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

[CJT]

So does steel.

[CWatters]

For what?

[jimbobmitchell]

e:

e quoted text -

Steel burns? I guess I learned something new today.

[JoeSpareBedroom]

Steel burns? I guess I learned something new today.

============

Anything will burn at the right temp.

[jimbobmitchell]

So there's no difference between burning and melting?

[CJT]

Sure there is. Steel will do either (or both) depending on the conditions.

[Stormin Mormon]

Does that mean that communities have to take down all the guard rails?

[HerHusband]

First, I'm no engineer, but a few thoughts did come to mind...

You didn't mention the height of the deck or the span between support posts, but in general there shouldn't be much of a load on a deck (no walls, ceilings, roofing, etc.). Assuming a typical residential situation, of course.

If the post footings are below the frostline in the area, they shouldn't heave anyway should they? But even so, PVC is fairly flexible, and even concrete flexes a small amount. With "typical" structural movements, I don't see this being an issue. They're not gonna "snap like twigs" at the first sign of stress.

If the PVC is filled with concrete, I don't see why it would be any different than a cardboard sonotube, except you would not need to remove the PVC after the pour. Even if the PVC doesn't offer any structural support, the concrete piers inside the PVC should support the weight of the deck.

If you're concerned about "flex" in the column, you should install rebar reinforcement whether you're using PVC, a sonotube, or even building a square box for the concrete pier.

Unless this is a second story deck, or one perched out over a hillside, "most" decks only sit a foot or two off the ground and are supported by multiple pier posts. I doubt there will be any significant sideways loads to buckle the columns.

A recent article in Fine Homebuilding showed a system that used PVC pipe to support a small shed (with no concrete). If it works for a shed, it's bound to be adequate for a deck.

Unlike a steel post, PVC won't rust. However, regular PVC will be damaged by ultraviolet light from the sun, and will get brittle. You could avoid this by using grey PVC conduit which has protection against ultraviolet light.

Around here, many decks are built with nothing more than a 4x4 post sitting in precast concrete pier blocks sitting on the ground and have lasted for decades. Also, many mobile homes sit on concrete blocks that are just dry stacked piers with no reinforcement (ours was setup that way for 13 years, with one end nearly four feet off the ground, and we survived two earthquakes with no problems). These are obviously UNDER engineered situations, but it's also possible to OVER engineer the support for a deck as well...

Anthony

[scrumble]

The problem with fire and steel beams/columns isn't so much from structural weakness when the steel becomes hot, although that would be a concern where the intention is to contain the fire for a length of time, but more from the actual expansion of the steel, either from it pushing other structural elements, or itself buckling when the expansion at both ends is restricted.

[scrumble]

But surely if that was the only reason for the concrete it would far easier and cheaper just to coat the steel with an intumescent paint?

[Thomas G. Marshall]

I agree, but further, this is an odd "argument".

I look also at the things that might happen in a basement. Heavy things could be moved around and dent the column. It would take a hell of a whack to do that, but it seems to me that filling the thing with concrete is an incredibly cheap way of ensuring that a column doesn't collapse in such an [albeit rare] event.

Further, having seen what my 3 1/2 and 1 1/2 year old consider game for smacking with a rock or hammer, I'm thinking that substantially removing the notion of disaster with concrete innards is a great idea. And I can imagine a young person able to swing a sledge hammer thinking it a "funny idea" to send a shock wave through the house, thinking perhaps that the lolies were solid steel or somesuch.

[Thomas G. Marshall]

Matt Whiting said something like:

...[snip]...

In my original post I believe I was clear in using the steel column reference only in what the person I met had used as his reasoning for filling them with concrete.

I pointed out that HE was under the impression that the strength of the steel column was primarily the concrete within it, and he believed that the steel was a mere form for it. Hence he figured that using nearly anything as a form, e.g. PVC, would be enough for the deck supports.

I am not sure of the diameter. I am not sure of the length. I am not sure that they were also the footings (though I hope not, since I doubt he understands the notion of "big feet" and I doubt he was using hugely wide posts).

But I have seen concrete cracks in all kinds of things, and I can't help but imagine a crack going clear through one of those things causing a disaster. YMMV {shrug}

[Thomas G. Marshall]

CWatters said something like:

I mentioned the deck in the subject and in the paragraph you snipped. But I was not as clear as I could be: he was using them as the posts.

[salty]

What can I say? Filling lolly columns with concrete FOR FIRE SAFETY is part of THE CODE in many places.

[Matt Whiting]

What is a lolly column? Is that anything like a lally column?

What place has this in the fire code?

Here is a reference that clearly states that the main purpose for filling the columns is strength. And it discusses that the concrete can actually be detrimental in a fire in certain conditions although it certainly is beneficial in other conditions.

Bottom line is that filling the columns with concrete is not done primarily for reasons of fire resistance. It is for strength first and fire resistance is a nice fringe benefit.

Matt

[George]

Exactly, the perfect example is to consider what is actually happening when you squeeze the lever on a cutting torch.

[jimbobmitchell]

.

That, imho, is a perfect example of melting, not "burning"

[Nate Nagel]

some of it does indeed burn. if you don't believe me, hold a match under a wad of 0000 steel wool. Outside, or in a deep sink...

nate

[PeterD]

Reference aside, do you know what cutting torch is how it works?

A cutting torch *burns* the steel using a stream of oxygen that is triggered by the lever, though the steel must be hot enough that it will burn first. However, not all the steel is burned, some does fly off as molten metal.

Melting a material changes its state. The state can be restored by lowering its temperature.

Burning a material changes the material's composition usually combining it with oxygen to form a new substance (iron oxide, for exmaple) Once a material is burned lowering its temperature won't restore it.

[RicodJour]

And how many angels did you get to dance on the head of that pin? You're engaged in sophistry. We are discussing steel columns and house fires. For all intents and purposes there is no burning of a steel column. But feel free to continue the debate - let me know when you feel you're winning.

R