The Federal Highway Administration (FHWA) reports that 7% of the nation’s 623,218 bridges are “structurally deficient.” Additionally, the 2021 Report Card of the American Society of Civil Engineers (ASCE) found that 42% of the 617,000 bridges it tracks in the U.S. are more than 50 years old. According to the FHWA, the average lifespan of a bridge is 42 years, meaning many of the country’s major highway bridges are over or approaching the end of their lifespan.
Bridge inspections ensure public safety and are a matter of law. The National Bridge Inspection Standards (NBIS) were established by the FHWA in 1971 to protect the public and prevent structural and functional failures. NBIS requires bridge inspections every 24 months. However, some bridges are inspected more frequently, and newer bridges are inspected less frequently. Privately owned bridges and some international crossings are not required to adhere to NBIS requirements, including those on public roads.
Except for federally owned bridges, the state’s Department of Transportations (State DOTs) are responsible for bridge inspections, but state DOTs may hire third-party engineers to conduct inspections. Ultimately, bridge inspections and their subsequent repairs are tied to federal funding. States simply don’t have the funds to make necessary repairs without federal assistance. The 2021 report card shows that $125 billion is needed to address the backlog of bridge repair.
Types of Engineers Involved in Bridge Inspections
Inspections are conducted to look for any design or construction flaws, stress, fatigue, excessive wear or other potential in-service problems that require maintenance. The NBIS sets minimum standards for bridge inspections, but states, federal agencies, and tribal governments can adopt more stringent guidelines. Bridge Inspectors are certified professionals who meet at least the minimum standards set by the NBIS or the minimum standards in the state where they work. They conduct the inspections and document the findings.
Structural engineers and civil engineers are uniquely qualified to become bridge inspectors based on their education and training; however, they still must meet the standards for qualification and training according to the NBIS or state regulations.
Structural engineers have experience in designing structures for stability and safety. They work with other engineers to design all types of structures, including bridges. Civil engineers have a broader scope of practice that includes infrastructure, such as bridges, tunnels, freeway interchanges, and other traffic-related structures. Adding bridge inspection to their repertoire enables structural and civil engineers to expand their practice or change their focus entirely.
Frequency and Type of Bridge Inspections
NBIS requires bridge inspections every 24 months for highway bridges on public roads that exceed 20 feet in total length. With express approval by the FHWA, some bridges can be inspected at greater intervals, such as newly constructed bridges. Underwater components are inspected every 60 months, or five years. Some bridges are inspected more frequently, especially when damage has been identified.
There are five types of bridge inspections: initial, routine, in-depth, damage, and special.
- The initial inspection provides the structural inventory and appraisal data, establishes baseline structural conditions, and identifies and lists any existing problems and their locations.
- Routine inspections are the most common and are required every two years, with a few exceptions. These inspections determine the physical and functional condition of a bridge.
- In-depth inspections are close-up, hands-on inspections to identify problems not found or discoverable during routine inspections. In-depth inspections often relate to bridges with components that are underwater.
- Damage inspections are emergency inspections following an accident or resulting from an environmental disaster like a flood, tornado, or fire.
- Special inspections are conducted on a regular basis to monitor known or suspected deficiencies.
The Bridge Inspection Process
Bridge inspections pose some unique challenges, the first of which is traffic. Bridge inspections often require special equipment, such as snooper trucks which allow for under-bridge inspections. Finally, bridges with underwater components require engineers who are also certified as underwater divers. The costs associated with bridge inspections is another consideration.
The methods used during inspections depend on the type of inspection and suspected or identified damage. All inspections include a visual inspection. What other inspection methods are used is determined based on the type, location, accessibility, and condition of the bridge.
Here’s a closer look at several bridge inspection methods mandated by the NBIS.
- Visual Inspection. This is the primary method used during routine inspections. Visual inspections include cleaning, probing, sounding, and measuring. Inspectors look for visible signs of wear and damage, such as noticeable cracks, rust, and misalignment.
- Destructive Testing. In some cases, destructive testing is performed in specific areas or materials identified during the visual inspection. Destructive testing is also used to identify areas where rehabilitation work is needed.
- Non-Destructive Testing (NDT) or Non-Destructive Evaluation (NDE). These technologies are used to assess areas that can’t be seen during in-depth damage, and special inspections. NDT includes state of the art techniques such as impact echo, infrared thermography, ground penetrating radar, and strain gauges for concrete structures and elements. Ultrasonic, eddy currents, acoustic emissions, strain gauges, and x-ray technologies are used to test steel structural components.
- Sounding, Cleaning, and Scraping. These methods are used during routine inspections to evaluate areas of delamination or concrete degradation. Steel members are tested using cleaning and scraping and dye penetrant and magnetic particle testing to identify cracks and significant erosion.
- Load Testing. This simulates stress on the bridge to ensure it can handle expected traffic loads. Load testing identifies deformations and strains under a given load condition.
Key Standards and Regulations
In the US, bridge inspections fall under state and federal guidelines. Most inspections are guided by the NBIS; however, some states have additional standards. The most recent updates to inspections standards were made in 2022 and concern technological advances, research, and inspection program administration. For example, drones (unmanned aircraft systems) can be used in bridge inspections but cannot replace visual or physical testing methods.
National Bridge Inspection Standards
The federal NBIS resulted from a bridge collapse in Ohio that killed 46 people in 1967. The accident shed light on the fact that the number of US highway bridges was unknown as well as the condition of those bridges. The NBIS set standards for the proper safety inspection and established a training program for inspectors. Additionally, the NBIS requires that all information collected on bridge composition and conditions be reported to the FHWA. The data are maintained in the National Bridge Inventory (NBI) database. The NBI database provides information for the appropriation of federal funding.
In addition to these procedural components of the NBIS, the NBIS sets forth the types of inspections, frequency of inspections, and inspection procedures.
State and Local Regulations
Most of the bridges are owned by state and local governments. Even so, they must meet the minimum NBIS inspection guidelines. However, many states have requirements above those of the NBIS for bridge inspections. These can include:
- Expanded definitions to include shorter bridges.
- Increased frequency such as annual inspections or even shorter timeframes.
- Varied inspection types to include in-depth inspections more frequently.
Safety Guidelines and Risk Assessment
A risk-based inspection determines the risk profile, implementation of effective controls, and the balance of residual risks. In essence, the inspection considers the overall risks and weighs those against the costs of repairs to prioritize maintenance and/or bridge replacement. Risk-based assessments help queue projects when limited resources, such as funding, are unavailable.
Challenges in Bridge Inspection
The biggest challenges facing the bridge inspection industry are aging infrastructure, advances in technology, and funding. Aging infrastructure requires more frequent inspections with already limited funding. New technologies potentially help mitigate some of the funding issues but are not fully embraced by national inspection standards—the NBIS.
- Aging Infrastructure. As of June 15, 2024, almost half of all bridges were reported in “fair” condition. The 2021 Report Card of the ASCE reported an increase in bridge structural deficiencies and a decrease in bridge improvements. This increases the need for more frequent and in-depth or special inspections.
- Technological Advancements: The NBIS approved the use of proven technologies to supplement bridge inspections. However, technology cannot replace visual or physical types of inspections. New technologies can only be used to assess bridge components where their use improves efficiency of the inspection without sacrificing thoroughness and effectiveness.
- Budget and Resource Constraints: Depending on the complexity of the bridge, inspections can be very expensive. For example, heavily traveled bridges require traffic mitigation or control, some inspections require renting specialized equipment, and underwater inspections require advanced training in underwater diving and inspecting. These all add to the cost of the inspection and limited funds available to pay for inspections.
- Navigational Traffic and Terrorism: Recent accidents like the Francis Scott Key Bridge collapse and escalation of political tensions with the middle east and other adversaries of the United States have provided additional support for more frequent bridge inspections. These inspections have need to be expanded to include approach protections to single failure bridge designs. Single failure bridge designs imply that one structural component failure can cause partial or total bridge collapses.
Ready to Jump Into Bridge Inspections?
Public safety in one of the primary responsibilities of the federal government in the U.S. and that includes the safety of our bridges. To maintain a safe crossing requires regular and consistent bridge inspections to assess structural damage, general wear and tear, and prioritizes repairs and replacements. Structural and civil engineers play a vital role in ensuring the safety and longevity of bridges. These engineers have specific qualifications due to their training and expertise in structure and infrastructure components of bridge construction.
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