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]

What about 10D nails instead of 8d. What about pre-drilled pilot holes to reduce nail spacing and get a few more nails in. That's all I can come up with. It's not much, but maybe a marginal increase in capacity.

 

[Ron]

Use screws.

 

[jplay2519]

I don't like predrilled holes for nails. Maybe premarking them but wouldn't predrilling reduce capacity?
The first part of the post is about screws, so you don't even get close to the required forces.

 

[oldestguy]

Ring shank nails? I bet careful spacing can put many more in at closer spacing. PLUS GLUE!

 

[gte447f]

I believe prebored holes can be used if limited to 75% of the nail shank diameter. If you have the NDS commentary, I believe it may be in there somewhere (I do not have a current copy). Definitely premark the locations. I had a very similar project a while back. The field crew marked a template to locate all of the nails properly and accurately on the plywood gusset before installation.

 

[gte447f]

Section 11.1.5.3 of he 2001 NDS has the 75% diameter limit for prebored holes (for G<0.6, 90% for G>0.6). No reduction in lateral design values. The 2012 edition doesn't have that provision, but it may still be in the commentary (I don't have the commentary).

 

[gte447f]

In older versions of the NDS Commentary there is also a table (C12.4-1) that gives recommended values for nail spacing, end and edge distances for non-prebored and prebored holes (it is available on Google). The recommended spacing for wood side members for nails in a row (load parallel to grain) are 15D and 10D respectively for non-prebored and prebored holes. It is 10D and 5D for steel side plates.

 

[dhengr]

Jplay2519:
What would happen if you used 1/8” or 3/16” thk. stl. gusset pls., 11.8” high and 18” long on each side of the truss? These could even be cut to extend out onto your vert. end bearing blks. Generally speaking, you can accomplish more with smaller, closer spaced fasteners 8d or 10d (real nails) than with three lag screws or other screws. The larger fasteners tend to concentrate larger loads at a single location, maybe with some lumber defects, or an oversized bored hole and you pay a load/price for that. They tend to split lumber when installed in the narrow edge. I would have a left and right hand gusset pl. so I could stagger nails btwn. the sides, and I would predrill the gusset pls., in the shop, using a drilling template. Thus, the gussets would act as a nailing template out in the field. Not inexpensive, but it may help solve your loads and nailing problem.

 

[jplay2519]

Thanks for those. The newer codes do have the 75% diameter for the similar materials but the spacing has gone down to 4d max for geometry factor, 3D minimum. That's a huge drop, and since the wood is old I assume that I may have to go with the larger spacings to keep the wood from splitting.

 

[jplay2519]

dhengr: I was contemplating the steel plates if it can get me the capacity I neeed. Hate to get into steel with these because it's residential and the contractor is going to push back on it or try to go crazy on the price. I wouldn't have as much of an issue if they hadn't put that gypcrete to add all the dead load on these floors. sigh.

 

[gte447f]

jplay2519, I do not think geometry factor applies to nails less than 1/4" diameter, so the 4D and 3D spacing limits for geometry factor don't really apply either, and I think those spacings certainly might lead to splitting of the wood. The applicable spacing limits are just "spacing shall be sufficient to prevent splitting of the wood" (NDS 2012 Section 11.1.6.1). The table (C12.4-1) I mentioned before from the 2001 NDS commentary (I think) has recommended values to avoid splitting of the wood in lieu of specific code requirements. It also says, "designers should note that specific type, moisture content, and grain orientation will affect spacing between fasteners in a row".

 

[KootK]

There's an alternate analysis method available in the unlikely event that you don't have mechanical runs in the way. Use an 8' sheet of plywood for your reinforcement and turn the last 1/3 of your truss into a box beam. This should lower the forces needing to be transferred out of your diagonals substantially and allow for a more distributed, comfortable fastener layout. There will be some complications at the last vertical web but I'll not get into that unless this is a solution with legs.

Hitting those 1.5" webs up in a dark plenum isn't an easy ask. I'd be hesitant to ask for anything too fancy in that regard. Also, if at all possible, I'd favor a solution that doesn't mangle the existing truss plates. Odds are those would work for the added load regardless of what your numbers say. And they'll absolutely be your best friend with regard to system stiffness. Save them if at all possible.

 

[Agent666]

Does the original member not have enough capacity, or is it simply the original connections that you are trying to address with this solution?

If you are trying to panelise the last truss panel, don't you need another vertical member at the left end of the ply to nail off to.

The loads noted on the drawing are they the original truss loads, or the shears you are calculating in the panel? In my mind if transferring a vertical shear across the panel, assuming no loads over the width of the panel, then why are the top and bottom loads so different, the shear flow top and bottom, and left and right should be equal?

 

[Agent666]

Remember your ply will also only carry the load added after its added (unless you jack all the load out of it). There are loads locked into the original diagonal and its connections. Don't know if this hinders or helps, but it's something that should be carefully considered as your composite section after the modification isn't carrying all the load.

 

[gte447f]

KootK, all good points that you make. One bit of feedback from a recent project regarding the difficulty of installation in a tight, dark, hard to reach area... it is a challenge for sure, but contractors on my project had success by pre-drilling lead holes in the plywood gusset and using a pneumatic palm nailer (instead of a regular nail gun or hammer). The palm nailer can fit into tight spaces and the pre-drilled lead holes acted as a guide to set the point of the nail in (blindly by feel if necessary).

 

[KootK]

If you are trying to panelise the last truss panel, don't you need another vertical member at the left end of the ply to nail off to.

I don't believe that you do. It would certainly be simpler and more intuitive to model and analyze that way, but I don't believe that it's necessary. Vectors and such.

then why are the top and bottom loads so different, the shear flow top and bottom, and left and right should be equal?

I assumed that the bottom chord force was a typo. Unless it's an unusually loaded truss, I'd expect that to be a zero force member axially.

KootK, all good points that you make.

Thanks, yours too. And I agree that the process you've described instills more confidence that I'd have in my typical, local, residential construction crew. I'm going to challenge you a little more on this though because, well, that's just what I do. The pic below shows the lowest profile hand nailer that Google was able to show me in 30 sec or less. Even at that, I contend that any nail going into a sheathed top chord is likely to be oriented at 30 degrees to the horizontal, at best. I think I'd prefer to see screws installed by hand, at least until some penetration is achieved.

 

[gte447f]

KootK, You are probably right about not being able to drive nails into the top chord horizontally with the palm nailer. If I ever face this situation again, hopefully I will remember this. Maybe a trial mockup out in the garage is called for... when I get the time

 

[jplay2519]

Agent666 The existing truss has the last vertical web and part of the bottom chord rotted out due to bad construction practices (they didn't put any flashing at the deck and some rot took place). The trusses are 24" on center, 20 psf dead load, 40 psf live load, bottom chord 5 psf dead load. I dropped the RISA file in.

Kootk I thought about a plywood beam but I didn't like the fact of not having the assembly per the APA PDS Supplement 2 Design and Fabrication of Glued Plywood-Lumber Beams 1992. Also we couldn't get that large panel in unfortunately.

gte47f thanks for the spacings, those make more sense for splitting spacing, they keep me from fitting what I need too through haha. I thought about clenching for double shear but not sure if I'm just doubling the load of the nail capacities, they don't have any capacity generation for that in the code.

 

[KootK]

The existing truss has the last vertical web...rotted out

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