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Re: Vierendeel truss in wood

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Scott,
It sounds like you are planning on doing the glulam truss design and detailing yourself, which is different from how I have seen it done around here (SC), but I know things are done differently in different areas. Here, for an architecturally exposed truss, I specify the profile of the truss, the web configuration, the desired member sizes, some information about the appearance of the connections, and then the glulam supplier has their engineer design the truss and its connections and they provide sealed shop drawings for review. I like this method a lot, since the connections can get quite intricate where trusses frame into other trusses at hip and valley conditions. Those guys make these things fit together for a living so they are quite good at it.

That being said, one time an architect asked me if we could do a glulam Vierendeel truss, so I called the glulam supplier that seems to get most of the glulam jobs around here (Unit Structures) and asked them. They came back with the answer: "Absolutely not!"

That was the end of that. We wound up going with steel Vierendeel trusses clad in steel studs and gyp board.

My trusses had a much longer span than yours however - about a hundred feet down the length of a church sanctuary. It seems that your 32' span and small tributary width could almost be handled by the two chords acting as beams (ignoring Vierendeel truss action). This would probably show up in your analysis model - even if you fix the joints, the chords would probably carry the great majority of the load as bending members since their stiffness is so much greater than your webs.

Rick Burch
Columbia, SC



Scott Maxwell wrote:

I have project where the architect would like to create/use a Vierendeel
truss out of timber/glulam members.  I thought that I would get others
thoughts as my gut is not giving me the "warm fuzzies" (I am in the
process of creating a model to determine what kind of moments I would
dealing with at the connection of the vertical members to the top and
bottom chords, so I only have my gut at this point).

The configuration proposed is basically comprised of two 15" deep by 5
1/8" wide glulams as the top and bottom chord.  At three places along the
span there are three vertical timber members that are spaced about 6" or
so apart (i.e. three locations of three vertical members each).  The
connection of the vertical members (9 members total in groups of three) is
done by way of a vertical row of two bolts/pins.  The vertical members are
about 4" to 6" wide (don't totally recall and don't have the drawings
right in front of me).  The span of the truss is on the order of about 32
ft.  The truss is carrying about 6 foot (horizontal projection of 12/12
roof) tributary width of stick framing roof joists.  The ground snow load
will likely be on the order of 25 psf.

Now, I know wood is not really that great for "moment connections".  But,
I also know that there will be some rigidity there from how this will be
put together.  I am just not completely sure how much I can count on (and
don't know yet how much I will really need until I run the model).

Oh, likely DougFir...if it matters.

Any thoughts?

Thanks,

Scott
Adrian, MI


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