Closing a bridge when live load carrying capacity drops
Here are some references about the minimum load carrying capacity of a highway bridge to keep it open for public traffic:
1994 AASHTO Manual for Condition Evaluation of Bridges, Section 7.4 (attached) states: "A birdge owner may close a structure at any posting threshold, but bridges not capable of carrying a minimum gross live load weight of three tons must be closed."
2003 AASHTO Manual for Condition Evaluation and Load Resistance Factor Rating (LRFR) for Highway Bridges, Section 6.8 states: "Bridges not capable of carrying a minimum gross live-load weight of three tons must be closed. Owner may close a structure at any higher posting threshold."
1995 FHWA Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation's Briges, Item 64 states: "If the bridge will not carry a minimum of 2.7 metric tons of live load, the operating rating shall be coded '000': and consistent with the direction of the AASHTO Manual, it shall be closed."
2008 AASHTO The Manual for Bridge Evaluation Section 6A.8-1 states, "Bridges not capable of carrying a minimum gross live load weight of three (3) tones must be closed. A Bridge Owner may close a structure at any higher posting threshhold."
2008 AASHTO The Manual for Bridge Evaluation Section 6A.3 states, "When Rating Factor (RF) for any vehicle type falls below 0.3, then that vehicle type should not be allowed on the span.