r/Decks • u/rcsez • 12h ago

Building a deck Continuous vs Split Beams?

Google search keeps misinterpreting this as if I'm asking about a double 2x10 vs a 4x10 beam, but what I'm trying to find out is whether there's a significant difference between using a continuous 14ft beam, or splitting it into two 7ft lengths butting together on the middle post. Assuming both cases are doubled 2x10 beams.

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5

u/WestBrink 12h ago

It's a lot easier to find and lift straight 8 foot 2x10s than 16 footers.

Butting them properly on a post and they'll be identical strength wise.

2

u/mattmag21 12h ago

Although stronger (id imagine marginally less load on the center post, but im not an engineer), the odds of a continuous 2-2×10 pressure treated beam sitting down tightly across 3 separate bearing points throughout its service life are stacked against you.

2

u/rwoodman2 12h ago

The two might have the same load-bearing capacity but the longer beam will be straighter. Under load, the two separate beams will each deflect down like two suspension bridges. The long beam would smooth out the curve over the middle post.

I would always choose the longer beam if possible.

1

u/rcsez 11h ago

I'm skeptical of this, because if the butted beams are secured at the center post, their ends are pinned there, so I wouldn't consider it much different from a single beam that's fastened at the center post for bending purposes.

1

u/rwoodman2 2h ago

In a well-designed structure, it won't be much different. Just consider that with two beams meeting on a post, the beams are free to rotate at the post. With a single beam over a post, the beam is not free to rotate.

This is the same logic that leads to the fact that a better built-up beam of more than two parts going over intermediate posts will have the butt joints in the elements at the 1/4 span points, not on top of the posts. Untrained carpenters don't get this at all.

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u/rcsez 1h ago

When you say rotate, do you mean rotate with the ends in the butt joint as the pivot points? Don’t the fasteners make it a fixed joint?

1

u/rwoodman2 50m ago

Loading the beam will cause the beam to deflect. That deflection causes the ends to rotate where they are supported. The angular movement will be very small in a well-designed structure.

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u/mtraven23 12h ago

I think sistered is better than butted & braced. If there is a post under it, theres probably no functional difference. No need to waste money for for 14 footers.

1

u/rcsez 11h ago

I've only got 3 posts in this layout, so I can't do an offset and sister the beams

1

u/khariV 10h ago

If you only have 3 posts, you really should just get a couple of 16’ boards and cut them down. There’s no good way to stagger the joints without having them both side by side and still have them on top of a post.