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

2.7 LOAD DURATION
6-9-3 T Feldborg, M Johansen
Deflection of trussed rafters under alternating loading during a year
See 3.19 Trussed rafters
19-1-1 R O Foschi, Z C Yao
Duration of load effects and reliability based design (single member)
Introduction
The introduction of reliability-based design in wood structures requires
consideration of the effects that load duration has on degradation of
strength over time. Normally, when no duration of load effects are present
(as in the case of steel structures), the estimation of reliability over the service
life requires only probabilistic information on the material's shortterm
strength and on the maximum load which the structure would receive
over the life period considered.
When strength degradation is present, the reliability estimation is complicated
by the fact that load history must be taken into account, considering
not only load magnitudes but also the duration of each load period. Of
course, central to the solution of the problem is the availability of degradation
or "duration of load" model which would allow the estimation of the
degradation effect for any load sequence. The authors, in a separate paper,
discuss different duration of load models, their relative advantages and
shortcomings, and their ability to represent experimental trends for constant
loads (Paper 19-9-1).
A second consideration in reliability estimation is the procedure chosen
for the calculations. Simulation is a straightforward procedure which can
account for the variability in the material as well as the possible loading
sequences. This procedure, however, is normally tedious and expensive.
Calculation of reliability indices based on algorithms like Rackwitz-
Fiessler's should be preferred as they are much faster and, in general,
equally accurate. However, application of such algorithm presents special
problems in the case of interaction between strength and load sequence.
Finally, reliability calculations for wood structures must take into account
the behaviour of the structural system rather than only that of a sin-
gle member. In fact, the usually high variability in single member properties
is compensated by the action of a redundant system, and consideration
of such action is the only way in which a realistic reliability assessment
can be made for structural applications of wood.
An example of reliability estimations for timber systems has been presented
by Foschi. Nevertheless, many codes are based on "single member"
design, with "load sharing" factors applied to take into account the added
reliability of a system. In this context, reliability of single members is a
first step in the consideration of the more general problem.
The objective of this paper is the discussion of single member reliability
in bending, using the Canadian damage accumulation model, and the
development of techniques based on the Rackwitz-Fiessler algorithm for
the reliability index.
19-6-1 J Kuipers
Effect of age and/or load on timber strength
Introduction
During many years much attention has been paid at many places in the
world on the effects of long-duration-loading on timber. Most of the investigations
on the subject dealt with the measurement of time-to-failure of
test specimens under constant loads of relatively high levels. Such investigations
suggested that a long-duration-strength of about 55 % of the
short-duration-strength should be used to determine safe working stresses.
Such working stresses for permanent loading on structures, built for an intended
lifetime of 50 to 100 years turn out to be on a level of, very roughly,
25 % (to 35 % for joints) of the short-duration-strength values. The
question arises if such low load levels do have a comparable damaging effect
on the initial strength or that they, being so low, do not have any
strength decreasing effects at all.
J Vermeyden comes to the conclusion that not much research had been
carried out to study the effect of long-duration of loading on the remaining
strength of timber structural elements. The available information deals
with the effect of repeated loading and of relative short duration of constant
loads, sometimes to high load levels, but not with the effect of longduration-loading
to about the allowable levels. Nevertheless Vermeyden
comes to the expectation that such preloading very probably will not have
a significant strength reducing effect.
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.57