Abstract
The seismic behaviour of caisson foundations supporting typical bridge piers is analysed with 3D finite elements, with due consideration to soil and interface nonlinearities. Single-degree-of freedom oscillators of varying mass and height, simulating heavily and lightly loaded bridge piers, founded on similar caissons are studied. Four different combinations of the static ( $$\text{ FS }_\mathrm{V}$$ ) and seismic ( $$\text{ FS }_\mathrm{E}$$ ) factors of safety are examined: (1) a lightly loaded ( $$\text{ FS }_\mathrm{V}= 5$$ ) seismically under-designed ( $$\text{ FS }_\mathrm{E} < 1$$ ) caisson, (2) a lightly loaded seismically over-designed ( $$\text{ FS }_\mathrm{E} >1$$ ) caisson, (3) a heavily loaded ( $$\text{ FS }_\mathrm{V} = 2.5$$ ) seismically under-designed ( $$\text{ FS }_\mathrm{E} < 1$$ ) caisson and (4) a heavily loaded seismically over-designed caisson. The analysis is performed with use of seismic records appropriately modified so that the effective response periods (due to soil-structure-interaction effects) of the studied systems correspond to the same spectral acceleration, thus allowing their inelastic seismic performance to be compared on a fair basis. Key performance measures of the systems are then contrasted, such as: accelerations, displacements, rotations and settlements. It is shown that the performance of the lightly loaded seismically under-designed caisson is advantageous: not only does it reduce significantly the seismic load to the superstructure, but it also produces minimal residual displacements of the foundation. For heavily loaded foundations, however ( $$\text{ FS }_{V} = 2.5$$ ), the performance of the two systems (over and under designed) is similar.
Published Version
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