VHS are individual multi-axle vehicles that have been introduced by the transport industry over the past ten years. Examples include dump trucks, construction vehicles, stationary waste vehicles and other transport vehicles. VHS generally conform to the bridge`s Formula B and are therefore considered legal in all states, unless the laws of one state expressly exclude the use of such vehicles. Condition B: The State shall have its own models of assessment vehicles for legal loads and shall verify that the State`s regulatory load models include the applicable AASHTO SHV load models specified in Annex D6A and Figure 6B.9.2-2 of the 1st edition of the MBE (Figure 6B.7.2-2 of the 2nd edition of the MBE), and that the state`s regulatory load models were included in the assessment/reservation analyses of all bridges. VHS types, such as six- or seven-axis VHS that are excluded under state laws, do not need to be considered. The purpose of this memorandum is to clarify the FHWA`s position on the analysis of specialized towing vehicles (VHVs) as defined in the AASHTO Bridge Evaluation Manual (MBE) when operating and deploying bridges to meet the requirements of the National Bridge Inspection Standards (NBIS). The objective of the NBIS load capacity and deployment provisions is to ensure that all bridges are adequately assessed to determine their safe load-bearing capacity under tension, taking into account all full legal loads, including routine government permits, and that bridges are properly displayed in accordance with the MBE as required. VHS generate higher force effects and thus lead to lower nominal loads for some bridges, especially those with a shorter span or shorter load length such as transverse beams, compared to AASHTO Type 3, 3-S2 and 3-3 legal loads and HS20 design load. Therefore, VHS, i.e. SU4, SU5, SU6 and SU7 or NRL, should be included in rating and booking analyses in accordance with Article 6A.2.3 and Article 6B.9.2 of the 1st edition of the MBE (Article 6B.7.2 of the 2nd edition of the MBE), unless one of the following two conditions is met: Project 12-63 of the NCHRP (report 575, 2007) examined the evolution of truck configuration and state legal loads and noted that: that AASHTO Type 3, 3-S2 and 3-3 legal vehicles are not representative of all legal loads, especially VHS. As a result, legal load models for VHS were developed and adopted by AASHTO in 2005, recognizing the immediate need to integrate VHS into a state`s sustainability process when VHS is operated in a state. SHV charging models in the MBE include SU4, SU5, SU6 and SU7, which represent four- and seven-axis SHV respectively, as well as an NRL (Notional Rating Load) model, which surrounds all four one-piece charging models and serves as a screen charge.
If the load factor of the NRL model is 1.0 or more, the individual loads SU4, SU5, SU6 and SU7 do not need to be evaluated. However, if the load factor for the NRL is less than 1.0, the single charges SU4, SU5, SU6 and SU7 must be taken into account in the use and booking. The choice of the load bearing capacity method should be consistent with the FHWA Policy Memorandum on Bridge Load Ratings for the National Bridge Inventory of October 30, 2006. The SHV load models apply to the permissible load level, load assessment and load and resistance factor assessment in accordance with sections 6A and 6B of the MBE. Group 2: Assess bridges not included in Group 1 by December 31, 2022. We ask that you share this memorandum with your state or federal agency partner. Any issues that cannot be resolved at the ministerial office level should be directed to Lubin Gao at lubin.gao@dot.gov or 202-366-4604. If a re-evaluation is warranted for either group due to changes in structural condition, use or configuration, or other requirements, the re-evaluation should include VHS. Condition A: The State verifies whether the laws of the State prohibit the use of VHS; or The above schedule will be included in the review of Measure 13 under the National Bridge Inspection Program (NFIP); In particular, all bridges that meet the Group 1 criteria are expected to be resilient to VHS by the end of 2017. Please work with your State to assist it in developing appropriate measures to meet these deadlines. If your state is currently developing or implementing a Corrective Action Plan (CPA) for span bridges, the CPA should be reviewed and amended as necessary to reflect the VHS assessment for these bridges and schedules.