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Concrete and Timber Composite
Construction for Enhanced Strength, Stiffness and Service
Life for Timber Bridges Introduction. The benefit of having a concrete deck has been demonstrated in paper No. 2 by Greg Nolan. From investigation of existing timber bridges with concrete decks it has been demonstrated that the timber beams can be effectively protected from rainwater and as a consequence there is much less decay. It can be concluded that the life expectancy of concrete decked bridges can be two or three times longer than timber decked bridges, that is 50 to 75 years, making such bridges "permanent" rather than "temporary" construction. The benefit of composite action between deck and supporting beams are well know and exploited in concrete and steel construction. The benefits of composite action in concrete decked timber bridges are significant. Composite concrete and timber bridges have been built in the USA and in New Zealand for many years and more recently in Australia. Having demonstrated the effective protection provided to timber bridges by a concrete deck the use of composite construction would seem to follow logically.The following paper presents some results from experimental research into composite construction with manufactured beams and unseasoned natural round timber beams. Composite
Concrete-Timber Systems Investigated. Development of the concrete-timber bridge superstructure system in the RTA of NSW using natural round timber beams cast with a concrete deck to form precast units, has been adapted for Tasmania and is available from Tas Span. The composite concrete-timber systems tested at the Centre for Building With Wood are summarised in Figure 1.  The LVL-beam systems could also be glue laminated softwood or hardwood. This system is considered appropriate for road and foot bridges. The log-deck system developed was developed for low cost forestry road applications, but is also appropriate for road bridges in other settings. Shear Transfer System
Investigated. The LVL-beam system has been tested with no active shear coupling relying only on friction. The use of "shear dimples" to improve the shear transfer was also investigated. The philosophy behind the small dimples is to transfer the shear forces over as large an area as possible, keeping in mind that wood is a relatively soft material with respect to connections. Figure 2 shows the dimples system tested.  The log-deck systems have been constructed and tested with no connectors and the shear connector system shown in Figure 3.  Testing Set-Up and
Results. All the specimens were tested with a central point load, which is a severe test configuration, but not inappropriate for short road bridges. The test results are presented in Figure 4 for the LVL beams with no shear connectors, and Figure 5 with dimple connectors. The log-deck results are presented in Figure 6.    The strain profile measured in Figure 7 are typical of the test series where active shear connectors are provided. The embedment of the LVL into the concrete deck was sufficient connection to produce significant composite action as seen in Figure 7. The inclusion of the shear dimple connectors in the LVL increased the ultimate load by about 250 % demonstrating the effectiveness of such simple shear connector devices.  Discussion of
Results. The test series demonstrate the effectiveness of simple shear connectors in producing composite action. For the log-deck tests the results show no significant contribution from the concrete deck unless active shear connection devices were present. The most effective method was chain-saw dovetail notches, and steel dowel pins with a 75 mm hole as a shear dimple. Both these connector systems produced composite action with stiffness about three times greater than the log stiffness alone. Conclusions. The combination of protecting timber beams from the weather plus improved structural action suggests that composite concrete-timber bridge systems are worth further investigation for economical bridge construction. The effect of cyclic loading and the timber shrinkage need further research. |
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Timber Research
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