Structural Design

Responsibilities of the Structural Design Division include:

  • Ensure the structural integrity and safety of over 700 public bridges and retaining walls throughout the County
  • Arrange and oversee biannual bridge inspections
  • Issue and review temporary bridge overload permits
  • Provide support to other agencies as needed with retaining walls, buildings and bridges 
  • Develop maintenance and construction programs

BIANNUAL BRIDGE INSPECTIONS

All bridges which span lengths over 20 feet are required to be inspected every two years by federal law. These inspections must comply with the Federal Highway Administration's National Bridge Inspection Standards. The costs for these inspections are covered 100 percent by Federal funds. Baltimore County follows the same procedures for their minor bridge structures (those between eight and 20 feet). The inspections for minor bridges are funded by the County, and when available, State aid.

BRIDGE DESIGN CRITERIA

The American Association of State Highway and Transportation Officials (AASHTO) maintain the building codes for both roadway and bridge construction in the U.S. The same design criteria, right-of-way acquisition process, and environmental review and permitting processes are followed for all projects.

FOR ROADWAY BRIDGES

We refer to the AASHTO Standard Specifications for Highway Bridges manual and its successor, the AASHTO Load Resistance Factor Design Bridge Design Specifications, which provided criteria governing the structural engineering and geotechnical aspects relating to bridge design.

FOR ROADWAY WIDTHS OVER BRIDGES

Both of the above manuals refer to the AASHTO Policy on Geometric Design of Highway and Streets. This document governs road design. These criteria were incorporated into the Design Standards and Policies for Rural Baltimore County roads.

Bridge widths are determined by six factors:

  1. Functional classification—Defines roads according to the character of service they are intended to provide. Most County roads are classified as local, collector or arterial roads.
  2. Average daily traffic volumes—This data is maintained and provided by the Bureau of Traffic Engineering and Transportation Planning. Twenty-year predicted counts are also developed and incorporated into the bridge design.
  3. Percent trucks—Provided by the Bureau of Traffic Engineering.
  4. Design speed—Determined by the provided guidance of AASHTO for different types of roadways.
  5. Geometric conditions—Relates to the existing alignment of the roadway across the structure. For bridge replacement projects, we look to do the least amount of approach roadway work possible to meet the minimum design criteria. Occasionally a bridge needs to be slightly wider to accommodate the curved roadway or for safety reasons relating to line of sight requirements.
  6. Community input—Provides us with information relating to local conditions and concerns of citizens impacted by the replacement of an existing bridge.

Bridge length is based on the needs of the feature crossed:

  • Bridges over streams—Sized on hydraulic characteristics of the stream passing through the crossing.
  • Bridges over railroads—Sized by the clearance envelope required by the railroad.

BRIDGE MAINTENANCE AND REPAIR

Routine maintenance and smaller repair projects are handled on an ongoing basis. These projects generally require minimal design work and address defects without altering the structure of the bridge or wall.

BRIDGE REPLACEMENT

The decision to replace a bridge is based on the findings of the biannual inspection reports. County engineers prioritize replacement candidates based on structural condition, ability to repair effectively and economically, and availability of funding. Once engineers determine a structure needs to be replace, design proceeds through the following steps:

This initial engineering stage consists of data collection (traffic volumes, location of existing right-of-way and adjacent property lines, wetland delineations, field surveys, etc.)

During the preliminary design phase is when community input is solicited to analyze alongside the collected data to determine the full extent of the project. Alternative options are evaluated and the preferred alternative is developed into a preliminary type, size and location plan. Most bridge replacement projects are designed to be hydraulically in-kind such that the flood characteristics of the new structure match the existing conditions. All new structures on new roadways must be designed to pass a certain flood based on the "design storm."

After receiving concurrence on the type, size and location of the proposed structure, the final design phase begins. During this phase, all elements of the proposed structure are designed in depth and the plans are refined, including specific details and notes. Once all approvals and permits are received, the contract documents for advertisement are developed.

Once the contract is awarded, notice to proceed is given. Construction is scheduled as best as possible in consideration of environmental restrictions, priority and community impacts.

FUNDING

When federal funds are used, projects must be reviewed by the Maryland State Highway Administration. There is also some additional environmental review documentation required on federal aid projects.

  • Bridges under 20 feet in length are 100 percent funded by the County. 
  • Bridges over 20 feet in length and meeting certain criteria qualify for Federal Bridge Replacement funds. These federal funds typically pay for 80 percent of the design and construction costs associated with a project. The County is committed to using federal funding whenever possible.

RURAL ROADS AND BRIDGE PROJECTS

Rural roads are defined as a County-maintained road within Baltimore County's rural conservation zones outside of the Urban-Rural Demarcation Line (URDL).

Design Standards for rural roads and for bridges and culverts along rural roads are included in Chapter Eight: Roads and Streets, and Chapter 10: Structures, in the Baltimore County Design Manual, adopted on August 2, 2010.

LEARN MORE

Once a year, the Division of Engineering and Construction conducts an open workshop to receive public input on all projects located outside the County's URDL. Public meetings may also be conducted for individual projects. Bureau of Engineering personnel are available to speak at civic groups by appointment.