Tgi Framing

Joe Carola

If an Architect draws a plan with 14" I-joists and the existing house has 2x10 joists with the plans showing no step in the new room, obviously the new foundation can't match the existing. It's not a matter of you guessing it might work, it will work and always works in this situation every day, no problems.

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Joe Carola

hughjazz

On my own personal house I ran the tgi the long way, (still engineered) In my family room in the basement I didnt want to have a 1' bulkhead in the middle of the room

knucklehead

You are right I shouldn't have said that without knowing the details.Never assume. Sorry.

Tim Garrison PE

I've engineered many floor systems and have learned a thing or two over the years.

The original question was about the direction of the joists. It doesn't matter which way they run as long as they're not overspanned and are properly supported.

Which brings up the issue of overspanned. Joists can span 20' or more and calc out. However, the issue becomes bounciness which is not a strength issue but rather a comfort issue. Blocking, glueing, screwing the subfloor all do very little to stiffen long-spanned joists.

My rule of thumb for wood I-joists is 14' max. span. Beyond that you're risking bounciness even though span tables or software may say okay.

I'll post a story on this topic shortly. It was one of the only times in my career that I was worried about a law suit.

tccoggs

Tim Garrison PE

That's a great point, tc. When faced with long span joists, I generally go with open web joists.

Timuhler

The house we just finished framing has 14" I-joists over the garage 16" oc. There is a bonus room above. The garage is 22' deep (which the I-joists span) and we added a 2' cant out the back, so the joists are 24' long.

There is very little discernable vibration and once furntiture is in and the drywall is on the underside, I bet no one notices it.

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scottalan

Have you actually had problems with people using your pictures?,whats the deal with the copyright.

Also your work look's incredible

Timuhler

I personally have had no problem that I know of, but I know some who have had problems. One of the updates to Picasa allowed a watermark.

whats the deal with the question?

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scottalan

Just seems kind of pretentious to me lol

Timuhler

Yeah, I debated whether to do that or not. What made me decide to do it is that some who contribute to the forums have had competitors take pictures off their websites and put it on their own. I figured I should make it harder on them.

Also, I've been getting into photography and on a lot of the sites like SmugMug, Flickr, etc, the photographers have a watermark. Of course theirs is cool and mine is a name

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scottalan

I must say with the quality of your work I can see why you would want to watermark it,

72chevy4x4

you noted that Picasa allows the option to automatically watermark-that's a great no-brain feature.

nice looking work!

TOH quite some time ago used 1/8" straight steel and bolted them to the sagging floor joists-I don't recall the span but it was likley a very old house.

loneframer

Tim, what's your opinion on X-bridging?
I personally have had great sucess with it on conventional lumber as well as engineered I-joists. In my own home, which I designed and built myself, I used 14" TJIs on 16" centers which puts the diagonals at roughly 45 degrees. I had several rooms thet spanned between 18' and 20' with a narrow area that spanned 21'6". I did 2 rows of 1x3 bridging spaced 4' apart down the center of the 20'span, keeping the rows continuous the full length of the building. In a few areas, I used joist blocking to allow ductwork ang plumbing to pass through. The bridging was stapled in with 7/16 crown staples 1 3/4" long, 3 per end, to prevent splitting of the LVL flanges. 3/4 T&G subfloor glued and nailed.
One other point with long spans is that multiple spans with a single joist will help tremendously to alleviate bounce in the spans. I used 38 foot joists with intermediate bearing, resulting in two 18'+ spans, with 2 rows of bridging each and the floor was very solid with no noticable bounce or vibration to speak of.
I'm interested in your thoughts on these matters.

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Timuhler

There was a thread at Breaktime awhile back that had to do with a study or studies on floor vibration. It seems that the conclusion was that blocking had no effect on it. I'll have to email the guy who posted the paper. He works as a truss designer.

I don't really have any thoughts on it because I'm not well studied on that kind of subject. If I have questions, I'll talk to the I-joist engineer at the yard that supplies us.

What I do know is that you can span longer and the floor feel stiffer when it is not just a simple span, but the joist spans multiple bearing points. Say for instance that we want to go 60' with a joist, and the largest span is 20'. You can get that out of a smaller joist depending on how the other 40' is spanned.

Seems like we had one plan where we spanned the garage (20'ish) with 11 7/8" I-joists 12" oc. I don't remember feeling bounce in that floor.

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Tim Garrison PE

Excellent questions. Here's my take.

Deflection has to do mostly with 1. span and 2. depth of joist. The relationships are exponential, i.e. a small increase in span makes a big increase in deflection; and a small increase in a joist's depth makes it a lot stiffer (more resistant to deflection). Blocking or bridging affects neither, so, theoretically anyway, blocking or bridging shouldn't make a difference in a floor's resistance to deflection.

There is another factor at work, however: dynamic response. This is how a floor (or any structural system) behaves under cyclic loading, such as walking. If a person walks (especially a heavy person) at the right speed, he can "excite" the floor system to bounce a lot. You may have heard of "natural frequency." That is the speed required to cause maximum bounce. Speeds below and above a system's natural frequency tend to cause cancelling dynamic motion and the bounce is less.

This is the very phenomenon that caused the old Tacoma Narrows Bridge (aka "Galloping Gertie") to buck and heave and ultimately break up in a wind storm. The wind excited the suspension wires at exactly the bridge's natural frequency. Buck and heave she did, well over fifteen feet vertically.

Back to our floor. Adding blocking to a floor system changes its natural frequency. Which could push the natural frequency out of the range of a person's normal walking speed. So, even though blocking doesn't do anything to bolster deflection resistance, it could change the natural frequency enough so that the floor never gets "excited".

Now that's exciting!

One footnote about long span floors. When I say 14' max span rule of thumb, I don't mean that longer spans can't be done correctly. Of course they can. It's just that you really have to pay attention to deflection criteria, and sometimes even dynamic response, or you might just wind up with your own personal Galloping Gertie. And if that happens, the fix will be substantial and expensive.

knucklehead

What does TGI stand for?

loneframer

Would you agree that X-bridging forces the deflection caused by walking across the floor to be transferred to adjacent joists and cause them to share that deflection, therefore lessening the severity of "give" in the floor?

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knucklehead

No.

I mean yes.

neolitic

I know that before we put bridging
in at my son's house a little Jack Russell
running across the floor sounded like
freaking a Gene Crupa solo, and would
bounce the needle on the turntable.

What ever the tests say, I was
in a basement once to nail off the tails
of the western bridging, when they rolled
a cart of drywall through the room.
I could see the gaps open and close
as the joist twisted a bit at the bottom.

Lone, I guess you and I will
just keep on bridgin' along!

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