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Old 09-15-2008, 10:56 AM   #1
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Proper size for TJI's


An architect is designing a new 4000 sq. foot house for us. We should the preliminary plans to a contractor we interviewed. The house will be 66' x 40'. The plans currently show 2 x 10" FJ's. The contract said that with 10" our floors will eventually sag. He recommended 12" TJI's. (Plans also car for 3 #5 rebar continuous and 3 steel beams.) Should we ask the architect ot change the FJ's?

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Old 09-15-2008, 11:04 AM   #2
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Proper size for TJI's


What is the span of the 2x10 floor joists, and what is the spacing between joists? Do your plans specify a lumber species and grade?

If they're not overspanned, sagging will not be an issue. That's BS.

There are benefits to I-joists of course, but there are downsides as well.

With some more information from you, I can give you better information.

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Old 09-15-2008, 02:33 PM   #3
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Proper size for TJI's


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Originally Posted by thekctermite View Post
What is the span of the 2x10 floor joists, and what is the spacing between joists? Do your plans specify a lumber species and grade?

If they're not overspanned, sagging will not be an issue. That's BS.

There are benefits to I-joists of course, but there are downsides as well.

With some more information from you, I can give you better information.
Please bear with me as I'm very new to looking at plans, etc. The FJ's are listed as being 16" OC. The lumber is not specified yet although we had thought that the engineered wood was best. (Is that what TJI's are? If so, what are the downsides?)

The plans say "Teco each FJ @ steel beam (typ)". There are 3 steel beams that run the 38' length - two of which cross 4" steel columns attached to concrete footings. I'm not certain how to determine the span, but if the FJ's are attached to the steel beams then I assume the span is 22' in one room. The other areas have shorter spans of 15'6", 12' & 14'10".

The plan also calls for 5/8" ab's at 36" oc. Don't know what that is.

I hope this makes sense!

TIA again!

The #5 rebar is placed at 24"oc.
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Old 09-15-2008, 03:15 PM   #4
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Proper size for TJI's


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Please bear with me as I'm very new to looking at plans, etc. The FJ's are listed as being 16" OC.
Pretty normal spacing
The lumber is not specified yet although we had thought that the engineered wood was best. (Is that what TJI's are? If so, what are the downsides?)
Engineered wood is a great option, but it isn't always cost effective, especially in short spans. It is also terrible in a fire when compared to thicker dimensional lumber like 2x10's...Lumber chars and stands for a while, whereas engineered lumber fails.
I'm not certain how to determine the span, but if the FJ's are attached to the steel beams then I assume the span is 22' in one room.
22' is an impossible span for 11-7/8" I-joists spaced 16"oc in most cases. You might want to look into this with your architect. Even if it barely makes it, it will feel very bouncy.
The other areas have shorter spans of 15'6", 12' & 14'10".
These spans could be done with 9-1/2" I-joists or 2x10's on the 12' and 14'-10" spans, and probably the 15'6" span. 15-1/2 feet is outer limits for 2x10's at 16"oc...It depends if it is living area or bedrooms, and the desired performance.
The plan also calls for 5/8" ab's at 36" oc. Don't know what that is.
That's anchor bolts. They anchor the walls to the foundation.
The #5 rebar is placed at 24"oc.
Unrelated to what we're talking about here.
Hope this helps.
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Old 09-15-2008, 05:33 PM   #5
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Proper size for TJI's


Are you going to install tile? if so, I'd go with engineered lumber (it's stable). In any case, ask the architect for his calcs or have him show you the tables he used.
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Old 09-15-2008, 06:33 PM   #6
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Proper size for TJI's


I'm a big fan of engineered wood, so don't let this sound like I'm not...I spent three years as a lumber salesman and did engineered lumber layouts and engineering.

Engineered lumber is no more "stable" than dimensional lumber, except it won't check, cup, or twist. On the same span, spacing, and load, a 9-1/2" I-joist will have slightly less deflection (bounce) than a nominal 10" dimensional member. Based on that, it would make it a better floor system for tile. But, don't ever assume that just because someone sells you an I-joist floor that it can't have incredible amounts of deflection, which would make it awful for tile. It is 100% reliant on the designer to make it a good floor.

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