|TO:||Bob George/Chuck Ruth|
|FROM:||M. Myint Lwin|
|DATE:||June 17, 1998|
|SUBJECT:||Load Rating of Bridges|
"Why do we need to load rate new bridges which have been designed to HS20 or HS25 Loading?" has been a topic of much discussion in the last few years. In April 1998, the State Bridge Engineers from Alaska, Oregon, and Washington sent a joint letter to Mr. David Densmore, Chief Bridge Engineer, FHWA, for clarification. Mr. Densmore agreed that the inventory rating corresponds to the design load and the calculations made in the design document the inventory rating for new bridges. However, Mr. Densmore contended that, in order to comply with the NBIS and the AASHTO Manual for Condition Evaluation of Bridges, calculations must be made to determine and support the operating rate of each structure. In summary, it remains the FHWAs position that calculations made for design are sufficient to document the inventory rating and calculations must be made to determine the operating rating of bridges in operation.
In view of the FHWAs position and the heavy design workload in the Design Section, I establish the following policy regarding the load rating of bridges:
"The Bridge Preservation Section is responsible for the bridge inventory and load rating of existing and new bridges in accordance with the NBIS and the AASHTO Manual for Condition Evaluation of Bridges, latest edition.
The bridge Design Section generally will not be required to load rate new bridges/designs. However, for the more complex structures where computer models are used in the design/analysis, a copy of the computer models shall be made and submitted to the Bridge Load Rating Engineer in the Bridge Preservation Section."
This policy takes effect immediately and supersedes any conflicting instructions in the current Bridge Design Manual.
The following presumptive ratings may be used pending load rating of new bridges:
For HS20 Designs: Inventory Rating = 36 tons and Operating Rating = 60 tons.
For HS25 Designs: Inventory Rating = 45 tons and Operating Rating = 75 tons.
(LRFD HL93 Loading may considered as equivalent to HS25 for presumptive ratings.)
The current AASHTO Manual for Condition Evaluation for Bridges is presently being revised under NCHRP Project 12-46 to incorporate procedures for the load rating of bridges using the LRFD philosophy.