Box Truss Floor Joist - Ok to trim? 1

[jdubs88]

I am working with a builder who is removing an exterior wall for an addition and installing a LVL over the opening. There are box truss floor joists running perpendicular and need to tie into the LVL. In order to do that, the box trusses will need to be trimmed back about 3.5" to make room for the new beam. I have not had a situation like this and have two questions:
1) Is it appropriate to trim back the truss, cut the diagonal brace so it still lands in the new corner, replace the (2) 2x4 vertical members, and attach plywood on both sides of the truss to act as a new gusset (see attached sketch)?

2) If #1 is proper procedure, we would install a L4x4x5/16 angle near the bottom of the LVL to act as a ledger for the box trusses. Is there any danger in installing this ledger that close to the bottom of the LVL (see sketch)?

 

[jayrod12]

I'm not a huge fan of modifying those trusses, but you're in a bit of a tough spot otherwise so I can see that working. The issue is appropriately connecting the plywood to the 2x4 members. Since the 2x4 members are on flat, there's not a ton of space to get nails in.

I'd be trying to talk them into a header of sorts. You could bury part of the beam into the truss, but I wouldn't be screwing with that top plate. I'd look at cutting the vertical 2-2x4 just below the upper plate and having the beam start there. Then it would still be top chord bearing and you wouldn't need any special connection between the two.

If you do end up going the route of cutting the entire truss, I would not be using the angle to connect them. I'd be using a proper face mount hanger. Simpson stringtie has many options for this depending on the load level

edit: If headroom clearance is a real issue, see if the layout can tolerate a post down somewhere to shrink the beam up. People always want it fully open, until they see the size of the beam required. Then I can usually sell them on a post in a non-descript location to shrink the beam up. Perhaps if they've got an island right there you could have them work a post into the edge/corner.

 

[KootK]

1) This kind of thing can be done if all of the design and construction details are tended to very carefully. Your proposal actually requires the last diagonal to be cut and rotated does it not? That's the part that bothers me the most. In rotating the diagonal, you may compromise the joint where that diagonal meets the bottom chord.

2) In practical terms, you'll not want the angle to tear off the bottom of the LVL in some combination of tension perpendicular to grain and cross grain flexure. It's a little better with LVL than with lumber because of the less grainy nature of the material. In theory, you can design for this if your fasteners are correctly designed. Still, I'd much rather see you use face mount hangers that would drag the load up higher into the body of the LVL beam.

You've done a good job of identifying some of the critical issues here.

 

[KootK]

If you go the big gusset route, I see abandoning the last diagonal and working with a model like the one shown below. You'll quickly get a sense for if the number of fasteners required in each web is within the realm of possibility. And, even if it does work out, it'll be a ton of effort for the contractor. If you can make jayrod's pseudo top chord bearing idea work, that's surely the way to go.

 

[jdubs88]

@KootK: I was thinking the diagonal would have to be cut and the miter angles on the end adjusted for the shift to stay flat with the top and bottom chord, but without having to rotate the 2x4. Do you expect the rotation to be required for it to fit?

I agree with you and jayrod12, hangers seem like a much better option than the ledger.

@jayrod12: you are probably right about talking them into adding a column, but they already had the beams onsite before opening up the ceiling and discovering the box trusses so their options are a bit limited. I could have them push the beam out so the trusses don't have to be cut, but then the load from the walls and roof above would be applied to the face of the LVL instead of the top.

 

[KootK]

Do you expect the rotation to be required for it to fit?

Yes, I expect that you'd have to rotate that last diagonal some if I understand the situation correctly.

 

[jayrod12]

I'm for pushing the beam outwards. Full depth hangers like that will stabilize it enough. Fasten the top of the beam back into the top of the trusses if desired to prevent LTB failure.

 

[KootK]

If I were to do the gusset thing, I'd take it back an extra web from what I've shown for good measure.

How is it that you're able to shift the beam outwards on an exterior wall without mucking up the facade? Canopy?

If the beam goes outward, keep in mind that the load path for the stuff above would then pass through the hangers en route to the LVL.

 

[jdubs88]

They are adding a big open room onto the back of the house where the existing exterior wall is being removed so the new opening will now be "interior" and be connecting the existing living space to the new open room
The load path change is what is giving us pause about pushing the beam outward.

 

[KootK]

You can get a pretty skookum face-mount hanger over that kind of depth.

How were you originally going to support the upper wall while the truss was cut back to make room for the LVL? Shoring up multiple stories through the trusses a panel point to two back of the wall?

 

[jdubs88]

Yes, they would have to build a second floor shoring wall above the one currently built in the first floor which presents even more of a headache. From a construction standpoint, it seems like pushing the beam outward and using some heavy duty hangers would allow them to install the new beam alongside the existing exterior wall (prior to removing it) and attach the box trusses much easier than attempting to cut back every single truss (it is a 24' opening so there would be a lot of trusses to cut back). As long as the new load path doesn't reduce their clear span capacity they should be ok.

 

[jdubs88]

I was able to talk them into jayrod12's idea of leaving the top chord untouched and resting it on the LVL (see sketch). Drops the LVL lower but they have tall ceilings. Should be much easier on the contractor as no plywood gussets or hangers will have to be used.
Really appreciate all the input and advice on this. Thank you both. Let me know if this sketch is not what you were describing.

 

[KootK]

Nice. Any suggestion that gets implemented as a real life solution gets a little purple star in my book.

Because your beam is long, deep, and slender, I would suggest one minor improvement as shown below to preclude the, admittedly remote, possibility of tension side lateral torsional buckling.

And thank you for reporting back on the final solution. It add a lot of value to a thread to have that kind of closure.

 

[dhengr]

Jdubs88:
Aren’t those trusses ‘2x4 parallel chord floor trusses,’ not “box trusses,” which connote a box beam with plywood side sheets or web elements? That certainly seems like a much better solution to the problem than your first shot was, except…
1.) You do not want to disturb that top chord bearing joint at all, for fear of reducing the intended design strength of the toothed connector pls. Twisting those pls. or memebers around at all well loosen them, and that wouldn’t be good. Thus, stop the new vert. 2x4 chord member short of that main diag. chord member, so you never apply a load perpendicular to its wide face. Square cut its upper end at the top of the LVL’s. and screw it to the face of the VLV.
2.) Take special care and instruction to the builder that the horiz. square cut of the two vert. end bearing members of the old truss must be cut square, true and clean for good bearing and loading into the joint and toothed pls. just above. That’s not some chop job, and must be done below the existing toothed pls. wherever their bot. elev. is. Those connector pls. will probably be quite large, so check their bot. edge location.
3.) Take special care in edge nailing to the members of the existing truss. They are really prone to splitting if you don’t space your nails/screws sufficiently. More, smaller nails/screws is generally better in those kinds of details, than fewer large nails/screws. Truss people generally don’t like you edge nailing to their chord members due to the splitting problem. Same goes for any nailing into the remaining pieces of the two vert. end bearing members of the old truss. They will be prone to splitting.
4.) Koot’s tension strap on the bot. chord of the truss is really a good idea. Maybe something like Simpson’s DTT1Z, or equal. Maybe you could find a 3” wide “L” shaped strap which would fit on the top face of the bot. chord.

 

[IRstuff]

Seems to me that the two short pieces with the required square cuts might be more easily implemented by ripping a 4x6 down in width, to ensure squareness and not have to worry about getting the two pieces to match in length.

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[jayrod12]

That was exactly what I had envisioned, and I do really recommend including something like Koot's sketch to prevent the beam from rolling out. That's a big beam and we want to make sure it stays where intended.

I think that's going to make the contractor's life significantly easier.

 

[KootK]

Seems to me that the two short pieces with the required square cuts might be more easily implemented by ripping a 4x6 down in width, to ensure squareness and not have to worry about getting the two pieces to match in length.

A 4x6 would be great but those two pieces are actually existing members cut back.

 

[KootK]

@OP: now that I look a little closer/longer, I see that end vertical that you intend to add and attach to the LVL. With the right connections, that could address the same roll out issue that I was worried about. That may well have been your intent all along. I still like my way better but, then, I'm seriously biased. Tomayto, tomahto.

 

[jdubs88]

@KootK: Yes, that was part of the plan for the end vertical but some more legit tension straps seem like a great thing to throw in for good measure.

@dhengr: Those are good things to be mindful of, thanks for the tips. I will be sure to talk to the contractor about his cuts and staying clear of those connector plates. I don't think we will have to do any edge nailing now that the we moved past the plywood gusset idea, which is another benefit of avoiding that mess.

 

[IRstuff]

It's typically pretty hard to get square cuts, in situ; the guys working on my house used a sawzall.

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