WSD is based on the allowable stress of a material under working loads. It obeys Hooke's law i.e the material will not fail if the stresses are within the elastic limit called as yield stress of material.  While LSD considers both the ultimate limit state and the serviceability limit state, and applies safety factors to ensure safety under extreme conditions. afety factors are applied to the loads and stresses in the structure to ensure that they do not exceed the ultimate limit state or serviceability limit state.