What is preset in bridge bearing?

“Preset” is a method to reduce the size of upper plates of sliding bearings in order to save cost.

The normal length of an upper bearing plate should be composed of the following components = length of bearing + 2xirreversible movement + 2xreversible movement.

Initially the bearing is placed at the mid-point of the upper bearing plate without considering the directional effect of irreversible movement. However, as irreversible movement normally takes place at one direction only, the bearing is displaced/presetted a distance of (irreversible movement/2) from the mid-point of bearing in which the length of upper plate length is equal to the length of bearing + irreversible movement + 2 x reversible movement. In this arrangement, the size of upper plate is minimized in which irreversible movement takes place in one direction only and there is no need to include the component of two irreversible movements in the upper plate.

Note: “Preset” refers to the displacement of a certain distance of sliding bearings with respect to upper bearing plates during installation of bearings.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.