Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
SUMMARY This report is a synthesis of the practices of U.S. state governments in restricting weights of vehicles that can cross highway bridges and culverts to levels below legal loads. Bridges and culverts restricted for vehicle weights are called load posted structures. The load post- ing practices of bridge owners include the identification of structures to post for load, the evaluation of safe load capacities of these structures, and the implementation of restrictions on vehicle weights at structures. Practices for load posting operate within a system of legal loads established in law and regulation of federal, state, and local governments. Posting for load is one possible outcome of statesâ larger activities in evaluation of safe load capacities of bridges and culverts. States post for load, but also grant permits that allow overweight vehicles to travel on designated routes. Overall, states identify and regulate routes that can carry overweight vehicles, routes that can carry legal weight vehicles only, and routes or individual structures that must be restricted to less than legal loads. This synthesis report addresses the practices and the context of load posting of highway bridges and culverts. Practices include methods of load rating, the role of safety inspections, the recognition of deterioration in structures, and the evaluation of safe load capacity for structures. The context includes laws and regulations that limit vehicles loads, exemptions to load limits for some vehicles, permitting for overweight loads, coordination with local governments and, when limits on load are violated, fines that states impose. The synthesis gathers information on loads and posting for load from the United States Code (USC), U.S.DOT National Bridge Inventory (NBI), statesâ statutes and administrative codes, state department of transportation (DOT) manuals and published advice to commercial carriers, and a survey of statesâ representatives to the Subcommittee on Bridges and Structures (SCOBS) of AASHTO. Load posting is a restriction of the weights of vehicles to values below legal loads and below routine permit loads. It includes the placement of signs at structures stating the limits on vehicle weights. There are several reasons for load posting. Some structures were designed for loads equal to or less than H15, a single-unit truck with gross weight equal to 15 tons, and are posted for load as a result. Other structures carry additional dead weight and are posted for the reduced remaining capacity for live load. Still other structures have deterioration or damage that weakens load-carrying components and are posted for load in consequence. Some low strength structures are closed, rather than posted. Load posting is an outcome of load rating. Bridge owners determine the safe load capacities of bridges and culverts. For most structures, load capacities are greater than legal loads, and posting for load is not required. Bridge owners must act when load capacity is less than legal loads. Owners can replace, repair, shore (as a temporary fix), close, or post the structures with low capacity. Load posting is a decision in asset management that proceeds from a finding in load rating. For more than 80% of bridges and culverts, load rating is an exercise in computational structural analysis. For 2% of structures, information on design is missing or incomplete, and STATE BRIDGE LOAD POSTING PROCESSES AND PRACTICES
2 safe load capacities are estimated from observed conditions of structures and known traffic loads in service. For less than 1% of structures, load ratings are determined using load tests. For 16% of structures, no load rating analysis is performed. Most computational load ratings use approximate structural analysis with live load dis- tribution factors; the same type of analysis used in design. The basis for a structureâs design is often the basis for its load rating. Methods for allowable stress design, load factor design, and load and resistance factor design provide corresponding bases in allowable stress rating, load factor rating, and load and resistance factor rating. The engineering practices for load rating and load posting are similar among U.S. states. AASHTO publishes a Manual for Bridge Evaluation (MBE) that provides methods for load rating, live loads for rating computations, and guidance on load posting. States employ AASHTOâs manual, sometimes with modifications such as additional state-specific live loads or state-specific policies for weight limits at load posted structures. Load postings address present-day conditions of structures, and are based on load ratings that are computed for present-day conditions. Conditions that alter strength such as deteriora- tion in components, and conditions that alter loads such as additional dead load on structures, are recognized in load rating computations. Safety inspections provide information on present-day conditions. Safety inspectors alert load raters about new conditions at structures, and load raters review reports of safety inspec- tions. Safety inspectors and bridge load raters compare observed conditions with current load ratings to determine when to update load ratings. Bridges and culverts are posted for load when safe load capacity is less than legal loads. Laws of U.S. states set limits on single-axle weight, tandem-axle weight, and gross vehicle weight (GVW). States also limit the combined weight of axle groups based on the count and spacing of axles in a group. Statesâ legal loads are influenced by limits for loads on interstate highways as set in USC Title 23. The general limits in USC Title 23 are 20,000 lb for single-axle load, 34,000 lb for tandem-axle load, and 80,000 lb for GVW. The majority of U.S. states use these same limits for state legal loads. Legal loads on non-interstate highways can be higher than loads on interstate highways as set in USC Title 23. In 13 states, the single-axle legal load for non-interstate highways is greater than 20,000 lb; the largest among these is 24,000 lb. In 16 states, the tandem-axle legal load for non-interstate highways is greater than 34,000 lb; the largest is 48,000 lb. In 18 states, GVW for non-interstate highways is greater than 80,000 lb; the largest is 164,000 lb. States provide exemptions from load limits for some vehicles, which are tied to vehicle use, to the commodity being transported, or to the vehicle owner. States exempt some farm equipment and construction equipment; some raw products from farms, forests, or mines; and some vehicles owned by public utilities, or state or local governments. No U.S. state government sets legal loads for state highways at values less than the general limits specified in USC Title 23. Some local governments limit loads on their roads to values less than state legal loads. Vehicles that exceed limits on legal loads routinely travel on U.S. highways, including interstate highways. USC Title 23 includes grandfather protections for state legal loads that were in effect in year 1956. Title 23 lists state-by-state exceptions for loads on designated route segments, and exceptions in 22 states for longer combination vehicles. The gross weights of such vehicles are as great as 164,000 lb.
3 Overweight permits are issued by states for single trips, multiple trips, or unlimited trips within a fixed period of time, often one year. States issue multi-trip permits for routes that have been evaluated for common configurations of overweight vehicles. The load ratings of bridges and culverts along these routes have been evaluated for overweight vehicles and found to be adequate. States can issue multi-trip permits without further evaluation of struc- tures. This synthesis report includes information on overweight permit loads. Many states issue multi-trip permits for vehicles with GVW equal to or greater than 100,000 lb. When posting for load is required, signs stating limits on vehicle weights are installed at structures. States set fines for violations of weight limits, with larger violations incurring larger fines. The median fine among U.S. states is $0.20 per pound of excess weight. Some states set additional fines for violation of load limits at posted structures. Three levels of government, federal, state, and local, have three ranges of responsibility in load posting. The federal government has direct control of few structures, but establishes regulations that affect the eligibility of states for federal aid to highways. Federal regulation addresses execution and reporting of safety inspections, load ratings, and load postings of bridges and culverts. Under federal regulation, state governments must inspect, rate, and post state-owned struc- tures, and must ensure the inspection, rating, and posting of local government structures. Coordination between state and local governments varies. In many states, local governments inspect, rate, and post structures on their road systems. In addition, in many states, local gov- ernment bridge owners receive advice and assistance from state governments for local bridge programs. In a few states, the state government inspects, load rates, and posts all structures; both those that are state-owned and those owned by local governments. Federal regulation requires the reporting of the load posting status of bridges and culverts. The federal NBI has information for bridges and culverts on public roads with a span greater than 20 ft. The NBI includes information on structure type, condition, and year built; on structure owner, route, and average daily traffic; and on load rating values, rating methods, and load posting status. NBI data are examined in this synthesis report to learn the prevalence of load posting and the relation of load posting to structure type, owner, condition, and other attributes of structure. Using year 2012 NBI data, it was found that 10% of all U.S. bridges and culverts are posted for load. Sixteen percent of local government structures are posted for load. Local governments (cities and counties) own five of every six structures posted for load. Among state-owned bridges and culverts, slightly more than 3% are posted for load. The posted structures are distributed unevenly among states, with 27 U.S. states having less than 1% of state-owned structures posted for load. Load posting is rare among bridges and culverts on U.S. interstate routes (0.26% are posted for load) and U.S. numbered routes (0.94% are posted for load). On state highways, 5% of structures are posted for load; on county highways, 17% of structures are posted for load. Three of four posted structures have daily traffic of fewer than 400 vehicles per day. Four of five posted structures have fewer than 20 truck crossings per day. Ninety-five percent of load posted structures are bridges, not culverts. Among bridges, 12% are posted for load; among culverts, 2% are posted for load. Three of four load posted structures are in fair or good general condition. Seventy-seven percent of load posted structures were designed for unknown loads or for loads less than or equal to H15. More than 50% of timber beam bridges, and more than 50% of steel thru-truss bridges, are posted for load.
