Inspection, Evaluation, Inspection, Evaluation, Rehabilitation, Maintenance Rehabilitation, Maintenance and Retrofitting of Bridgesand Retrofitting of Bridges

Inspection, Evaluation, Rehabilitation, Inspection, Evaluation, Rehabilitation, Maintenance and Retrofitting of BridgesMaintenance and Retrofitting of Bridges Causes of Bridge FailuresCauses of Bridge Failures

• floodsfloods• ScourScour• WindWind• EarthquakesEarthquakes• CorrosionCorrosion• FatigueFatigue• Failure of a fracture-critical memberFailure of a fracture-critical member• Over LoadingOver Loading• Poor Design (buckling of compression members lack of Poor Design (buckling of compression members lack of

redundancy etc)redundancy etc) Bridge EvaluationBridge Evaluation

• Bridge RatingBridge Rating• Types of Bridge RatingTypes of Bridge Rating

Bridge InspectionBridge Inspection A special task force to investigate the collapse of the bridgesA special task force to investigate the collapse of the bridges A committee, formed at the direction of this task forceA committee, formed at the direction of this task force All bridges longer than 20ft were required to be described and All bridges longer than 20ft were required to be described and

registered in a national database and thus the National Bridge registered in a national database and thus the National Bridge Inventory (NBI) was establishedInventory (NBI) was established

Several studies about the nation’s bridges have been Several studies about the nation’s bridges have been published in the last few yearspublished in the last few years

A jurisdictional breakdown of these figures indicates:-A jurisdictional breakdown of these figures indicates:-

• Out of about 275,000 interstate and state highway bridges, about Out of about 275,000 interstate and state highway bridges, about 75,000 or 27 percent are substandard75,000 or 27 percent are substandard

• Also, out of 310,000 city, country and township highway bridges, a Also, out of 310,000 city, country and township highway bridges, a shocking 146,000 or 47 percent, are substandardshocking 146,000 or 47 percent, are substandard

For preserving the safety and serviceability of such a vast For preserving the safety and serviceability of such a vast number of bridges, obvious need for a systematic inspection number of bridges, obvious need for a systematic inspection and reporting system for these bridges and a program for and reporting system for these bridges and a program for their maintenance and repairstheir maintenance and repairs

Most Common Bridges Built Type & Most Common Bridges Built Type & Classified Structurally DeficientClassified Structurally Deficient

Bridge TypeBridge Type Number built Percent of Average Percent Number built Percent of Average Percent Structurally Structurally

1950-1987 total built1950-1987 total built year built Deficientyear built Deficient

1950-19871950-1987 1950-1987 1980-1950-1987 1980-

19871987

Steel stringer (SST)Steel stringer (SST) 69,88569,885 24 24 19641964 23 23 77 Continuous steel strnger (CSST)Continuous steel strnger (CSST) 32,227 32,227 11 11 19671967 11 11 11 Timber stringer (TST)Timber stringer (TST) 31,08331,083 10 10 19631963 52 52 3030 Prestressed concrete stringer (PCST)Prestressed concrete stringer (PCST) 27,92327,923 9 9 19711971 4 4 00 Reinforced concrete slab (RCSL)Reinforced concrete slab (RCSL) 24,16224,162 8 8 19661966 11 11 22 Continuous reinforced conrrete slab Continuous reinforced conrrete slab 18,57318,573 6 6 19671967 4 4 00 Prestressed concrete multiple box Prestressed concrete multiple box 16,37716,377 5 5 19731973 5 5 11 Reinforced concrete stringer (RCST)Reinforced concrete stringer (RCST) 12,50012,500 4 4 19661966 10 10 22 Reinforced concrete T (RCT)Reinforced concrete T (RCT) 11,36111,361 4 4 19641964 6 6 11 Continuous reinforced concrete TContinuous reinforced concrete T 58275827 2 2 19631963 3 3 11 Prestressed concrete slab (PCSL)Prestressed concrete slab (PCSL) 57065706 2 2 19731973 3 3 00 Prestressed concrete T (PCT)Prestressed concrete T (PCT) 50175017 2 2 19721972 5 5 00 Total of 12 typesTotal of 12 types 260,641260,641 88 88 -- - - -- Total of all typesTotal of all types 296,668296,668 - - -- - - --

What is a Substandard BridgeWhat is a Substandard Bridge

A substandard bridge is defined by A substandard bridge is defined by its functional obsolescenceits functional obsolescence

A bridge is judged to be structurally A bridge is judged to be structurally deficient it has been restricted to deficient it has been restricted to light vehicleslight vehicles

Bridge EvaluationBridge Evaluation Bridge RatingBridge Rating. . Structural deficiency of a bridge Structural deficiency of a bridge

may have resulted from several factors such as:-may have resulted from several factors such as:-

• Designs for lighter loads (e.g. H15, HS15, H20 etc) Designs for lighter loads (e.g. H15, HS15, H20 etc) compared to loads in use at present (e.g. HS20 or compared to loads in use at present (e.g. HS20 or HS25, etc)HS25, etc)

• Designs according to codes, specifications, or Designs according to codes, specifications, or stress levels that are no longer applicablestress levels that are no longer applicable

• Reduction of the live load capacity as a result of Reduction of the live load capacity as a result of aging, deterioration, or damage to structural aging, deterioration, or damage to structural members. An increase in dead load, such as that members. An increase in dead load, such as that due to a thicker new deck or a new layer of due to a thicker new deck or a new layer of wearing course on an old deck without removing wearing course on an old deck without removing the old layer of wearing course, would increase the old layer of wearing course, would increase the dead load moment, with consequent decrease the dead load moment, with consequent decrease in the live-load moment capacity of the bridgein the live-load moment capacity of the bridge

Superstructure Consists of

DeckDeck StringersStringers Floor BeamFloor Beam BearingsBearings Truss MemberTruss Member

Step-by-Step Procedure for the Step-by-Step Procedure for the Rating Analysis of a BridgeRating Analysis of a Bridge

• Analyze the bridge completely for the Analyze the bridge completely for the service loads (H20, HS20, etc) i.e. service loads (H20, HS20, etc) i.e. determine moments, shears, stresses, etc., determine moments, shears, stresses, etc., in various bridge componentsin various bridge components

• Calculate the member forces (or stresses, Calculate the member forces (or stresses, as required) resulting from dead loads only.as required) resulting from dead loads only.

• Calculate load-carrying capacity of members Calculate load-carrying capacity of members based on their actual section properties and based on their actual section properties and allowable (or code-prescribed) stressesallowable (or code-prescribed) stresses

• Calculate the member capacity available to Calculate the member capacity available to carry live load. This quantity is simply the carry live load. This quantity is simply the difference between the quantities difference between the quantities calculated in steps 2 and 3calculated in steps 2 and 3

Live-load capacity =Live-load capacity = Total member capacity (step 3) – Total member capacity (step 3) – Forces due to dead loads Forces due to dead loads

(step 2)(step 2)

• The rating factor is simply the ratio of quantities The rating factor is simply the ratio of quantities calculated in steps 4 and 1, and may be variously calculated in steps 4 and 1, and may be variously expressed as:expressed as:

For bending members, such as slabs, beams, and For bending members, such as slabs, beams, and girders:-girders:-

RF =RF = Available moment capacity to carry live-loadAvailable moment capacity to carry live-load

Design moment due to live-load plus impactDesign moment due to live-load plus impact

For axially loaded members such as truss members:-For axially loaded members such as truss members:-RF =RF = Stress available to carry live-loadStress available to carry live-load

Design stress due to live-load plus impactDesign stress due to live-load plus impact

Step-by-Step Procedure for the Step-by-Step Procedure for the Rating Analysis of a BridgeRating Analysis of a Bridge

Types of Bridges RatingsTypes of Bridges Ratings

Inventory RatingInventory Rating Operating RatingOperating Rating

Bridge PostingBridge Posting What is done after the rating analysis of a What is done after the rating analysis of a

deficient bridge has been performed? Generally deficient bridge has been performed? Generally speaking, a sign indicating the safe load limit is speaking, a sign indicating the safe load limit is posted near the approach of bridge. The posted near the approach of bridge. The practice of posting safe loads varies widely, for practice of posting safe loads varies widely, for the value of the load limit itself would have two the value of the load limit itself would have two values – one based on the inventory rating and values – one based on the inventory rating and the other based on the operating rating. the other based on the operating rating. Essentially, posting load limits for a bride is a Essentially, posting load limits for a bride is a jurisdictional policy decision to be made by the jurisdictional policy decision to be made by the bridge owners. Bridge posting may be made if:-bridge owners. Bridge posting may be made if:-• The legal load exceeds the inventory ratingThe legal load exceeds the inventory rating• The legal load exceeds the operating ratingThe legal load exceeds the operating rating• Or the value of load lies in between the Or the value of load lies in between the

inventory and the operating ratingsinventory and the operating ratings

Bridge RehabilitationBridge Rehabilitation Many of the structurally deficient bridges Many of the structurally deficient bridges

are good candidates for rehabilitation.are good candidates for rehabilitation. Experience has shown that old bridges, Experience has shown that old bridges,

when properly rehabilitated, often serve as when properly rehabilitated, often serve as well as new bridges.well as new bridges.

The two terms generally used in the The two terms generally used in the context of strengthening bridges are context of strengthening bridges are rehabilitation and retrofitting.rehabilitation and retrofitting.

They involve engineering solutions that They involve engineering solutions that these are completely different from these are completely different from routine maintenance or repair work.routine maintenance or repair work.

Difference between Rehabilitation & Difference between Rehabilitation & RepairsRepairs

A distinction is made between rehabilitation and repairs, A distinction is made between rehabilitation and repairs, depending on the scope of the work involved. The term depending on the scope of the work involved. The term “repairs” often refers to corrective action or steps taken for “repairs” often refers to corrective action or steps taken for common maintenance for an otherwise functional bridge. On common maintenance for an otherwise functional bridge. On the other hand, the terms “rehabilitation” implies the other hand, the terms “rehabilitation” implies implementing a planned upgrade of a bridge to a desired level implementing a planned upgrade of a bridge to a desired level of service. It generally involves significant improvements in of service. It generally involves significant improvements in structurally deficient or functionally obsolete bridges to structurally deficient or functionally obsolete bridges to enhance their load or traffic-carrying capacities, or to prolong enhance their load or traffic-carrying capacities, or to prolong their service life. Common bridge repairs would include items their service life. Common bridge repairs would include items such as patching a portion of the deck, repairing or replacing such as patching a portion of the deck, repairing or replacing a damaged component of the superstructure, repairing and a damaged component of the superstructure, repairing and cleaning frozen bearings, etc. Rehabilitation, on the other cleaning frozen bearings, etc. Rehabilitation, on the other hand, refers to, in layman’s terms, giving new life to an hand, refers to, in layman’s terms, giving new life to an otherwise structurally deficient or functionally obsolete otherwise structurally deficient or functionally obsolete bridgebridge

RetrofittingRetrofitting The term “retrofitting” also involves The term “retrofitting” also involves

strengthening a bridge, and therefore may strengthening a bridge, and therefore may be considered simply as rehabilitation. The be considered simply as rehabilitation. The difference between rehabilitation and difference between rehabilitation and retrofitting is often blurred, and academic retrofitting is often blurred, and academic at best. The goal in both cases is to at best. The goal in both cases is to upgrade a bridge to a desired level of upgrade a bridge to a desired level of service. In the current engineering service. In the current engineering practice, however, the seismic safety or practice, however, the seismic safety or upgrade, a task referred to as seismic upgrade, a task referred to as seismic retrofit. It involves uniquely engineered retrofit. It involves uniquely engineered solutions to modify dynamic characteristics solutions to modify dynamic characteristics of bridge components and member of bridge components and member connections. The goal of a seismic retrofit connections. The goal of a seismic retrofit is to provide additional capacity is to provide additional capacity requirements and, through modifications, to requirements and, through modifications, to force the bridge to the desired behavior force the bridge to the desired behavior during a seismic event.during a seismic event.

There is no such thing as an everlasting bridge; all There is no such thing as an everlasting bridge; all bridges age. With aging of the nation’s highway bridges age. With aging of the nation’s highway bridges, their rehabilitation has become a problem bridges, their rehabilitation has become a problem of significant dimension, which has been of significant dimension, which has been compounded by an ever-increasing demand for compounded by an ever-increasing demand for wider and heavier load-carrying vehicles. This wider and heavier load-carrying vehicles. This development simply translates into a demand for development simply translates into a demand for increasing the load-carrying capacity of bridges by increasing the load-carrying capacity of bridges by rehabilitating them. Rehabilitation of a bridge rehabilitating them. Rehabilitation of a bridge could often be a more cost-effective alternative could often be a more cost-effective alternative than its replacement, an idea that has gained than its replacement, an idea that has gained significant importance because of dwindling significant importance because of dwindling revenues and budgetary constraints. As a result, revenues and budgetary constraints. As a result, rehabilitation work has gained significant rehabilitation work has gained significant momentum, and increasing percentages of momentum, and increasing percentages of infrastructure budgets are being spent on this type infrastructure budgets are being spent on this type of work.of work.

Rehabilitation of BridgesRehabilitation of Bridges

Not all bridges can be rehabilitated Not all bridges can be rehabilitated economically, however. In some economically, however. In some cases, a bridge may be evaluated as cases, a bridge may be evaluated as so deficient that its rehabilitation is so deficient that its rehabilitation is precluded as a cost-effective precluded as a cost-effective alternative, justifying replacement alternative, justifying replacement of the entire bridge.of the entire bridge.

Rehabilitation of SuperstructuresRehabilitation of Superstructures The nature and scope of work involved in The nature and scope of work involved in

rehabilitation very widely, depending on rehabilitation very widely, depending on the nature and scope of structural the nature and scope of structural deficiency of the functional obsolescence deficiency of the functional obsolescence of a bridge. Structural strengthening of a bridge. Structural strengthening techniques are generally divided into four techniques are generally divided into four categories:categories:• Addition or modification of a member or Addition or modification of a member or

support.support.• Reduction of dead loadReduction of dead load• Application of external post-tensioningApplication of external post-tensioning• Increased bridge stiffness in either the Increased bridge stiffness in either the

transverse or the longitudinal direction.transverse or the longitudinal direction.

Maintenance of BridgesMaintenance of Bridges A bridge is like a price of equipment in that A bridge is like a price of equipment in that

it shows signs of wear and tear over time. it shows signs of wear and tear over time. Its endurance depends on several factors.Its endurance depends on several factors. The original strength of its design and The original strength of its design and

materials. materials. The amount of strain from the traffic and The amount of strain from the traffic and

weather and maintenance. weather and maintenance. The wear and tear caused by traffic, bridges The wear and tear caused by traffic, bridges

are always exposed to environmental are always exposed to environmental hazards such as rain, shown, floods, fire, hazards such as rain, shown, floods, fire, earthquakes, and debris accumulation.earthquakes, and debris accumulation.

ConclusionConclusion Bridge rating involves structural capacity Bridge rating involves structural capacity

analysis of bridges to determine their overall analysis of bridges to determine their overall response to various life loading conditions. response to various life loading conditions. Inventory rating refers to the capacity rating Inventory rating refers to the capacity rating that indicates the load level considered safe for that indicates the load level considered safe for a bridge for an indefinite amount of time. a bridge for an indefinite amount of time. Operating rating is the capacity rating that Operating rating is the capacity rating that indicates the absolute maximum load level that indicates the absolute maximum load level that is save but not must be exceeded during the is save but not must be exceeded during the service life of a bridge. Bridge rating is service life of a bridge. Bridge rating is required to be performed on all bridges that do required to be performed on all bridges that do not measure up to the current AASHTO not measure up to the current AASHTO standard. Bridges with rating below inventory standard. Bridges with rating below inventory level would be restricted to reduced load level, level would be restricted to reduced load level, and posted as such. Such bridges would need and posted as such. Such bridges would need to be rehabilitated (or retrofitted) to provide an to be rehabilitated (or retrofitted) to provide an improved level of service.improved level of service.

As bridges grow older, there rehabilitation and retrofitting becomes increasingly important. Deterioration of concrete, corrosion of steel, heavier life load, increase traffic density, poor resistance of fatigue, wind and seismic load, damage due to fire and collision, and normal wear due to aging or some of the reason for rehabilitating or retrofitting of a bridge. Experience with earth quake has indicated serious inadequacies in many bridges word wide in terms of their safety and integrity during earth quake (Priescley and Seilble, 1994). Most of these bridges were designed when the state of knowledge required for earth quake resistant design was still evolving. As a result, many bridges are now being retrofitted (or replaced) to endure seismic forces according to the new standard.

With the growing inventory of the nation’s bridges, With the growing inventory of the nation’s bridges, inspection, evolution, rehabilitation, and inspection, evolution, rehabilitation, and maintenance of bridges evolved into discipline of maintenance of bridges evolved into discipline of special significance. Related to the life cycle cost of special significance. Related to the life cycle cost of a bridge, these functions now from the core of the a bridge, these functions now from the core of the bridge management systems of the country. With bridge management systems of the country. With the continuing monitoring and energy crisis and as the continuing monitoring and energy crisis and as bridges continue to out performed their designed bridges continue to out performed their designed life, rehabilitation and maintenance of bridges have life, rehabilitation and maintenance of bridges have assumed new dimensions as viable money saving assumed new dimensions as viable money saving alternative. With new technologies and experience, alternative. With new technologies and experience, the art of ridge repair and rehabilitation is continuing the art of ridge repair and rehabilitation is continuing to be enhanced with worldwide application. The to be enhanced with worldwide application. The topic of bridge strengthening and rehabilitation has topic of bridge strengthening and rehabilitation has been comprehensively discussed by several authors.been comprehensively discussed by several authors.

Inspection, Evaluation, Inspection, Evaluation, Rehabilitation, Maintenance Rehabilitation, Maintenance and Retrofitting of Bridgesand Retrofitting of Bridges