hollow sections

In your building code. In Canada, that would be CSA S16-01. The shear strength is expressed by: Vf = Φ.Aw.Fs.

Fs depends on the ratio h/w. Its maximum value is 0.66 Fy. Aw is the area of the web, or in this case the area of the vertical walls of the hollow section.

HSS 4x4x1/4 would have a shear strength of 4 x 1/4 x 0.66Fy in each wall. The shear strength of the HSS would be 1.32Fy. If the yield strength is 45 ksi, the shear strength of the section would be 1.32*45 = 59^k (59,000#).

What is Φ ?

For the example you showed, is Φ = 4 ?

And area = 1/4 ? How did you get that?

Φ is the resistance factor which can have different values depending on the material. For concrete it is 0.6. For structural steel, it is 0.9.

For HSS 4x4x1/4, the area subject to shear stress is the area of the two vertical walls, or 2x4x1/4 = 2.0 in². If Fy is 45 ksi, Fs = 0.66Fy = 29.7 ksi. So the shear strength by CSA S16 is Φ*Aw*Fs = 0.9*2.0*29.7 = 53.5 kips. My apologies, I forgot to include Φ in my earlier calculation.

bruce,

This would give me the shear strength value.

If say a load of 2000 lbs is acting on this square tube , normal to one of the walls and parallel to the other (trying to shear it) , would my max. shear stress be 2000 / area of vertical walls ?

No. That would be the average shear stress. Shear stress varies over the wall height.

In a rectangular bar, the maximum shear stress occurs at mid-height and is 3V/2bh where V is the shear, b is the breadth and h is the height of section, so the maximum shear is 50 percent more than the average.

In a WF or I beam, the shear stress at the middle of the web is greater than the shear stress at the flanges, although less than 50 percent greater.

Shear stress can be calculated using the equation s = VQ/Ib where V is the shear at the section, Q is the statical moment of the area above the point in question, I is the moment of inertia of the section and b is the width over which the shearing stress acts.

In calculating shear strength by Limit States Design, the variation of shear stress over the height of web is not considered. But if you wish to know the exact state of stress at any point, you must use elastic theory to calculate it.