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roof construction oar 12-05-2006
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Posted by oar on December 5, 2006, 3:43 pm
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?


Posted by longshot on December 5, 2006, 4:17 pm

>
> 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?
>

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




Posted by RicodJour on December 5, 2006, 4:22 pm
oar wrote:
>
> 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?

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


Posted by Bob Morrison on December 5, 2006, 5:33 pm
In a previous post oar wrote...
> 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?


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.


--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com

Posted by Robert Allison on December 5, 2006, 6:01 pm
Bob Morrison wrote:
> In a previous post oar wrote...
>
>>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?
>
>
>
> 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.
>
>

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.

--
Robert Allison        
Rimshot, Inc.
Georgetown, TX

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