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While this is not an engineered beam and may not be
approved by all inspectors, a careful builder can construct a strong and
durable beam of almost any length from 2x joist lumber, plywood and construction
adhesive. A three joist beam with 2x12's and two layers of 1/2" plywood
as shown above will have roughly the same strength as a 6x12 beam of the
same lumber and grade. It will also be the same width and can therefor use
the same hardware such as post brackets, hangers and post caps.
Layout such a beam from straight solid lumber of
reasonable length. Cut 8' rips of plywood for internal spacers. Assemble
the beam by laying up 2x, plywood and 2x with each joint in the joists no
closer than 4' from another joint. Plywood joints are not as critical but
keep them at least 2' away from a joist splice. Run a line of construction
adhesive between each layer. Space the galvanized nails approximately 4"
to 6" apart in a staggered pattern 1-1/2" to 2" from both the top and
bottom of the composite beam. Nail from both sides. Nails should be long
enough to penetrate the last joist on the other side of the beam (4 -1/2"
to 5" for the beam above).
The glue and the nails pin the joists together and
get them to work as a single unit much like a
LVL
beam (but not as pretty or with the guaranteed and tested strength of
the LVL). 4x and 8x beams can be assembled in a similar fashion. PT lumber
and plywood can be used for exposed conditions. Be sure to use compatible
fasteners such as stainless steel or
hot-dip
galvanized.
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