Author Topic: girder beams & cantilevers  (Read 4615 times)

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Offline dug

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girder beams & cantilevers
« on: October 21, 2008, 06:45:24 AM »
Have purchased the 20 by 30 1 1/2 story plans and am anxious to order lumber but I have a few things to work out. This is the beginning of many questions I am sure to have so your patience is greatly appreciated. If I ask anything that is already covered in the plans feel free to say 'check the plans knucklehead!'.
OK, 1st. question-
I plan to use a third beam in the center of the house. On top of this beam, in the loft section, will be a load bearing wall supporting the loft joists. According to my source (code check complete) 2 2 by 12's should be plenty of lumber for the beams, at least thats how it reads to me. I was planning to use 2 2 by 12's with 1/2 in. ply between to fit a std. 4 by bracket. Does that seem alright?
Also- I am using the pier foundation with 8 in. concrete form tubes, as specified in plans. With a 3 1/2 in. beam that will leave the pier extending 2 1/4 in. outside the beam. I was planning to run some sort of skirting (probably metal) from the beam to the ground so this would be a problem. Could I cantilever the floor joists that amount beyond the beams? How do most people deal with this issue?

Offline John Raabe

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Re: girder beams & cantilevers
« Reply #1 on: October 27, 2008, 10:44:34 AM »
The 20' wide 1-1/2 story plans are designed for an open floor plan with full span joists at both floors. The 20' wide Universal Cottage plan (with two full floors of load) has an option for an interior bearing wall and beam. The spacing of the bearing points and location and size of the posts, beams, and foundation pads or piers are design specific.

If you choose to do an interior bearing beam and wall system a double 2x12 girder can be used for the bearing girder but it may not be strong enough for the spans you wish it to carry. I use wider girders in the Universal plans but the spans there are up to 14'.

We cannot do engineering for such a redesign but this would be a simple consult with a local engineer to determine your beam and joist options and check it with the loads on the foundation piers. The engineer will also know your local snow and wind loads and can help to design for these if needed.

Make sure you have your floor plan modifications worked out completely and know where you want the posts before you go to the engineer as this cannot be changed without redoing the engineering.

On the issue of cantilevering the joists over the beams. As a general rule you can safely extend a set of joists over a beam equal to the depth of the joists before having to design it as a cantilever.
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Offline John Raabe

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Re: girder beams & cantilevers
« Reply #2 on: October 27, 2008, 11:59:04 AM »
If you want to do a preliminary sizing of the joists yourself, you can use the calculator here: http://countryplans.com/smf/index.php?topic=2729.0

For a midline beam under the main floor the load going to the beam would be 500 pounds per linar foot. (PLF). For the loft floor the load would be 400 PLF. The calculator above cannot size girders but doing a quick calc here your double 2x12 girder (DF or SP #1) could span 10' on the main floor and a bit further on the loft floor.

This will give you a general idea of what might work.
None of us are as smart as all of us.

 

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