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Re: Looking for Structural failure pictures

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Daryl,

When you say "cavity" wall are intending to mean a composite, multi-wythe
masonry wall?  That is what it seems like you intent by your comments.

I ask because to my knowledge a cavity wall is generally not treated as a
composite, multi-wythe wall.  That is a cavity wall is generally not
designed to have both wythe act together to make a stronger wall system by
effectively creating a wall section with a larger cross section...in
otherwords, shear is not transfered across the interface between the two
wythes.  Cavity walls CAN BE and ARE designed so that the two wythes work
in "parallel"...in otherwords, the ties are axially stiff enough to
transfer load between the two wythes so that the two wythes each resist
the out-of-plane loads (i.e. wind loads) proportional to their flexural
stiffness.

A lot of "cavity" wall as designed as "veneer" walls.  That is the out
wythe is just along for the ride.  Like you, this is what I typically
design when doing a brick and block wall...i.e. the brick is just a veneer
and the block is the structural "element".  This is definitely a "veneer
wall" but is also commonly called a "cavity wall".  The difference is that
a "cavity wall" could also be designed so that the brick could resist some
of hte out of plane loads, but generally NOT in a composite manner (i.e.
transfer [beam] shear across the interface).

FWIW, "early" cavity walls were constructed in a manner such that they
could act compositely.  "Early" cavity walls would have headers that would
cross the joint/interface.  But in todays world of metal ties, it is
generally no longer the case.  If you want a composite wall, it will
generally have a collar joint with with either headers or ties crossing
hte interface...but generally not a cavity (i.e. open joint).

Regards,

Scott
Adrian, MI


On Sat, 19 Feb 2005, Daryl Richardson wrote:

> Lloyd,
>
>         I have some pictures you might find useful.  I am getting ready to
> move so I may have some difficulty in finding them but you are welcome to
> them if you're willing to post them.
>
>         Strange how threads on this list sometimes interact with one
> another.  The failure I have photographed, in my own opinion, happened
> because of ambiguity in one of our Canadian codes several years ago.  Let me
> describe the circumstances for your information.
>
>         Several years ago, about 1970 give or take a couple of years, we had
> a substantial wind storm in Calgary.  Wind gusts reached 85 miles per hour,
> which, at the time, was the design wind speed for Calgary.  Several
> buildings were significantly damaged.  One, a 16 storey building I was
> working on, had all the formwork for the roof, including all of the
> reinforcing, fully inspected and ready for concrete placement, blown off
> into the parking lot.  A few blocks away, a couple of campers, seeking
> shelter in their tent, found themselves and their tent in the middle of the
> Bow River.  A school gymnasium also had the wall blown out after the school
> was in service.  It is this school which I have photographed.
>
>         I was rather brazen about getting the photos of the school since it
> was not my project.  I simply walked up to the security guard with my hard
> hat and camera and said "Hi, I'm a structural engineer here to look at the
> building."  We also exchanged pleasantries when I left.
>
>         I don't know much about the design details because I had no
> involvement in either the design or the investigation of the failure.  Never
> the less, some structural deficiencies are fairly obvious to a structural
> engineer with 8 or 10 years of experience.  The following are some of my
> observations as I remember them after all these years.
>
> 1.)    The wall was about 24 feet tall and 30 or 40 feet long (this can be
> measured from the photos based on the 8" by 16" concrete block units).  This
> was the east wall of a flat-roofed building loaded by wind from the north;
> the building and the wind direction were essentially "square" with one
> another.  About 70% of the wall was sucked out and piled up in the school
> yard with nothing ending up inside the school.  Fortunately, it was a
> Saturday afternoon and there were no children in the school or the school
> yard.  I remember the wind vividly and no one would be outside in that wind
> storm.
>
> 2.)    The wall construction consisted of 6" hollow concrete block with,
> perhaps, 1" or 2" of fiberglass type insulation on the outside covered by 4"
> brick veneer secured to the concrete block by metal ties.  I don't remember
> the reinforcing but it was minimal.
>
> 3.)    There was an open web steel joist 2" or 3" away from the wall on the
> inside.  As I remember, none of the blocks was sufficiently anchored to the
> joist or the roof decking to remain suspended.  This suggests that the wall,
> which might have been intended to act as a simple span beam spanning from
> foundation to roof, may in fact have become a cantilever.  The north
> (windward) wall, which supported open web steel joists at 6' of 7' spacing
> would have been subjected to higher loading than the east (side) wall but it
> did not fail although it appeared to be of similar construction.
>
> 4.)_    The code of the day was very unclear regarding the difference
> between a block wall with a brick veneer and a cavity wall.  I was forever
> at odds with other engineers on this topic.  Unfortunately, there was no
> resource like this list to discuss the matter.  In the case of the school,
> there is a considerable difference between a 6" block wall with a brick
> veneer (which I would have considered this to be) and a 12" cavity wall
> (which, apparently the designers of record considered it to be).  A 6" block
> wall 24 feet high, of course, does not comply with any code I have ever
> read.
>
>         In my entire life I have never designed a wall that I considered to
> be a cavity wall.  On reflection, I guess, I would expect that in order to
> qualify as a cavity wall the ties between the brick and the concrete block
> (or between multiple brick wythes) would have to be  both strong enough and
> rigid enough to resist the "horizontal beam shear" calculated by use of the
> formula v=V*Q/I.  (Any comments on this, whether they apply to Canadian or
> American codes would be welcomed by me.)
>
>         Regarding the photos, Lloyd, they are in the form of 35mm color
> slides (probably 20 or more).  I could have them developed as prints, scan
> them and e-mail them to you (fairly expensive for me), I could scan them
> directly from the slides (I have a high quality scanner but, I think, the
> quality would still be not so hot; but it would be much cheaper for me.) or
> I could mail them directly to you if I had your address.  I think I would
> prefer the latter solution.
>
> Regards,
>
> H. Daryl Richardson
>
>
> ----- Original Message -----
> From: "L. Pack" <Lloyd(--nospam--at)pecid.com>
> To: <seaint(--nospam--at)seaint.org>
> Sent: Friday, February 18, 2005 2:09 PM
> Subject: Re: Looking for Structural failure pictures
>
>
> > On 18 Feb 2005, at 11:02, Szuchuan Chang wrote:
> >
> >> Lloyd,
> >>
> >> Will you also be able to establishe a web site where we can access to
> >> your collections?  Eventurally, SEAOC may want to have a central
> >> location to put these pictures together like a library.
> >>
> >> Thanks
> >>
> >> Szuchuan
> >>
> > I think that this is an excellent idea.  I would be willing to work toward
> > this end.  Back when I was in school, I would have loved having a
> > resource like a website with forensics or failures, to help me better
> > learn the subjects that I was studying.
> >
> > I've had very little response to my request, so I'm not sure if I can
> > find much to put this together.  I've googled this a little and have
> > yet to find much on this subject.
> >
> > Take care,
> > Lloyd Pack, P.E.
> >
> >
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