Floor joist questions

[beckhamk]

I am sure this will apply to any of the plans available here. But I would like to know what are the reason for using i-joists over 2x12's for the floor joists?  I would like to understand is one cheaper, better than the other, springy'er than the other etc.

This all revolves around the 20 x 30 plans.

[dug]

Unless you use a center beam (I did) 20 ft. is pretty much the limit for a 2 by 12 to span. Acceptable but too springy for some. I-joists will cover that span with more strength.

[davidj]

I-joists are light, stiff, straight and strong.  They are also expensive and the installation details are slightly more fussy.

On the recommendation of a couple of folks, I used them for the 2x10' span on my 20'x30' (I've got a center beam). I could install the 20' TJIs easily without help.   However, if I was doing it again I'd probably go with dimensional lumber, just to save the money, even though I quite liked them.  The plus side is that, combined with the 1 1/8", plywood subfloor, I've ended up with a floor that feels like concrete and should have no problems with the 12x12 slate I'm putting down or the cast iron clawfoot tub in the bathroom.

If you do use I-joists, make sure to line up the punch-out holes - I didn't, which meant I had to drill a bunch or zig-zag my wiring.  Also be sure to read the stuff about no holes near the support points - it's the obvious place to drill for wiring following a wall but it isn't allowed per the instructions.

[PEG688]

"I" joist are nice , but generally they are bouncy .  The span charts are made to sell joists , or to make there cost look better on paper, if you cut down the distance between beams by about 4 feet , or go up one size for depth you can reduce the bounce effect.  But I think due to the added costs the companies attempt to sell there joists with wider spans between beams than is effective.   Yes, they can span that far and not break , but who wants a floor that bounces , china rattles , grandfather clock rattle when you walk by them?


   

As far as lining up the hole, I don't think that is practical , you'd need to add a foot , maybe more to each joist lenght and then some how figure out what the starting point would be , to get the holes to line up.

  I think the joist are made as long runs and the machine cuts them to rough lenght with little re-guard to the hole pattern.   And lumber yards suppliers may stock "long lenghts" some times that they can cut to lenght as required.

  I don't recall any one ever lining up the holes when they roll joists, I guess it might be possible , but doubt it's a cost effective practice / practical policy. 

[beckhamk]

How much are these i-joists vs the 2x12's?

Seems that both i-joist and 2x12's will be bouncy for 20ft?

[PEG688]

How much are these i-joists vs the 2x12's?

Seems that both i-joist and 2x12's will be bouncy for 20ft?

Call a lumber yard and ask , different areas of the country will have different pricing.


  I'd guess the 2x12 would be stiffer , but again you need to get a apples to apples  comparison. 

  Meaning a "I" that's big enough to span the 20' , BUT be upgraded to be a better floor.   I don't have  a I joist span chart but I think you could span and meet the floor load with a smaller I joist maybe even a 9 1/2" one , but the floor would bounce.

I also don't think the manf. "make it easy" to compare 2x12 to "I" joist , there's no direct conversion chart that I know of.   

The charts have live load /dead load / other not so easy to figure out ways of laying out the joist , 12" OC ,  16"OC , 19.2 OC etc, so all that has to be sorted out when making the decisions.


   It's not as simple as a  2x8 will span X ,  2x10 will span Y , a 2x12 will span W.

So it depends 

[MountainDon]

Also remember when pricing 2x12 for a 20 foot span that means select structural grade which may need to be ordered in.

[Alan Gage]

If I remember right was quoted $2/linear foot for 14" i-joists.

Another benefit of using them over dimensional lumber, other than lighter weight, straighter, and stronger, is the lumber savings itself in the grander scheme of things. A 2x12 is a big chunk of wood. An i-joist can be made from much smaller trees.

I'm no expert on this but have been doing a little research and it sounds to me like if you follow minimum code you're going to have some bouncy floors whether you use dimensional or i-joists. The two most common span benchmarks are L360 (minimum code) and L480 (extra stiffness if you want to lay tile). The most useful charts show both ratings so you can see what you would need to do if you wanted to stiffer floor.

My dad has i-joists in his added on kitchen that are right at minimum code. You can feel the bounce when a dog trots across the floor and he doesn't like it. The only time I've ever noticed it though is when he's called it to my attention, I don't think it's that bad. He has the same setup in his bedroom above the kitchen and says he's never noticed the bounce there. I looked under the kitchen and none of the joists had been drilled or cut.

Alan

[John Raabe]

In my 20' wide plans I spec out mid-grade 11 7/8" I-joists (two brands) for the full span width (the lowest grade 14" I-joist will also work). The deflection is set at the over code level of 1/480 and I've had no concerns about springiness. That same span can be done by 2x12's but it has to be the Select Structural grade to meet the deflection requirement. That grade is harder to find and standard grade (#2 or btr) might be felt to have too much bounce.

When doing an unforgiving floor such as large tile or concrete it might make sense to run a center line girder and go to 2x10 for the joists. That still leaves enough room for decent insulation. Second floors don't usually have the same demands for loading and will in most cases be fine with the full width span. Note this is not an issue of structural strength, all those options have plenty of strength, but of a perceived issue of springiness - and perceptions vary.

[rwanders]

I have found that our local and big box suppliers tend to compete on price for dimensional lumber by stocking lower grades of limber and requiring special orders for longer 2x10 and 2x12s. It becomes rather frustrating when trying to pick through warped and knotholed lumber piles. In that case TJIs offer better and consistent quality to the other pros-----y

[Alan Gage]

To expand just a bit on what I said about my dad's "springy" floors. When a dog walks across the floor is about the only time you notice the flex. If people are walking on it it's not noticeable. A dog trotting across must be the right frequency to get in "rhythm" with the floor bounce.

Alan

[Squirl]

The deflection is set at the over code level of 1/480 and I've had no concerns about springiness. That same span can be done by 2x12's but it has to be the Select Structural grade to meet the deflection requirement. That grade is harder to find and standard grade (#2 or btr) might be felt to have too much bounce.

The code charts I have say that you can span 20'-7" @ 12" O/C #2 L1/360. I see that you go over this to 480. Is 360 still to bouncy?

Also according to the AWC calculator at L/480 it can still span 19'-1".  With two 6" wide foundation beams on either side, does #2 lumber still qualify for a 20 ft wide house?

[MountainDon]

Our cabin floor calcs out to L/488. I can not feel any bounce when either of us walk about. A number of people have commented on how solid it feels. However, we have a lamp, that unless the glass shade is positioned "just so", picks up the vibrations and rattles just enough to be an annoyance when all else is quiet.

[Don_P]

These are two different qualities, many floors that calc out fine get vibration complaints that are not "bouncy" problems. As we increase spans that becomes more of a problem. I have a glass front secretary that has the same problem in our bedroom. The floor is solid. If I stacked a few hundred more pounds on the floor it would kill the vibration.

Deflection is a serviceability not a structural concern as John said. With long spans the deflection and vibration are more noticeable. For a long span I wouldn't hug code minimum. At 20' l/480 is 1/2" allowable deflection under full load where at 10' it is 1/4". The vibration frequency is also affected. Pluck a guitar string and then fret it at its' midpoint, the frequency is much higher.
I joists are stiffer but of lower mass so they are easier to excite, as a string they are light and tight, they pluck easily. The floor is not neccesarily bouncy like a trampoline it is tight and easy to ring like a snare drum. The center girder is the easiest way to dampen that type of vibration, the next is depth, the next is mass.

[beckhamk]

Thanks for all of the good feedback, think we will stick with the I-joists.  How wide would the center line girder need to be?  I assume we could do the center line girder and use the 20ft I-joists?

[PEG688]

  It could be a 4x6 , with posts every 6 feet or so between posts.  That would make it feel more solid. And like has been stated,  it's not a it's going to fail thing,  it's more a comfort thing.   

I had the guy price me a "I" joist vs a 2x12  , of course I got in a conversation with some one else and never got  the specific "I" joist he priced , but the 2 by was about .70 cent more per LF.  I asked for a apples to apples size of joist to size of BCI , so I'm pretty sure the "I" joist was a 11" BCI with  1 3/4 flanges.   But I didn't confirm that.

BCI = Boise Cascade

     

[davidj]

I needed long (I think 18') 2x12 #1 for my rafters.  The #2 or btr where something like $20 each.  We tried to pick out #1s from the mix but there weren't enough, so they ended up special-ordering and could only get sel struct.  They were more like $35 each!  This is all in rural CA where there isn't a whole bunch of choice for this kind of stuff - I'm sure it would be cheaper in the city.

Our center beam under the floor has 3 4x4 posts and uses doubled LVLs and 4x4 posts.  The expensive bit was the special-order Simpson hangers for the ends, which hang over the concrete block wall.  It would've been much cheaper to cut a shelf in the block but I wasn't exactly sure how to do that so I just spent the money.

You can also see the footings for the 4x6's which hold up the door frame which in turn holds up the center post for the ridge beam.

The unused footings are taller because it made them easier to lay out.  Here's what it looked like before the concrete was poured.

[beckhamk]

davidj - are you doing a crawlspace or half basement and then framing the remaing 2-3ft with wood framing?

[MountainDon]

Here's davidj's topic

page three for the cabin start