The structural strengthening of bridges by post-tensioning

Artificial Load, 18782.3 Prestressing By Ballast Load, Railway Bridge Over The Elbe River2.4 Prestressed Arch Bridge By Force Regulation 2.5 Aare Bridge, Trusses Strengthened By Polygona

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The Cooper Union Albert Nerken School O f Engineering

THE STRUCTURAL STRENGTHENING OF BRIDGES BY

POST-TENSIONING

by

Derek Steven Constable Advised by Dr Cosmas A Tzavelis

A thesis submitted in partial fulfillment

o f the requirements for the degree o f

Master o f Engineering

December 16, 1999

The Cooper Union For The Advancement O f Science And Art

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UMI Number 1397436

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UMI Microform 1397436 Copyright 2000 by Bell & Howell Information and Learning Company All rights reserved This microform edition is protected against unauthorized copying under Title 17, United States Code.

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The Cooper Union For The Advancement Of Science And Art

Albert Nerken School O f Engineering

This thesis was prepared under the direction o f the Candidate's Thesis Advisor and has received approval It was submitted to the Dean o f the School o f Engineering and the hill Faculty, and was approved as partial fulfillment o f the requirements for the degree o f Master o f Engineering.

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to my father who gave me the inspiration and means to do this and to my mother who ju st gave without questioning

i

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Since the erection o f the earliest structures there has been the need for structural strengthening The necessity for strengthening originates primarily from insufficient load capacities, structural deterioration by environmental and service effects, design and construction inadequacies, or inadequate performance In the case o f bridges, the need has never before been so noticeable The performance o f our aging bridges is falling significantly short o f our needs

As o f June 30, 1996, 19.6 percent o f our nations bridges are or should be load posted because o f structural deficiencies or functional obsolescence The challenge is to address these bridge

deficiencies with limited funds A feasible and economic method to strengthen bridges is by posttensioning Post-tensioning is applicable to nearly all structural and material types However, bridge post-tensioning is wrongly often not regarded as the preferred alternative for structural upgrades Other strengthening schemes, partial structural replacement or total structural replacement are often uneconomically chosen over p o s t-te nsioning.

With the advent o f advanced structural analysis tools and field assessment instrumentation has come greater acceptance o f strengthening by post-tensioning As well, future technology should greatly increase its acceptability The future will bring forth advanced materials with greater environmental and service durability and more predictable mechanical characteristics as well as advanced health monitoring techniques that may more accurately assess the condition and

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capacity o f our bridges These technologies will enable more confident and economical decisions aimed at extending the service life o f structures In the near future, these two technologies will be applied in conjunction as smart fiber reinforced polymer composite tensioning systems.

This thesis addresses the situations where bridge strengthening may be needed, why and when strengthening by post-tensioning should be included in the alternatives for upgrading bridges which are structurally deficient and, if chosen, how to go about designing and constructing the strengthening system The argument is approached from multiple perspectives o f which economics, safety and mobility are always o f primary importance

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The Structural Strengthening O f Bridges By Post-Tensioning

TABLE OF CONTENTS

List o f Figures List o f Tables

3.1 Increase Bridge Load Rating3.2 Correct Inadequate Design And Construction3.2.1 Inadequate Steel Reinforcement3.2.2 Excessive Deflections

3.2.3 Seismic Retrofits3.2.4 Other Performance Improvements3.3 Emergency Repair

3.4 Strengthening For Construction3.5 Historically And Culturally Significant Bridges3.6 Cited References

4 The Theory, Design And Construction Concepts O f Bridge Strengthening By Tensioning p 62

Post-4.1 Post-Tensioning Construction Operations And Stages4.2 The Principle O f Prestressing

4.3 Active Versus Passive Strengthening Systems4.4 The Difference Between Post-Tensioned Concrete And Post-Tensioned S tre n gthen in g Systems

4.5 The Mechanics O f A Post-Tensioned Axial Load Carrying Member4.6 The Mechanics O f A Post-Tensioned Beam

4.7 Prestressing Steel Mechanical Properties4.8 Anchorages

4.9 Post-Tension Force Losses4.9.1 Friction Loss4.9.2 Anchorage Slip4.9.3 The Relaxation O f Steel Tendons4.9.4 Controlling The Post-Tensioning Force4.10 Protection O f Tendons And Anchorages From The Environment

IV

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4.11 Design And Construction Standards And Specifications4.11.1 AASHTO LFD And LRFD Standard Specifications For Highway Bridges4.11.2 Federal Procedures-96: Standard Specifications For Construction O f Roads

And Bridges On Federal Highway Projects4.11.3 A S ™ Volume 1.04, Steel

4.11.4 Discussion On Specifications4.12 Design And Construction Considerations

5 When To Use Strengthening - WhenNot To Use Strengthening p 162

5.1 Selection O f Post-Tensioned Strengthening Option5.2 Strength Evaluation By An Integral Field And Analytical Investigation5.3 Life-Cycle Cost

5.4 Build Then Forget?

5.5 Cited References

6 Case Studies p 1766.1 Case Study One: Strengthening Simple Span Composite Steel Beam Bridges By Post- Tensioning p 1766.1.1 Summary

6.1.2 Background And Need6.1.3 The Investigations' Considerations And Findings6.1.4 Recommended Design Procedure For The Strengthening Of Simply Supported

Exterior Beams6.1.5 Analytic Ultimate Strength Model O f An Isolated Post-Tensioned Beam6.1.6 Ultimate Strength O f An Isolated Post-Tensioned Beam Compared To The

Ultimate Strength O f A Bridge System6.1.7 Conclusions And Recommendations6.1.8 Cited References

6.2 Case Study Two: Strengthening Continuous Span Composite Steel Beam Bridges By

6.2.1 Summary6.2.2 Background And Need6.2.3 The Dual Strengthening System6.2.4 Experimental And Analytical Investigation6.2.5 Design Methodology For Strengthening6.2.6 Cited References

6.3 Case Study Three: Strengthening Bridge Pier Caps By Post-Tensioning p 2296.3.1 Background And Need

6.3.2 The Remediation Plan6.3.3 Strengthening O f The Pier Caps

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6.3.4 Conclusions And Recommendations

7 The Future O f Strengthening By Post-Tensioning p 2447.1 Fiber Reinforced Polymer Prestressing Systems

7.1.1 The Benefits O f Fiber Reinforced Polymer Prestressing Systems7.1.2 Fiber Reinforced Polymer Material Properties And Their Comparison To

Prestressing Steel7.1.3 Research And Development Needs7.2 Health Monitoring And Assessment Utilizing Smart FRP Prestressing Systems7.3 Post-Tensioned Steel Plate Girders For New Construction

7.4 Cited References

8.1 Design Conclusions8.2 Construction Conclusions8.3 Recommended Continued Studies

9.1 Bibliography

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LIST OF FIGURES

2.1 Prestressed Truss By H Rider, 18502.2 Elbe Bridge, Prestressing By An Artificial Load, 18782.3 Prestressing By Ballast Load, Railway Bridge Over The Elbe River2.4 Prestressed Arch Bridge By Force Regulation

2.5 Aare Bridge, Trusses Strengthened By Polygonal Cable Configurations, 19692.6 Aare Bridge, Details O f Cable Support At Midspan, 1969

2.7 Aare Bridge, Detail O f Cable Anchorage, 19692.8 Post-Tensioned Bridge Beam, 1984

3.1 Status O f Bridges Approved For The Highway Bridge Replacement And

Rehabilitation Program3.2 California’s Maximum Permit Load3.3 Pier Cap Strengthened By Post-Tensioning, Interstate 495, Maryland3.4 Pier Cap Post-Tensioning Anchorage Bearing Plate, Interstate 495, Maryland3.5 Pier Cap Post-Tensioning Tendons And Deviation Saddle, Interstate 495,

Maryland3.6 Pier Cap Post-Tensioning Anchorage Bearing Plate And Wiring For Strand

Monitoring, Interstate 495, Maryland3.7 Post-Tensioned Earth-Filled Arch, Bridge Number 3094, Maryland Route 147

Over Gunpowder Falls3.8 Earth Filled Arch Tie Rods, Bridge Number 3094, Maryland Route 147 Over

Gunpowder Falls3.9 Typical Voided Slab Plan And Section3.10 Prestressed Concrete Girder Damage Repair By Post-Tensioning3.11 Prestressed Concrete Box Girder Damage Repair By Post-Tensioning3.12 Thrust Pit Bracing Plan

3.13 Thrust Pit Section3.14 Post-Tensioned Slurry Wall Typical Elevation And Section3.15 Post-Tensioned Slurry Wall Horizontal Section

3.16 Post-Tensioned Slurry Wall Anchorage Details3.17 Truss Strengthened By Superimposed Arch, Baltimore County Bridge Number 18,

Sparks Road Over Gunpowder Falls, Maryland3.18 Superimposed Arch Splice To Truss Vertical Member, Baltimore County Bridge

Number 18, Sparks Road Over Gunpowder Falls, Maryland

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3 1 9 Superim posed Arch Bearing End, Baltimore County Bridge Number 18, Spades

Road Over Gunpowder Falls, Maryland

4.1 Active Versus Passive Strengthening System4.2 External Post-Tensioned Concrete Tendon System4.3 Prestressed Beam Equivalent Loads

4.4 Prestressed Beam Stress Distribution4.5 Prestressed Beam Tendon Deformation Under Additional Load4.6 Stress-Strain Diagrams O f Prestressing Versus Mild Steels4.7 Typical Stress-Strain Curves For Prestressing Steels4.8 Magnel Sandwich Plate Wire Anchorage (courtesy o f Troitsky, 1990)4.9 Strand Wedges

4.10 Strand Anchorages And Couplers4.11 Mono-Strand Anchorage

4.12 Strand Chuck4.13 Multi-Strand Tensioning Jack4.14 Mono-Strand Tensioning Jack4.15 Threaded Bar Anchorage4.16 Shell-And-Bar Strand Anchorage4.17 Shell-And-Bar Strand Anchorage, Interstate 495, Maryland, Pier Cap

Strengthening4.18 Threadbar Tensioning Jack4.19 Smooth Bar Anchorage Systems4.20 Tendon Deviation Support4.21 Friction Loss Along A Tendon4.22 Derivation O f Formulas For Calculation O f The Effects O f Anchor Set4.23 Percent O f Initial Prestress Force Loss Due To Anchor Slip

4.24 Comparison O f Strand Relaxation Losses4.25 Stress Relaxation Curves

4.26 Final Stress Ratio Versus Initial Stress Ratio4.27 Typical Tendon Stressing Log

4.28 Prorated Graph O f Jacking Force Versus Elongation

5.1 Bridge Field Inspection Report5.2 Methodology For Selection O f Bridge Improvement Option

6.1.1 Bridges Included In Regression Analysis For Distribution Fractions6.1.2 Regression Formula Variables

6.1.3 Regression Formulas For Force And Moment Fractions, Post-Tensioned Exterior

Beams, Skew o f 0 To 45 Degrees6.1.4 Post-Tensioned Beams And Moment Diagrams

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6.1.5 R ecom m ended Interpolation For Distribution Fractions At Locations Other Than

Mid-Span6.1.6 Idealized Composite Post-Tensioned Beam Failure Mechanism

6.2.1 Strengthening Method For Continuous Span Beams6.2.2 Strengthening Schemes

6.2.3 Effect O f Strengthening Scheme (a) Post-Tensioned End Span Exterior Beams6.2.4 Parameters Considered In Analysis O f Distribution Factors

6.2.5 Regression Formula Variables

6.3.1 Governor Thomas Johnson Memorial Bridge6.3.2 Deep Water Pier Cap Dimensions

6.3.3 Thomas Johnson Memorial Bridge Post-Tensioned Pier Cap6.3.4 Pier Cap Post-Tensioning System

6.3.5 Pier Cap Post-Tensioning System6.3.6 Costs Associated With Repair O f Bridge

7.1 PARAFTL Strand, Coupler And Spike Wedge7.2 FRP Reinforcing Fibers Stress-Strain Curves7.3 Testing O f Concrete Beam Post-Tensioned With PARAFIL Tendons7.4 Fiber Optic Wires To Be Placed Within A Laminate Composite7.5 Post-Tensioned Plate Girder Schematic

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1 INTRODUCTION

The United States’ transportation providers are faced with the enormous problem that many o f their bridges are structurally deficient or functionally obsolete This problem is currently an economic burden o f incredible proportions and, unless counteractive measures are taken, will become an even larger burden According to the 1997 report to the United States Congress, “The Status O f The Nation’s Highway Bridges: Highway Bridge Replacement And Rehabilitation Program And National Bridge Inventory”, 31.4 percent o f our bridges are structurally deficient or functionally obsolete Since the net material worth o f our nation's bridges is roughly estimated at

300 billion dollars, and one-third o f our bridges require replacement or rehabilitation, this equates

to upwards o f 75 billion dollars o f replacement or rehabilitation costs But, more important than the net material worth o f our bridges is their net worth to our economy, which is even larger and not quantifiable

While structural strengthening may benefit many o f these deficient bridges, the most accurate representation o f those bridges that may benefit from stren g th e n in g are those which require load posting The 1997 report indicates that o f all the nation’s bridges (581,862), 19.6 percent (182,726) are or should be load posted because o f inadequate load capacities Load posting requirements are indicative o f the inability o f bridges to serve their intended use This condition has resulted primarily from environmental and service deterioration, increases in legal trucking loads, changes in specifications and standards, and increased dead weight from either resurfacing

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or the addition o f bridge features Load posting has incurred sizeable costs to the freight industry

as motor carriers are required to take alternate routes over greater distances to avoid posted bridges

Bridges that require load posting fell into two groups The first group includes structurally deficient bridges that have deteriorated to the extent that they cannot carry the load for which they were designed The second group includes functionally obsolete bridges that are in good condition but whose current State legal load exceeds the originaL design load and therefore require posting (Federal Highway Administration, 1997)

There are three possible solutions to this problem The first solution is bridge replacement, an extremely expensive solution not only because o f the tangible costs o f reconstruction, but also because o f the intangible costs o f inconvenience to the traveling public in the form o f additional times and distances traveled as a result o f detours and increased fuel consumption (collectively termed road user costs) The second solution is posting load restrictions where trucks with loads exceeding the posted load limits would be required to take alternative routes attending intangible costs as indicated in the first solution The third solution is to strengthen these bridges (Podolny, 1990)

Today's challenge is to address these deficiencies with limited funds A feasible and extremely cost-effective method to strengthen bridges is by post-tensioning Bridge p o s t-te nsioning dates back to the late 1800's and early 1900's and has been used steadily since, but wrongly, is often not

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regarded as the preferred alternative Other strengthening schemes, partial structural replacement,

or total structural replacement are often uneconomically chosen over post-tensioning

The basic principle o f post-tensioning is “the introduction o f internal stresses o f such magnitude and distribution that the stresses resulting from additional loadings are counteracted to a desired degree” While most associate post-tensioning with only concrete, almost any material is

conducive to post-tensioning whether steel, masonry, timber, composites or synthetics

The state o f the art o f post-tensioning has changed little since its inception However, with the advent o f advanced structural analysis tools including finite element modeling and various field- testing and response data acquisition systems, there has come an increased understanding o f the responses o f various structural systems to post-tensioning Future technologies should greatly increase the acceptability o f post-tension strengthening The current focus o f bridge research and development is for more advanced materials and the health monitoring and assessment o f in-place structural systems With the introduction o f advanced materials will come materials with greater durability under environmental and service effects and more predictable performance

characteristics with respect to time, stress levels, etc With the introduction o f advanced health monitoring instrumentation and response data acquisition systems will come the ability to more accurately assess the condition and capacity o f our existing bridges from which more confident and economical decisions can be made to extend their service lives In the near future, these two technologies will merge and be applied in the form o f smart fiber reinforced polymer (FRP) composite tensioning systems

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This thesis’ first section, The History O f Strengthening By Post-Tensioning will give a brief background on how post-tension strengthening has been used to date Next, the section The

Need For The Structural Strengthening O f Bridges will present and assess the reasons why

strengthening is needed Then the discussion will turn to the heart o f the subject where the

section The Theory, Design And Construction Concepts o f Bridge Strengthening By Post-

Tensioning will present the principles o f post-tensioning, the mechanics o f post-tensioned

structural members, the materials used for post-tensioning, and design and construction

specifications and considerations From there, the all important question When To Use

Strengthening - When Not To Use Strengthening will be addressed with a discussion on the

required steps to be taken to make an informed engineering decision Then, various case studies will be presented which will demonstrate current findings and the attending design and

construction standards The section The Future O f Strengthening By Post-Tensioning will assess

future technologies and needs Lastly, the thesis findings will be presented in the section

Conclusions And Recommendations.

Introduction

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Cited References

Federal Highway Administration, The Status O f The Nation’s Highway Bridges: Highway Bridge

Replacement And Rehabilitation Program And National Bridge Inventory, Thirteenth Report to

the United States Congress, Government Printing Office, Washington D.C., May 1997

Podolny, Waiter, Federal Highway Administration Senior Structural Engineer, Introduction to

Prestressed Steel Bridges Theory And Design by M.S Troitsky, New York, Van Nostrand

Reinhold Company, 1990

Introduction

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2 THE HISTORY OF STRENGTHENING B Y POST-TENSIONING

The concept o f prestressing (prestressing is the collective term for both post-tensioning and pre- tensioning) can be traced to the early 1800s when England’s Squire Whipple overcame the brittleness o f cast iron truss tension members by prestressing them Soon after, bridge designers began prestressing bridge floor beams with prestressed king and queen post trusses In 1840, Howe o f the United States was granted a patent on a timber truss which was prestressed by vertical iron ties tensioned by torqued nuts

By 1850, it became widely recognized that cast iron was brittle under tension and prone to fatigue related failure When cast iron was used for tension and moment carrying members, prestressing was applied to keep the material within the compressive stress range From 1847 to 1850, Rider

o f the United States designed prestressed trusses with the upper chord and vertical members made from plates o f cast iron Prestressing was applied by torquing the diagonals to compress the upper chord and vertical members (refer to Figure 2.1, Prestressed Truss By H Rider, 1850)

The History O f Strengthening By Post-Tensioning

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Figure 2.1: Prestressed Truss By H Rider, 1850 (courtesy of M.S Troitsky, 1990)

In the second half o f the 1800s, engineers began using ballast loads to prestress bridge structures The stress distribution within these bridges was improved by the introduction o f forces at the supports In 1878, Germany’s Koepcke designed the Elbe Bridge with the bottom chords prestressed by an artificial load One lever adjustment increased this load by a factor o f two producing a prestress force that countered the dead load tensile stresses (refer to Figure 2.2, Prestressing By An Artificial Load, Elbe Bridge, 1878) Following the success o f the Elbe Bridge, Koepcke strengthened a railway bridge by adding a ballast load over its side spans The ballast load transferred the bridge into a three-hinged arch producing a horizontal force at its abutment thereby compressing the adjacent span (refer to Figure 2.3, Prestressing By Ballast

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Load, Railway Bridge Over The Elbe River) Koepke also developed a design for prestressing an arch bridge by force regulation using a ballast load that would act only when live load was applied (refer to Figure 2.4, Prestressed Arch Bridge By Force Regulation) (Troitsky, 1990)

332 6 ’ 145.6 332 6 '

Figure 2.2: Prestressing By An Artificial Load, Elbe Bridge, 1878

(courtesy of M.S Troitsky, 1990)

8

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78.71*

Figure 2.3: Prestressing By Ballast Load, Railway Bridge Over The Elbe River

Figure 2.4: Prestressed Arch Bridge By Force Regulation (courtesy of M.S Troitsky, 1990)

Prestressing really took off in the 1950s when Europe was searching for a fast and economical method to replace its war-torn bridges Post-tensioned segmental concrete and cable-stayed bridge construction were used It would not be until the 1970s that prestressed concrete would meet the same level o f acceptance in the United States

In 1963, a suspended span o f the cantilevered Australian King’s Bridge collapsed due to multiple

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flange fractures at the welded cover plate ends The suspended spans were innovatively strengthened by multiple post-tensioning designs Post-tensioning was used to decrease stresses

in the girders as well as to provide redundancy to fracture critical pin and hanger connections

The Aare River Bridge o f Switzerland, consisting o f two simple deck trusses, was strengthened in

1969 Each truss was strengthened with two 2.56 inch diameter high-strength steel cables with a polygonal configuration The cables were stressed to 60 percent o f their ultimate tensile strength and connected to every fourth vertical member o f the truss At mid-span, each truss is supported

by a vertical end saddle The end vertical o f each truss above the middle pier behaves as a pylon

on top o f which is installed a cable anchorage (refer to Figures 2.5,2.6 and 2.7) (Troitsky, 1990)

Figure 2.5: Aare Bridge, Trusses Strengthened By Polygonal Cable Configurations, 1969

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H V -Bolts bent plate

Figure 2.7: Aare Bridge, Detail Of Cable Anchorage, 1969

Also in 1969, Vemigana described the successful strengthening o f a five span reinforced concrete bridge in Ontario, Canada The five spans were post-tensioned by draped cables that converted

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the bridge from simple spans to one continuous span.

During the past three decades considerable fundamental research in the area o f prestressed steel construction has been carried out In 1968, an ASCE-AASHO Subcommittee prepared a review

o f developments on the use o f prestressed steel flexural members and available prestressing methods with comparative economics International conferences were organized in 1963, 1966, and 1971, and specifications were developed for designing prestessed steel structures In 1972, Belenya and Gorovskii o f Russia presented a comprehensive analysis o f steel beams strengthened

by prestressing Their analysis proved that prestressing can add as much as 90 percent additional capacity to a steel beam They recommended a tie rod length 0.5 to 0.7 times the span length and

advised considering P-A effects only when the depth-to-span ratio is less than 1:20 The

introduction o f prestressed steel construction has increased the span lengths and load capacities achievable by steel bridges and has increased its competitiveness with prestressed concrete

construction (refer to Section 7.3 o f this thesis, Post-Tensioned Steel Plate Girders For New

Construction) (Troitsky, 1990)

Timber prestressing has just taken off in the past two decades with the development of prestressed laminates coined stress-Iaminate The introduction o f stress-laminate has increased the span lengths and load capacities achievable by timber bridges

Masonry bridges, although seldom built today, require specific strengthening, rehabilitation and maintenance plans because most are upwards o f 75 years old and are listed on historic registers

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which dictate that they cannot be replaced with newer structures Post-tensioning has been used

on occasion to m aintain these structures

Within the Unites States, California has used post-tensioning the most extensively to strengthen several hundred bridges The State has used post-tensioning to strengthen simple and continuous span steel girder bridges as well as some reinforced and prestressed concrete girder bridges California has also used post-tensioning for the seismic retrofit o f bridge pier caps In the last 20 years, approximately 30 bridges have been strengthened to increase their live load capacity on

California’s “heavy-hauler” corridors (refer to Section 3.1 o f this thesis, Increase Bridge Load

Rating) The majority o f these bridges were post-tensioned with seven-wire strand with a straight

profile enclosed in grouted galvanized ducts (Mancarti, 1984 and 1990)

During the 1970s, T.Y Lin International used post-tensioning on a Puerto Rican continuous span plate girder bridge to remove approximately 6 inches o f dead load deflection at mid-span In the early 1980s, the post-tensioning system required replacement due to severe stress corrosion o f the unprotected cable system and improper anchorage detailing (the bolt holes were torch cut rather than drill cut) Post-tensioned epoxy coated tie-rods in a king-post arrangement were used for replacement (Sandoval, 1998)

Since the 1980s, Iowa has been using post-tensioning to strengthen simple and continuous span

steel girder bridges (refer to Section 6 o f this thesis, Case Studies One and Two) At least two

other Minnesota steel beam bridges have been repaired temporarily using post-tensioning and, in

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one case, salvaged cable and timbers were utilized for repair Maryland, Michigan and California have used post-tension strengthening on a number o f occasions to strengthen under-designed and

cracked concrete bridge pier caps (refer to Section 6 o f this thesis, Case Study Three).

In 1984, a four beam, two-lane composite bridge in Florida was repaired and strengthened by post-tensioning The post-tensioning, designed by the AASHTO Service Load Design Method, was applied to all four beams in each o f the three simple spans (refer to Figure 2.8, Post-

Tensioned Bridge Beam, 1984) The post-tensioning raised the capacity o f the bridge from an

HI 5-44 to an HS20-44 load rating at a cost o f approximately $20,000

14

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SfMl Neutral

Figure 2.8: Post-Tensioned Bridge Beam, 1984 (courtesy of M.S Troitsky, 1990)

As well, within the United States there are a scattering o f older steel truss bridges and concrete arch bridges that have been strengthened by post-tensioning Overall, nearly every state has used post-tension strengthening on at least one occasion

Today, post-tension strengthening is being used worldwide Foreign countries, particularly European countries and Russia, because o f their older infrastructure and limited funds, are leading the way in post-tension strengthening England holds annual conferences on strengthening and

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Japan is currently sharing in the lead with technological developments for advanced composite post-tensioning tendons and anchorages.

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Cited References

Dunker, K.F., Klaiber, F.W., Becker, BX and Sanders, W.W Jr, Strengthening O f Existing

Single-Span Steel-Beam And Concrete-Deck Bridges, Final Report - Part II, ISU-ERI-Ames-

85231, Ames, Iowa: Engineering Research Institute, Iowa State University, 1985a

Dunker, K.F., Klaiber, F.W., and Sanders, W.W Jr., Design Manual For Strengthening Single-

Span Composite Bridges By Post-Tensioning, Final Report - Part III, ISU-ERI-Ames-85229,

Ames, Iowa: Engineering Research Institute, Iowa State University, 1985b

El-Arabaty, H A., Klaiber, F W., Fanous, F S and Wipf, T J., “Design Methodology For

Strengthening O f Continuous Span Composite Bridges”, Journal o f Bridge Engineering, August

1996

Mancarti, Guy D., "Strengthening California's Steel Bridges By Prestressing", TRB Research

Record 950, Volume 1, Transportation Research Board, Washington D.C., 1984.

Mancarti, Guy D., "Strengthening Short Span Bridges For Increased Live Loads", Proceedings o f

the Third International Conference on Short and Medium Span Bridges, Toronto, Ontario,

Canada, 1990

Maryland Department O f Transportation State Highway Administration, The Task Force Report

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On Closure O f The Governor Thomas Johnson Memorial Bridge, Bridge No 4019 Carrying

Maryland Route 24 Over Lower Patuxent River At Solomon's Island, Final Report, Summer 1989

Sandoval, Luis, Federal Highway Administration Puerto Rico Division Bridge Engineer, interview, November 18, 1998

Spaans, Leo, Janseen & Spaans Engineering, “Innovative Post-Tensioned Steel Bridge On Indiana

E-W Toll Road”, presentation and paper for Transportation Research Board 7&h Annual

Meeting, Washington D.C., January 10-14, 1999.

Subcommittee Three On Prestressed Steel O f Joint ASCE-AASHO Committee, "Development

And Use O f Prestressed Steel Flexural Members", Proceedings ASCE Structural Division,

Volume 49, Number S T 9,1968

Troitsky, M.S., Prestressed Steel Bridges Theory And Design, New York, Van Nostrand

Reinhold Company, 1990

18

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3 THE NEED FOR THE STRUCTURAL STRENGTHENING OF BRIDGES

There are a variety o f reasons why bridges may need strengthening Most bridges need strengthening either to increase then- load capacity or to restore load capacity that has decreased from deterioration A general grouping o f the reasons bridges need strengthening includes:

• Additional live loads

• Additional dead loads

• Additional environmental loads (particularly seismic)

• Temporary construction loads

• Deterioration from environmental or service effects

• Changes in bridge specifications

• Inadequate or poor design

• Inadequate or poor construction

• Emergency repair o f damage or deterioration

• Add redundancy

• Control stresses to prevent fatigue cracking

• Add safety

• Halt crack growth and deterioration

• Improve performance and load sharing

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• Remove dead load deflection

• Decrease live load deflection

The following sections focus on the reasons why strengthening is needed and how strengthening can be facilitated by post-tensioning The section titles are:

• Increase Bridge Load Rating

• Correct Inadequate Design And Construction

o Inadequate Steel Reinforcement

o Excessive Deflections

o Seismic Retrofits

o Other Performance Improvements

• Emergency Repair

• Strengthening For Construction

• Historically And Culturally Significant Bridges

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3.1 Increase Bridge Load Rating

The 1997 report to the United States Congress The Status O f The Nation's Highway Bridges:

Highway Bridge Replacement And Rehabilitation Program And National Bridge Inventory

indicates that o f all nationwide bridges (581,862), 19.6 percent (114,332) are or should be load posted because o f inadequate load capacities (refer to Figure 3.1, Status O f Bridges Approved For The Highway Bridge Replacement And Rehabilitation Program)

Load posting requirements are indicative o f the inability o f bridges to serve their intended use.The primary causes o f this condition are environmental and service deterioration, increases in legal truck loads, changes in bridge specifications and increased dead weight from either resurfacing or the addition o f bridge features such as median barrier

Bridges that require load posting fall into two groups The first group includes structurally deficient bridges that have deteriorated to an extent that they cannot carry the load for which they were designed The second group includes functionally obsolete bridges that are in good

condition but whose current State legal load exceeds the original design load (Federal Highway Administration, 1997)

While most bridge engineers are familiar with how environmental deterioration, service deterioration, and additional dead load may decrease the live load capacity o f bridges, the history and effects o f bridge specification changes and legal truck load changes are less known

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Therefore, the remainder o f this section will address these topics.

Adopted changes in bridge specifications and legal truck loads are closely coordinated with each other because o f their economic interrelations Bridge owners prefer that bridge specifications remain unchanged and that legal loads are not increased because o f the associated costs to upgrade their bridges Motor carriers have consistently lobbied for increased legal loads to maintain their competitiveness in the freight industry As such, specification and legal load changes have always been controversial political issues Efforts have been made to minimize changes and mitigate the economic consequence o f changes Nonetheless, changes in the past decades have incurred sizeable costs Many bridges which were or are in otherwise good condition must be upgraded to meet current specifications and legal loads As well, bridges that were initially borderline with respect to load capacity and condition often fell below borderline once the changes are adopted Load increases have also lowered the service life o f many bridges due to increases in fatigue stress ranges and general wear and tear

Prior to 1957, the American Association o f State Highway Officials (AASHO) bridge design standards permitted exterior beams to be designed for a wheel load fraction considerably lesser than the interior beams Consequently, many composite bridges built prior to 1957 have exterior beams with depths 2 or 3 inches less than the interior beams The AASHO “Seventh Edition O f Standard Specifications For Highway Bridges”, issued in 1957, increased the wheel load

distribution fraction for the exterior beams and made the requirement that under no circumstance may exterior beams have less load capacity than interior beam s This design change had the

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greatest impact on bridges that had exterior beams with smaller section moduli than the interior beams - particularly those bridges with five beams or less For instance, the increase in design load for exterior beams was as much as 40 percent for a typical 50 foot span, two lane, four beam bridge (El-Arabaty et al, 1996).

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M W OWrFA M M M W O M tM M F A

M r M — O w n J H B B T W M i l — l w IMiimii M l

N g W rfW fN ^w M 120.911 m m 339.371 s n j t o 127.730 1755* 2a3.no 3*1 * 2 andcfaaaifiad

"mrti 1 iirTlnr— 0) 9 * 7 24.149 7 3 * 9 1 0 7 * 3 9 * 0 2 2 * 7 M * 1 0 U 1 I

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rnaMTQiHEBOHiy ofeiolaac hridpn *•* 22.716

23.043 3 4 * 3 7 9 * 2 2 3 * 0 24.023 33.933 0 1 * 0

N ^ o f b n d p e a W a n 1.664 17.737 4 9 * 4 100.769 1.434 16.995 04.001 1 0 2 * 0

Addhiooal bridg— W W lM bepoMd*

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V

3.460 3.976

bridges findad H R 1P uudaribe bridge

Number o f replaced or leM bStaM bridgae opeu lo traffic (SBRF A H B U F )‘

1 7 * 4 1 2 * 0 2 9 * 4 > 1 9 * 2 1 4 * 9 34.011

« A an nil nOy liO rinlrilgi — IW 4 by MWA iaam W ( l ) baabaaunartalad uvMMeebjrlw M r O) M rlnnl oc(5)

m w w i— * n M H W i n - M p t i O w M n Q y a b n lib W o e ia eu n B * M W |— oy.W iw nyfan

W m » | W M » M i n M t i B f c M W i H o l w w M W l r h l W I n B l W 2-10.2-11W

2 1 2 f e r * i t a M N B l r l W

4 ■! mill mil I il Oil i ■ 1 1 I b f IW SW POW fc Hipr lifdpi W — n n a n ihn w l i Wir

Figure 3.1: Status O f Bridges Approved For The Highway Bridge Replacement And Rehabilitation Program (courtesy of the Federal Highway Administration)

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Consequently, the bridge category having the greatest percentage o f noncompliant structures due

to changes in bridge specifications is steel beam composite bridges constructed between 1940 and

1960 (El-Arabaty et al, 1996) By current bridge standards, they are under-strength due to excessive flexural stresses in the exterior beams

Changes in legal truck loads have mostly impacted bridges located on Interstate roadways and the Interstates' primary connectors In 1983, changes in Federal law governing weight and size provisions made by the Surface Transportation Assistance Act o f 1982 (STAA) and the DOT Appropriations Act o f 1982 went into effect This Federal law governs the weight, length, width and height o f trucks using the Interstate System and other designated roadways within the

Federal-Aid highway system

Section 113 o f the STAA designates the following uniform m inim um weight requirements for the Interstate System’s bridges and non-Interstate bridges that are needed to provide reasonable access to the Interstate System;

axle weight tandem axle weight gross vehicle weightcompliance with the Federal Bridge Formula

Each state has designated truck weight and size limits that are either equal to or greater than those

20.000 pounds34.000 pounds80.000 pounds

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