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to earthquake ground motion. The procedure applies only to bridges that behave essentially as a single degree-of-freedom system and very regular bridges satisfying the following requirements:

• The number of spans per frame does not exceed six.

• The number of spans per frame is at least three, unless seismic isolation bearings are utilized at the abutments.

• The maximum span length is less than both 60 m and 1.5 times the average span length in a frame.

• The maximum skew angle is less than 30° and skew of piers or bents differs by less than 5° in the same direction.

• The subtended angle in the horizontally curved bridges is less than 20°.

• The ratio of the maximum bent or pier stiffness to the average bent stiffness is less than 2, including the effects of foundation.

• The ratio of the maximum lateral strength (or seismic coefficient) to the average bent strength is less than 1.5.

• Abutment shall not be assumed to resist the significant forces in both the transverse and longitudinal directions. Pier wall substructures must have bearings to permit transverse movement.

• For concrete column and pile bents, Pe < 0.2f Ag; r > 0.008; and D > 300 mm.

• Piers and bents must have pile foundations when the bridge site has a potential for liquefaction.

20.6.1.2.5 SDAP D — Elastic Response Spectrum Method

The elastic response spectrum method uses either the uniform load or multimode method of analysis by considering cracked section properties. The analysis shall be performed for the governing design earthquakes, either the 50% PE (probability of exceedence) in 75-year or the 3% PE in 75-year/1.5 mean deterministic earthquake. Elastic forces obtained from analyses shall be modified using the response modification factor R.

20.6.1.2.6 SDAP E — Elastic Response Spectrum Method with Displacement Capacity Verification

SDAP E is a two-step design procedure. The first step is the same as SDAP D and the second step is to perform a two-dimensional nonlinear static (push over) analysis to verify substructure displacement capacity.

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