Deck main girder question

New to the group. As a 15-year HVAC tech, I hope to help any and all with their HVAC woes. In the meantime, pop a cold one on me and grab a handful of pretzels... gotta question about building a deck for all you carpenters in here....
I've got a typical raised ranch with a typical 10' x 20' deck. Being an outdoorsy type, I plan to rip it down and replace it with a bigger 16' x 32'. My problem is that the damn septic tank is gonna screw up the works. I'll need to span over it. The tank is over eight feet long, which means the closest I can get two footing is about 12 feet (allowing for 18" between the footing and the tank, plus the diameter of the footing). Henceforth, my question:
How beefy a girder will I need to span 12 feet whilst supporting 9.5 feet of joists? I'd like to get away with 2 - 2" x 12"s with a 1" plywood flitch plate in between. That'll give me a 4"x12" girder which will fit nicely on a 4x4 post.
Any thoughts?
--
C.J.

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I'd rather see 3 - 2 x 12's on a 5x5 or 6x6 post. Any way of working some decking, maybe on an angle (who says decks have to be square?) or an outdoor staircase onto this girder, instead of having a girder protruding 2 &1/2' out into mid air would improve the asthetics.
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<snip>

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out
I'm planning on a staircase utilizing the last 3 feet of girder on one side. My design has 45-degree corners built into the cantilever, so there will be some load reduction. I might have to go with a triple 2x12 assembled with two 1/2" plywood flitch plates. Big money, and lots of labor.
Hey, there's always steel......
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C.J.


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I'm sure some of the engineers will be along soon. In the mean time, 2-2x12 seems light according to the Southern Pine Headers & Beams tables. They would according to this table support 40 psf live load, which is perhaps light for a deck.
TB
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Don't cover up the access caps for your septic tank so that it can be pumped and/or repaired. Pumping can be messy and you may not want access trap doors in your deck, or possibly tear down part of the deck. Also watch out for piping while you are digging the posts into the ground.

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CJ,
I'm just an amateur, so take my advice accordingly...

Any chance of running your joists in the opposite direction? You could easily span that 12' distance with 2x8's at 16" OC.
Otherwise, I would calculate the load as a normal house floor at 40 lbs per square foot live load, and 20 lbs per foot dead load (chairs, tables, grills, etc.). Probably overkill for a deck, but best to be safe.
If your 9.5' span is at the edge of the deck, only half of the load will be carried by this beam. The other half will be carried by the support on the other end of the joists. So... 60lb x 12' x 4.75' = 3420 pounds on that beam.
If the beam is in the middle of your 16' deck, it will be supporting half of the two spans (4' of each span), or 60lb x 12' x 8' = 5760 pounds on the beam.
According to the tables I have, using wood with a 900 fiber stress (weaker woods),
A 4x12 will carry 3856 pounds for a 12' span A built-up beam of two 2x12's will carry 3459 pounds for a 12' span. A 6x12 will carry 5852 pounds for a 12' span.
Naturally, a stronger wood like Doug-fir would be stronger than these figures. Obviously, the more you can do to reduce spans, the lighter the timbers need to be.
By the way, for outdoor projects, a solid beam is better than a built-up beam. Water can get down between the boards in a built-up beam and cause rotting.
Hope this helps,
Anthony
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IMHO if you want to carry the load of 9 joists across a 12' span, I would go with at least a triple 2x12, glued and bolted together on 6x6 posts resting on 12" concrete footings. Make sure you stagger the seams when building the girder. I built a deck last summer at my old house, and I used a triple 2x12 21' girder to support the load of the entire deck. It was a bit pricy but the deck was rock solid. It didnt move at all when you jumped up and down on it (and I am a big guy).
Thanks, Mike
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Just to add to the good advice. Lose the plywood flitch idea. Though bolt 3 - 2x12s and your golden.

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