My calculator comes up with breaking load of 150 pounds in the
center of a softwood 2x4 laid flat on 32" center. This is the
same as 300 pounds equally distributed (row of books) I wouldn't
go more than half that. 2 studs would be OK for lightweight loads
(Christmas wrapping paper?) use 3 to have a bit of safety factor.

Deflection is not really an issue in this application, who cares
if it sags a bit? But the sag will increase over time, usually at
least half again more than when you first load it.

Keep in mind that stud grade lumber is not rated for the sideways
load on the weak side since the sheetrock keeps it from moving in
that direction. Look for any shakes, splits, etc. I've been able
to easily break some of the cheap 2x4s with a quick snap over the
knee. Take each one and slam the other end on a concrete floor.
If it 'boinks', fine, if it 'buzzes' put it back. Don't be
surprised if some break.

BTW: text books are some of the heaviest things you can put on a
shelf. Those nice engineering text books run about 6 pounds per
inch of shelf!

Loren Coe wrote:
i know folks here talk about beam loading, both steel and
wood. i am building several large wall mounted shelves in
a frame home. they are metal L-brackets rated at 500-600lbs
each, topped with 3, 4 or 5 2x4 studs.

there is no spec, or minimum load for these shelves, just want
them to be heavy duty and safe. the brackets vary in width,
16-20 inches. this accomodates between 3 and 5 2x4's on their
side, with some amount of spacing between.

i would rather use 3 & 4, rather than 4 & 5 wood members, if
the wider spacing is no problem. this leads to the question:

with support at 32" intervals, how much load can a 2x4
support on its side?

btw, i am using 5/16"x3" lag bolts to fasten the brackets.

Thanks! --Loren