Truss design for moderate attic storage

[Special55]

OK not really trusses but I figured the title would get me the proper experts. I believe I read on one of the posts that someone here was a truss design engineer.

Anyway, I've been hanging around for some time getting great ideas and decided it was time to join. Hopefully I can add as well as take away.

I have a 26 x 28 garage. The roof is conventionaly built with 2x6 rafters and 2x6 ties 12OC (not 16OC) that span the 26FT length (Front to Back). The flats of the roof are front and back with the gable ends on the sides. It has standard asphalt shingle with a 5 x 12 pitch. The builder threw up some 1x just to tie things together a bit. I would like to use the middle 8FT section for storing car parts (not engine blocks or anything that heavy) and seldom used equipement. The remainder of the outer edges (front and back) can be filled with supports.

Putting a support post in the middle is not an option. I don't even know if this is doable but I figured someone here might have an opinion.

Any help is greatly appreciated,
Rich

 

[twostory]

You are looking for "Attic storage" trusses. I had some made to span my 26 foot wide garage. The pitch is 8/12. I have a 10 foot wide by 8 foot tall area in the truss. If only cost a $250 more than normal trusses, so it was not very expensive. My trusses can take 40 pounds/square foot, so I can store a lot of weight in my 10x30 ft room.

 

[Special55]

Thanx for the reply TwoStory.

Unfortunately cost prohibitted me from using trusses at the time of construction. The garage is already built with the materials listed in my original post.

I was just hoping there would be a way for me to add bracing to the rafters/cross ties that are already there to support some attic storage.

Rich

 

[StingRay]

I'm not a truss designer but I am an engineering tech. Your 2 x 6 collar ties must be spliced somewhere in that long a length. A collar tie or truss bottom chord is really usually only desinged as a tension member. They have allowed about 7 lb/sqft for cieling and insulation. This is however a distributed load and point loading is not recomended. The middle of the span is the worst place to concentrate any kind of load. A 2 x 6 is in itself not much on a 26 or 28 ft span. The truely weak link will be the splice. The fact that they are in tension because of the rafters pushing the walls out will improve to a small degree the collar ties ability to support a load but once again the weak link will be the splice. Think of the 2 x 6 collar ties like a rope. They could actually be replaced by a rope and the building would stand fine with the exception for wind loading conditions. If a heavy load is put on the collar tie it will want to deflect down and if pushed fail. It will sag the cieling, pull the tops of the wall inward and push the peak of the rafters upward. To carry any real usable load a support would be required underneath or joists would need to be sistered to the collar ties or the collar ties encorporated into a floor truss of sorts. You really should have an engineer assess your exact conditions and then make recomendations.

 

[Willy Victor]

You are looking for "Attic storage" trusses. I had some made to span my 26 foot wide garage. The pitch is 8/12. I have a 10 foot wide by 8 foot tall area in the truss. If only cost a $250 more than normal trusses, so it was not very expensive. My trusses can take 40 pounds/square foot, so I can store a lot of weight in my 10x30 ft room.

Twostory my building is 30X36 and I went with the attic trusses. I get usuable storage space of 14X36 with a 6/12 pitch. It's the only way to go.

Willy

 

[Special55]

Nobody has any additional opinions on this?

Given the previous truss threads I've read I find that surprising!

 

[astroracer]

Put up some OSB sheeting for a floor. Screw it down and I doubt you will ever have a problem. 12" O.C. is going to be pretty stout and anything you can carry up there isn't going to overload it. Just don't pile a ton of magazines or heavy stuff in one spot. Spread it out a bit. I plan to do the same thing with my addition and I will even build some shelves between the rafters for small items...
Mark

 

[StingRay]

A 12" engineered I joist spaced at 12" oc is good for a 19' 6" span @40 lb per sf live loading. Best I recall the deflcection was at least l360 and probably better. A regular 2 x 6 for 26 ft or greater is essentially good for nothing (extra loading). A 2 x 12 in dimensional lumber is rated for about 15 ft maximum span. You have half of that and nearly twice the span. It shouldn't be unreasonable to see that sheetrock and insulation are all that's been allowed for. Think twice before you try storing anything up there. There are ways to add structure that will allow the use of the attic for storage they will all likely require that existing structure be incorporated into trusses or extra material be sistered up or both. Nothing will likely be free or easy. Really do talk to an engineer or truss designer. Your particular circumstances need to be investigated for lumber type, construction method, condition, etc. No professional will likely offer up a design without knowing all of the variables.

 

[Special55]

I'm not looking for free/easy. I have the building skills to add bracing as necessary. The only reason I didn't build the garage myself was that a young guy starting up his business did it for $1200 more than my cheapest lumber pack bid.

I just don't have the knowledge of what bracing should be added. I was hoping to get some input from some of the very knowledgable folks here on the board. I'd rather put $500 into lumber than having an engineer come out and tell me to tear the roof off and rebuild it with trusses.

I always thought trusses were just 2x with bracing in the correct places. I was just hoping to add the braces to my exsisting 2x.

 

[cw_racefan]

Not sure if this is of any help, but I just looked at the design drawings for my trusses. They span 32' (32x36 garage). They are a 4/12 pitch, attic truss, which only gives me 3 1/2' of height in the storage area, but enough for what we needed. The specs are: 2x8 no. 1 bottom chord with a joint plate dead center, 2x6 top chords, and 2x4 webs. They are designed for a 20psf live load with the trusses @ 24" OC.

 

[StingRay]

Trusses are exactly that.....but truss design is a science and the size and position of the lumber are based on calcutations that take into account all live and dead load requirements, lumber size and grade, method of construction (type of connection at the truss nodes), safety factor, etc. Unless you have a professional that deals with this stuff give you a design that is specific to your load considerations (including snow and wind load), local/regional building code requirements and so on it's not just a matter of putting some extra bracing up there. As an engineering tech even knowing how to do the math I go to a civil engineer about stuff like this. There is more to be considered than some simple physics. When you induce loads not in the original design alot can change. When you are talking trusses things like assymetrical snow loads, wind loads and so on come into play. When you want to load the bottom chord or collar tie in anything but tension you are changing everything and all variables must be considered. Coming from an engineering background my best advice is that you find someone in this field to work with you. I can imagine alot of ways to do what you want that will probably work. I might even try some of them on my own building. I'd never give anybody else even a hint at what they might be for fear that if it ever failed I'd be held responsible.

 

[Special55]

Trusses are exactly that.....but truss design is a science and the size and position of the lumber are based on calcutations that take into account all live and dead load requirements, lumber size and grade, method of construction (type of connection at the truss nodes), safety factor, etc. Unless you have a professional that deals with this stuff give you a design that is specific to your load considerations (including snow and wind load), local/regional building code requirements and so on it's not just a matter of putting some extra bracing up there. As an engineering tech even knowing how to do the math I go to a civil engineer about stuff like this. There is more to be considered than some simple physics. When you induce loads not in the original design alot can change. When you are talking trusses things like assymetrical snow loads, wind loads and so on come into play. When you want to load the bottom chord or collar tie in anything but tension you are changing everything and all variables must be considered. Coming from an engineering background my best advice is that you find someone in this field to work with you. I can imagine alot of ways to do what you want that will probably work. I might even try some of them on my own building. I'd never give anybody else even a hint at what they might be for fear that if it ever failed I'd be held responsible.

I fully understand your reservations. That is why I was asking for peoples opinions not advice. Something like: If it were me I would .............

I was a Dealer Technician for 11 years and people asked me for help all the time. Sometimes you just can't help from a distance you have to look at something first hand. If that is the case then I respect it and thank you for your recomendation to hire an Engineer. If you are saying you have ideas but are afraid to share them due to possible litigation, where would I begin to look to find you?

Either way, thanx to everyone who has responded.

If there are additional opinions out there, feel free to share them.

 

[StingRay]

Like said before. Even understanding the math I still consult with a civil engineer that I know. I know that I don't know all the ins and outs that should really be considered in this kind of scenario so I ask somebody that does. I know enough to know that I don't know enough.

Hypothetically lets say you were to convert the collar tie into a floor truss of sorts making it like an engineered joist with similar specifications to a commercially available product. It might be strong enough to carry the load but will it laterally deflect and roll over at that span and fail without a particular type of bracing or a specific floor sheeting material installed via specific instructions. Real engineered floor systems consider that things such as having the bottom of the member sheetrocked are significant in such designs. I should think one would need to take more than just a look at it and any kind of real educated professional or contractor will need to get alot of information and crunch some numbers. If you want to throw a few hundred pounds in the middle well distributed your risks are low but the act of reinforcing improperly itself can add enough weight to the structure to cause a failure. If you want to store anything of substance in the middle an opinion is the last thing you should be looking for. Go to a pro in that field and get answers.