Moment x Moment?

[jopalu]

|
5' \|/ 2.4K
___________
M1 |
|
|
| 12'
|
|
|
|
M2__|__

I have a 2.4K point load at the end of the cantilever which produces a 12.0 k*ft moment at top of column.

Do I then determine M2 to equal 12 k*ft x 12 ft = 144 k*ft^2? This does not seem right to me. I am expecting a typical k*ft moment at the base. What am I doing wrong?

 

[civilperson]

Why take moment at top of column? Take moment at base of column, 2.4 K x 5' =12 'K. (the same moment will exist throughout the column), a constant moment since the location from the load is a constant 5'.

 

[rb1957]

M2 = M1 ... the loading of the column is an axial load of 2400 lbs + a moment of 12000 ftlbs (being equivalent to the 2400 lbs load offset from the column by 5' ...

divide the structure into free bodies, the cantilever is one and the column the other

good luck

 

[bjb]

bending moment =dxF,
where F= force
d=perpendicular distance from the point under consideration to the line of action of the force.

In your case,
d=5', F=2.4kips
M=5x2.4=12 k-ft.

When considering moments about the base of your column, you're not using the proper moment arm. Your moment arm should be perpendicular to the line of action of your force. Your correct moment arm is 5 feet. Since a bending moment is a force times a distance, the units on a bending moment should be force and length, not force and length squared. A good book on statics will go into more detail.

 

[haynewp]

It is the same if you neglect p-delta.

 

[mrMikee]

This problem reminds me of the jib cranes I designed years back. I found that deflection at the point of load was sometimes excessive even when designed to allowable stresses. In my opinion it's something that should probably be checked too.

Regards,
-Mike