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# Structural question about fence posts

## Structural question about fence posts

(OP)
I'm about to start my senior year in ME but I have questions about a small section of fence I'm building at a house I own.

The largest span I have between two 4x6 posts is 5.5ft, fence is 6ft tall. I'm trying to figure out what will fail first and then figure out how to reduce the probability of failure, all within reason (monetarily.) Code here in my community in south florida (hurricanes) says all I have to do is embed 4x4 in 2ft of 10" diameter concrete. I'm gathering that the most likely mode of failure is rot at the base of the fence post and then it snaps right where it goes into the concrete in high wind. So I have no problem calculating the force on this section of fence and the bending stress right where it goes into the concrete, I believe I'm fine to over 100mph with 4x6 posts with the 6" normal to the fence (its actually well protected from the east, and runs parallel east-west, so winds won't be as bad from the north/south.) My question is the bearing capacity of the soil and determining what, if anything, I can/need-to do to prevent the posts from beginning to tip in high winds if the posts don't break. The dirt down here is basically beach sand once you get down 6 inches. I believe the capacity of the sand is somewhere in the 10-15psi range. In any case, I'm trying to figure out if my 4x6 is more likely to snap at the base (assuming no rot of course) or if I should make some modifications to the hole (10" diameter now, but i'm only worried about north/south movement, would increasing the area of the hole normal to my expected force, and widening it at the top, be useful at all? I have no idea how to calculate this, it was much easier figuring out the bending stress on the posts.
Replies continue below

### RE: Structural question about fence posts

#### Quote:

1) ...figure out how to reduce the probability of failure, all within reason (monetarily.) Code here in my community in south Florida (hurricanes) says all I have to do is embed 4x4 in 2ft of 10" diameter concrete.

2) I'm gathering that the most likely mode of failure is rot at the base of the fence post and then it snaps right where it goes into the concrete in high wind.

1) Building codes are not "recipes" for successful projects. They are minimum requirements for life safety. If you want a better performance, exceed code requirements. Embed the posts in holes deeper than two feet.

2) IMHO, the most likely failure mode are the connections between the fence components and the posts. Poor connections are a common design weakness. Concerning wood rot, use pressure treated posts with more preservative than found at a typical lumber yard or big box store. Wood preservation is a technical subject in itself with standards for various applications and environments that have expected lifetime ranging from "none" to almost "permanent".

### RE: Structural question about fence posts

(OP)
I think we are on the same page about building codes being the minimum which is why I've already designed above and beyond. And what I was saying about the fence posts was from researching how most fail; the consensus seems to be post failure (either from winds higher than designed or post rot.) I'm just trying to determine if there is anything else I should do to increase longevity within reason. I actually don't know anything about wood preservation so that could potentially be a good avenue to explore. The other thing that comes to mind is determining if burying the post deeper or in a wider hole, for example, will provide any benefit or if the post is going to fail in a 2 ft hole anyways and a 3 ft hole adds no additional strength to the fence - but again, this i'm not sure how to calculate.

### RE: Structural question about fence posts

#### Quote (MB305)

1) I actually don't know anything about wood preservation so that could potentially be a good avenue to explore.

2) The other thing that comes to mind is determining if burying the post deeper or in a wider hole, for example, will provide any benefit or if the post is going to fail in a 2 ft hole anyways and a 3 ft hole adds no additional strength to the fence - but again, this i'm not sure how to calculate.

1) The recent (2019) publication "Guidelines for Selection and Use of Pressure-Treated Wood" by the respected USDA Forest Products Lab will give you a good overview on that subject, along with practical recommendations... for fence posts (including those surrounded by concrete), preservative level UC4A, or upgrade to UC4B. If you want essentially permanent fence posts, go to UC4C.

2) You are well on your way to becoming a practicing engineer. The math will give you the answer; verbal "opinions" from me, or others are worthless. Soil stress on a laterally loaded post look something like this:

For larger diameter concrete, quantity of excavation and concrete is proportional to the square of increased diameter. What counts, however, is cross sectional area of concrete to resisting overturning. That area is directly proportional to the increased diameter. Takes a lot of "extra" excavation/concrete to decrease soil stress... and you tend to wind up with a lawn full of oversized concrete circles.

For deeper embedment, quantity of excavation and concrete is directly proportional to increased depth. Concrete area to resist overturning is also directly proportional to increased depth. There is a more important benefit; moment arm for soil pressure to resist overturning is increased. This reduces soil stress,too.

www.SlideRuleEra.net

### RE: Structural question about fence posts

(OP)
Thank you for the information.

In regards to #2, I have seen that diagram and understand it, but I have not seen the calculations necessary to determine what force my soil (sand) can resist (assuming the post does not fail at ground level.) How would I calculate the total force being placed on the post underground when a lateral load is applied, the location of that vertical centroid, etc?

I assumed that cross sectional area perpendicular to the load (all else equal) was critical, which leads me to a question: the perpendicular cross sectional area of an oval should provide the same resistance as a circle of the same cross sectional area, correct?

My goal here is, assuming I've calculated the force that causes failure at ground level, to determine the optimal width/depth of hole that provides the same resistance so I don't waste any time digging deeper/wider than necessary (I assume it will be above code as my posts are above code.) But again, I haven't found those equations.

(And although it seems that deeper will provide more resistance than wider if quantity of excavation is the same, wider might be physically easier to do than deeper, and costs also increase both ways - larger machines, more labor, or more concrete. I want to look at it both ways.)

### RE: Structural question about fence posts

Good links. Soil properties are the "wild card" in post embedment. If you want to read about the necessary math for the calcs, here are a couple of links, one "new":

"Design Guide: Preliminary Embedment Depths for Concrete and Steel Poles"... principle is the same for wood poles.

and one "old": See the attachment below.

Concerning the 4 x 6 oversized posts, be sure to take advantage of the "Duration Factor" for short-term loading (wind) for wood. Perhaps oversized posts are not needed if properly treated 4 x 4 are used to prevent premature rot. You may still have a conservative design and get some cost savings, too, by using smaller posts.

### RE: Structural question about fence posts

(OP)
Based on those IBC calculations, even using double the lateral bearing pressure of our sandy soil for singular poles, it indicates something like 5.5ft deep for a 12" diameter hole to resist 440lbs at 3ft above the surface. It looks like I need a 18" diameter hole 4ft deep.

As for the reason I chose 6" x 4" wood, it was because there was a pricing error at Lowes where they sold me 10ft lengths for $15, cheaper than using 4" x 4" ($23 each, same grade, pressure treatment, etc) and a third of the price of full 6" x 6".

### RE: Structural question about fence posts

#### Quote (MB305)

IBC calculations... indicate something like 5.5ft deep for a 12" diameter hole to resist 440lbs at 3ft above the surface. It looks like I need a 18" diameter hole 4ft deep.

Unlikely. For example, a direct buried 30' long wood utility pole is installed in a hole 5' deep (10% of gross length + 2 feet). Wood guard rail posts at bridge abutments typically have embedment just over 3' deep (38"). There is probably an error in your use of the data, or the calcs.

I assume you are using the presumptive value of 150 psf/ft for lateral bearing pressure in sand (cohesionless soil). As and ME, you are likely making "allowable stress design" calcs... the value of 150 psf/ft can be increased by 1/3 to 200 psf/ft. If you are willing to accept 1/2" deflection at ground level, the value can be doubled to 400 psf/ft.

This number gives a triangular soil pressure distribution on the unloaded post... 200 psf at 1' depth, 400 psf at 2' depth, etc. When lateral load is applied, things change, a "short" post (this fence post qualifies) tends to rotate about a point that is between ground line and bottom of the post:

See the attached document for a summary of the calcs for Broms method of analysis of single (short) piles under lateral loading in cohesionless soil. Presumptive values (of anything) in building codes are conservative. If you obtain basic soil properties for the sand (unit weight and angle of internal friction), a higher value will likely be justified.

These calcs will give a value for the ultimate lateral force the post can resist... consider applying (or not) a suitable safety factor.

www.SlideRuleEra.net

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