Section 5: In-Depth InspectionsAnchor: #i1002437
Reasons for In-Depth Inspections
In-Depth Inspections are usually performed as a follow-up inspection to an Initial, Routine, or Event-Driven Inspection to better identify any deficiencies found.
Underwater Inspections and Fracture-Critical Inspections are both types of In-Depth Inspection. These are described in more detail below.
Load testing may also sometimes be performed as part of an In-Depth Inspection. However, load testing for determining bridge load capacity is costly and its results open to interpretation.Anchor: #i1002457
Underwater Inspections are a type of In-Depth Inspection. Perform these at least every sixty months or more frequently if conditions warrant. Perform an Underwater Inspection on structures where the submerged portions of the structure have a history of water depths of at least four feet year round or where the submerged elements are in less than four feet of water, but wading would be unsafe due to channel bottom conditions, high current or localized scour.
A majority of the bridge structures in the United States, some 86 percent in the National Bridge Inventory, are over some type of waterway. The lower elements and foundations of many of these bridges are permanently inundated, so there is no opportunity to view the effects of scour or damage to the structure. To determine the condition of these bridge foundation elements and ensure the safety of the traveling public, perform Underwater Inspections on a regular basis. As a result of several bridge collapses during the 1980s, the National Bridge Inspection Standards3 (NBIS) were revised to require a master list of bridges requiring Underwater Inspections, and to establish an inspection frequency not to exceed five years.
The master list of Underwater Inspections is reviewed and updated during Routine Inspections. Once a bridge is added to the master list, it remains there until it is no longer in use. Some bridges must be inspected at intervals more frequent than the required sixty months due to the susceptibility to scour or other factors such as the age of the bridge, configuration of the substructure, environment, adjacent features, or existing damage. The frequency, type, and level of inspection are left up to the owner of the structure.Anchor: #i1002482
Underwater Inspection Methods
There are currently three methods used to conduct Underwater Inspections. These are:
- Wading -- The most basic of the three methods, wading requires only a probing rod and wading boots to be effective. This is performed during Routine Inspections.
- Scuba diving -- A method that allows a more detailed examination of substructure conditions at the mudline. The diver has freedom of movement and may carry a variety of small tools with which to probe or measure.
- Hardhat diving -- Involves the use of sophisticated diving equipment and a surface supplied air system. This inspection method is well suited when adverse conditions will be encountered, such as high water velocity, pollution, and unusual depth or duration requirements.
The choice of which method to employ depends largely on accessibility and the required inspection detail.Anchor: #i1002512
Levels of Underwater Inspection
Standard levels of inspection originated in the U.S. Navy. Three levels have been established as the result of the process through time.
- Level I -- A simple visual or tactile (by feel) inspection, without the aid of tools or measuring devices. It is usually employed to gain an overview of the structure and will precede or verify the need for a more detailed Level II or III inspection.
- Level II -- A detailed inspection which involves physically cleaning or removing growth from portions of the structure. In this way, hidden damage may be detected and assessed for severity. This level is usually performed on at least a portion of a structure, supplementing a Level I.
- Level III -- A highly detailed inspection of a structure which is warranted if extensive repair or replacement is being considered. This level requires extensive cleaning, detailed measurements, and testing techniques that may be destructive or non-destructive in nature.
Underwater Structural Elements
The elements of a bridge structure that may be located below the water line are abutments, bents, piers, and protection systems. Bents are distinguished from piers in that they carry the loads directly to the foundation rather than using a footing.
Abutments normally do not require an Underwater Inspection, but in rare instances may be continuously submerged. Although usually founded on piles or drilled shafts, abutments occasionally rest on spread footings in rock. Scour is almost always the primary consideration when an underwater abutment inspection is being conducted. Local scour is often detectable during diving inspections, although sediment will eventually refill a scour hole between the events that cause the scour. More general scour, or channel degradation, will usually be undetectable to the diver and must be determined from known channel cross sections or historical data.Anchor: #i1002552
Underwater Inspection Devices
Divers may use several types of sounding or sensing devices in underwater investigations. The most common device is the black-and-white fathometer. It uses sound waves reflected from the channel bottom and records the depths continuously. It provides an inexpensive, effective means of recording channel depths, but does not detect a refilled scour hole. Another device is the color fathometer. It uses different colors to record different densities and in this way often detects scour refill. Other devices include ground penetrating radar, which works well for shallow water but has limited usefulness in murky water, and fixed instrumentation, which is reliable but requires periodic monitoring and resetting to be effective.Anchor: #i1002562
Underwater Structural Materials
Piers and bents, if located in a navigable waterway area, are often subject to material defects, collision damage and scour. Concrete is the most common type of material encountered in Underwater Inspections, followed by timber, steel, and masonry. Common defects in concrete substructures include cracking, spalling, laitance, and honeycombing. Minor or even moderate damage to concrete can be tolerated if it does not endanger the reinforcing. Corrosion of the reinforcing can lead to serious difficulties.
Timber has frequently been used for piles, especially in fenders or protection systems. The most common type of damage to timber members is from biological organisms, such as fungus, insects, and marine borers. In order to control infestations, timber is usually treated to poison the wood to block a food source for organisms. In time the treatment may leach out of the wood or the treatment layer may be penetrated. Pay particular attention to the area of the waterline and the vicinity of connectors where this type of damage may occur.
Steel substructures are very susceptible to corrosion near the waterline or between the high and low water levels. In this area, oxygen and frequent wet/dry cycles promote deterioration at an accelerated rate. Measure steel to determine the possibility of section loss.
Masonry substructures are rare and if present, are almost always on very old bridges. These elements are subject to damage similar to concrete, in addition to the loss of joint mortar and individual pieces.Anchor: #i1002589
Fracture-Critical Inspections are a type of Special Inspection. These inspections are usually limited to non-redundant load path tensile stress areas. Perform Fracture Critical Inspections every twenty-four months. Perform them more frequently if conditions warrant. A Fracture Critical Inspection is a hands-on (within arms length of the component) inspection of a fracture critical member or member components. It may include visual and other nondestructive evaluation. Methods of Non-Destructive Evaluation (NDE) of steel members may include dye penetrant, magnetic particle, or ultrasonic techniques.Anchor: #i1002599
History of Fracture Critical Considerations
Early development of modern steel design focused on stress and strain. Little was known or recognized about the potential adverse effects of multiple stress cycles. Early materials such as wrought iron were not capable of great unit strength. Early designs lacked the sophistication that would allow a designer to closely address details. Even after the introduction of electric arc welding in the 1880s, most steel bridges were simple-span, composed of built-up and riveted members.
Design of continuous beam highway bridges began after welding technology was improved. Some of the first welded steel beam bridges were in Texas. The use of continuity resulted in more flexible structures that were more subject to deflections and rotations. The use of welding, particularly in Texas, resulted in simpler bridges and more consistent construction quality.
As steel production and availability improved, along with higher strength steels, design engineers were quick to accept the obvious benefits. However, no material is perfectly homogenous, and the fact that steel could have hidden flaws was essentially ignored by designers. After World War II, there was a massive expansion of highway bridge construction. The popularity of personal motor vehicles increased as a result of more highways and thus more roadways and bridges were needed. The construction material of choice was initially steel throughout much of the country. However, Texas designed many smaller structures with concrete, which is still serving well in many locations. Steel bridges, particularly trusses, were used for longer crossings, usually for streams and rivers.Anchor: #i1002619
A number of steel structures failed in the 1950s and 1960s due to various causes, but the failure of the Silver Bridge at Point Pleasant in West Virginia in 1967 got national attention. This truss collapsed suddenly due to the brittle fracture of an eyebar link, resulting in the loss of 46 lives and closure of a major route. As a result of this terrible tragedy, the National Bridge Inspection Standards4 (NBIS) were developed as part of the Federal-Aid Highway Act5 of 1968. In addition, significant additional research efforts were initiated in fracture mechanics. As a result, the effects of multiple stresses at less than yield of the materials were understood more thoroughly.
Redundant and non-redundant members were first recognized in the twelfth edition of the AASHTO Bridge Specifications6 in 1977. The first guide specifications for fracture critical bridge members was issued by AASHTO7 in 1978.Anchor: #i1002652
After design engineers began to recognize the problems associated with multiple stresses at less than allowable values, further information was developed to assist in the design process and in evaluation of existing structures. After notable failures, it was recognized that many existing bridges may be nearing failure due to fatigue. Fracture-Critical (FC) members were recognized and defined as a steel member or component whose failure in tension would result in the total or partial collapse of a bridge. These are commonly referred to as non-redundant members. Methods were developed to help determine which structures must be further evaluated by designers for susceptibility to fatigue problems. Designers began to include Fracture Control Plans in bridge design details.
The most common types of FC members are tension flanges and parts of webs of flexural members such as beams and girders. Tension members of trusses, particularly eyebars, which commonly make up the lower chords of old trusses, can also be FC. Other tension members of trusses, such as diagonals, are also FC. Concrete members are not often used in tension. The design of flexural concrete members with multiple reinforcing bars precludes the possibility of abrupt failure due to their internal redundancy.
The following circumstances determine FC members:
- Define all two-girder bridges as FC. Fracture of lower flanges in positive moment areas (mid spans) and upper flanges in negative moment areas (over supports) can be expected to lead to collapse of the structure. However, cracks over interior supports may lead to subsequent higher positive stresses in the spans with no catastrophic collapse. Therefore, these FC components receive more frequent periodic In-Depth Inspections.
- Define the majority of steel caps FC. The exceptions are those where support columns provide load path redundancy.
- Define a floorbeam as FC if one or more of the following conditions exist: Flexible or hinged connection to support girders, or floorbeam spacing greater than 14 feet, or no stringers connected to the floorbeams supporting the deck, and stringers not continuous over floorbeams.
- Define lower chords of trusses as FC. This determination is based on the fact that most truss bridges employ only two trusses and most are simple-span.
- Do not define secondary members such as diaphragms and stiffeners as FC. They are rarely used in a manner where failure would lead to a structure collapse. However, use caution in evaluating certain truss members that may appear to be secondary when, in fact, their attachment to main FC members can provide a starting place for the main member failure. The only exceptions to this are diaphragms used in horizontally curved fracture critical units. These elements are almost always classified as primary members due to the forces they are carrying and are also considered to be FC.
- The tied arch is a variation of the through arch with one significant difference. In a through arch, the horizontal thrust of the arch reactions is transferred to large rock, masonry, or concrete foundations. A tied arch transfers the horizontal reactions through a horizontal tie which connects the ends of the arch together, like the string on an archer's bow. As can be imagined, the tie is a tension member. If the string of a bow is cut, the bow will spring open. Similarly, if the arch tie fails, the arch will lose its compression and will collapse. The tie girder is FC.
In order for a bridge to be classified as fracture critical, it must have an element that if failed would cause total or partial collapse of the bridge. With this in mind, it is crucial to recognize and identify the type redundancy present in a bridge. Redundancy allows the load that was previously carried by the failed member to be redistributed to other members, thus avoiding failure or collapse.
There are three basic types of redundancy present in bridges:
- Load path redundancy
- Structural redundancy
- Internal Redundancy
Load Path Redundancy
Bridges with three or more main load-carrying members or load paths are considered load path redundant. If one member were to fail, load has a better chance of being safely redistributed to the other members, and bridge failure may not occur. An example of load path redundancy is a multi-girder bridge. Definitive determination of load path redundancy requires structural analysis with members eliminated in turn to determine resulting stresses in the remaining members. In extreme cases where girder spacing exceeds fifteen feet a three girder bridge will also be classified as fracture critical.
Bridges which provide continuity of load path from span to span are referred to as structurally redundant. Bridges where girders are continuous across internal span two-girder bridge designs are structurally redundant. In the event of a member failure, loading from that span can be redistributed to the adjacent spans, and bridge failure may not occur. The degree of structural redundancy can be determined through computer programs which model element failure. Some truss bridges have structural redundancy, but this can only be determined through analysis.
Internal redundancy exists when a bridge member contains three or more elements that are mechanically fastened together so that multiple independent load paths are formed. Failure of one member element would not cause total failure of the member. Internal redundancy of a member can be decreased or eliminated by repairs that involve welding. The welds provide paths for cracks to travel from one element to another.
Presently TxDOT only considers load path redundancy for the classification of fracture critical members.Anchor: #i1002712
Inspection Procedures for Fracture Critical Members
Inspection procedures begin with proper advance planning. Important planning aspects, usually based on an office review of the structural plans, include:
- Identify possible FC members.
- Note the particular members in the structure that may require special field attention, such as built-up tension members composed of few individual pieces.
- Pre-plan necessary access to the members, including special equipment needs such as a snooper truck, ladders, bucket truck, air monitoring device, or climbing gear.
- Many structures designed for urban situations with necessary complex alignment geometries result in FC members. Proper inspection of these bridges may require closing a traffic lane and require a night time inspection due to high ADT, during normal business hours. Coordinate safe traffic control in advance with the local district and Area Engineer offices and their Safety Review Team.
- Use a railroad flagger coordinated with the proper railroad company if the structure crosses and is within 25 ft. proximity of a railroad track. Every individual entering a railroad right-of-way must is required to complete, and have certification of completion of the on-line Safety Awareness Course. This course can be found at www.contractororientation.com.
- Identify and make available any necessary special tools and equipment that may be required in addition to the normal inspection gear. A high-pressure washer is often useful in cleaning areas where a large accumulation of debris might obscure view of FC areas. Non-destructive test equipment such as ultra-sonic or ultra-sound devices may be advantageous in some areas, particularly inspection of box-type bent caps and pin-and-hanger connections.
The actual field inspection of all FC members consists of several steps. The most important step is a visual inspection. The inspector notes any:
- Visual cracks and their direction and location
- Evidence of rust, which may form at a working crack
- Weld terminations in a tension area
- Interrupted back-up bars used for built-up-member fabrication
- Arc strikes, scars from assembly cables or chains, or other physical damage
- Cross-section changes which may cause a sudden increase in the stress pattern
Fatigue and Fatigue Fracture
Members subjected to continued reversal of stress, or repeated loading such that a range of change in stress occurs, are subject to a behavior called fatigue. Members that have a relatively constant, steady stress are not subject to fatigue. The term has been in use for almost a century and is currently defined by the American Society of Testing Materials (ASTM 1823-96e1) as "the process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations." Fatigue can result in:
- Loss of strength
- Loss of ductility
- Reduced service life
Fatigue fractures are the most difficult to predict since conditions producing them are often not clearly recognizable. Fatigue occurs at stress levels well within the elastic range, that is, less than the yield point of the steel, and is greatly influenced by minor imperfections in the structural material and by fabrication techniques.
Fatigue fracture occurs in three distinct stages:
- Local changes in atomic structure, accompanied by sub-microscopic cracking
- Crack growth
- Sudden fracture
Fatigue fracture almost always begins at a visible discontinuity, which acts as a stress-raiser. Typical examples are:
- Design details such as holes, notches, or section changes
- Flaws in the material such as inclusions or fabrication cracks
- Poor welding procedures such as arc strikes, gouges and start-stops
- Weld terminations
Certain structural details have been long recognized as stress-raisers and are classified as to their potential for damage. These details appear in the current AASHTO Bridge Specifications8, and other technical publications. Ensure that these common details should be familiar to the fracture critical bridge inspector.
Proper consideration of member detail and sizing during design will help control stress level and, thus, control crack growth. The stress range, or algebraic difference in the maximum and minimum stress, also is important. The most effective way to control cracking and eventual fracture is sensible detailing. Details such as out-of-plane bending in girder webs and certain weld configurations can cause crack propagation and fracture.
Design for fatigue also includes observing a Fracture Control Plan. The Fracture Control Plan identifies the person responsible for assigning fracture-critical designations. It establishes minimum qualification standards for welding personnel and fabrication plants. It also sets forth material toughness and testing procedures. The specific members and affected sections are also identified in the plan. During fabrication, these members are subject to special requirements.
A fatigue failure is classified as a brittle fracture and is always an abrupt fracture. A brittle fracture is distinguished from a ductile fracture by absence of plastic deformation and by the direction of failure plane, which occurs normal to the direction of applied stress. Other failure surfaces due to high stress are usually at an angle to the direction of the stress and are often accompanied by a narrowing or necking of the material. Brittle fracture failures have no narrowing or necking present to indicate potential failure.
The three main contributing factors to brittle fracture are:
- Stress level
- Crack size
- Material toughness, sometimes called fracture toughness
Small, even microscopic cracks can form as a result of various manufacturing and fabrication processes. Rate of propagation, or growth, of cracks also depends on the stress level and the material toughness. Material toughness is the ability of a material to resist when stressed or the ability to absorb energy and plastically deform without fracturing. This resistance is primarily determined by chemical composition and to some extent by the manufacturing processes.
Usually, higher strength steels are more susceptible to brittle fracture and have lower toughness. Toughness can be improved by techniques such as heat treatment or by quenching and tempering or modifying the steel composition with varying amounts of alloys.Anchor: #i1002923
Be familiar with the characteristics of good and poor structural details and identify those details in the field. Welding creates the details most susceptible to fatigue and fracture. Therefore, it is imperative to recognize features prone to FC failure.
Major FC problem areas are at weld discontinuities or changes in geometry such as:
- Toes of fillet welds
- Weld termination points
- Welds to girder tension flanges from other connections such as stiffeners or diaphragms
- Ends of welded cover plates
Welded cover plates on rolled beams were a very common detail until fatigue failures began to be recognized by bridge engineers. Whether the weld is terminated or continued around the end of the cover plate, the condition is at best Category E fatigue detail, which has a greater susceptibility to crack.
Weld attachments to a girder web or flange can reduce fatigue strength as the length of the attachment increases. Welds two inches or less fall in Category C and those greater than four inches in length reduce to Category E. Such details are commonly used to attach diaphragms for lateral stability and wind bracing to main structural members, either at the flange or web. Details such as run-off tabs and back-up bars may also provide possible stress riser discontinuities if not smoothed by grinding after removal.
Be familiar with acceptable and unacceptable fillet weld profiles in order to recognize potential problem areas in the field.Anchor: #i1002973
Fatigue in Secondary Members
Secondary members may also have fatigue problems. For instance, main girder stress reversal may induce vibrations in lateral bracing or diaphragms. In many cases the number of stress reversals in the secondary member is a magnification of those stresses in the main member. The attachment of plates to a girder web may cause out-of-plane bending in the web, a situation not usually considered by the designer.
In general, secondary members themselves are not subject to a FC inspection. However, some secondary members, even though designed only as secondary members, such as lateral wind bracing in the lower plane of a girder system, will act as primary members. These cases generally occur in curved or heavily skewed structures. A curved bridge will have twisting or torsional effects due to the live loads that are partially resisted by the diagonal lateral wind bracing. These braces, particularly those near supports, should be inspected for possible fatigue cracks.Anchor: #i1002988
Proper Welding and Repair Techniques
Proper welding of structural steel members is a tedious process under the very best of conditions, which are usually found in the fabrication shop. Closely examine any field welding, whether it is a welded girder splice, retrofit detail, or repair, for visible problems. Many shop splices are accomplished by automatic welding machines under controlled conditions and can be smoothly ground to eliminate surface discontinuities. Field splicing operations are subject to exposure to the elements and difficulties in stabilizing the pieces to be joined. In addition, the welding is usually done by hand and, therefore, subject to human error. Fortunately, welded field splices for bridges constructed with state supervision in Texas have always been subject to careful inspection and must be done by certified welders. The welded field splices for these bridges are usually of the same quality as shop splices and are often further inspected by radiographic (X-ray) techniques.
Be aware of problems that may arise from the use of improper field repair processes. Often a well-intentioned repair can actually make a member even more susceptible to a fatigue fracture.Anchor: #i1003003
FC Inspection Techniques
FC inspection techniques may include non-destructive testing to determine the condition of a structural member. There are several types available, including radiographic, ultrasonic, dye penetrant, and magnetic particle inspection. All are acceptable methods, but each has limitations and may not be suitable for a particular situation. One single technique may not be sufficient to assess damage and a combination of more than one may be advisable. Perform these types of inspection only if you have undergone the proper training. The selection of the type of non-destructive testing method for a particular location is a function of the detail. For instance, potential cracks at the ends of welded cover plates are often inspected by the use of radiographic methods. Cracks in pins are best inspected by ultrasonic techniques. Subsurface defects such as inclusions may be found by magnetic field irregularities, and cracks adjacent to fillet welds at tee-joints are usually inspected by dye penetrant. These methods are all described in more detail in the Bridge Inspector's Reference Manual 9.