On the seismic response of under-designed caisson foundations

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 ($$\text{ FS }_\mathrm{V}$$FSV) and seismic ($$\text{ FS }_\mathrm{E}$$FSE) factors of safety are examined: (1) a lightly loaded ($$\text{ FS }_\mathrm{V}= 5$$FSV=5) seismically under-designed ($$\text{ FS }_\mathrm{E} < 1$$FSE<1) caisson, (2) a lightly loaded seismically over-designed ($$\text{ FS }_\mathrm{E} >1$$FSE>1) caisson, (3) a heavily loaded ($$\text{ FS }_\mathrm{V} = 2.5$$FSV=2.5) seismically under-designed ($$\text{ FS }_\mathrm{E} < 1$$FSE<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 ($$\text{ FS }_{V} = 2.5$$FSV=2.5), the performance of the two systems (over and under designed) is similar.

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