Simplified wind net pressure coefficients for the design of portal frames

When using the wind Code CP 3 Chapter 5 Part 2, the traditional method of designing purlins, side rails and main portal frames consists of ignoring localised wind pressure coefficients. This is based, in part, on recommendations given by the UK steel design codes, which state that localised pressures do not need to be considered for the design of purlins and side rails. The introduction of the new wind Code BS 6399 Part 2 has led to a change in the pressure coefficients to be used in design. These new coefficients have been derived from full-scale measurements on buildings and an improved understanding of wind behaviour. This paper compares the coefficients given by the new and old codes, and discusses the validity of ignoring the localised pressures. Equivalent, but conservative, sets of uniform net pressure coefficients are also presented, based on the new coefficients given in BS 6399 Part 2. These new coefficients simplify the calculation procedure and can be used for the design of the cladding, purlins, side rails and main structural frames. It is shown that if these simplified coefficients are used, the specification of the cladding, together with the number and layout of purlins and side rails, will not significantly change from current design practice using CP3 Chapter 5 Part 2.

Colin G. Bailey, PhD, BEng
Consultant, Building Research Establishment (BRE)