Abstract A plasticity-based model is developed under the framework of the slip line theory to estimate the ultimate bearing capacity of a skirted strip foundation resting on cohesionless soil under both static and seismic conditions. The analysis is performed considering a both-side failure mechanism beneath the foundation. The bearing capacity is eventually derived by satisfying the force equilibrium of the soil wedge trapped between the skirts. A pseudo-static analysis is performed to encompass the effect of an earthquake in the present analysis. The enhancement in the bearing capacity of a skirted foundation compared to a conventional surface foundation is demonstrated with a comprehensive parametric study. The obtained results are found to be lesser than that obtained through the upper bound solution reported in the literature but closer to the available experimental studies. Reduction in the bearing capacity of a skirted foundation during any seismic event is found to be lesser due to the presence of the skirts.