Subsidence
 of the soil can create difficulties on large lengthy construction
projects.
Long arching bridges often have a hump or dip somewhere in the length
near the middle.
Not saying this is true of all arching bridges but you can watch the oil
drip line in the middle of the lanes and pretty well pick those spots.

Stan Wilder

On Fri, 31 Jan 2003 10:30:15 -0500 David Brodbeck <gull@CYBERSPACE.ORG>
writes:
> On Fri, 31 Jan 2003, Bill Marshall wrote:
>
> > It was built up as 2 towers that curved in toward each other.
> Where they
> > met, there was a final 8' keystone to go between the two sides and
> link them.
> >  The sides ended up only 3' apart, though, so they had to jack the
> sides
> > apart to get the keystone in.
>
> Not unexpected, considering how different the forces in an arch are
> with
> and without the keystone.  The amazing feat of accuracy, I think,
> was
> getting the two sides to line up horizontally.  If they hadn't lined
> up
> there wouldn't have been much to do to fix it, and there were no
> laser
> alignment tools back then.
>
> Where I used to live there were a lot of abandoned factory
> smokestacks,
> built mostly in the 1920s I believe, and made out of reinforced
> concrete.
> Every one of them had a slight crook in it at some point partway up
> -- not
> one was perfectly straight, even to the naked eye.
>
>              _ _
>   __ _ _   _| | |  | David M. Brodbeck (N8SRE)
> Ypsilanti, MI
>  / _` | | | | | |
> +-----------------------------------------------------
> | (_| | |_| | | | @ cyberspace.org
>  \__, |\__,_|_|_| "To the optimist, the glass is half full.  To the
>  |___/             pessimist, the glass is half empty.  To the
> engineer,
>                    the glass is twice as big as it needs to be."
>
>

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