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RE: Question on wood Roof Trusses

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Roger:

Either you missed the point or I didn't explain it well.  Web bracing
suggested by the truss manufacturer is usually using a 1x4 or a 2x4
connecting a series of webs together.  Now if the ends of these
1x4's(2x4's) are not connected to anything else, when the failure happens -
they all fail at once.  For calculation purposes to size a connection of a
web brace connecting a number of webs together, lets say that each of the
braced web members has a compressive force of 2000 lbs, which I somehow
obtained from the little fuzzy computer printout from the truss company.
By rule of thumb, I assume that to stabilize this brace I only need a
resistance of 2% or 40 lbs per brace.  Now there are 10 braces that have to
be braced, so 10 x 40 lbs = 400 lbs.  Now I can connect this 1x4 or 2x4
brace to my handy gable wall, but I have to check the gable wall stud and
I'll probably find it won't work, so I'll have to make it bigger.  Now I'll
also have to determine the kind of connection from the 1x4(2x4) to the stud
and I'll have to check the connections of the stud at the top and at the
bottom.

Now if I don't do all this and there's only a couple of nails holding the
web brace or the top and bottom of the stud is only toe nailed or even if
the stud is turned flatways and this project is at Lake Tahoe, where by the
way, this is a fairly common scenario, we have the makings of a neat little
collapsed roof.  And when this happens, because all of these 10 webs are
connected together - they all go down together.

By the way Roger, the L/d of a 1x4 connecting webs at 24" on center is
about 32; I sure that that's good for something.

Samir Ghosn's latest comments hits the nail about this whole situation
right on the head.  The only saviour for all of us is a building department
that won't issue the permit until all of the submittals have been
submitted.  Of course, the building department could also refuse to give
the occupancy permit until the reviews are completed.  Instead of a mayor
calling the building department, it could be a county supervisor protecting
his contractor buddies.

In our local county we have a building department that works very closely
with the structural engineering community.   In fact, each month we have a
meeting at the building department where we'll have presentation from
manufacturers (which like to give us free lunches) and general meetings
where we have to bring our own lunch.  The truss web bracing problem has
had a number of discussions and some of the engineers brought in their own
solutions so that they could have standardized calculations and
connections.  It has been an eye-opener for all of us.  The kinds of
bracing that you mentioned we've also looked at, but I think that whatever
solution we come with for the particularly project shouldn't be to
sophisticated or it won't get done.

Neil Moore, S.E.


At 03:51 PM 1/16/2001 -0500, you wrote:
>Neil,
>
>For the record, I never said to brace one slender web member against another 
>slender web member. That is, of course, dangerous!  What I said was that it 
>is that the bracing can be self-equilibrating.  There are many ways that
this 
>can be accomplished, including:
>
>   double king or queen post the slender member
>
>   providing X-bracing between the slender member and the top or bottom
>      chords of the truss with horizontal struts between the adjacent 
>      trusses
>
>   any method that will keep the braced points of the slender member in
>      alignment with the ends of the slender member
>
>By failing to completely show how the bracing is attached, the truss
designer 
>is providing an incomplete design!  That is like us showing a slender steel 
>column for one of our buildings and saying, "Bracing is by the steel 
>fabricator."  That the plated truss industry has unilaterally decided to 
>foist off the problem of stability bracing onto someone else is irrelevant.
>
>A. Roger Turk, P.E.(Structural)
>Tucson, Arizona
>
>Neil Moore wrote:
>
>>>Tarek:
>
>If I remember correctly, one of the TPI publications has an example of how
>ALL of the webs can buckle at once.  If the web bracing isn't anchored to
>something, then this can happen.  Most of the time, the real loads never
>approach the design loads and any buckling problem has moved into the
>factor of safety area. 
>
>If people are designing trusses in snow areas, say 160 pcf, then our
>discussions and the possible reprecussions become important.  My original
>involvement in the web bracing responsiblity problem came about 6 years ago
>in a court case.  That's were I found out that the responsibility is kinda
>hidden in the TPI specification, which the UBC refers you too.  How many
>people owned the TPI spec?  Further, it is also difficult to interpret the
>results of the computer output.  That presentation can be, and may already
>have been, improved upon.  I've read where some engineers are going to get
>tough about this and make the truss company provide the web bracing
>anchorage to the building or to the roof or somewhere.  Good luck!.
>
>It still is important to review the truss company's layouts; you might be
>surprised at what you will find.<<