ABSTRACT
The load rating of an existing bridge is a way of rigorously defining the live load carrying capacity of the structure. Rating is critical to insure the structural integrity of the bridge and thus, the safety of the public, by insuring that critical stresses are not exceeded accounting for both inevitable section loss of bridge members and occasional live loads in excess of the original design loads. While design of new structures is occasional, rating of existing structures is common and ongoing. Each member of a bridge must be individually rated consistent with the allowable stress, shear and displacement. The procedure for calculating the bridge rating is extremely labor intensive, and requires details of the original design parameters, as well as current condition of every component of the bridge. In its simplest terms, rating is done by analyzing the structure using a Cooper E-80 loading, and calculating the maximum stress caused by that loading configuration. You then divide the maximum allowable stress by the maximum calculated live load stress, and apply the ratio to the “80” in the E-80 load. Thus, the basic rating equation in its simplest form is where σ all = maximum allowable stress; σ DL = stress caused by deal load and σ LLmax = maximum calculated stress cause by live load. Although this appears simple, in practice the calculations are complicated by the fact that all stresses must include those caused by, in addition to live and dead loads, also impact, wind, centrifugal, forces from welded rail and other lateral and longitudinal forces. This makes the procedure cumbersome, time consuming, confusing and prone to error. This paper will explore the entire route in detail, and attempt to explain each required formula application in simple, logical and incremental steps. Efforts are currently underway to write a robust rating software tool to make the procedure quicker and more reliable. ( σ all − σ DL σ LLmax ) 80 = E Rating https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/eq153.tif"/>
