Section 5: In-Depth Inspections
Anchor: #i1002437Reasons for In-Depth Inspections
In-Depth Inspections are usually performed as a follow-up inspection to an Initial, Routine, or Damage 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 interpretation of the results are sometimes open to question
Anchor: #i1002457Underwater Inspections
Underwater Inspections are a type of In-Depth Inspection. These are regularly performed every five years. The frequency can be less than five years if conditions warrant.
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. If the condition of these bridge foundation elements is to be determined and the safety of the traveling public, ensured, Underwater Inspections must be performed on some regular basis. As a result of several bridge collapses during the 1980s, the National Bridge Inspection Standards3 (NBIS) were revised to require the development of a master list of bridges requiring Underwater Inspections, and an inspection frequency not to exceed five years.
The master list of bridges needing Underwater Inspections is compiled and updated during Routine Inspections. Once a bridge is added to the master list, it will remain there until it is no longer in use. Some bridges must be inspected at intervals more frequent than the required five years 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.
Anchor: #i1002482Underwater 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.
- 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: #i1002512Levels of Underwater Inspection
Standard levels of inspection originated in the U.S. Navy. Three levels of inspection have been established as the result of the process through time. These are:
- Level I -- Consists of 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 an important 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: #i1002552Underwater Inspection Devices
There are several types of sounding or sensing devices are available for use by divers in underwater investigations. Most common is the black and white fathometer, which uses sound waves reflected from the channel bottom and records the depths continuously on a strip chart. It provides an inexpensive, effective means of recording channel depths but will not detect a refilled scour hole. Other devicesmethods are color fathometers, which use different colors to record different densities and in this way can often detect scour refill; 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: #i1002562Underwater Structural Materials
Piers and bents, if located in a navigable waterway area, are often subject to material defects or collision damage as well as 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. Particular attention should be directed 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, the presence of oxygen and frequent wet/dry cycles promote deterioration at an accelerated rate and steel should be measured 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 much the same type of damage as concrete plus the loss of joint mortar and individual pieces.
Anchor: #i1002589Fracture-Critical Inspections
Fracture-Critical Inspections are a type of Special Inspection. These inspections are usually limited to non-redundant tensile stress areas. They are regularly performed every five years. The frequency can be less than five years if conditions warrant. Methods of inspection may include dye penetrant, magnetic particle, or ultrasonic techniques.
Anchor: #i1002599History 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 require 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. Railroads were reluctant (some still are) to use continuity and welding. The result of the use of continuity was more flexible structures that were more subject to deflections and rotations. The result of the use of welding, particularly in Texas, was 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 highways 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: #i1002619Fatigue Failures
A number of steel structures failed in the 1950s and 1960s due to various causes, but the failure that got national attention was that of the Silver Bridge at Point Pleasant, West Virginia in 1967. 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. The immediate result was the impetus for the National Bridge Inspection Standards4 (NBIS) as part of the Federal-Aid Highway Act5 of 1968. In addition, after the failure, 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.
The first recognition of redundant and non-redundant members was presented 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: #i1002652Fracture-Critical Members
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 member or component whose failure in tension would result in the 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 (FCP) in bridge design details.
Most common types of FC members are tension flanges and sometimes 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 possibility of abrupt failure due to their internal redundancy.
The following rules-of-thumb usually determine FC members:
- Two-girder bridges are defined 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 sometimes lead to subsequent higher positive stresses in the spans with no catastrophic collapse. Therefore, these FC components receive more frequent periodic In-Depth Inspections.
- All steel caps are defined as FC. While this statement is bold, an exception is difficult to imagine.
- 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.
- Secondary members such as diaphragms and stiffeners are not FC. They are rarely used in a manner where failure would lead to structure collapse. However, caution must be observed 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.
Redundancy
The concept of structural redundancy is well known. Any statically indeterminate structure may be said to be redundant, to varying degrees, depending upon its supports. A two-span straight girder is redundant. However, a two-span curved girder is also redundant, but the support reactions are determinate. These definitions of redundancy are of little value to the field inspector who must make a determination of FC potential for various members in a bridge. There are two types of redundancy that concern the FC inspector:
- Load Path Redundancy. Superimposed traffic loads are supported directly by the deck, which in turn is supported by longitudinal stringers or beams. A bridge with a single box girder would therefore be non-redundant since a failure in the box would collapse the bridge. Likewise, a two-girder bridge is non-redundant since one girder cannot assume all of the load for which two are designed. However, it can be argued that a continuous two-girder bridge is structurally redundant since a girder failure would not cause collapse, but the structure would sag excessively. Three or more girders will usually have enough load capacity due to inherent design factors of safety to avoid collapse. The failure of one girder will immediately cause the loads to be shared by the other girders. However, the FHWA considers three-girder bridges with more than 15-foot girder spacing to be FC. The strength of the deck system should be considered for this case. Some deck systems for wide beam spacings are two-way slabs and others have stringer and floor beam systems with one-way slabs. Those with two-way slabs will still have a load-path redundancy, while those with stringers and floor beams will be more unstable after failure of one girder in a three-girder system.
- Internal Redundancy. This term refers primarily to built-up members, such as riveted plate girders. A single plate or shape in the built-up member might fail without causing collapse. However, even members such as this must sometimes be considered non-redundant, since like two-girder structures, failure of one portion of the member can overload the remaining portions such that there is not sufficient remaining capacity to prevent total failure. Usually, if the cross-sectional area of the largest shape or plate in a built-up member is less than about 30 to 40 percent of the total member area, then the member may be considered to be have internal redundancy.
Inspection Procedures for FC Members
Inspection procedures begin with proper advance planning. The more important planning aspects, usually based on an office review of the structural plans, are
- 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 ladders, bucket truck, or climbing gear.
- Many FC members are a result of structures designed for urban situations with necessary complex alignment geometries. Proper inspection of these bridges may require closing a traffic lane. Safe traffic control must be coordinated in advance with the local district and Area Engineer offices and their Safety Review Team.
- If the structure involves a railroad, a railroad flagger must be coordinated with the proper railroad company.
- 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 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-Prone Details
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
- 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. Most of 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 becomes 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 (FCP). The FCP 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 FCP. During fabrication, these members are subject to special requirements.
Fatigue failure is always an abrupt fracture, called a brittle 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.
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 brittle fracture. 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.
Anchor: #i1002923Weld Details
Inspectors concerned with FC inspections must acquaint themselves with the characteristics of good and poor structural details and be able to 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.
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 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.
Inspectors should familiarize themselves with acceptable and unacceptable fillet weld profiles in order to recognize potential problem areas in the field.
Anchor: #i1002973Fatigue 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: #i1002988Proper 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. Any field welding, whether it is a welded girder splice, retrofit detail, or repair, should be closely examined 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 bridge are usually of the same quality as shop splices and are often further inspected by radiographic (X-ray) techniques.
The inspector should also 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 brittle fracture.
Anchor: #i1003003FC 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. Usually these types of inspection are best left to personnel who have undergone the proper training. The selection of the type of non-destructive testing method for a particular location is usually 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 Training Manual 909.
3. The National Bridge Inspection Standards, Code of Federal Regulations, Title 23, Part 650, October, 1988
4. The National Bridge Inspection Standards, Code of Federal Regulations, Title 23, Part 650, October, 1988.
5. Federal-Aid Highway Act of 1968.
6. Standard Specifications for Highway Bridges, AASHTO, 1977.
7. Guide Specifications for Fracture Critical Non-redundant Steel Bridge Members, AASHTO, 1986.
8. Standard Specifications for Highway Bridges, AASHTO, 1996