Chris B wrote
> Why were bridges built like that? Do the roof and walls add to the
> structural strength?
>
> It's surely not worth sheltering traffic for the short distance of the
> crossing.
Without seeing the image ( I obviously missed the link) and with much of my
professional working life being in forestry, timber utilisation, and so on; I
can
think of at least two good reasons for covering timber bridges.
1) Where the timber is inherently of low natural durability and no
preservative treatment is / was available, keeping the decking and structural
timbers dry would extend their useful life a lot. There would of course also
possibly be timber-eating insects as well though.
2) Where the decking / running surface is timber, it is very important in
winter to keep the surface protected from radiation frost. Timber is a
relatively good insulator and tends to not accumulate heat. So that in clear
cold nights, even when it's not terribly cold, a layer of frost / ice will form
on
the timber surface.
This is very slippery for traffic and contributes to bad accidents.
I know of one really nice timber bridge over the Selwyn River in Canterbury
which was made of preservative treated laminated pine and was intended to
show-case the versatility and excellence of timber lamination. Physically it
performed very well.
However, during even slightly frosty weather, even though it had a tarsealed
running surface laid over the top, it used to become treacherously slippery
and several bad accidents occurred.
.
Eventually the bridge was dismantled and replaced with a concrete bridge.
Brian Swale ( working on dialup again :--( )
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|