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I am using some 11-7/8" x 2-1/2" TJI I Joists. They say they can span 19' 4" when spaced 16" O.C. Does this mean they will have the same stiffness as if they were only spanning 12'? Or 15'? Or is this saying that they slowly get more and more soft and spongy up to the point of 19' 4" that after that they may break or no longer meet the deflection rating?

I'd like to span 19' to prevent adding in more basement support walls, but I do not want to design for this and then realize I have a cheap & weak feeling floor. I would like to know how the rating works in terms of changing how the floor feels along that maximum span.

  • Who is “they” and what loading did they specify? You should get this checked by a qualified engineer / surveyor, if it fails you will be liable – Solar Mike Jan 18 at 16:45
  • The makers of the engineered joists have literature that states what the allowable spans are. I have used them in the trade quite often. Nowadays those specs are included in the building code, but when the material first came out, the local building inspectors deferred to the specs of the manufacturer. – Jack Jan 18 at 17:12
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The joists will be more springy than with a shorter span. It will carry the load that they are designed for. If you want to reduce the springiness, set the joists at 12" centers instead of 16" and add 2 rows of bridging instead of one through the center. This of course will cost more and carry a much heavier load but will reduce deflection.

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There are several factors in designing floor systems

  1. Normal Loading consists of Live Loads (people, furniture, etc.) and Dead Loads (carpet, subfloor, insulation, ceiling finish below, etc.)

    The Code requires a minimum of 40 psf Live Load and 10 psf Dead Load. Check to see if your construction exceeds this minimum. You can google how much plywood weighs per square foot, insulation per square foot, carpet per square foot, ceramic tile per square foot, etc. (Ceramic tile weighs 3 times what carpet weighs.) Also, consider the live load: do you have a big family, treadmill, etc. that will be in this area.

  2. Acceptable Deflection - Little known fact: the code does not specify a required deflection. It only says to account for deflection.) So, when you look up the maximum span for your I-joists, check to see how much deflection.

    Lumber can deflect up to L/180 without failing, but it will feel spongy. Normal design standards is L/360 ...and some ceramic tile floors require L/720. (For L/360: 19’ x 12” per foot / 360 = 0.63” or about 2/3 of an inch.) Consider what type of floor coverings you’ll have in this area: carpet or ceramic tile?

  3. Traffic patterns will reveal a spongy floor. You’ll notice a spongy floor when walking down a hallway or across a room before you’ll notice much deflection in a child’s bedroom, etc.

    Also, what rooms are over the middle 1/3 of the 19’ span is important. Greatest deflection occurs in the middle third of spans.

By the way, we always add an extra joist or two under refrigerators, tubs, hot water tanks, waterbeds, etc. AND we decrease the spacing in Kitchens, Entry’s, Family Rooms, etc. (where people gather.)

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