I just saw an article about an allegedly "wonderful" pedestrian/
bicycle bridge designed by architects. It was awful. So I, snarling
with rage, designed this very different one:
It's kind of like London's Tower Bridge's middle, but with a roadway
outward from each side of the center towers and
span. It has a
raised center section that is a permanent elevated walkway. The
technical term is a two-leaf bascule, but the road is also divided
between the lanes so widely that the bridge towers are in the
"median". There are fixed bridges with the supports between the
lanes. but no drawbridges that I found.
The arch of the bridge is part of the support for the raising
mechanism, and is not a true arch, rather it has three straight
segments. At each end is a partly-sloped "upright" section, which
serves as a stair for pedestrians. Across the top is a flat walkway.
The ground-level roadway is built in two straight lanes, one on each
side of the arch, parallel to it. Each lane is broken in the middle of
the channel, and each half of the drawbridge pivots at ground level
at the edge of the riverbank. For each half of the bridge, the two
lanes are tied together under the arch by a girder between them.
The drawbridge's lifting mechanism pulls up on the girder that
connects the two lanes, and is anchored where the arch's upright
section meets the flat top of the arch. When the bridge is drawn up,
the girder is up in the corner of that intersection, and the ends of the
roadway are sloping up above the permanent pedestrian walkway,
one on each side, roughly parallel to the stairways at each end.
The raising mechanism is either a cable on a winch, or a set of two-
part linkages, powered by hydraulics. Counterweights are as
needed, as on any bascule.
The towers on each end can be built vertical on the river side, with
the other side sloped for stairs, and braced upstream and down by
additional sets of stairs, with a flat section over the roadways. The
road lanes can be used by pedestrians while lowered, while the
arch walkway ensures that they can always cross (those on wheels
will have to wait, unless elevators are put in the towers).