Sistering Deck Support Beams

[Brent Altman]

I have the need to span a large area for an overhead deck. Is there an advantage to sandwiching the beam on both sides with a “partial” sister. For example a four foot sister in the middle of a twelve foot span. Is there a formula for calculating the load improvement.

[JT]

Sounds like you are looking to add stiffness to a
marginal span. Unless you have the structural
engineering expertise to calculate the effects of
such non-standard constructs, I’d say avoid them.
There is an excellent article in the current issue
of Journal of Light Construction about “bouncy”
floor systems and how to avoid them. The thing I
found interesting is that floors with a natural
(i.e. resonant) frequency of 8-10 Hz were judged
to bouncy. This, they point out, happens to be the
resonant frequency of our intrenal organs. At about
15 Hz floors were judged acceptable. They suggest
upsizing from the normal L/360 deflection to L/480
for floors not supported by a girder. It gets more
complicated of there is a girder.

[Bob Haddleton]

A good source of information is your local town or municipal building inspector. He (or she) is the person who must be satisfied with the construction details. I agree with JT who said to avoid sistering. It pays to do it right, and right is per your local building code.

[Marc Laplante]

Hi-I’m building a storage shed/addition to my 1 car detached garage. It is 8′ by the width of the garage, which is 14′. The ledger is 2by6, as are the joists, which are spaced 16″ O.C. 7′ out from the ledger are 3 8″ sonotubes 4′ in the ground. A made-up beam consisting of a 4by4 sanwiched between 2 2by4’s(thru bolted with 8 3/8 by7″ bolts,washers,lockwashers, and nuts) is secured to the 8″ piers and is holding up the floor system, whith bracing at @ the 4′ mark.

Being used only as a shed for bikes,a lawnmower,snoblower(I live in Ct.) workbench,ect.

I thought this would be enough, especially so since I got the joist span from a book on how to safely build a deck. Which, being attached to the garage, makes it a deck,I guess. My question is this-is what I’ve done strong enough for a storage shed or is it, as I have been told, to be considered a deck and therefore not safe, because I used 2by6 construction 16″O.C. throughout and primarily because I used an undersized made-up beam WHICH I DRILLED THROUGH, THEREBY WEAKENING THE STRENGTH OF THE BEAM. All of this was told to me by a retired carpenter who happens to be a neighbor of mine. He also told me that because I don’t have a ceiling ledger I would have to bolt the new walls to the exixting building in at least 4 places using 3/8by4 lag bolts. AND I was also told that I just might have to use 2by8’s 2′ O.C for ceiling rafters-I had been planning to use 2by6’s 2’O.C and a small wall directly above the existing wall. The roof will be a shed design with a 3″in12″ pitch over all.

Please advise as this is getting confusing.

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