|
Posted by CAD Wizards on January 2, 2007, 6:09 pm
MichaelB wrote:
> La-bob@cox.net wrote:
> > I'm getting ready to have a carport added to our home. One end will be
> > perpendicular to the house and tied into the existing wall/roof
> > structure. The other to a new, standalone wall structure. I would
> > like to not have any posts on the entry side of the carport. Rough
> > dimensions are 23' deep, 32' wide. The roof (6/12 gable) will be built
> > with light weight engineered trusses (no attic space). My questions
> > center around sizing the car entry side beam (~29' unsupported span).
> > My intention is to use a 30' LVL beam. I called the parish (Louisiana)
> > inspection department last week, and was given design loads of 20 LL
> > and 10 DL (although most of the experts were out on vacation and was
> > asked to check again with them after the holidays). Using Non-Snow
> > tables in a manufacturer's beam specifiers guide, I could then find a
> > beam to match these requirements. This yielded a fairly hefty triple
> > ply beam, leading to me to investigate what could be done to reduce
> > this size or at least try to understand the real load better (which may
> > be a moot point when I talk again with the inspection department in the
> > next week or so):
> >
> > 1. If I just calculate a Dead Load from scratch (truss, felt, shingles,
> > soffit, fascia, etc.) for the roof structure, I get a number on the
> > order of 7 psf. Ok.
> > 2. I'm not sure what the basis should be for the Live Load. My initial
> > assumptions are that there is no snow/ice load, no access/storage to
> > the attic, that wind load is more directed at an upward lift of the
> > structure, possibly some load from workers on the roof, and possibly
> > some load from rain in a very heavy storm. I must be missing something
> > (or a calculation error). Even if I assume a 1/2" of rain water on the
> > roof, I get loads less than 5 psf. So, what am I missing and/or why
> > would the Live Load required be so large (in fact, it always seems to
> > be much larger than the Dead Load for the tables I see - even for
> > southern Louisiana)?
> >
> > Thanks.
>
> Without going into any of the above numbers, some of which strike me as
> 'light', you need to factor in 'safety' margin. This is to cover all
> the inadequacies of both material and workmanship that is typical in
> the field, and for the crazy things that a percentage of the public
> will do. IINM, a factor of 2-3 is typcal, but Bob will probably be
> around shortly to edify us.
I agree with MichaelB's response to your post. When engineering for
almost any application, there is a factor of safety that must be
considered to prevent failure. Ther are a few applications which are
engineered to allow components fail at a certain point, but in your
case, you don't want it to fail.
I reside in State College, PA (formerly in the Poconos) so my designs
typically call for a heavy snow load. In your location, you have to
worry about wind as Bob mentioned earlier.
In either case, a 29' clear span is a large span for a residential
application so the call for a triple ply beam does not surprise me, in
fact it may not be enough.
Without running the #'s, I wouldn't be surprised if it is necessary
to use a small scale steel beam to minimize the appearance and
aesthetic impact of using a large beam. You may want to consider
consulting an professional engineer for this project. Good luck, and
let us know how everything works out!
Sincerely,
Joseph Podwats
CAD Wizards
(570) 510-0238
www.CADwizards.com
| 
Yahoo! 
Google 
Windows Live 
del.icio.us 
digg 
Netscape 
XML Sitemap