Disaster waiting to happen? Using PVC for deck supports???

[Thomas G. Marshall]

There was a person I was speaking to recently who was proud of what he considered to be a very bright move on his part.

He had noticed that the lolly-columns in his basement had been filled with cement prior to being used. He assumed that the strength of the support was in the cement, and therefore concluded it was a good idea to try filling PVC tubes and using those as posts (note-not as sonno-tubes, but as fully structural support posts).

I pointed out that I thought that the strength of a lolly was in the steel, and that the cement was there merely to ensure that it was never dented, causing it to fold like an aluminum can. I was concerned that the first major frost heave under his deck that is able to stress the ledger enough to pull outward a small amount would cause his pvc+cement "posts" to break.

Was I right?

[salty]

The concrete in steel lolly columns is to help keep them from collapsing in a fire.

[EXT]

It probably depends on the diameter of the PVC tube. A 3" or 4" will not have much strength, especially if there is not a continuous large diameter re-bar down the middle. I can see the PVC cracking in cold weather, particularly if the top is able to soak up some water that freezes.

A 6" or 8" PVC tube with three or four re-bars, probably would be no more than a vinyl coated concrete column, and look good compared to bare concrete. The concrete will take most of the load. I can see the same problems with freezing water however. It could be difficult to make a connection with the foundation unless the post is run all the way down to the footing.

[RicodJour]

I think you're right that there's cause for concern, I think you're wrong about the failure mechanism unless the footings are seriously sub-standard, and I think there's a lot of information left out. The diameter of the PVC, whether it's Schedule 40 or 80 or even the thin stuff that they use for drainage, how deep the footings are, how high above grade the deck is, etc.

R

[curt]

If I remember correctly, Schedule 80 is called for on all deck support posts.

[Thomas G. Marshall]

Implication question: Is PVC meeting schedule-80 requirements considered acceptable for deck supports?

[Thomas G. Marshall]

[e-mail] said something like:

...[snip]...

....so the preventing a dent thing is secondary, or not a concern...?

[JoeSpareBedroom]

My interpretation is that the dent thing is completely irrelevant.

[Dick Adams]

Ignorance is bliss.

I used PVC for my mail box post. Went down 18 inches, carved out a 3" foot lip, crisscrossed 8" bolt at 6" and 10", filled with concrete and rebar. My mail box was getting knocked down by cars sliding on ice.

Someone was knocking down mail boxes with a baseball bat. The police found him when I told them to check for people with forearms. This clown broke his forearm and his wrist.

But as a deck support, that's a joke.

Dick

[Pete C.]

There are code approved PVC pipe based deck support columns available. The compressive strength of PVC pipe is tremendous as long as you keep it from buckling. I once tested a 1' length of 4" sch40 PVC pipe in a hydraulic press and it too over 30 tons of force before it started to fail (Enerpac press with pressure gauge). The compressive strength of concrete is also very high. Combined strength is more than adequate and PVC handles cold pretty well also.

[Matt Whiting]

It also does provide much improved buckling resistance.

The PVC filled column is a very dumb idea unless an engineer has computed the capacity of the column and sized it appropriately for the anticipated loads.

Matt

[Matt Whiting]

No, preventing buckling is a significant advantage provided by filling a steel column.

Matt

[Matt Whiting]

Your interpretation is incorrect.

Matt

[JoeSpareBedroom]

OK.

[Glenn]

Interesting concept. Never did that but with 2 or 3 rebar in it, maybe a #8 right up the middle, it seems like just a neat form that you don't have to remove.

[Glenn]

Whoa, if you are using a steel column, why the concrete? Sky scrapers sit on steel columns with out concrete in them. What am I missing?

[Matt Whiting]

I'd hardly call it tremendous. 8300 psi exceed most standard concrete, although high strength concrete is available now that substantially exceeds this value. And this falls FAR short of even standard A36 steel.

[Pete C.]

8300 psi? I come up with more like 37,500 psi since this was 4" dia sch 40 PVC pipe with no filling i.e. 60,000# load on something like 1.6 square inches of PVC total.

[Lou]

What am I missing here? The extra cost alone makes it not worth doing. Not to mention how you would attach them without making it look jerry rigged. Even if you got past those two items, what would you make the rest of the deck out of to look right? I suppose if your only going 12" off the ground that it wouldn't matter but it sure sounds like your taking a step backwards. Lou

[salty]

Uh, yes. And the buckling it prevents would be the result of fire. Filling the columns with cement is to protect the columns from buckling in a FIRE.