Beams- Spread or doubled/tripled together

[leinom]

Hello again, got my concrete poured and 4x4's in last night ( I am taking pictures but its all on my friends camera). My friends who are helping me build my cabin, and they both have some experience building-they both say to put the 2x10s on the sides of the posts nailed into each side, so making beams with a 4 inch gap in between. After browsing the gallery I'm seeing most people nail the boards together an set them on top of the posts. I honestly believe it will be about as strong either way. Thanks for answering all my newbish questions!

Mark

[JRR]

I think you're right ... about as strong either way.  If you do put them 4" apart, you could take a few scrap pieces of 4x4 and put in between along the span as vertical blocks .... up to four, say .... now you are beginning to make a "box", and I would suspect this would be stiffer than either of the other two options.

(But I'm guessing!)

[glenn-k]

The whole thing may slide right down the post shearing the nails and leave your house sitting on the ground with a bunch of posts sticking up through the floor.   The plans outline the proper procedure.

Here is some info from our site.

http://www.countryplans.com/builtupbeam.html

http://www.countryplans.com/cgi-bin/yabb2/YaBB.pl?num=1148346241

Note that you can use the search function on the line that says • Click SEARCH to find a topic quickly.  - Don't use the spyglass one.

http://www.countryplans.com/search.html

Some do bolt them to posts but I would at a minimum notch the side of the post.  Probably just me. :-/

[John_M]

I strongly agree with Glenn.  If you nail them, the only thing holding your cabin up will be the nails!!  You often see this type of construction on decks, but at the very least, they use carriage bolts to hold the beams to the posts.  Plus, most decks are not built to hold a tremendous amont of weight like a cabin would!!

So....boards nailed to the side of the posts....bad idea!!

Boards carriage bolted to the posts...better, but still a bad idea!!

Boards built up and resting on top of posts....good idea!!

You can attach them with these:

http://www.strongtie.com/products/connectors/BC.html#gallery

...or at least a version of this.

[leinom]

I'm going to order the plans today or tomorrow. Yeah for sure I'm just going to do it right and put doubled up two by 10's on top of my 4x4s and use those hangers that sit on the posts. I'm still wondering though... why do you need to put half an inch of plywood between? Is it just to bring it to nominal dimension? Thanks again!

[glenn-k]

That's about it - but sandwiched it does provide a bit more resistance against deflection - a 4 x 10 is about 3 1/2 to 5/8 inches thick.  Two 2x10s are only 3 inches thick.

[jraabe]

Just to be clear. 2 - 2x's with a space between have about the same beam action as two nailed together when the load above is equalized on both. The problem comes with the ends of the joists and the fact that they are held up only by the shear of the nails into the posts (as Glenn notes). Get these supported (perhaps by another 2x nailed to the post) and you will be fine.

Put a 2x4 bottom in with a 1/2" reveal and it will be stronger yet.

Here is a sketch of such a beam:

[glenn-k]

That's why he's the boss.

[n74tg]

Interesting idea John.  On the house I am building I was planning on using three 2x10 as my floor girders.  The columns are spaced at 10' intervals and they are built out of concrete block.  I can make the bearing plate on top of the columns any size needed.  

Would there be any advantage to building my girders in the configuration you drew above rather than in the 4.5" thick (three 2x10's) more standard configuration?  Just to make certain we are all on the same page here, I'm thinking of turning one 2x10 flat and putting it between the two outside (vertical) 2x10's.

[John_C]

4.5x10  would be stronger/stiffer than putting one of the 2x10's on the flat.

[glenn-k]

I would say correct on that, John C.   and add that 2 2x10's with the 2x4 nailed and glued as John mentioned would be stronger than 2 2x10's nailed together plain as the 2x4 will be in tension.  I don't see much benefit to going wider than that.  From my perspective - not an engineer. :-?

[jraabe]

Here is a diagram that helps explain this. Most of the work of a beam is done at the top and bottom. The middle of a beam is only there to keep the top and bottom in position and stable under side loads.



A taller beam (around 3x taller than width for wood) is usually the most effective trade-off between slenderness and strength. Each of these shapes can be structurally analyzed and optimized for a specific span, material and load. (I once knew how to do these calculations but now send them to an engineer.  )

[glenn-k]

Even though time is sapping you of your desire to do the heavy lifting, -- you still draw nice pictures.