|FHWA Policy Memorandums - Office of Engineering|
Superseded on 11/15/2011 by Revisions to the Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation's Bridges (Coding Guide) Items 63 and 65, Method Used to Determine Operating and Inventory Rating
|Subject:||ACTION: Bridge Load Ratings for the National Bridge Inventory||Date: December 22, 1993|
|From:||Chief, Bridge Division
Office of Engineering
|Reply to Attn of: HNG-33|
|To:||Regional Federal Highway Administrators
Federal Lands Highway Program Administrator
This is a follow-up to our memorandum dated November 5 on the subject topic in which we announced that the FHWA has chosen the load factor method as the standard for computing inventory and operating ratings for the National Bridge Inventory (NBI). It was also advised that, while load factor ratings are required for the NBI, States may elect to use other methods for the purpose of load posting, such as the allowable stress or LRFD method.
From the reaction to our memorandum, there are a number of concerns and technical issues being raised about the rating of bridges using the load factor method. Some of these will be addressed by the AASHTO "Manual for Condition Evaluation of Bridges" due to be published and distributed this month by AASHTO. However, we would appreciate receiving any comments and suggestions that your office or the divisions may receive from the States.
The AASHTO condition evaluation manual provides guidance on load factor ratings, but not for prestressed, wood and masonry bridges. However, this office is working with the AASHTO Subcommittee on Bridges to develop guidance for bridges built with those materials. Information on the rating of prestressed concrete bridges is also available in the 1989 AASHTO "Guide Specifications for Strength Evaluation of Existing Steel and Concrete Bridges." Our draft guidance has been prepared, and after being reviewed and coordinated with AASHTO, it will be sent to field offices for application by the States.
There have also been concerns expressed about meeting the schedules for rating new or NHS bridges or for those in need of rerating because of condition, replacement or rehabilitation. Our schedule was established in order to complete within a reasonable period of time the rating of all bridges in the NBI, starting with NHS bridges, for an HS (MS in metric terms) vehicle load by the load factor method. If a State is unable to comply with the schedule established in our November 5 memorandum, it is requested that the Division Office discuss the issue with the appropriate State officials and agree upon a plan for accomplishing the load rating of bridges within a reasonable time frame.
An acceptable plan should indicate an active program for rating/rerating bridges beginning in FY 1994, and for the entering of ratings into the NBI beginning with the FY 1995 NBI update due April 1, 1995. The plan should also reflect a concerted effort to complete the ratings for NHS bridges by the FY 1996 NBI update due April 1, 1996. Please advise us of any agreed upon plans for completing the load rating of bridges on a schedule other than that specified in our November 5 memorandum.
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