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



 
 
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  #31  
Old May 20th 08, 03:28 PM posted to alt.building.construction,alt.home.repair
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Posts: 6,757
Default Disaster waiting to happen? Using PVC for deck supports???


Matt Whiting wrote:

Pete C. wrote:
Matt Whiting wrote:
Pete C. wrote:
Matt Whiting wrote:
Pete C. wrote:
"Thomas G. Marshall" wrote:
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?
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.
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.

http://www.harvel.com/piping-clear-pvc.asp
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.
Material properties don't depend on the size of the pipe. Did you even
open the reference I provided? Do you understand MATERIAL properties?

Matt


Do you understand that under testing in my 50T press, it took a load of
~37,500 PSI on the 1' length of 4" sch 40 PVC pipe before it failed?
Material properties are irrelevant, that was the actual result of the
test I noted.


37,500 psi is a pressure, not a load.


It is both.

Material properties are not only
relevant, they are essential to almost all structural engineering
calculations (I know as I have a masters in civil/structural).


And what I presented was not a structural engineering calculation it was
simply the results of a real world test that point out that sch 40 PVC
pipe is a lot stronger than most people think.

And the
behavior of a short column is MUCH different than a long column. Ever
heard of Euler?


No kidding, and again, I didn't present any structural engineering info,
simply actual results of a real world test showing the surprising
strength of PVC pipe.


A nearly pure compression test (which is what a 1' long 4" pipe
comprises) has almost no relevance to the case of a column that has a
substantially different slenderness ratio and thus subject to a buckling
failure mode as well as possible bending moments due to eccentric loading.


Right, but again, I didn't present anything to the contrary. I indicated
that code approved PVC pipe based deck supports were available, which
they are, and an example showing that PVC pipe is stronger than people
think.


I didn't see a column length in the OP, but when "basement" is the
description rather than "crawl space" it is likely that the length is at
least 6' and possibly 8' or even more. This is far from being a 1'
column. Concrete filled PVC could work if sized properly, but using the
same size as the existing steel column is a fool's errand. Again, the
OP didn't mention the size of either the steel or the PVC, but I'll bet
there were likely nearly the same size. If that is the case, then the
PVC is almost certainly inadequate, unless the steel column was grossly
oversized for the required load.


Probably, and I didn't suggest the OP's scenario was proper or safe. I
simply noted that "Using PVC for deck supports" as in the subject line
is possible and code approved if done properly, and not a "Disaster
waiting to happen" as was also in the subject line. Can't seem to find a
link to the product at the moment. Believe I read about it in Fine
Homebuilding or perhaps JLC.
Ads
  #33  
Old May 20th 08, 08:40 PM posted to alt.building.construction,alt.home.repair
CJT
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Posts: 1,155
Default Disaster waiting to happen? Using PVC for deck supports???

CWatters wrote:

wrote in message
...

On Sun, 18 May 2008 18:28:48 -0400, Matt Whiting wrote:


Thomas G. Marshall wrote:

said something like:

...[snip]...



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

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



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

Matt


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.




PVC burns doesn't it?


So does steel.

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  #34  
Old May 20th 08, 08:41 PM posted to alt.building.construction,alt.home.repair
external usenet poster
 
Posts: 18
Default Disaster waiting to happen? Using PVC for deck supports???


"Thomas G. Marshall" . com
wrote in message news:ib_Xj.134$ay2.84@trndny01...


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).


For what?



  #35  
Old May 20th 08, 08:48 PM posted to alt.building.construction,alt.home.repair
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Posts: 17
Default Disaster waiting to happen? Using PVC for deck supports???

On May 20, 3:40*pm, CJT wrote:
CWatters wrote:
wrote in message
.. .


On Sun, 18 May 2008 18:28:48 -0400, Matt Whiting wrote:


Thomas G. Marshall wrote:


said something like:


...[snip]...


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


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


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


Matt


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.


PVC burns doesn't it?


So does steel.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. *Our true address is of the form .- Hide quoted text -

- Show quoted text -


Steel burns? I guess I learned something new today.
  #36  
Old May 20th 08, 08:53 PM posted to alt.building.construction,alt.home.repair
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Posts: 2,906
Default Disaster waiting to happen? Using PVC for deck supports???

wrote in message
...
On May 20, 3:40 pm, CJT wrote:
CWatters wrote:
wrote in message
.. .


On Sun, 18 May 2008 18:28:48 -0400, Matt Whiting
wrote:


Thomas G. Marshall wrote:


said something like:


...[snip]...


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


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


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


Matt


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.


PVC burns doesn't it?


So does steel.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .- Hide
quoted text -

- Show quoted text -


Steel burns? I guess I learned something new today.

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

Anything will burn at the right temp.


  #37  
Old May 20th 08, 10:44 PM posted to alt.building.construction,alt.home.repair
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Posts: 17
Default Disaster waiting to happen? Using PVC for deck supports???

On May 20, 3:53*pm, "JoeSpareBedroom" wrote:
wrote in message

...
On May 20, 3:40 pm, CJT wrote:





CWatters wrote:
wrote in message
.. .


On Sun, 18 May 2008 18:28:48 -0400, Matt Whiting
wrote:


Thomas G. Marshall wrote:


said something like:


...[snip]...


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


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


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


Matt


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.


PVC burns doesn't it?


So does steel.


--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .- Hide
quoted text -


- Show quoted text -


Steel burns? I guess I learned something new today.

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

Anything will burn at the right temp.- Hide quoted text -

- Show quoted text -


So there's no difference between burning and melting?
  #39  
Old May 21st 08, 03:44 PM posted to alt.building.construction,alt.home.repair
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Posts: 810
Default Disaster waiting to happen? Using PVC for deck supports???

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 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.


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
  #40  
Old May 21st 08, 07:45 PM posted to alt.building.construction,alt.home.repair
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Posts: 2
Default Disaster waiting to happen? Using PVC for deck supports???

Thomas G. Marshall wrote:
RicodJour said something like:
On May 18, 8:45 pm, wrote:
On Sun, 18 May 2008 18:28:48 -0400, Matt Whiting
wrote:
Thomas G. Marshall wrote:
said something like:
The concrete in steel lolly columns is to help keep them from
collapsing in a fire.
....so the preventing a dent thing is secondary, or not a
concern...?
No, preventing buckling is a significant advantage provided by
filling a steel column.
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.

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.aconcern, 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. {shrug}


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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(\_/) Procrastinator bunny says:
( . .) "I'll conquer the internets tomorrow"
C(")(")
 




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