Multi-Span Glulam Beam and Uplift 2

Design it as a cantilever and hope for the best. Maybe a connector that allows vertical movement at joint three. It would be good to know the cantilever deflection as a gauge for whether or not ignoring the tension will be a problem.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

I would try to tie it down on that end. Releasing it will change the behaviour of the other span, and may also cause distress in finishes.

Given that the uplift is within the spec. for the column cap (ECCQ66) uplift and the post base (CBSQ66) uplift I think it would probably be fine to keep it intact from a connection standpoint but I just wanted to see if conventional wisdom would suggest otherwise.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

if you're comfortable that you have plenty of dead load at support 3 to resist uplift once it is anchor-connected, I might anchor it down too. If the reliable dead load was marginal, I'd consider if there was a window or something above support 3 that might be sensitive to a little movement if lots of live load was moving around in the large span..... ( on a reread, this assumes you have a floor beam as it says in the side notes...but maybe it's a roof beam as it says in the title, and suggested by the 15% duration....not sure how to read your output)

if your large span deflection is ok as a simple span, I'd be comfortable cutting it at support 2 and addressing your lateral connection.... minding the finishes, as noted above.

I'd be less concerned about roof.....

This comes up in the design of MPCWT all the time. The classic condition is a gable roof truss that cantilevers one side to shelter an open air deck space. And the uplift gets ridiculous owing to the fact that you might have have a 40'/8' split. If you take your truss model and replace the 100% rigid supports with springs -- even very stiff springs -- the uplift reaction fades away in a hurry. The amount of movement required is probably on the order of the combined nail slip for base/cap connections. It's quite analogous to the backstay effect in high-rise shear walls.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

further on the software, I guess by weyerhauser (?), it mentions that compression face bracing must not be less than 31 feet o.c..... I'd review that. If you cut the beam at support 2, your compression face bracing issues are reduced.

@ Triangled

Sorry about the mix up on the notes. I initially choose a floor beam in the Forte software, I should have probably chosen Roof Beam instead. I reran it with the roof beam option and the numbers are exactly the same the only difference is the option to assign a pitch to the beam.

Forte is fairly decent for some things but I don't like that it doesn't allow one to specify enough details (ie. custom unbraced lengths etc...). I'll have to recheck this in Woodworks sizer for the bracing requirements.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

Sizer's output is pretty close to Forte, the small differences may be the way in which it is calculating the design span. The upside with Sizer is that I can specify the top and bottom bracing. However, the selection of the Glulam type and grade is confusing in my opinion.



A confused student is a good student.
Nathaniel P. Wilkerson, PE

I do not understand why uplift would constitute a failure... just go to a 24FV8 or a Parallam and tie the third support down. No worries.


Mike McCann, PE, SE (WA)

I had a set of pre-engineered plans (by the lumber supplier)come across my desk with a similar situation. The second span was much shorter so the tie-down force was huge (on the order of 10k). I ended making it a cant. (like Kootk suggested) and upsizing the beam to compensate for the increased main span deflection.
You could also do a similar thing by leaving out the third support and kerfing the top of the beam to weaken it sufficiently.

I believe it calls it a failure because the software doesn't size column caps and just checks bearing stresses based on the support length you define. Since it can't find a connection, it defaults to a fail warning. I ignore this all the time and make sure my connection can transfer the uplift, which is usually pretty easy. I agree that Forte doesn't let you specify enough details. It is good for quick and simple beam runs, but I prefer RISA 3D for more complicated scenarios. I do like that it does pattern loading, though.

Software only assists in analysis. The design is left to the design engineer.