Section 2.0 - Loads

ID #1109

Section 2.6.6 Calculating the topographic factors

Question: Section 2.6.6 (2.6.6.1 and 2.6.6.2) My colleagues and I have been discussing the proper method of determining which topographic category to apply to a given site. Sections 2.6.6.1 and 2.6.6.2 are somewhat vague as to the correct method of approach. For example, section 2.6.6.2 subsection 2 reads, "The hill, ridge or escarpment protrudes by a factor of two or more above the average height of the surrounding terrain features within a 2 mile radius." The word "surrounding" seems to suggest that only features around the central feature and not the central feature itself are included in "the average height." Is this correct? Also, in determining this average height, are all surrounding peaks in a two mile radius included, or only those along the lines of multiple wind directions? For example, we have encountered locations in which using the average height of surrounding peaks to determine the topographic category yields a category 1. However, along a particular 2 mile upwind path the topography clearly suggests a category 3 or 4. Should the topographic category be determined based on a single wind direction, i.e. that of the worst case scenario?In the case that one considers only a particular wind direction, should the topographic features be considered only in the 2 mile upwind segment, or should the entire 4 miles (2 miles upwind + 2 miles downwind) be used?Are the peak heights of the surrounding terrain features measured from the lowest point within the two mile radius? I restated and numbered each of my questions below to make them easier to reference in your response. 1.) In section 2.6.6.2 subsection 2, the word "surrounding" Seems to suggest that only features around the central feature and not the central feature itself are included in "the average height." Is this correct? 2.) In determining this average height, are all surrounding peaks in a two mile radius included, or only those along the path of a particular wind direction? 3.) Should the topographic category be determined based on a single wind direction, i.e., that of the worst case scenario? 4.) In the case that one considers only a particular wind direction should the topographic features be considered only in the 2 mile upwind segment, or should the entire 4 miles (2 miles upwind + 2 miles downwind) be used? 5.) Are the peak heights of the surrounding terrain features measured from the lowest point within the two mile radius? 6.) Should the base of a crest height measurement be taken from that point where the slope exceeds 0.10 or from the base of the feature itself? 7.) Finally, Section 2.6.6.1 seems to indicate that wind speed-up effects need only be taken into account if all four conditions are met. Is this correct? RESPONSES: 1. Yes 2. The surrounding peaks along the path of each wind direction considered shall be used. ASCE 7 may be used as a guide for a specific site. 3. Each wind direction must be considered to determine the direction that results in the maximum response per Section 2.6.9. 4. The topographic features in the two mile upwind direction shall be considered. 5. The peak heights of the surrounding terrain and the central feature shall be determined from the lowest point within the surrounding terrain. 6. The base of a crest height measurement shall be taken from the base of the feature. 7. Yes

Tags: topographic

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Last update: 2012-06-08 00:07
Author: John Erichsen
Revision: 1.7

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