Re: arch versus cantilever

["John S. Denker" <jsd@MONMOUTH.COM>]

Karl Trappe wrote:
> ... you can take two cantilever-like structures and create
> an arch.  That arch can support considerable weight, and can even be used
> to make a bridge.

Horsefeathers.  Try it sometime.

Let T designate the table, i.e. the footings and foundations.
Let X designate one of the blocks.
Let Z designate an identical block.
Let G designate the gap to be spanned.
Let L designate the load.

                      L
              XXXXXXXXZZZZZZZZ
        TTTTTTTTTT        TTTTTTTTTT
        TTTTTTTTTT        TTTTTTTTTT
        TTTTTTTTTT        TTTTTTTTTT
        TTTTTTTTTT   G    TTTTTTTTTT
        TTTTTTTTTT        TTTTTTTTTT
        TTTTTTTTTT        TTTTTTTTTT
        TTTTTTTTTTTTTTTTTTTTTTTTTTTT
        TTTTTTTTTTTTTTTTTTTTTTTTTTTT
        TTTTTTTTTTTTTTTTTTTTTTTTTTTT
        TTTTTTTTTTTTTTTTTTTTTTTTTTTT

As soon as you put the slightest load on this structure,
block Z tips.  Taller cantilevers don't work any better.
Try it sometime.

If the gap is smaller and/or the blocks are longer,
you can support some weight, but if you try to support
"considerable" weight it is monstrously inefficient.

Arches operate on a completely different principle.
The face of one block pushes on the face of another.

So-called cantilever bridges work on yet a third
principle, and have essentially nothing to do with
a stacked-block cantilever.
  http://www.google.com/search?q=cantilever+bridge

==========================

In my previous note I suggested a calculation to
demonstrate the difference between a cantilever and
an arch.  Here I suggested an experiment.  Why don't
we act like physicists and try analyzing the physics
of the situation (theoretically and/or experimentally)
before resorting to ad-hominem attacks and attempted
ridicule?  Restating a physically-incorrect notion
more and more loudly isn't going to make it physically
correct.