Determination of Togographic Factor, Kzt in ASCE

[imsengr]

Hi everyone,

Does anyone know to determine the topographic factor, Kzt per ASCE 7-05. Most of the time we do not even have the information, such as whether our structure is on an escarpment, or on a 2-D Ridge or 3-D Axisymmetrical Hill. Does anyone have any luck using Figure 6-4 on Page 45 of ASCE 7-05. Do we just assume that the effect of wind speed-up shall not be required to be considered and just use 1.00? Or do we take a figure like, 0.85, just to allow for some uncertainty in the effect of wind speed-up, but not have to go through the process of determining Kzt? The process does look a tad involved, and we might not have all the information hand to use the tables correctly.

Any advice or suggestions, please? What is the best thing to do in these cases? What is your own experience in this?

Thanks, y'all.

 

[csd72]

You need to have a good read of the commentary for this section and go throught the numbers at least once so that you can get a feel for it.

I have not used the topographic factor for ASCE7-05, but in my experience with other codes these factors are for major geographical features and are not usually significant.

I would not think this would be significant for any sea cliff/escarpment less than 300' high or a hill less than 500' or a slope less than 1 in 4, but you should really do some numbers to get a feel for this.

I always check the location of my jobs on mapquest/google to see if they are in an area that may have these issues. If it is in one of these areas then try to get your hands on a contour map so that you can do the calculation.

 

[civilperson]

The factor only applies if ALL the conditions apply, 1-5 of 6.5.7.1. The factor if used is ALLWAYS greater than 1.0, (use 1.0 if no speed up is attributed to geography). The 0.85 that you cited is not a easily arrived at value since K1 K2 and K3 are multiplied together, added to one and then squared.