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