Page 130 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 3 BRIDGE FAILURE STUDIES AND SAFETY ENGINEERING 105

3.13.3 Suggested Preventive Action against Fire Damage

1. Emergency fire extinguishing equipment should be provided near bridges which are located
more than one hour away from a fi re station.
2. Transverse and longitudinal wind analysis and uplift with the correct intensity need to be
considered in the design.
3. Member sizes and joints must be designed for the AASHTO LRFD load combination
strength III.

4. Timber bridges should be painted with fire resistant paint and regularly sprayed.
3.14 SUBSTRUCTURE DAMAGE DUE TO EARTHQUAKE AND PREVENTIVE ACTIONS
3.14.1 Progressive Collapse of Piers

1. Bridges are known to move sideways and collapse under their own weight during seismic
events. The sway collapse mechanism of bridges depends upon the type of substructure
(Figure 3.13).
During earthquakes which last for more than a few seconds, active and passive resis-
tance, for instance, of integral and semi-integral abutments on piles would not be the same
as abutments with expansion bearings. Similarly, the response of piers with pile or frame
bents is different than that of say a hammerhead piers. Earthquake damage records show that
earthquakes can cause foundation settlement due to liquefaction, damage piers, and cause
cantilever mechanisms.
2. Ground deformation: One of the major causes of destruction during an earthquake is the
failure of the ground surface. The ground may fail due to fissures, abnormal or unequal settle-

ment, or complete loss of soil shear strength. A loose saturated sand deposit when subjected
to vibration (or cyclic loading) tends to compact and decrease in volume. If drainage is un-
able to occur, the pore water pressure increases. Based on the effective stress principle the
shearing strength of saturated sand is given by the well known shearing strength equation:
3 (%n 6 u) tan7

During longer shaking of loose saturated sand deposits, increased pore pressure (u) becomes
equal to the overburden stresses (%n) and the ground may lose its shearing strength resulting in
settlements and tilting of structures. Loss of strength during cyclic loading occurs in clays also,
but loss of strength does not occur until after large strains have developed.

Figure 3.13 Foundation failure as a result
of earthquake.