What are the advantages of assigning the central pier and the abutment as fixed piers?

it can crrie lord equly and distributly lord eqvanly.

The bearing initially is placed right in the middle point of the upper bearing plate.
No directional effects of irreversible movement is considered.
But since the irreversible movement usually takes place in one direction only the displaced direction is placed away from the midpoint.
In such cases the length of the upper plate is equal to the length of the length of the bearing + irreversible movement +2 x reversible movement.
(i)                  For abutment pier to be assigned as fixed pier while the bridge is quite long, the longitudinal loads due to earthquake are quite large.
As the earthquake loads are resisted by fixed piers, the size of fixed piers will be large and massive.
In this connection, for better aesthetic appearance, the selection of abutment as fixed piers could accommodate the large size and massiveness of piers.
Normally abutments are relatively short in height and for the same horizontal force, the bending moment induced is smaller.
(ii)                (ii) For the central pier to be selected as the fixed pier, the bridge deck is allowed to move starting from the central pier to the end of the bridge.
However, if the fixed pier is located at the abutment, the amount of movement to be incorporated in each bearing due to temperature variation, shrinkage, etc.
is more than that when the fixed pier is located at central pier.
Therefore, the size of movement joints can be reduced significantly.