The bridge was sized to fit exactly on 9 regular baseplates (32 x 32 studs each).
The pattern of the arch is simple- each of the 11 segments are pinned together, and the geometry of the holes (i.e. hole spacing) is identical. 6 of the sections are 2 studs wider than the other 5 sections. This enables them to be interlocked and pinned together (e.g. the smaller ones slot into the larger ones).
The deck is composed of parallel sets of LEGO train track. There are support hangers at each joint (every 16 studs), and the tracks are also linked together with plates.
The deck is placed under tension by the arch because it wants to spread outwards. Gravity pulls the arch downwards, and therefore the ends want to spread out.
I suspended the deck by simply "eye-balling" it. Each section of string was adjusted until the deck appeared somewhat level.