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Re: Pitched Tapered Glu-Lam in a three pinned Arch

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I'm not sure of your geometry.  Are you saying that the top of the glulam frame
is curves in an arc like a bow-string truss, and the bottom is a vaulted ceiling
like a scissor truss?  And, that the '3 pins' you refer to are located at the
tops of the columns and at the peak with the frame fabricated from 2 symmetrical
halves joined at the peak??

If this is the case, then  the top of the glulam is in tension over the columns
(for gravity), and I assume this is what you're referring to.  In the 'classic'
example, the top of the tapered peak tries to 'lift off' the curved glulam,
creating radial tension.  In your case, you don't have a 'peak' over the
columns, you have a single sloped cantilever.  Radial tension will result only
if the glulam plies are curved at this location and the forces try to increase
the radius of curvature (otherwise the radial forces are compressive).  To
determine the radial forces, it's a simple computer analysis using a mesh of
quad-node shells and look for tension at the nodes perpendicular to the glulam

Regardless of the geometry, if radial tension exists that exceeds the allowables
in the UBC (around 15 psi) , then it must be controlled.   Lag screws installed
from the top of the beam are effective to 'stitch' the plies together.

This is one of those fun types of projects that make engineering interesting.
Don't you think?

Good luck.

Lew Midlam, PE

Paul McEntee wrote:

>  Well, I had a little problem dropped on my desk about a week ago, and I've
> been picking the brains of glu-lam fabricators as well as a few people at
> AITC, and no one has been able to an answer that I or the engineer I work
> under is willing to accept, so I'll throw it out to the list.  And I'll do my
> best to avoid referencing any of the good Clinton jokes I've heard this
> week.We're designing an addition and a couple of new buildings that match and
> existing church.  The new structures will have TS columns at the perimiter,
> and pitched and tapered glu-lams for the main spans, about 40 foot spans with
> an eight foot overhang.  Two glu-lams will make up a three pinned arch, and
> we'll have some type of tie bar across the middle so we're not bending the
> cantilever columns (at least not with gravity loads).  My problem is that,
> unlike examples from AITC, the radius of the beams are on the top - ie, in
> compression - and the tapered face will be in tension due to bending.  This is
> exactly the opposite of most textbook examples.  There is also the additional
> bending and compression due to the arch, and the question of where is the best
> elevation along the beams to tie the too together (at the top of the columns,
> at midspan, etc.).If any of you timber gurus out there could point me toward a
> design reference that handles more complex design of glu-lams like this, I
> would appreciate it.Thanks in advancePaul McEnteeBiggs Cardosa & AssociatesSan
> Jose, CaliforniaCan a throw in a funny anecdote from work today?We're
> retrofitting this bridge, and the basic scheme is to infill the space between
> columns with these monstrous concrete walls.  We were going over what calcs
> needed to be done according to Caltrans specs, and one of them was to show
> that the walls & footing won't overturn.  So a co-worker of mine says
> something along the lines of:  "It's ridiculous that we have to show that
> these walls aren't going anywhere.  I mean, it would take an act of GOD to
> make this thing overturn."Senior engineer on the project looks at him and says
> "Yeah, it's a good thing we're not designing for an earthquake or anything
> like that."