bmancanfly had written this in response to
http://www.thestuccocompany.com/main...an-368757-.htm :
Thanks for the reply Rico
I was planning on getting a 3.5" X 12" LVL. Since a dimensional beam
could handle a 12' span I figured an LVL of the same dimensions could
easily handle 12' 8" .
I tried getting the local home center to help me figure the exact
dimensions that I would need but they wern't very helpful.
-------------------------------------
RicodJour wrote:
On Apr 16, 9:06=A0pm,
(bmancanfly) wrote:
I'm opening a wall between two rooms. =A0The neW opening
will be 12'
8". =A0I have a header span table and it indicates
that the
maximum
span for a 4" X 12" header is 12'. =A0The header
is only
going to
support the 10' long ceiling joists above (no roof or
floor above).
If I increase the header to a 6" X 12" will this
work for
a 12' 8"
span? =A0Or will I need an LVL. =A0IF SO WHAT SHOULD THE
DIMENSIONS OF
THE LVL BE? =A0Thanks in advance
I guess what I'm asking is how do I span a 12' 8" opening?
People post such questions and often believe they are providing
sufficient information. They rarely are. You've addressed
dimensions, but not loads, and that is critical to answer any
structural question.
You have a ceiling, which makes the space above an attic.
Is that attic space now used for storage? Will it be?
Did the original builders, or later remuddlers, decide to piggyback
some of the roof load onto that interior wall?
How do you plan to deal with the increased point loads at either end
of the new beam?
You mentioned 10' ceiling joists - is that 10' on each side or 10'
total?
For an accessible attic space, whether you use it for storage or not,
you should allow some load in your calculations - say 20 pounds per
square foot. That's the Live Load, and it's not much as one box of
books easily exceeds 20 PSF.
The Dead Load, or weight of the structure itself, is generally in the
10 to 15 PSF range.
If somebody did piggyback some of the roof load onto the interior wall
(you'll see near-to-vertical framing between the roof and the interior
wall location below), then you will have to add some of the roof loads
(Snow, Wind, Dead Loads) to your calculation.
The Total Load is the sum of the the Live, Dead and Roof Loads.
The contributory area is the area that will add load to your new
beam. Generally that is half of the span on each side. If your house
is 20' wide, the beam in the middle will pick up half of the load on
each floor joist from each side, multiplied by the length of the beam.
The last piece of information you will need is the desired stiffness
of the new beam. If you have plaster ceilings you will need a stiffer
beam to minimize cracking due to beam deflection. Generally 1/360 is
a good compromise and that reflects a deflection of about .42"
over
your 12' 8" span (many people would choose a stiffer beam with
less
deflection).
You now have enough information to contact your local lumberyard/LVL
purveyor and ask them to size the beam for you. This is a free
service. They will plug in your numbers and their proprietary
software will crunch the numbers for your specific situation and their
specific products.
Ask them specifically what the beam end reactions are and how many
supporting 2x4s you will need to carry the beam at each end. You will
need at least two.
If you go with LVLs you will probably need, at a minimum, two
1.75" x
9.25". Remember that a beam gets far stiffer with increases in
height
(depth) - changes in width have relatively less impact on stiffness.
R
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