Wood Truss Plates ??? 1

[sgc]

Greetings,
I am kinda new at this site so forgive me if I have this entered more than once.
I want to beef up the load handling ability of some existing 2" x 4" syp trusses which are constructed with small metal plates. I would like to cut larger (much larger than the metal plates) pieces of plywood and glue and attach these new oversized wood truss plates over the existing metal plates.
Is liquid nails the best glue to use?
Can I use a staple gun or is nails the best choice? What length?
Will 1/4" plywood be the best thickness plywood to use?
I considered getting larger metal plates with predrilled holes but the cost gets way high for say 12" or larger plates, then I would have to hand nail these plates to boot.
Any advise welcomed.
Thanks,
SGC

 

[jheidt2543]

The load capacity of a truss is not just dependant on the gusset plates that connect the members. It depends more on the applied loads, the geometry of the truss and the size of the truss members. There is a fair amount of analysis that must be done, just to figure out the existing capacity of the truss, then you must decide how the truss capacity can be increased, if at all.

Your best course of action is to go to a strucural engineer and explain what you want to TRY to do and have him check to see if it is possible.

 

[Watermelon]

I'm with jheidt on this one: I wouldn't try to design electrical equipment, you might want to consider staying out of the structural arena.

In looking for a structural engineer, I'd recommend you call a truss manufacturer and see if his structural engineer might be able/willing to help.

 

[DaveVikingPE]

"Is liquid nails the best glue to use?"

Depends on the purpose. It's great stuff, but not for truss plates. Notice, for example, that Liquid Nails does not recommend its for parquet floor tile.

"Can I use a staple gun or is nails the best choice? What length?"

AHHHHHHHHHHHHHHHHHHHHHHHHHH!

sgc, truss construction is a specialized trade, so much so that many of the truss manufacturers' plates and construction methods/machine processes are patented. There are, of course, design procedures backed up with good structural engineering theory.

My free advice: You will save money and agony, mind you, by purchasing additional trusses and installing them in-between the existing ones if you're trying to strengthen a floor or roof - if you augment an existing truss you will void any warranty, etc. that came with it.

 

[sgc]

Thank you for your input on truss support. So good to have access to expertise!
My situation is that I have a house that is occupied and need to add on a room which means I am going to have to have to tie onto the existing roof. Simply put I have a 24' wide house that I want to be a 36' wide house. I have drawn out the change and discussed this with an architect. HIs advice was that I go in and use plywood to reinforce the trusses. I don't think I can install more trusses and still occupy the building. But I can get to the trusses in the attic. I know truss design is complicated but isn't there some seat of the pants carpenter strategies for increased support that I can use. The local archtect I consulted said the change I wanted to make was not going to increase the real load on the roof and only for extra insurance I could go in and further support the rafters.
Any Thoughts fellas?
Thanks,
SGC

 

[Trussdoc]

SGC, I have made a career of fixing carpenters' "seat of the pants" work on trusses.

Truss manufacturers are in a very competitive business, and they don't put any more wood into a truss than they absolutely have to to remain within specs, or at least within code. Changing the parameters, such as span, changes everything.

As jheidt and watermelon said above, you need a structural engineer with truss experience to design the changes. You have multiple unknowns in this equation...where is the roofline of the addition tying in? are you creating a hi-lo condition that will trap snow? do you have adequate headers supporting the wall where it will be penetrated for access to the new addition?or is the wall being removed?

 

[boo1]

Have you considered handframing and ignoring the trusses?

 

[sgc]

boo1, I wish I knew in my mind how I could handframe and still keep the current roof intact enough to keep rain out. I can't work quickly anymore so what I do may take a while to accomplish.

Thanks, SGC

 

[boo1]

The new plywood I-joists are very light and are easy to handle. Just an idea!

 

[sgc]

Greetings again,
The question was "Why am I wanting to alter the original trusses?"
I have added a room the lenght of my house. I want to add a center section to the roof to give it a barn style look (not a gambrel style). This would mean coming off the existing peak of my house and using the now same height rafter on the addition as the level platform to put a 4' high wall on these two peaks then a rafter on top of that to form a 12' wide and 4' high center section on the house getting the barn look I want.
I have calculated the total weight of the project to be 5,000 pounds. Half that load will transfered down a block wall so about 2,500 pounds will be placed down the peak of the existing house.
Now I said all that to get back to my original question. I currently live in this house and the attic has a/c duct work etc. in it. So sliding 2" x 10" I beams or the like in there is out of the question. There should be a way to superimpose more strengthening trusses on top of the existing metal truss plates, or more support members, or plywood sheathing sections could be added to help strengthen the existing rafters.
I have had a local architect look at the design and he has reassured me that what I want to do will work fine (not in writing). I have gone to the two truss plants here in town and neither one of them will gaurantee me that it will work and have given me no solution as to beefing up existing trusses.
There is no doubt in my mind that I can go in that attic, and add by glueing and nailing strategic support members, plywood truss plates, to help support the additional load.
My initial idea was to cut plywood truss plates, glue and nail them in the proper places.
I searched the net and found an enginerring study that was stating glued and nailed wood truss plates deflected less over a five and ten year study than did metal or nailed only wood truss plates.
So it comes down to this, I want to exert a total weight of 2,500 pounds spread down the peak of the roof of a 44' long, 24' wide building that has manufactured 2" x 4" trusses 5/12 pitch with no support bearing walls in the interior. I am located in south Georgia which only gets a whisper of snow every 5 or 10 years but we do get wind gusts from some bad rain storms.
I know there is a way to get in there and help support those existing rafters.
Is there anyone out there that can visualize this problem and advise me?
I don't think that I have to worry about the 2" x 4" bottom chord shearing at the walls. I would think it would be more of the whole triangle trying to spread at the bottom or sag at the middle. The only way that can happen is if the joints move, by further strengthening the joints I should be able to overcome the additional stresses.
I humbly, humbly ask for practical help, my health is failing and I have a house that is hacked up and I need to move on with this thing and my wife is set on this type of design. I don't want to leave her with a potential problem in later years. The weather is getting good enough for me to proceed, any suggestions will be GREATLY appreciated.
SGC

 

[sgc]

Greetings from SGC again,

I still have no takers on the above situation. For a truss to fail, or collapse the joints have to move or flex. What can be done to strengthen the truss joints on existing factory trusses assembled with mimimum metal plates?

Thanks again,

 

[AJGIII]

sgc:

You are not listening. There is NO SEAT OF THE PANTS MODIFICATION.

Your architect is giving you BAD ADVICE. If licensed, he/she should be disciplined by your state licensing board.

You need to listen to TRUSSDOC and the TWO truss manufacters (Hmm, you'd think they might know something since they design and build trusses) and hire a qualifed, experienced engineer to design the modification. You are STUBBORNLY ignoring advice that could prevent serious injury to you and your family.

 

[sgc]

AJG III:
The urgency for safety and need for completion leads me again to approach the local truss plant people and ask if they can advise me. The first time I asked them, their answer was it falls outside of the design parameters of their software or the build capability of their truss jig (one or the other).
Maybe they will contact another affliate if they can't get me a certified design.
I know there is a workable solution that can be installed while still living in the building.
Thanks again for the advise.

 

[boo1]

Contact a local PE in your area. As AJGIII indicate there is no easy solution. The 2500 pound dead load is only part of your total load. The live load from you design wind speed wind (Chatham County's )to ~100 mph plus 120 mph three-second (depending on area). The dead load and live load are combined. Load combinations for allowable stress design are specified in '97 UBC 1612.3.1. They are:

D
D + L + (Lr or S)
D + (W or E/1.4)
.9D ± E / 1.4
D + .75 [L + (Lr or S) + (W or E/1.4)]

 

[RDK]

Cannot you simply buy new trusses with a 36’ span?

This would involve removing the existing roof system with the enclosed HVAC and replacing the mechanical and electrical.

While it sounds like a lot to do, it would be easier than having some half-baked fix that would not last.

Get a local structural PE to look at what you are doing. That’s the best advice we can give you here.



Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[johnson]

Hello,
is there any method ( i mean any easy manual method or thumb rule)to calculate the forces in the simple truss members ( truss sloping in one direction only).
Thank you
Johnson

 

[DTGT2002]

SGC,

Movement or separation of the joints is certainly ont the only way a truss can fail. If the truss is overloaded, individual members of the truss can start to bow, and deflect and fail, certainly moving joints as the truss collaspes. In typical buildings, with stick framing by hand, there is typically a lot of redundancy, studs at 16" O.C. when 19.67" would work, sheathing and diagonal bracing when only one is required, just as a standard practice. BUT trusses, espically those used in residential construction lack this redundancy. When one plate goes, there goes the truss. When one member overloads, there goes the truss. I do not question the ability of the truss to carry the original loading, but introducing additional loading could be catastrophic.

But let's get back to ideas for shoring up your trusses. Now to back the architect,( I don't consider the Architect to be the Structural Engineer's enemy, most of the time) what exactly did he/she recommend? Was it, increase the plate size, or was it add plywood? I could see a capacity increase by fully sheathing each truss on each side and nailing to each member of the truss, effectively confining and bracing all members. But you would need the services of a PE licensed and insured to work with the Public, not so much me, an engineer working on his PE at a Formwork Firm. There is one direction you might head.

Another would be to cut down the span of your trusses. Are there walls that are non load bearing below the trusses? It would be possible to make these wall load bearing. It would be traumatic, as the sheetrock etc would have to be removed and the wall rebuilt, but cutting down the truss span could help. This stongly depnds on the layout and style of your trusses, a good place to have the assitance of your truss manufacturer.

I am sorry I cannot give you a solution. I could not even if you paid me. I am onbligated by my profession and my ethics, as are the other civil/structural engineers not to misuse my engineering knowledge. Sending you down the wrong path could kill you or your family.

Continue pursuing the truss companies, but look for a local Professional Engineer, and arrange to have a meeting ad his/her convienience to discuss fees, the scope you would require and whether or not the engineer feels comfortable dealing with your problem.

The money you spend will let you sleep safely at night, and not worry about the future of your house, unless the raining down there like it is here in Atlanta, whereas we both need to put our houses on stilts.


Daniel

 

[Miper]

sgc,

There may be a solution, but only if you can get to every joint and member of every existing truss. With a 5/12 roof, that may not be possible at the outside wall.

If you can get to them, then there is a conservative approach to strengthening your trusses that will work safely. It may be overkill, but it answers the objections that the pros are raising.

First, as mentioned above, adding gussets alone won't add much to the strength of the existing trusses, unless they were done all wrong to begin with -- not likely.

If you can slide sticks of lumber in and around the existing utilities in your attic, then you can add doublers to the existing truss members. So if the existing trusses are made of 2 x 4 members, then additional 2 x 4 members can be added to each truss member to effectively double the truss strength. Use your plywood gussets on these doublers, preferably on both sides.

Attachment of the doublers and gussets should be by something more substantial than ordinary Liquid Nails. I haven't found that stuff to be strong enough to stop a stair squeak, let alone hold up a roof. There are lots of better construction adhesives. Make sure you use a waterproof and freezeproof one. Some mechanical fasteners (nails, screws, staples - whatever) should also be used.

This approach will double the strength of your existing framework. I have no idea whether this will be strong enough for your house addition, especially if you really have no load bearing internal walls. But this approach at least enables you to realistically strengthen your existing trusses.

If there is to be no load bearing wall where the original trusses were originally supported, then you still need a licensed structural engineer to say how much additional strength is required. The additional 12 ft of width will put considerable additional bending loads into those old trusses.

 

[sgc]

Greetings Miper,
I would say half of this building has a suspended ceiling so I could get to the bottom members quite easily there and the other half I can probably get to with a little patience.
Miper I handed the drawing to a P E, and at first glance he said that added weight was not the problem but it would be the moments due to wind that might be of concern.
On a sidebar, I perceive some real resentment from earlier responces about using staples to attach wood gusset plates. But I recently had to remove a few pieces of OSB and the ones attached with nails came off easily but the ones that were stapled I had to really struggle with and ruin the OSB to get them off. Now what I want to know is, what is wrong with using staples?
I haven't heard back from the engineer yet so any input is appreciated, and I certainly thank you for your responce.

 

[Miper]

As you have seen first hand, there is obviously nothing wrong with using appropriately sized staples. Besides,if you are planning to glue the gussets, then it will be the glue that holds everything together. If it's a good glue, the staples are only there to clamp it all together until the glue dries. Being a belt-and-suspenders type, I would also recommend using adhesive coated staples.

I have the impression that a number of the comments you received above weren't very helpful, which is why I entered the discussion. Maybe we can't solve your whole problem, but we could at least try to point a way through it.