Michigan Bridge Inspections
Driving over bridges is as routine as getting in your car and driving to the store. Everywhere we drive there are bridges providing easy access over areas that would otherwise cause us to reroute and make our commutes to wherever we are going a hassle.
However, like any other man-made structure, bridges are subject to the constant stresses of traffic, weight, and weather. While working hard to safely support our daily lives, they are constantly fighting the war on age. Unfortunately, it took an event like the fatal bridge collapse over the Silver River in Ohio in the 1960s to remind us that bridges are susceptible to failure.
As a result of this failure, which caused 46 deaths, Congress established the Highway Act of 1968 which led to the establishment of the National Bridge Inspection Standards (NBIS). This federal law requires all bridges with a span of at least 20 feet to be routinely inspected at a maximum of 24 months. Since the inception of the bridge safety inspection program there have been a number of improvements and requirements added to the program, except they were after more failures of structures that caused fatalities.
“In 1986, the requirement to identify non-redundant steel bridges and perform hands-on inspections was implemented as a result of a fracture-critical steel girder failure that killed three people,” Rich Kathrens, P.E., Spicer Group Structural Project Manager said. “In 1987 ten people were fatally injured during an extreme flood event causing erosion (scour) along the bridge supports resulting in a bridge collapse.”
Again, the NBIS was enhanced to provide additional inspection requirements for bridges over water. And one of the more recent events to impact the bridge safety program was the sudden Minnesota bridge collapse in August of 2007 killing 13 and injuring 145 people.
“After this collapse, Congress required the Federal Highway Administration (FHWA) to significantly enhance their oversight responsibilities to ensure all States are completing the requirements of the NBIS and ensuring the safety of the traveling public,” Kathrens said. “FHWA has implemented a data-driven and in-depth review for every NBIS requirement as part of their annual review process, and this process is consistent with every State’s Bridge Inspection program across the nation.”
Steel Inspection with access provided by MDOT’s Reachall Crew.
In Michigan, there are 11,000-plus bridges that carry traffic and span greater than 20 feet. Generally, 60 percent of these bridges are locally owned and maintained, and the remaining 40 percent are owned by the Michigan Department of Transportation (MDOT). Any bridge falling into this category is required by Federal law to be inspected every two years.
Spicer Group’s structural engineers have been conducting bridge inspections to determine the structural integrity of bridges for decades. Safety inspections are simply an assessment of a bridge’s condition. The most common means of evaluating a bridge for safety is the completion of a routine inspection. A routine inspection as defined is “regularly scheduled inspection consisting of observations and/or measurements needed to determine the physical and functional condition of the bridge, to identify any changes from initial or previously recorded conditions, and to ensure that the structure continues to satisfy present service requirements.” A routine inspection is typically a visual inspection, and the first inspection is typically called the initial inspection and establishes a baseline for the structural conditions.
“The goal of a routine and initial inspection is to assess the physical and functional condition of a bridge, Kathrens says. “This inspection consists of rating the three major structural components of a bridge such as the deck, superstructure (girders), and substructures (piers and abutments).”
According to Kathrens, there are several safety and functional elements that are evaluated during the inspection and some of those include approaches, bridge railing, waterway characteristics, slope and channel protection and clearance requirements. The overall bridge’s condition is determined by National Bridge Inventory (NBI) ratings using a 0–9 scale.
The conditions encountered during a routine visual inspection may require additional investigation to determine the overall severity and quantity of the defects. An in-depth inspection is used to identify deficiencies to enhance the quality of the data collected during a routine inspection. Typically, routine inspections have little impact to the traveling public. However, when there are indications of deterioration located during the routine inspection, a detailed inspection will be recommended which can typically result in specialized access equipment and traffic control to allow the inspector access to complete the in-depth inspection. Should a bridge suffer from damage such as a high-load hit or an environmental cause such as a flood, an emergency inspection can be performed.
Spicer Group crew members sounding concrete checking for delaminations on bridges in Battle Creek along I-94.
Generally, routine inspections of concrete, steel, and timber members include limited cleaning to determine the extent of the defects such as section loss, cracking, or timber rot. For structures crossing waterways, data is collected to evaluate the stability of the channel and this includes stream bed cross sections and completing scour inspections using waders, boats and probing along the foundations. In-depth inspections include more intense cleaning, and hands-on inspection of the members to accurately quantify the extent of the defects. Section loss for steel members include the use of a ultrasonic thickness gauge to determine the amount of steel remaining. Potential cracks are evaluated using dye penetrate or mag particles for verification and to determine the crack limits.
The information collected during all types of inspections are recorded electronically in the field and judgment rendered as to the need for additional investigation such as a structural analysis of the given structure as a result of observed deficiencies. In some cases, the defects encountered result in temporary or permanent load restrictions for safe operation of the structure.
It’s important to note that Spicer Group is also using the latest in surveying technology to support bridge inspections especially those crossing relatively deep fast moving waterways.
“In addition to regular use of 3D river bottom scanning using multi-beam sonar, Spicer Group’s fleet of drones are beginning to be used to capture high-resolution imagery of hard-to-access bridge components, that otherwise would not be able to be inspected,” Eric Barden, P.S., Spicer Group’s Director of Survey and Geospatial Services said. “Bridge Safety Inspections play a critical role in ensuring the safety of the traveling public and in collecting accurate data for bridge owners to manage their structures.”
Bridge inspection requires all types of access, including waders, boats, and sometimes divers are necessary to perform underwater inspections.
Inspectors measure differential settlement at bridge approach railing.