Earthquake response of shallow foundations with low vertical gravity load

This paper presents the development and response of a Ruaumoko model for the
representation of uplift and reattachment of a shallow foundation. The vertical load in the foundation is used to control the characteristics of moment and shear loads, as these values must reduce to zero when the entire footing has detached from the underlying soil. Through extension of the capabilities of Ruaumoko this response was able to be represented.

A simple elastic two bay portal frame structure was created and shallow foundations were
attached to the base of each column of the portal frame and subjected to cyclic loading. Uplift modelling was shown to have a significant effect on the shear and moment in the footings. If the point of detachment and reattachment of the foundation was at different horizontal and/or rotational displacements the result was residual horizontal and rotational displacements at the end of loading. This shift in displacements occurred in conjunction with a shift in shear and moment in the footing.