Effect of Critical Contact Area Ratio on Moment Capacity of Rocking Shallow Footings

The rocking of a relatively rigid footing on soil can be visualized as a moving contact problem, with a contact area moving from one side of the footing to the other as the footing rocks. Thus the partial separation of footing (uplift) and the resulting moment capacity can be more closely related to the critical contact area ratio, A/Ac (ratio between the actual footing area (A) and the area required to support the vertical and shear loads (Ac)). This paper compares the moment capacities calculated by using A/Ac with those obtained from centrifuge and 1-g experiments conducted on rocking shallow footings for a range of A/Ac values. It is shown that the moment capacity of a rocking footing is well defined and, unlike bearing capacity, moment capacity is less sensitive to the uncertainty in soil properties as A/Ac increases.