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Beam Sizing – Help Please

Shoeman | Posted in Construction Techniques on April 15, 2004 03:44am

I don’t get into this kind of stuff much in my remodeling work, so thought I would ask for input here. 

My parents have an old 23′ x 24′ cabin on their property.  The land slopes off such that one end of the building basically sits on the ground and the other end is about 3′ feet off the ground.  It is basically one large empty room that will be used for family gatherings.  It has a hip roof – about 4/12 pitch I think.  Currently has 3 old rough cut beams that are rotting away that run the 23′ length.  These beams support 2×8’s that are laid out at 19.2″ (that’s right – someone was using their black diamonds on this one) centers, running perpendicular to the beams (24′ way).

I would like to shore up this building.  Cribbing for temp support.  New concrete pads, posts, and beams.  What I am looking for is suggestions on beam sizing and spacing as well as post sizing and spacing. 

2×12’s for beams or 4×6’s or ? 

4×4 posts or 6×6 posts or ?

I plan on using all treated lumber, and appreciate any input anyone cares to give me on this.  This place is basically an old garage or something that someone moved onto the property long before my folks moved in.  I would just like to give it some strength before we end up with the whole fan damily in there when it rains on the fourth of July or something.

Thanks in advance,

Shoe

 

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Replies

  1. csnow | Apr 15, 2004 06:49pm | #1

    The 2X8s are the floor joists, right?

    So you have 3 beams perpendicular to the joists, under the joists, holding up the structure?

    I wonder if it would be possible to simply place new beams near the old beams, jack them level, and set new posts.  Then remove the old rotted beams and posts.

    By setting one new beam at a time, you might be able to eliminate the need for temporarily supporting the entire structure (at once) on cribbing.

    If the floors are bouncy, you could half the floor joist span by setting 4 beams instead of 3.

    1. Shoeman | Apr 16, 2004 01:47am | #2

      Thanks for the input.  Yes you have the picture right.  My intention is to add new beams next to the old ones one at a time.  Don't plan on raising whole building at once.  Cribbing I mentioned is just as needed to work one beam at a time. 

      I am curious what the strenth of a 2x12 is verses a 4x6.  Was considering putting in 5 beams where it currently only has three.  Must have one beam down the center where the floor joists lap.  Wondering about cutting notch in top of 4x4 to attach 2x12 or maybe a notched 6x6 holding a 4x6.  or a 4x4 under a 6x6.

      Sure there are many ways to do this.  Just wanted to brainstorm with some of the folks here.

      The only thing I am pretty sure of is that I will end up pouring concrete pads and attaching the posts to them with a metal connector.  Can't really dig full frost footings as this cabin is close to the lake and run into water real fast.  Also easier not to have to dig a 42" deep hole when you only have a foot or two of headroom.

      Thanks again.  Like I say, just trying to brainstorm

      Edited 4/15/2004 6:55 pm ET by Shoeman

      1. wrudiger | Apr 16, 2004 04:12am | #3

        If you have Microsoft Excel this is the tool for you!

        http://www.wwpa.org/techguide/suite.htm

        It gives you the engineer's answer to your questions, covering sheer, bending and deflection.  Even if you are like me and don't completely understand what those mean you can easily see if your choice will work.  You can compare 2x to 4x to ... to your heart's content.  Also choose the wood grade, account for the incising from the pressure treating process, etc.  Covers posts, beams, joists, rafters...

        1. Shoeman | Apr 16, 2004 03:57pm | #11

          Thanks for the link.  Should prove quite helpfull for a number of things.  Will have to look at it when I get some time this weekend.

          Thanks again,

          1. csnow | Apr 16, 2004 04:19pm | #12

            Right, I would vote for deep built up beams as well.  Fewer posts that way.  Keep the moles happier.

            Getting below frost is desirable, though it is not that unusual for seasonal structures like this to 'float' on the surface.  Over time, it would probably go out of level.  At my 'family compound', many of the buildings will get an occasional 'jack and block' to set them level again.

      2. DanH | Apr 16, 2004 05:28am | #4

        Very roughly, when you double the width of a beam you double it's strength. When you double it's height, you quadruple it's strength. Tall and thin is always better than short and thick.

        1. Shoeman | Apr 16, 2004 08:30am | #6

          Thanks for the input.  I was thinking that the taller would be stronger.  I would, of course, put in plenty of blocking. 

  2. User avater
    coonass | Apr 16, 2004 05:40am | #5

    Shoeman,

    We build on pier and beam alot to get away from the termites and water. With a 4x6 I would span 6', with a 6x6 about 8' OC. Usually use CMU's but sometimes use 6x6 as post. I like to set the post on tic tac toe rebar halfway in a 2' x2' x2' deep hole and then pour. Get a little shear on each post this way that adds up.

    Everybody on the crew gets more money for "mole" work.

    KK

    1. Shoeman | Apr 16, 2004 08:40am | #7

      Thanks for the input.  I was also thinking rebar in a 2x2x2 pad.  Was thinking that I would put the post on top on a pin or a metal connector though, rather than in the concrete, just to keep it from rotting.  Was thinking about shear and that is why I was thinking either cheek or saddle cuts on the posts with through bolts and/or bracing.

      Now I have one response for the tall thin and one for the square.........Hmmmmmmmm back back to square one I guess.  Actually, glad to hear your comments on your pads, as that is just what I was thinking.

      Anyone else care to chime in?  Still not sure if I want to sink the post in concrete or put them on top.  Also, guess I am still a bit in the air about the beams - tall and thin or short and thick - guess it doesn't really matter, think either way I will end up over building.

      As for the extra pay for the moles - I have some very able bodied nephews that like to go to Grandma and Grandpa's and play at the lake.  In fact they are the ones that convinced me that we need to heat and insulate once the thing is shored up.  Makes for a nicer place to hang out for a few when snowmobiling.

      Thanks again for all the input.

      1. User avater
        coonass | Apr 16, 2004 02:31pm | #10

        Concrete masonary unit (block)

    2. Shoeman | Apr 16, 2004 08:41am | #8

      I forgot to ask.  What is a CMU?

  3. User avater
    hammer1 | Apr 16, 2004 09:47am | #9

    If I understand your situation, you are building three carrying timbers. One on each end and one in the center of the building? I would use four concrete piers or sona tubes per beam dug below frost level. 8" tubes minimum. Do not put your posts in or on the concrete. I would build the beams just like a house carrying beam. Three 2x10 nailed together, joints broken to fall on the posts should be plenty and easy to work with. 4x4 posts are fine if structurally solid. Add bracing on the posts. Something similar to what is under this deck.

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