Preparation of Nairobi County Wind Map as Part of the Kenya National Annex to En 1991-1-4

Abstract

Structural engineering education and practice in Kenya has been based on British Standards which are developed and maintained by the British Standards Institution (BSI). Given that BSI has decided to change to Structural Euro codes, the way forward for engineering design training and practice is to adopt the use of Euro codes. For Kenyan engineers to switch to the use of Eurocode 1991-1-4 on Wind loads, they is need to develop a National Annex to that code to take care of the local wind characteristics. Part of the information to be put in the annex is a wind map that enables evaluation of wind loads by Eurocode 1991-1-4 This research was carried out as an initial step in the development of the Kenyan National Annex (NA) of the EN1991-1-4 by developing a wind map illustrating the basic 50 year return period design wind speeds in Nairobi County. Hourly wind speeds were obtained from the Kenya Meteorology Department and the environmental correction factors (both terrain and altitude correction factors) determined. The combined factors were then multiplied to the hourly wind speeds and the maximum wind speeds from each storm derived. These were then subjected to extreme value analysis to come up with the basic hourly wind speeds which were then multiplied by a factor of 1.06 to get the 10-minute wind speeds from which a wind contour map was drawn. The results show that the 10-minute wind speeds within Nairobi County vary from 18m/s to 22m/s. Currently, a 3-second gust speed of 28m/s is used in the design of structures within Nairobi Country. When converted to the 10-minute wind speed using the durst curve shown in figure 2.1, it resulted to a 10 minute wind speed of 19.63m/s. This is comparable to the 10 minute wind speeds obtained from this research.