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Performance of Timber Beam Bridges in
Tasmania, Australia.
Peter J. Yttrup, Gregory Nolan,
University of Tasmania,
Australia.
Abstract
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Introduction | Form of
Tasmanian Bridges | Life Cycle | Traditional
Inspection and rating
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Investigation
Methodology | Conclusions | References |
Timber beam bridges are used extensively for road and
rail bridges in Tasmania. Road, rail and forestry bridges
number about 4,000 in the State. The investigation reported
in this paper has field load tested about 50 bridges and
tested to failure under laboratory conditions about 200
bridge beams. The investigation demonstrates that modern
structural analysis methods do apply to traditional timber
bridges, and preliminary indications suggest many road
bridges are kept in service for too long, potentially
compromising safety.
Introduction
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Abstract | Form of
Tasmanian Bridges | Life Cycle | Traditional
Inspection and rating
|
Investigation
Methodology | Conclusions | References |
Tasmania is the smallest State in Australia but is
rich in forest resources. Because of the low cost and
availability of native hardwoods, principally eucalypts,
these have been used extensively for bridge construction in
the past and currently. The existing road network has about
3500 bridges, constructed with timber, servicing the State
highways, rural roads and forestry access roads.
In Tasmania, like elsewhere in Australia and the world,
bridge engineers have focussed on steel and concrete as the
premier materials with timber bridges considered prime
targets for replacement. Typically, the management of timber
bridges, including maintenance, inspection, load rating and
replacement was delegated to skilled inspectors or bridge
foremen. Even where timber bridges are a major part of the
total stock of bridges they seem to have lost appeal to the
structural bridge engineer.
Recent political changes have forced asset management
techniques on to State and local authorities. Asset managers
usually have no allegiance to concrete, steel or timber,
their concern is with managing the existing assets
effectively. The predominance of timber bridges in some
areas, even at State level, has focussed attention on this
asset, not as a target for replacement but for proper
management. The old statement "itís a timber bridge
and must be replaced", is increasingly being confronted with
the question "why?".
The questions asked by asset managers in their attempt to
manage the existing stock of bridges are, "is it safe?";
"what is the remaining service life?"; "what is the current
value of the bridge?; and "what will the replacement costs
be?". The attention has been shifted from the material of
construction to performance and fitness for service
issues.
The timber bridge research project conducted at the
University of Tasmania has investigated the structural
behaviour and strength of timber beam bridges. Truck load
testing, element testing from dismantled bridges and
synthetic load testing of computer models of bridges have
been employed. The following paper gives an overview of this
work.
Form of Tasmanian Timber Beam Bridges
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Abstract | Introduction | Life
Cycle | Traditional
Inspection and rating
|
Investigation
Methodology | Conclusions | References |
Timber beam bridges have been used in most States of
Australia, and although similar, do vary in some details
from State to State. The typical timber beam bridge, as used
in Tasmania, is shown in Figure 1. The timbers used are
native hardwoods which in Tasmania are not as hard, strong
nor durable as those found on mainland Australia.
The combination of low durability timber, high rainfall, and
design details that create water and dirt traps, cause
biodegradation and maintenance problems and shorter service
life. The high shrinkage of the hardwoods can also cause
maintenance problems.
Figure 1 - Tasmanian Timber Beam Bridge.
Figure 2 - Life Cycle of Timber
Bridge.
Life Cycle of Timber Beam Bridges
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Abstract | Introduction | Form of
Tasmanian Bridges | Traditional
Inspection and rating
|
Investigation
Methodology | Conclusions | References |
The "rule of thumb" in Tasmania is for a service life
of 5, 10, 20 and 40 years for running planks, deck, beams
and piles respectively. This life cycle is shown in Figure
2. Although the capital cost of timber bridges is relatively
low their maintenance cost is high.
A relevant question asked by bridge asset managers is "when
is the structure just safe and in need of replacement?"
Traditional Timber Bridge Inspection and Capacity
Rating
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Abstract | Introduction | Form of
Tasmanian Bridges | Life Cycle | Investigation
Methodology | Conclusions | References |
The 5 : 10 : 20 : 40 year rule mentioned above is not
formalised, but does appear to influence bridge appraisal
work. Timber beam bridges are inspected periodically to
certify the bridge capacity, or if necessary, to impose load
limits or plan other remedial works. Bridge inspection
relies heavily on visual cue such as fruiting bodies or
stains associated with wood decay, or distortions to the
form of beams indicating internal collapse often associated
with advanced decay. Boring to measure the extent of decay
and depths of sound wood are also common procedures. The
limitations of such methods are several, as discussed by
Yttrup and Law (1991).
The current method of capacity rating of a timber bridge is
more an art than a science, and is based on experienced
inspectors. The procedure is largely intuitive and not based
on engineering principles.
The demand for better capacity rating and appraisal of
existing timber bridges by asset managers, combined with
declining availability of skilled inspectors, is a challenge
for engineers.
Timber Bridge Investigation Methodology
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Abstract | Introduction | Form of
Tasmanian Bridges | Life Cycle | Traditional
Inspection and rating
|
Conclusions | References |
When bridges are due for replacement, or become
redundant by road realignment, they are considered for
further investigation in the University of Tasmania timber
bridge research project. This investigation will typically
consist of a "behaviour load test", using a gravel truck for
load, prior to demolition, with recovery of all components
for laboratory structural testing and analysis by computer
models to predict the capacity of the bridge.
Truck Load Testing
The deflection response of the bridge beams is measured
using a 20 tonne truck load. The 20 tonne, dual axle, gravel
truck is used because it is readily available. The test
vehicle is weighted to determine actual axle loads. The
truck is placed at three positions across the deck at the
mid-point of each span of the bridge. The three positions
are down stream, central and up stream. The deflection of
all the beams is measured at midspan and near to the face of
the supports, usually by simple optical methods. This
process is quick, taking less than one hour per span.
Beam Testing
The beams from the bridge are recovered at demolition
and transported to the University of Tasmania laboratories.
Each beam is then load tested to determine stiffness and
strength in a 100 tonne purpose built test rig. The beams
are supported near their ends on sound wood and loaded by a
single mid span point load. Figure 3 shows some typical load
deflection results.
The nominal modulus of elasticity (M of E) and modulus of
rupture (M of R) for 95 beams tests are shown in Figure 4.
The term "nominal" is used for M of E and M of R because the
beam sectional properties used are the external gross
dimension, with no reductions for decayed wood areas.
The plot of results in Figure 4 has significant scatter, but
a crude relationship still exists between strength and
stiffness. The "ductility" of the beams seen in the load
deflection plots in Figure 3 is a characteristic of natural
round hardwood with large deformation occurring at
essentially constant load. Such ductility is not typical for
manufactured timber beams that are very brittle.
The load redistribution that can occur in timber beam
bridges due to the element ductility is potentially
significant and very beneficial. The very low number of
bridge failures that have occurred, even where beams are
broken, is possibly contributed to by this ductility.
Figure 3 - Typical load
deflection results.
Figure 4 - M of E versus M of R
test results.
Computer Model
The timber beam bridge computer model as described by
Yttrup and Evans (1992), is used for structures like the
Tasmanian bridges. This model is calibrated with the
stiffness and strength as measured from the actual
elements.
The computer model represents the "re-assembled" bridge
which is loaded with the "same" truck loads as used in the
field. The deflected shape of the bridge measured in the
actual load tests are compared with the models predictions.
A set of results are shown in Figure 5, and generally good
agreement is achieved.
Having reconciled the actual with the predicted bridge
deflection behaviour, the model is used to "load test" the
bridge to estimate the ultimate strength of the bridge. The
simplest definition of ultimate bridge strength is that load
which first causes an element action to equal its ultimate
strength. Some typical results are presented in Table I for
a gravel truck loading pattern. Any truck load configuration
and placement can be used, as well as investigation of the
post failure behaviour of the bridge due to individual
element failure.
The gravel truck produces an action effect on a typical
timber bridge of about 80% of the T44 bridge design load.
The result in Table I demonstrates that these bridges have
very low strength.
Figure 5 - Deflected shape
comparison
Table I
Conclusions
|
Abstract | Introduction | Form of
Tasmanian Bridges | Life Cycle | Traditional
Inspection and rating
|
Investigation
Methodology | References |
The indication to date is that timber beam bridges in
Tasmania have a low "factor-of-safety" when approaching the
end of their service life. The behaviour of drivers of heavy
trucks to choose a central path when crossing old timber
bridges, that is, the path producing the lowest action
effects in the bridge elements, probably contributes to the
low occurrence of significant failures. Also, the load
redistribution and sharing that is possible due to the
ductility of natural round hardwood bridges beams is also
significant in explaining the low failure rate.
The "behaviour load test" using a gravel truck can be used
to indicate the condition of a bridge, and change in
condition, by the "deflection signature". Also, broken or
severely decayed beams can be seen as an anomaly in the
deflection profile of the bridge under test load.Traditional
bridge inspection can be enhanced by the diagnostic
potential of the load test data.
The traditional timber beam bridge, as used in Tasmania,
will probably be replaced by other forms of construction in
the future other than in historical or National Park
settings. The new generation of concrete deck and timber
beam composite construction will possibly be used
extensively in the future, as these bridges have low cost,
like the traditional timber bridge, but have much longer
life and very low maintenance.
References
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Abstract | Introduction | Form of
Tasmanian Bridges | Life Cycle | Traditional
Inspection and rating
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Investigation
Methodology | Conclusions |
Yttrup P.J., Evans T.D., 1992. The Development of a
Computer Model for a Corbelled Timber Beam Road Bridge. In:
Proceedings, Timber Bridges Conference, The University of
Melbourne, Australia, November 4th - 6th. PP 67-77.
Yttrup P.J., Law P.W., 1991. The Durability and Structural
Performance of Timber Railway and Highway Bridges in Eastern
Australia. In : Proceedings, 1991 International Timber
Engineering Conference, London U.K., September 1991, Vol. 3,
PP 311-318.
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