It is self evident that a structure should be "safe" during its lifetimes, i.e. free from the risk of collapse. There are, however, other risks associated with a structure and the term safe is now replaced by the term "serviceable".
A structure should not during its lifetime become "unserviceable", i.e. it should be free from risk of collapse, rapid deterioration, fire, cracking, excessive deflection etc.
A structure should be designed to be safe under all conditions of its useful life and to ensure that this is accomplished certain distinct performance requirements, called "limit states", have been identified. The method of limit state design recognizes the variability of loads, materials construction methods and approximations in the theory and calculations.
Limit state may be at any stage of the life of a structure, or at any stage of loading. The limit states which are important for the design of reinforced concrete are at ultimate and serviceability. Calculations for limit state involve loads and loads factors, and material factors and strengths.
Stability, an ultimate limit state, is the ability of a structure or part of a structure, to resist overturning, overall failure and sway. Calculations should consider the worst realistic combination of loads at all stages of construction.
Deflection is a serviceability limit state. Deflections should not impair the efficiency of a structure or its components, nor cause damage to the finishes. Generally the worst realistic combination of unfactored imposed loads is used to calculate elastic deflections. These values are compared with limit states of deformation.
Dynamic affects to be considered at the serviceability limit state are vibrations caused by machines, and oscillations caused by harmonic resonance, e.g. wind gusts on buildings. The natural frequency of the building should be different from the exciting source to avoid resonance.
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