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Concrete In Australia : June 2013
30 Concrete in Australia Vol 39 No 2 FEATURE: BRIDGES Design and performance advantages of elastomeric shear keys in small concrete bridges David Herbert -- Structural Consultant, CDE Design Solutions is article looks at the transverse distribution of live loads in a standardised precast concrete bridge construction system which prefers to use elastomeric shear keys between deck beam units for this purpose, for the perceived benefits, which it outlines. It examines the determination of inter deck-beam shear transfer forces, which in turn allows the determination of the transverse deflection profile, transverse moments and the proportion of live loads distributed to each deck-beam, or deck-beam rib. e magnitude of shear transfer forces is then compared with typical capacities available in elastomeric shear keys and their concrete restraints. e analysis method described is amenable to manual computation and is therefore a viable alternative to using a computer model of the structure. is is seen to be appropriate for the scale of the typical bridge structures involved. It also confers on the designer a more intimate understanding of the load paths being utilised. e article concentrates on twin ribbed deck beams with St Venant torsional stiffness taken as negligible. e appendix however, looks at a hollow plank deck where the St Venant torsional stiffness is dominant. e numerical process which provides a solution here is believed to be previously unpublished and extremely practical. 1.0 INTRODUCTION Financing the replacement of ageing timber bridges on rural roads throughout eastern Australia has become an acknowledged crisis. e current rate of replacement is dependent on a series of ad-hoc schemes set up by national and/or state governments, and (overall) is not keeping up with the rate at which further bridges are reaching the end of their useful lives. e construction authorities for these bridge replacements are usually shire or regional councils, many of which cover large areas with the need for significant numbers of bridge replacements, but with small population bases. Figure 1. Nowendoc Road single lane bridge featuring 10 m, 20 m, 18 m spans over Mummel River for Greater Taree Regional Council. Figure 3. The single lane Lavinia-Murray Bridge with three 10 m spans, built for Great Lakes Shire Council. Figure 2. A 12 m span, double lane bridge on Thunderbolts Way over Mia Mia Creek, built for Walcha Shire Council. Figure 4. A double lane bridge with one 12 m span at Hill End Road over Spring Creek, built for Bathurst Regional Council.