roof construction

[oar]

Hi all,

I have to span a square of about 50 by 50 feet with a roof. The roof should have a cupola in the middle.

I am wondering if I can simplify construction of the roof and reduce cost by putting up four columns arranged in a much smaller square in the middle of the larger square. These columns would form the base of the cupola and be tied together on the top by a sturdy wooden square frame.

What I am hoping to do is to eliminate large and complex trusses by having rafters running from the exterior wall to the columns and tying into the frame on the columns directly (somewhat like in a shed roof). The cupola will then be erected above the square frame on top of the columns. The rafters would have to be about 25 feet long and span empty space.

If simple rafters cannot span this, would it at least result in less complex and smaller trusses that are easier to transport and handle. Can I do this and achieve significant cost reduction in the construction of the roof.?

If this particular construction does not work can columns be used to simplify the construction of the roof in some other way?

What do you think?

[longshot]

Truss manufacturers will figure this out for free if you are going to buy trusses from them.

[RicodJour]

Well, you've left out a whole lot of information, like what general shape you want the roof to be, how big the cupola is, where you're located (snow loads, hurricane, seismic?), what the structure is going to be used for (affects code requirements), etc.

Generally speaking 50' trusses are going to be a problem to transport and you'll certainly need a crane. The four columns in the middle would result in shorter, lighter, less deep, more transportable and easier to handle trusses. Cheaper to buy and most likely cheaper to install. It's hard to say without more information, but most likely the four columns and their required footings wouldn't entirely offset the cost savings of the shorter trusses, so you'd probably come out ahead in the construction cost.

You might want to take some sketches to a local truss supplier and have them use their software to work up a couple of variations so you can get a more precise idea of what you're dealing with price wise. That's usually a free service, so there's no reason to delay contacting them.

After that, it's time to talk to a local engineer, architect and/or contractor to determine the rest of the design and get some budgeting numbers.

Many neophytes assume that all columns are bad, but they can actually help organize a plan. I'd suspect that might be the case in your situation, but without knowing the building's purpose it's not possible to say.

R

[Bob Morrison]

Your "design" does not appear to have any provisions for dealing with lateral forces. I think you should hire a local engineer to investigate this. The structure you propose is way too complicated to discuss in a newsgroup.

[Robert Allison]

I believe that the OP is asking for a design. That too cannot be done in a newsgroup.

To the OP: Try the least expensive route first; Call a truss company and talk to the truss designer. Discuss what you want and ask what can be designed for your project. He can give you some details without actually designing the project. If you can't live with what he can do for you, then you should contact a structural engineer to design it for you.

The truss manufacturer will not charge anything to tell you what he can do. He will not charge you for the design of the truss system if you buy the trusses from him. The engineer will cost you money right from the start.

On the other hand, the truss manufacturer will assume that the building that you set his trusses on is designed to withstand the forces that will be placed upon it. That includes not just the weight of the trusses, but lateral forces, wind uplift, etc. If the building is not engineered for that, you may have to contact an engineer anyway, to make sure that it will all work together.

[oar]

Thanks for all responses.

The roof would be a pyramid with a roof pitch of about 4 in 12 along the shortest rafter (parallel to the exterior walls) and less elsewhere. This is for a house to be built in Florida ("Cracker style") so no snow but hurricanes.

I am in the early stages of design. The columns would be masonry 2 by 2 feet fully rebared and grouted and anchored to the foundation slab with rebar. The columns would be about 16 feet tall and form a square of 11 by 11 feet. If these are connected rigidly at the top they should be resistant to lateral forces.

I agree that this has to be worked out by an engineer though. I am just soliciting opinions to see if such a design makes sense from a cost perspective. I want to have some columns in any case but having them exactly in the middle makes the interior design somewhat harder.

Thanks again.

Mike Meyer

[marson]

[RicodJour]

That's a big if, particularly for site built block columns. R/C concrete or steel would be my first choice.

If you're thinking that you're going to have a 50' x 50' open space with only four columns and no other walls to stiffen the structure to resist the lateral loads that Bob "If I Design It, It Will Stand" Morrison referred to, you might have a hard time...errrr....a too easy of a time getting it off of the ground!

First step. Truss manufacturer in the area where the house will be. Next step, if you can't accept the truss manufacturer's "can't be done", is an optimistic engineer.

Report back here when you get the results from the truss manufacturer. Thanks.

R

[oar]

I forgot to say that there will be load bearing perimeter walls around the 50 by 50 square (it's a house), so the space is not open. Nothing in between would stiffen the structure though.

I'll report back if I get the opinion of an engineer but it will take quite a while.

[oar]

OK, I get what you are saying: the perimeter walls have to be stiffened against lateral forces.

I did not think about that and actually planned on having more than half the space without walls of any sort. Thanks for pointing out that this is not possible. If I were to stiffen a 10 foot high perimeter wall with a perpendicular load bearing wall, how long would that wall have to be about: 5 feet, 6 feet, 7 feet?

How many such walls would I need along the 50 foot length: More than one?

Obviuosly nothing exact can be said without having all the details but if it's pretty clear that I need two such walls I might have to rethink the design in general.

Thanks.

[RicodJour]

Don't bet on that. If you have way more money than you know what to do with, you might be able to make it work, but the walls and roof structure would have to be so heavily built that it would be ridiculous.

Post this in alt.architecture. Put "Ping Don" as the subject line. Don is a designer who worked in FL for 20 years and will know exactly what you are up against with code and hurricanes. He also used to work for a truss company. Tell him he still owes me twenty bucks.

OK.

R

[RicodJour]

Ping Don

Thanks

R

[oar]

Thanks, this is all very valuable information.

[Bobk207]

......as you said

..."Obviuosly nothing exact can be said without having all the details "........

the details & requirements are coming out slowly.....I thought this was a giant outdoor / patio / bbq area at first...like in a public park....not a house

If you're really serious out this potential project I would suggest getting (paying) a home designer to bounce some ideas off of (realtime) so you can arrive at "doable" design starting point.....details & changes / tradeoffs can be made from there.

otherwise I can see this getting a little frustrating for the guys in this group

......"If I were to stiffen a 10 foot high perimeter

depends on the number of stiffner walls you have & what they're made out of....brick, concrete, timber?

If the perimeter walls were r/c & thck you won't need to stiffen them....

cheers Bob

[oar]

The perimeter walls would be autoclaved aerated concrete block mortared and possibly surface bonded in addition. Interior load bearing walls would be standard concrete block surface bonded only (no mortar) but rebared as needed and fully grouted (for thermal mass).

The need to stiffen these walls does put a crimp in my plans since I was hoping to generate a very large open space. I might have to rethink this but once the structure is properly stiffened the idea of using columns to simplify roof construction and installation.

I'll draw up several designs and then pay an architect to talk over how realistic these are.

Thanks for all the advice, it is certainly extremely helpful.

[Kevin Thibeault]