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MECHANICS - EXAMPLE

    Example


Fixed Pole with Load
 

A 12 foot pole rigidly fixed to the ground is loaded with a 300 lb horizontal force at its top. The bottom half of the pole is hollow and the half is solid, as shown in the diagram on the left.

What is the maximum bending stress at any location in the pole?

   
  Solution

 

 

The maximum bending stress is a direct function of the maximum bending moment and the member cross section, as given by,

     σ = My / I

Since there are two different cross sections, both will need to be checked.

     

Free-Body Diagram
 

First, the moment distribution from the bottom to the top of the pole needs to be determined so that the maximum can be identified. The pole can be considered as a simple cantilevered beam which makes finding the moment diagram simple. If a cut is made at an arbitrary location, x, then the moment at that location is

     Maa = (300 lb) (12 - x ft)(12 in/ft)
            = 43.2 - 3.6x  kip-in

The maximum in the bottom section is 43.2 kip-in at the ground. The maximum for the top section is 21.6 kip-in at the mid-point joint.

The moment of inertia for both sections are,

     Itop = πd4/64 = π24/32 = 0.7854 in4

     Ibottom = π/64 (34 - 2.54) = 2.059 in4

     

Pipe Cross Sections
 

The stress in both sections are

     σtop-max = My / I = 21.6(1)/0.7854
                 = 27.50 ksi

      σbottom-max = My / I = 43.2(1.5)/2.059
                       = 31.47 ksi

The maximum stress occurs in the bottom section.

      σmax = 31.47 MPa

     
   
 
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