Ripping TJIs to Stiffen a Floor?

[StructDave]

An attic has been built with finished rooms on a ceiling of 2x4s spanning 17ft. As expected, the ceiling sagged. The owner is into a fully gutted renovation of the storey below the attic (not modifying the attic itself). The attic kneewalls are built tight to the roof rafters.

So I'm looking at how to stiffen the attic floor from below. We're not trying to remove all the sag, as lifting the attic floor would force the rafters up. So 1” to 2" total sag will remain.

Options under consideration:
A - We considered sawn lumber sisters for the 2x4 ceiling joists. They could bear on new internal framed walls, so clear span about 16'-4". Since the attic is only 10ft wide (loaded width between the kneewalls), I would try 2x10 sisters. It seems heavy-handed.
B - considered perpendicular beams at the third-points, under all 2x4s. This seemed to be the least effort, except the ceiling would have to be furred out flush to underside of the beams.
A1 - considered using TJIs for the sisters, to gain stiffness and save weight. Calculations give 9 1/2" deep TJI. To sister the 2x4 joists, the TJI has to go up beside the 2x4, which leaves each one with eccentricity. And, how to fasten the top chord of a TJI (essentially, plywood) to a 2x4?
A2 - Can we rip a 16" TJI, down the middle of the web? Then would use each half to sister a 2x4. I’m attaching a sketch. Fastening the OSB web directly to the face of a 2x4 with screws seems straightforward, and we can include glue at the joint. We would have to make sure there's enough overlap at the ends of span for shear transfer, and block the TJIs from twisting (either for this option or for A1).

I'm liking option A2 but can't find where anyone else has tried it. And I don't expect the supplier to be responsible for the TJI after we modify it!

Has anyone tried something like this?

 

[RPMG]

I vote dimensional lumber and nail gun.

And against using the existing 2x4 for compressive strength.

 

[KootK]

I vote dimensional lumber sistering too. I'd only go for the ripped TJI thing if you did the gluing that you mentioned, and that seems like a lot of work.

TJI has to go up beside the 2x4, which leaves each one with eccentricity. And, how to fasten the top chord of a TJI (essentially, plywood) to a 2x4?

I'd probably fasten with something like Simpson SDS screws. The eccentricity could be addressed with a couple of bridging/blocking lines.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[NorthCivil]

Really don't like that ripped TJI detail. Seems like a lot of work. You get a strange, eccentric profile from it prone to torsion.

Another vote for sistering dimensional lumber, and if the client is really complaining about lack of head height and doesn't mind the bill, you might be able to shave 2" from the dimensional lumber depth by using LVL or LSL members to sister.

 

[JoelTXCive]

Fine Homebuilding ran an article on this showing various options for stiffening.

Your floor is sagging, but some of the methods might still apply.

See attached.......

 

[XR250]

I think A2 is very clever and would likely be very effective. Trying to sister to sagged joists is a pain, but curving the web of a TJI would be pretty easy and would give you alot of purchase for nails. I normally do option "B" in these situations. Maybe some W5's would do the trick.
I might run the TJI idea by the framer next time this comes up.

 

[MikeHalloran]

Given the client's wish to not increase the depth much, I'd look at sistering with steel.
Maybe C4's on each side of each joist, or maybe a channel formed in a fab shop, that can punch all the nail or bolt holes you want before bending the flanges up.



Mike Halloran
Pembroke Pines, FL, USA

 

[Ron]

Agree with MikeHalloran. You can sister with wood, but using steel gives bigger bang for the buck. Have the shop pre-punch holes in the channel or you could flitch both sides with plate.

 

[mjr6550]

Over 16' span is pushing it for 2x10's. You only have 10' of floor load, but the knee walls can transfer roof loads down to the joists, unless the roof is stiffer than the floor. You will also get some additional deflection over time added to what you have. I would second the option of using LVLs for sistering. You may be able to use 7-1/4", which may help headroom. Depending upon how many you need the extra cost may not be a big factor, especially since labor will be a big part of the cost.

I would not touch the ripped TJI options. I don't like to rely on gluing in the field, especially with typical residential remodeling contractors. Too much potential for problems, and more labor. Also, 3/8" OSB does not have much capacity for nails or screws.

I have looked at using steel joists in the past, but sourcing can be a problem.

 

[hokie66]

Steel would be my choice, but if I am going to do it, I would want to take all the sag out before putting the load on the new joists.

 

[NorthCivil]

mjr - the existing 2x4 floor joists have been holding everything up for decades.

At my firm I'm the "alteration guy". I find you can almost never get an existing structure up to modern codes - and there are provisions in the code that address this and give you a bit of leniency. Sometimes you can't hit the lenient benchmarks either.

This leaves you with 2 options. Offer the robust solution, and the GC decides to not touch it and leave it as "existing". Or offer a compromise solution, and the structure is left in a significantly better state than when it was opened.

I still haven't fully digested this internal battle over codes, guidelines, adequate structure and ethics... Some guys prefer to go "by code or I don't approve!" Which sometimes will result in the GC yeilding to your desires, and sometimes will have the GC leaving structures in the terrible existing shapes they are in. What is in the best interest of the public?

 

[EngineeringEric]

Why not perpendicular LVL above the 2x4 @ 1/4 points and use hurricane ties to support the 2x4s?

 

[StructDave]

Thank you all for very helpful observations and comments!
Concerns about gluing, the thin web, etc are well-founded. Use of steel was considered (we can get anything fabricated, but first would try a standard CFC or Z-Girt).
Our code allows "existing construction" to remain, with load management/reduction, which we can use if necessary for this small area.
At this point I am putting forth a solution with TJIs in every second joist space, using blocking to support the sagged rafters (and reinforced webs, where blocking is connected). We get half as many TJIs to fit in, but likely one size deeper which enables blocking to be connected, and no need to directly connect top chords of TJIs to the 2x4s.