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.

Overweight Permit Loads are classified as Routine or Superheavy. Routine Overweight Permit Loads may be allowed in the regular traffic stream. An escort is required if the load is also over-length or overwidth. Use the standard AASHTO load distributions since there may be a legal truck alongside the Routine Overweight Permit Load truck crossing a bridge at the same time.

The term Superheavy Permit Load designates total loads over 254,300 lbs. gross. It consists of a 14,300 lb steering axle followed by four groups of three axles, each totaling 60,000 lbs. Any configuration with multiple axles with a gross load of over 254,300 lbs. is considered a Superheavy load and requires structural evaluation of individual bridges. Loads with individual axles or axle group weights that exceed the maximum permit weights are also considered to be Superheavy. Any load exceeding 200,000 lbs. with a total overall length of less than 95 feet is also considered Superheavy.

More information on superheavy permit requirements and procedures is given on the Texas Department of Motor Vehicles – Motor Carriers website at http://www.txdmv.gov/motorcarriers.

The Superheavy Permit may require a specific configuration such that the load straddles multiple lanes. This loading may be such that other legal trucks will not be alongside the Superheavy load and gives better load distribution. In such situations, the AASHTO load distributions used for Superheavy loads are, therefore, usually single lane.

A printout of the proposed list of bridges to be crossed is reviewed by the TxDMV and the Bridge Division. It may be necessary to structurally evaluate only a portion of the bridges on an extensive proposed Superheavy route. For any bridges on the route with a Deck, Superstructure or Substructure condition Rating of 4 or less, review the actual written Bridge Inspection Record. This bridge-by-bridge evaluation is one of the primary reasons that the data in the Bridge Inspection Management System must be accurate and up-to-date.

Occasionally a request is made for a Routine Overweight Permit or a Superheavy Overweight Permit to cross a load-posted bridge. TxDMV does not allow overweight permits for posted bridges.¹ However, Section 623.0113 of the

Texas Transportation Code

allows TxDOT to issue weight tolerance permits for overweight vehicles to cross load-posted bridges only when there is no other route.

Certain other bridges that are not load posted may not be capable of carrying Routine Overweight Permit Loads or Superheavy Permit Loads. Bridges that are in this category include but are not limited to continuous flat slabs with original H-15 designs. These bridges have short spans and were designed with the single H-load pattern truck placed along the span for maximum design conditions. Many of these bridges when rated with the now required HS-load pattern, and even using LF analysis, will rate at significantly less capacity than other types of bridges designed with H-load patterns. These bridges, though not currently load posted, must be carefully evaluated when overload permits are considered. This is the primary reason that the original design loads as shown in the Coding Guide should be entered correctly. Often these bridges have been widened, and occasionally, the widening design load has been incorrectly entered as the original design load.