Inspection of Bridge Components: An Introduction
With the recent launch of the National Bridge Inspection Standards (NBIS), 2022, there have been updates to the administration of inspection programs for highway bridge components and elements across the United States.
While outlining the updates, this blog post introduces you to the inspection and evaluation methods for the three major components of a bridge. Let us take a closer look.
What are bridge components?
Put simply, bridge components are the main parts of a bridge. Most bridges can be primarily divided into the following three parts or components:
The bridge component on which the live traffic load is directly applied is called the deck. A deck should provide a smooth surface for pedestrians or vehicles using the bridge.
On some bridges, the deck transfers the live load directly to the superstructure. However, on other bridges, such as concrete slab ones, both the deck and the superstructure may distribute the live load to the substructure.
Additionally, a deck may be composite or non-composite. Composite decks are integral to the superstructure and contribute to the latter’s structural capacity, while non-composite decks are the exact opposite.
Based on the materials used, there are generally three types of decks:
Steel decks: Steel grids or solid steel plates are used to compose steel decks. Some common types of steel decks are corrugated steel flooring and orthotropic steel decks.
Timber decks: Timber flooring are of several types, including plank decks, stressed-laminated decks, nailed-laminated decks, and structural composite lumber decks.
Concrete decks: Concrete being weak in tension, it’s used along with steel reinforcement (often in the form of steel-deformed reinforcing bars or rebars) when manufacturing a deck to avoid tensile stresses.
Common types of concrete decks include precast prestressed decks, precast conventionally reinforced decks, and precast prestressed deck panels that have cast-in-place toppings.
Fiber-Reinforced Polymer (FRP) decks: Despite being more expensive than concrete, FRP material is used for bridge decks because it’s easy to install and is more resistant to earthquakes. Its weight is lighter, too, thereby enabling transport and traffic to move efficiently.
The bridge component that supports the deck by transmitting loads from the deck across each span to the substructure is called the superstructure. It may carry loads through compression, bending, tension, or a mixture of all these three methods.
According to the Specifications for the National Bridge Inventory (SNBI), 2022, the superstructure includes:
Members above the spring line for arch bridges
Beams or girders for integral superstructures
Members above the bearings for bridges with non-integral superstructures and substructures
Slabs of concrete rigid frame bridges
Girders, legs, and knees for steel and concrete rigid Delta-Frame or K-Frame bridges
Generally, the different types of superstructure are:
Movable and floating bridges
Single web beam or girder bridges
Box beams or multi-girders
The superstructure’s way of transmitting the load to the substructure differs based on the type of the superstructure. For example, loads from the superstructure are carried vertically to the substructure in slab bridges and single web beam and girder bridges.
However, in the case of arch bridges, such loads are transmitted diagonally to the substructure.
The component of a bridge that includes all the elements supporting the superstructure is called the substructure of the bridge. It transmits loads from the superstructure to the foundation rock or soil.
The SNBI states that the substructure includes:
Members below the beams or girders for integral superstructures
Members below the bearings for bridges with non-integral superstructures and substructures
Members like thrust blocks below the spring line for arch bridges
Legs of concrete rigid frame bridges
Foundation piles exposed by scour or erosion
Basically, there are two categories of substructures:
Abutments: These retain the approach slabs or embankments and provide support for the ends of the superstructure. Some common types of abutments include spill-through abutments, shelf abutments, and cantilever abutments.
Piers and bents: These provide support at intermediate points for the superstructure along the spans of the bridge. Piers may be column, cantilever, or solid shaft ones, while bents may be either pile or column bents.
Source: Minnesota DOT
Performing a component-based bridge inspection
Before moving on to the various inspection methods for bridge components, let’s examine the possible deficiencies and their respective locations on the three major components of a bridge.
Depending on the component to be inspected and the type of bridge inspection, inspectors may need to use only one method or all inspection procedures. These are:
Visual or hands-on
The inspection of bridge components and members is, for the most part, done visually. Visual inspection is used to detect most of the obvious defects and deficiencies in bridges for the first time. A routine visual inspection typically entails reviewing past inspection reports and visually assessing the bridge components.
To identify defects that aren’t readily detectable using routine inspection methods, a hands-on inspection may be necessary. Hands-on inspection requires inspectors to be close enough to touch the areas of the bridge being inspected.
Upon identifying the defects visually, bridge inspectors may need to use certain tools and equipment to thoroughly determine the type, size, location, and extent of a suspected defect. They should carefully measure and record each deficiency they find when performing a physical inspection.
For example, inspectors may be required to remove paint using a wire brush, sand blasting, or grinding to reveal any suspected defects like cracks. However, excessive grinding, brushing, or hammering may make it difficult to find surface cracks by closing these up.
Several advanced destructive and non-destructive methods are available for inspecting decks, superstructures, and substructures. Such advanced methods include:-
Sonic and ultrasonic testing: Timber decay, cracks, discontinuities, sub-surface damage
Vibration: Timber deterioration
Field ohmmeter: Timber decay
Ground-penetrating radar (GPR): Location of fractures and subsurface cavities in bedrock, delaminations, contaminants
Probing: Timber decay
Computed tomography: Interior deficiencies of steel bridge members
Acoustic emission testing: Fatigue cracks in steel members, plastic deformation, weld discontinuities, corrosion
Drilling or boring: Internal timber decay
Infrared thermography: Subsurface delaminations
Dye penetrant test (PT): Surface flaws in steel members
What are bridge component condition ratings?
Component condition ratings are a set of codes indicating the current field conditions of the bridge components.
When performing a bridge inspection, inspectors need to evaluate the condition of each component and its respective elements. According to the National Bridge Inventory (NBI), they assign an overall Federal Structure Inventory and Appraisal (SI&A) condition rating to the following major bridge components:
Deck: Item No. 58
Superstructure: Item No. 59
Substructure: Item No. 60
The new SNBI lays down the following bridge inspection condition rating codes for bridge component items:
Each component condition rating code considers the type, location, and extent of the defects on the component being evaluated. Such defects are problems with the bridge components, possibly caused by damage, deterioration, or inherent defects.
For example, let us say you’re inspecting a reinforced steel (RC) bridge deck about 250’ long and 30’ wide, where you’ve noted the following defects:
Location 1: Full width transverse cracks, 0.008” wide, spaced at 3’ to 5’. ~100 sq. ft. total area.
Location 2: Spalls up to 22”x18” and >1” deep with exposed rebar (no section loss) and patched areas that are unsound. ~40 sq. ft. total area.
According to this evaluation, the inspectors would need to prepare a summary of their findings for their bridge condition report as below:
As you can see, the deck is subject to isolated moderate defects that can be characterized as “some moderate defects”. Therefore, the deck would be assigned a component condition rating of ‘5’ (refer to Table 2).
It’s also important to note that condition ratings of ‘3’ or less for the superstructure or the substructure are classified as critical findings.
Making the inspection of bridge components more convenient
In order to function well, bridge components and elements need to meet tough standards. It’s vital to closely monitor and measure the deterioration, wear, and other types of defects and deficiencies the components may be subjected to.
That’s why inspectors and inventory managers need the best commercial bridge inspection software to guide their operations.
inspectX, for instance, lets you switch from the conventional pen-and-paper mode of inspecting bridges to using tablets.
On the tablet, you can collect field data offline. For example, taking photos of components and associating those photos with the respective NBI items or defects are possible in inspectX, irrespective of the presence of Internet connectivity.
What’s more, it allows you to collect and record field data according to the recently released SNBI coding guide. As a result, using inspectX, you can perform component-based bridge inspection more efficiently.