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Old 02-24-2014, 12:10 AM   #1
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Sister joists or to add joists in between...


I have a rambler with a daylight basement. The joists are exposed in the basement and easy to get to. The house was built in the late 60's. I want to stiffen up my floor (and level one part of it), for future tile or stone installation.

Currently I have a subfloor made up of approx 2x6 T&G planks (my home builder friend called it "car decking"). The T&G is covered with a layer of 5/8 plywood screwed to the planks.

I also screwed the planks down to the joists. The joists are rough-cut 3.75" x 13.75". They are spaced 48" on center and span 22 feet. I do not want to add posts or a supporting beam that would take up headroom.

The floor feels fine when walking normally, but there is noticeable bounce with kids running and jumping. Finding similar lumber to properly sister the joists will be difficult. I was thinking of the following options.

1) sister both sides of each joist with 2x12 lumber

2) sister with LVL beams of the same size as the joists.

3) add I-joists or LVL beams midway between the exiting joists.

The last option would probably be cheapest if I used I-joists, but attaching the subfloor to the joists would be difficult in some parts of the house. I am leaning towards option 1 as the simplest choice.

Option 2 would work well I think, but will be the most expensive and I'd need to special order the beams and get them delivered. Does anyone have any good input? Am I missing a good option?

This is a bit of an offshoot from another thread I started over a month ago. I got some really good info there, but I'm trying to narrow my options and I wanted to start a new, more focused thread.


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Old 02-24-2014, 02:04 AM   #2
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Sister joists or to add joists in between...


Hello,
I am certainly not a structural engineer and simply just a meer homeowner of a 1950 Ranch house.

All I can say is WOW! 4"x14" joists/beams on 48" centers?

I would think the best and cheapest option would be double up 2"x12"'s and split the gap and put them 24" on center. LVL's are much stronger but can be expensive.

I think "sistering" to your current beams is a waste of time and material when you would still have a large open unsupported area approx 40" between the joists/beams. I think the cause of your "bounce" is from this open space.

This is just my opinon and you should probably speak with a structural engineer to verify any advice from here.

/Rick

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Old 02-24-2014, 03:07 AM   #3
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Sister joists or to add joists in between...


Thanks. I'm pretty sure it's the joist span and not the spacing. My previous house actually had similar construction. It was also a 1960's rambler. Maybe it was common here at that time. Believe it or not the spacing between joists on that house was even further. More than 6 feet in some places, and that floor was very stiff except in one spot where a joist end had rotted. I even installed some large ceramic tiles that held up beautifully. The difference in that house was that the span on each joist was much shorter.
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Old 02-24-2014, 03:14 AM   #4
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I apologize as I failed to notice the span length.

I still think splitting the spacing is the best option but at that span I think you would be forced to use LVLs.

Are your current joist/beams single, solid pieces or multiple laminated pieces?
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Old 02-24-2014, 03:31 AM   #5
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Sister joists or to add joists in between...


If you use the 2x12 sister joists, I suggest that you use PL Construction adhesive zigzagged along the length, then Screw them to the existing 4 x 14 beams. This will strengthen them , and won't need new support posts to hold them up or to transfer the load .

ED
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Old 02-24-2014, 05:36 AM   #6
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To do this properly, you shouldn't try to guess what size of additional timbers you need, or whether to sister or not.

The critical aspect of this problem is the maximum permitted deflection you would be allowed for the finish you choose. In structural terms, the deflection is usually given as a proportion of the span, eg for normal floors it's something llike span/300 or thereabouts.

It will be less than this for a brittle finish. If a tile supplier gives you a limit for deflection, it is then possible to work back to find the size/spacing of beams to comply with the given deflection. But that is generally an SE's job, as the calcs. involve knowing the loading, timber species and various geometric properties of the cross-section of the beams.
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Old 02-24-2014, 06:03 AM   #7
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Sister joists or to add joists in between...


Tony g.--The deflection for tile is 360 and natural stone , 720---

I believe the deflection of the subfloor between the joists (spaced are 48") will be to much for tile---the 48" span will need reducing even if the joists are beefed up---I'm not an engineer----just a carpenter that sets a lot of tile--Mike---
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Old 02-24-2014, 06:23 AM   #8
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Originally Posted by oh'mike View Post
Tony g.--The deflection for tile is 360 and natural stone , 720---

I believe the deflection of the subfloor between the joists (spaced are 48") will be to much for tile---the 48" span will need reducing even if the joists are beefed up---I'm not an engineer----just a carpenter that sets a lot of tile--Mike---
We really need Daniel H's input on this.....he is the 'expert'.

It's been discussed before...sistering joists...etc. What we learned from Daniel, glueing and such does not really add any strength...it still comes down to the deflection properties of the added wood....

As mike pointed out...the span between joists is what is hurting you. 48"? Just wow....

You might be ok on deflection when on top of the joist...but it's the deflection between joists. For reference....my new addition has 2x12's 12"OC. On top of that...1 1/8" T&G plywood. The max span for 1 1/8" it is 24".....but yet I'm just 12".

In your case, you have 3/4" planking with 5/8" plywood on top...total of 1 1/8" across 48".....

If it was me....I would be getting more 2x14 and putting them right between the existing joists....that would then get you to 24' OC....the minimum I suspect.

48"? Just wow........

Dan....we need your input.
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Old 02-24-2014, 06:29 AM   #9
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+1

Dan is the man on this stuff.
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Old 02-24-2014, 06:40 AM   #10
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tony g. is also an engineer----one of them will chime in---
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Old 02-24-2014, 07:15 AM   #11
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Assuming reasonable properties for your beams, modulus of elasticity 1.5 million psi, maximum allowable bending stress 1500 psi, and given the span of 22 feet and 48 inches OC, and using 40 psf live load, 10 psf dead load, you get an L/D of about 300. This isn't too bad, not good enough for tile, but good enough for hardwood. You also need to check between span stiffness of the subfloor for whatever type of surface you put down, I would check with the manufacturer of the surface material since you have a somewhat unusual spacing.

The moment of inertia of each beam is 812 in^4. The maximum deflection, which controls stiffness, is inversely proportional to the moment of inertia. Each 2x12 with nominal size 1.5 inches wide and 11.25 inches deep will add about 180 in^4, so two of them sistered either side will add about 360 in^4. This will result in a total of about 1170 in^4, which will be about 45% stiffer than the current arrangement, which would cover you for standard size tile, but not for stone or large format tile that requires L/D = 720 or better.
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Old 02-24-2014, 07:24 AM   #12
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All floor systems have resonant frequencies - the fact that a floor seems to shake when someone runs / jumps is to be expected, and doesn't by its self indicate there is a problem with deflection.

2X6 T&G SYP boards 4' OC will make a 1/360 deflection, but the question in this case is what wood / grade the boards are. Same question with the beams. If there is any deterioration of any of the elements, then that would have to be evaluated as well.

In this case, the deflection for something like a point load or line load can be directly determined since the underside is totally accessible, but you'd still need a floor system modeling program to see how it stacks up to your requirements.

I wouldn't recommend doing anything without an engineer's evaluation.
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Old 02-24-2014, 02:04 PM   #13
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Thanks all. Dans info was very helpful.

The planks appear to be Doug fir, which would be appropriate for the area. The joists appear to be red cedar. It's all in very good condition.

I have seen the advice to "call a SE" many times on this forum. A problem I have had is finding a structural engineer who would do the job for less than a couple thousand bucks just to take a look. I have called a few and they don't seem to think that doing this type of evaluation for a homeowner is normal.

Another thing I think a couple people assumed was that I have a 3/4 inch plank subfloor. It isn't. It's 1.5" x 5.5" planks (actual dimensions). I don't know if this is uncommon elsewhere, but I've even seen these same sized t&g planks for sale at the local HD, so I assume it's normal in this area.

Like I said, my last house had them with much larger spacing between joists and it was plenty solid. Here's a link Gary in WA posted awhile back that shows such spacing is acceptable. http://publicecodes.cyberregs.com/ic...9_5_sec003.htm
In fact, there is a 15' x 6' area of 6 inch tiles in the main entry area of this house that is sitting on just the planks and what looks like 1/4" plywood. It's been there for over 60 years and not one cracked tile (it could use new grout though). Here's a pic of the underside, in case anyone is interested.
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Old 02-24-2014, 03:09 PM   #14
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A problem I have had is finding a structural engineer who would do the job for less than a couple thousand bucks

Then you could do the calcs yourself - seriously. There's plenty of info on the interweb on how to do it. All you need is a calculator, a little time, and some patience.

You would need to look at the deflection in two parts - first the 2x6s spanning 4ft, and then the main beams spanning 22ft.

Think about the 2x6 t&g planks first and consider an imaginary 12" wide section. Your live load will probably be 40psf, so each 12" wide section will carry 1x4x40 lbs, = 160 lbs live load.

Then you have to work out the dead load. This is the weight of the timber planking itself plus the weight of your tile or stone or whatever on top. A good average for timber would be 40 lbs/cubic ft, so a 12" wide section 2" thick will weigh 1 x 1/6 x 4 x 40 = (say) 30 lbs. The weight of the stone will depend, among other things, on the thickness, but could be in the order of 30 psf. So for a 12" wide section, the stone could weigh 1x4x30, say 120 lbs.

So your total loading per 12" width of floor (live load + dead load) will be
160 + 30 + 120 = 310 lbs.

Next you have to work out the deflection which this load will cause on the 12" width of planking, to make sure it doesn't exceed the deflection allowed for your stone.

There are standard formulae for these things and for a uniform load, the maximum deflection is; 5 x W x L x L x L/ 384 x E x I.

This might look complex, but break it into parts; W = your total load - 310 lbs; L = your span, 48" ; E = the 'modulus of elasticity' of the timber (don't ask!) and 'I' = the 'moment of inertia' of the cross-section of your planking.

'E' can be read from published tables - it varies depending on the species of timber, but a conservative value would be 1,500,000 psi (look back at Dan's post);
'I' for any rectangular section is (width x thickness^3)/12, so the 'I' of your 12" width of planking 1.5" thick will be (12 x 1.5 x 1.5 x 1.5/12) = 3.37 in^4

Now put all this into that formula for deflection and you get

(5 x 310 x 48 x 48 x 48)/384 x 1,500,000 x 3.37 = 0.08" - this is the maximum deflection you would get in the middle for that load.

Finally, you work out this a fraction of the span , = 0.08/48 = span/600.
This is only slightly worse than the maximum allowed for stone. But in practice the deflection will be less than this because the planks will be continuous over the main beams. There are also other factors which will serve to give you a smaller deflection in practice.

There - I've done half the job for you. I can't do any more because it's late and near bed-time here. You can do the figures for the main beams. By the way, as I've done half, can I have $1,000 please into my PayPal account

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Old 02-24-2014, 03:20 PM   #15
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Sister joists or to add joists in between...


Option 3 will require lots of shimming. Also, full depth joists can be almost impossible to get into place in an existing structure without jacking the existing joists or at least a lot of sledgehammer work.

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