Q. What is Preset in bridge bearing?

'Preset' is a method to decrease the size of upper plates of sliding bearings in order to save cost. Normal length of an upper bearing plate must be composed of the following components: length of bearing + 2 x irreversible movement + 2 x reversible movement. Initially bearing is placed at the mid-point of upper bearing plate without considering the directional effect of irreversible movement. Though as irreversible movement generally takes place at one direction only, bearing is presetted/displaced a distance of (irreversible movement/2) from the mid-point of bearing in which length of upper plate length is equal to length of bearing + irreversible movement + 2 x reversible movement. In this arrangement size of upper plate is minimized in that 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 at the time of installation of bearings.