the fact that
the structures tested may be stronger than average.
Usually a prototype test of a timber structure is undertaken when there
is some uncertainty concerning a particular failure mode, such as that associated
with a complex joint. If this is the case, then it is acceptable to
consider only the load factors relevant to that failure mode or to reinforce
the structure in such a way that failure can occur only in the structural
mode under investigation; in such an event, the acceptance of the remainder
of the structure must be based on conventional design computation
procedures.
19-6-2 R H Leicester
Confidence in estimates of characteristic values
Abstract
This paper describes three algorithms that may be used to estimate five
percentile values with 75 percent confidence. A method for assessing the
effectiveness of estimating algorithms is described. The suitability of using
a 75 percent confidence level is discussed.
Conclusions
A method has been proposed for comparing the effectiveness of estimating
algorithms, and has been applied to three such algorithms. The use of a
very simple estimator algorithm was found to give reasonable results and
hence in practice the use of more complex algorithms may not be warrant-
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.74