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Old 06-04-2013, 01:10 PM   #1
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Help with my floor framing


I'm planning the floor framing for one of the rooms on my first story. I've decided to go with solid 2x10 joists, 16 o.c. in the orientation shown below. I'll be using Simpson LB210 top flange hangers for connections. I could use some help with the following:

1. There will be a 24'4" flush girder that runs the length of the room. It will hang from the sill beams at either end and be supported by a pier about 8'1-1/2" from each end. I intend to build the girder from laminated 2x10s, but I'm not 100% sure how many I'll need. I'm building to the 2009 IRC and table R502.5(2) mentions "building width". The footnotes state that building width is measured perpendicular to the ridge. In my case, the roof ridge runs perpendicular to the girder. The girder spans from one exterior wall to the other (minus the thickness of the wall itself), so the building width will be around 25'. The footnotes also state that for widths between those shown, spans are permitted to be interpolated. If I did the math correctly, 3-2x10 will have a maximum allowable span of 8'4-5/8" with building width of 25'. So with my piers at 8'1-1/2" from each end, I should be good. Am I calculating this and using this table correctly? Any ideas what deflection limit this table is based upon?

2. A friend of mine suggested switching to 2x8s for the floor joists and possibly the built up girder as well. He seems to think that doing so will cut down dramatically on the costs, but still provide a solid floor. I've run the numbers on AWC's span calculator and come up with a maximum span of 10'2" using both Douglas Fir and SYP 2x8s. (Deflection limit of L/720, Live load 40psf and Dead load 20psf) So this would seem to work for my joist span of 5'10-3/4". Matter of fact, if I'm using the calculator correctly, my floor would be good in excess of 100psf live load. Does this sound right? Looks like my built up girder would have to be 4-2x8s in this case.

I suspect that the size of the girder is more important since it seems that either 2x8 or 2x10 joists would work. The last thing I want is an undersized girder causing bounce in my floor. Anyone have any opinions on all of this?

BTW: There are no walls bearing on top of this floor. The floor will most likely be sheathed with 1-1/8" Advantech. This is a kitchen, so there will be some heavy appliances present.


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Old 06-04-2013, 03:17 PM   #2
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Help with my floor framing


Additional thoughts...

Any benefit to adding a third pier to this design?

Or running the joists the other direction entirely?

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Old 06-04-2013, 03:26 PM   #3
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Help with my floor framing


I would add 2 more piers, one at each end of the center joist (girder), I would stick with the 2x10's for the center joist and if you're really concerned with costs you could go with 2x8's for the floor joists, but personally I'd use 2x10's for the whole project.
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Old 06-04-2013, 03:47 PM   #4
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Are I-joists an option? You may be able to span that without the girder.
Edit: I just checked, you can definitely make that span with a 9 1/2" I-joist.
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Old 06-04-2013, 04:06 PM   #5
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Quote:
Originally Posted by Clutchcargo
Are I-joists an option? You may be able to span that without the girder.
Edit: I just checked, you can definitely make that span with a 9 1/2" I-joist.
A 9 1/2" I-joist wouldn't work because I only have room for 9 1/4". Have to match the elevation of existing floors in adjoining rooms.
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Old 06-04-2013, 04:12 PM   #6
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what is the flooring? I ask because of the L/720 deflection limit which normally you'd find for marble/stone flooring. the IRC deflection limit for floor joist is L/360.

what is the space being used for?
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Old 06-04-2013, 04:24 PM   #7
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Quote:
Originally Posted by GBrackins
what is the flooring? I ask because of the L/720 deflection limit which normally you'd find for marble/stone flooring. the IRC deflection limit for floor joist is L/360.

what is the space being used for?
This space will be a kitchen.

As far as the finished floor, that's unknown at this time. My other half hasn't decided yet. I'm trying to build the floor to accomodate whatever she decides on. (Hence the L/720 limit) If she chooses slate, I'm covered. If she chooses hardwood, I'm over-built, but still covered.
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Old 06-04-2013, 06:07 PM   #8
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can you drop the girder under the joists? having the joists span the full distance will actually increase the solidness of your floor.
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Old 06-04-2013, 06:21 PM   #9
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Quote:
Originally Posted by GBrackins
can you drop the girder under the joists? having the joists span the full distance will actually increase the solidness of your floor.
This floor is over a dirt crawlspace. The distance between the bottom of the joists and the ground will be about 18". If dropped, the bottom of the girder will be about 9" from the ground. Per code, the girder would then have to be pressure treated lumber. There would also be no crawling underneath it. So could I drop it? Yes. Given the clearances though, I don't think it will be very practical. Would having a continuous joist add significantly more stiffness than hanging two smaller ones from the girder?

Any insight on the size of the girder or the number of piers I should install to support it? I'd really like to confirm my understanding of those IRC tables.
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Old 06-04-2013, 06:36 PM   #10
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based upon the prescriptive header/girder span tables as found in R502.5(2) of the 2009 IRC I concur with your (4)2x8 girder or (3)2x10. this would be based upon the 20' building width which would have a 10' tributary load on the girder, whereas your layout would have a 7' tributary load (more or less).

http://publicecodes.cyberregs.com/ic...9_5_par024.htm
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Old 06-04-2013, 07:06 PM   #11
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Quote:
Originally Posted by GBrackins
based upon the prescriptive header/girder span tables as found in R502.5(2) of the 2009 IRC I concur with your (4)2x8 girder or (3)2x10. this would be based upon the 20' building width which would have a 10' tributary load on the girder, whereas your layout would have a 7' tributary load (more or less).

http://publicecodes.cyberregs.com/ic...9_5_par024.htm
The allowable spans in this table are over unsupported distances, correct?

So if I add additional piers, the girders span would decrease and I could possibly use fewer plys, right?
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Old 06-04-2013, 07:07 PM   #12
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the spans are from support to support, if you decrease the distance between supports then you may be able to reduce the number of plies necessary
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Old 06-04-2013, 07:20 PM   #13
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Quote:
Originally Posted by Pittsville View Post
The allowable spans in this table are over unsupported distances, correct?
I'm not being sarcastic or an AH, because this thread intersts me from a design perspective without eng specs and Gary is teaching me alot....
but is not that what span means

Edit..Sorry X-posting late
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Old 06-04-2013, 07:21 PM   #14
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Help with my floor framing


Last two advice/questions than I'm done for the night...

Would there be any structural issue with adding two layers of 1/2" plywood to the girder to pack it out to 5.5"? (So it sits nicely over a 6x6 post.)

At the ends of the girder I was thinking of using a top flange simpson hanger to hang the beam off of the sills. (Still using the piers for center support of course) Would it be better to do this or would it make more sense to install piers closer to the ends and let the last couple inches of girder cantilever out? In other words, should the extreme ends of the girder be fastened or can they "hang free"?
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Old 06-04-2013, 07:33 PM   #15
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adding layers of plywood does nothing per the prescriptive code, and very little if engineering calculations are ran. we pad out multiply beams all the time to come up with the same width as columns and attachment of column caps.

the ends of the girder needs to be supported, unless they are engineered to handle a cantilevered load.

if you are flush framing and the joists and girders are framed from the same size members then wouldn't the girder sit on top of the sill plates? I'm guessing you have a foundation wall around the perimeter of the building.

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