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Queries on ASD - 2

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John Smith wrote:

. > ****ITEM 6
. > Chapter F - Sect F4 allowable shear
. > 
. > a)Is it assumed that for solid sections like circular, sqaure and
. > rectangular sections that the allowable shaer stress is Fv = 0.40Fy
. > based on the Gross Sectional area
. > 
. > b) Is there any reduction in the gross area of a circular hollow
. > section in determining the shear area and is the allowable shear Fv =
. > 0.40 Fy
. > 

I would be very careful about applying the allowable shear of Sect. F4 to 
other than flanged sections with the shear area being based on the area 
*of the web*.

If you look at the flexural shear stress diagram of a flanged section using 
VQ/Ib, the shear stress in the flanges is very low, then jumps up markedly at 
the flange/web interface and remains almost constant for the height of the 
web, making the average shear stress in the web [V/t(w)*h(w)] being almost 
equal to the maximum shear stress.  This, of course, does not apply to shapes 
of other cross sections.  For a rectangular section bent about its major 
axis, the shear stress is parabolic and the maximum is 1.5*V/A.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona