"Rick" wrote in message
ink.net...

wrote in message
...
Rick wrote:

...at first glance I'd say they calculated the load capacity of
the
existing configuration and used that to spec a clear span.

Well, if two 9' 2x10s have S = 3x9.25^2/6 = 42.8 in^3 and M =
1000S
= 42800 in-lb = W9x12/8, W = 3169 pounds, it looks like the 2x4
post
(it might support 5250 pounds) isn't the limiting factor. The
uniform
load on an 18' beam would be 2x3169/18 = 352 pounds per foot.

This was my initial approach, too.

When you calculate the bending stress (S=67.7) and deflection on the
glulams specified, (E=1.8E06) with a 352 pounds/foot load, Fb is
about 2000 psi (2400 psi allowed) but the deflection is about 0.9
inches, assuming a 16 foot span (garage door width).


It does meet 1/180 of the span. Still, even 352 pounds per foot is low
if the header is supporting a roof. If the opposite wall is 22 feet
away, that's only a 32 psf total load.

Oh well....


If you use the 1/360th of span as the limit, and 16 foot unsupported
span you get about 210 pounds/foot.

The 1981 NAHB ap note says an 18' 2x10 Hem Fir beam with a 1/2"x9"
plate
can support 215 lb/ft of uniform load, so a 0.5x352/215 = 0.82"
plate
(or 2 246 pound 7/16" plates) might support 352 lb/ft. They
suggest
1/2" bolts every 20" near the top and every 40" near the bottom
and
something like 2 2x6 jack posts with 3/4" plywood plates at each
end.

Please apply the usual disclaimers.

Ditto


Nick