A different kind of fencepost problem

May 3, 2007 10:57 AM

Stuctural-engineering filter: I need some help figuring out requirements for a temporary art installation that needs to stand up to the wind.

The installation will resemble a fence made of T-posts and panels of vinyl banner material. Each panel will be approximately 2 m square (I have some flexibility on this), and will be supported by T-posts left and right, held to it by zip-ties through grommetted holes at 60 cm intervals. I'll be cutting slits in the panels to allow some wind to pass through, but it needs to be able to stand up to 60 kph winds. My questions are:

1. How deeply do the T-posts need to be sunk into the ground?
2. Can I double-up panels on T-posts, so a single T-post is supporting panels on both sides, or should each T-post be holding up only one side of one panel?
3. Another way to approach #2 is this: below a certain panel size, I could double up (I imagine). What is that size? I'm interested in minimizing the time and effort it will take to set all these T-posts.

I'll be grateful for any other suggestions along these lines. I've thought about anchoring the T-posts with cables and stakes front and rear, but site considerations make that impractical, and I am hoping it would be unnecessary.

posted by adamrice to science & nature (8 comments total)

How high do you anticipate having the posts above the ground? Two meters? Are the posts going to be wood or metal? I assume that this is just regular grassy ground and that the posts are going to be pounded in? Or do expect to dig a hole, put in the post and backfill?

There should be no larger torque due to having two panels back to back.
posted by JJ86 at 11:20 AM on May 3, 2007

JJ86: A T-post is a metal post with a T-shaped cross-section, typically used to put up fences quickly. It's pounded into the ground with a sort of tubular hammer.

I don't see how there wouldn't be greater torque when doubled-up, since each T-post will be supporting twice as much surface area. The panels are not back to back, but side by side. Perhaps I'm missing something.

The posts will run high enough above ground to support the panels all the way up, so yes, 2 m, give or take. I can get them in 2-foot increments (probably 1-foot if I scrounge).
posted by adamrice at 11:42 AM on May 3, 2007

60 kph? that is only about 37 mph, not much of a wind. At that speed the horizontal force on each panel will be about 160 lbs. At 50 mph it will be more like 275 lbs.
posted by caddis at 11:43 AM on May 3, 2007

I guess the only thing missing is the quality of the soil. If you have a well packed soil, maybe about .5m underground should do. A looser fill soil may require closer to 1m.
posted by JJ86 at 11:58 AM on May 3, 2007

I'm thinking at that height, a t-post isn't a great option. A tubular steel post might offer less flexing. Most "T-posts" are too flexible.
posted by JJ86 at 12:01 PM on May 3, 2007

Flexing is OK. Breaking/uprooting is not. This is only going to be up for 4 days.
posted by adamrice at 12:17 PM on May 3, 2007

Well if it is windy, flexing isn't ok because it will loosen the soil thus destroying the structural integrity. In that case you may need to add another foot underground.
posted by JJ86 at 12:22 PM on May 3, 2007

Caddis's numbers are for a flat surface perpendicular to the wind. If a single panel is supported by two posts, then each post would support about 80 lbs. If you double up so that each post has a panel on each side, then each post supports 160 lbs.

Cutting half-moon slits will reduce the wind load, but the calculation is difficult and depends on the spacing and size. But using the worst case numbers above will give you a safety margin.

A rough rule of thumb for posts is that you want 1/4 to 1/3 of the total length in the ground. So you would need 2.7 to 3.0 meter posts with 2.0 meters out of the ground. The ability to remain upright depends on the softness of the ground.

To give you a rough idea, you could put a trial post in the ground, grasp it at shoulder height and lean backwards, putting as much of your weight on it as possible. Try it again with two people.
posted by JackFlash at 12:25 PM on May 3, 2007