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South African Pavement Engineering Manual
Chapter 10: Pavement Design
Category A Category B
Category C Category D
Figure 5. Typical South African Roads for Road Categories
3.2 Analysis and Structural Design Periods
The analysis period incorporates one or more structural design periods and is often related to the geometric design
life of the facility. If the geometric design is expected to be appropriate for a long period, because the traffic
demand can be forecasted with a high degree of confidence, then the analysis period can be as long as 30 years.
With an uncertain geometric design life, in a changing traffic situation, a shorter analysis period, such as 10 years,
can be considered. In many instances, however, budget constraints dictate the analysis and structural design
periods. The economic analysis of the life cycle cost of the road is done for the analysis period.
The following guidelines apply to the selection of a structural design period for the different road categories:
• Category A roads: A structural design period of 25 years, ranging between 15 and 30 years, is recommended
because:
− The road alignment is fixed for a long period with a high certainty.
− It is not acceptable to road users to have heavy rehabilitation on recently constructed important roads.
− The cost of disrupting the high volumes of traffic offsets any advantage of a shorter structural design period.
• Category B roads. A structural design period of 20 years, ranging between 15 and 25 years, is recommended.
Factors that may encourage a shorter design period are:
− A changing traffic situation, which may cause the geometric design to become outdated.
− A shortage of funds for the initial construction cost.
− A lack of confidence in the design assumptions.
If a shortened structural design period is selected, the design should be able to accommodate staged
construction. See Section 3.3 for further discussion.
• Category C roads. Financial constraints may dictate the selection of a shortened structural design period of 10
years. If it is foreseen that structural rehabilitation will be difficult, a longer period of 20 years may be
appropriate.
Section 3: Design Considerations
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