"Peter James" wrote in message
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J wrote:
"toller" wrote in message
...

Actually it has much less than half the bending strength. Resistance to
bending is proportional to the cube of the depth.

Admittedly it was 30 years ago and things change, but when I took
mechanical engineering it was proportional to the square.
You integrated the material multiplied by it's distance from the
center; that means squared, no?

Perhaps you didn't do that well in mechanical engineering... :-)

Moment of inertia (I) for a rectangular beam with width b and height
h has been I = bh^3/12 for as long as I can recall.

And for deflection (d) of a simply supported beam we use the formula
d = PL^3/48EI

Other formulas for bending (canteliever, multiple supports ...) vary,
but
all the ones I can recall off the top of my head include I in the
denominator.
Therefore I am certain that I did mean cubed and did not mean squared.

-j




Well - you're confusing bending strength and deflection. In your first
post you said:

Quote: Actually it has much less than half the bending strength.
Resistance to bending is proportional to the cube of the depth.

This is incorrect - bending strength is governed by stress, which is
derived from bh^2/6 - the square of depth, as "toller" noted.

But in your second post you introduced deflection:

Quote: Moment of inertia (I) for a rectangular beam with width b and
height h has been I = bh^3/12 for as long as I can recall.

This is correct - for deflection.

Yep, I got a step ahead of myself and forgot I even mentioned bending
strength.

-j