Maintaining an Effective Bridge Inventory
ABCs of Bridges
According to the Federal Highway Administration’s (FHWA) Bridge Inspector’s Reference Manual, there are three basic types of bridges: arch-supported, beam-supported, and cable-supported bridges. Components common to each bridge type include:
A. Deck - Top layer that provides a smooth and safe riding surface to accommodate bridge traffic.
B. Superstructure - Supports deck loads across bridge span to the bridge supports.
C. Substructure - Transfers superstructure loads to the foundation soil or rock on piers, abutments, and footings.
As with all structures, bridges respond to the forces that act upon them, chiefly the forces of compression (inward or pushing force), tension (outward or pulling force), and shear (a transverse or sliding force in a member). Bridges must resist these forces caused by today’s demanding transportation system, which includes higher volumes of traffic, with heavier vehicle loads over a longer service life.
“The bridges we use today are experiencing a life cycle counter to that of our bodies. When we’re young, our bodies are designed to absorb the shocks and scrapes. As we age we tend to take longer to mend and recover; therefore, we take fewer risks and try to reduce the stresses to our bodies,” said Structural Engineer Jeff Johnson, PE, who has designed and performed inspection services for hundreds of bridges across the nation. “On the contrary, we’re putting more stress of increased traffic volumes and vehicle weights on our bridges as they age.”
Bridge Inspection
“Bridge owners today must now be prepared to educate their citizens about the condition of their bridge inventory in order to maintain public confidence in the safety of their structures. Providing citizens an overview of basic bridge terminology, bridge types, widely accepted inspection tasks, and rehabilitation and maintenance approaches can be helpful in increasing the public’s knowledge level.” Structural Engineer |
The FHWA requires an inspection every two years for bridges spanning 20 feet or more to evaluate the condition of the bridges, detect any deficiencies, and observe any variations from the time of the last inspection. Bridges that are older, have critical elements, or are in poor condition require more frequent inspection.
During a typical bridge inspection, the inspection team:
- Reviews previous reports and original design plans.
- Notes the deck condition and deviations from the original plans (surface widening, repairs, signage additions, etc.).
- Inspects substructure and superstructure.
- Notes scour or movement of the substructure due to settlement.
- Reviews bridge framing to verify all members are functional, in good condition, and free of defects or deterioration that would reduce their load-carrying capacity.
- Assigns condition codes to each element in a standardized report.
- Recommends and prioritizes courses of maintenance, rehabilitation, and replacement actions.
- Provides follow-up inspection of critical issues.
Preserving Bridges through Maintenance and Rehabilitation
“Although bridge inspection is important for ensuring safe public infrastructure, we need to take a more holistic view of our bridges,” Johnson said. “We need to look at larger transportation trends, projected demographics and growth objectives, and create bridge maintenance and rehabilitation programs to take a more proactive approach.”
Bridge maintenance and rehabilitation programs are integral to extending the service life of a structure. Removing debris from expansion joint systems to ensure that they will function properly, re-painting steel components of a structure that are exposed to de-icing salts to reduce corrosion potential, and maintaining a functioning sacrificial wearing surface for the bridge deck (ex. concrete or an asphalt overlay with a waterproofing membrane over a corrugated metal deck) to protect the structural deck are examples of effective maintenance activities.
Rehabilitation work generally stems from observations made during the bridge inspection. Rehabilitation can be relatively minor in nature, such as expansion joint replacement, deck patching, and railing improvements. Major rehabilitation activities, including deck replacement, girder repairs, and major substructure repair, can be necessary in order to realize the full service life of the structure.
By gauging traffic patterns and projected development, a city or county may be able to better prioritize their infrastructure funding and plan for their objectives. Using planning, transportation engineering, asset management, and funding resource services can help accomplish this level of understanding.
Funding and scheduling these maintenance and rehabilitation programs may be a challenge due to competing priorities and diminishing funding support; however, they ultimately provide today’s bridge owners a critical means to maintain an effective bridge inventory.
For more information about the National Bridge Inspection Program, visit the Federal Highway Administration's website at www.fhwa.gov/bridge, or contact SEH's Structural Engineers. SEH is also available to discuss rehabilitation option for your bridge inventory.
Minnesota, Nebraska, South Dakota - Jeff Johnson, PE, 800.325.2055
Illinois, Indiana, Wisconsin - Chris Blum, PE, 800.732.4362
Colorado, Wyoming - Mark Hildahl, PE, 800.490.4966
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