Page 132 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 3 BRIDGE FAILURE STUDIES AND SAFETY ENGINEERING 107

• Using dampers
• Strengthening of members and connections.
3.14.5 Seismic Resistant Design for Movable Bridges
Bridges need to be analyzed in both open and closed positions, and for positions in between.
AASHTO movable bridge specifications specify that the seismic load used for the open position

may be reduced by 50 percent if the bridge is in that position for less than 10 percent of time.
Counterweights on bascule bridges represent large seismic inertias. The effect of the large
mass on the girders should be explored. When the span is raised, the span drive braking machinery
becomes effective. Seismic acceleration forces on brakes need to be investigated.
Several methods of seismic retrofit are outlined for bearings and expansion joints within the

FHWA Retrofit Manual. Primarily, the bearings, joint restrainers, and minimum seat widths for
seismic Zone 2 criteria retrofit need to be addressed.

Because of stringent operational and mechanical tolerance requirements, movable bridges
need to be evaluated not only for stress, but more importantly for displacements. Effi cient func-
tioning of the expansion bearings therefore is important. In bascule bridges, there are heavy
counterweights that significantly affect the seismic behavior of the long structure.

3.14.6 Fragility Analysis of R.C. Bridge Pier Considering Soil-Structure Interaction
Seismic fragility methodology for highway bridges: Bridgefragility curves, which express
the probability of a bridge reachinga certain damage state for a given ground motion parameter,
play an important role in the overall seismic risk assessmentof a transportation network.

3.14.7 Case Studies of Seismic Failures
Table 3.11 gives details of seismic failures for numerous bridges located in USA and abroad.
Although earthquakes have been known to cause damage for hundreds of years, it is only since
the Santa Barbara and Norma Prieta earthquakes in California that interest in design and studies
by NCHRP have been initiated.
3.14.8 Some Case Studies of Recent Collapsed Bridges

1. Gujarat India Earthquake: Bhuj suffered major damage. Widespread liquefaction and lateral
spreading of soils have been reported in Rann of Kutch (India) and in many parts of the
Southeast Sindh. Craters several feet wide developed on and around Badin-Kadhan road.
Fault rupture results from ground vibration due to the upward transmission of the stress
wave from rock to the softer soil layers. These stress waves are body waves that reach the
surface at an angle depending upon the distance of the surface point from the epicenter or
point on the surface over the origin. These body waves may generate two other surface waves
that are confi ned to elastic-half-space and are known as “Raleigh wave” and “love wave.”
The seismograph may also record the ground motions of these waves, which are complex
in nature.
Rocks in the region are primarily Jurassic to Cretaceous age sedimentary and volcanic
rocks. The earthquakes in India and Pakistan are the result of the compression thrust of the
Eurasian Plate with the Indian Plate. The neotectonic geology of Kutch (Malik, et al, 2000)
consists of a series of folds and faults with a general WNW/ESE trend.
The kinetic energy of the waves is dissipated in the earth’s crust with distance from the
source, and its magnitude is registered at various intensities at the locations through which
the body waves pass.
2. Balakot Bridge failure in the Pakistan Earthquake of 2005: Several bridges that were not
designed for seismic resistance were severely damaged in the northern region of Pakistan
during the 7.6 intensity earthquake. Balakot Bridge suffered the greatest damage and the
main highway was shut down.