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Inside diameter does matter
Roger,
The moment of inertia of the cross section of a cylinder is as follows
pi x (OD^4 - ID^4)/64 (in inches)
That is : pi multiplied by (Outside diameter to the power of 4 minus Inside diameter to the power of 4) divided by 64
and Deflection under load is an inverse function of the moment of inertia.
So inside diameter does make a significant difference.
In an approximation of the example given (I used 200mm for the whole length at 8mm and 5mm wall thickness) the deflection increases from 2.8arc secs to 4.25 arc secs - not quite a doubling.
For these particular dimensions in this particular range the relationship is semi linear - ie very approximately - double the wall thickness double the strength
What does occur that is relevant to OD in that if you were to take the same pipe but make it smaller in OD to say 150mm the strength would drop and the deflection is increased by 50%
But by using the same wall thickness pipe but increasing diameter by a relatively small amount you will increase the pier's rigidity by a significant amount.
The amount is a function of the mathematics
Cheers
Rally
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