Limit states design

Fri 26 Oct, 17:52

The purpose of desigh is to achieve acceptable probabilities that a structure will not become unfit for its intended use - that is, that it will not reach a limit state. Thus, any way in which a structure may cause to be fit for use will constitute a limit state and the design aim is to avoid any such condition being reached during the expected life of the structure.
The two principal types of limit state are the ultimate limit state and the serviceability limit state.

1. Ultimate limit state

This requires that the structure must be able to withstand, with an adequate factor of safety against collapse, the loads for which it is designed to ensure the safety of the building occupants and/or the safety of the structure itself. The possibility of buckling or overturning must also be taken into account, as must the possibility of accidental damage as caused, for example, by an internal explosion.

2. Serviceability limit states

Generally the most important serviceability limit states are:

1) Deflection -the appearance or efficiently of any part of the structure must not be adversely affected by deflections norshould the comfort of the buiding users be adversely affected.
2) Cracking - local damage due to cracking and spalling must not affect the appearance, efficiently or durability of the structure.
3) Durability - this must be considered in terms of the proposed life of the strucutre and its conditions of exposure.
4) Extensive vibration - which may cause discomfort or alarm as well as damage.
5) Fatique - must be considered if cyclic loading is likely.
6) Fire resistance - this must be considered in terms of resistance to collapse, flame penetration and heat transfer.
7) Special circumstances - any special requirements of the structure which are not covered by any of the more common limit states, such as earthquacke resistance, must be taken into account.

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 class selection. Check must also be made to ensure that all other relevant limit states are satisfied by the result produced.
Exept in speciaal cares, such as water-retaining structures, the ultimate limit state is generally critical for reinforced concrete although subsequent serviceability cheks may affect some of the details of the design. Prestressed concrete design, however, is generally based on serviceability conditions with checks on the ultimate limit state.
In assessing a particular limit state for a structure it is neccessary to consider all the possible variavle parameters such as the loads, material strengths and all constructional tolerances.

Source: Reinforced Concrete Design to Eurocode 2, Bill Mosley, John Bungley and Ray Hulse