TJI Web Stiffeners

In finishing my basement, I framed a wall parallel with and directly under a TJI (real TrusJoist-brand) joist. So, the top plate of my wall is attached to the underside of the bottom flange of the joist above for a length of above 13 feet. As per normal practice, I made sure everything was a nice snug fit. But now that I think about it, I'm concerned that with the frame wall below the joist and whatever forces are applied above it, the joist may get "crushed" if the web fails when, say, we move a piano on top of that area. I'm considering whether I should apply several web stiffeners in series along the length of the joist where it is over the wall below. Any thoughts? The joist is not carrying any point loads in that area (like a paino), just the weight of the floor, normal furniture, a partition wall and people above it.
Thanks!
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In a previous post snipped-for-privacy@gmail.com wrote...

Can't hurt to add the web stiffeners, but they are probably not required.
If it were my house I would forgo the extra work.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Bob Morrison wrote:

Thanks, Bob. I figured I was being a little paranoid, but I like to overbuild generally and wanted to ask people who might know a little more than me.
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I'm assuming that those plywood and 2 x 3 composite floor joists have been perfected but let me tell you about an experience I once had. This was probably at least 15 years ago.
I went into this nearly new house to add a island cabinet. The next day the tilesetter said it was so much out of level that he couldn't install the tile top. I leave everything level. (???)
Went back and it was tilted. Went down stairs and looked down the joists from the beam outward. The cabinet shouldn't be enough load to bend joists but it was bent. But it wasn't just there. The whole house was falling in. Looking down the joists was a condition I had never saw before. From the beam to about 3 feet in, the joists were down a measured 1/2 inch. (string line) Then toward the center it seemed to not be as much change to a total of about 3/4 inch. The plywood didn't show that it slipped in the 2 x 3's groves but the plywood its self racked. I got the owner down there on the step ladder and showed him. I never heard what they did, I would have wanted real 2 x 10 or 12 (don't remember which) rolled in beside every one which means all the wiring and plumbing going through them would have to be reworked. Bar joists would be the easiest, Just lay them onto the outside mudsill and inside beam and jack them up.
What a mess.

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Everybody with a Quonset hut, a glue gun and a table saw seems to be in the business of engineered joists now. Designs are proliferating but the MacBlo product has a great track record. I use them a lot and have never had a problem. If only they would cantilever more easily....
--


MichaelB
www.michaelbulatovich.ca
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Having read of a variety of issues and manufacturing problems with I-joists, I made sure I found a builder that was foremost a quality framer and secondly into the detailed fit and finish. I'd rather have a house with good bones but a cosmetic blemish or two than the reverse. That said, the point of my question was how to avoid causing an issue with what by all appearances is a quality installation of a solid product (TrusJoist). My sense is that if for no reason than easing my own concern, I'll install a series of stiffeners at key locations before I close up the ceiling. Done correctly, it can't hurt and the cost in both time and materials is minimal.
Michael Bulatovich wrote:

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In a previous post Michael Bulatovich wrote...

For that one you sometimes have to nail on a couple of 2X's to make the system work.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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A friend calls'em "squish blocks" ; ) It's still a bunch of work, especially if you're going to cantilever them all.
--


MichaelB
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In a previous post Michael Bulatovich wrote...

Michael:
Most I-joists will cantilever farther than the manufacturer's software will tell you they can (usually 3 feet). Having said that, there is a practical limit of 4 to 5 feet (based on deflections) for the typical 9-1/2" and 11-7/8" joist sizes. Deeper joists can be made to cantilever farther than 4 to 5 feet.
Speaking of cantilevers, here's a trick:
If your backspan is less than 2x the cantilever, and the end of the backspan is attached to a beam, then you can install joist hangers on the beam upside down to resist the uplift caused by the cantilever.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Bob Morrison wrote:

Could you expand on that slightly, where does the 2x material go and what function does it serve? I understand that in a cantilevered beam, the top is in tension and the bottom is in compression.

What connection carries the load when the backspan is loaded and the cantilever is not?
Thanks, Wayne
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Look at page 12 here:
http://www.ilevel.com/literature/TJ-4002.pdf
--


MichaelB
www.michaelbulatovich.ca
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In a previous post Wayne Whitney wrote...

Wood is usually weakest in tension. The 2x's get nailed to the top flange if the I-joist is to be cantilevered. My favorite method is actually for cantilevered decks. The I-joist stops at the exterior wall and PT sawn lumber (doubled if necessary) is cantilevered out. In this case I often align the sawn lumber with the bottom of the I-joist. This gives a "step- down" between the floor level and the deck level, which can reduce the incidence of water intrusion.

You have to look at the "uplift" capacity of the hanger in question. Remember this is for the condition where the backspan is quite short - perhaps even equal to or shorter than the cantilever.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Bob Morrison wrote:

Is uplift more of an issue than shear at the support of the cantilever?
Matt
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I thought it was the shear on these things.
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MichaelB
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In a previous post Matt Whiting wrote...

Matt:
I assume you mean shear in the joist at the support.
Generally, the answer is no. Here's our model:
__________________ | w | =================^ L ^ a A B C
Shear for span AB is w*L/2 (same as if there was no cantilever). Shear for span BC is w*a or P if loaded on the end. Reaction at B is w*(a+L/2)
Shear at the cantilever can be a problem if there is a heavy concentrated load (like from a wall above) on the cantilever.
If AB < 2*BC then you might have uplift at A when you load BC only. Span AB will behave as a simple span when there is no load on BC.
For simple loading conditions (uniform only and discounting the weight of the joist), the maximum design bending moments will be wL^2/8 for AB and wa^2/2 for BC.
Member stiffness is often the governing design criteria for sizing the joists or the cantilever.
Of course this all gets more complicated when you start looking at load combinations. D+L full length, D+L on AB and D on BC, D on AB and D+L on BC. Throw in a snow or wind load then things really get interesting!
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Bob Morrison wrote:

Yes, I was thinking shear in the joist at the support due to the larger reaction at B. However, I guess this is more properly a web buckling issue than a shear issue.
Matt
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In a previous post Matt Whiting wrote...

Yes you are correct. The stiffeners at the support for the cantilever are often called squash blocks as Michael pointed out a few posts ago.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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I said "squish blocks" ; )
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I knew of another building in this area that was almost new and falling apart. Example, one hallway was 2-1/4" out of level in the length of a 4' level. Don't remember what the joists were in this case but he put it on the market after I was pointing out some of the sags in various areas. Like one wall was 3-1/2" out of plumb. They must have put it on the wrong side of the partition mark and nailed it where it landed. Asking price, $850,000.00. He was a lawyer and probably got it. <G>
Like many of you, being a perfectionist, I've seen jobs that make you shudder.

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It's not the job so much that makes me shutter, it's the exceptance of a poor job that bugs me. If people didn't except a poor job, the poor quality would come to a halt. Lou
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