Re: Movable span bridges

["Derick Wuen" <cullend@webone.com.au>]

Thanks to all those who posted replies or emailed. Corrections welcomed.

1. Summary of Movable Span Railway Bridges, so far

Location        Waterway                Type            Comment
NSW
Shea's Ck   Alexandria Canal     Hinged
Menindee    Darling River            Hinged
Grafton        Clarence River         Hinged
Pyrmont       Darling Harbour       Swing        Tram then Monorail
Glebe Is.      Sydney Harbour       Swing        Tram
Gladesville   Sydney Harbour      Swing        Tram

NSW-Vic
Tocumwal    Murray River            Vertical Lift
Murrabit       Murray River            Vertical Lift
Robinvale    Murray River            Vertical Lift     never used for rail
Yelta            Marray River            Vertical Lift      never used for
rail

S.A.
Paringa       Murray River            Vertical Lift
Jervois        Port River                 ?                        see note
(a)
Robinson    SA Co Basin            ?                        see note (a)
Fisher         SA Co Basin            ?                        see note (a)

Tasmania
Bridgewater    Derwent River    Vertical Lift
Bridgewater    Derwent River    Swing                replaced by lift bridge
Hobart             Constitution Dock Hinged(?)

Note (a) SA Port Adelaide area bridges details unclear. See ARHS Bulletin
July 1970 for photos of Robinson and Fisher bridges. Both look immovable,
but enclosed tallish sailing vessels. Jervois Bridge carried Semaphore
branch and/or trams?

Is there really nothing in Qld?

The old road bridge over the Swan at Fremantle was a timber swing bridge I
dimly recall, and so there was a chance that the rail bridge may have been
at one time. But my references only bemoan the fact that the rail bridge
obstructs navigation of the Swan. So none for WA.

A hinged movable span in Auckland, NZ was also reported.

2. Concepts

The suggestion that the scope of the enquiry into "movable spans" might
include those bridges which had collapsed caused a little flurry of thought.
But once it was clear to me that the next logical step after collapsed spans
was potentially movable spans, I stopped worrying. Even the Sydney Harbour
Bridge is potentially movable, and thus the term "movable spans" covers all
bridges. But we all know what is "really" meant.

Likewise the suggestion that the Spit tram punt might be a movable span.
Same function; different solution, as far as I'm concerned.

In the summary I have added movable span type. I think there are 3 broad
types:

. Swing, where the span rotates horizontally to clear a passage unobstructed
vertically;

. Hinged, where one end of the span rotates vertically around a hinge at the
other end. Some in this NG have referred to this arrangement as a bascule
bridge. The arrangement leaves a vertically unobstructed passage.

. Vertical lift, where the span lifts vertically to clear a passage but only
to a limited amount.

Each type has advantages and disadvantages.

>From a mechanical engineering point of view, the swing and hinged types
require precision only at the point of travel where the road or rails have
to line up for on-bridge traffic. Furthermore, both can give unlimited
overhead clearance when opened, a big advantage.

But both tend to obstruct clear passage horizontally:

. for example the big central pier on which the Pyrmont swing span is
located.

. hinged types have to rotate almost 90deg to provide a clear passage, and
complicated systems of counterbalances are needed as the moment around the
hinge decreases the more vertical the span rotates to. For example the
curved counterweight tracks on the Sheas Creek bridge decreased effective
counterbalance the more the span rotated. Additionally some designs of
hinged bridge provide for the hinge to move away from the waterway to
increase clearance, causing further complications.

The vertical lift bridge can only provide a limited amount of vertical
clearance. Furthermore, the lift mechanism is required to be accurate for
the whole lift. Each corner of the lift span must be lifted at exactly the
same rate, otherwise the span will jam in the tracking of the four lift
towers.

It is therefore a mild surprise to find so many vertical lift spans. These
spans are closely related to many road only vertical lift spans, so the
clearance and engineering accuracy disadvantages of the type were well
understood and under control.

3. Safeworking

A movable span bridge in a section of railway is a serious obstacle to
safeworking. Study of safeworking methods is likely to prove rewarding.

4. Trams

When trolley equipped trams crossed movable spans, what were the
arrangements for mechanical and electrical continuity for the trolley wires?

Regards