Question

Explain what is meant by ultimate limit design and what is meant by serviceability limit state design.

Answer 1

Limit state in general is defined as a state of imminent failure.This failure can be in terms of strength, serviceability, durability etc. The design philosophy which is based on limit state is called ultimate limit state design method. Once the limit state is reached, the structure will not perform satisfactorily in terms of either safety or serviceability. Ultimate limit state design considers ultimate strength of structure and also takes care of serviceability requirements of structure. Accordingly, two limit states are defined- LIMIT STATE OF COLLAPSE and LIMIT STATE OF SERVICEABILITY.

Ultimate limit state design approach is a probabilistic design approach as it considers that there is always a chance of variation in properties of materials and calculated loads. To take care of these, partial safety factors were introduced which when multiplied by calculated parameters gives values of those parameters for design purposes.

LIMIT STATE OF SERVICEABILITY

Serviceability refers to the performance of structure in a satisfactory manner. LIMIT STATE OF SERVICEABILITY implies a state beyond which structure will not perform as desired which leads to discomfort and anxiety. Some of the parameters which are taken careof in limit state of serviceability are deflection, cracks, crack width, vibrations etc. Serviceability is always considered at service loads and not design loads.