Topographic Factor Ktz and the topography

[Torres_Structural]

Hi everyone, I have the question in regards to the ASCE 7-22 chapter 26 Topographic Factor Ktz. In the section it does not mention how far back we need to considered any topographic effects that can speed up the wind velocity. I'm working on a project, and it has an escarpment and a ridge, which are about .75 miles from the site. Is there a standard rule for how far out from the site you need to consider these changes in topography? Does this have anything to do with the 2600 ft radius that is mentioned in the commentary for the wind exposure category? TIA

 

[DaveAtkins]

I think I have used the topographic factor only once in my long career. That was on a project where the building was definitely on top of a rise. I would say 0.75 miles away is far enough away that you can ignore it.

Sometimes I wonder if I am being a little too careless or lazy by not using the factor, but honestly, I never use it.

I am wondering what others will have to say.

DaveAtkins

 

[dauwerda]

I don't have ASCE 7-22 yet but I think its safe to assume this didn't change from 7-16.

To take this into consideration, the escarpment or ridge needs to be isolated - meaning there aren't any other similar features upwind of the escarpment/ridge within 2 miles or 100 times the height of the feature (whichever is smaller). The structure also needs to be located on the upper half of the ridge/escarpment (sounds like yours is not actually on the escarpment, so this effect would not apply in your case).

Here is the section of code:

26.8.1 Wind Speed-Up over Hills, Ridges, and
Escarpments. Wind speed-up effects at isolated hills, ridges,
and escarpments constituting abrupt changes in the general
topography, located in any exposure category, shall be
included in the determination of the wind loads when site
conditions and locations of buildings and other structures meet
all of the following conditions:

1. The hill, ridge, or escarpment is isolated and unobstructed
upwind by other similar topographic features of comparable
height for 100 times the height of the topographic feature
(100H) or 2 mi (3.22 km), whichever is less. This distance
shall be measured horizontally from the point at which the
height H of the hill, ridge, or escarpment is determined.
2. The hill, ridge, or escarpment protrudes above the height
of upwind terrain features within a 2-mi (3.22-km) radius in
any quadrant by a factor of 2 or more.
3. The building or other structure is located as shown in
Fig. 26.8-1 in the upper one-half of a hill or ridge or near
the crest of an escarpment.
4. H∕Lh ≥ 0.2.
5. H is greater than or equal to 15 ft (4.5 m) for Exposure C
and D and 60 ft (18 m) for Exposure B.

 

[Celt83]

Assuming items 1,2,4,and 5 don't kick you out of needing Kzt, then you can can quickly determine what "x" distance gives |x|/μLh = 1 which reduces K2 to 0 making Kzt = 1.

ex.
H = 80
Lh = 100
Assume exposure D
μ = 1.5
|x|/μLh = 1
|x| = 150 ft

∴ any structure within the upper half-height or 150 ft from the crest would be considered "near"

 

[structSU10]

For what it is worth skyciv has a free wind calculator that based on the geographic point you put in it will show - from various directions - the topography and attempts to determine the Kzt for the project site. I haven't played with it too much but it is a nice visualization at the very least.