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Posted by J on June 15, 2005, 3:29 pm
> J wrote:
> >
> >>>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
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