Section 4: CalculationsAnchor: #i1005101
General Calculation Requirements
Provide bridge load rating calculations in accordance with currently accepted TxDOT bridge inspection procedures as described in this manual and in other associated documents such as the Bridge Inspector's Reference Manual, 7 the Manual for Bridge Evaluation,8 and the Standard Specifications for Highway Bridges.9 Note that the methods of calculation are different for on- and off-system bridges.
A qualified licensed Professional Engineer must sign, seal and date all calculations and/or documentation referring to load rating capacity. Date, sign and seal documents in accordance with the requirements given in this chapter and with the Texas Engineering Practice Act.10 It is acceptable to only initial and date the calculations and seal only the Bridge Summary Sheet, which summarizes the results of the calculations, rather than sign, and seal each page of the calculations themselves. It is also acceptable, if you are in agreement with the previous calculations, to provide a statement concurring with the existing calculations. Import all calculations, along with pertinent plan sheets, into the Bridge Inspection Management System. If the engineer concurs with existing calculations, scan and import the original calculations and concurrence statement into the Bridge Inspection Management System. Sign, seal and date any concurrence statement.Anchor: #i1005123
Calculations for On-System Bridges
Perform bridge load rating analyses (calculations) and submit them in the findings. If no deficiencies are noted, limit the analysis to the superstructure of the bridge. According to the Manual for Bridge Evaluation11, substructures should be rated if the engineer believes that the condition of a substructure element has the potential to control the load rating for the entire bridge. However, provide load rating analysis for any element that has any condition rating of 4 or less. All on-system bridge records must have documentation in the files to support any recommended changes in load ratings.
Perform all load rating calculations for on-system bridges not designed using Load and Resistance Factor Design (LRFD) using the "Load Factor" method as illustrated in AASHTO’s Manual for Bridge Evaluation 12 with no exceptions. Load rate on-system bridges designed by LRFD using the Load and Resistance Factor Rating (LRFR) method as illustrated in AASHTO's Manual for Bridge Evaluation. When on-system ratings are calculated, present them to the State in HS loading for Inventory and Operating Ratings and in Rating Factor for SHV and EV Ratings. Some analyses may involve bridges that have section loss or damage to structural members. In these cases, verify and document the conditions of members and incorporate those findings into the analysis. The Inventory Rating (Item 66) can be assumed to be at least equal to the design loading if no damage or deterioration exists and the original design load was HS-20 or HL-93.If assumed load ratings of HS-20 or HL-93 based on the original design loads are to be used, then add plans sheets, preferably including the superstructure, with the design load called out to the Bridge Record by scanning the applicable plan sheets and importing them into the Bridge Inspection Management System.
Special attention is called to the coding of Items 41, 41.1, and 41.2 (operational status and posting limits). Verify these items and revise them, if needed, for all bridges to ensure that all posting, posting recommendations, closures and closure recommendations are properly reflected in the Bridge Record. Notify the District Bridge Inspection Office immediately of any bridges recommended for closure, and include details and calculations.Anchor: #i1005145
Calculations for Off-System Bridges
Provide bridge load rating calculations in accordance with the State's bridge inspection policy. Only Load Factor analysis is acceptable for most off-system structures; Allowable Stress methods should only be used for timber and masonry bridges. The Texas Bridge Load Rating Program (TBLR) which calculates load ratings using a Working-Stress analysis is acceptable. For all timber, steel, and truss bridge files, document calculations for load ratings of all structural elements that apply, including the deck, stringers or beams, truss members, bent caps, and piling or columns. The TBLR program will give an Inventory (INV) and Operating (OPR) rating both for H and HS truck loading. Use Figure 8-1, the Simplified Load Posting Procedure in the Bridge Inspection Manual, applying the H-loading to the table, to select the proper load posting sign type and weight limits. Load rating documentation, including assumptions, is required in the files for all bridge class structures (including culverts). Bring into conformance to current TxDOT policy concerning assumed ratings, etc. any assumed load ratings for concrete structures. For all off-system bridge files, for all bridge types, include documentation in the files to support changes in load ratings. When off-system ratings are calculated, present them to the State in HS loading for Inventory and Operating Ratings and in Rating Factor for SHV and EV Ratings.
Special attention is called to the coding of Items 41, 41.1, and 41.2 (operational status and posting limits). Verify these items and revise them, if needed, for all bridges to ensure that all posting, posting recommendations, closures and closure recommendations are properly reflected in the Bridge Record. Notify the District Bridge Inspection Office immediately of any bridges recommended for closure, and include details and calculations. The State will set a time to meet with the Engineer to review the findings, and the State will notify the bridge owner.
Figure 8-1. Simplified Load Posting Procedure