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Concrete In Australia : June 2013
34 Concrete in Australia Vol 39 No 2 FEATURE: BRIDGES component of the M1600 LL (that is as specified in AS5100) is compared with permissible deflections in Table 3. 5.0 ELASTOMERIC SHEAR KEY e elastomeric shear key, as used in the system chosen for these analyses is a 50 diameter, 60 durometer hardness elastomeric section restrained in grooves in the deck-beam edges. A nominal direct load is applied to prevent separation of adjacent deck-beams, and a shear force between the deck- beam edges must be transferred via the elastomeric shear key. e maximum value of this shear force, as found in the above analysis is 0.3417 l1 for the four beam cross section, and 0.330 l1 for the two beam cross section. en, with the maximum value of l1 = 42 kN/m (occurring for a 9.6 m span), the maximum shear force on the elastomeric shear key encountered in the cases analysed is 0.3417 x 42 = 14.35 kN/m, but for further analysis let s say 20 kN/m. As shown in Figure 22, there is a minimum direct load to ensure positive pressure at all four concrete/elastomer contact points. e Mohr s circle shown on Figure 22 would apply if the shear key were a 50 x 50 square section, but it indicates that as long as positive pressure is maintained at the contact points, no tension occurs in the elastomer, and the shear stress remains constant regardless of the magnitude of the direct load. However, with a 50 diameter shear key the shear stress is increased by a factor of 1.414 to 282.5 x 1.414 = 400 kPa (compared with 20/.05 = 400 kPa). In the case of the system that is being used in this example, the direct load is provided by HT threaded bar tendons (Pult. = 530 kN) which can be stressed to 50% ultimate (based on calculated extensions measured with provision for joint compressions) using a lubricant and a torsion wrench. en with a generous allowance for losses, a final load of 160 kN per tendon remains. Transverse bar tendons and 4 m centres provide a uniform distribution of compression with a 45° dispersion angle. is results in a final load per metre across the joints of 40 kN. While there is no specific test for acceptance of elastomers used in this configuration, the lack of any tensile stresses to promote splitting, and the order of magnitude of the compressive stresses (< 1 MPa), appear to be generally acceptable in relation to the requirements for elastomeric bearings set out in AS5100 Part 4. Further, the experience gained from the use of elastomeric shear keys also indicates that they are able to accommodate severe unintentional abuse without any sign of distress. APPENDIX A Torsionally, stiff deck-beams, or planks, with transverse LL distribution provided by shear only, are typically squarish in cross section, so that more planks and shear keys are required in a deck section than if ribbed deck-beams were used. e width of ribbed deck-beams is governed by road transport regulations. erefore, the use of the analysis set out in this article may lead to a large number of simultaneous equations. is is not in itself a problem, but it compromises the previously mentioned benefits claimed for being able to use convenient Span EI ΔAV = 0.450 (l1 -3.7) (L/π)4 (excl. UDL) EI/rib ΔAV xDLA Span/600 9.6 1503 32000000x0.0137/2 .0069x1.3 = .0089 .0160 11.6 2744 32000000x0.0234/2 .0073x1.3 = .0095 .0193 13.6 5105 32000000x0.0367/2 .0087x1.3 = .0113 .0227 15.6 8564 32000000x0.0536/2 .0100x1.3 = .0130 .0260 17.6 13874 32000000x0.0739/2 .0117x1.3 = .0152 .0293 19.6 21339 32000000x0.0981/2 .0136x1.3 = .0177 .0327 Table 3. Average serviceability deflections including the dynamic load allowance. Figure 22. There is a minimum direct load to ensure positive pressure at all four concrete/elastomer contact points.