TJI - ForteWeb - Lateral Bracing 1

[BSVBD]

ForteWeb TJI Software by Weyerhauser

For decades, I understood that TJI's, in general, do not require Bottom Edge Lateral Bracing throughout the span(s), even at mid-span.

The Weyerhauser website states... Trus Joist® TJI® joists are made with no observable twist and have minimal material variation between joists within any joist series/depth. Consequently, they do not see any significant benefit from bridging. Our code report (ICC ESR-1153) specifically states that bridging is not required for floor and roof TJI joist applications. Weyerhaeuser has found that the addition of bridging does little to improve floor performance and may create squeaks. It is the policy of Weyerhaeuser to recommend against installation of bridging unless an unusual condition exists that would be improved by this addition.

I understand web stiffeners & blocking at bearing locations, intermediate bearings and cantilevers, etc.

HOWEVER... The ForteWeb Software "Report" says, Bottom Edge Lateral Bracing Intervals of 4'-8" over a 16'-0" span!

I DID contact Weyerhauser ForteWeb Support, but, that takes time... "Submit a question" (online)... "Our support team will call you back" (phone)...

AND... I also like to (additional) get Eng-Tip opinion(s).

SO... Despite "generic" program reccommendations / specifications, what do YOU do?

Do you bridge, brace or block at mid-span anyway? (increse cost?)

Comments? Reccommendations? Thank you!

 

[YoungGunner]

I've normally seen the program not require any bottom bracing. Even now I just opened up to see what joist I last modeled and it's required bracing spacing is the full length of the joist - in other words, not bracing. Wondering if you have the settings correct on the "Spans and Supports" tab to set the lateral bracing to full length, or if you have any special circumstances like additional loads, a cantilever?

 

[jerseyshore]

I'm with YG, my guess is you have the lateral bracing table messed up or have a cantilever.

 

[jayrod12]

Or is there uplift on your joists from anywhere?

 

[XR250]

Maybe if it is 2-span continuous then the bottom flange goes into compression (similar to a cantilever)

 

[met33]

Any updates on this lateral bracing issue?

I think that XR250 is correct. For continuous spans, ForteWeb calls for additional bottom edge bracing. I assume because, under unbalanced loading conditions (e.g., one span heavily loaded, and the adjacent span lightly loaded), the bottom flange of the lightly loaded span can go into compression, and therefore could need additional bracing to prevent lateral-torsional buckling.

There is also bottom edge compression at intermediate supports in continuous spans even under uniform loading. Boise Cascade says that "In joist applications, the required connection of the bottom flange to the bearing support is typically considered adequate bracing in these areas." (tech note PDF at link: Link)

As mentioned, this can also happen in cantilevers (bottom edge compression that requires additional bracing), as Weyerhaeuser discusses here:

https://www.weyerhaeuser.com/blog/cantilever-length-restrictions-in-forte/.

That article points out that simple spans have the compression edge at the top, so the floor sheathing provides the necessary lateral bracing.

Below are some ForteWeb results for simple and continuous spans. See the "Lateral Bracing" for the bottom edge, circled in red. For two continuous spans of 15', the bottom required edge lateral bracing is 6'10" o.c.

It's strange that Weyerhaeuser would say that bridging is unnecessary unless "an unusual condition exists". Continuous spans seems like a common condition that, apparently, requires additional bottom edge bracing? Unless they are using the terms "bridging" and "bracing" to mean different things in this context?

 

[Bob_D]

Stumbled across this thread as I had the same question.
Research of Weyerhaeuser site yielded the answer:
Apparently "bracing" is temporary, required during construction (prior to sheathing).

 

[RontheRedneck]

I've heard some discussion that bracing the bottom flange helps control vibration. But I can't point you to specific reference that covers the subject.

 

[lexpatrie]

I'll supplement at the risk of repeating myself, residential code the TJI is supposed to be covered on the bottom in the newer codes (gypsum board, it's a fire thing) so you'd likely have effective continuous bottom side restraint anyway, and TJI use a smaller C[sub]r[/sub], repetitive member factor as well, so there are some trade offs.

Maybe someone else can provide the code reference. I think it's in floor framing.

 

[ChorasDen]

The bracing interval in ForteWEB is based on a back calculation from chapter 3 of NDS, which displays the required bracing interval to achieve sufficient capacity to resist buckling forces induced by the design compression moment in the member, whether that be a positive or negative moment, both are checked. I-joists require bracing of the compression element when the local moment is sufficiently high to induce elastic buckling and/or lateral torsional effects, and ForteWEB's report is intended to convey the required bracing interval to achieve appropriate bracing based upon the design moment.

lex, not necessarily true, as not everywhere in the U.S. has basements. Out west, we have a very low frost line (~18"), so basements are uncommon, post and beam and crawl space is much more common, and thus, not typically drywalled when you have a 24" deep crawl space.

Repetitive member increases are not allowed on i-joists, check chapter 7 of the NDS.

residential code the TJI is supposed to be covered on the bottom in the newer codes (gypsum board, it's a fire thing)

I do not have any knowledge of this provision, I would be curious to see it, regionally, I know of some local requirements that occur in certain jurisdictions (Colorado and New Jersey come to mind), but nothing that I have seen in the typical i-codes. Not to say it isn't there, just that I am not familiar.

 

[Eng16080]

...the TJI is supposed to be covered on the bottom in the newer codes (gypsum board, it's a fire thing)... Maybe someone else can provide the code reference. I think it's in floor framing.

I do not have any knowledge of this provision, I would be curious to see it, regionally, I know of some local requirements that occur in certain jurisdictions (Colorado and New Jersey come to mind), but nothing that I have seen in the typical i-codes. Not to say it isn't there, just that I am not familiar.

Refer to Section R302.13 of the IRC code. This code provision is something that I've found most architects/engineers/building inspectors don't know about. The I-joists don't necessarily have to be covered in drywall. They just have to be fire protected in an approved manner. Drywall is probably the simplest way.

Most people aren't aware of this provision because it's not that obvious in the code, and I-joist manufacturers aren't overly excited to call attention to this downside of their product. With that said, the information can be found without too much trouble if you know to look for it, and I will say that I was made aware of this at an I-joist seminar put on by Weyerhaueser, so maybe I shouldn't be too harsh.

I went down a slight rabbit hole in the past concerning this. Not surprisingly, with the super thin plywood web of I-joists, unprotected I-joists perform terribly in a fire which originates below them. I came across a fire fighting blog/website where somebody referred to I-joists as "firefighter killers." Somewhere else I read that a town fire department (somewhere) won't enter certain newly constructed homes with I-joist construction at the 1st floor.

 

[lexpatrie]

Sorry, In my mind I said basements above. I'll not edit it now to change it.

I think I got diverted on C[sub]r[/sub] for I joists and Composite lumber (LVL, LSL, etc). If I recall correctly both had C[sub]r[/sub] at some point (or they listed C[sub]r[/sub] as 1.0 for "consistency") and the value for LVL is 1.04 (might have been toned down over the years?). Not fantastically helpful.

Eng16080, that sounds like fire-engineering.com, I've only dealt with I joists a few times (typically fires), and one design project before I became a P.E., they aren't that popular (to me) in the Mid-west.

Nice work with all the embedded images folks, I really appreciate that. Very helpful.

 

[XR250]

More reasons I hate these things - especially in crawlspaces. Fortunately, I usually can talk most folks into using dimensional lumber in those locations.