Attaching Posts to Piers for a Driveway Entrance

[flyingparks]

Wondering if anyone has attached posts for a driveway entrance to piers. I would put footing (which I'm debating is necessary) below frostline, then a 10-12" pier to receive an 8x8 post. I don't want to sink the post in the ground and I don't want to do any masonry work around the base of the post. I will eventually reach out to an engineer because that is how this county operates but I figured I would ask it here first. Is this connection possible with a knife plate? Wind load is 135. Any advice is much appreciated. I love this forum.

 

[Don P]

Never done one, it sounds interesting
Its basically a flagpole or cantilever condition. The force at the post to pier connection is at its simplest, the load at the tip of the post x the post height. 135 mph is about 45 psf. Just to throw some numbers at it for example's sake, assume there's around 10sf of sail up there and its tall enough for trucks, say 14', so 45 psf x 10sf x 14 ft=6300 ft-lbs at the post to pier connection. This is just a torque wrench standing up and there is the force on the connection. That isn't a simple knife plate. Check out some signs and lightpole connections next time you're out. How to achieve that in wood is beyond me but an interesting problem.

Assuming that is solved with some type of rigid connection.

To resist that lateral force at the pier to soil it is about the pier diameter and depth, the soil lateral bearing capacity and whether the pier is constrained at the groundline by a slab or unconstrained, just soil surrounding it. The codes and equations pertaining to all this start around 1806 here;
IBC, Foundations, piers


When its all done and said and assuming the piers are unconstrained I think you'll be around 6' deep in this example using a 1' dia pier and can get it down to around 4.5' deep using a 2' dia pier.

Do keep us posted on what you come up with , sounds like a fun project.

[flyingparks]

Don P,

Thank you so much for that wealth of information. I've got to get better at knowing how to calculate loads. I do know that a 1/4 fillet weld (which would probably be utilized in this application) is good for about 3.7 kips per inch. A kip is equal to 1,000 pounds. This is the connection I envision. a 9"x9"x1/2" piece of flat bar with 2'-3' long #5 rebar welded to each corner. This is wet set into the concrete pier. the knife plate is then welded to the flat bar. As the base of the knife plate is 8"x8" I then have 32" of welding surface. My math tells me that that equals over 50 tons. But the connection of the knife to the bottom plate of the knife plate has 16" of welding surface. So 25 tons. I guess it matters tremendously on the pier. And of course the real problem of how to resolve that load in the timber.  

 

 

At this location in the Rocky Mountains I won't be able to go too deep without equipment. Once the owners like the design I'll chat with my engineer and will definitely post an update.

I have a friend in the commercial concrete business and I remember asking him a few years ago about BIG stoplight poles. He told me that they don't dig very dig to put the piers in but they go very, very wide.

[flyingparks]

This image is public record. No mention of loads, but this is the type of foundation or a typical street light in Seattle...not where I'm installing this but a starting point. 

 

I like the idea of setting j bolts and attaching the knife plate directly to those so I can have some adjustment in the nuts. Also eliminates the need to weld on site.

[Don P]

The steel part is easy, the wood part is tough. Those 2 dowels are in tension perp to grain, it'll just split under that type of load. You can look up perma column but I think it'll be basically the same problem, from memory those were rated in the inch-lbs range. Maybe a tall boot of some sort, engineer land.

[Ruffgear]

Don, could you use bolts instead of dowels to keep from splitting?

[DWyatt]

Seems like the better option might be what Don is alluding to, rather than having a knife plate, you could have a sleeve that goes around the post. If you got an 8x8 square tube, you could shape the end of the leg to fit into the tube rather than relying on the shear strength of the wood fiber at the knife plate connection.

[flyingparks]

Seems like the better option might be what Don is alluding to, rather than having a knife plate, you could have a sleeve that goes around the post. If you got an 8x8 square tube, you could shape the end of the leg to fit into the tube rather than relying on the shear strength of the wood fiber at the knife plate connection.

Agreed. Was resolve to the fact when I went to bed that a knife plate is not the proper connection. A "sleeve" would work but I worry about how high it will need to go and drainage from the post where the sleeve stops. I guess one way to solve that problem would be to relieve the post at the bottom so the sleeve would be flush with the post. That might look cool too.

[Jim_Rogers]

The wind load is going to be in two directions only, front to back or back to front. There will be very little wind load left to right or right to left.
In stead of a tube sleeve that could hold water, put a flat plates on two opposite sides of the post. I would do front and back
These would be through bolted and stagger the bolt pattern so that then upper and lower bolts are not in the same grain line of the post.
I think there is a place near by me that has done this with light poles in a parking lot. I'll see if I can get a picture, later today.

Jim Rogers

[Bandmill Bandit]

What is the reason for not cementing the poles in the ground? 

I think I would be inclined to sink the poles about 3' into a 4' X 16"  hole that has about 10-12 inches of 2" drain rock in the bottom of the hole and then fill the hole with concrete to 3 or 4 inches above grade. Have done that many times with good results. Probably the simplest and likely the strongest and with the drain rock in the bottom the posts will last a long time too. I think it would be your most cost effective method as well.  

[Jim_Rogers]

I was wrong it is not two plates, but it is a sleeve. they hid that fact with an outside sistered slab of 2by stock.
Here is the lamp post:


 

here is the base:


 

 

 

to hide the wire going up they cut a groove up the side and put a piece of wood over it:



 

Notice how they staggered the bolts.

Jim Rogers

[flyingparks]

What is the reason for not cementing the poles in the ground?

I think I would be inclined to sink the poles about 3' into a 4' X 16"  hole that has about 10-12 inches of 2" drain rock in the bottom of the hole and then fill the hole with concrete to 3 or 4 inches above grade. Have done that many times with good results. Probably the simplest and likely the strongest and with the drain rock in the bottom the posts will last a long time too. I think it would be your most cost effective method as well.  

Just not a fan of that. But I guess I could look into it further. I always seems like the post is susceptible to rot when imbedded in concrete. Especially right where the top of concrete meets the post. Do you have that experience or are you treating your posts. Thanks for the feedback.

I was wrong it is not two plates, but it is a sleeve. they hid that fact with an outside sistered slab of 2by stock.


Jim, Thank you. It seems like the sleeve will be the way to go. That, or imbedding in concrete. I'll let you know what the engineer calls out.

[Bandmill Bandit]

When the post are encased in concrete so that there is no escape route for water AND the top of the concrete is not shaped to direct water away from the post then you are absolutely correct. However if the post is sitting on a good base of washed drain rock so that concrete does not encase the bottom of the pole you have a very solid reliable pole placement that has good water drainage away from the pole, you wont have trouble with rot.

I did quite few pole building in the 80s that way and so far no rotted off posts. This was a primary method recommended by Alberta Agriculture for pole structures back then and is still a standard method in this region. Fortis also uses if for transmission line construction.

PS I always made sure the drain rock came up around the bottom of the pole a few inches just to be sure there is good drainage out the bottom. More drain rock is better than less.         

[Don P]

I'm not opposed to washed rock under or around the post but the post itself needs to sit on a footing of some sort to distribute the load effectively (This could be treated wood, the footprint is what we're after). This is a short explanation by the awc of that code section (I linked to that part of the codebook above, see it for more detail), this is from DCA6 on the awc website under the publications tab;








The collar there is an uplift restraint, there are different designs.

[Cosmo52]

 

 
This is a similar structure that was used during a wedding.  Braces could be added to the feet to compensate for wind loading and four piers in the ground would give it good stability.  The final iteration of this project is a second identical bent that is attached by three stretchers per vertical posts with through tenons and will sit on 4 piers.  (The feet are temporary and will be discarded/recycled) The additional work obviously adds time and cost to the project but is an option.

[Don P]

Or, just to add more job creep, it could be 2 bents braced in both directions... and a roof to protect it all :D

[flyingparks]

Yea...I tried to push that hahaha. What a beautiful covered bridge it could be...

[Bandmill Bandit]

2" inch washed rock properly contained in place is all you need for a footing. I'd have to look it up to be sure but my memory says that an 8x8 set on 12" of 2 inch washed rock will give you the equivalent of a 18" footing for the pole assuming a rest angle of 45°.

You need to drill your hole the size of support base you want based on the depth of rock you will use. Washed rock footings are actually a better design idea than concrete in many applications. You don't get near the shift and movement as a solid concrete footing can deliver in a lot of situations.    


This diagram shows a concrete footing but the principle is relevant to the topic. The yellow zone is your critical gravel zone. After that more is just better to a dimension roughly equal to 3 X the area of the bottom of the post and crushed stone is better for the application. It is also the reason for the crushed stone ballast used under railway ties. There is no other substrate on the planet that gives the same footing stability and weight carrying capacity of wash rock.

A concrete footing in this application actually creates a zone at the bottom of the post that can cause moisture to accumulate around the base of the pole on top of the concrete and be drawn upward into the pole from the base there by encouraging rot etc. Rock will ensure a dry pole base most of the time and allow air to move around the base of the pole to accelerate drying after wet periods during the year. 

I have used 3/4" minus road finish grade gravel as a filler compacted around the post with a hydraulic post tamping tool quite often. This work very well BUT you have to be very thorough in your tamping of the gravel. You want to be as hard as concrete when your done. The advantage is that water moves through the gravel well.         

[Don P]

I don't disagree, I've built prefabricated basements on gravel footings several times. I would put a footing under the post on the gravel to distribute the load. It doesn't have to be concrete, I've used treated and post frame suppliers are advertising recycled high density plastic cookies now. Yes the cone of influence is a 45 degree angle. On those precast basements under places of concentrated point load the engineers called out a prefab concrete cookie under the precast wall. What I referenced above is from the building code, so there is the easier path. I did just get a plan through that had more than the 24" prescriptive max depth of gravel under the slab by referencing a prescriptive code section on deeper 1/2" minus compacted gravel under footings so there is room for interpretation by the building official.

That is a very good point about tamping the fill. If you're expecting to pick up any lateral resistance from a buried post tossing fill in the hole and walking away won't get you there, I've seen that from owner builders more times than I can count. The buried sonotube or post is just giving vertical support at that point. Fenceposts get better treatment than their houses.

[flyingparks]

Saw something like this...Through bolted in a few spots. My eye caught it at the last second while I was driving. Next time I'll take a picture.