[Univ of Cambridge] [Dept of Engineering]

Behaviour of reinforced concrete cantilevers
under concentrated loads

Hai-yun Lu


Bridge engineers have encountered difficulty in designing and assessing reinforced concrete bridge cantilevers against accidental wheel loads from deviated vehicles.  This is because of the lack of understanding of the behaviour of reinforced concrete cantilever slabs under concentrated loads.

This dissertation examines both the elastic and collapse behaviour of reinforced concrete cantilevers under concentrated loads.

To study the elastic behaviour, a patch load is transformed into a number of point loads so that Jaramillo's elastic solution, which has been incorporated in four Matlab programs, can be used to compute the deflections, bending moments and twisting moments due to a patch load applied on an infinitely long cantilever.  The results from elastic analysis are plotted in graphs which provide a general view of the elastic behaviour of cantilevers under patch loads. Engineers can use the proposed Matlab programs in the design of cantilevers.

To investigate the flexural failure of reinforced concrete cantilevers under concentrated loads, both upper-bound and lower-bound flexural strengths are assessed.  Yield-line analysis is performed to evaluate the upper-bound flexural strength.  Five new yield-line mechanisms are proposed which aim to improve the flexural strength predictions.  Engineers can apply these new yield-line mechanisms in the assessment of bridge cantilevers.  The lower-bound flexural strength is computed using discontinuity-line analysis, a state-of-the-art lower-bound method.  New techniques to postulate discontinuity-line patterns are proposed for improving the lower-bound flexural strength predictions and for analysing edge-stiffened cantilevers.  A lower-bound method is proposed for use by engineers to calculate the effective length of a reinforced concrete cantilever resisting concentrated loads.  Key parameters influencing the lower-bound flexural strength predictions from the discontinuity-line analysis are discussed.  The discontinuity-line method gives engineers an insight into the possible load-carrying mechanisms in bridge cantilevers and hence allows them to obtain flexural strength predictions with an adequate safety margin.

The shear behaviour of reinforced concrete cantilevers under concentrated loads is also studied, and two empirical methods which relate the topology of critical sections for shear to the schematic load paths are proposed.

An experimental program is carried out to investigate the effect of slab reinforcement ratios, the presence of an edge beam and the load configuration on the elastic and collapse behaviour of reinforced concrete cantilevers.  Non-linear finite element analysis is performed to model the behaviour of the cantilevers tested for verifying its applicability.  Modelling techniques are recommended and the limitations are also discussed.

Finally, all the theoretical, empirical and numerical methods mentioned above are used to predict the collapse load of all the cantilevers tested and the results compared.  Guidelines are provided for engineers to design and assess bridge cantilevers.

[Cambridge University | CUED | Structures Group | Geotechnical Group]

This page is maintained by rcb@eng.cam.ac.uk (last update 12 August 2004)