On Fri, 28 Jun 2019 12:10:09 +0100, Recliner
wrote:

On Thu, 27 Jun 2019 18:52:37 +0100, Guy Gorton
wrote:

On Thu, 27 Jun 2019 13:38:13 +0100, Basil Jet
wrote:

On 27/06/2019 11:48, David Cantrell wrote:
On Tue, Jun 25, 2019 at 04:02:38PM +0100, Guy Gorton wrote:

The only links Gatehampton has with transport.London is that the
Thames is flowing towards London and the railway is running away from
Paddington! But that is my excuse for mentioning this lovely viaduct
by Brunell which was built at two diffrent times, using different
brickwork techniques, the two being attached to each other their full
length.

The Grosvenor bridges crossing the Thames outside Victoria were built
similarly, one track at a time, so that the old structure could be
replaced without having to close Victoria station.


I've always wondered if there is any advantage in having a multi-track
bridge as opposed to several single track bridges with inch gaps between
them. You'd think narrow bridges would be preferable with respect to
maintenance, and also preferable in most conceivable catastrophes, but
one wide bridge seems to be the usual choice.

Brunel used both. Maidenhead is the first going west and it was made
by extending the existing spans on both sides. Very interesting to
look at from underneath the river arch. Quite distinct differences in
the constructon techniques. Gatehampton is two bridges glued together
so that at rail level there is no break. The next set is at Moulsford
where there are two independent viaducts with some construction
differences and some connecting mini-arches. Other Brunel bridges
over the Thames demonstrate his versatility - wrought iron at Windsor,
steel on the Henley branch, steel(?) at Bourne End, although the
last-named is probably before his time, it being on the Wycombe
Railway.

I think it's quite common to have a separate structure when the
formation is widened (eg from one track to two, or two to four). But
if building a two-track bridge from scratch, I can't see any benefit
from building it as two physically separate, adjacent single track
bridges. That would almost certainly cost more, be heavier, and
require more maintenance. In particular, it might be very hard to get
maintenance access to the structures that are only an inch apart. How
would they be inspected, painted or repaired?

Brunell's bridges over the Thames that I mentioned are a mixed lot.
At Windsor, the bridge carried two tracks but a long time ago it was
reduced to single line but the bridge is unchanged. Wargrave was also
double track, now single, and half the bridge was removed.
Tha main line bridges all started as double track when built in 1839
and all were widened to 4 tracks in 1890.
Maidenhead was widened on both sides and the two new arches are
slightly different from the original arches - they have a slightly
wider span at river level. Whether there is anything left of the
orginal parapets underneath the ballast, I would not know.
At Gatehampton, the additionial structure is entirely on the Down side
of the line. There is no visible structure berween the old and new
spans which are of exactly the same dimensions. The old span uses
skew-brickwork, the new straight brickwork, so the change is very
visible. Again, I would not know if there are any remnants of the
original parapet under the ballast.
At Moulsford there are two seperate viaducts over the flood plain and
the river. This time, the original viaduct is on the Down side.
There are different construction techniques used on the two structures
but they have essentially the same dimensions.
Brunel was a versatile man!

You may well ask why I have all this somewhat trivial knowledge. The
answer is that back in 1999 I decided that Thames bridges were an
interesting topic so I set about taking photos of all of them from
Albert Bridge, Windsor to Wallingford, (OS map 175 plus a bit at the
west side). 26 bridges used by the public. The Powerpoint show has
been given to many groups with a wide range of interests so I had to
develop talks to suite the interests of the groups.

Guy Gorton