Check out the CREEPY tapered columns on the upper deck and how much
those crazy things were holding up.  Those tapered columns tell me why
the Embarcadero (built with normal columns in 1961) did not collapse
while this one did.  It is amazing it didn't collapse BEFORE the
earthquake with its incredibly shoddy design, which is described in
detail all the way through.  I would love to hear any mtr engineers
commentary on what in all was wrong with this design.  What I knew
that stood out was the shallow columns, weak columns on upper
structure and the reinforcement even looked poor.

Basically, the Cypress Viaduct collapsed because its design, which was
finalized in 1953 and was considered to have three times the strength it
needed to handle the seismic loadings then accepted for design, did not
take into account the amplified ground motions which resulted from the
poorly consolidated soils in that part of Oakland.

The Cypress was also built without shear keys connecting the lower deck
to the columns and the upper deck.  Shear keys are basically lengths of
reinforced concrete which are designed to oppose shearing stresses.  In
the case of the Cypress during the Loma Prieta earthquake, the shearing
stresses came from the lower deck responding to seismic loading by
moving in one direction, while the columns holding the upper deck tried
to stay in the same place due to inertia.  Because there was no steel
reinforcement to resist the shear stresses at the column bases, there
was just unreinforced concrete in those locations to oppose the cyclic
loading caused by the earthquake.

Concrete is very strong in compression but very weak in tension, which
is why steel reinforcement has to be provided whenever concrete is
likely to be in tension.  Thus, after a suitable number of cycles of
alternating tension and compression, the concrete just crumbled and that
in turn caused the columns to fail and the upper deck to come pancaking
down.

There were localized areas of the viaduct which collapsed in a different
way due to other mechanisms which came into play because of different
local ground conditions.  But, as a generalization, the majority of the
viaduct came down because of the missing shear keys.  They simply were
not provided at the time of design in 1953 because they were not
considered to be necessary.

Love those single bar railings!

Jim K. Georges

At no time did I ever hear the Nimitz Freeway (I880) referred to as the
Cypress Freeway.

There was a Cypress Street Viaduct, I think.

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The overriding point is that all of the double-decker freeways in the
Bay Area that did not collapse had a narrow escape.  This is why they
have since all been substantially modified or demolished.  Part of the
Century Freeway lost its second story while the tail end was demolished,
the entire Embarcadero was demolished, the Bay Bridge west approach
viaduct is being reconstructed and seismically retrofitted, the Bay
Bridge east span is being replaced, etc.

Some research reports on the Cypress Viaduct collapse can be found here:

http://nisee.berkeley.edu/elibrary/Text/8515

http://nisee.berkeley.edu/elibrary/Text/8516

http://nisee.berkeley.edu/elibrary/Text/11275

http://nisee.berkeley.edu/elibrary/Text/12436