## floor spans

Started by Kris, February 10, 2012, 05:40:05 PM

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#### Kris

i cant attach a photo right now,,,so i will try my best to describe the image..

a 18 x 24 ft cabin {will have a second story}
5 piers on each side {total of 10}

the piers are inset one foot on each side from the outside perimeter of the 18 ft..so the span between the piers are 16 ft.

if i used 2x8x18's for the floor joists, is the 16 ft span to far?.i want a firm floor......should another row of piers be added down the centre of the foundation?

thanks anyone that can answer that.....

kris

#### Kris

well...after some thought.....i guess i would just use the same I-joists that i would be using for the second floor...........sound right?

#### MountainDon

If you are doing a second story, IMO, the foundation should be located under the side walls, no cantilever. 2x8's would be too small for a 16 foot span on 16" centers. 2x10  #2 SYP or DougFir can span 16 feet,m 16" OC. For 18 ft, 2x12  #2  10" OC DougFir or SYP works.

Handy calculator here

Piers/girder are not prescriptive (code); if there are inspections in your area they may not be permitted. In any event if you use piers see this thread for what is a pretty good bracing system. Scroll down the page slightly from where the page opens.  Note Don_P's comment is Reply #83 further down the page.

Just because something has been done and has not failed, doesn't mean it is good design.

#### Kris

thanks abunch for the info..........i will re-think my foundation........

#### Squirl

If you want stiff floor don't forget to adjust the deflection on the calculator. The higher the number the less deflection, the stiffer the floor.  I believe tile manf. have a min. of L/480 recommended while code is only L/360.  Also, if you are designing it yourself, don't forget to properly size the girders for a clear span 2 story building.  Over the years I have seen many spend a lot of time devoted to figuring out floor span sizing and ignoring girder span sizing.

#### MountainDon

From John Bridge (tile site)    maximum deflection for tile is L / 360, and for natural stone is L / 720   But no matter what, the better the L/ deflection the less chance of being unhappy with the floor.

Our floor is something like L/420. It feels quite solid (2/4 OSB, hardi backer cement board, ceramic tile). There are harmonics that can cause one particular table lamp shade to vibrate depending on how it sits.

Once the floor plan is decided upon, work on structural design from roof on down.
Just because something has been done and has not failed, doesn't mean it is good design.

#### Don_P

You mentioned a second floor and the desire to cantilever the joists over the foundation. The cantilever projection can be equal to the joist depth, more than that and you are into engineering. With all that the span between supports can pretty easily be reduced to about 15', that is #2 grade 2x10's of most species @ 16" on center and it is around the limit. You'll not regret a mid girder or 2x12's will make it a good bit stiffer.

#### Mathew23

Quote from: MountainDon on February 10, 2012, 08:03:32 PMIf you are doing a second story, IMO, the foundation should be located under the side walls, no cantilever. 2x8's would be too small for a 16 foot span on 16" centers. 2x10  #2 SYP or DougFir can span 16 feet,m 16" OC. For 18 ft, 2x12  #2  10" OC DougFir or SYP works.

Handy calculator here

Piers/girder are not prescriptive (code); if there are inspections in your area they may not be permitted. In any event if you use piers see this thread for what is a pretty good bracing system. Scroll down the page slightly from where the page opens.  Note Don_P's comment is Reply #83 further down the page.

Great idea, I'll try it out