Structural dynamics of modular bridge expansion joints resulting in environmental noise emissions and fatigue
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Whilst the use of expansion joints is common practice in bridge construction, modular bridge expansion joints are designed to accommodate large longitudinal expansion and contraction movements of bridge superstructures. In addition to supporting wheel loads, a properly designed modular joint will prevent rain water and road debris from entering into the underlying superstructure and substructure. Modular bridge expansion joints (MBEJs) are widely used throughout the world for the provision of controlled pavement continuity during seismic, thermal expansion, contraction and long-term creep and shrinkage movements of bridge superstructures and are considered to be the most modern design of waterproof bridge expansion joint currently available. Modular bridge expansion joints are subjected to more load cycles than other superstructure elements, but the load types, magnitudes and fatigue-stress ranges that are applied to these joints are not well defined. MBEJs are generally described as single or multiple support bar designs. In the single support bar design, the support bar (beam parallel to the direction of traffic or notionally parallel in the case of the swivel joist variant) supports all the centre beams (beams transverse to the direction of traffic) using individual sliding yoke connections (for the swivel joist variant, the yoke connection is characterised as a one-sided stirrup and swivels rather than slides). In the multiple support bar design, multiple support bars individually support each centre beam using a welded connection. Environmental noise complaints from home owners near bridges with modular expansion joints led to an engineering investigation into the noise production mechanism. It was generally known that an environmental noise nuisance occurred as motor vehicle wheels passed over the joint but the mechanism for the generation of the noise nuisance has only recently been described. Observation suggested that the noise generation mechanism involved possibly both parts of the bridge structure and the joint itself as it was unlikely that there was sufficient acoustic power in the simple tyre impact to explain the persistence of the noise in the surrounding environment. Engineering measurements were undertaken at two bridges and subsequent analysis led to the understanding that dominant frequency components in the sound pressure field inside the void below the joint were due to excitation of structural modes of the joint and/or acoustic modes of the void. This initial acoustic investigation was subsequently overtaken by observations of fatigue induced cracking in centre beams and the welded support bar connection. A literature search revealed little to describe the structural dynamics behaviour of MBEJs but showed that there was an accepted belief amongst academic researchers dating from around 1973 that the loading was dynamic. In spite of this knowledge, some Codes-of-Practice and designers still use a static or quasistatic design with little consideration of the dynamic behaviour, either in the analysis or the detailing. In an almost universal approach to the design of modular bridge expansion joints, the various national bridge design codes do not envisage that the embedded joint may be lightly damped and could vibrate as a result of traffic excitation. These codes only consider an amplification of the static load to cover sub-optimal installation impact, poor road approach and the dynamic component of load. The codes do not consider the possibility of free vibration after the passage of a vehicle axle. Codes also ignore the possibilities of vibration transmission and response reinforcement through either following axles or loading of subsequent components by a single axle. What the codes normally consider is that any dynamic loading of the expansion joint is most likely to result from a sudden impact of the type produced by a moving vehicle ‘dropping’ onto the joint due to a difference in height between the expansion joint and the approach pavement. In climates where snow ploughs are required for winter maintenance, the expansion joint is always installed below the surrounding pavement to prevent possible damage from snow plough blades. In some European states (viz. Germany), all bridge expansion joints are installed some 3-5mm below the surrounding pavement to allow for possible wear of the asphaltic concrete. In other cases, height mismatches may occur due to sub-optimal installation. However, in the case of dynamic design, there are some major exceptions with Standards Australia (2004) noting that for modular deck joints “…the dynamic load allowance shall be determined from specialist studies, taking account of the dynamic characteristics of the joint…” It is understood that the work reported in Appendices B-E was instrumental in the Standards Australia committee decisions. Whilst this Code recognizes the dynamic behavior of MBEJs, there is no guidance given to the designer on the interpretation of the specialist study data. AASHTO (2004), Austrian Guideline RVS 15.45 (1999) and German Specification TL/TPFÜ 92 (1992) are major advancements as infinite fatigue cycles are now specified and braking forces considered but there is an incomplete recognition of the possibility of reinforcement due to in-phase (or notionally in-phase) excitation or the coupled centre beam resonance phenomenon described in Chapter 3. This thesis investigates the mechanism for noise generation and propagation through the use of structural dynamics to explain both the noise generation and the significant occurrence of fatigue failures world-wide. The successful fatigue proofing of an operational modular joint is reported together with the introduction of an elliptical loading model to more fully explain the observed fatigue failure modes in the multiple support bar design.
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