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AASHTO Load Rating Provisions for Implements of Husbandry (2020)

Chapter: Chapter 4 - Conclusions and Suggested Research

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Page 101
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2020. AASHTO Load Rating Provisions for Implements of Husbandry. Washington, DC: The National Academies Press. doi: 10.17226/26001.
Page 101

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101 Conclusions and Suggested Research The following conclusions can be reached based on the research work reported here: • Load test results show that the dynamic impact of IoH load is lower than the 33% used for bridge design, owing to low speed. • Wider gauge width of IoH results in a lower live load distribution factor and narrower gauge width leads to a higher live load distribution factor. • Tracked IoH may be modeled conservatively using concentrated wheel or axle loads. More refined analysis may be performed to reduce conservatism of concentrated loads. • Dual spread between two tires in one wheel may be neglected up to 2 ft center to center of the two tires. More refined analysis is needed for larger spreads. • Single-tire-steering-axle IoH have a different load distribution from multitire-steering-axle vehicles. • Available WIM data show that IoH load is at low volume. Calibration based on these data recommends Tier 1 load at 115% of FBF be load rated using current AASHTO MBE live load factors for legal load, for consistent bridge safety. Tiers 2 and 3 above Tier 1 are recom- mended to be load rated using 95% of current AASHTO MBE live load factors for routine permit and special permit loads. Further research efforts relevant to the subject here are suggested to address the following needs. • More WIM data with IoH recorded are needed for engineers to better explain IoH behavior in terms of vehicle configurations and traffic volume. • It is important for bridge owners to continue data collection on IoH operation when a load- rating or permit-reviewing program is commenced. IoH traffic has been growing in both weight and volume. Such data collection and the subsequent analyses will help safe operation as well as adequate forecasting for possible future developments. C H A P T E R 4

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Farm equipment, generally referred to as “implements of husbandry” (IoH), has been considered to be local vehicles on farms that do not use public roads often, if at all. As a result, state and local jurisdictions vary widely in managing IoH. Apparently, the growth of IoH has far outpaced that of other legal highway vehicles, warranting concern with bridge safety.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 951: AASHTO Load Rating Provisions for Implements of Husbandry (1) proposes new IoH load-rating provisions for the AASHTO Manual for Bridge Evaluation in load factor rating and load and resistance factor rating (LRFR), along with related revisions to the AASHTO LRFD Bridge Design Specifications; and (2) develops protocols to evaluate IoH with various configurations for load rating and overload permits.

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