pipe shear stress 2

[sda97]

How is shear calculated on a circular pipe (column or beam)? I have one reference(Design Manual for Structural Tubing -1974) that says to use half the gross section - I am assuming gross area. This is shear across the member,not torsional shear. Why use half the gross section? Is there a shear stress distibution problem. maybe the pipe wall will bend before it pipe shears. Are ther any values for pvc pipe?

 

[jec67]

The formula for shear that you have staed is correct: fv=(2V)/(area). This formula is derived from mechanics of materials: fv=(V*Q)/(I*t). If one runs through this formula using the appropriate values for a pipe cross section, the resulting formula will be fv= (2V)/(area).

Note: applying the same technique to a wide flange beam results in fv= (V)/(area of web)(approximately).

I hope this helps.

 

[GoodnPlenty]

Shear area is a term that represents the efeective area in direct shear. In wide flange shape the effective shear area (Av) is approximtely Av = tw x d,

In a round shape the effective shear area Av is 60% the gross area (Ag)...... Av=0.6 x Ag This is a geometric property and is independent of material ie steel, aluminum, pvc, etc

This is different then the rolling shear eqaution given by jec67. Effective shear area is also used to calculate shear deformations. The VQ/IT is correct equation for rolling shear and 2V/A is the derivation of VQ/It for rolling shear on a rectangular shape- I don't believe that this is correct for a round hollow section.

Good luck

 

[jec67]

In response to GoodnPlenty's posting:

For a circular section: fv = VQ/It

Q = (2r/pi)*(pi*r*t)

I = pi*r^3*t

t = 2* pipe wall thickness

Substituting these equations into VQ/It results in:

fv = 2V/A, where A = 2*pi*r*t (the xsectional area of the pipe).


For a rectangular section, using the same process:

fv = 1.5*V/A where A is the area of the rectangle

 

[sda97]

I did subsequently find in the Specification for the Design of Steel Hollow Structural Sections that the shear stress is 2V/A (V/(pi)Rt). Thanks for input. I am still looking for shear values for PVC maerial.

 

[jec67]

The same equations will hold for any material in the elastic range of stresses. I suggest that you consult a pipe supplier to find an allowable shear value. YOu might also try Ryerson's, which is a stell supplier. I seem to recall they had some values for plastics in their catalog.

Good luck.

 

[sda97]

I am not familiar with Ryerson's. Do you have any additional information on the name/website? I have seen some tensile strength values but I am not sure if this equates to yield strength.

 

[jec67]

I took a look at Ryerson's on line. they do not seem to have any stress values for pvc.

You may want to post a question under one of the mechanical forums, perhaps even the pressure vessel forum. Someone there may be able to give some advice as to what to use for a design value.

I hope this helps.

Please let us know what you come up with for a design value.

Regards

 

[sda97]

I did find some pvc stress values on manufacturer's websites. The ASTM 1784? could also consulted. The tensile strength (yield strength)is about 6600 -7450 psi range. Using the formulas discussed in the other posts, the shear stress can be calculated.