Wood Truss Repair-Wood Gusset Plate Design 1

[RickinPA]

I have a truss problem and was hoping that someone could point me in the right direction to come up with a solution.

I am helping some friends get there house ready to sell and went in the attic and saw that someone had chopped a couple of trusses apart to install a set of pull down stairs. It is a simple gable roof using fink trusses. They cut out the bottom cord and 1 web member from each truss. I am a civil engineer but my specialty was geotechnical engineering and I haven't resolved truss load conditions in 35 years so I am woefully unqualified to come up with a fix.

I have tried to find the truss manufacturer to no avial. The house is 22 y/o, the builder is long gone, and the truss stamps are illegible as to who built them even if they are still in existence. I have contacted many people but keep getting told that they can't do anything to help.

I have a good friend who is a structural engineer and he designed the fix for the bottom cord using very conservative wood properties, 1/2" diameter bolts, etc. So no problem there.

The web member that was removed is creating the most difficulty. It seems that the solution would be to put back the missing web member and then tie the joints together with a gusset plate. I was thinking a plywood gusset plate for simplicity and wouldn't really care how big it was. However, I can't seem to find a direction to go to get someone to help.

Does anyone here have any ideas on how to proceed?

Thanks for any help you can provide.

 

[BAretired]

You need to find a structural engineer who is willing to seal the drawing of remedial work. Nobody wants to buy a house with questionable repairs to the roof.

BA

 

[Ron]

Sounds like the pull-down stair was installed perpendicular to the trusses....they are designed to fit between two trusses that are spaced at 24"o.c.

If so, take down the stair, restore the trusses to their original design and reinstall the stair in the proper orientation.

Have structural engineer do the repair design as BAretired noted.

 

[woodman88]

First check if a non-engineer can do residential building design in your state. If so, Have your structural friend calculate the web force. Add replacement web and plywood to one face. Using NDS nail values, nail the web, top chord and bottom chord for this force. Use no less than 2.5" o.c. for the nail spacing. You should get all the calculations for the repair for your records. If not, see if your friend will seal and sign the repair.

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.

 

[BAretired]

Only one face? I would prefer to see plywood gussets on both faces with nails in double shear. A little glue between gussets and members wouldn't hurt either.

BA

 

[MiketheEngineer]

I have had some luck "beaming" across the bad truss bottom chord and transferring those loads to the good exisiting trusses and then fixing everything else. I am assuming they only cut one truss. Won't always work - depends on how stressed everybody is.

I generally prefer to use screws in existing trusses. Nailing is often difficult - lack of room, etc. And the vibrations it generates can start cracking drywall - esp the bottom chord.

Testing has shown that adding glue doesn't really help much - seems that the glue "cracks" or fails before the fasteners fail - or at least that was what I was told. That said - I still like a good dose of Liquid Nails. That stuff is amazing.

 

[hawkaz]

Mike,
How have you justified the existing press plates on the adjacent trusses? From calcs I have seen, they design these right down to the wire.

Recently, I had a 4' deep truss spanning 17' (depth needed for architectural purposes). The owner wanted to put a new mechanical unit on this roof. The truss members worked easily for the new loads, but the truss calcs indicated that the press plates were designed to .99 unity.

 

[MiketheEngineer]

Hawkaz

You do have to check everything!! Sometimes - you get lucky - many times not so much

 

[RickinPA]

All:

Thanks so much for the input. I spent many hours this morning tracking down a structural engineer who was well versed in residential wood framing AND was interested in doing the work. A very difficult combination! Anyhow, I did find someone. He discussed several possible solutions but until he came and viewed the situation and went back an made calculations he couldn't come up with a definite solution.

The one interesting thing I have had to do was to explain to the homeowners why they have a problem. This condition has existed for 10 years, and last year we had a series of snow storms that left about 48-inches of snow on the ground and roofs. No problem at all with the roof. They look at me with a very odd expression as I try to tell them it really is a problem.

Not really knowing how truss capacities are determined, I assume that they are modeled as simply supported structures. If this is so that doesn't take into account all of the redundancies built into homes by default, such as interior walls that, while not designed as such, still act as load bearing walls, drywall, roof sheathing, etc. Am I correct in these assumptions? I know that modeling this is difficult at best but the reality is that all of these components contribute in some fashion.

Anyhow, I will post an update on the solution once it is known, which looks like it will be in a couple weeks.

And again, thank you for all of the input. It really pushed me to searching hard to find the right person to do the work.

 

[BAretired]

You are correct about redundancies acting in ways which were not originally contemplated. If the truss is supported in the span by a wall or partition, the statics are changed in a way which is sometimes difficult to assess. Roof deck can sometimes act like a thin shell and carry load for years even if the trusses are damaged.

However, a Fink truss with one diagonal missing is an unstable structure and needs to be addressed.

BA

 

[XLjunkie]

The fix for this deficiency is relatively straight forward. The bottom chord fix is the most critical and should be corrected first. Cutting the bottom chord turns the entire truss frame into what i call "fire wood". No structural strength exits in the truss frame if the tension chord is cut.

The nice thing as you mentioned is for the last decade they didn't have any problems. Its cause the system is stronger than the individual components. Sheathing, factor of safety in the adjacent trusses, and other redundancies (partitions are a huge redundancy, as someone mentioned above) all kick in to keep the roof up in view of the damaged component.

When replacing missing webs, tell the contractor to ensure a tight fit between the new member and the existing members. A max. gap of 1/8" is recommended between new and existing members.
Plywood is the method of choice for this reinforcement. Preferably both sides. If using a gusset on one side, i recommend driving the nails through the lumber face and clinching them on the plywood face.

If you find a truss engineer, they should be able to detail this for you easily.

 

[woodman88]

When doing the “beaming” type of fit (I require all the webs to be intact), I use add-ons (they should extend to a bearing or pass a truss panel point) on the existing adjacent trusses. By attaching the “beam” to these add-ons the tension force is assumed to be transfered just by the add-ons and not the adjacent trusses chords. So the only additional force that is transferred to the adjacent trusses is the vertical reaction from the “beam” which the add-on transfers to the truss panel points/bearings. If this vertical load increases the panel point vertical load 5% or less I will not reinforce the adjacent trusses for this load.

When clinching of nails at the plywood or OSB face I use a decease value (usually 66% for the clinched side) for the connection. This is due to the damage I have seen done to the plywood and OSB when the nails are clinched.

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.