Bridge Inspections

The Code of Federal Regulations Subpart C, entitled National Bridge Inspection Standards (NBIS), applies to all structures defined as "bridges" located on all public roads. As per subpart 650.305, each bridge is to be inspected biennially, at regular intervals not to exceed two (2) years in accordance with the American Association of State Highway and Transportation Officials (AASHTO) Manual. The results of the inspections are forwarded to the State for their use in updating their statewide database. The County also felt it prudent to include approximately fourteen (14) of its larger un-inventoried structures in the inspection cycle. The State now requires an Underwater Inspection be performed. All materials between the splash zone line and the mud line, including concrete, steel and timber must be investigated for any indications of corrosion, erosion, vermin attack, spilling, cracking, scouring, or other deterioration. "The State Highway Administration has "determined that St. Mary's bridge inspection program is in substantial compliance with the NBIS requirements."

Inventory and Posting

The Engineering Services Division is responsible for ensuring that bridges incapable of supporting legal highway loads are properly posted. The bridge inventory details the bridges, which are posted as of January 1, 1999. For more information regarding these structures please contact the Engineering Division at 301-863-8400 or view our current Bridge Inventory for an inventory of bridge structures (posted weight limitations and locations etc.) within the County's inspection program. St. Mary's County's bridge posting program seeks to make known the safe load carrying capacity of structures which cannot presently withstand the maximum legal load permitted on the highway. The sign restrictions meet federal bridge inspection standards and requirements. To minimize inconvenience and difficulty, truckers are urged to consult a map and this table in planning or predetermining available routes for a particular vehicle and load.

NOTE: This restricted bridge list is not static. As bridges are replaced or rehabilitated, the load limit restrictions may be removed or changed. Also, as existing bridges deteriorate or are damaged, load limit restrictions may have to be placed upon them. It is imperative that all users of this table make certain that they have the latest version. St. Mary's County will make every effort to keep users informed of any changes but cannot accept responsibility or liability for any user not utilizing the latest version of this document. In all cases, with the possible exception of gross load posting as described below, the weight limit signs posted at St. Mary's County bridges shall take precedence.

Bridge Rating

A bridge's sufficiency rating affects its eligibility for Federal funding for maintenance, rehabilitation, or replacement activities. For bridges to qualify for Federal replacement funds, they must have a rating of 50 or less. To qualify for Federal rehabilitation funding, a bridge must have a sufficiency rating of 80 or less. The sufficiency rating factors are:

S1, the structural adequacy and safety factor (ie. superstructure, substructure, culvert, and inventory rating)
S2, the serviceability and functional obsolescence factor (ie. deck rating, structural evaluation, deck geometry underclearances, waterway adequacy, approach and road alignment, ADT, bridge roadway width, lanes on the structure, approach roadway width, structure type, vertical clearance over deck)
S3, the essentiality for public use factor (includes detour length)
S4, the special reductions factor (is a function of detour length, traffic safety features, and structure type)

Though the sufficiency rating provides a consistent standard for the evaluation of sufficiency to remain in service, it is not comprehensive enough to provide performance-based information on each element of a bridge. Similar to sufficiency rating, health index provides a single numeric indicator of the structural health of the bridge. This indicator is expressed as a percentage from zero, which corresponds to the worst possible condition, to 100, which is the best condition. Health index is a function of the fractional distribution of the bridge element quantities across the range of their applicable condition states. Concretely, the health index value of an entire bridge is calculated as a weighted average of the health indexes of its elements, where elements are weighted by their total quantity and relative importance (i.e., failure cost). Consequently, the bridge health index is a function of the failure cost of each element of the bridge, quantity of each element on the bridge, and condition state of each element. Likewise, the health index of each element is a function of its unit failure cost, its quantity on the bridge, and its condition state on the bridge.

Permitting

In order to replace existing culverts that do not permanently disturb additional wetland areas ("in-kind" replacement) the County entered into a Regional Letter of Authorization with the Maryland Department of Natural Resources. The resulting Memorandum of Understanding is accordance with the appropriate Natural Resources Article and Code of Maryland Regulations. Unfortunately, very few of the culverts and their respective out-falls evaluated to date meet current design standards and must be upgraded in length, size, and/or material type. As such, these structure replacements do not qualify for permitting under the RLOA and must be separately permitted.

Typically, a Corps of Engineer jurisdictional wetland delineation, hydrologic and hydraulic computations, safe fish passage provisions and a formal design is required to replace the larger culverts that permanently disturb existing wetland areas. The Engineering Division evaluates, designs and processes between 5 and 10 Federal permits each year to assist the Highway Maintenance Division crews. The Department's Development Review Division currently has a staff member who has completed the Interagency Wetland Delineation Training Course sponsored by the Corps of Engineers, U.S. Environmental Protection Agency, U.S. Fish and Wildlife Service, Natural Resources Conservation Service and the Maryland Department of the Environment.