The ABC’s of Bridge Safety

By Paul Kivisto, P.E., Senior Structural Engineer, WSB

Did you know that the average life span of a bridge is 60 – 70 years? There are a number of ways to reach or exceed that average. Newer materials like high performance concrete and epoxy/stainless steel rebars certainly improve the quality and longevity of a bridge. Regular bridge inspections are not only legally required, but by inspecting and documenting issues with your bridges you can hopefully slow the expected deterioration and make repairs early which often costs less.

MnDOT, counties, cities, townships, and tribal governments are all responsible for managing bridge assets within their jurisdiction. Many of these entities have inspectors on staff to perform inspections, while others contract out that work to private firms like WSB. Maintaining regular and accurate inspection data is necessary to ensure public safety, reduce liability, maintain accurate budgeting, and comply with state laws and regulations.

Five important aspects of bridge inspections

About Documents, Frequency, and Inspection process

MnDOT releases the Bridge and Structure Inspection Program Manual (BSIPM) which summarizes inspection guidelines. In Minnesota, all bridges and culverts over 10’ long and carry traffic or are over roadways must be inspected. An initial inspection is required within 3 months of opening to traffic. The typical inspection frequency is 24 months, with shorter time periods for bridges in poor condition. The bridge owner can request longer than 24-month cycle for culverts. The latest version is May 2021 and can be viewed here:  

Bridge Condition Codes

BSIPM Chapter B – Bridge Inspection Field Manual and Chapter D – Recording and Coding Guide should be used by inspectors during all inspections. Physical condition and geometric properties of each bridge are recorded on inspection and inventory forms.

  • Overall condition codes from the National Bridge Inventory (NBI) range from 9 (new) to 0 (closed) and track the overall condition of the superstructure, substructure, deck, culvert, and other elements.
  • More detailed element level inspections record percentages of elements in conditions ranging from 1 (new) to 4 (severe deterioration).

The owner must report inspection and inventory data through the Structure Information Management System (SIMS) database to MnDOT every year.  MnDOT in turn provides the data to the Federal Highway Administration (FHWA).

Channel Inspections/Waterway/Scour

One of the leading causes of bridge failures is scour or undermining of the substructures in, or adjacent to, water. Scour is the term used to describe the erosion of soil surrounding a bridge’s foundation. When fast-moving water moves sediment from around the foundation, it creates scour holes adjacent to substructures that must be identified and monitored. Degradation of channels can cause shifts in the channel and may increase the risk of undermining. Bridges that cannot be inspected by wading and probing must be included in the statewide underwater inspection program on a 48-month cycle. Channel cross-sections are required for scour critical bridges and recommended for unknown foundations and scour susceptible bridges.

Additional causes of bridge deterioration to be aware of include:

  • Rebar and steel corrosion due to chlorides
  • Delamination and spalling on concrete members
  • Shrinkage cracking in concrete
  • Fatigue cracking on steel beams
  • Vehicle impacts to members
  • Leaking expansion joints
  • Bearings moved out of position
  • Timber decay

Drone Inspections

Increasingly, drones are used to provide access to hard-to-reach portions of bridges. They have photographic and video capabilities that allow them to identify cracking, spalling, and other deterioration that is otherwise challenging to document. Drones have the additional benefit of using Infrared (IR) cameras to identify delamination and distress below the surface.

Engaging with the Inspection Data

Agencies should use inspection data to help identify maintenance, preservation, improvement, and replacement projects. It is critical that accurate inspection data be maintained to track deterioration over time and prioritize maintenance work. Additional detailed inspections may be needed to help put together a system-wide scoping analysis or bridge management plan (BMP).  A thorough BMP can help identify funding needs over a given budgetary cycle.

Paul has more than 36 years of experience in bridge construction, bridge management, bridge inspection, and bridge design. In his role as Metro Region Bridge Construction Engineer for MnDOT’s Bridge Office, Paul was responsible for writing foundation recommendations, bridge preservation and improvement recommendations, constructability reviews, recommended repairs, and identified projects for inclusion in MnDOT’s bridge preservation program.

[email protected] | 612.201.9163

Sustainable Design

By Steven Foss
Feb. 6, 2015

Our environment – natural and built – is a complex network of components, creating unique and dynamic landscapes. Sustainable design focuses on maintaining and improving environments through a collaborative approach, considering how they fit within the greater ecosystem, and employing devices that are environmentally conscious and friendly. Sustainable design strategies typically include reducing carbon footprints; improving energy efficiency; and enhancing or protecting natural habitats while still providing economic, environmental, and social benefits.



Environmental benefits of sustainable design

The major goal of sustainable design is to preserve and improve our environment while reducing our carbon footprint and minimizing the use of natural resources. When sustainable design solutions are incorporated through project development, communities and the environment benefit through one or more of the following scenarios:

  • Protecting/conserving the ecosystem
  • Improved air and water quality
  • Reduced volumes of waste
  • Conserving natural resources

Social benefits of sustainable design

Implementation of sustainable design not only provides environmental benefits to our communities, but also improves our quality of life, health, and well-being. Improving the environment and integrating sustainable practices can have the following results on individuals and communities:

  • Improved active and passive spaces for social interaction and circulation
  • Improved emotional function
  • Reduced stress
  • Improved work effectiveness
  • Stronger sense of belonging and connection to the environment

Economic benefits of sustainable design

Incorporating sustainable design, through integrated design processes and innovative use of sustainable materials and equipment, can also generate economic benefits such as:

  • Reduced infrastructure needs
  • Lower annual costs for energy, water, and maintenance/repair
  • Reduced “heat island” effect
  • Improved ability to attract new employees/residents
  • Reduced time and cost for project permitting
  • Improved use of former sites (such as brownfields)
  • Reduced construction costs through reuse of construction materials
  • Increased property values


Sustainable design transforms conventional thinking about our landscape, infrastructure and buildings. It presents significant opportunities to improve our quality of life through environmental, social and economic benefits.

The following is a list of materials and tactics that can be incorporated into sustainable design practices:

  • Preserving existing tree cover and biodiversity
  • Vegetated swales/rain gardens
  • Dry and wet ponds
  • Green roofs
  • Underground storage and permeable pavement
  • Enhanced tree plantings (Silva Cells)
  • Infiltration devices
  • Alternative energy (wind, solar, biomass, geothermal, hydroelectric)
  • Conversion of mowed/maintained turf to low-maintenance native grasses
  • Stormwater capture and reuse for irrigation
  • Use of recycled construction materials


Wildlife Passage Bench

By Roxy Franta
February 1, 2016

A passage bench, or “critter crossing,” is a gravel walking path incorporated into the riprap along a bridge abutment to allow wildlife to pass under bridges uninterrupted. Many species of wildlife travel along the natural shoreline of streams, but modern infrastructure often breaks the continuity of the shoreline. Traditional riprap is not passable by many animals, forcing them to leave the shoreline and cross at busy bridge approaches. Wildlife on roadways presents safety hazards to both the animal and drivers, but including a passage bench into bridge design is an easy solution to this common problem in Minnesota.

(photo credit: Minnesota Department of Natural Resources)

Why include a passage bench?

The idea of a passage bench was developed by the Minnesota Department of Natural Resources (DNR), the U.S. Fish and Wildlife Service (USFWS), and the Minnesota Department of Transportation (MnDOT) in 2005. Continued observations prove this feature to be successful, providing benefits such as:

  • Movement of animals under the bridge, increasing road safety of bridge approaches
  • Safe footing for bridge inspectors and fishers
  • Access for maintenance
  • Flexibility in design for cross-section of normal channel and flood profile
  • Riprap design change from aggregate base to geotextile base
  • No extra cost or time to install
  • Wildlife conservation by minimizing deaths from collision

Designing a bench

In 2011, the passage bench became part of the MnDOT Standard Plan Set for use on all bridges. Specifications for passage bench design can also be found in the “Best Practices for meeting DNR General Permit 2004-0001, March 2006” guidelines.

The passage bench is a level trail from one side of the bridge to the other. It should be constructed of any size aggregate that is “walkable,” to allow wildlife proper footing as they pass under the bridge. The bench should be placed at or slightly above the adjacent bank elevation to create a continuous path that mimics the natural contours of the streambank and should be tied into the groundline outside the bridge area. A typical game trail width of three feet has proven to be successful for animal movement.

Common mistakes

The passage bench is a relatively simple addition to bridge construction, but there are some common mistakes to avoid during installation.

  • Having the riprap block the path. It’s important to incorporate the passage bench into both the grading and bridge plans to ensure it is completed and continuous across the entire area. Drainage outfalls and associated riprap along the abutment should be placed below the bench.
  • Too high or too low. The height of the passage bench often doesn’t match the natural streambank. A good indicator for the bench elevation is to place it at or just above the vegetation line.
  • Lack of guidance for wildlife. Though not a requirement in Minnesota, the right-of-way (ROW) fence should be used to guide wildlife to the passage bench. ROW fencing is often extended parallel to the roadway until it reaches the stream at the bridge. Instead, the fence should be turned up at the abutment and installed tight against the bridge corner. If it is a divided highway, a median fence should be installed as well.
  • Too skinny. MnDOT specifications suggest a minimum bench width of three feet. Benches that are too narrow are subject to longitudinal scouring during flood events and are not used as often by wildlife.
  • Construction phasing. Contractors often wait to install the passage bench until all other work is complete, but this makes it impossible to mechanically install it once the bridge beams are set in place. Installation should occur with riprap installation and should be a planned part of construction phasing.


MnDOT funded a study of the passage bench and collected data on wildlife movement and utilization in 2009. They concluded that a wide variety of species use the passage including black bear, red fox, gray fox, bobcat, whitetail deer, and even humans.

Roadway networks have caused fragmentation in the natural environment, but as a construction planner, design engineer, or contractor, you can help minimize this conflict between wildlife and highway operation with the inclusion of a passage bench.


Envision: The Age of Sustainable Infrastructure is Here

By Brandon Movall
Aug 1, 2016

With the state of America’s infrastructure declining due to climate change and limited funding, today’s engineers and scientists must adopt creative and sustainable solutions. In 2011, the American Society of Civil Engineers (ASCE), the American Council of Engineering Companies (ACEC), and the American Public Works Association (APWA) came together to revolutionize the way engineers plan, design and build. The result was Envision, a holistic rating system for sustainable infrastructure.

Envision is a rating system to help project teams incorporate higher levels of sustainability at each step of a project, from assessing costs and benefits over the project lifecycle to evaluating environmental benefits and using outcome-based objectives. Envision considers social, environmental, and economic factors of projects (a process called the Triple Bottom Line), rather than only focusing on economic factors. Envision uses a scorecard of 60 credits divided into five categories that reflect all aspects of the Triple Bottom Line:

  • Quality of Life
  • Leadership
  • Resource Allocation
  • Natural World
  • Climate and Risk

By tallying the credits achieved throughout the project lifecycle, Envision is able to effectively rate proposed infrastructure options in a way that is easy to communicate to clients, consultants and owners.

While there are many sustainability rating systems out there, there are a few things that make Envision the best option:

  1. Envision rates all types of civil infrastructure, such as transportation, water, energy, information, and landscape infrastructure.
  2. Envision covers the entire life cycle of a project, from the first meeting of the project team to post-construction maintenance.
  3. Envision is free to use. Anyone can sign up for an Envision account and have access to the guidance manual and scorecard. The only costs involved are if a project is registering for awards through Envision, or if you want to get special training and become an Envision Sustainability Professional (ENV SP). These are optional and are not necessary to use the Envision system on a project.

In addition to individual users, many companies and public agencies across the United States have implemented Envision into their planning, design and construction processes. Benefits to a company or agency include discounted ENV SP certification rates, discounted project award registration rates, exclusive content from the founding organizations, and more. As part of our commitment to bettering ourselves, our clients, and our world, WSB is proud to be recently certified as an Envision qualified company.

To change the world, we must change our practices. Envision is one large step toward planning, designing and building a sustainable future. For more information about Envision in general, visit For more information about Envision at WSB, please contact Katy Thompson, Brandon Movall, Stephanie Hatten, or Ann Wallenmeyer.


“2013 Report Card for America’s Infrastructure.” 2013 Report Card for Americas Infrastructure. ASCE, n.d. Web. 28 July 2016.

“Envision.” Institute For Sustainable Infrastructure. N.p., n.d. Web. 28 July 2016.

Bridges – An Overview

by WSB Municipal Engineering
Dec. 22, 2016

What legal responsibilities do bridge owners have?

Any municipality that owns a bridge in Minnesota must appoint a bridge program administrator. This administrator needs to be a professional engineer with a bridge background, as they are responsible for ensuring their bridges are inspected, load rated, and load posted (if required) according to state and federal law.

What does a bridge safety inspection involve?

A bridge safety inspection is an evaluation of the physical condition of a bridge. The inspection involves a visual and hands-on evaluation of all bridge components. The inspector looks for issues such as corrosion, deterioration, settlement, damage, or scour, and the results are detailed in a report based on each component. Following a bridge safety inspection, the overall condition of the bridge is compiled in an online database. Bridges are required by law to be inspected either annually or biannually, depending on the bridge type and condition. Special inspections such as an underwater inspection may also be necessary for bridges with components that are not visible during low water conditions.

How does a bridge owner know when it is time to replace a bridge?

The answer to this question varies based on the volume and type of traffic over the bridge. Bridges should always be replaced before the safety of the traveling public is at risk. Every bridge is assigned a sufficiency rating score, which varies from 0-100 and factors in the condition of the bridge, traffic volume importance of the route, and load carrying capacity. A bridge’s sufficiency rating is used to determine when it should be replaced and when it qualifies for funding. Bridges are also replaced when they are no longer able to meet traffic needs. Bridge owners can significantly extend the life of bridges by performing routine maintenance such as painting, cleaning, and crack sealing.

What is a bridge load rating?

A bridge load rating is a calculation that determines the safe load carrying capacity of a bridge. Bridge load ratings are based on the original capacity of the bridge while factoring in any deterioration or changes to the bridge’s condition that have occurred over time. A bridge load rating calculation is required when the bridge is first constructed and whenever the condition or configuration of the bridge has changed. The results determine if a bridge should be load posted and if it is safe for special permit vehicles to cross the bridge.


  • Load Rating: A calculation to determine the safe load carrying capacity of a bridge.
  • Load Posting: Restricting the weight of vehicles that cross a bridge in order to prevent overloading.
  • Sufficiency Rating Score: A numerical value on a scale of 0-100 that considers a bridge condition, traffic volume importance, and load carrying capacity.

Co-authored by Jay Kennedy and Diane Hankee.

The text of this article contains general information and is not intended as a substitute for specific recommendations. Your professional staff is more familiar with your community and can provide specific recommendations. Guidelines and regulations change and may be different from when this article was published. 

Integrated Design Approach

By Robert Slipka
Feb. 6, 2015

Integrated design brings together a diverse team of design professionals on one project. Projects benefit from this approach because a wider range of experts is contributing throughout the project as a team, rather than acting independently.

Early integration is crucial to reduce the potential for expensive conflicts as design progresses or implementation begins. The integrated design approach involves all parties, including design professionals, clients/owners, permitting agencies, and others. Involvement may also include cost analysis specialists, construction managers, and contractors.

No matter what that project type, an integrated approach helps ensure a holistic outcome rather than a culmination of interdependent elements. Below are two examples of what teams could look like.

Example 1

A site development project is led by a landscape architect or civil engineer with direct integration of specialists such as environmental scientists, ecological specialists, engineers, building architects, electrical engineers, irrigation designers, and the client (including their operations and maintenance staff).

Example 2

A roadway corridor project is led by a transportation engineer and/or a planner. The team for this type of project may integrate urban designers/landscape architects, engineers, environmental scientists, right-of-way specialists, and representatives from numerous government agencies.

Design charrettes and brainstorming sessions are often utilized heavily in the beginning phases of project planning and design. This helps the team identify key goals, strategies, and desired outcomes of the project while also establishing areas of conflict or design implications. Including a diverse range of professionals means a better likelihood of achieving creative solutions that might not be explored in a conventional, non-integrated approach. As the project develops into the construction documents phase, continued collaboration is required to ensure compatibility of spatial character, uses, spaces, materials, and other factors. This approach can also identify conflicts that might not otherwise be identified until late in design or into construction, avoiding unanticipated costs or redesign.

Although an integrated approach provides better results, it is important for consultants and clients to judge how extensively integration needs to occur based on costs and benefits. Some projects are smaller in scale or fee, which can make an elaborate integrated approach difficult to justify. Clients should also be aware that the term “one-stop shop,” often utilized to describe multi-disciplinary firms, does not necessarily mean that an integrated design approach is used for projects. If it is unclear or unproven, clients should ask the consultant to describe how the various team members will be integrated throughout the design process. The ultimate goal is to achieve higher quality projects with increased cost effectiveness to clients.