# Homework Help: Static engineering v -- stress/buckling

1. Jan 19, 2012

### oxon88

1. The problem statement, all variables and given/known data

A column has the dimensions shown below...

Column material properties:

young's Modulus E = 200 GN m^-2

Yield stress = 140 MN m^-2

2. Relevant equations

a) what is the minimum length of the column at which buckling is likely to occur?

3. The attempt at a solution

i got an answer of: 2*((118.74 * Sqrt((2.33x10^3)/(2800x10^-3))) = 6850.56 M
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jan 19, 2012

### nvn

oxon88: Your current answer is wrong. Try again. First, list relevant equations. And then show how you computed each numerical value used in your final equation.

By the way, MN*m^-2 is called MPa. And GN*m^-2 is called GPa. Always use the correct, special name for a unit. E.g., 140 MPa, not 140 MN*m^-2.

3. Jan 20, 2012

### oxon88

1. The problem statement, all variables and given/known data

a) what is the minimum length of the column at which buckling is likely to occur?

3. The attempt at a solution

least second moment of area. I = (B^4 - b^4)/12 = (80^4 - 60^4)/12 = 2.33x10^6

area. A = B^2 - b^2 = 80^2 - 60^2 = 2800

σ = 140x10^6

E = 200x10^9

effective slenderness ratio. E.S.R = sqrt(pi^2.E/σ) = sqrt(pi^2.(200x10^9)/(140x10^6)) = 118.74

effective length. L = (E.S.R). Sqrt(I/A) = 118.74(sqrt(2.33x10^3)/(2800)) = 3427.7

effective length = 1/2 column length. = 3427 x 2 = 6854 mm

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I also tried another method and got the same answer......

Least value of radius of gyration. K = (1/2).sqrt((B^2+b^2)/3) = 1/2 sqrt((80^2+60^2)/3) = 28.868

effective length. L = (E.S.R x k) = 28.868 x 118.74 = 3427.73

effective length = 1/2 column length. = 3427 x 2 = 6854 mm

4. Jan 20, 2012

### oxon88

ok. so i worked it out.

5. Jan 20, 2012

### nvn

By the way, always leave a space between a numeric value and its following unit symbol. E.g., 5.94 m, not 5.94m. See the international standard for writing units (ISO 31-0).

Last edited: Jan 20, 2012
6. Jan 20, 2012

### oxon88

ok. So if the column was to be 5.94 m. What would be the mode of failure? and at what load would you expect the failure to occur?

I guess it would be buckling?

7. Jan 23, 2012

### oxon88

any ideas?

8. Jan 24, 2012

### oxon88

useless forum....

9. Jan 24, 2012

### nvn

10. Mar 28, 2013

### Rafeng404

Hi all, I have the same question and was getting the same answers originally. I assume that your new answer is based on changing the formula to a hollow tube. I was wondering without being told in the question how we are supposed to determine what shape it is... Or am I being a bit thick...

11. Jul 2, 2013

### forexpipz

Wrong formula

The least second moment of area I about the xx axis is not what you stated so your value is incorrect. I get L = 5.236389088 m

12. Jul 4, 2015

### David J

I am just wondering if this thread is still active as I am studying this question and facing similar problems to those on here

13. Jul 4, 2015

### SteamKing

Staff Emeritus
The last post was two years ago, so, no, it's not still active.

However, you can and should create your own, fresh thread. It's not like you can create only so many threads at PF before you run out.