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

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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?
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On Sun, 18 May 2008 18:02:54 GMT, "Thomas G. Marshall"

The concrete in steel lolly columns is to help keep them from collapsing in a fire.
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snipped-for-privacy@dog.com said something like:
...[snip]...

....so the preventing a dent thing is secondary, or not a concern...?
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wrote in message said something like:

My interpretation is that the dent thing is completely irrelevant.
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JoeSpareBedroom wrote:

Your interpretation is incorrect.
Matt
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said something like:

OK.
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Thomas G. Marshall wrote:

No, preventing buckling is a significant advantage provided by filling a steel column.
Matt
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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?
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Perhaps four years of engineering school? Or even two semesters of structural member strength? <g>
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wrote:

Cute. When you have 1300 degrees on the wood it's supporting, the column is the last thing I will be concerned with. In a total of some 45 years of construction, I have never seen this in my neck of the woods. Perhaps some are over educated? <g>
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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.
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On May 18, 8:45 pm, snipped-for-privacy@dog.com wrote:

In a wood framed building? I guess if the fire started at the base of the column that might be a good argument, but in any real situation the house would be engulfed in flames long before the plastic temperature of the steel was reached.
The concrete fill is to prevent buckling, partially from being dented, but also to prevent localized failure which can occur at lower loads than the straight compressive strength of the material(s) would indicate. http://en.wikipedia.org/wiki/Buckling
R
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RicodJour said something like:

This makes sense to me, though I'm hardly as versed in structural analysis as the other denisins of this ng. Fire seems a concern, though secondary, only because the plasticity of steel could never be reached in a basement without the rest of the house already having been reduced to dust. Or so it seems to me.
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Thomas G. Marshall wrote:

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.
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snipped-for-privacy@dog.com wrote:

It also adds tremendously to normal buckling resistance, not just buckling resistance due to fire reducing the capacity of the material.
http://www.aisc.org/Content/ContentGroups/Documents/Connections_IV_Proceedings/51.pdf
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said something like:

PVC burns doesn't it?
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CWatters wrote:

So does steel.
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Steel burns? I guess I learned something new today.
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Steel burns? I guess I learned something new today.
=========== Anything will burn at the right temp.
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So there's no difference between burning and melting?
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