ASCE 7-10 Wind Directionality Factor Question 1

[JDCWYO]

I have a question about the wind directionality factor (Kd) in Table 26.6-1 of ASCE 7-10. I am calculating the wind loads for an air cooled heat exchanger which consists of a bundle resting on a plenum which is constructed using cold-formed steel panels approximately 2’-0” tall. The cold-formed panels are similar to a scaled-up metal stud in profile. The plenum panels are supported by W-shape columns at each corner of the assembly. The LFRS consists of the sides of the diagonal steel angles between the columns and uses the sides of the plenum as beams which carry the flexural loads and the compression loads from the braced frames formed by the angles and columns.

The bundle consists of two steel box headers between which a series of finned tubes span. The tubes also rest on intermediate supports which span between the sideframes and are spaced at approximately 5’-0” O.C. The header and sideframes are detailed such that 1/4" of horizontal movement due to thermal expansion is possible before the tube fins touch the inside of the sideframes. These sideframes are also cold-formed steel panels, approximately 1’-9” tall, which are similar to a scaled-up metal stud top track in profile.

The bottom of the plenum consist of light gage steel panels and the top of the structure above the bundle tubes is exposed to the atmosphere.

Table 26.6-1 specifies a Kd value of 0.85 for building MFWRS’s and 0.90 for square tanks. In my mind, neither one of these options really describes the structure I am working on. It is obviously not a building, but at the same time it is not a tank full of fluid. The sideframes are 1/4" from the tips of the tubes and the tube fins, if placed tip to tip, provide approximately 1 1/4" between the tubes full of fluid.

In reading the commentary provided by the Committee, I understand that the closer Kd gets to 1.0, the more the MWFRS has the same resistance to wind loads from any direction. The stiffness of the braces in each orthogonal direction of the frame I am designing do not have the same wind load resistance, so in my mind I should be closer to a Kd value of 0.85. At the same time, the bundle is full of tubes and fins which would not deflect as much as building interiors when exposed to the horizontal deflection of the sideframe panel under wind load, and in my mind this would justify a Kd value closer to 0.9 or 0.95. The plenum below the bundle does contain a significant amount of interior elements, so in my mind, a value of 0.85 would be appropriate for that consideration.

Given all of this, I am having trouble settling on a value of 0.85 or 0.9 or a value between these two for Kd. What do you think would be an appropriate value to use for Kd in this design?

 

[briancpotter]

The wind directionality factor is based on the probability of the structure being oriented the worst possible way for building loads. From Ellingwood (1999):

"The wind speed used to compute the nominal wind pressure is based on the annual extreme wind speed, irrespective of direction. Likewise, the pressure coefficients, obtained from wind tunnel testing or field instrumentation, are found by enveloping the measured pressure data and thus represent pressures developed from the most unfavorable orientation of wind. Research (e.g., Simiu et al. 1981, 1985, 1986; Wen 1983, 1984) has indicated that the probability of a coincidence of ‘‘worst-wind’’ and ‘‘worst building orientation’’ is sufficiently small that some reduction in overall calculated wind pressure might be taken in design."

When has the same properties in every direction (ie: something round), this probability goes up, and thus Kd goes up. Deflection properties of the structure have nothing to do with it, and shouldn't be taken into account.

Since your structure seems to fall into a square "Chimney, tank, and similar structures", this implies a Kd value of 0.90, based on ASCE 7-10.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[steellion]

You could just use Kd=1.0. Is the design that tight?

 

[SAIL3]

a good and clear description by Brien......I would use a value of 1.0 and move on....I typically use 1.0 irregardless and put the extra SF in my hip pocket to cover the probability future changes etc.....unless one has control over the foundation design and the load combination used in it's design, I believe it is recommended to use a value of 1.0....

 

[JLNJ]

I would use 1.0 as well and be done with it. You are not building a watch, and the size of the project and the difference between factors doesn't justify any angst over the loading.

We really don't understand wind all that well anyway. If we did, the code would give us more than two exposure categories to choose from.