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Question about shear stress in steel plates, tees and angles

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I have a basic question about shear in steel plates,
tees and angles.

Web shear stresses at a vertical cross section through
a W shape beam are fairly constant and this (almost)
constant shear stress is approximated with the
equation fv = V/dt.

If I chop the flanges off the W shape then the
distribution of the shear stresses through the web
changes substantially - shear stress varies from zero
at the top and bottom of the web to a maximum of fv =
1.5 x V/dt at mid-depth.

Page 16.3-10 in the Third Edition LRFD Manual states
that the maximum shear stress in an angle bent about
one of its principal axes is  fv = 1.5 x V/bt.

Page 16.3-10 also states that the maximum shear stress
in the leg of an equal leg angle bent about one of its
geometric axes is fv = 1.35 x V/bt.

All of the above equations are derived from the basic
equation fv = VQ/It.

Page 16.3-10 also says (to paraphrase) a uniform shear
stress equal to fv = V/bt may be assumed in the legs
of angles due to inelastic material behavior and
stress redistribution.

Here?s my question:

Can you always use the simplified equation fv = V/bt
to compute shear stress (and shear capacity) in a
steel plates, stems of tees and legs of angles (as
long at the members in question are adequately
braced)?  Are there any situations where you MUST use
the equation fv = VQ/It when checking shear stress. 
When using ASD design, does the maximum shear stress
have to be kept less than 0.40 x Fy or is it
acceptable (because of inelastic material behavior and
stress redistribution) to verify only that the average
stress is less than 0.40 x Fy?

TIA,

Cliff Schwinger




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