CHAPTER

A Review of Chapters, River
Bridges, and Conclusions

11

11.1  Introduction to chapter 11
This final chapter is divided into two parts.
Part 1: It deals with the summary and review of the first 10 chapters.
Part 2: It deals with rapid construction on rivers, using alternative float-in method to transport
assembled bridge to the bridge site and timber and aluminum bridges.
Part 2 is followed by overall conclusions.

Part 1
11.1.1  Summary of earlier chapters
For early completion or for rapid construction, the main factors and issues discussed in the earlier
chapters may be summarized as the five M’s, namely:
Management team of design-build engineers,
Modern materials using high-performance steel (HPS), high-performance concrete (HPC), and
composites,
Method of assembly of modular construction in factory or on site,
Method of transport using self-propelled modular transporters (SPMT), and
Method of erection by lifting into position, roll-in, roll-out or lateral slide-in.
For bridges on rivers, a float-in method can be used.
Both full and partial accelerated bridge construction (ABC) methods are discussed.
Partial ABC is a compromise between conventional and ABC methods. It is applicable when sophisticated transport and lifting equipment is not available and where the bridge owner wants to keep the
in-charge consultant. Some factory fabrication of girders would still be used.
Since the scope of each project is slightly different, full ABC may not always be applicable. The
following types of conditions would exist:
 

1. A new bridge on a new highway. Coordination with highway construction on one or both sides of
the bridge will be required. Bridge construction activities may not be on the critical path. Also, no
demolition work is needed.
2. Existing bridge requiring superstructure replacement only. Only superstructure demolition may be
required. The ABC can be done using staged construction with limited lane closure.
3. Existing bridge requiring both superstructure and substructure replacement. Staged construction
may be required since existing footing width may interfere with the new footings. Demolition of
entire abutment footing would require shutting down of the entire bridge rather than lane
closure.
Accelerated Bridge Construction. />Copyright © 2015 Elsevier Inc. All rights reserved.

489


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CHAPTER 11  A Review of Chapters, River Bridges, and Conclusions

4. C
 onstruction duration for deep foundations such as minipiles or long piles or drilled shafts/
caissons will not change for full or partial ABC.
5. Funding will be unaffected in each case.
 
Construction season may be geared to local weather and factory manufacture in doors will be an
advantage. Also, roll-in, roll-out method may be more expensive than lateral slide-in but has the advantage that the assembled bridge can be lifted and placed over the bearings without relocating the existing
utilities.
Training programs in ABC may be necessary. Use can be made of the Federal Highway Administration (FHWA) conferences, and lunchtime seminars organized by FIU and other universities engaged in
research in the new technology.
A variety of case studies are presented for superstructure or substructure replacement using prefabrication, self-propelled modular transporters, roll-in and roll-out methods, and lateral slide-in methods.
A glossary of ABC terminology applicable to all the chapters is listed for ready reference in Appendix

2 ABC.
Part 1 will provide brief summaries of the chapters and will be the review of Chapters 1–10.
The chapters that follow this introductory chapter on modern ABC will cover the following
themes:
 
Sections 11.2 address coordination with highway construction schedule.
Sections 11.3 to 11.8 address scour issues related to river bridges and design of countermeasures.
The details related to scour are based on author’s textbook on Bridge and Highway Structure
published by McGraw=Hill 2010.
Section 11.9 provides details of case studies.
Section 11.10 is for the conclusion of the chapter 11.
In addition, Section 11.11 discusses future developments of ABC.
Finally, Section 11.12 discusses acknowledgements and future revisions of codes/
Section 1 Innovative Construction Methods (chapters 1 to 4)
(Chapter 2), Recent developments in ABC concepts
(Chapter 3), Research and training in ABC structural systems
(Chapter 4), Introducing innovative ABC techniques
(Chapter 5), Modular bridge construction issues
Section 2 Recent Developments in ABC Concepts (chapters 5 to 7)
(Chapter 6), Rapid bridge insertions following failures
(Chapter 7), Planning and resolving ABC issues
(Chapter 8), ABC Prefabrication of the superstructure
Section 3 Modular Bridges (chapters 8 to 11)
(Chapter 9), Prefabrication of the substructure and construction issues
(Chapter 10), Alternative ABC methods and funding justification.
 
Chapter 1 presents an introduction to modern ABC with discussion of the many advantages and
deterrents. Deterrents include administrative and planning bottlenecks, construction easements and
right-of-ways, permit approvals, and utilities relocation issues. Timely labor availability, weather problems, and the large storage yard areas required at the site are addressed. In addition, the need exists for
certification and training, laboratory testing related to the structural behavior of field connections of



11.1  Introduction to chapter 11

491

subassemblies, and mathematical modeling. Design and construction codes, continuous funding, heavy
cranes, and erection equipment such as trolleys and SPMTs are required to properly implement ABC.
Major benefits include reducing traffic impacts, and the use of prefabricated bridge components
made of HPS, HPC, and other new materials and equipment. Application of the latest techniques in
concrete manufacture, including the use of lightweight concrete and other hybrid materials, will contribute to durability and possibly early completion of projects.
It was shown that applying the ABC methodology will result in 50% more completed bridges each
year. This will help the economy by reducing wasted man-hours due to traffic jams during construction;
commuters will get to work faster, which will benefit commerce by promoting faster delivery of goods.
Primary and secondary consideration for the selection of suitable projects for ABC, in terms of
benefit, are addressed. Tables 1.2(a) and 1.2(b) give a format of criteria and allocating points in the
point system.
Please see references to FHWA publications in Appendix 1 (Bibliography) for Chapter 1 for details.
In Chapter 2, we address recent developments in ABC concepts and their application to infrastructure. We noted that it might be possible to reduce the number of failures with ABC by applying recent
advancements in technology and innovative methods. The failed bridges that were built using old technology can be rebuilt on a fast track using ABC.
It appears that there are hiccups that may be holding up a more rapid switch to ABC. A slow but
gradual shift from conventional methods to full ABC (with many projects utilizing a partial ABC
approach) has been observed. Each management subsystem, such as partial ABC, can be used to
accommodate different circumstances and physical conditions. ABC-related design needs to be made
part of the American Association of State Highway and Transportation Officials (AASHTO) and state
bridge design codes and specifications. Deterrents and bottlenecks such as maintenance and protection
of traffic (MPT), construction easements, right-of-way, permit approval, and utilities relocation need to
be resolved, and administrative procedures further simplified to facilitate ABC.
There are many feasible applications of the latest techniques in concrete manufacture, composites,
HPS, and hybrid materials that need to be promoted. Integrated software that would cover all aspects

of ABC, including design calculations and drawing preparation, should be investigated and developed
to save engineering man-hours.
A surge has been seen in the manufacturing of bridge components and construction machinery
worldwide. FHWA has prepared a comprehensive ABC manual. AASHTO grand challenges by the
AASHTO Technical Committee for Construction (T4) present additional goals to strive for. For bridges
located on rivers, a survey of scour countermeasures that are being used nationwide was conducted. A
form was successfully developed to assist in the field assessment of scour at bridges. Introducing more
rapid inspections to identify deficient bridges by using remote sensors is emphasized.
Full-scale testing of joints in precast curved decks for both rectangular and curved decks is required.
Modifications to analytical methods applicable to discontinuities of components need to be developed.
Chapter 2 also addresses design-build contracting system and the role of the Design-Build Institute
of America (DBIA) in promoting ABC. Construction Manager/General Manager system is described in
Chapter 2.
Chapter 3 emphasizes ABC logistics and training and research aspects. For promoting ABC,
design-build method of construction management is described in this chapter. The role of the transportation agency in patronizing and promoting ABC is the most critical. The consultant and specialized
subconsultant roles come next; they introduce key innovations in design and field connections.


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CHAPTER 11  A Review of Chapters, River Bridges, and Conclusions

This chapter reviews bridge rating procedures to identify deficient bridges and how to prioritize
bridge repair. Structural health monitoring methods using remote sensors will help prioritize bridges
for rehabilitation. The key factors dictating a particular type of delivery method include time restraints,
level of risk, budget, and level of quality.
Preparing an evaluation matrix for selecting the type of fix or replacement would be helpful. Innovative techniques need to be popularized and adopted as routine bridge construction. Most accidents
occur during bridge construction; hence rapid constructability requirements during erection need to be
met, and preventive measures in design and construction to prevent failures need to be introduced.
ABC planning, analysis, and implementation methods vary for each of the structural systems and

lead to many diverse applications for small, medium, and long spans, each of which has different construction durations and their own specialized construction methods.
Constraints in implementing ABC include MPT, approach slab construction, permits, and utility
relocation; these are unavoidable constraints and should be on critical path for early completion.
The nature of manufacturing precast products creates a proprietary system and monopolistic environment, which may lead to unemployment of some number of construction workers. Overemphasis of
incentives/disincentives may pressure the contractor into adopting unrealistic schedules at the expense
of quality control.
Certain improvements for economical design include the following:
 
•An upgrade to most modern construction equipment would be required.
•Current plan preparation and presentation should reflect ABC.
•Payment and accounting of pay items need to be accelerated.
•Arching action in deck slabs should be utilized—there is reserve strength that is being neglected.
•Deck overlays for riding surface quality—latex-modified concrete (LMC) or corrosion inhibitor
aggregate concrete may be used.
•Bridge deck expansion joints for precast deck units should be investigated.
•Compliance with permitting regulations—environmental permits may hold the start of
construction.
•Insurance against risk and liabilities is critical.
 
Chapter 3 describes training programs in ABC organized by DBIA. It also addresses construction
permits issues for air quality and water quality etc. to be award by the Department of Environmental
Protection (DEP). Chapters 7 and 9 also describe environmental issues. Chapters 3 and 4 and Appendix
1 (Bibliography) provide a list of relevant references on all aspects of ABC.
Chapter 4 discusses how maintaining the right-of-way philosophy is achievable through innovative ABC techniques. The chapter deals with design-build construction management, addressing
modern concrete technology and the philosophy of maintaining the right-of-way at all times for all
citizens. There has to be a reward to promote innovation and encourage the undertaking of some risk.
The most recent initiatives and innovative techniques are described; they are promoted by federal
agencies like FHWA and AASHTO as well as individual states, which are promoting the implementation of ABC for faster bridge delivery. This chapter discusses a comparative study of conventional
and innovative methods, along with a review of new design methods and the development of diverse
repair technologies.

We look at modern construction equipment, the use of recyclable materials, and examples of recent
ABC applications in the United States. The scope of design-build (D-B) contracts and considerations


11.1  Introduction to chapter 11

493

of engineering ethics are also addressed. We also discuss the important issue of ensuring adequate
returns of the hundreds of billions of dollars of yearly investments in infrastructure through rapid
bridge delivery.
Innovations help in upgrading the quality of construction and in completing the project in a timely
manner. A list of advancements in ABC methods include:
 
•Preventing bridge failures by minimizing the identified deficiencies through maintenance
•Use of advanced methods, including computer-aided analysis and design techniques
•Closer interaction between design documents and construction.
 
Continued research efforts are required in resolving technical issues. Common examples of innovative concepts that require continued study are ground-penetrating radar (GPR), staged construction,
overhead and utility lines, environmental permits, road closures versus detours, precast and composite
decks, and the use of stainless steel.
On the administrative side, new procedures for asset management, award of simultaneous multiple
contracts, and accelerated highway construction (AHC) to accompany ABC were introduced in this
chapter. Using nanotechnology to reveal cracks and corrosion, searching for photographic evidence of
defects, and using remote sensing technologies would certainly help in rapid bridge inspection and SHM.
There have been developments in the use of self-consolidating concrete (SCC), lightweight aggregate concrete (LWAC), recycled concrete aggregate (RCA) concrete, accelerated cure cast-in-place
(ACCIP) concrete, blended cement concrete (BCC), fiber mesh concrete (FMC), reactive powder concrete (RPC), and rapid setting concrete (RSC).
Researchers have developed special repair materials. They include nonshrink, multipurpose and
high-strength repair mortar. Cementations materials concrete utilizes fly ash, blast furnace slag, and
silica fume.

Examples of proprietary bridge systems include the robotic steel beam assembly system by Zeman,
which has added a new dimension of structural steel fabrication and erection. The system is designed
for fully automated assembling, tack-welding, and full welding of structural steel elements. Other systems include recycled plastic lumber bridges, lightweight titanium pedestrian bridges, Inverset, Effideck
bridge decks, Exodermic bridge decks, and full-depth precast concrete deck panels (FDDP).
Chapter 5 addresses construction and rehabilitation using prefabrication, prefabricating bridge elements and systems (PBES) and various other improvements in the manufacture of ready-made bridges.
Prefabrication of bridges and their technical issues are discussed in Chapter 5. There are many bridge
companies in this area, such as CON/SPAN and Mabey-type temporary bridges. Modular design and
prefabrication have a number of benefits, including shorter production cycles and enhanced sustainability. This results in lower production costs, with savings to be passed on to the client. The use of
eco-friendly materials and prefabricated design also make these modular structures ideally suited to the
changing demands of the transportation industry.
Consider sample projects using the following bridge elements as guidance in the absence of an ABC
code of practice:
 
•Precast foundation elements
•Precast pile and pier caps
•Precast columns
•Precast full-depth deck slabs


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CHAPTER 11  A Review of Chapters, River Bridges, and Conclusions

•Cored slabs and box beams
•NEXT beams and deck girders
•Full-span bridge replacement units with precast deck
•Bridges installed with SPMTs
 
Cost evaluations are important and should be accurate. They should include the following
categories:

 
•Time and materials estimates
•Roadway user costs
•Maintenance of traffic costs
•Safety costs
•Agency costs
•Life cycle costs
 
If the cost of ABC is not greater than 30% of the conventional bridge construction cost, strongly
consider ABC. The benefits are in early delivery, improved quality, and longer bridge life. Take advantage of existing technologies, such as Inverset and prefabricated fiber-reinforced polymer (FRP) deck.
ABC methods have evolved ahead of the design codes. Research is required in many aspects,
including:
 
•Developing strengthening methods and corrosion mitigation techniques, including fabricating
stronger girders by eliminating the need for shear stiffeners with the use of folded web plate in
steel girders
•New methods to monitor and strengthen foundations against scour, earthquake, and impact
•Developing and reviving the concept of full canopy on bridges to facilitate mobility, improve
drainage, prevent skidding, and eliminate the use of deicing agents
 
Chapter 6 deals with rapid bridge insertions following failures. This chapter addresses the reasons
for numerous failures of bridges in United States and abroad, which can be prevented by the introduction of new technologies of ABC. Maintenance can avoid failures or at least warn of failures in advance.
Use of remote sensors to monitor structural health is desirable. Early failures in conventional construction can be attributed to a variety of reasons, both administrative and technical. Studies shows failures
resulting from inadequate oversight of projects, a lack of supervision at the site, design errors, lack of
comprehensive codes, contractor’s last-minute decisions to meet hasty schedules, and limited resources.
Most failures occur during construction due to lack of redundancy in design, inadequate construction,
and lack of contracting experience and knowhow. Alternate ABC contracting methods are described in
Chapters 6 and 9.
As discussed in this chapter, bridges on rivers failed more than those located on intersections, due
to soil erosion. Use of HEC-23 countermeasures is on the rise. Deep foundations are preferred over

shallow foundations for bridges that are scour critical. Scour countermeasures need to be designed
according to HEC-23 and provided to protect footings. When replacing an existing superstructure, deck
elevation may be raised by 1–2 ft. Some progress has been made in making bridges seismic resistant by
using lightweight materials and isolation bearings. The large volume of site work in conventional construction is minimized by ABC, which requires as much work offsite as possible.


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Use of modern technology: Bridge engineering is changing with time. New technology and innovative ideas developed in the last two years need to be adopted. In planning bridges, cost is still the main
criteria. Much of the cost goes into the foundation and substructure concrete construction.
The use of new and stronger construction materials such as HPS, HPC, and Ultra HPC and FRP
decks should be encouraged, as these are more durable. Shallow-depth girders will result, which are
lighter in weight. Galvanizing will reduce corrosion. Currently, rolled sections in HPS 70W and 100W
are not available or are too expensive. Welded girders in HPS are being used.
Prestressed concrete box girders are stronger in torsion and cost-effective, especially with the use
of lightweight concrete; they also lower maintenance costs. Also, composite construction, for example
using the Inverset system, is more economical and on the rise. Precast integral abutment construction
requires greater attention. Peak stress and deformation can be checked prior to lifting. The location of
cranes needs to be identified on contract drawings.
Project management, quality control, and MPT are of critical importance. It was observed that the
professional relationship between owners, contractors, and consultants needs improvement through
increased communication. ABC design-build methods are a step in the right direction.
Widening of highways in urban areas is not always an option. Right-of-way and legal issues are
involved to acquire new land. Hence, underpass and/or double-deck highways are often used to overcome the additional lanes problem and traffic congestion once and for all.
Safety checklists: It is critical for personnel to be safe and healthy at construction sites. A checklist
of do’s and don’ts needs to be prepared and issued.
Erection methods for curved girders are also described in Chapter 6.
Chapter 7 addresses ABC planning and construction issues. Our failing infrastructure and transportation problems are discussed in this chapter. Before launching a multimillion dollar project, it is the

professional responsibility of engineers to conduct an effective planning exercise. The continued and
ever-increasing infrastructure difficulties faced by the public are highlighted. The economic and public
comfort benefits derived from early completion of projects are reviewed. A survey of ABC projects
successfully completed in many states illustrates an increased interest in adopting the new technology.
Major contractors and fabricators have welcomed the increased responsibility of the design-build system in which their decision making is appreciated. The progress of design-build system and MPT
issues are described in Chapter 7.
Various aspects of the contractors’ role such as that of Construction Manager/General Manager are
described in Chapter 7.
Partial ABC: For rehabilitation of an existing bridge, an engineer’s options are restricted as compared to the options available for design of a new or replacement bridge. But partial ABC is still possible. The huge funding issues can be partly overcome by public-private partnerships.
The focus of Chapter 8 is on bridge superstructure prefabrication, several aspects of prefabrication
of the superstructure and includes the stakeholders of ABC. The reasons for its success are as follows:
 
•The contracting industry is not afraid to take the lead in the management of small- and mediumsized projects and is willing to take the necessary risks and meet challenges that inevitably
arise.
•There have been developments in special transportation methods for long and wide loads using
SPMT. In addition, heavy capacity cranes for lifting and erection are now available.


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CHAPTER 11  A Review of Chapters, River Bridges, and Conclusions

•Organizations such as FHWA (with their Every Day Counts Program and ABC Handbook),
Transportation Research Board (TRB) and AASHTO have been a motivating factor.
•The design-build contract system helps in the adoption of prefabrication. According to SHRP2
Project R04, ABC is the clear choice. Life cycle costs are significantly reduced.
 
Promoting modular construction: European practice is to standardize the design of bridges on typical intersections (limiting them preferably to two spans) and wherever possible on river bridges as well.
The location of abutments can be adjusted to utilize standard precast girder lengths. The location of
field connections are also kept unchanged as determined from analysis.

A list of recent innovations is presented for selection and for further action and implementation,
such as:
 
•PBES
•Connection details for PBES
•NEXT beams, spliced girders, bulb tee, and Wolf girders
•Structural placement methods
•Launching, sliding, and heavy lifting.
 
On-site construction under open sky is far more difficult than factory manufacture. New bridges
have become more complex since the bridge practice of a century ago, when cast-in-place (CIP) construction was the only option. Today a medium-size factory would likely have the necessary facilities
for indoor fabrication. Some of the difficulties involved in on-site construction include:
 
Extreme events and climatic hazards: Most of North America has a subzero cold climate for four
months of the year, and southern states have high temperatures in the summer. This may slow
down the speed of outdoor work. In large factories, temperature change does not affect the
schedule for construction. Also, activities on the critical path are not affected.
Labor availability at remote locations: Most bridge sites are located on distant highways. Hundreds of members of the labor force cannot be relocated. The factory is their regular workplace.
Storage of construction materials: A special building is required on-site for storing construction
materials such as aggregates, cement bags, ladders, machinery, and dozens of other appliances.
Temporary pathways need to be constructed. This adds to the schedule.
Formwork: This is an expensive item of CIP construction. It needs to be erected for the deck slab
and for the CIP girders. This adds to the cost of work and affects the schedule.
Exposure to rain and sunlight: Due to the exposure of steel and cement to the elements, corrosion of
steel and wetting of cement, etc., takes place, which lowers the quality of work and is not desirable.
Mobilization: For CIP, a temporary administration building needs to be set up. This adds to the
overhead.
 
Other issues covered in Chapter 8 include the following:
 

Wider use of the P3 system: With the P3 approach, required funding is made available to replace
hundreds of these bridges more quickly.
Introduction of new maintenance and planning techniques: The causes of structural deficiencies,
functional obsoleteness, and bridge failures need to be investigated. Methods to prioritize the
planning of structural systems and introduce rapid construction need to be researched.
Structural health monitoring (SHM): This is a new technology for bridge inspections that uses
lasers and remote sensors. Bridge inspectors need to be trained in the use of computer software


11.1  Introduction to chapter 11

 

497

that operates such remote sensors, as well as radar technology and Lidar techniques that can
quickly obtain information about the fatigue and stress-strain history of a bridge. This approach
will make bridges safer and reduce life cycle costs.
Overload prevention and review of live loads: In light of the latest advancements in the truck
industry, it has become important to assess the magnitude of axle loads on highway bridges and
also update the military live loads on military routes due to new tanks. American Society of
Mechanical Engineers design codes need to be reviewed.
Use of high-friction surface: Introducing the British-invented surface treatment on asphalt
pavement to create a high-friction surface and favorable friction properties between vehicle tires
and bridge deck surfaces will help in braking and prevent skidding. Binders such as thermosetting
epoxies are used. Maryland has successfully introduced plates on their intersections.

Chapter 9 deals with substructure prefabrication techniques and construction management. The
progress in using prefabrication has been slower for substructure construction compared to that for
superstructure construction, especially for longer span bridges. It is easier to transport horizontal bridge

beams and slab panels on an SPMT than vertical pier bents due to their size. Also, post-tensioning may
be required for the substructure panels to make them watertight.
For emergency bridge replacements on important routes after floods, earthquakes, or accidents, etc.,
prefabrication of both pier and abutment members would help. Other key aspects of prefabricated substructure planning and management include:
 
Soil report: Since foundation design requires soil investigation, this operation should be started
well in advance by the owner even before the award of the contract.
Utility pipes: Advance coordination with the utility companies for supporting their pipes and
transferring from pavement elevation to deck elevation is required.
Deck drainage: The method of disposal of rainwater from the deck into public sewers also needs
to be planned.
Electrification: If deck lighting is provided, the power supply needs to be arranged from the
electric supply company and negotiations need to be started in advance, as the prefabrication
activity is in progress.
Precasting concrete and welding: Although prefabrication in a factory may not take as much time
as cast-in-place construction under the site conditions, the time required for the plan layout of
rebars, the curing of concrete components, and the welding of steel members, etc., remains
unchanged.
Planning: The additional time required to plan a route, obtain permits for heavy and wide loads,
and apply for police escort, as well as the hauling distance for the prefabricated bridge from the
factory to the site need to be taken into consideration.
Hauling heavy loads: Loading prefabricated components onto the SPMTs and unloading them at
the site as well as the required lifting and placing operations by the special cranes on-site need to
be taken into account in the overall schedule.
Stay-in-place formwork: The additional time and cost for hauling, lifting, and placing needs to be
analyzed in comparison to the cast-in-place construction erection time for temporary formwork or
using permanent stay-in-place formwork to make prefabrication as economical as possible.
Modular construction: The greatest benefit of prefabrication is for small spans, where the hauling
and lifting problems are fewer and pier construction is avoided. Arch structures combine superstructure girders with substructure curved columns and are more aesthetically pleasing.



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Leading prefabrication companies: There are a wide variety of bridge manufacturing companies
in the United States who have developed specialized bridges for repeated use. Examples include
High Steel Structures, Acrow, Jersey Precast, and CON/SPAN.
Need for standardization: AASHTO specifications have recommended minimum vertical and
horizontal clearance requirements. Similarly, many states have developed standard details for lane
widths, shoulder widths, and bicycle tracks, spaces for plants and flowers, etc. Span length
alternatives to conform to the width of the highway can be used to standardize bridge lengths.
Such ready-made standard span structures using concrete and steel can be made available off the
shelf and ready for delivery to sites, as required. A choice of colors is also available for aesthetic
requirements.

Quality control: Construction drawings for precast substructures are more specialized than conventional construction drawings. Typical review comments on reinforced concrete detailing of abutment
walls, pier caps, and columns are therefore necessary. Examples of necessary quality control measures
are review by expert bridge engineers of the connection details, location of hinges, seismic detailing,
lifting points, etc. Case studies of a variety of bridges using PBES for the substructure in the United
States are summarized in Chapter 9.
Foundation drawing reviews: Expert reviews can raise a number of issues and lead to various
recommendations. Some of these include the following:
 
•Foundation design is too expensive; footings are too big/deep. Review soil report.
•Monitor compaction before placing footings. Consider soil improvement techniques.
•Always get soil borings and a geotechnical report before foundation design and have geotechnical
oversight and testing during construction.

•Use deep piles or drilled piers.
•Use caissons or auger piles.
•Use tied spread footings.
•Check for retaining wall failure from settlement and overturning.
 
Alternate ABC contracting methods are also described in Chapter 9.
Various aspects of engineering management are presented, such as asset management (Chapters 2
and 10), disaster management (Chapter 3), design-build (Chapter 4), bridge failures and risk management (Chapter 6), and construction management (Chapter 9).
Chapter 10 addresses evaluation criteria for deficient infrastructure and alternative ABC methods.
ABC technology is still developing, although significant progress has been made in prefabrication. This
chapter highlights important alternatives to the well-established use of factory prefabrication and
SPMT, such as transportation by barges and lateral slide-in methods. The many impacts of rapid construction and traffic volume and lane closures are also given in Chapter 10.
In some cases it may be warranted to use lateral slide-in methods due to the limitations of transporting large bridges by road or on rivers for long distances and fitting them on SPMTs or barges. The
method consists of site casting adjacent to the existing bridge on temporary bents, followed by the use
of mechanical devices for sliding or the use of cranes to lift the superstructure in position. Case studies
have shown that many states have successfully used this modern technology.
A few states like Utah and Oregon have developed special provisions as part of their construction
specifications. In some cases it is possible to construct abutments prior to slide-in of the new bridge,
leading to further time savings.


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499

The use of new types of concrete materials for deck slab will enhance deck life and reduce life cycle
costs. Examples are Exodermic decks, Effidecks, and FRP and HPC deck panels. Several types of
lightweight precast girders such as NEXT beams, Wolf girders, and T-Bulbs have helped in providing
rapid delivery of bridges and reducing initial and life cycle costs.
The important and sensitive issue of generating additional funding through the P3 system is also

discussed in this chapter. Public investment has promoted much needed and timely reconstruction of
thousands of structurally deficient (SD) bridges.
The ASCE Report Card for infrastructure has put pressure on federal and state governments to take
the necessary measures to replace, repair, and rehabilitate the growing number of deficient and obsolete
bridges nationwide. FHWA’s “Every Day Counts” Program and FIU seminars on ABC have been training bridge engineers in new technology. Also, courtesy of FHWA, Website resources are now available.
These will help in training of contractors in the use lateral slide-in methods. The conclusions for this
chapter are presented at the end of the chapter. A wide range of appendices are presented on the following topics and are referred to in the text in the chapters:
 
Bibliography
ABC Glossary
Bridge Inspection Terminology
Three-Credit University Course in ABC
Training Courses and Workshops in ABC
Survey Form for Structural Countermeasures
ASCE Report Card—Innovations and New Technology
Rapid Construction of Timber, Aluminum, and Lightweight Bridges
TEMPLE-ASCE One-Day Course on Rapid Bridge Construction
 
A three-credit course syllabus shows the importance of theoretical and practical aspects and how the
AASHTO Load and Resistance Factor Design (LRFD) Specifications and Load and Resistance Factor
Rating (LRFR) Provisions need to be supplemented.
The salient features of the topics of the course are as follows:
 
Introduction and Objectives of Rapid Bridge Construction

 

Overview of Highway User’s Comforts; Examples of Bridge Failures, Deficient and Functionally
Obsolete Bridges; ASCE Report Card on Infrastructure.
Problems with Detour and Lane Closures for Considerable Length of Time.

Initiatives for ABC by FHWA, TRB, Selected States and Universities.
Bridge Inspections, Site Surveys, Testing, Alternates and Preliminary Designs.

11.1.2  Funding aspects
Economic Considerations in Planning, Value Engineering, Public-Private Partnership (P3).

11.1.3  Project management aspects
(Lump-sum, Design-bid-build, Design-build, CMAR etc.)
DBIA Recommendations


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11.1.4  Accelerated construction techniques
Use of SPMT, Slide-in, Roll-in, roll-out, Float-in, Bridge in a Back Pack, partial ABC.
High-capacity cranes, Rollers, etc.

11.1.5  Modern durable construction materials
Concretes in Deck, Prestressed Girders, Abutments, Piers and Foundations (HPC, UHPC, FRPC,
CFRPC, GFRPC, etc.)
Steel Girders (50W, HPS 70W, HPS 100W)

11.1.6  Type of superstructure and geometry of bridge
Slab-beam, Truss, Arch, Segmental, Cable-Stayed etc.

11.1.7  Type of substructure
Shallow Foundations, Piles, Drilled Shafts etc.


11.1.8  Funding constraints
Prioritization, P3 requirements.

11.1.9  Typical ABC construction specifications
•Bidding procedures
•Materials control, Precasting in Factory and Field Conditions
•Quality considerations
•Insurance, Warranty, and Surety Issues

11.1.10  Project management aspects
Technical Proposal to Client, Structural Planning, Feasibility Studies, Preliminary and Final designs,
Post design Services.

Part 2
11.2  Coordination with highway maintenance schedule
Maintenance and protection of traffic is a primary requirement during reconstruction. Work on the
bridge would affect traffic flow on the highway and vice versa. The volume of tasks for fixing the highway pavement and resurfacing takes much more time than required for the bridge repairs itself.
In practice, highway maintenance requires the following tasks, which can be performed parallel to
those on the bridges to avoid frequent lane closures.
 
•Cracks may happen in the road surface due to frequent rains and snow.
•Due to accidents, the median barrier can get damaged.
•Variable message sign structures may be added.
 


11.3  ABC applications for bridges located on rivers

501


Bridges are essential parts of the highway. Both the highway and the bridges require maintenance.
If a number of bridges need to be fixed on a busy highway, the work will most likely be done in the
window available in the same construction season. The schedule needs to be planned by the highway
agency.

11.2.1  Construction season restrictions
For deck repairs, longitudinal joints require cast-in-place concrete pours. All outdoor work, which
involves minor or major repairs (both for highway and for bridges), must be carried out in reasonable
weather conditions. Weather may vary according to the location of each state. Work will not be possible
if it rains the day wet concrete is planned for use or in extreme cold or hot weather. The bridge construction schedule and activities on the critical path must be planned while keeping an eye on highway repair
work and severe weather conditions.
When bridges on a given highway are due to for repairs or replacement, often some of the highway
sections and pavements also require fixing. Generally, bridge deficiencies and highway wear and tear
go together. In the interests of minimizing adverse impacts on the traveling public, the owner would
like to perform both the highway and bridge work at the same time using the same contractor, who can
deploy its resources (equipment and labor) in an efficient manner. This enables all of the work to be
done in the shortest possible time.
A review of research challenges by a key AASHTO subcommittee (T-4) on this developing subject
presented some areas still in need of further investigation. For more efficient project management and
faster implementation, improved coordination and communication skills need to be researched so that
ABC methods can be made more economical. A comprehensive construction code that spells out practical steps based on past experience for a refined and rapid type of construction needs to be
developed.

11.3  ABC applications for bridges located on rivers
Application of ABC for construction of bridges on rivers is possible, but the scope of work is greater
and it may take longer to complete than for bridges located at intersections. This is due partly because
banks of flowing rivers are subject to erosion and scour. Deep foundations, including longer piles, are
required, which increases the duration of construction. Moreover, the foundations need to be shielded
against erosion during floods, which requires special items such as river training, scour countermeasures, and retaining walls along the eroded banks. For the slide-in method, temporary bents adjacent to
the existing bridge need to be constructed in the river conditions. The additional work for extra items

gets added to the construction schedule and is discussed below.
Environmental Permit requirements: For continuous span bridges, piers are required in the middle of rivers. An application for a construction permit should be made to the state DEP, well ahead of
the start of the project. Otherwise, objectives of ABC will be defeated.
Documents need to be prepaid in support of the application forms for review by DEP. Meetings are
held to minimize obstructions to the flood flow such as eliminating piers and increasing opening sizes.
Cofferdams for foundation construction are required, and in some cases the river needs to be diverted
through an auxiliary bridge. Prefabricated members may not require SPMTs for the entire distance, but


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CHAPTER 11  A Review of Chapters, River Bridges, and Conclusions

transportation on barges will shorten the distance of travel from the factory to the site. Deep foundations, scour countermeasures, retaining walls, use of barges, etc., are major items that will increase the
overall duration and cost of the project. Nevertheless, the ABC design-build method will still be a great
help compared to the conventional method.

11.3.1  Scour and erosion of foundations at river bridges
In the United States, over 36,000 bridges are either scour critical or scour susceptible. Some examples
of recent bridge failures are:
 
•Schoharie Creek Bridge, located on the New York State Thruway, in 1987
•US 51 Bridge over Hatchie River in Tennessee in 1989
•Damage to bridges located on the Mississippi River in 1993
•Interstate 5 NB and SB bridges over Los Gatos Creek in California in 1995
•Route 46 Bridge on Peckman’s River Bridge in Passaic County in New Jersey in 1998
•Ovilla Road Bridge located in Ellis County in Texas in 2004
 
In this chapter the latest technology for scour countermeasures is introduced. Applications for
FHWA Circulars HEC-18 and HEC-23 are discussed. Since scour is a major problem for bridges

located on waterways, familiarity with the methods presented will benefit the engineer in terms of
safety and economical foundation design and also assist in solving constructability issues. According
to the AASHTO LRFD Specifications (Section C3.7.5):
 
Scour is the most common reason for the failure of highway bridges in the United States.
 
Scour excavates and carries away material from the bed and banks of a stream. Small brooks,
streams, rivers, and oceans all possess different degrees of kinetic energy. Scour or soil erosion at a
bridge is caused by the dynamic effects of water in motion.
Erosion is a very old subject that is currently analyzed using scientific disciplines such as:
 
•Hydrology
•Bridge hydraulics
•Soil cohesion
•Scour analysis
 
Erosion can be minimized by installing “countermeasures.” Stone lining or shielding of soil is the
oldest form of countermeasure. A countermeasure is defined by HEC-23 as:
 
A measure incorporated at a stream/bridge crossing system to monitor, control, inhibit, change,
delay, or minimize stream and bridge stability problems and scour.
 
The type of countermeasure is dependent upon:
 
•The nature of scour contraction or local
•Clear water or live bed
•Aggradation or degradation
•Meander or
•Debris accumulation.
 



11.3  ABC applications for bridges located on rivers

503

Scour-critical bridges across the United States are currently being retrofitted using different standards for countermeasures. The design procedures depend on individual bridge owners, representing
hundreds of city, county, and state government agencies. Design guidelines are being applied differently in different states for old bridges.
The author carried out research on this subject for a joint publication with Anil Agrawal with a goal
of developing a “Handbook for Scour Countermeasures.” The NJDOT Bureau of Research sponsored
the project, and a detailed report is now available on their Website for general use. Some of the countermeasure details provided in this chapter are based on the handbook. See />sportation/refdata/research/reports/FHWA-NJ-2005-027.pdf.

11.3.2  Factors affecting magnitude and rate of scour
Soil profiles for typical scour-critical bridges: The soil profile for a particular bridge site should be
based on boring logs. While detailed geotechnical and borehole testing needs to be carried out to obtain
site-specific information, studies will often utilize existing maps and information available with the
state and the U.S. Geological Survey (USGS). Different soil and rock materials will exhibit different
rates of scour. The kind of geologic material, coupled with the intensity and duration of a flood, will
determine scour depth.
Soil classification: Soil types are broadly classified as:
 
•Noncohesive (e.g., gravels, sands, and silts)
•Cohesive (silts, clays) materials
 
The magnitude of scour for cohesive and noncohesive soils is different, but scour takes much longer
in cohesive soils, resulting in longer bridge life. In cohesive soils such as clay, both the local scour and
contraction scour magnitudes may be similar, but scour takes place considerably later than in the noncohesive sand. The threshold of movement of particles of both cohesive and noncohesive materials
depends on:
 
•Particle size

•Density
•Shape
•Packing and orientation of bed material
 
Noncohesive sediments: Examples are sands, gravels, and silts that have a granular structure. Such
soils are considered to be the most susceptible to scour. The unbounded individual particles are susceptible to erosion when the applied fluid forces (drag and lift) are greater than the stabilizing forces (gravity and friction with adjacent bed particles).
Most fine-grained sediments (clay, silty clay, and clay mixtures) possess some cohesion, the clay
content being of great importance. Cohesive sediments typically require relatively large forces to
detach the particles and initiate movement, but relatively small forces to transport the particles
away.
Type of bed material: The bed material is comprised of sediments (alluvial deposits) or other erodible material. If bed materials are stratified, there is a greater risk of scour breaking through the more
resistant layer into the less resistant layer. A survey of U.S. bridges indicates that bridges founded in
sand have the most scour problems, as summarized in NCHRP 24-7.


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CHAPTER 11  A Review of Chapters, River Bridges, and Conclusions

The clay content in the soil increases cohesion, and relatively large forces are required to erode the
riverbed. Higher pulsating drag and lift forces increase dynamic action on aggregates until the bonds
between aggregates are gradually destroyed and aggregates are carried away by the flow. An approach
estimating scour in cohesive soil is to couple erosion rate with the cumulative duration of flows that
exceed the threshold velocity for particle movement.
Soil types: Jean-Louis Briaud at Texas A&M University has proposed the SRICOS method of scour
measurement. Streambeds that either consist of bedrock or contain a high percentage of oversized
cobbles and boulders are the most scour-resistant materials. To determine rock quality careful evaluation is needed to assess factors such as:
 
•Strength
•Fracture frequency

•Weathering
•Slaking
 
A breakdown of scour problems by soil type is given in Table 11.1. However, since geologic conditions vary widely across the country, the statistics for each state will vary.

11.3.3  River inspections
In the absence of scour analysis, the Recording and Coding Guide of the National Bridge Inspection
Standards (NBIS) or AASHTO Guidelines should be used to classify the bridge if it is scour critical.
Items 61, 71, and 113 of the Recording and Coding Guide can be of significant concern during underwater inspections (see the Appendix for items 61 and 71). Item 113 is used to determine the Scour
Rating. A list of Bridge Inspection Terminology and Sufficiency Ratings used by PennDOT is given in
Appendix 3.

11.3.4  Damage from flood scour
Minimal marine life disruption and quick construction are being achieved by gabion baskets, articulated concrete, or cable-tied blocks in lieu of traditional riprap. Sheet piles were used for the new
Schuylkill River Bridge and SEPTA’s 30th Street Station Bridge (by the author). The author

Table 11.1  Soil Types with Scour Problems (NCHRP 24-7)
Sediment Type
Sand
Cohesive
Mixed
Gravel
Bedrock
Uncertain
Silt
Total

Percent
48
19

13
10
5
5
0
100


11.4  Planning of bridges over rivers

505

prepared a “Handbook for Scour Countermeasures” for NJDOT jointly with CUNY, which was
approved by FHWA, and helped developed Sections 45 and 46 of the NJDOT Bridge Design
Manual.

11.3.5  Structures on water crossings
Study of scour critical bridges in the Northeast USA: In the past twenty years, the author investigated
the effects of flash floods and 50-year floods and the many damages caused to the superstructure, substructure and foundations.
Research reports were prepared for the identified scour critical bridges in Massachusetts, New
­Jersey, Pennsylvania, Delaware and Maryland.
Some of the findings are presented here.
 
•Unless founded on rock, all structures crossing water shall be supported on piles or have other
positive protection to prevent scour of the substructure.
•Cofferdams should be evaluated with regard to need, type, size, constructability, and cost.
Alternative types of construction such as causeways, caissons, or drilled shafts should be considered and compared to conventional cofferdam costs.
•The estimated maximum depth of scour should be used to determine overall structure stability.
Piles should be socketed into rock if scour can affect their stability. Recommendations for details
will be contained in the foundation design report (FDR).

•In addition to areas for repairs identified in the last underwater inspection and evaluation report, a
field inspection and a new underwater inspection need to be carried out for field verification of the
latest substructure conditions underwater.
 
Figure 11.1 shows damage to pier concrete due to fast moving flood water making the bridge unsafe
for heavy traffic.

11.4  Planning of bridges over rivers
Use of Float-in Method: The modular substructure and superstructure transportation problem to the
river site will be solved by the float-in method (as shown in Figure 11.1 to avoid non-composite concrete
cracks). It will help immensely in conducting complex construction over rivers. Existing continuous
spans may be replaced by a single span, using high-strength construction material such as HPS and
HPC that are fabricated in the factory rather than on the site.
The latest methods for repairing deteriorated concrete and repointing mortar joints, the applicable
design details from AASHTO, and the applicable state bridge design manual can all be used as necessary. If the current NBIS rating given in the inspection report is higher for the abutments than for the
piers, the load rating will be upgraded by performing the recommended repairs.
Perform substructure repairs both above and below the waterline for the following
conditions:
 
•Abutment showing deterioration
•Abutment back wall with wide cracks at the north and south ends


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CHAPTER 11  A Review of Chapters, River Bridges, and Conclusions

FIGURE 11.1
Damage to pier concrete due to fast moving flood water making the bridge unsafe for heavy traffic.


•Concrete aprons at the piers exhibiting wide cracks
•Deteriorated expansion joint and back wall elements
•Removal of buildup of sand debris at piers
•Removal of any tree trunks or tree roots between piers
•Tooth dam at abutment not functioning and needs to be replaced
 
An estimate of the cost and repair quantities is required in each case for scour-critical bridges.
Unlike the HS-25 live loads, which are defined fairly accurately, flood magnitudes are unexpected and
of unpredictable magnitudes and are difficult to compute accurately for the required 50-, 100-, and
500-year intensity floods.
StreamStats hydraulic analysis software: There is a need for preparation of several databases
(including demography changes in urban river locations) as required by the latest STREAMSTATS
software, which is developed using hydrologic studies on rivers in each state by the USGS. Updates are
required for the following reasons:
 
•The Rational Formula for computing flood discharge (which generally has been used) is approximate and has led to failures of bridges subjected to floods nationwide.
•Also, HEC-18 provisions have recently been revised.
•DM-2 and DM-4 specifications for flood computations and countermeasures design based on the
old version of HEC-18 need to be updated so that foundations of bridges located on rivers can be
safe in accordance with the new specifications.
 
Due to increased corrosion of steel bridges on waterways due to daily evaporation, the life of such
bridges is adversely affected. During floods, countermeasures should not get displaced. Sediment
deposits in some rivers require dredging and clearing of stones under bridges before it is too late.
Hence, frequent inspections may be required.


11.5  Issues of scour-critical bridges

507


11.5  Issues of scour-critical bridges
Examples of original planning defects are narrow openings and shallow foundation depths. Old bridges
are likely to have planning defects as compared to new bridges. The effects of floods include both
aggradations and degradation. In earlier days there were no scour analysis criteria. As per AASHTO it
is now required to conduct such analysis. There is a considerable increase in velocity due to increase in
discharge or reduction in flow area under the bridge.
V = Q/A



where V = Peak flood velocity
 
Q = Flood discharge and
A = Cross-sectional area of bridge opening.
 
Potential issues include the following:
 
•The original design of the flow area may be inadequate.
•The river has meandered over a long period and direction of flow is skewed.
•Debris may accumulate during floods, reducing the size of the opening.
•Heavy storms, increased snow and subsequent melting, global warming of glaciers, and changes
in demography will increase discharge.
 
When both Q increases and A decreases simultaneously, the magnitude of increase in velocity will
make the bridge “scour critical.”
Scour is very much a site-specific issue as no two rivers are alike even though bridges may be alike.
Physical parameters include:

 


 

River configuration types
Straight
Braided or multichannel
Meandering
River bottom types
Aggrading
Stable
Degrading

Scour at bends due to river meander: Special consideration must be given to scour for bridges
located on bends. Maynord’s equation (Maynord, 1995) is helpful to determine flow depths for these
conditions and is equally applicable at abutments and piers. When scour occurs at the confluence of two
rivers, flow depths can be estimated using Ashmore and Parker (1983) or Klaassen and Vermeer’s
(1988) equations; they will be applicable at abutments and piers. It is noted that the magnitudes of scour
due to thalweg effects and migration of bed forms are typically small and are usually neglected.
Bridges on waterways have the following additional problems:
 
•Restrictions from environmental agencies in placing piers in riverbeds, resulting in longer spans
•Difficulties in designing and constructing deep foundations in flowing water
•Difficulties in maintaining bridge substructure underwater and in painting of corroded girders


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CHAPTER 11  A Review of Chapters, River Bridges, and Conclusions

11.5.1  Scour analysis

Codes and design guidelines: The following FHWA and AASHTO publications serve as major resources
for scour analysis and design:
 
Riverine Flow HEC-18, “Evaluating Scour at Bridges”
HEC-20, “Stream Stability at Highway Structures”
HEC-23, “Countermeasures”
Tidal Flow HEC-25, “Tidal Hydrology, Hydraulics, and Scour at Bridges”
AASHTO LRFD Bridge Design Specifications
Model Drainage Manual (AASHTO)
 
Also, the Maryland, New Jersey, Pennsylvania, and Florida state codes, among others, can be
­consulted. NCHRP materials and CIRIA (British code) may also be useful.

11.5.2  Design floods
The aim should be to design bridges for all times and for all occasions. AASHTO (LRFD) load combinations for extreme conditions are applicable. The extreme-event limit states relate to flood events
with return periods (usually 100 years) in excess of the design life of the bridge (usually 75 years).
Foundations of new bridges, bridges to be widened, or bridges to be replaced should be designed to
resist scour based on 100-year-design flood criteria, reviewing conditions that may create the deepest
scour at the foundations. The author designed Peckman’s River Bridge on Route 46 in North Jersey
after Hurricane Floyd had subsided. Figure 11.2 shows damage even to the girders from
overtopping.
Check flood for bridge scour: The foundation design should be reviewed using a 500-year check
flood, or 1.7 times a 100-year flood, if 500-year flood information is not available.

FIGURE 11.2
Hurricane Floyd High water elevation causing damage from over topping.
(Photo by the author during peak floods.)


11.5  Issues of scour-critical bridges


11.5.3  Evaluation of the need for countermeasures

509

 
. Post-flood inspection in shallow water
1
2. Post-flood inspection in deep water
 
The location of a bridge influences the formation of scour at its foundation. Bridges located on a
straight, meandering, or sloping thalweg, at a confluence, or downstream of a dam will all have different degrees of scour.
 
Bridge footings: Countermeasures are required for scour-critical bridges. Ideally, a
­recommended scour countermeasure will permanently eliminate a bridge’s potential vulnerability to scour damage for the peak floods. A permit from the state is required to install
armoring.
 
Periodic inspections after major floods or coastal storm surge.
 
Bridge Scour Evaluation: A nationwide survey was conducted by the author’s research team
for the types of scour countermeasures being used by each state. Appendix 9 shows a questionnaire
survey form for scour countermeasures. The feedback received gave useful information on the type
of countermeasure used and its performance. Scour-critical bridges located on streams with high
flood velocities can cause major foundation settlements. Bridges with narrow waterway openings
and soft erodible soils contribute to bridge collapse.
The magnitude and depth of erosion depend upon discharge volume and velocity. Rivers, rivulets,
streams, brooks, and channels are all subjected to scour to varying degrees. A channel, for example,
may have a small discharge but a high velocity. Hence, scour is present in all types of rivers, narrow or
wide. The two main issues are hydraulics and soil science, i.e., the interaction between water and soil.
Nonerodible rock will not be subjected to scour.

The following items discuss procedures for the assessment of scour at all bridges that are 20 ft or
greater in length that spans water. There are two basic types of assessment:
 
•Field-viewed bridge site assessments, for which USGS personnel visit the bridge site
•Office-reviewed bridge site assessments, for which USGS personnel compile data and do not visit
the bridge site
 
Both types of assessments are primarily focused at meeting the requirements of the FHWA mandate. Date of bridge construction and the accessibility of the bridge substructure units for inspection
determine which type of assessment a bridge receives.
Pennsylvania State Scour Code: A web-based Scour-Critical Bridge Indicator (SCBI) Code as
developed by USGS is used. The SCBI Code indicates the vulnerability of the bridge to future scour. It
is based on the FHWA code (NBI Item 113) (FHWA 1989) and PennDOT’s interpretation of the FHWA
Code (Bryan Spangler, PennDOT, written communication, 1999).
The SCBI Code contains a whole number between 9 and 2. Each code number has one or more
cases. Scour Assessment Rating (SAR) is computed from select collected and compiled structure components, and hydrologic and hydraulic data. Agency personnel assign the final SCBI Code and an SAR
(between 0 and 100) on the basis of their review of all data.
The SCBI Code and SAR calculator use various factors from the field or office scour evaluations to
determine the SCBI Code for individual subunits and the bridge. Field view, soil maps, and previous


510

CHAPTER 11  A Review of Chapters, River Bridges, and Conclusions

inspection reports are required. The calculator allows inspection personnel in Pennsylvania to determine overall bridge rating when:
 
•Review of bridge reports identify undetermined historical data or revised field data
•Site conditions change
•New scenarios have developed
•New bridges are constructed

 

11.5.4  Types of scour
According to HEC-18, general scour is a lowering of the streambed across the stream or waterway at
the bridge. This lowering may be uniform across the bed or nonuniform. General scour may result from
contraction of the flow or other general scour conditions such as flow around a bend. Total scour is the
sum of long-term degradation, general (contraction) scour, and local scour.
Contraction scour is the component of scour resulting from a contraction of the flow area at the
bridge. It causes an increase in velocity and shear stress on the bed at the bridge. If the abutments are
located outside the width of a channel, no contraction takes place and there will be no contraction scour.
The initial scouring in low flows is known as “clear water” scour. Clear water scour is scour at the
pier or abutment (contraction scour) when there is no movement of bed material upstream of the bridge
crossing at the flow causing bridge scour. If the flow continues to increase, “live bed” scour can occur,
which is general movement of the bed. Live bed scour depth increases with the increase in the size of
bed material D50 in the riverbed, while clear water scour decreases as mean bed material size Dm
increases. The increased velocity affects the stability of the streambed. For HEC 18, scour depth is
computed from Laursen’s equation for channel contraction within the total bridge opening. In terms of
magnitude, it may be higher at the piers or at the abutments.
Local scour is removal of material from around piers, abutments, spurs, and embankments caused
by an acceleration of flow and resulting vortices induced by obstructions to the flow. Local features at
a bridge such as abutments, piers, weirs, cofferdams, and dikes may obstruct and deflect the flow. The
substructures increase the local flow velocities and turbulence levels, giving rise to vortices that may
increase the erosion of the riverbed.
At the piers, local scour is computed using the Colorado State University (CSU) equation. It is
dependent upon many factors including length of pier, width of pier, and the angle of attack. Abutment
scour is computed from Froehlich’s and Hire’s equations. It is dependent upon many factors including
length of embankment.
The flow of water is on both sides of a pier, generating vortices and eddy currents, while the flow is
on one side only for abutments. This results in higher scour depths at piers than that seen for local scour
at abutments.

Ultimate scour is the maximum depth of scour attained for a given flow condition and may require
multiple flow events, in cemented or cohesive soils over a long period.
A flow diagram was developed by the author for the NJDOT Bridge Design Manual Section on
scour at bridges. The flow diagram shows that several types of analyses need to be carried out:
 
Inspection reports
Geotechnical analysis
Hydrologic analysis
Hydraulics analysis


11.5  Issues of scour-critical bridges

 

511

Scour analysis and
Detailed countermeasures design.

The complete original procedures for determining the SCBI Code can be found in Cinotto and
White (2000). The SCBI Code algorithm used by the web-based SCBI Code and SAR calculator was
modified from Cinotto and White (2000) to eliminate the comparisons of USGS and PennDOT data.
Geotechnical: Both boring information and grain size analysis would be needed for accurate determination of scour. Collection and processing of geomorphic, hydrologic, and hydraulic data for assessment of scour at bridges require borehole information for soil characteristics.
The size of the opening or degree of obstruction from abutments, piers, and foundations will influence the velocity of water. The catchment area of a river, its source of supply, demography, storms, and
melting of glaciers will influence the volume of discharge and erosion. The use of a gabion mat and
baskets on a New Hampshire project for scour analysis on Monatiquot River is shown in the author’s
textbook “Bridge and Highway Structure Rehabilitation and Repair” published by McGraw-Hill, 2010.
Figure 11.3 shows use of Gabion Mat between abutment walls.
w

Flo uot
tiq
na
Mo river

A

0'

40.

A
Plan

58.7'
Underside of
deck beam

6.0
(TYP.)

Flow

1.0' TO 2.0'
3.0
(TYP.)

3' × 3' Gabion wire basket
Anchor block (TYP.)
Section A–A

East abutment

FIGURE 11.3
Use of gabion mat between abutment walls.

10.8'

Gabion mat


512

CHAPTER 11  A Review of Chapters, River Bridges, and Conclusions

11.6  Rapid repairs and replacement of bridges on rivers
11.6.1  NBIS condition rating
The overall condition of substructures of 19 bridges was assigned the following NBIS condition
rating:
 
Very good condition: No problems noted.
Good condition: Some minor problems.
Satisfactory condition: Structural elements show some minor deterioration.
Fair condition: All primary elements are sound, but may have minor section loss, cracking, or
spalling.
Poor condition: Advance section loss, deterioration, or spalling of primary structural elements.
 
Aggradation is a common problem due to the tree environment close to the river banks.

11.6.2  In-depth bridge inspections
The purpose of the inspections is to identify levels and areas of deterioration of all structural and nonstructural substructure elements in order to develop repair recommendations and details. This effort

also includes correlating probing measurements taken near the pier edges with the previous substructure inspections. FHWA has adopted three diving inspection intensity levels. The first two levels can be
described as follows:
 
Level I: Visual, tactile inspection (100% “swim-by” at arm’s length)
Level II: Detailed inspection with partial cleaning (100% “swim-by” with 10% cleaning)
 
FHWA Level II requires that portions of the structure be cleaned of marine growth to identify possible damaged and/or deteriorated areas that may be hidden by surface growth. The cleaning must be
performed on at least 10% of all underwater elements. The equipment used to inspect the majority of
the bridges consists of a small boat, sounding rod, hand tools, and line-tended SCUBA.
FHWA Level III diving inspections: This is a highly detailed inspection with nondestructive testing.
Testers are inspecting a critical structural element where extensive repair is contemplated. Based on underwater inspection reports, the defects in Table 11.2 are typical of what may exist for a scour-critical bridge.

11.6.3  Concept study report and plans
Based on the findings of the condition assessment, potential repair/remediation recommendations will
be developed. Fluctuating river elevations will be taken into account when developing repair recommendations and reviewing the construction feasibility.
Evaluation shall include but not be limited to:
 
•Evaluation of constructability and construction staging
•Community impacts during construction: impacts to emergency vehicle response, tourist industry,
traffic delays, pedestrians and bicyclists, local businesses, and noise
•Development of construction cost estimates for each feasible alternative along with the anticipated
construction schedule


11.6  Rapid repairs and replacement of bridges on rivers

513

Select bridge location and span


Is bridge location
acceptable to scour?

NO

No risk

YES
Structural planning for waterway opening

Field data collection.
Obtain river x-sections.

Obtain hydraulic & flood data
Obtain geo-technical data.
Check bridge for AASHTO Extreme loads.
Set Foundation elevation. Design Spread
Footing or Deep Foundations.
Check stream stability

Field Surveys
FEMA Insurance Study
AASHTO Model Drainage
Manual
Geo-technical Report

AASHTO LRFD,
Section: 2 & 9
HEC-20


hydrologic analysis river or
tidal Hydraulic analysis.

Identify abutment or pier. Perform scour analysis.

HEC-18

Inspection & maintenance

FIGURE 11.4
Flow diagram for scour analysis.

•MPT schemes shall be prepared for each feasible alternative
•Right-of-way requirements
•Environmental impact to the waterway and endangered species
 
A Draft Concept Study Report, including plans and recommendations, should be created to provide a concise aggregation of the important elements of the condition evaluation and an overall