- The philosophy of the limit state method of design represents a definite advancement over the traditional design philosophies.
- Unlike WSM which based calculations on service load conditions alone, and unlike ULM, which based calculations on ultimate load conditions alone, LSM aims for a comprehensive and rational solution to the design problem, by considering safety at ultimate loads and serviceability at working loads.
- The LSM philosophy uses a multiple safety factor format which attempts to provide adequate safety at ultimate loads as well as adequate serviceability at service loads, by considering all possible 'Limit State'.
A limit state is a state of impending failure, beyond which a structure ceases to perform its intended function satisfactorily; in terms of either safety of serviceability i.e. it either collapses or becomes unserviceable.
There are two types of limit states:
Ultimate limit states (limit states of collapse):- Which deal with strength, overturning, sliding, buckling, fatigue fracture etc.
Serviceability limit states: - Which deals with discomfort to occupancy and or malfunction, caused by excessive deflection, crack width, vibration leakage etc., and also loss of durability etc.