Civil Engineering Reference
In-Depth Information
(1989)
The Cypress Viaduct was California's first continuous double-deck freeway structure.
Construction commenced in 1955 and it was opened to traffic in June, 1957. It was
situated just west of downtown Oakland and extended approximately 2.4 km (1.5
miles) in a north-south direction. Both the upper, southbound, and lower, northbound,
traffic lanes were supported above ground level by a series of reinforced concrete
bents. The upper frame incorporated shear keyed, essentially pinned, joints positioned
so that the upper part of each bent was statically determinate.
Some recognition of the potential hazard posed by a structure which had been
designed to much lower seismic load demands than would be appropriate for a
structure conceived more recently had resulted in some retrofitting of the viaduct.
This work comprised tying the spans together longitudinally. No strengthening of the
joint areas around the shear keys was done.
The majority of the fatalities which occurred as a result of the 1989 Loma
Prieta earthquake resulted from the collapse of the upper deck of the Cypress Viaduct
onto the lower elevated deck, trapping and crushing vehicles in the northbound lanes.
The fact that other nearby bridges and buildings survived the seismic shaking focused
attention on the configuration of the Cypress Viaduct, the details of the structure and
the site on which it was founded. An extensive investigation was mounted by many
groups of researchers including those from the University of California, Berkeley. As
a result of these investigations the characteristics of the structure and its behavior in
the October 17, 1989 earthquake were clarified.
The majority of the collapsed bents followed a common pattern of failure
involving slipping along a plane of weakness which existed in the stub region of the
lower column to beam joint, just below the shear key at the bottom of the upper bent
column (Figure 4-4). This plane of weakness in the joint region was created by
closely spaced lower girder negative moment steel reinforcement which was bent
down into the column. Insufficient transverse reinforcement was provided to prevent
the wedge of concrete outside the plane of the bent down girder reinforcement from
sliding on the sloping failure surface under the combined effects of the lateral seismic
loads and the weight of the upper deck.
Lessons Learned
The reinforcement in the columns and girders was poorly detailed, even allowing for
the lack of understanding, at the time the viaduct was designed, of the inelastic
response demands on reinforced concrete structures in seismic zones. Confirmation
of the necessity to design for realistic earthquake generated forces and displacements
was one of the lessons learned from the experience of the Cypress viaduct collapse.
Another was the penalty of delaying seismic strengthening once it is established that
a structure is not up to current design standards.
