When a bridge was originally designed, the designer often had to select the next size of reinforcing bar, size of steel beam, or thickness of cover plate to meet the design stress criteria. Sizes that were larger than the theoretically perfect size of member result in Inventory Ratings significantly higher than the design loading. However, the design loading and date of original construction are important parts of the bridge data since they often provide a basis for determining initial routing of overload permits.

If the original design load was an H-load, such as H-15 or H-20, then the equivalent HS Inventory Rating will usually be significantly less, numerically. For example, an H-15 design might rate at HS-12. However, this difference means that the total inventory HS-load capacity is 43,200 lbs. (two 19,200 pounds tandem axles and one 4,800 lbs. single axle, totaling 21.6 tons) as compared to the H-15 design of 30,000 pounds (15 tons).

Determine the original design load from a review of the bridge plans if available. If the structure substantially matches an old TxDOT standard bridge, and is from an appropriate time period, then the design load for that standard can be used for Design Load, Item 31 (SNBI: B.LR.01). (Note: this would be difficult to do with Pan Girders.) Enter an appropriate notation about this in the Bridge File. Use caution accepting the design load when the Design Load references a modification by THD Design Supplement No. 1¹² due to circumstances described above.

Do not consider temporary repairs for Inventory or Operating Ratings. However, take temporary repairs into account when assigning the operational status code of Item 41 (SNBI: B.PS.01) to the structure. Temporary repairs are to be considered for the operational status code only until a more permanent repair is made. Do not use temporary repairs for more than four years.

Use all field information and conventional analysis techniques when the design loading is unknown or deterioration exists. Even when the design loading is known, the only acceptable method for accurate load rating is to perform calculations based on the plans and known field measurements.