On 2008-03-11, L D'Bonnie wrote:

If the 2x4's are orientated vertically and are resting on the floor,
they will support most of the weight. Think of a hundred lb weight
hanging on a 2x4 in free air, just your hand holding it from falling
over. It will not require 100 lbs of effort on your part to stop the
2x4 from falling. the force you are exerting is not vertical, the
2x4 takes care of that. You effort is only opposing the overbalance
of all load being on one side.

OK, so suppose your 100 lbs of weight is from a shelf bracket
supporting a shelf 1' wide. Let's take the worst case, that all the
weight is at the front of the shelf. Then that's a moment of 100
ft-lbs.

If the shelf bracket has two fasteners into the 2x4 that are 1' apart,
then the load on the top fastener is 100 lbs of tension. If you are
holding the 2x4 5 ft from the ground and have the bottom end against
your foot, you have to apply a tension force of 20 lbs.

Now if the 2x4 is glued to the face of the masonry wall, and is 8'
long, then a tension force of 25 lbs at 4' from the bottom gives 100
ft-lbs. If the adhesive is along the entire length of the 2x4, then
that's a tension load on the adhesive connection of only 3.125 lbs/ft.

Yours, Wayne