Civil Engineering Reference
In-Depth Information
Figure B.51
Elephant foot mode in steel piers of the Collector from Port Island to Kobe during the 1995
Kobe (Japan) earthquake: buckling at the pier base (
left
) and at intermediate height (
right
) (
courtesy of
Dr. Matej
Fishinger)
In many steel bridges, unzipping of corner welds in fi lled/unfi lled box piers has caused collapse; the
weight of the heavy deck squashes the piers. This type of failure mechanism was observed in the
Tateishi Viaduct during the 1995 Kobe earthquake as shown in Figure B.52 .
Several cases of symmetric buckling of reinforcement and compressive failure of piers may be, at
least in part, attributable to high vertical earthquake forces both in Kobe and Northridge (Broderick
et
al
., 1994 ; Elnashai
et al
., 1995). Three out of four RC piers supporting the I10 (Santa Monica freeway)
collector-distributor 36 suffered varying degrees of shear failure due to the short shear span that resulted
from on- site modifi cation of the original design (Figure B.53 ).
B.3.4 Joint Failure
Beam - column connections (or pier -cross beam connections) are subjected to high levels of shear. The
heavy damage infl icted on several RC bridges in the San Francisco area during the 1989 Loma Prieta
earthquake dramatically brought this problem to the fore. Current design philosophy is to attempt to
over-design connections in order to force inelastic action in beams and columns. Without adequate
transverse reinforcement, concrete diagonal cracks are opened in the joint regions, where shear stresses
produce excessive tension cracks, as shown in Figure B.54 .
A further factor that may precipitate joint failure is insuffi cient anchorage of reinforcement in the
end regions. Sliding shear at intentional fl exural hinges has also been observed, and is possibly the
main reason for the collapse of the Cypress Viaduct (Figure B.55 ).
