# Calculating buckling strength of steel pipe for non-axial load

1. Aug 1, 2013

### sudnak

Hello All,

I am in the process of creating a 33' tall chaotic pendulum, and cannot find any calculations for the buckling strength of pipe. The pendulum swings from side to side, but is stopped as it goes too far one way or another by a limiting cable attached to a sleeve over the pendulum. So, essentially I have a pipe supported on one end and free on the other, and there will be a force applied to the end of the pipe. I need to know how to calculate the amount of force that different size pipes can take before they buckle. The force from the load, calculated at the fixed end of the pipe, is about 2800N. See picture for details. So, what formulas can I use to find this? Would I calculate section modulus and moment of inertia? Thanks for all the help!

2. Aug 2, 2013

### nvn

sudnak: Ensure d2/t does not exceed 40, and then your steel pipe will not flexurally buckle, where d2 = pipe outside diameter (OD), and t = pipe wall thickness.

Yes, compute moment of inertia (I). Also compute bending moment (M) on the pipe at the cable. Then compute sigma = M*c/I. Ensure sigma does not exceed 170 MPa.

Because you have the cable tensile force, you can compute bending moment M, but you will need the distance from the pivot to the cable attach point, the distance from the pivot to the mass, the mass of the mass, and the angle between a vertical line and the pipe when the cable tensile force is 2800 N.

Last edited: Aug 2, 2013
3. Aug 2, 2013

### sudnak

What is c in your M*c/I?

4. Aug 2, 2013

### SteamKing

Staff Emeritus
c = OD/2, where OD is outside diameter of the pipe.

5. Aug 2, 2013

### SteamKing

Staff Emeritus
You haven't specified the type of piping material or grade you are using. For ordinary pipe material, the maximum tensile strength is lower than that for ordinary steel (30 ksi for Grade A pipe v 36 ksi for ASTM A-36 steel). I would limit the max. bending stress to 120-125 MPa (18 ksi) to give yourself a reasonable factor of safety.

6. Aug 2, 2013

### sudnak

Awesome, thank you so much for the recommendations, I cannot tell you how much it helps!

7. Aug 8, 2013

### mauchitoq

Hi, i have some annotations:
- In this case, the kind of stress is bending, not buckling.
- I recommend you to consult the "Specification for Structural Steel Buildings (ANSI/AISC 360-10)": http://www.aisc.org/WorkArea/showcontent.aspx?id=26516 (go to F.8 chapter).

8. Aug 9, 2013

### nvn

mauchitoq: No, sudnak is correctly referring to flexural buckling, as well as bending stress. This was covered in post 2. Recheck, e.g., the subject title of the thread.

Last edited: Aug 9, 2013
9. Aug 9, 2013

### mauchitoq

nvn: it's true. But in steel structural engineering, buckling usually is about the behavior explained with Euler formula, and not with the stress failure criteria.