Wood Truss Repair - Plywood Gussets and Wood Adhesive 2

[marinaman]

I have a project where I have existing wood trusses that need repair. Part of the repair includes adding new web members to the existing trusses. The two new web members are both in compression. I have checked the other existing web members and their connections, with the new truss configuration, and they are acceptable.

The question is one of how to fasten the ends of the new web members. I am going to use plywood gusset plates on each end of each new web member, both sides of the truss. The existing chords are 2x6 and the new and existing webs are 2x4. I have 1,750 lbs of compression in the new webs. The new webs are to be built tight to the intersection of webs and chord at the top chord and tight to the bottom chord. Given that, developing 1,750lbs with nails and plywood gussets will take several 10d nails per web/plywood interface. Over the thumb, if a 10d nail is good for 100lbs shear, I'd be looking at 18 nails (9 on each side) just to fasten one member to the gusset plates. I dont want to put so many nails thru the web that the member itself is destroyed.

I'm trying to think thru a better gusset/web/chord connection. What about only (4) nails per side, but, add heavy duty wood adhsesive to each plywood/web/chord interface. Wood adhesive shear strength is >450psi. If I lap my plywood over my web by at least a foot, I'd have 3.5" x 12" = 42 square inches of adhesive per side. Say the GC does just an average job of getting adhesive on the gussets....so say I only have half the area covered in adhesive, that would give me (2) x (21 square inches) = 42 square inches x 450 psi = well beyond my 1,750 lbs I need.

Do you guys use a combination of adhesive and nails when repairing trusses with plywood gusset plates?

 

[hawkaz]

I would definately use the adhesive, but I wouldn't count on it for design purposes.

9 nails each side isn't bad- I would just do that. Make the gusset big enough that you get good nail spacing.

 

[msquared48]

If you use nails long enough to clindh the ends, If I remember correctly, I believe you get 20% or so more load capacity. Have to check the code though.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[MiketheEngineer]

Adhesive is good - but I have seen studies where it does not appear to make sense. BUT I would use it anyway. Also, I like all purpose screws (think drywall) only because in existing trusses - they will be less likely to crack drywall. When you you whale on nails - bad things can happen. Also I suggest structural grade plywood - probably 5/8 or 3/4 for your app.

We use to reduce the load by 50% for compression if the members all "touch". Idea here is that the member really needs no connection except to keep it in place OR in case of a reverse force that might put it into some tension??

Have fun!!

 

[jeffhed]

I have done a few truss repairs in the past using plywood gussets and nails like msquared48 suggested. Use plywood gussets on each side of the truss and nails long enough to penetrate all the way through and clinch the end. This gets each nail to act in double shear instead of single shear, reducing the amount of nails you need. Use adhesive but do not count on it for your design calculations. Haven't heard of reducing the load by 50% for compression, but it sounds like it has some merit. However, be sure to check the truss uplift forces and make sure that uplift forces don't cause a load reversal in the new web members resulting in a tension force instead of compression.

 

[marinaman]

Let me ask you guys this....

In the old NDS, a 12d nail is 3 1/4" long. Shear value, single 1/2" member to a single 1 1/2" member, is 101 lbs.

If I'm reading this correctly, in 12.3.3, if my nail penetrates the 1/2" gusset on one side, penetrates, the 1 1/2" thick truss member, then penetrates, the 1/2" gusset on the other side of the truss, and, extends beyond the second 1/2" gusset by three nail diameters and is clinched, I get to multiply my capacity by 1.75.

In calculation form.... (101 lbs/nail for SYP) x (2 shear planes) x (1.75 due to clinching) = 354 lbs/nail

On an 1,800 lbs load, I'm looking at needing only (5) nails. That sounds better to me. Plus, I will require the adhesive.

Is this in alignment with how you guys interperate the load calc for this case?

 

[msquared48]

No. The way I read it is 1.75 times the 101 value to get a pseudo double-shear situation, for a value of 176#/nail.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[MiketheEngineer]

I believe M^2 has it about right - though I didn't run the numbers...but past experience would seem to bear that out.

I still like screws better unless the trusses are setting on the ground and no drywall to crack.

 

[woodman88]

Here is a link to a good basic artical on wood truss repairs.

http://www.structuremag.org/article.aspx?articleID=797

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[marinaman]

What confuses me about the NDS requirement is that it states, "....design values, Z, shall be 1.75 times the nominal design value Z.....for each shear plane"

When I read that, it sounded like, out of the NDS chart, for a connection where 1/2" member is fastened to another member, SYP, the capacity is 101lbs for a single shear plane....then double the load for two shear planes....then multiply by 1.75 because they're clinched.

NDS could do a better job of writing that out.

 

[woodman88]

The 1991 NDS allowed doubling of the single shear values for clinching of 12d or smaller nails.
The 1997 NDS allowed only a 1.75 increase of the single shear values for clinching of 12d or smaller nails.
The 2001 NDS (and after?) eliminated the increase of single shear values for clinching of 12d or smaller nails by requiring the use of the double shear equations. But still only allows clinching of 12d or smaller nails. The double shear equations (see Table 11.3.1A) double the side member values but keeps the main member values the same as the single shear equations. So that if the main member shear does not control the double shear value you can have a doubling of the side member values from the single shear to the double shear values.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[RFreund]

Can you use the Simpson steel plates for a retro fit like this?

Either:
Typical steel plate

http://www.strongtie.com/products/cfs/tp-tpa.asp

or
"Press-on Plate"

http://www.strongtie.com/products/ics/trussplates.html

EIT

http://www.HowToEngineer.com

 

[CANPRO]

Reading this thread makes me wonder why everyone is so quick to specify wood glue but then ignore its strength. This is the way I have always done it as well, but when you look up the values for some of this glue, it doesn't make sense to me to ignore. See the link to typical wood glue....

http://dashboard.lepageproducts.com//upload/english/general/4fde2f5c-3ae2-4db7-87ae-8dc7c711ceb4.pdf

Worst case is when two pieces of frozen wood are glued together. The minimum bond strength to pass this test is 150 lbs (over 1.5 sq.in), so you get 100psi. The product claims to hold 762 lbs or 508psi. Seems like you could assume the minimum passing requirements of 100psi for design and potentially have a factor of safety of 4 or more depending on conditions at time of application.

Using OP's original geometry and assumptions to get 42 sq.in you really only need the glue to hold 42psi, which is less than half of the mimimum passing strength published by the manufacturer (not to mention what they claim their product can actually hold).

So why worry about whether the nail can take 101 lbs or 176 lbs when really the glue is all you need? Personally I would still spec' glue and 8 or more nails because at the end of the day its not that much work, but I was just wondering why people are so quick to dismiss the strength of glue. I understand the worry about the contractor cleaning surfaces, properly applying glue, etc....but don't you have the same worry about nails? They could easily use an air-nailer with a narrow shank nail, drive it half way through your first plywood gusset and now you're left with much less than 100 lbs. Sorry about the rant, but the more I think about it the more I like the glue...

 

[RFreund]

I think the hesitation with glue is that it is hit or miss. You are putting a lot of faith in the contractor. If I was doing the repair myself then sure I'd rely on the glue. However if there is some sawdust on the plywood or the glue drys some before it is put in place or the contractor grabs whatever glue he has on the truck, etc it may not perform. I used to do a lot of demo and if glue was used in the original construction it was usually hit or mis, sometimes it came up no problem other times you had to get the chisel out. However pulling up flooring that someone went to town on with a nail gun (or staples) always came up the same way.

EIT

http://www.HowToEngineer.com

 

[TXStructural]

Having designed repairs for hundreds of these: plywood and bolts. Or find a contractor that can install new plates. We used these methods with great success. When we tried plywood, glue, and nails, the nail patterns were excessive and keeping the glue fresh until the plywood was in place and assuring that the glue didn't get smeared away from the area between the members bat difficult.

You should avoid imparting fixity in the members, which glue and large numbers of nails will do.

The glue you choose would need to be structurally-rated. The closest tube product we found was a 3M synthetic rubber adhesive with no creep.

Unless Simpson has changed, they do not want the plates used for structural repairs. You would need to use real truss plates.

 

[woodman88]

In regard to the use of structural glue in plywood/OSB wood truss repairs. When I have specify the use of structural glue in these repairs. I do it as an addition to the nails (the nails having been designed for the full transfer of the loads) to reduce the splitting of the 2x wood members. I have done this only when the repair required a tight nail spacing.
I do not like the use of bolts in plywood (or 2x and /or truss frame) repairs. The use of Simpson 1/4” SDS wood screws or similar would be my second option to repair with. But with the Simpson 1/4” SDS wood screws or similar or 1/4” or larger bolts you have to consider the group action factor in your design. I have always wonder why Simpson did not go with a 7/32” diameter to avoid this issue.
As most of the repairs I have been involved in has been a “I need the repair by tomorrow morning!” or even sooner. I usually specify the total number of nails from each gusset or add-on. Giving the nails room for a spacing of 3” in 2x4's, 2” in 2x6's and 1” in 2x8's or larger staggered from each gusset or add-on. So that if a 2x4 required 8 nails from each gusset I would size the gussets to cover 24” along the length of the member that is the full 3.5” width.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.