MPC floor truss repair options

[shorton2]

Hi guys:

I'm a ME but don't normally do much structural stuff. I built my basement home with MPC floor trusses above the basement. I have discovered one of them has a break.

Trusses are top chord bearing on one end, 18' span, 20" deep, 16OC. Design was supposed to be for l/480, I dont' have the LL andDL load numbers handy. The break is almost dead center on the bottom chord. It was right at a wood knot, I expect/hope that was the culprit since this is a high stress location. The break point (crack) has separated by about 3/16". No other breaks are evident in adjoining trusses. This floor system is about to get a 5/8" double layer of drywall on it's bottom side (basement ceiling) and I had checked the truss design could accomodate that weight recently. I had asked them to be designed to commercial floor standards at the time.

One problem with the repair is there is no way to do the typical gluing of plywood to the sides of the truss to repair it. It has chases with every kind of mechanical sytem in it including plumbing, HVAC ducts (steel/solid), gas and electrical. Relocating those is not much of an option. And becasue of the truss strongbacks and the mechaincals, I really don't have room to attach anything structural to the sides of the broken truss. A 2x4 would be pushing it, unless it was attached laying flat, to the sides of the bottom chord.

It seems that a decent solution would be to apply a flat steel "strap" to the bottom of the broken member with screws. I could make this as long as necessary, up to the length of the member. Since that member should be in tension the shear strength of the fasteners would carry the load. Or some way to attach secondary bottom chord members to the sides of the original.

I may need to jack up that area of the floor system to relieve the stress while attaching such a reinforcement.

Can anyone offer any experience with this type of truss repair?

Thanks,
Scott

 

[msquared48]

If you know the manufacturer, it would be best to contact them for their input as to how to do the repair.

Making the repair any other way than what they recommend could void any warranty that there is on the truss.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MiketheEngineer]

Based on a typical total residential floor load of 55 psf - I get the axial load to be in the 1900-2000 lb range - or about 380 psi on the 2x4 - very low - so it must have been a pretty good sized knot. Most 2x4's used in floor trusses have a 1200-2000+ psi capacity. What grade is it?? A commercial load of say 100 psf Live Load would basically double those numbers.

A 14 or 16 guage piece of steel 3.5 or 4'' wide would be able to handle the tension load. Getting it to grab the existing 2x4 chord is another problem - mostly in terms of deflection/slippage.

I am going to assume that the truss has deflected lower than the others.. So you will need to shore it back in place. Be careful you don't start cracking drywall and other things above. Go slow - take a couple of days if need be.

Assuming you can make this strip as long as you need it and get all the screws or bolts into you want - it will still relax once you release the shoring. A lot of that relaxation will come from the screws/bolts slipping in their holes or the steel straigthening out. Try to minimize that by screwing directly through the steel - use a punch to start the hole.

BTW - you could put steels straps on both sides of the truss - if that is easier or better. Keep screw lengths short as possible and as far apart as possible so you don't chew up the wood.

I also assume you can't put any wood under the bottom chord - because of your new ceiling. You might be able to get something on top. A Structural grade 3/4'' piece of plywood should work. It could be wider than 3.5'' too. Again - you are going to have to install a bunch of screws - probably at least 20 or so. Don't have time to check sizes and values - you can do that easy enough.

Glue and screw is called for here. While I have been lead to believe that the glue will creep over time - this is based on older info. I think some of the new glues have some pretty impressive characteristics and would prevent any sliding or relaxation.

BTW - Check to make sure there are no bearing walls right above or heavy appliances, kitchens, baths, etc.

Good luck.

 

[shorton2]

Mike (M): Did contact the company. Trusses long out of warranty and they wont' do anything on out of warranty trusses. I thin they might give me somve "verbal" help though, just not in writing.

Mike (te). Thank you for these details. That's a big help.

The wood knot didn't look that big to me so that concerns me as to why this member snapped. This floor system has one open room above it, no walls or other dead load. Just furniture and people, nothing exotic up there, a couch, and a normal weight family . The truss that failed is almost in the center of the room above that truss system.

re current deflection, it wasn't sag that caught my eye, I just happened to notice the crack when I was mounting a small projector to the basement ceiling. It hasn't deflected much, it's in line with the others by eye looking across the bottoms of the trusses anyway.

I'm afraid I don't know the wood grade. It may be speced on the truss documentation.

I see your point regarding post deflection/slippage. Simpson makes some side-mount straps (CTS218) that list a worst case allowable loads of 2350 for a 2-sided installation. They also make a variety of straps that I could fasten to the bottom with load ratings up to almost 6000 (MSTC66).

The drywall is going to be installed with an isolation clip and a steel hat channel, so I have a little "breathing room" under the truss surface for repair materials. If I put anything on top it would have to be fairly short due to the truss members. Steel straps woudl probably be better than that. With my "breathing room" under the truss, it is possible I'd have enough room for a pair of 2x4's on their sides on each side screwed and glued. I'll take a look at that.

But if I understand your correctly, and based on the 18' 20" truss, a 3990+ lb rated strap (380psi x 2 x (1.5 x 3.5)) should be sufficient?

If I go with steel, I though if I got extra long straps (accomodating more fasteners) and used a countersunk square drive #10 floor screw instead of nails, that ought to hold the strap from post slippage better than just screws or nails. Or use Simpson structural screws, but they aren't countersunk heads.

Just need to figure out how to raise this one up a little to install. How many spans to raise along with it.

Thanks again very much for the input and help.

Scott

 

[msquared48]

Good to see that you have covered your bases. Good hunting.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[shorton2]

One other point, after thinkign about it, the nailer plates woudl probably make it difficult to get a board up against it tight enough to glue on the sides. Unless I route out a nailer plate thick recess for the nailer plates to sit into.

 

[MiketheEngineer]

The Simpson straps are great - but they can slip - just a bit is too much. But installing them will definitely secure the truss from any type of failure. Based on what you describe - a more typical load would be a 40+10+5 (LL+TCDL+BCDL) for a total of 55 psf or about 1950 lbs axial in your case. What part of the world are you in??

If you are not seeing any signs of sagging - just fix it best way you can and go on. Measure it with a string line. Again, a little bit maybe too much.

 

[shorton2]

Mike:

I'm in Tennessee.

Got home to check them. It is not sagging at all. If anything it's bowing up a little. One thing occured to me, we installed HVAC in the basement a year+ ago. Things started drying out in the wintertime as we pulled the moisture from the basement. Caulking cracking from trim contracting, etc. It was a dry basement in the first place, but the HVAC brought the relative humidity down to around 20% in the winter. Maybe that casued some shrinkage in that member that caused the break. Anyway, it is not sagging at all. Guess that's good.

I've attached a link to the truss document for the particular truss that failed. I put a mark on it where the break is, and indicated where the main HVAC duct is above it.

Also linked a couple of photos. One from the top where you can see the small knot, one from the side. In the side shot that yellow string is sagging a little but when pulled tight on each end of the truss, it's still a little below the truss bottom. As far as I remember these trusses were not prestressed/curved in any fashion.

Curious that its not sagging. But good. I did mean for them to make the floor stiff when I designed the house. You can see the design loads in that truss document. If I'm reading it right, that failed section (E-G) was designed for 2782 lbs of tension.

Looks like I'll be fine just reinforcing it with the steel band. No need to jeackit up, but I'm surprised that memeber apparently shrank that much. I think

http://tn1.companypostoffice.com/scott/TrussF4.pdf

http://tn1.companypostoffice.com/scott/DSC04391.JPG

http://tn1.companypostoffice.com/scott/DSC04393.JPG

 

[Ron]

Sounds like you might have had an improper classification of the lumber. You can pursue legally, or just accept the issue and move on.

 

[shorton2]

Too much trouble, too much time passed for that. Just need to fix the issue. If all of them were failing maybe, but one of hundreds, if I can fix it I'm OK with that.

 

[MiketheEngineer]

It was designed for a total of 75 psf - 50 LL and 25 DL - kind of unusual - but not unheard of.

Many floor trusses are built with a bit of camber - usually equal to dead load or slightly more.

Pic 4393 is most interresting - looks like "bad" lumber that should have been culled - at least for a bottom chord. What is the "junk" right above and to right of the break?? Looks like shattered or split wood??

 

[shorton2]

Mike, I don't see what you are referring to in the photo. The only thing above it is the HVAC duct. The crack is pretty clean looking. No stretched fibers, just a clean break. Like something brittle was broken in half. Jagged but popped in 2. At least around the perimiter you can't see any sign of tearing, fibers. I don't see any junk, not sure what you mean.

photo 4391 is taken from the side A, looking down at the top of the bottom chord. Photo 4393 is taken from the side B, looking horizontal, a pointing a little up. That help?

 

[shorton2]

Truss company came to similar soultion I/you did. Recommended using a CTS218 on each side and a third on the bottom, but with Simpson screws. I think I'll tweak that recommendation a little and use a longer flat strap (MSTC40) on the bottom. The combined ratings of the 3 straps (after reducing them 60% from Simpson table values) will exceed the member design substantially. Best to be conservative. The longer, flat strap on the bottom will get the bottom plate screws away from the side plate screws so they aren't all right there together.

At this point it's still puzzling what casued the failure. Lower humidity as a contributing factor seems plausible, and of concern if so since the whole structure has been subjected to same.

But hopefully just a bad spot in the wood and bad luck as to it's location in the high tension member.

 

[hokie66]

Mike,
I had the same reaction as you, but if you look closer, the "junk" is just a reflection off the aluminum foil.

 

[Ron]

The crack is a direct tension failure.

 

[shorton2]

That's what it looked like to me. Still think it's due to a faulty piece of wood?

 

[MiketheEngineer]

Looks like it to me