Misconceptions often arise about the relationship between Operating Ratings and Overweight Permit Loads. The primary difference is that overweight Permit Load analysis usually assumes only one load on the bridge, which, therefore, allows the use of single-lane load distribution. The Operating Rating is based on the standard AASHTO load distribution given in the current

LRFD Bridge Design Specifications

for multi-lane distribution for bridges over 18 feet in width. This distribution implies two or more of the Operating Rating trucks being on the bridge side-by-side at the same time.

The other major difference is that Operating Ratings and Overweight Permit Loads use different load multipliers, resulting in Overweight Permit Load analysis being significantly more liberal than Operating Rating analysis. Review the current Operating and Inventory Ratings, the age and type of structure, the span lengths, and the Condition Ratings for any structure proposed on a permit route. For any Condition Rating of 4 or less, request more detailed information on the structure, including the written inspection comments. Reduced strength in a portion of a bridge can often be avoided by controlling the load path and load distribution of the Overweight Permit Load across the bridge. Additionally, consider the effects of scour and lateral stability of the substructure.

There are other major differences between Operating Ratings and Overweight Permit Loads

The Operating Rating is usually based on Load Factor (LF) criteria, which use multipliers of 1.3 applied to both the dead and live loads. The live load has an additional allowance of up to 30 percent for impact. Note that Inventory Rating uses a significantly higher live load multiplier of 2.17. The result for either Operating Rating or Inventory Rating is compared to the yield or ultimate strength capacity of the members. A “phi” strength reduction factor (usually from 1.0 to 0.85) is also applied for concrete members.

Overweight Permit Load analysis usually assumes a factor of 1.0 applied to both the dead and live loads. Ten to 30 percent is added to the live load for impact, depending on the speed control and type of load suspension system. Stresses are compared to an allowable maximum of 75 percent of the yield capacity of steel members or 75 percent of the ultimate capacity for concrete members. The reciprocal of 75 percent is 1.33; thus it can be seen that Overweight Permit Load analysis with Allowable Stress (AS) methods has essentially the same factor of safety as an analysis using LF criteria.