It's always better to have the beam bearing on the post instead of relying on bolts to bear the weight. The sandwhich style beams will not meet code here & doubtful wherever you are.

I would suggest you move your post inward on each end letting the beam cantilever 2-3' to cut down on the span between post. Even a 6x12 is overspanned at 24', no way you could span 24' with cedar 2x12's & expect it to hold roofing material & decking above too.

If you use a glue/lam beam it will need to be a treated glu/lam to be used outside.

What were you going to use for the footing to post connection?

Just to clarify, the span is actually 21.5'. The footing connection is a Simpson Strong-Tie Z-MAX Galvanized 14-Gauge 6 in. x 6 in. Adjustable Post Base (from HD) using a 6" Galv bolt and 3/4 nut.

Let's say I reduced it 2' would it be significantly better? That would require me (because of the concrete slab I'm trying to cover) to increase the depth about two feet, but I think I could make that work, as well.

Take a look at the history on my project:

http://ths.gardenweb.com/forums/load/porch/msg0314220528218.html

I went with 6x12"x24's. They haven't sagged a bit. I would pass on 2x12's as the deflection get's pretty high with cedar. I'll monitor this a bit if you have any questions. It took me a couple years to plan, but it has held up perfectly.

I was actually inspired by your project to consider cedar, but given the really high cost of the 6x12x24 I decided against it.

I'm also looking to put a lot more weight on it than you did, I believe, as I'm going make it water tight on top. (I am doing some cedar to pretty it up underneath.)

Anything you would advise based on what you did, or something you might do different?

Henry,

A couple of thoughts come to mind.

Remember that this thing is heavy enough to wreak significant harm if it's underbuilt. I understand that snow load may not be much of an issue, but make sure you're thinking about how winds will impact this thing too. If it were me, I'd be looking at beam span tables for normal #40 loads. That'd probably be more appropriate of a comparison than what I was going for. I'm guessing that it'll push you into a 14" or 16" LVL scenario for that span. Doubled up 2x12"s might get you there, but I'd seriously think about moving in the posts a couple feet and cantilevering the beams. It'd probably hold it up, but I think it'll visually deflect and look funky.

Second thought is that there isn't any angle bracing on your short axis. That is critical to making this thing free standing. Definitely don't rely on the footers to keep this thing from racking horizontally.

Thanks for the input.

So, here is the revised plan:

The major beam is going to be a 6x12x24 Treated Glu-Lam.

The space between the posts has been reduced to 20'.

I have also reduced the weight of the roofing and decking by about 200 pounds by using a composite and a lighter weight roofing material.

I am going to add a cross-bracing at the two short ends. I also have a contractor that I've hired to help me.

Follow-up questions:
-Can I go "down" to 4x4 on the wide axis posts-to-beam braces?
-Will a 2x6 be an acceptable brace for the short end?

I’m wondering if there’s any chance you may have your plans for this project as I am just about to embark on the same type of project. I’m using 6 x 6 posts on the end still be 10 feet apart and lengthwise they’ll be 19 to 20 feet apart. I was going to use just for posts to try and avoid a middle post. I was looking at 2 x 10 x 24’s or 2 x 12 24’s On each side of the post. This is going to be a pergola type with the two by twos running across the top for shade only.