Design period may be defined as
It is the number of years in future for which the given facility is available to meet the demand.
The number of years in future for which supply will be more than demand.
Why Design period is provided ?
Design period is provided because
- It is very difficult or impossible to provide frequent extension.
- It is cheaper to provide a single large unit rather to construct a number of small units.
Factors affecting design period
Following are some factors which affect the design period of the structure.
- Life of the structure
Life of structure is the number of years in future for which the design period is physically suitable to provide the intended facility. So it should be less than life of structure.
- Ease or difficulty in extension
For the projects whose extension is easily possible, it is kept low. For example we can install new tube wells at any time, so we do not need to install all tube wells which would be required after 20 years.
But for the projects whose extension is difficult, their design period is kept greater. For example dams and reservoirs cannot be extended easily.
- Rate of population growth
If the rate of population growth is higher, then for that region shorter design period is required.
- Lead time
It is the time from the commencement of a project to its completion. Design period should be greater than lead time.
- Economy of scale
The decrease in average cost as the size of facility increase is known as economy of scale.
If the economy of scale is small, smaller design period will be used. It is economical to build a large structure, for longer design period.
- Performance time
Structures are checked under working condition for sometime, which should not be considered in design period. During this time it is not providing facility to community.