The 2022 version of ASCE 7 Guidance has changed Section 26.8 conditions for calculating the topographic factor Kzt by dropping the requirement that the topographic feature must be deemed an isolated hill or ridge for the speed up to occur. In effect this means that any hill or ridge must now include the calculation of a speed up factor.
This change is long overdue, since there is no evidence that the speed-up does not occur for the second and subsequent hills in a range of hills. In fact the original formulation of the wind speed up had a provision for this condition (rolling or undulating hills). The ICE site specific procedure developed in 2012 includes the rolling terrain specification in all site specific assessments which we carry out.
As discussed in our previous post (Wind Speed-Up Formulation Testing), ICE presented an analysis of the recent tests of the speed-up equation by Australian researchers which shows that the ICE site specific procedure predicts speed-up values which agree with the described wind tunnel and numerical modeling results.
The discussion paper referenced in the previous post shows that the speed up for the second and subsequent hills in rolling terrain does require modified values for the γ parameter which are about 25% less than the values for an isolated hill, which results in smaller speed-up in the rolling terrain case. In effect the change to Kzt in the 2022 version will result in over-statement of the speed-up compared to experimental data.
The effect in practice of the new 2022 version of ASCE 7 and derivative codes is that existing towers designed with previous versions may be under-designed according to the new code. On the other hand, towers in rolling terrain situations designed with the 2022 code will face up to 30% excessive wind pressure determinations.
Tower designers and owners in the rolling terrain situation should re-evaluate the design basis, which could be done with a proper site specific assessment.