DESIGN SPECIFICATIONS FOR HIGHWAY BRIDGES - IISEE

seismic earth pressure or hydrodynamic pressure need not be applied to the
structures.
6.2.2 Inertia Force
(1) Inertia force shall be calculated in terms of the natural period of each design
vibration unit specified in Section 6.2.3. For Level 1 and Level 2 Earthquake
Ground Motion, Sections 6.2.3 and 6.4.2 shall be referred to, respectively.
(2) Inertia forces shall be generally considered in two horizontal directions
perpendicular to each other. It can be assumed that the inertia forces in the two
orthogonal directions, i.e. the longitudinal and transverse directions to the bridge
axis, act individually. However, in the design of substructure in which the
horizontal component of the earth pressure acts in a different direction from the
bridge axis, the inertia force can be assumed to act in the directions of the
horizontal component of earth pressure and its perpendicular direction.
(3) In the design of supports, vertical inertia forces shall be taken into account, in
addition to the inertia forces in the two horizontal directions mentioned in the
above (2).
(4) The inertia force of a superstructure shall be assumed to act at the height of the
center of gravity of the superstructure. However, in case of a straight bridge, the
acting height of the inertia force can be considered to be at the bottom of its support
in the case of the longitudinal direction to the bridge axis.
6.2.3 Calculation Method of Natural Period
(1) Natural periods shall be appropriately calculated with consideration of the effects
of deformations of structural members and foundations. In case of unstable ground
during an earthquake, natural periods shall be obtained without reducing
geotechnical parameters specified in Section 8.2.4.
(2) When a design vibration unit consists of one substructure and its supporting
superstructural part, natural periods can be calculated by Equation (6.2.1)
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