I hope I get terminology right - our condo is four buildings connected
by continuous roof over atrium, two story. Finally, board has gotten
around to repairing a long-rotted support beam and correcting this
sagging portion of roof. I've commented before that the sag was held up
for years by downspout tubing that someone installed between the ceiling
and railing around upper deck. Duh! The old beam rotted entirely away
along the end of a skylight that blew away last year ) Rather than a
solid beam, contractor is using 4 1x12's glued and nailed to each other.
2x4 along bottom of each side will hold the rafters - rafters have a
corner cut out that the 2x4 fits into. The new beam spans about 15',
but I haven't measured. The beam goes between two concrete? encased
steel beams, one of which crosses the center of the atrium. The
steel/concrete beam across the center gets support from a two story wood
partition that isn't entirely healthy and takes a lot of stress in high
wind. The span for the new wood beam from outer to center beam is about
22'. New skylight, about 5x15, will be installed, one end of which sits
at about the center of the new beam.

This laminated beam will probably be many times stronger than the
original solid beam. It's actually becoming more common to see this
sort of construction now.

On 2005-11-16, louie wrote:

This laminated beam will probably be many times stronger than the
original solid beam.

Why is this? The grain is running in the same direction in both
cases. Is it simply that any defects are offset in the laminated
beam?

Cheers, Wayne

Partly that, but mostly a beam's strength comes from it's depth not
width. The moment of inertia (directly related to load carrying
ability) of a certain cross-sectional shape = width x height^cubed /
12. So a 3/4" x 12" beam has a moment of inertia of 108, which is
equivalent to a 6x6 beam.

That said, you DO need SOME width to prevent twisting & buckling, but
it's not the primary load characteristic of a beam. Also, many times,
the beams are laminated with plywood or OSB, which does vary the grain
direction.

Why is this? The grain is running in the same direction in both
cases. Is it simply that any defects are offset in the laminated
beam?

Pretty much, yes. That and the fiber-stresses in a solid
beam tend to all be uniform, and those is a laminated beam
are averaged out, so a laminated beam will seldom twist
with changing humidity.

"louie" wrote in message
oups.com...
This laminated beam will probably be many times stronger than the
original solid beam. It's actually becoming more common to see this
sort of construction now.


He didn't describe an engineered laminated wood beam , he described a
composite beam made up of four pieces of dimensional lumber. This would be
a little stronger than a solid beam but not as strong as a truly laminated
beam but will be dimensionally stable (less likely to twist or warp). This
construction may be adequate if there is not a large load on that beam and
would be much less expensive than a solid or laminated beam. If the
contractor is good, he at least alternated the crown of the planks before
gluing and nailing. (nails may be incidental until the glue cures depending
on how much and what kind of glue)

It's probably adequate construction but likely does not exceed design
requirements. The real thing is to find the source of the water that rotted
the original beam and make sure that is resolved too.

PipeDown wrote:
"louie" wrote in message
oups.com...

This laminated beam will probably be many times stronger than the
original solid beam. It's actually becoming more common to see this
sort of construction now.



He didn't describe an engineered laminated wood beam , he described a

She )

composite beam made up of four pieces of dimensional lumber. This would be
a little stronger than a solid beam but not as strong as a truly laminated
beam but will be dimensionally stable (less likely to twist or warp). This
construction may be adequate if there is not a large load on that beam and
would be much less expensive than a solid or laminated beam. If the
contractor is good, he at least alternated the crown of the planks before
gluing and nailing. (nails may be incidental until the glue cures depending
on how much and what kind of glue)

The board/beam now has some big, fat bolts through it about every 3'.
The rafters, which appear to be about 12" apart - didn't measure - each
have those metal thingys that anchor them to the beam. The metal
thingys are kind of a "U" as the rafter sits in them. It appears it
might stay up a while ) I've seen so much half-assed work it worried
me, but this contractor is pretty cool. Even cleans up each day.
Inspector comes today.

It's probably adequate construction but likely does not exceed design
requirements. The real thing is to find the source of the water that rotted
the original beam and make sure that is resolved too.