embedment depth of posts

[tabio]

I am required to show calculations of the embedment depth of posts on an aluminum fence 6' tall. There are two conditions:

1. the posts are anchored on a 6" slab on grade, I proposed 4" anchorage depths on non-shrinking grout.

2. The posts are anchored on a compacted sandy soil, I proposed 12" embedment depth surrounded by 12" diameter x 24" depth concrete.

In addition to the stresses at the base of the posts (which I already have),the building official requested engineering calculations for the embedment depths. I have no idea how to calculate them. Could any one give me some directions as to how to get a formula to calculate them?

 

[Ginger]

Tabio

For Option 1

If the post is bolted down onto the top of the concrete then this will act as a column base taking moment. The method of analysis is to balance the compressive stress in the concrete below the base plate with the tension force in the anchor bolts when the moment is applied at the base.

If theleg are set into the concrete then a pocketed base approach is necessary. Both of these design methods are set out in The Steel Designer's Handbook -published by the Steel Construction Institute (UK).

For Option 2 I suggest you look at the design of king post- type retaining walls, The buried footing can most economically be designed using the approach by Clayton & Milititsky. There should be several reference books which outline his approach. I use CIRIA Report SP95.

Andy Machon

 

[haynewp]

UBC has information for calculating the required embedment lengths and diameters for circular embedded foundations.

 

[Qshake]

There are many details available on and off the internet for this.

For a slab 6" thick I would hope that you will just bolt to the slab rather than create a recess for water to accumulate. One way or the other slab embedments will crack especially if they are near the edge.

For the other condition, standard embedment for post is approximately (approximately because all locations may differ somewhat) 2'-3' and may or may not be embedded in concrete depending on what the post is used for (line post or pull post) In rocky, shallow areas a concrete block is used as well.

Get out and search around!

 

[atilashorse]

I have a similar problem: I need to calculate the embedment of railing posts in concrete slab for balconies.

In this case, the post is subject to a lateral load, creating a moment at the post base, which inturn subjects the concrete slab to shear. My problem is to determinet the are of concrete subject to shear. Any ideas?

Please this does not refer to anchor bolts in axial loading, but a post embededed in concrete, under bending moment.

 

[BigH]

Ramcode - yours would have been a wonderful new thread since it is a different problem.

Seems kind of wierd that building department wants calculations for a fence post. Do they require similar of telephone/hydro poles?? - which are likely based on charts developed locally over the years. Do a search of columns in this forum and you'll find many discussions in the past of posts - but yours is so small. How many posts to install? I'd go at least 2 ft into ground - then show that this will give you adequate "capacity" in vertical loading - you don't mention horizontal loading - but unless there are impact problems or the posts are holding up "solid" sheeting for fence, there should be little horizontal loads so that isn't really an issue.

My initial thoughts!

 

[Focht3]

Yeah, seems like a real waste of time unless it's a "fencecrete" wall or some other condition exists that you haven't described. Generally speaking, two feet should be more than adequate unless the posts are in soft sands -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[atilashorse]

Sorry if I could,nt explain the problem clearly.
Calculations are required becuase this is not a fance, but a stair railing, or guardrail. Posts are installled by drilling the concrete steps, placing the guardrail posts inside the drilled holes, and filled with grout.

The problem is that local code requires a 200 lb lateral force, and at the same time, the posts are installed very close to the edge of the concrete steps, causing possible breakout of the concrete.

this does not have to do with fence on soil, direct burial of posts or anything like that.

My concern is what I described in particular, and in general, how to determine concrete breakout patterns when subject to different types of loading, escept pullout, which is quite simple.

I have started a new thread as you sujested, but no one has replied.

Thank you for the advise. If there are any further explations, i will welcome them.

 

[atilashorse]

Aditional clarification:

I am referring to the failure mode of the connection between the concrete slab, and the embedded post, which in this case is very close to the slab edge (2 to 5 in). Post is 2 in diameter.

It is important to calculate because this applies to balcony guardrails. Somebody falling from a 20-story-high balcony because of a failed guardrail connection, certainly won't consider proper design of the connections a waste of time.