The relative importance of each limit state will vary according to the nature of the structure. The usual procedure is to decide which is the crucial limit state for a particular structure and base the design on this, although durability and fire resistance requirements may well influence initial member sizing and concrete grade selection. Checks must also be made to ensure that all other relevant limit states are satisfied by the results produced. Except in special cases, such as water- retaining structures, the ultimate limit state is generally critical for reinforced concrete although subsequent serviceability checks may affect some of the details of the design. Prestressed concrete design, however, is generally based on service¬ability conditions with checks on the ultimate limit state.
In assessing a particular limit state for a structure it is necessary to consider all the possible variable parameters such as the loads, material strengths and construc¬tional tolerances.