## 20811 Minimum Seat Requirements

In SDRs 1 and 2, the minimum seat width is

where L is the distance between joints (m), H is the height of the tallest pier between joints (m), and B is the width of the superstructure (m).

In SDRs 3 to 6, the minimum seat width is either Equation 20.38 or 1.5 times the displacement of the superstructure at the seat according to the following equation:

where Ae is the displacement demand from the seismic analysis.

20.8.1.2 P-A Requirements

In SDRs 3 to 6, the displacement of a pier or bent in the longitudinal and transverse directions determined by Equation 20.40 shall satisfy

where Vn is the lateral nominal shear strength of the pier, W is the dead load of the pier, and H is the height of the pier from the point of fixity for foundation.

The basis of this requirement is that maximum displacement is such that the reduction in resisting force is limited to a 25% reduction from the lateral strength assuming no postyield stiffness. The inequality in Equation 20.42 is to keep the bridge pier from being significantly affected by P-A moments.

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