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Posted by Jonny on October 25, 2006, 8:55 pm
> My house (built in '86) has a family room (size 16' x 26') above a
> two-car garage. All that's supporting the floor across the span of the
> garage are 2x8 floor joists running across the 16' length. Because
> there are no supports in the garage, the family room floor is bouncier
> than I'd like. What I'm thinking of doing is putting a beam down the
> 26' length in the garage, supported at each end by a metal post. The
> floor to finished ceiling height in the garage is 10'. Anyone have any
> suggestions on what size beam to use? My first thought was a steel
> I-beam, but maybe I can do it myself with (say) three 2x12s bolted
> together. Of course, that might depend on whether I can find 26' long
> 2x12s. Think 2x12s will do the job and provide rigidity over that
> large a span? I guess manufactured beams would be another possibility.
> If so, what size?
>
In the case of what you're anticipating doing, the 2X12 would only serve as
a nailer, not doing the actual support. You would need a steel plate 3/4" I
beam. The 2X12s could be put on both sides of the I beam via bolts. The
2X12s don't have to be continuous, can have 2 on either side of the I beam.
3 2X12s over each other is doable, but the span is too great.
--
Jonny
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Posted by on October 26, 2006, 11:51 pm
Jonny wrote:
> In the case of what you're anticipating doing, the 2X12 would only serve as
> a nailer, not doing the actual support. You would need a steel plate 3/4" I
> beam. The 2X12s could be put on both sides of the I beam via bolts. The
> 2X12s don't have to be continuous, can have 2 on either side of the I beam.
> 3 2X12s over each other is doable, but the span is too great.
> --
> Jonny
Thanks for the suggestions! I understand suggestion #1 (use
manufactured beams) and #2 (sister the 2x10s), but I don't fully
understand #3 (I-beam with 2x12s on each side). If the 2x12s aren't
doing the actual supporting, why bother with them at all? I don't have
to nail anything to the beam, it can be exposed.
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Posted by PPS on October 30, 2006, 5:36 pm
I believe what Bob was referring to (in #2) by doubling the floor joists,
was to add additional section modulus to the joists in the middle of the
span (where deflection and hence, bounce, is greatest.) The greatest moment
occurs at mid span, doubling the joists there will give you something
similar to a 4x10 section.
You could probably build up a beam out of shorter 2x's (considering only if
it was glued and screwed or nailed well), but you could not count the outer
layers as being effective (they would act as a splice section only). And you
would also have to create columns at each end to carry the load of the beam
into a concrete footing (can't go just into the garage floor unless you are
prepared for cracking and settling.)
Bob's suggestion was probably the most cost effective (considering all the
alternatives, including the beam itself, the additional cost of the columns,
and footings necessary to support the loads) and would produce a very rigid
floor above.
>
> Jonny wrote:
>
>> In the case of what you're anticipating doing, the 2X12 would only serve
>> as
>> a nailer, not doing the actual support. You would need a steel plate
>> 3/4" I
>> beam. The 2X12s could be put on both sides of the I beam via bolts. The
>> 2X12s don't have to be continuous, can have 2 on either side of the I
>> beam.
>> 3 2X12s over each other is doable, but the span is too great.
>> --
>> Jonny
>
> Thanks for the suggestions! I understand suggestion #1 (use
> manufactured beams) and #2 (sister the 2x10s), but I don't fully
> understand #3 (I-beam with 2x12s on each side). If the 2x12s aren't
> doing the actual supporting, why bother with them at all? I don't have
> to nail anything to the beam, it can be exposed.
>
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Posted by Bob Morrison on October 31, 2006, 10:34 am
In a previous post PPS wrote...
> I believe what Bob was referring to (in #2) by doubling the floor joists,
> was to add additional section modulus to the joists in the middle of the
> span (where deflection and hence, bounce, is greatest.) The greatest moment
> occurs at mid span, doubling the joists there will give you something
> similar to a 4x10 section.
>
That is exactly what I had in mind. This is one of those things that
might fall under the heading of "Engineer's Secret Tricks"
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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Posted by on October 31, 2006, 5:45 pm
OK..the votes were in and Bob's idea seemed to prevail. So...I went
ahead and pulled down the first of about 12 sheets of drywall and to my
surprise found not 2x10s, but trusses! (I was assuming 2x10s because
that's what the builder used on the main floor, which also has long,
unsupported stretches.) What they are are two 2x4s laid flat (top &
bottom) connected on each side by pieces of metal. I took a picture,
which I'll send to anyone who is interested. What does everyone think
now? Is sistering the joists out and we're back to the beam?
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