Buildup Beam Questions

[rlbow43]

I'm going to build 2 16' 4X12's to span 32', with 3 4x6 posts. I've seen tables that would make me think that a 4x10 would do the trick, but since the difference in cost is not that much am thinking of overbuilding just to make sure.

I've tried to find something that lists the nailing, or screwing, pattern & o.c. spacing, AND the size of nail/screw.

I've seen both 16d and 20d as the nail size to use. Nothing on screws, although seem to remember reading that you could use screws.

I've seen a pattern of 32" o.c. alternating, AND 16" o.c., not alternating. What I've read seems to say 2 fasteners at each end (on each side I assume) no matter how big the beam. Seems like on a 2x12 you might want 3 fasteners on each end.(?)

For the spacer between the 2x12, should the 1/2" plywood be glued? If so, what type of glue? This beam is for a patio roof that I cannot attach to the house. If I were to use 4 or more posts so I could use a smaller beam I would create traffic issues - the posts would be about 4' in front of sliding doors. If I could attach a ledger to the house, I wouldn't have this problem, but a 4' roof overhang and a 2' eave makes that not practical in this case.

I'd appreciate any comments or "points in the right direction".

This is my first post on this forum. Glad I ran across it yesterday. Topics are interesting and the replies are full of good information.

Thanks.

[Don_P]

This is from HUD;

"Built-up beams are constructed by nailing together of two or more plys of
dimension lumber. Since load sharing occurs between the plys (i.e., lumber
members), the built-up girder is able to resist higher loads than a single member
of the same overall dimensions. The built-up member can resist higher loads only
if butt joints are located at or near supports and are staggered in alternate plys.
Each ply may be face nailed to the previous ply with 10d nails staggered at 12
inches on center top to bottom. The design method and equations are the same as
those for floor joists; however, the adjustment factors applying to
design values and loading conditions are somewhat different."

Make all splices directly over a support unless an engineer tells you otherwise. The code's minimum pattern is to nail each layer 32" oc at top and bottom and staggered. two nails at each end and splice. The 10d specified is a 3"x .128" nail, if you use a screw use a structural screw not a deck or drywall screw, they tend to be brittle.

As long as the load is acting on all plies, forcing them to act in unison, the nailing is simply holding them together. A construction adhesive will make it stiffer.

http://www2.iccsafe.org/states/newjersey/
Table R602.3(1)