Development of foundation design for the West Seattle Freeway Bridge required extensive geotechnical investigations including a seismic intensity determination. This bridge is one of the first major structures to be designed in accordance with the guidelines developed by the Applied Technology Council (ATC-06). Design constraints, procedures, solutions, and details are described.

The West Seattle Bridge is a 1700 m (5,580 ft) long structure carrying a major arterial over an important ship channel (Figure 1). The 32 m (105 ft) wide deck carries six lanes of highway traffic and will serve an estimated daily volume of 80,000 vehicles by the year 2000. Navigation clearance over the ship channel (West Waterway) required a 180 m (590 ft) main span with 43 m (140 ft) vertical clearance. The structure chosen for construction is a twin, single-cell, concrete, box-girder main span unit of three spans--114, 180, and 114 m (375, 590, and 375 ft). Structure depth varies from 3.7 m (12 ft) at midspan to 9.1 m (30 ft) over the piers. The principal dimensions of the main span unit are shown in Figure 2. Typical piers are shown in Figures 3 and 4.