Floor Framing Spacing / Sheathing
TroutMullins
| Posted in Construction Techniques on
A general question about floor framing:
It seems that relatively standard framing is using 11 7/8″ I joists at 16″ centers, and seems that standard subfloor on this is 23/32″ T&G OSB or similar.
Would you frame a floor with I joists at 24″ centers? If so what thickness subfloor would you use? I know the span rating would need to be 48/24, but 23/32″ satisfies this criteria. Based on a few calculations, the stiffness of the sheathing need to be 1 1/4″ to get an equivalent stiffness…
It seems that this is all about how the floor “feels”, not if it is strong enough.
Is it cost effective to frame at 24″ centers? less framing, but thicker sheathing.
Replies
Techniques and requirments are very regional. Most work done here in Central Texas uses open web trusses usually 16" or 18" deep on 24" centers with 3/4" decking. I personally find that kind of bouncy and have seen lots of failed tile bathrooms due to excess deflections.
My personal approach is using girder trusses 18" deep on 19.2 centers with 1 1/8 Advantech decking(much different than OSB). Result in very stiff floor
Bruce
Not a good idea to go with 24" OC no matter the sheeting product used (Steel not withstanding). The span dictates the size of the I Joist and the engineered flooring plan for I Joists will dictate your spacing and usually must be followed for your framing inspection to pass. The max spacing for the 11 7/8" I joist is 19 3/16+" OC. 16" OC is better but not the norm, you usually have to specify this yourself or inform the engineer doing the floor plan of your intent. In my area the building materials supplier, engineer the floor for you when you buy their product through them.
You guys are planning on some very stiff floors.
I have built many a building on 2' centers, with 2x10's.
The flooring has never been more than 3/4" nominal.
117/8 I joists are plenty strong to 2'centers, depending on the lenth.
11/8 " floors are way over what is needed, unless you are driving tanks accross.
All this discussion including my own is all fluff because all of this depends on spans more than on joist size as we all know.
I like to build stiff floors, bouncy is generally not good-at least in my customers opinions.
I was having a block, couldn't remember the word"span". Thanks for the help.
Seems like all I have these days are mental blocks....I think.
"The max spacing for the 11 7/8" I joist is"???There are three different I-joists available that I know of for different load ratingsSo it ain't THE 11 7/8" I joist
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"What ain't the 11 7/8" I joist"? Sorry, I don't understand what your asking or telling me here. I know there is more than 1 size of I joist and that each gives you a different span distance. Or are you talking about different types of I joist materials & thicknesses? The Manufacture specs allow for Georgia Pacific wood I beams to be 24" OC but the engineers here generally don't spec it, nor would most homeowners want it. I believe the company even prefers to see it at 19.2". Look at their specs on the link ( http://www.gp.com/build/product.aspx?pid=1390 ) and you can see the difference between a wood I joist at 24" OC and 19.2" and 16".
Edited 11/25/2008 8:01 pm ET by losh
Or are you talking about different types of I joist materials & thicknesses?
Without putting words into out wize sage Piffin's mouth, I have to wonder if he is thinking as I am of the three different dimensions of 11.875 I-joist products GP (georgia-pacific) makes. They have differing flange and web dimensions, but have the one uniform depth. Other manufacturers have differing standards, and products, too.
I'm leary of any one solution "wedded" to a particular joist thickness. Always seems to not quite work out due to the needs of mechanicals of one kind or another. All of which just makes me appreciate open-web floor joists all the more.
But, that's me, I have biases; others differ.Occupational hazard of my occupation not being around (sorry Bubba)
bingo!
Yes, you got oit.TJ 115, TJ 125, and TJ 135 as I recall, all with increasingly thicker webs and flanges.
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You know, as I do, that an increase in joist depth usually leads to more 'Strength' or simply put; longer spans and less deflection.
But I just can't grasp why bigger (usually wider and easier to hit with the gun) top and bottom chords of a TJI can give it significently more strength without changing the depth. Can anyone offer an explanation that a guy like me can understand?
I agree with most of what has been posted above. But really want to stress that the floor framing has to be designed as a "system". Taking into account lifestyle of the occupants, mechanicals, budgets and of course span and spacing. Not everyone is aware that you can usually get a increase in spans by simply glueing the subfloor down . let a plumber hack your joists to death and them 14'' TJIs start looking real good. All ingerdiants in the big recipe.
Just don't forget your stair opening might need to be bigger cause you will need an extra step with the big joists.
why and how?Because the design book says so
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so you don't know either
Educated guess! The wider and taller top chord allow for a larger cross sectional (Triangle) aspect which in turn carry more load themselves while simultaneously sending some of the applied force into the web, which in turn transmits some of this load back into the lower chord. The link I posted earlier mentioned that materials used in the chords also affect the strength
Ok, I'll try to keep it simple, The "stiffness" is defined by the strength of the material, which is called the modulus of elasticity, and represented by "E", multiplied by the moment of inertia, or "I" of the cross section. "I" is found by using calculus, and we won't get into how to calculate it. (Actually it's been twenty years since I calculated an "I" for anything, and I probably can't do it without looking up how to do it in a text book. I just look them up in table supplied by the manufacturers, and plug them into the equations.)
The "E" value is a property of the material the joist is made of, and if the type of material doesn't change, the "E" doesn't change. So we won't discuss it.
But, we will discuss what the "I" represents, because it does change as the chords change. "I" is the moment area of the, joist, as a factor of the square of the distance to the area. Just think of the distance from the center of the web to the center of the chords, as the length of a lever, and you should be able to visualize it.
If the top and bottom chords, are larger the value of "I" goes up. If you double the area of the top and bottom chords, by making them wider, while keeping their thickness the same, then joist is roughly twice as strong.
Ok, so how did I do? Simple enough, or clear as mud, and more confusion now than when I started?
Ok, so how did I do? Simple enough, or clear as mud, and more confusion now than when I started?
I think you did pretty good, you have shed a lot of light on the subject.
or think of it this way- the web (of an I joist, truss, steel beam or what have you) essentially deals with shear. the ability of a member to resist bending mostly depends on the amount of material at the top and bottom.
when a beam is loaded (in the most simple case), it bends into a kinda smiley shape, right? the bottom edge of that beam is being stretched (working in tension) and the top edge is being compressed (working in compression).
think of bending a paper clip- the coating on one edge kind of flakes and tears as that edge is being stretched, while the other edge is actually bunching up as it is being compressed.
if you precisely measure a simply loaded i-joist, you'll find the top edge is shorter than it was when the joist was flat, the bottom edge is longer, and the very centerline is exactly the same length.
the centerline (called the neutral axis) of a simple beam isn't being stretched or compressed. it has zero linear load.
so the amount of linear force in psi being exerted on the beam (in tension on one side, compression on the other) increases from zero at the neutral axis to its maximum at the "extreme fibers", or very edges. putting more material at the edges therefore resists bending.
k
incidentally, "I" is base x height cubed divided by twelve. bending strain, or the second moment of inertia, is equal to M times Y divided by I. M is the internal moment force generated by the load, and Y is the distance from the neutral axis to the extreme fiber.
I am fat! I weigh 280 pounds and I was tired of walking on floors that caused dishes to rattle. So for a little more I speced really deep joists and 2 inch thick sub floors with near 1 inch thick finish floors.. 3 inches thick of flooring! In the end it wound up much cheaper than plywood and lumberyard floor trusses because I eliminated all the middle men and bought from local sawmills..
Now cars could drive across my floors without a worry..
Let me put it this way...I'm an engineer and can easily calculate what required spacing and spans of I-joists will provide sufficient strength and good performance, but and unsure from a "feel" standpoint what floor sheathing thickness would be required if 24" spacing is used. Any other thoughts on how thick the sheathing should be?Would you rather frame a floor with 2/3 of the framing members (24" oc) and use thicker sheathing? or 16" oc and thinner sheathing?
16" centers and 3/4" material to cover it. Done deal, and stiff.
rather frame a floor with 2/3 of the framing members (24" oc) and use thicker sheathing? or 16" oc and thinner sheathing?
Floor dead on for framing, and more subflooring, not less.
But, that's a personal bias.
Also, that framing is not just sized to meet span, but has had some educated examination as to mechanicals, and their penetrations (so as to not have under-sized framing in spots).Occupational hazard of my occupation not being around (sorry Bubba)
I prefer 19.2" centers or larger, noticeably easier to work between for other trades.My take is that you can stay with 3/4" decking material (although I used 1 1/8" on my house... ) and get the desired stiffness by going with deeper joists, not thicker subfloor.Maybe you can find Boss Hogg's thread on floor vibration. There's some good stuff in it.
Would you rather frame a floor with 2/3 of the framing members (24" oc) and use thicker sheathing? or 16" oc and thinner sheathing?
From the perspective of a remodeler the number of framing members will still be a relatively small factor in the overall project. Often we end up adding additional joists for this reason or that, and while 3/4" decking is more common, I'm a fan of 1-1/8" ply.
If there's a lot of plumbing involved, the wider the spacing the easier it is to get pipes in.
If there's a lot of large duct work, wider spacing may be better.
When I'm nailing down hardwood, I'm a fan of the thicker ply.
When I'm putting down large floor tiles, the stiffer the floor the better since failed grout lines is easy to prevent with a little extra stiffness in the floor.
If I'm overseeing other carps I'd rather keep to 16" or 24" centers since that's what many guys are used to so it's second nature. If it's a remodel I will often use 12" centers if framing depth needs to be kept to a minimum, after carefully thinking through the requirements of the mechanical, plumbing and other trades.
If insulation value is a key concern, wider spacing and deeper framing is the way to go.
If sound insulation is a major factor I might favor framing slightly shorter, but spaced closer to support the extra weight of a sound deadening layer of concrete, or sound channels.
The same goes for radiant heat. I'd rather get the tubes into a layer of concrete rather than have deeper joists and staple it up.
If the budget is tight sometimes deeper joists spanning more of the floor are the least cost, and other times not.
If a large beam is used to split the floor framing, it makes sense to use joists that are the same depth or more to keep the beam up out of the way.
Yep, that just about covers it--it depends on the situation. :-)
Good building!
Beer was created so carpenters wouldn't rule the world.
It depends on what finish flooring is going to be used. Worlds of defference in Carpet, Ceramtiles, and solid wood flooring on that deflection issue.
Another option to consider - that I have done often - is cubit spacing @19.2"
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