The wall seen at in the middle distance is a transverse beam, connected to a pylon top similar to that seen in the preceding picture. The structure of a deck consists of an upper and lower plate connected by a number of continuous beam webs. A short portion of these beams have been constructed here. These longitudinal beams contain tubes for cables, tensioned after the concrete cures and before the falsework is removed. This complex structure reduces weight and cost.
Light weight is also important to reduce inertial effects during an earthquake.
These reenforced concrete structures are actually quite elastic under moderate shaking. This observer was walking under the previous structure at this location during the 1989 Loma Prieta earthquake. One of the techniques of seismic retrofiting in older structures is to lace the deck sections together with steel cables in order to prevent sliding to the point where the deck would fall, as happened during the Silomar and Northridge California earthquakes. Another retrofit is to place a continuous steel jacket around each critical column and fill the interval with concrete grout. This is being done at the Highway 24 Rockridge Station overpass in Oakland and is intended to prevent the kind of catastrophic pylon failure seen in the viaduct collapse in Kobe, Japan.