[TowerTalk] Mast Analysis

[Kurt Andress]

Dave D'Epagnier wrote:
> 
> A while back someone posted a web site where there is an article describing
> how to calculate mast yield. I've lost my bookmark for this site. I'm trying
> to figure out which would be stiffer: an aluminum mast with 2" diameter and
> 0.375" wall or a 2.5"diameter 0.25" wall mast. Can anyone point me in the
> right direction?
> thanks,
> Dave
> K0QE
> 

Dave,

A convenient way to compare the stiffness of different
members is to look at the combination of their "E" and "I"
values. 

Commonly referred to the "EI" of something, it is merely
found
my multiplying the E by I.

E = the material's modulus of elasticity and I = the moment
of inertia of the section (how the material is distributed
about its centroid). The "E" provides the stiffness of the
material that is in the section, and "I" accounts for  where 
the material is located in the section.


The modulus of aluminum is generally accepted as, 
 E = ~10.0 MSI (Million Psi)

The moment of inertia for round tubing, 
I = PI * (OD^4 - ID^4)/64     where, PI = 3.1416


For 2 In. O.D. x .375 In. wall round aluminum tube: 

	E*I = 10.0 Msi * .666 In^4 = 6.66 

For 2.5 In. O.D. x .250 In. wall tube:

	E*I = 10.0 Msi * 1.132 In^4 = 11.13

The larger tube is 1.67 times stiffer.
As you can see, "I" goes up with diameter to the fourth
power, so making things bigger stiffens them pretty quickly.


Just for comparison,
A 2.0 In. O.D. x .250 In. wall steel tube would be:

	E*I = 29.0 Msi * .537 In^4 = 15.57

Be sure you use the same units of measurement in all
calculations. Psi & inches, or Psf (pounds per square foot)
& feet.

Enjoy your project!

-- 
73, Kurt, K7NV 

http://www.freeyellow.com/members3/yagistress/

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