Metal Plate Truss Top Chord Notched/Cut

[dylansdad]

I was asked to perform a site visit as a contractor was renovating a hotel. The bathrooms were to receive new shower pans and upon removing flooring, etc., it was observed that some of the floor truss top chord have been compromised (in particular, one has been totally removed). I was asked if there were a generic fix for this situation as I am sure there are plenty that I did not observe. I have heard of plywood sheathing applied as a potential remedy, but I really have no idea how we can fit that into the space(s) as they exist at the present time. Also I am not sure how to "run the numbers" on this to justify a repair as noted. I would appreciate any thoughts.

 

[kostast88]

Is the metal web truss prefabricated? If so I would ring the supplier to ask him that, as it is made by them it would be their responsibility. If you do anything there you automatically take the responsibility.

If it's custom made and you feel brave you could add a metal plate, just like the ones it already has: The top chord is in compression. The bottom chord is in tension (simply supported assumption).

The bottom tension is resisted by the fasteners, and transfered through the plate to the next piece of wood. I expect top and bottom axial forces to be (sort of) similar magnitude. Therefore the identical detail you see on the bottom chord, could be replicated to the top chord. But this time for compression. You can run the numbers to confirm. Use nails/screws, avoid bolts.

 

[dylansdad]

Thanks for the response. Here is a photo that did not attach to my previous post of one that is completely cut through. Yes, these were prefabricated (building is 20 years old). Thanks again.

 

[phamENG]

dylansdad - be cautious going into this. If you don't feel comfortable, defer to another engineer. No shame in recognizing your limitations. But there is shame in ignoring them and getting it wrong. These things are tricky, and you need a good understanding of the design principles that go into them.

It's clearly prefabricated, and as it's a renovation I'll bet discussing it with the fabricator is probably off the table as they have no part in the project. However, you could suggest to them that, if the hotel was built in the last 20 to 30 years, they try to look up the fabricator and have them weigh in, for a fee, and provide details. If they can't be found, you could check with local truss shops and see if you can sub this to them with you as the EOR. They can run the numbers through their system at your direction, you can verify the results, and get it fixed. But you need to be sure you understand what's being done, of course.

I'd start by exposing the full length of the truss and doing an analysis - don't use a simple truss model, model it with the actual member splice locations. That will help you get on the road to a repair detail. Make sure you add a joint where this is broken - there is moment in the webs (supporting continuous loads between panel points) and you don't want to account for continuity that's no longer there.

 

[kostast88]

That 2nd photo got me...

I believe you already told the owner & cordoned off the area?

For the 2nd photo I would now be thinking a proper steel flitch plate.

+1 for everything phamENG said.

 

[KootK]

1) FWIW: I was a prefab truss designer long ago and far away. Back then, I never would have proposed what I'm about to. Empty brain and small testes back in 90's I'm afraid.

2) My proposal where the problem is not with consecutive trusses:

a) Shore the truss being repaired.

b) Cut out the affected top chord panels in their entirety. Maybe within 4" of the adjacent panel points or whatever.

c) Slip in a new section of top chord between cuts. It needs to be clean and snug. This is meant to deal with the axial load in the chords.

d) Sister a 2x strong orientation on either side of the spliced top chord segment and extend it one panel point beyond the panel points that bound the repair. Nail the sistered bits regularly, both to the top chord and any crossing webs. This is meant to deal with the transverse loads on the chords and to stabilize the chord splice dealing with axial.

e) Add robust, lateral blocking near the chord cuts to add a bit more lateral stability to the final assembly. Do this on both sides of the truss and tie it into the floor sheathing.

f) Add a bunch of stongbacks between the repaired truss and the neighboring ones.

g) Make it clear to the client that this is a strength only repair and you are not able to guarantee stiffness performance.

h) Get eyes on the final repair(s), either your own or those of someone you trust.

3) These setups have a pretty good level of system behavior that goes unaccounted for in 2D design. The likely worst case here, I feel, is that the floor sags a bit but does not fall down. This concept informs my proposed repair.

 

[dylansdad]

yes, we have locked down the whole floor from construction traffic (there are 5 floors) and are assessing all rooms at the present time. Seems the plumber went rogue over the weekend. Thanks for all responses as I attempt to evaluate this mess.

 

[dylansdad]

KootK, thanks for your reply and your time offering a repair proposal. The problem is, is this hole/notch cannot be moved/covered/replaced (your 1)c) above). It is for a shower drain pan.

 

[phamENG]

It is for a shower drain pan.

Sounds like they need to redesign the drain pan.

 

[KootK]

Yeah, that narrows your options. Remove this truss, sister LVL to the adjacent trusses, and frame the other way with 2x6 in between? Not an easy thing to do with other MEP in there already.

 

[XR250]

Are there any interior walls you can use to cut the span?

 

[RontheRedneck]

I'm kind of late to the game. But wanted to share some thoughts in case they help.

We deal with this all the time. There are no easy solutions. And certainly no "generic" repairs.

Digging way back in my memory, there's a formula you might find helpful.

X = the bending moment in foot pounds where the chord is cut.

Y = The distance between the centerlines of the top and bottom chords in inches.

X/Y = The approximate force in the chords at that point. That can give you an indication of what you're up against.


Possible solutions -

As XR250 noted, sometimes we can pick up a bearing somewhere to shorten the span.

Sometimes we set an LVL alongside the truss and push it up against the subfloor, so it effectively replaces the truss. If the LVL is a bit shorter in height it's easier to slide in.

If the cut is near the end of the truss where forces are low, it might be possible to do a scab on one side. Or 3/4" plywood.

I sometimes dream of going in with a sawzall and cutting the PVC pipes out, then repairing the truss. Let the freaking plumber figure out what to do then. But my employer has never let me do it.