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When the piers are built directly into deck without bearings, the monolithic construction creates a portal structure which modifies the bending moment envelope in the deck when compared with bridges with bearings. For instance, hogging moments are increased in supports with the decrease in sagging moments in mid-span of bridge deck. On the other hand, the shear stiffness of piers is a major concern because it tends to resist length changes of bridge deck which could not expand and contract readily.
In order to retain the bending stiffness of piers and to destroy the shear stiffness of pier simultaneously, the piers are split into two parts. The split pier act like the flange of an “I beam” which is effective in resist bending moment. The web of the “I beam”, which is responsible for shear stiffness, is purposely removed so that the resulting split piers could deflect readily under length changes.
This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.