CE 73 : Prestressed Concrete

Course Project Report

Analysis and Design of aPrestressed Concrete Bridge

DEPARTMENT OF CIVIL ENGINEERING

CENTRAL MINDANAO UNIVERSITY

Authors:Cleo BatocailJulie Mae ClarabalGlennie LumampaoJethrine Claire MadarangEdgardo Vargas Jr.

Instructor:Engr. Richard Aquino

27 January 2013

Certification

This is to certify that part or parts of our work was not copied from some-body else work. A proper and full referencing was included for all ideasincluding plans, drawings, pictures and diagrams taken from the internetand other sources. For the materials which is quoted essentially word-for-word is given in quotation marks and referenced.

Signed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Date . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Signed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Date . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Signed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Date . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Signed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Date . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Signed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Date . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

Summary

3

Contents

1 Project Background 91.1 Project Description . . . . . . . . . . . . . . . . . . . . . . . . 91.2 Objectives of the Study . . . . . . . . . . . . . . . . . . . . . 91.3 Scope and Limitation . . . . . . . . . . . . . . . . . . . . . . 91.4 Project Outline/Workflow . . . . . . . . . . . . . . . . . . . . 10

2 Prestressing Materials and Systems 112.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.1.1 Concrete . . . . . . . . . . . . . . . . . . . . . . . . . 112.1.2 Nonprestressing Reinforcement . . . . . . . . . . . . . 122.1.3 Prestressing Tendons . . . . . . . . . . . . . . . . . . . 13

2.2 Prestressing Systems . . . . . . . . . . . . . . . . . . . . . . . 132.2.1 Classification and Types of Prestressing . . . . . . . . 132.2.2 Pretensioning vs. Post-tensioning . . . . . . . . . . . . 142.2.3 Anchorages . . . . . . . . . . . . . . . . . . . . . . . . 142.2.4 Jacking Systems . . . . . . . . . . . . . . . . . . . . . 15

3 Bridge Engineering 163.1 Design Parameters . . . . . . . . . . . . . . . . . . . . . . . . 173.2 Substructures . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.2.1 Wing wall . . . . . . . . . . . . . . . . . . . . . . . . . 173.2.2 Abutment . . . . . . . . . . . . . . . . . . . . . . . . . 173.2.3 Piles, Piers and Caissons . . . . . . . . . . . . . . . . 18

3.3 Superstructures . . . . . . . . . . . . . . . . . . . . . . . . . . 183.3.1 Bridge Deck . . . . . . . . . . . . . . . . . . . . . . . . 183.3.2 Girders . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4 Design Methods and Procedure 204.1 AASHTO and NSCP Codes . . . . . . . . . . . . . . . . . . . 20

4

4.2 Loadings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.3 Prestress Losses . . . . . . . . . . . . . . . . . . . . . . . . . . 204.4 Design for Flexure . . . . . . . . . . . . . . . . . . . . . . . . 204.5 Shear and Diagonal Tension . . . . . . . . . . . . . . . . . . . 204.6 Deflection Calculations . . . . . . . . . . . . . . . . . . . . . . 204.7 Development, Splicing and Bar Cut-offs . . . . . . . . . . . . 20

5 Plans and Specifications 215.1 Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.2 Elevations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.3 Cross sections . . . . . . . . . . . . . . . . . . . . . . . . . . . 235.4 Framing Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . 235.5 Design Assumptions and Specifications . . . . . . . . . . . . . 23

6 Results and Discussions 246.1 Design Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6.1.1 Dead Load . . . . . . . . . . . . . . . . . . . . . . . . 246.1.2 Live Load . . . . . . . . . . . . . . . . . . . . . . . . . 246.1.3 Impact Load . . . . . . . . . . . . . . . . . . . . . . . 24

6.2 Design Envelope . . . . . . . . . . . . . . . . . . . . . . . . . 246.2.1 Bending Moment . . . . . . . . . . . . . . . . . . . . . 246.2.2 Shear . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6.3 Bridge Girder Design . . . . . . . . . . . . . . . . . . . . . . . 246.3.1 Prestressing Force . . . . . . . . . . . . . . . . . . . . 246.3.2 Non-prestressed Reinforcement . . . . . . . . . . . . . 246.3.3 Prestressing Tendon and Profile . . . . . . . . . . . . . 256.3.4 Shear Reinforcement . . . . . . . . . . . . . . . . . . . 25

6.4 Bridge Deck Design . . . . . . . . . . . . . . . . . . . . . . . . 25

7 Conclusion and Recommendations 267.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . 26

5

List of Figures

6

List of Tables

7

Acknowledgements

We would like to express our heartfelt gratitude firstly to Our Heav-enly Father for the guidance and wisdom he have given and the bless-ings He bestowed upon us enabling to complete this book. To ourloving parents for the financial, moral and emotional support as weeagerly continue these lifetime challeges everyday of our lives as a stu-dent. Without their guidance and love we cannot continue this step-ping stone as Civil Engineering students.

To our Professor in CE 73 , Engr. Richard J. Aquino for beingour guide and humble mentor in every step of the way. For lettingus know that learning is the most important aspect as a student andnot just having good grades. For sharing his knowledge and all hiscapabilities as we learn from him in our mistakes. And lastly for ourgroup who worked well together as a great team, as a family, as anordinary student who worked hard within our limits. Enabling to finishthis project was a great privilege and success to enhance our skills assimple students and future engineers.

8

Chapter 1

Project Background

To overcome concrete’s weakness in tension, Professor Magnel develop theconcept of prestressing in which internal stresses are introduced by meansof high strength reinforcement. Prestressed concrete was developed literallyto overcome the reinforced concrete’s limitations especially on cracking.Thisimprove shear and torsion strengths as well as protecting the reinforcementsteel.

1.1 Project Description

JR Borja Prestressed Concrete Bridge with a two way lane.

1.2 Objectives of the Study

• To be able to know the difference between reinforced concrete designand prestressed concrete design.

• To have an experience in analyzing and designing a prestressed bridgewhile at the same time learning.

• To plan and design a Prestressed Bridge Structure that is safe to useand well planned.

1.3 Scope and Limitation

This Project is limited on designing and analyzing a prestressed concretebridge with the use of AASHTO specifications and NSCP code, that isespecially would not go beyond our objectives.

9

1.4 Project Outline/Workflow

I. Design Parameters of Prestressed Concrete BridgeII. Structural Designs using AASHTO Specs and ACI CodesIII. Computation of LoadingsIV. Structural AnalysisV. Design

10

Chapter 2

Prestressing Materials andSystems

2.1 Materials

This typical bridge is mainly a prestressed concrete material.

2.1.1 Concrete

”Concrete is an artificial engineering material made from a mixture of port-land cement, water, fine and coarse aggregates, and a small amount of air.It is the most widely used construction material in the world (MicrosoftEncarta 2009)”.

Concrete is the only major building material that can be delivered to thejob site in a plastic state. This unique quality makes concrete desirable as abuilding material because it can be molded to virtually any form or shape.Concrete provides a wide latitude in surface textures and colors and can beused to construct a wide variety of structures, such as highways and streets,bridges, dams, large buildings, airport runways, irrigation structures, break-waters, piers and docks, sidewalks, silos and farm buildings, homes, and evenbarges and ships.

The following are the main materials from which concrete is made:

11

Cement

”Cement, is any material that hardens and becomes strongly adhesive afterapplication in plastic form. The term cement is often used interchangeablywith glue and adhesive.In engineering and building construction, the termusually refers to a finely powdered, manufactured substance consisting ofgypsum plaster or portland cement that hardens and adheres after beingmixed with water (Microsoft Encarta 2009).”

Aggregates

”Is a granular materaial, such as sand, gravel, crushed stone and iron blastfurnace slab, and when used with a cementing medium forms a hydrauliccement concrete or mortar (NSCP,2010 6th Ed.).”

Water

”Water used in mixing concrete shall be clean, free from injurious amountsof oils, acids, alkalis, salts, organic materials or other substances deleteriousto concrete or reinforcement(NSCP,2010 6th Ed.).”

Admixtures

”Is a material other than water, aggregate or hydraulic cement used as aningredient of concrete and added to concrete before or during its mixing tomodify its properties (NSCP,2010 6th Ed.).”

2.1.2 Nonprestressing Reinforcement

Steel reinforcement consisting of bars, wire fabrics and welded wires all ofwhich are manufactured according to ASTM Standards.

The most important properties of reinforcing steel are:1. Youngs’s Modulus, Es

2. Yield Strength , fy3. Ultimate Strength , fu4. Steel grade designation5. Size or diameter of bar or wire.

To increase the bond of concrete and steel, deformations are rooled untothe surface of the bar and must conform in accordance to ASTM specifica-tions inorder to be accepted as deformed bars or reinforcement.

12

2.1.3 Prestressing Tendons

Because of the high creep and shrinkage losses in concrete, effective pre-stressing can be achieved by using very high strength steels in the range of270,000 psi or more. Such high-stressed steels are able to counterbalancethese losses in the surrounding concrete and have adequate leftover stresslevels to sustain the required prestressing force.

2.2 Prestressing Systems

2.2.1 Classification and Types of Prestressing

Prestressing is applied to concrete by means of high-strength tendons (usu-ally steel)in tension, passing through the concrete. This can be achievedin one of two ways, namely pretensioning and post-tensioning, the maindistinction between the two methods being whether the steel is tensionedbefore or after the concrete is cast,hence the terms pre and post.

Types of Prestressing

1. Full Prestressing: sufficient precompression to ensure crack-free at fulldesign load. Freyssinet 1930.2.Partial Prestressing: Precompression is not sufficient to prevent cracksunder full design load. Therefore, the member will normally contain someconventional reinforcement bars.

In many cases, partial prestressing improves the structural performanceand is commonly used.

Other method of Prestressing:

a. Thermal prestressing by application of electirc heat. Electric and me-chanical combination has been also used in USSR.

b. Prebending high strength steel beam and encasing its tensile flange inconcrete. Releasing the bending will place the concrete under compression.

c. Chemical prestressing by means of expansive cement which expand chem-ically after setting and during hardening started in France 1940 and knownas self stressing.

13

2.2.2 Pretensioning vs. Post-tensioning

The term ”pretensioning” means the pretensioing of the prestressing steelnot the beam itself. Steel is tensioned before casting of concrete mix. It isnormally performed at pre-casting plants where a precasting stressing bed,of a long reinforced concrete slab is cast on the ground with vertical anchorbulk heads or walls at its ends.

In Summary: Pre-tensioned

• Stress strands

• Cast concrete

• Cut strands

• Force transferred to concrete by bond.

On the other hand in Post-tensioning, the steel is tensioned after concreteis cast. Concrete is cast and hollow ducts are placed during casting in theconcrete. Tendons are located inside the hollow ducts. A tendon comprisesa number of individual strands in the same duct. After the concrete has at-tained the required strength the tendons are stressed using prestressing jacksat the ends of the concrete member (from one or both ends simultaneously)and anchored.

In Summary: Post tensioned

• Provide tendon inside duct

• Cast concrete

• Tension tendon after concrete attains required strength

• Inject grout into duct

2.2.3 Anchorages

Anchorages is a prestressing system loacted at the end of a high tensilestrength section. After the wires are stressed, these anchor serves as lock atboth ends. Such anchorages are supported by large and stable bulkheads tosupport the exceedingly high concentrated forces applied to the individualtendons.

14

Figure 2.3: Type of Achorage

2.2.4 Jacking Systems

One of the fundamental components of a prestressing operation is the jackingsystem applied,i.e.,the manner in which the prestressing force is transferredto the steel tendons. Such a force is applied through the use of hydraulicjacks of capacity 10 to 20 tons and a stroke from 6 to 48 in.,dependingon whether pretensioning or post-tensioning is used and whether individualtendons are being prestressed or all the tendons are being stressed simulta-neously.

Figure 2.4: Types of Jacking Systems

15

Chapter 3

Bridge Engineering

An engineering bridge is a structure built to span physical obstacles suchas a bodies of water, valley, or road, for the purpose of providing passageover the obstacle. There are many different designs that all serve uniquepurposes and apply to different situations. Designs of bridges vary depend-ing on the function of the bridge, the nature of the terrain where the bridgeis constructed and anchored, the material used to make it, and the fundsavailable to build it.

Bridges were made to provide trade to other places to boost economicdevelopment and easier transportation of goods from one place to another.Prestressed Bridges was made possible through the advancement of thescience of materials in creating long span bridges in utilizing compressivestrength of concrete and through proper design, eliminates deflection andcracking.

Figure 3: Cable Prestressed Concrete Bridge in China

16

3.1 Design Parameters

3.2 Substructures

3.2.1 Wing wall

Wingwalls are a necessary component of most FIAB bridges to retain the fillthatsupports the roadway. Currently, wingwalls do not get a lot of attentionfrom the designer, and are almost an afterthought to the overall design ofthe structure. However, wingwall orientation and connection details canhave an impact on the forces induced in,and the distribution of, the forcesthroughout the structure.

The following are types of Wingwall:

• A cantilevered wingwall is a retaining wall that is poured integrallywith the abutment stem and thus moves with the abutment stem withrespect to the soil. Cantilevered wingwalls may or may not have pilesplaced directly beneath the wall.

• An independently supported wingwall is a retaining wall that has itsown foundation and is not connected to the moving abutment stem.

• An in-line wingwall is a retaining wall that is parallel to the centerlineof bearings.

• A U-wingwall is a retaining wall that is parallel to the roadway.

• A flared wingwall is a retaining wall that lies between an inline and aU-wingwall.

3.2.2 Abutment

In engineering, abutment refers to the substructure at the end of a bridgespan whereon the bridge’s superstructure rests. Single-span bridges haveabutments at each end which provide vertical and lateral support for thebridge, as well as acting as retaining walls to resist lateral movement of theearthen fill of the bridge approach.

Multi-span bridges require piers to support ends of spans unsupportedby abutments.The term may also refer to the structure supporting one sideof an arch, or masonry used to resist the lateral forces of a vault.The wordderives from the verb ”abut”, meaning to ”touch by means of a mutualborder”.

17

3.2.3 Piles, Piers and Caissons

Piles are a type of deep foundation that is drilled using a hydraulic drillingmachine below the earths surface unto a stable strata. Bridges are usingthese piles to assure the stability because these are made from prestressedconcrete to withstand vertical forces.

A pier is a raised structure, including bridge and building supports andwalkways, typically supported by widely spread piles or pillars. The lighterstructure of a pier allows tides and currents to flow almost unhindered,whereas the more solid foundations of a quay or the closely spaced piles of awharf can act as a breakwater, and are consequently more liable to silting.

In geotechnical engineering, a caisson is a watertight retaining struc-ture[1] used, for example, to work on the foundations of a bridge pier, forthe construction of a concrete dam,[2] or for the repair of ships.[3] These areconstructed such that the water can be pumped out, keeping the workingenvironment dry. When piers are to be built using an open caisson and itis not practical to reach suitable soil, friction pilings may be driven to forma suitable sub-foundation.

3.3 Superstructures

3.3.1 Bridge Deck

A bridge deck or road bed is the roadway, or the pedestrian walkway, sur-face of a bridge. The deck may be of concrete,wood which in turn may becovered with asphalt concrete or other pavement.

The primary function of a bridge deck is to support the vehicular verti-cal loads and distribute these loads to the steel superstructure. The deck istypically continuous along the span of the bridge and continuous across thewidth of the span.

In most applications, the bridge deck is made composite with the steelsuperstructure through positive attachment to the girders, such as usingshear connecters to attach the concrete deck slabs to steel girders. In suchcases, the deck serves as part of the top flange in the composite section andcan be utilized for strength and stiffness. The deck is subjected to local flex-ural bending of the slab spanning over the girders in the transverse directioncaused by the vehicle wheel loads. When the deck is made composite, it isalso subjected to longitudinal stresses caused by flexure along the span.

18

3.3.2 Girders

A girder bridge is perhaps the most common and most basic bridge. Alog across a creek is an example of a girder bridge in its simplest form.In modern steel girder bridges, the two most common girders are I-beamgirders and box-girders.

19

Chapter 4

Design Methods andProcedure

4.1 AASHTO and NSCP Codes

4.2 Loadings

4.3 Prestress Losses

4.4 Design for Flexure

4.5 Shear and Diagonal Tension

4.6 Deflection Calculations

4.7 Development, Splicing and Bar Cut-offs

20

21

Chapter 5

Plans and Specifications

5.1 Plan

5.2 Elevations

22

.

5.3 Cross sections

5.4 Framing Plan

5.5 Design Assumptions and Specifications

23

Chapter 6

Results and Discussions

6.1 Design Loads

6.1.1 Dead Load

6.1.2 Live Load

6.1.3 Impact Load

6.2 Design Envelope

6.2.1 Bending Moment

6.2.2 Shear

6.3 Bridge Girder Design

6.3.1 Prestressing Force

6.3.2 Non-prestressed Reinforcement

1. At supports

2. Midspan

3.Bar cut-offs

24

6.3.3 Prestressing Tendon and Profile

6.3.4 Shear Reinforcement

6.4 Bridge Deck Design

25

Chapter 7

Conclusion andRecommendations

7.1 Conclusion

7.2 Recommendations

26