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Deflection in TJI joists over long span

15 years ago

The first floor joists on my plans are calling for 16 inch (in height) TJI joists, 16 inches on center. I'm concerned about deflection. The joists span the width of the foundation which is 24 feet (the length of the foundation is 34 feet). We are trying to not use a center beam with lally columns in the basement. Does anyone have a similar situation or can anyone talk from experience if the scenario I describe will be be a problem with regards to walking across the floor and feeling it "shake"....

thanks.

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Comments (6)

mightyanvil
15 years ago
The deflection or bounce of the I-Joists you describe is dependent not only on the joist depth and spacing but the size and strength of the top and bottom chords.

To make it simple, you only have to ask the designer what "deflection limit" was used when the joists were sized (L/360, L/600, L/750, etc.) The manufacturers actually have different charts for each of these deflection limits so it is essentially the first decision the designer had to make.

For this span, L/750 would give you a max. deflection of about 3/8" which would feel pretty stiff in my opinion. A limit of L/360 (the minimum required by code) would give you a max. deflection limit of about 7/8" which would feel pretty bouncy in my opinion.

Not everyone agrees what deflection limit is acceptable for a given span but no one will ever say it's too stiff.
mightyanvil
15 years ago
My guess is that they are Weyerhauser TJI 230's with a deflection limit of L/480 which would be about 5/8" in your case. I would consider that border line but if they are 210's at L/360 (minimum code requirement) I think you will have a problem. The building code has never been concerned with bouncy floors. I would recommend 360's. This is sometimes a difficult thing to explain to contractors who think the building code is a manual for house construction.
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Your question is unclear. What do you mean by trusses in the floor structure? TJI is a trademark for I-joists made by Weyerhaeuser. It is best to call them by their generic name in order to avoid misunderstandings. I don't know what you are referring to by "steel beams". Are they composite steel and wood joists or floor trusses? Are the joists dimensioned lumber with a beam in the middle that long span I-Joists would replace? Whatever the advantages, you can make a serious mistake using a manufacturer's span tables to design I-Joists. The most important issue is limiting deflection so that the floor will not feel bouncy. That sometimes requires a deeper I-Joist but the tables and the building code won't help you make the floor feel comfortable. Who is designing the structure of the house? What does that person think about the change?
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You should not be able to measure that much deformation in a newly framed floor. Are you assuming the floor below is level? Did you use a string or a laser device? As Mushcreek mentioned the L/360 maximum code deflection limit assumes the joists are at the full design live load (30 psf) plus dead load (10 to 20 psf) and you have already exceeded it with only the unfinished framing dead load so this is a measurement issue, a material issue or an installation issue. L/360 is the code maximum but at long spans that is never stiff enough to limit floor shake/vibration to a comfortable level (the code does not address this critical issue because it does not involve life safety) so L/480 or better should be used as well as a 20 sf dead load. The tub is too close to the support wall to be an issue for the joists but it might be an issue for the support beam. The quick and easy way to test for adequate stiffness is for a 200 lb person to stand at the mid span on their toes and drop their heels to the deck. Care should be taken to avoid back injury (i.e. this should be done by a young person). If the floor shakes it is not stiff enough for normal use. For a 24 ft span, I would have used L/480, 40 psf live load and 20 psf dead load. The TJI span table for that criteria would limit the span to 23'-6" and the bold type would alert me that the initial dead load deflection would exceed 0.33". I would have then selected the next stiffer joist which is 14" deep rather than decreasing the spacing to 12" o.c. Owner comfort should the design goal rather than cost efficiency.
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pearl2007
Original Author
15 years ago
I was looking at the 360s... but I'm wondering if the 560 would be overkill? The 360 is rated for 26'-3" span at 16" oc... but the 560 can span 29'-8" for 40 psf live load......Probably wouldn't get much bounce with the 560 over 24 feet... but like I said... I wonder if it's overkill....??

Then there is the cost reality... would the 560 (or the 360) be more or less $$ than going with a 9 and 1/2 inch height joist (16" oc) and using center beam with lally columns (considering labor costs and material cost comparisons) ?
iliya1
15 years ago
Weyehaeuser used to give away computer software that would produce a "rating" of how a particular floor design would feel to an "average" person. The higher the number, the more solid the floor would feel. It is a GREAT program for an experienced builder. I don't have it here, it is at work and I can't remember its name. If you search the iLevel website, you may be able to find it. If you are interested, email me and I'll send you more info when I get to work.

Here is a link that might be useful: iLevel website
mightyanvil
15 years ago
How are the other floors and roof framed? Is there no need for an intermediate support?
mightyanvil
15 years ago
You would be wise to not try to design the structure of your house yourself; that is a job better done by an experienced designer. Just tell the designer what floor stiffness you want. As a double check let us know the deflection limit of the final joist selection.

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