shear resistance of tubing

huangyhg

shear resistance of tubing?
hi,
i'm a new  civil engineer and i'm trying to check the structural resistance of a lifting frame that is fabricated entirely from 3" diameter schedule 80 carbon steel tubing.  
the frame is a simple design.  it has a 3 foot long backbone and a 3 foot long cross-piece at each end.  picture an i-beam viewed from the end, this would be the frame in plan view. it is lifted from above with a crane via an eyelet welded to the backbone.
the cross-pieces will support a heavy plate on each side of the backbone.  i want to check the moment and shear resistance of the tubing since we are concerned with the safe maximum load on the frame.
so my question is:
does anyone know where to find the moment shear resistance of the tubing?
thanks.

for circular tube or pipe you may use following formula for shear stress:
fv = p*(d^3-d^3)/[12*i*(d-d)]
where:
fv - shear stress
p - shear
d - outside diameter
d - inside diameter
^3 - third power
i - moment of inertia of pipe
(ref: ncees handbook for structural engineers)
i would assume that the lifting hooks will be on the ends of the two cross pieces. however it is arranged it is likely that moment capacity will govern. so you just need to find the moment capacity. this is given by s x sigma where s is the plastic section modulus and sigma the yield stress. you can either get s from published tables for various sections (depends on the thickness of your tube) or the formula for s is given in most basic mechanics texts for various shapes. i'm not familiar with us designations of steel strengths so you'd need to find out what the yield strength of your particular steel is.
carl bauer
i need to know how many lbs a 2 1/2" square tube 1/4" wall  and 8' long can hold horizontally.
this is trying to answer "johnmoore23"'s question.
since i do not know what kind of material you are using so i will leave the tension yield strength as s and shear strength as t.
the moment of ineria of the tube i=1.92 in^4
the cross-sectional area of the tube a=2.25 in^2
the shear area of the tube as=1 in^2 (web area only)
the distance from extreme fiber to center of section c=1.25 in
first, check flexural strength.  the critical section is at the fixed end.
         moment m=8*12*p=96p (k-in)  
where, p=ultimate load at the tip of this tube (kip).
the stress at the extreme fiber of the critical section is
         s=m*c/i=98p*1.25/1.92=63.8p (ksi)
thus the ultimate load p=0.157s (kips)
second, check shear strength and assume von mises criteria is used.
         t=p/as=p/1=p (ksi)
by von mises criteria t=s/3^0.5
thus     p=0.577s
then, we found that the flexural strength actually controls this ultimate load and the ultimate load is
         p=0.157s (kips)
for example, your tube has a yield strength of 60 ksi.  the ultimate load at the tip is
         p=0.157*60=9.4 kips
however, this calculation is purely based on mechanical calculation without any safety factor or considering any code requirement.  you may also need to check the fixed and loading points to prevent local buckling and web crippling under bearing force.  if your force is not at the center line of the tube, you will need to check the torsion and torsional buckling, too.
good luck.