Truss Plywood Gusset Repair 1

[jplay2519]

I'm trying to provide a connection detail to replace the end web member and I'm using a plywood gusset(probably 23/32" thick) and the forces are too great to get in enough fasteners based on their minimum spacings (Simpson SDS, SDWH, SDWS etc) you need like 3" min end distance then 6" spacing, you can hardly get 2 fasteners into the web and at 300 lb or so you don't even get close to the required forces. With 8d nails it's only like 91 lbs each so you need more than 10 nails and at 2" on center staggered (installed from each face) you couldn't get more than 8 nails in. I always try to design the amount of attachments per member but based on the configuration its pretty much not possible. Any way to analyze this? It's a standard way of reinforcing the trusses, just get less room when its a floor.

 

[gte447f]

How are you proposing to deal with that? Blocking on the outside of the gussets?

That is what I did in a similar scenario. New end verticals nailed to the outside of the plywood gusset. May have to notch out for the ribbon boards.

 

[jplay2519]

I'm replacing some of the webs members and part of the bottom chord, that's why I need this plywood gusset. Need to connect everything to transfer the load back through. I am planning on telling them to jack up the truss 3/8"-1/2" to release the load and then install the repairs. I think it's better to release then load and replace than try to just shore it and cut out members and replace like that.

 

[jplay2519]

Thoughts on putting steel straps over plywood over the chord?

 

[Agent666]

Steel straps on the face of the plywood doesn't really address the plywood to wood connection capacity as far as I'm concerned.

Another idea might be to look at a small welded truss out of lightweight angles or SHS, recreate the last timber bays geometry and lap as far as you need to onto the chords beyond the first panel to transfer the chord and web loads to the new frames. Place a frame on either side for symmetry. With some cunning detailing you might be able to detail it so it all bolts together to ease getting it into place.

 

[charliealphabravo]

Thoughts on putting steel straps over plywood over the chord?

As the thickness of the side plate increases, the failure mechanism tends more toward a hybrid shear and pullout mode. For thick wood-to-wood connections, there will be one or two plastic hinges near the boundary between the main and side members (Mode III or IV). A metal plate added to the outside of the plywood side member will not participate much in the controlling failure mechanism.

 

[jplay2519]

Thanks. The strap seemed like a reach but just brainstorming. As far as the steel frames I couldnt do that bc my members are flat so we are installing into the narrow face

 

[gte447f]

jplay2519, if you want to explore squeezing more capacity out of your original plywood gusset idea via squeezing more and more nails into it, consider 2 rows of nails in each member. You and others may think that is crazy, but per that same Table C12.4-1 from NDS 2001 Commentary, the recommended spacing (gage) between rows of nails is 5D and 3D for non-prebored holes and prebored holes respectively when the rows are not staggered from one another. For staggered rows, the spacing goes down to 2.5D for both non-prebored and prebored holes. Granted, 2 rows of nails in a 1.5 inch wide member may seem a little crowded, but per the recommended edge distances and row spacings from that Table C12.4-1 it might be an option. Obviously a template for accurate nail placement would be necessary.

Worst case (i.e. 10d nails in non-prebored holes and non-staggered rows) would be:

edge distance + gage + edge distance = 2.5D + 5D + 2.5D = 10D = 1.48" (for 10d nail) < 1.5"

If you used 8d nails, pre-bored holes, and staggered rows, you would have more extra space to play with.

 

[KootK]

What length of the truss end are you able to sheath in this situation? I still feel as though some version of box beaming the thing will be the end game here, be it with plywood, LVL, or steel. I don't even feel that there should be such a thing as a 12" floor truss really. And I used to build the things. You get bad angles and plates working really hard.

 

[charliealphabravo]

Although not conventional for residential, a metal side plate (18ga) gives the most bang for the buck. It basically forces failure Mode I and increases fastener capacity by over 5x that of wood-to-wood assuming some other mechanism doesn't become primary.

 

[jplay2519]

gte447f I looked at that and thought it would be crowded and noone would think it was a good idea. I that may be what is done to provide more nails. Would try to use 8d though so that I could fit more in. If we're installing from both sides that spacing between fasteners would be on one side or would you need to have that same spacing from either side? (meaning if your spacing is 2" on center can you put one on the other side between or do you have to do 4" on center and then put one in between to be 2" from both sides). I'm thinking the latter but want to squeeze in as much as I can and want to take advantage of that.

KootK the issue is the removal of the drywall below, I could probably get away with 24" max before they start to ask why we're going so deep. I know I can always pull the "bc I said so" but I got an engineer living in the space below and he won't accept a grandstand like that.I do like using an lvl or something to get more thickness and strength out of each connector, just not sure if a short piece like 18" would be enough.

charliealphabravo The steel plate would give much more capacity and may be the option I go with if I don't want to have the nails in so tight.

 

[KootK]

won't accept a grandstand like that.

Grandstand? Hogwash. Rip off the ceiling and fix it simply and convincingly. All the goofing around at the truss end isn't cheap either and you're time isn't -- or at least shouldn't be -- fee.

 

[gte447f]

jplay2519, if you use 8d nails, I wouldn't worry about staggering the nails on opposite sides of the truss because at 2.5" long, you can drive 8d nails from opposite sides of the repair and not worry about them overlapping in the middle, since the truss and 2 pieces of 3/4" will be 5" thick.

Also, with prebored holes you could go down to 10 times diameter spacing or about 1-3/8" for 8d nails.