Section 7: Fracture-Critical Inspections
Fracture-Critical (FC) Inspections limited to non-redundant load path steel tensile stress areas. These inspections are typically performed every 24 months but can be performed more frequently if conditions warrant. A Fracture Critical Inspection is a hands-on (within arm's 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: #i1037810
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:
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- Steel girders in two-girder bridges not evaluated for system redundancy in accordance with the TxDOT Bridge Design Manual are 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. Anchor: #MMFVOQHQ
- The majority of steel caps are FC. The exceptions are those where support columns or multiple cap members provide load path redundancy. Anchor: #NGIFTSTG
- A floorbeam is 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. Anchor: #YTESKHVA
- Lower chords of trusses are FC. This determination is based on the fact that most truss bridges employ only two trusses and most are simple-span. Anchor: #LFWELCBL
- 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. Anchor: #TTOVXWRR
- 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:
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- Load path redundancy Anchor: #FDIHCOMN
- Structural redundancy Anchor: #HTEQOWNU
- 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: #i1037887
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:
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- Identify possible FC members. Anchor: #RAKWCCBM
- Note the particular members in the structure that may require special field attention, such as built-up tension members composed of few individual pieces. Anchor: #KNLTMVLJ
- 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. Anchor: #KMTWCJLQ
- 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. Anchor: #NNSCHJCG
- Use a railroad flagger coordinated with the proper railroad company if the structure crosses or is within 25 ft. of a railroad track centerline. Every individual entering a railroad right-of-way 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. Anchor: #IWMUQBLO
- Working over railroad tracks requires significant coordination with rail companies. The issuance of inspection work containing FC members that cross railroad tracks will be done so as to allow for an eight (8) month lead time prior to the earliest inspection due date to accommodate this extra coordination. Beginning at the one-month anniversary of the execution of a Work Authorization for inspection of these bridges, and continuing every subsequent monthly anniversary until all bridges are inspected, an update from the consultant will be provided to the TxDOT Project Manager as to the status of obtaining the Right-of-Entry permits. TxDOT will in turn report monthly to the FHWA the status of obtaining such permits for in-house and consultant inspections. This reporting will continue until inspections are completed. Anchor: #UFJLWEOI
- 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:
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- Visual cracks and their direction and location Anchor: #RXHLSRCY
- Evidence of rust, which may form at a working crack Anchor: #VTLPXMDX
- Weld terminations in a tension area Anchor: #XRLUWPOL
- Interrupted back-up bars used for built-up-member fabrication Anchor: #KVLAVBTW
- Arc strikes, scars from assembly cables or chains, or other physical damage Anchor: #LFOGKUGW
- 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:
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- Loss of strength Anchor: #JGECHMNE
- Loss of ductility Anchor: #YYMYJUHG
- 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:
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- Local changes in atomic structure, accompanied by sub-microscopic cracking Anchor: #PEKOUWYI
- Crack growth Anchor: #YWADAQXT
- Sudden fracture
Fatigue fracture almost always begins at a visible discontinuity, which acts as a stress-riser. Typical examples are:
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- Design details such as holes, notches, or section changes Anchor: #RMURXRIX
- Flaws in the material such as inclusions or fabrication cracks Anchor: #XYPJEPHP
- Poor welding procedures such as arc strikes, gouges and start-stops Anchor: #JMEFDGDG
- Weld terminations
Certain structural details have been long recognized as stress-risers and are classified as to their potential for damage. These details appear in the current AASHTO Bridge Specifications5.
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:
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- Stress level Anchor: #OQLCVDYY
- Crack size Anchor: #VHMJJGXD
- 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: #i1038033
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:
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- Toes of fillet welds Anchor: #TMDBJMXT
- Weld termination points Anchor: #VEATLATI
- Welds to girder tension flanges from other connections such as stiffeners or diaphragms Anchor: #ODDRXLKN
- 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: #i1038065
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: #i1038103
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: #i1038198
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 Manual6.