I am designing a post-tensioned straddle bent. The superstructure has 6 girder lines. Is it possible to define a design truck using fixed axle spacing so it appears "stationary" to reflect the live load reactions from girders to a post-tensioned straddle bent? For example the bearing locations = axle fixed spacing from the left end are: G1-G2-G3-G4-G5-G6 = 63.5'-7.5'-7.5'-7.5'-7.5'-7.5'.
Accurately determining live load effects on intermediate piers always represented an interesting problem. The live load case of loading producing the maximum girder reactions on the substructure varies from one girder to another and, therefore, the case of loading that maximizes live load effects at any section of the substructure also varies from one section to another. The equations used to determine the girder live load distribution produce the maximum possible live load distributed to a girder without consideration to the live load distributed concurrently to the surrounding girders. This is adequate for girder design but is not sufficient for substructure design. Determining the concurrent girder reactions requires a three-dimensional modeling of the structure. For typical structures, this will be cumbersome and the return, in terms of more accurate results, is not justifiable. In the past, different jurisdictions opted to incorporate some simplifications in the application of live loads to the substructure and these procedures, which are independent of the design specifications, are still applicable under the AASHTO-LRFD design specifications. The goal of these simplifications is to allow the substructure to be analyzed as a two-dimensional frame. One common procedure is as follows:
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