I Don't Have To Use Clear Span...I Suppose

Started by VannL, February 24, 2017, 12:54:29 PM

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VannL

Well, as will happen in a home build, I am beginning to rethink something. And that being my floor joist situation.

I am building a 20 x 40 1 1/2 story based on the plans from here (Thanks John!). I have a full foundation of cinder block walls (see my post in my signature). My idea was to keep the under side of the house clean and simple and use floor joists (I joists) to run the 20 foot clear span. After a little education, especially around pricing of those joists, I am reconsidering things.

Let me bounce something off you guys.

I am looking at pouring a few cement pads and use 4 x 4's up a height of about 5' 4" to hold up a beam, effectively cutting the clear span in half, 10 feet on each side of the center beam. My main points are wanting to:
- Cut costs from the I Joists
- Ease of installation (never installed I Joists before)
- Keep the deflection (the bounce factor) to an absolute minimum, or less

I have a few questions, but PLEASE offer up anything you might have to say on this topic.

1. Do I get 12 foot lumber and rest them on the center beam, connecting them together, or do I go with hangers?
2. Looking at the material from the plans and the pictures I see from other builds, it looks like I can get some 2 x 12's and make my own center beam. Yes?
3. How many supports would I need for a 40 foot run?

Once again, you guys are great on information and my wife and I do appreciate it.
If you build it, it will be yours!

glenn kangiser

Hangers could be used to give more head room, but if that is not an issue I think choices are open. :)

I haven't looked at John's original suggestions in quite a few years.
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.


ChugiakTinkerer

To keep the beam spans within approved code you should have a look at the Girders and Headers table in the IRC: http://codes.iccsafe.org/app/book/toc/2015/I-Codes/2015%20IRC%20HTML/index.html

This info you need is actually in Chapter 6: Wall Construction.  Scroll down to Table 602.7(2) to find allowable girder spans for interior walls.  Your beam (girder) will be supporting just one floor, and you would calculate your pad and post spacing based on the beam you use.  The table assumes built-up beams made from dimensional lumber, just like you are considering.  Based on your lumber and cement costs you could estimate the least expensive way to construct and support a beam.  If you want to have four interior post supports, you would want any girder combination that spans at least 8'.  For a 20' wide building that means you could go with two 2x12, three 2x10 or larger, or four 2x8 or larger.

For your floor joists it might be better to stay with the full length in dimensional lumber.  Lumber yards in my area have 20' 2x joist boards for about the same price per lineal foot as a 10' or 12' board.  In your situation I would prefer to use a 20' and not have to puzzle out how to butt or lap two shorter joists.  That means your joists rest on top of the interior beam.  It will save the costs of additional hangers, but means that your crawl space is reduced by the depth of the beam.

If you opt for a beam sitting below the joists, you'll also have to figure out how to support each end at the cinder block.  I think there are brackets designed for hanging such a beam, or you could probably have one custom made.  If designed for ahead of time, foundations are made with a pocket in the concrete or block for placing and supporting the beam.
My cabin build thread: Alaskan remote 16x28 1.5 story

Don_P

Generally if you can use a dropped girder it is preferable. The joists are bearing on wood instead of hanging on nails in shear, cheaper and stronger, easier on running HVAC and plumbing most of the time. If you lap joists over the girder don't let the tail of a joist overhang the girder more than a few inches or it'll create a squeeker. I've sawn and chipped to make pockets before. The girder should be restrained at its ends by some means as well as blocking between joists over the girder. I believe the girder table is fine either way but just for more info there is a load difference on the girder depending on whether the joists are full length or lapped. A continuous joist places 5/8 of the joist load on the girder, with 2 simple spans it bears half the load.

DaveOrr

My place is 24'X36' and I used 2 6"X6" posts with 2"X10"'s let into either side to create a beam.
I then ran my floor joists to hangers.









Dave's Arctic Cabin: www.anglersparadise.ca


VannL

Ok, after reading everyone's replies, and walking my situation through a few calculations, measurements and such, here is where I think I am headed.


  • I will use small concrete pads, like DaveOrr's. I will dig a 1x1x1 foot cube out of my sandy clay ground for the four posts, with gravel in the bottom for drainage. I will use the 5000 PSI mix.
  • From there, a metal bracket in the concrete to hold a 4x4 post up to a height of 5' 4". The top of this will have a bracket for the girder.
  • The girder will be made from 2x12s, 12 and 8 feet long to keep the joints staggered well, sandwiching 1/2" ply to keep the dimensions correct. The distance between my posts will be about 7' 9.5", so that should keep within the IRC of 8'+, if I read the table right for 2 2x12.
  • I will go with the hangers as I am past the point of redesigning my cinder block walls and I've done hanger installations before and am familiar with it.
So, that is what I have on the plate at this point. Thoughts, ideas, comments?
If you build it, it will be yours!

Don_P

Your footing needs to be larger and the 5000psi concrete is overkill. Your soil is good for ~2000 lbs per square foot or about 15 psi (see why I'm saying big psi concrete is overkill?), check the load and make sure the area of the footprint can resist the load. Then backtrack from there, the concrete should be as thick as the distance from post edge to footing edge. put 4 pcs of rebar in the footing in a # pattern 3" off the bottom and 3" from any soil.

ChugiakTinkerer

Since this is just to support a floor your loads shouldn't be too much.  Figure on 40 lbs per ft (psf) live load and 10 psf dead load.  Total design load for the floor is 20' by 40' by 50 psf = 40,000 lbs.  The beam will have to support half that load, or 20,000 lbs.  With four posts, each post and pad will need to support at least 5,000 lbs.  Building stresses being what they are, loads are never perfectly distributed, so you should probably add a 50% margin in there, making a design load of 7,500 lbs.

As Don P said, most soils are good for at least 2,000 psf.  In order to support a load of 7,500 lbs you need a pad with an area of 3.75 square feet.  A 24" x 24" pad will do the trick.  The pad doesn't need to be completely made of concrete, you can put crushed rock in the bottom first.  It will allow for drainage and when compacted it will lock into a strong load-bearing base.  I think you can get maximum load-bearing without too much investment by doing the following:

1) Dig out a 24" x 24" square at least 6" deep, then fill it with 3/4 minus crushed rock and compact it with a tamping post.
2) Make a form out of 2x6 and center it on the gravel.  The inside should be 12" x 12", with rebar reinforcement set 3" above the rock.
3) Mix and pour whatever concrete is handy, and set a post bracket in the center.
4) Set your 4x4 post in the bracket and proceed from there.

If you want to go all out, you could pour concrete in place of the crushed rock, as long as it is the same dimensions.  Actually that is probably a better thing to do as you can then use rebar to tie the 24" footing with the 12" pier.
My cabin build thread: Alaskan remote 16x28 1.5 story

VannL

If I understand, this would be a 24x24 6 inches high with a 12x12 6 inches high on top of that, but poured all at once. That makes sense.
If you build it, it will be yours!


ChugiakTinkerer

#9
Yep, that's it.  Tie them together with some rebar and pour self a footing and pier in one go.  :)

You'll want to do some compacting to the ground before you make the forms.  It certainly wouldn't hurt to put some gravel down too.  Actual forms for the footing probably aren't required, you could just dig a 24" square and use the earth as your form.  Do you have to deal with a code inspector?  Getting too free-form probably won't fly in that case.

Edit: Meant to comment earlier on your girder.  The joints in the built up beam where one 2x12 butts up to the next need to be located above a post so that the ends of each 2x is supported.  This means you need to have 2x long enough to span three posts.  So if you have 40' of beam and there are 4 posts then you have 5 spans each of 8'.  You would start building the beam with a 16' and an 8' board of 2x12 (cut to exact length so that the joint falls at the middle of the supporting post).  You would then finish the beam with 16' boards until you reach the last one which will be 8'.  See here: http://www.countryplans.com/builtupbeam.html
My cabin build thread: Alaskan remote 16x28 1.5 story

VannL

We live out in Rural Alabama so we do not have inspectors, no permits, and no plans inspections. This is a great thing, but it does mean I have to be extra careful at what I am doing.

As for the forms, we have clay where the house is going so forms aren't needed. I can just dig and pour.

Thanks for everyone's help on this. I can't wait to get the footings started this weekend.
If you build it, it will be yours!

ChugiakTinkerer

Here's some good discussion on why it's important to get girders designed correctly. 

http://countryplans.com/smf/index.php?topic=10511.0
My cabin build thread: Alaskan remote 16x28 1.5 story

VannL

Man, if only you and DonP were my neighbors. LOL

Great info. I can see the possible sag that an unsupported joint would create now. Just have to figure out how to get 16 foot sticks of 2x12 home in my pick up truck. "Look out folks, here he comes!" LOL
If you build it, it will be yours!

Don_P

I'm easily swayed on a cold morning ;D
I've got to move a 16' scaffold plank and ladders Sat to help hang a sign. I really need to weld up a set of racks for this truck.


VannL

Don, I promise not to mention the 62 degree high for Saturday and Sunday, oh, and the clear sunny skies. I won't say a word.  ;)
If you build it, it will be yours!

DaveOrr

Quote from: VannL on March 01, 2017, 03:15:59 PM
Don, I promise not to mention the 62 degree high for Saturday and Sunday, oh, and the clear sunny skies. I won't say a word.  ;)

It was -53F with the windchill here this morning.
   c*
Dave's Arctic Cabin: www.anglersparadise.ca