Bridges

Slender reinforced concrete (RC) columns are prevalent in modern infrastructure due to a need to accommodate elevated highway interchanges and deep gullies. The destabilising effect due to second order effects on slender RC columns can be considerable. While the moment magnification procedures are well established and have become common industry practice, the parameters relating to real bridge articulations is often ambiguous (particularly those relating to the unsupported length of column and the effective length factor). A comparison is made using basic engineering principles with the methods available in AS5100.5-2017 and other international standards including AASHTO and Eurocode 2. The investigation gives special attention to the effective length factor k and unsupported length of column Lu in the analysis and design of a single circular cantilever column founded on a single circular pile and with elastomeric bearings provided at the top restraint (between superstructure and substructure). The impact of soil stiffness, column cracked stiffness and elastomeric bearing stiffnesses for varying column diameters on k and Lu are assessed and results provided. Firstly, the results broadly indicate that the depth to fixity of piles can be defined by a number of pile diameters below FSL (and subsequently Lu can be determined) for columns embedded in soils with varying stiffness and columns of a specified cracked stiffness. Secondly, the results from basic engineering principles indicate that as elastomeric bearing stiffness increases, the effective length factor of the column decreases for reasonably sized columns and commonly used bearings. Finally, a comparison is made between the moment magnifier method and second order elastic analysis method as permitted by AS5100.5-2017 and other international standards to demonstrate the economic and sustainability benefits of using higher order methods based on project need.