Literal impact (the engineering term of art) is usually the controlling force for these types of structural calculations. The force varies with respect to veloc

Literal impact (the engineering term of art) is usually the controlling force for these types of structural calculations. The force varies with respect to velocity-squared, and due to the dynamic nature of landing, the forces are not likely to be distributed evenly (high pressures at unpredictable locations), so you end up with the controlling condition needing to be much stronger than perhaps even supporting the (static, evenly distributed) weight of a fully loaded vehicle.