S cousins,P loads are hypothetical design vehicles. The P load design vehicle consists of a steeringaxle and between two to six pairs of tandems. The number of tandems used is based onthe configuration that produces the maximum stress in a span. The abbreviation LL isused to reference live load in the text. The abbreviation DL is used in reference to deadload in the text.Earthquake loading is a product of natural forces which are dependent on the geographiclocation of the bridge. In general, there are three major forces with which the bridgeengineer must be concerned: Seismic Forces, Wind Forces, and Channel Forces. Like thevehicle live loads discussed before, seismic, wind, and channel forces are temporary loadson a structure which act for a short duration. Superstructure elements, though areaffected by seismic forces in many ways. An earthquake exerts forces on a bridge that are defined as a function of the followingfactors:* Dead weight of the structure* Ground motion (acceleration)* Period of vibration* Type of soil presentThese factors are used to determine the response of the bridge to an assumed uniformloading on the structure. This response takes the form of an earthquake loading which isapplied to the structure to calculate forces and displacements on bridge elements. TheAASHTO specifications provide two methods for calculating this loading. The methodused is dependent of whether the bridge is single span or multispan and the geometriccharacteristics of the structure. Since single span structures can be considered to beextremely stiff and their ability to withstand earthquakes is deemed adequate, AASHTOdecided to segregate their analysis from bridges with two or more spans.(1)The analysis of multispan bridges varies depending on the type of geometry present andthe degree of seismic activity at the bridge site. The regular bridges are those withconsistent and similar superstructure cross sections and intermediate su...
