For a structural designer, it is most important to ensure that the structures and facilities designed by him are:
- Fit for their purpose
- Safe
- Economical and durable.
Thus, safety is most important responsibility of a structural designer. But it is more difficult to say how safe a design is, because of natural variation of material strengths, and also loading. Below are uncertainty affecting safety of a structure:
- Uncertainty about loading
- Uncertainty about material strength
- Uncertain about structural dimension and behavior.
All these uncertainties makes impossible to guarantee that the structure is safe. Thus, it becomes most important for the designer that he ensure that the there is no risk of failure or risk of failure is extremely small, despite uncertainty.
To be assured that the risk of failure is minimum, the designer has to design the structure based on its characteristic strength (also called characteristic resistance). Suppose, a beam shall be designed to resist certain characteristic loading, its characteristic resistance shall be greater than the bending moment produced by the characteristic loading. But the characteristic strength of resistance of the material is itself not fixed, it varies with elements even of same materials.
The maximum loads also vary and thus varies load effects such as bending moment with different structural elements in their service life. The variation also occur element to element on loads applied to the structure and load distribution through the structure. Thus, if a weak structural element gets exposed to heavy load, which exceeds its strength, the structural element will fail.
Thus, safety factors are used to get the characteristic strength (resistance) or characteristic load, so as to minimize the effect of uncertainties in loading conditions and material strength.
Characteristic strength of steel is defined as that value of resistance below which not more than a prescribed percentage of test results may be expected to fall. In design, usually characteristic yield strength is defined as yield stress below which not more than 5% of the test values may be expected to fall, i.e. the strength of materials is expected to be exceeded by 95% of the cases.
The characteristic load is that value of the load which has an accepted probability of not being exceeded during the life span of the structure, i.e. the load on structure will not exceed the characteristic loads 95% of the time.
Limit states are the conditions in which a structure is considered to be failed to serve the purpose for which it was designed and built. There are two limit states which are considered at the design stage:
Limit State of Strength:
- Strength (yielding, buckling)
- Stability against overturning and sway of structure
- Fracture of structural element due to fatigue
- Plastic collapse of structure
- Brittle fracture of structural element
Serviceability Limit State:
- Deflection of structural element
- Vibration of structure or some part of it
- Fatigue checks including repairable damage due to fatigue
- Corrosion on steel
- Fire hazards in structures
Thus, we can say that limit state of strength refers to loss of equilibrium of structure and loss of stability of the structure. Serviceability Limit State refers to the limits on acceptable performance of the structure.
Limits such as corrosion, brittle fracture are not covered in design calculation. These can be taken care of by maintaining quality of materials. Like, corrosion can be covered by protection methods such as painting, brittle fracture is covered by material specifications like ductile material.