Post extension engineering thread ... (perma-columns, sonotube, etc.)

[gjbuilder]

(Thanks to this forum and its community - much appreciated)

So ... a perma-column is basically a "post extension" and you could make a post extension in a lot of ways. A concrete post extension (perma-column or DIY with sonotube) or a steel beam post extension ... or even just another piece of wooden post. Just different ways of extending a post (since part of it was cut off for being rotten).

Three initial engineering questions come to mind:

First, *if* you properly connect a post extension (below) to an existing post (above) does it really become, effectively, one post ? I understand all of the lateral strength benefits that a sunken pole give to a pole barn (thanks to research on this forum) and so I start to worry that a post with a joint in it can provide those same benefits.

Second, shouldn't my post extension go as deep or even deeper than my existing poles ? That's difficult to do with perma-columns since they are only 5' long and 12-18" of them will be above ground anyway ... if you have poles buried 6' or deeper in the ground, don't you need a 7-8' perma-column ? Is there something about these engineered posts that would lessen your depth requirements for seismic and wind load ?

Third, would pouring concrete around a post extension defeat the purpose ? Let's say I use a perma-column and I dig out my hole 18" in diameter, attach the perma-column and then fill in the hole with concrete. That really appeals to me because it is so solid and I am effectively building a mini-pier under the column ... but maybe that would be a very bad idea because you would then lose all flex at ground level, since the perma-column would be totally immoveable, and you're basically turning your in-ground pole into a very expensive pier ... not only would you lose all the lateral benefits of the in-ground pole, but now you've got your bracket up at 12-18" high ... basically little stilts on top of piers.

Am I correct to be cautious about a pole-extension inside a pier like that ?

Thanks, as always.

 

[gjbuilder]

For the archives / other readers...

Answer to #1, above, is yes. If you properly attach the column extension to the remaining portion of your wood pole, above, it does effectively become one column.

As for #3, perma-column rep tells me that wouldn't be negative in any way (pouring a pier around the perma-column) but I'm not convinced ...

 

[theoldwizard1]

First, *if* you properly connect a post extension (below) to an existing post (above) does it really become, effectively, one post ?

Unless you live in a active earthquake area I would not give it another thought. Even then, those attachment points are designed to take a certain amount of seismic activity.

Second, shouldn't my post extension go as deep or even deeper than my existing poles ? That's difficult to do with perma-columns since they are only 5' long and 12-18" of them will be above ground anyway ...

60" - 18" = 42" Plenty in most locations.

Third, would pouring concrete around a post extension defeat the purpose ?

I would say yes.

If you want a larger area for the post to rest on, rent a Bobcat with an auger of whatever size is appropriate, drill the hole, insert the tubes and pour.

 

[rieferman]

As for #3, perma-column rep tells me that wouldn't be negative in any way (pouring a pier around the perma-column) but I'm not convinced ...

When in the business previously, here's how it was explained to me....

In a normal building scenario, you wouldn't pour concrete around the perma-column. In these cases, the entire building together is a system working together. Between the uplift anchors on each post, the footers at the bottom of each hole, the skeleton of the building, and the sheathing (plywood or steel), there's no strength need for concrete around the perma-column.

In another case... let's say an open air pavilion... we WOULD pour concrete around the perma-columns because the "system" of the building is much less of a system once you eliminate the skeleton and sheathing. Therefore, to get the rigidity and resistance to racking, we poured around each post. We showed engineer stamped drawings to support this approach, and had success in many townships across Mid Atlantic states. There may be other ways to get the same result (e.g. deeper post holes) but this route provided a clean look and was efficient to do in terms of time and dollars.

 

[rieferman]

Second, shouldn't my post extension go as deep or even deeper than my existing poles ? That's difficult to do with perma-columns since they are only 5' long and 12-18" of them will be above ground anyway ... if you have poles buried 6' or deeper in the ground, don't you need a 7-8' perma-column ? Is there something about these engineered posts that would lessen your depth requirements for seismic and wind load ?

Seismic and wind load requirements are what they are for your area, regardless of what pole you put in the ground.

Agree with oldwizard that in most areas the 5' concrete column (with 12" above ground approx and remainder below) meets/exceeds local code requirements. But longer are available too if locality mandates it. We had occasion to use longer perma-columns (e.g. in cases where we needed to reach virgin soil) and had good experiences in those cases.

In terms of the strength of the perma-column, I was told (by a non-employee, but expert on the topic)) that wood posts would break before the joint between wood and concrete during break testing. That's hella strong.

 

[gjbuilder]

In another case... let's say an open air pavilion... we WOULD pour concrete around the perma-columns because the "system" of the building is much less of a system once you eliminate the skeleton and sheathing. Therefore, to get the rigidity and resistance to racking, we poured around each post. We showed engineer stamped drawings to support this approach, and had success in many townships across Mid Atlantic states. There may be other ways to get the same result (e.g. deeper post holes) but this route provided a clean look and was efficient to do in terms of time and dollars.

Ok, so in your mind, concrete around the perma-columns ONLY makes them stronger - there is no way it weakens them.

Ok, thank you.

 

[rieferman]

Ok, so in your mind, concrete around the perma-columns ONLY makes them stronger - there is no way it weakens them.

Ok, thank you.

Agreed. Since there are cases where we would install them with concrete collars, I would think that it would be fine (even if overkill) in other cases as well.

 

[pmiranda]

I guess at some point you might have so much concrete that it is too heavy for the soil, but you have to have pretty bad soil for that to be true. Also, if you ever need to remove or replace it, it will probably have to be taken apart instead of cleanly removed in one go.