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Simple Truck Load Testing as part of
Timber Bridge Appraisal Methods Introduction Timber bridges in Tasmania are built using timber of only moderate durability. As a consequence, their structural capacity reduces with time due to decay processes. Timber bridge inspection to determine the structural condition of bridges, and to rate their load carrying capacity, is an essential part of timber bridge management. Traditional inspection methods use visual examination, soundings, and boring to rate the condition of bridge elements. Traditional inspection methods, when performed by experienced person, is very reliable. Most bridge elements, particularly beams and pile, can be rated using traditional methods. However, some defects that affect structural capacity are "invisible" to traditional methods, and the use of simple truck load testing is proposed as an adjunct to the traditional methods to improve the reliability of timber bridge inspection and capacity rating. The following truck load testing methods are applied only to timber bridge beam testing. Loss of
Strength of Beam Natural round bridge beams are supplied green and have very high moisture contents. In service the beams will dry, particularly the outer layer. Shrinkage of the timber is associated with this drying. The differential shrinkage and growth stresses in the log cause checks to occur. The check pattern due to drying in the section of natural rounds is shown in Figure 1. A natural round will check in random way, as shown in Figure 1(a) and a trimmed section as in Figure 1(b).  The effect of checks is to reduce the shear transfer area to small regions at the root of the checks, with the shear transfer occurring by "fibre inter-lock" as shown in Figure 2. The checked, but not decayed, log has a reduced shear strength but sufficient for normal beam action to continue.  In bridges the checks quickly become contaminated with soil, dust and hold water, an ideal incubator for fungal decay to develop. Decay organisms need a food source (wood), water, oxygen and favourable temperatures for decay to occur, all of these conditions occur at the root of seasoning checks. The shear transfer fibres at the root of checks are attacked by decay much more quickly than wood elsewhere in the log section. The consequences is a reduction in the shear strength of logs to the point where the beam ceases to behave as an integral whole but rather as a series of loosely coupled sections. The life cycle of a natural round bridge beam is shown in Figure 3. with the structural capacity dictated by compression failure for very young beams through to shear failure for old beams.  Structural Consequences of Deterioration of Log
Beams The continuous loss of strength of beams presents a challenge for the inspection process, when is a beam(or bridge) just unsafe to be in service ? Considering the simple case of a log checked at its mid-depth, as shown in Figure 4. The change in the strength and stiffness can be simply calculated on the assumption that the beam halves (a) work together and (b) do not work together, as shown. The loss of strength and stiffness is significant. The sinister aspect of this type of failure is that the checks, a natural feature of timber, may not be "seen" by the traditional inspection methods, but are readily detected by load testing due to their loss of stiffness.  The
Simple Truck Load Test A twenty tonne conventional gravel truck, with weighbridge ticket, is parked on the bridge at mid span in three positions. Upstream, centre and downstream positions as shown in Figure 5. The deflection of the beams at mid span and also near to the supports is measured by suitable methods. The simple stick-and-tape method is shown in Figure 6 is often sufficient.   The presence of a single beam with low stiffness in a bridge is shown in Figure 7. The change in the deflection profile is a good indicator of the presence of a more flexible beam present in a bridge, higher flexibility being indicative of shear failure. The magnitude of the deflection in the bridge is also indicative of the "health" of the bridge and is also useful information. General deterioration of a bridge is associated with reduced stiffness. Deflection ratios (span/deflection ) of 1 in 500 is indicative of a good structure. 1 in 300 is indicative of a bridge near to the end of its service life, and 1 in 250 or more indicates a bridge in advanced decay.  Conclusions Simple truck load testing is a very useful method for use in the appraisal of the load capacity of timber bridges. The load testing should always be supplementary to a detailed inspection by traditional methods and not used to replace such inspections. The management of timber bridges should be by proven inspection methods supplemented by truck load testing, particularly for bridges in the later stages of their service life.
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Timber Research
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