beam and arch question

[plantguyforfun]

Ive been trolling your forum and scouring the web for a while now trying to find some answers ... I hope someone can help....and possibly tell me something i "want" to hear! lol



Im in the process of building one of these greenhouse designs above ( 14' x 20' or similar design) and need some advice on two different elements. The beam and the arch.

Project background- this is a 20×14 greenhouse, of which i already have the foundation and block work done. Across the bottom and top will be windows spanning the length ( either recycled house windows or framed polycarb) to maximize ventilation.

The beam- Im really hoping to be able to clearspan the 20' being that there isnt too much of a load on it.

I ended up getting a good deal on some 18ft cedar 6×6 and thought i could bold a couple together to make a 6×12in beam. However after more thought im not sure this is an option without a center post.

Now im thinking of a triple 2×12 or possibly making a gluelam (perhaps fir or cedar 1×4s 5 thick?) and sealed.

the Arch -12-14ft span at a 1' 6" radius this will be holding a polycarb film and assuming a minimal snow load due to the temp of the greenhouse.

Ive seen gothic style greenhouse build with trusses of sort ( two 1.5 inch rips off a 2x with blocks in between. Which i dont think would work for my application and would rather them solid.

Codes in my area are sort of lax but would rather be on the safe than sorry side of this equation.

What are the best or possible ways to make these beams and recommendation on wood species

Also failed to mention that im and artist ( with a background in custom framing) on a shoestring budget.... and live in bfe.... so delivery costs on 20ft anything are pricey.

I live in the midwest with an avg snowfall of 26in

thanks for taking the time to read this and the patients and time to help!!

[akwoodchuck]

Pretty sure a 4-ply 2x12 beam would carry...I recently put one 20' in a garage build carrying a roof beam in studio apartment above, not worried about it. Snow loads are similar. Another way to skin that cat would be to continue the arches all the way to the back wall, then build the clerestory on top, all tied together with gussets, truss-style. Long as you follow rafter sizing tables, should be fine. Myself, I'd just throw a couple posts in. 

[plantguyforfun]

Thanks a lot for taking the time to reply woodchuck!

Thats what i was thinking for the beam.... that or ive also seen a few homemade glulams for sort that i think would work ( they were essentially 7 ( 2x4) glued together. 

And you are right... should i ever see any deflection i can always toss in a post.

At the moment im still trying to figure out the arches though.  Im not sure what size they would need to be ... being they would mostly be holding up a little snow in the winter.  And was contemplating building out some glue lams for them as well  maybe 3x5??

Thanks again for your help.

[Don_P]

It takes more information to get there. Where are you... what is your snow load? Your dimensions and drawing have me a little confused. Is the greenhouse foundation 14' x 20'? Is the arch spanning ~7' and the shed roof is spanning ~7'?

It's generally a really bad idea to try to build a gluelam out of boards stacked flatwise, most people cannot reliably achieve a glue joint that is as strong as the wood. When you build up beams, vertically laminating the boards, then the field connection is mostly simply holding the boards together in alignment. That is why we do built up beams. This beam is also supporting half the shed roof as well as half the arch roof, about 7' of width, times 20' of length=140 square feet.  I suspect you can keep the roof load light by bleeding heat and melting snow... maybe 25 psf, that is a guess, if you shortchange it better have some props ready for winter. Anyway, 140sfx 25 lbs per sf=3500 lbs on a 20' span. A 4 ply 2x12 does work well with those assumptions.

I have assembled a sunroom that was made from small dimension oak gluelams, the one I did was shed styled but they did make an arched model. The laminations were 1/4-5/16" thick x ~2-1/2" wide and were sized deep enough for the span. With a plywood form and wedges I've glued up arched door trim, the process would be similar but I'd use epoxy to have a better chance at making a structural glue joint.

[akwoodchuck]

At the moment im still trying to figure out the arches though.  Im not sure what size they would need to be ... being they would mostly be holding up a little snow in the winter.  And was contemplating building out some glue lams for them as well  maybe 3x5??

I would go half the width and twice the depth, or 1.5" x 10"... 1/4" rips as Don stated to pull that radius...40 or so per arch, but I'd recommend titebond ultimate over epoxy. Epoxy has a bit of a learning curve, is messy, pricier, a bit time- sensitive, and degrades rapidly under uv exposure. Appearance-grade spf, as knot-free as you can get, would be an excellent choice of material. Easy way to glue up is to apply adhesive to your rips with a roller, gang them together, then wrap with pallet wrap, then clamp the whole thing to your form.

[Don_P]

 I think that's another formulation of PVA, the reason they are not approved is creep under long term load...but epoxies are not approved either to my knowledge. Weldwood I think makes one structural adhesive I've seen on the consumer market. That's a cool idea on the roll and wrap, I try to cover the floor with cardboard or plastic for this kind of mess. I was kind of leaning towards western redcedar for decay resistance in the greenhouse. A bandsaw and planer will be invaluable/ necessary for this.

[plantguyforfun]

Thank you both again for helping me out here... your insight is invaluable!

My snow load is 20-30psf...
the arch is spanning 8' the shed roof 6'
was planning on running 2foot on center

It seems as if the consensus would be to run 4- 2x12s  for the beam.... one problem solved

My original intention with the arch was to provide more headspace (aside from appearance)  so i can stack in more plants vertically... which wouldnt give me that much more space than if i ran them straight....  but wouldnt look as nice. lol    Perhaps i should make this easier on myself and raise my kneewall in the front a bit and run them as 2x8s??   I suppose i can always go back later and arch it out in better weather... otherwise i wont get to put fish in  my system until next fall haha    This realization sort of makes me sad... i think the arches would be fun to look at and fun to build....

Thanks again for taking the time to help me!!!!!!!