Superseded by National Bridge Inspection Standards.
LRFR and Extended Frequency Inspections Policies
FHWA outlines the basic framework for approval of an extend bridge inspection policy in our Technical Advisory 5140.21. As part of that TA we state that bridges with an inventory rating less than the State's legal load will not be eligible for an extended frequency bridge inspection (longer than 24 months). There have been some questions of how to apply this criteria to load resistant factor rating (LRFR). We have been working on this for some time and the below is our interpretation. This is what we will be using as a criteria in our concurrence of an extended bridge inspection frequency policy that uses LRFR. This does not replace polices based on load factor rating (LFR) and LFR policies are still valid.
Subject: Load and Resistance Factor (LRFR) rating criteria for extended bridge inspection intervals.
Extended bridge inspection frequency criteria are to meet the requirements set forth in FHWA Technical Advisory T5140.21 "Revisions to The National Bridge Inspection Standards (NBIS)." One of the requirements in this Technical Advisory states that "(b) Bridges that have inventory ratings less than the State's legal load" would not be eligible for an extended frequency. We interpret this to mean that the bridge must carry the design loads and/or the State legal and unrestricted annual permit loads. As per section 6A.1.7.1 of the AASHTO Manual for Bridge Evaluation, First Edition 2008, the LRFR Design Load Rating is a first-level assessment of bridges based on the HL-93 loading and LRFD design standards using dimensions and properties of bridges in their present as-inspected condition. Bridges that pass the design level check (RF ≥ 1.0) at the inventory level will have a satisfactory load rating for all legal loads that fall within the LRFD exclusion limits.
The extended inspection interval criterion for LRFR then becomes:
Design load rating at inventory level: RF using HL-93 ≥ 1.0
- A load factor of 1.75 is considered to be the design inventory level, as per table 6A.4.3.2.2-1 of the AASHTO Manual for Bridge Evaluation, First Edition 2008.
- In the case where the State legal and unrestricted annual permit loads produce effects larger than those of the HL-93 design envelope, the State legal and unrestricted annual permit loads shall govern. In this case the 1.75 load factor shall be used with either a higher enveloping HL-93 loading or the State legal and annual permit loads using the 1.75 loading factor to determine the inventory rating factor.
- In the event that the effects of the State legal and unrestricted annual permit loads can be shown to be less than the HL-93 design envelope the State may request the use of a HL-93 rating factor of less than 1.0 as a criterion. Such requests will be handled on a case-by-case basis.