Chain Link Fence Engineering 1

[medeek]

This is something new for me, most of time is spent dealing with wood structures but today I'm faced with a 10' high chain link fence (tennis court) in a high wind locale (155 mph ult., 120 mph ASD).

I just downloaded a copy of the Wind Load Guide for Selection of Line Post and Line Post Spacing from the Chain Link Fence Manufacturers Institute. This seems like a good starting point. It's a 38 page document so it may take a while to wrap my head around all of it.

Right out of the gate I can see it references the ASCE 7-10 and the 2009 IBC as well as ASTM F567 for footings.

I'm not particularly fond of tables if I can derive the values directly through equations. My goal with this project is to create a spreadsheet that will generate the correct line post spacing, footing depth and size given typical inputs for fence height, privacy slats, basic wind speed, exposure, icing conditions, post diameter and group type etc...

Are there any other references or engineering resources that deal specifically with chain link fences that I have missed? Existing software/tools?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[martintstruct]

The guide is useful for figuring the wind are on chain links based on the percentage of wind each configuration catches. You should use new wind speed/code to calculate pressures and figure the line post strength as shown in the example in the guide. The footing I think is better to calculate per pole embedment section in IBC. The whole chain link fence line post guide I believe also just got updated to say this; I don't have the updated guide yet though.

 

[medeek]

It is very simple to use but I don't really see any comments about how to handle slats. These will increase the wind pressure but I don't think they should be considered a solid fence since they will be somewhat porous.


A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

I am somewhat confused now. Example 3 in this guide goes through and calculates the footing depth of the line post using Eq. 18-1 from the IBC. To obtain the wind pressure the guide uses Fig. 29.4-1 (Solid Freestanding Walls) with the values tabulated for different exposures and velocities in Table 13. The problem is these values appear to be the "Strength Level" pressures and not the ASD pressures. They are then applied directly in Example 3. My thinking is the force "P" used in equation 18-1 should be an ASD level force, correct me if I'm wrong.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

My suspicions are correct. I found an older version of the guide (2008) and calculated the same fence using the ASCE 7-05 wind values (ASD), you get a different value than the updated guide using ASCE 7-10. Basically, it appears they attempted to switch to the ASCE 7-10 but did not adjust their equations to account for the "ultimate" wind speed. To use the guide properly you first need to convert your wind speed into the ASD value 135 mph -> 105 mph otherwise you will be oversizing your line posts and your post foundations, what a mess.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[BUGGAR]

A local tennis court here just put up rubberized canvas sheets covering all their fences (some local privacy issue) so you may want to consider the fencing solid. If it can happen, it will.

 

[medeek]

The footing depth equation found in the IBC (eqn. 18-1 and 18-2) are pretty straight forward. Unfortunately, 18-1 will require an iterative approach. I also noticed the referencing of ASTM F-567 minimum footing depth eqn. which can give a much more conservative footing depth. Do others use the ASTM F-567 or just go with the IBC equations?

Based on Table 9 from the Chain Link Fence Guide a 2" mesh with #9 gage wire has a C_f1 = 7.26. The reciprocal of this coeff. is the porosity of the fence = 0.138 or 13.8% of the wind load of a solid fence.

Similarly a fence with vertical privacy slats might have a porosity of 70% and hence a C_f1 = 1.42 based on my searches on the internet and various vertical slat descriptions and specifications.

The bottom line is a vertically slatted fence is going to have a much higher wind load than a standard fence without slats. My problem is that using the guide as a template I am getting some ridiculously close line post spacing numbers when I use a C_f1 = 1.42. Perhaps I am missing something here.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

Here is what I have thus far:

Comparing my footing depths with what was done previously by another engineer I seem to be getting depths that are quite a bit deeper for the same footing diameter (ie. 36" -> 54"). Is there some additional reduction in loading due to wind or something else I am possibly missing?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[BUGGAR]

Your gust effect factor reflects a flexible structure. Shouldn't you consider some flexibility?

 

[medeek]

I have set the gust effect factor to .85 to reflect a rigid structure. Once again the natural frequency of the fence will be greater than 1Hz and per Sec. 26.2 if is permissible to use a gust effect factor of .85.


A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[azcats]

Did the previous engineer double the allowable soil resistance per section 1806.3.4?

 

[medeek]

He must have, however it is very hard to tell from his single page write up. He essentially quotes a value of 200 psf/ft and then his sketch shows the line post footings at 36" deep.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com