Limit State of Design

Limit State of Design

Structures must perform satisfactorily in normal service, and must also have a sufficient margin of safety against collapse. Two type of limit state:

  1. Serviceability Limit State (SLS)
  2. Ultimate Limit State (ULS)

Serviceability Limit State (SLS)

It includes criteria on the deflection of structures when this may affect performance, such as damaging floor finishes or interfering with the drainage of rainwater, on their susceptibility to vibrate excessively and critically on the durability

Ultimate Limit State (ULS)

The structure must not collapse when the loads applied to it are factored up by load factors, and the strength of its constituent material is factored down by material factors.

Load factors depends upon the likely intensity of any load.

There are low and high variability of these factors. For example: for a bridge, self weight of the bridge is known and there is less variability in this load when calculated, thus lesser load factor like 1.15 is assigned. But for the live loads, the nature and variations varies so the load factor is choose from 1.3 to 1.5.

Similarly for material factors, for the strength of nut and bolts from a steel manufacturing company, its variation in strength is controlled during quality control so the result will have significantly same strength thus lesser material factor is given like 1.05 to 1.1 while a cast in place concrete’s strength varies from condition and nature thus factor of like 1.5 is given.

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