Expert: Dave Johnson - 6/1/2009

Question
Hi Dave, You were quite helpful on a prior question so here
I am again. I live in a rural setting and want to put a
sort of arbor over my driveway gate. The gateway is 20'
wide. I will describe the appearance I am after facing the
gateway head on. I would set one set of 6x6 12'high
uprights one on either side of the gateway set in concrete.
I want to put a cross beam fastened to these uprights with
carriage bolts with the top edge flush with the top of the
posts. The cross beam needs to extend 2' to either side of
the posts (a 24' length) with corbeled ends. I would use
2x12 x12' material spliced to make the 24' length. Then
repeat this setup 2' away from the first set. I want to cut
a shallow arch into the cross beams including curving the
top of the ends downward as well to give a Spanish detail.
I then would cap the arbor with 5' lengths of 2x8 set on
edge in sets of two spaced 6" apart, with each set spaced
2' apart with the ends cut in a corbeled shape. An oval
sign is to be hung from the center point with a southwest
design.

The problem, of course is that 20' span. I'm worried there
will ultimately be some sag.I don't see how knee braces
would fit into the design to shorten the span. I could
double up on 2x12s making them into 4x12s. I'm interested
in any advice,
Thank you

Answer
Hi Trilby,
I like the design.

I think if you use (3) 2"x12" laminated, breaking joints,gluing and screwing or nailing and don't cut an arch, you'll be okay. You can achieve the effect you're after by making the top cuts you mentioned and adding some curved pieces on top of the beams. The curved pieces on top won't need to be 24', because you'll be cutting a curve on the top ends.
The pieces you cut from the top could be applied to the bottom ends. I realize this is a lot of "add ons " but I'd hate to see you have a sag later on...

Best of luck, and if you need more, please mail me again..
Dave


Just had another thought, if you sandwich some 1/4" steel between the layers of  2"x12" you will get a stronger beam. Plus, if the steel is 8" wide , you could then do what you wanted originally...

Dave