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SEISMIC CONSIDERATIONSFORPRE-FABRICATED

ACCELERATED BRIDGE CONSTRUCTION

CONTENTS

1. Introduction

2. Scope and Objectives

3. Precast Concrete Components and Systems in ABC

3.1 Precast Concrete Bridge System3.2 Geotechnical Consideration

3.3 Precast Concrete Deck Panels

3.3.1 Precast Deck Panels

3.3.2 Panel-to-Panel Connections3.3.3 Panel-to-Girder Connections

3.3.4 NCHRP 12-65 System3.3.5 Applicability of Precast Deck Panel in ABC

3.4 Superstructure

3.4.1 Precast Concrete Girders3.4.1.1 Types of Girders

3.4.1.2 Techniques to Increase Span Length

3.4.1.3 On-Going Researches

3.4.1.4 Applicability of Precast Beams in ABC3.4.2 Spliced Girders

3.4.2.1 Types of Girders3.4.2.2 Construction Detail3.4.2.3 Construction Issues

3.4.2.4 Connection Detail

3.4.2.5 Application of Spliced Girder in Seismic Regions3.4.3 Precast Segmental Box Girders

3.4.3.1 Structural Concepts

3.4.3.2 Construction Issues

3.4.3.3 Seismic Consideration3.5 Substructure

3.5.1 Precast Bent Cap

3.5.2 Integral Piers3.5.3 Precast Segmental Columns

3.5.4 Connection Details

4. Seismic Design Principle

4.1 Design Principles

4.2 Capacity-Based Approach

4.3 Force-Based Approach

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4.4 Base Isolation

5. Seismic Analysis and Design Procedures

5.1 PCI Girder Bridge

5.2 Spliced Girder Bridge

5.3 Segmental Box Girder Bridge

6. Seismic Design Requirements

7. Experimental Studies

8. Bridge Information System

9. Summary

APPENDICESA. Literature Review

B. Precast Concrete Segmental ColumnsC. Analysis Tools

D. Experimental Data

E. Design Guidelines and Commentary

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1.INTRODUCTION

Accelerated Bridge Construction (ABC) scheme is an agenda to realize the idea of Get In, GetOut, and Stay Out.

This monograph discusses seismic analysis and design of bridges built following accelerated

bridge construction schemes.

Some of the considerations for accelerated construction are:

Improved work zone safety.

Minimizing traffic disruption during bridge construction.

Maintaining and/or improving construction quality.

Reducing the life cycle costs and environmental impacts.

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2. SCOPEAND OBJECTIVES

The objective of the present study is to propose seismic design guidelines that can be used forbridge systems consist of pre-fabricated components in accelerated bridge construction. The area

of interest corresponds to the intersection of three different aspects as in Figure 2-1.

Implementation of ABC includes various aspects in planning, designing, construction, financing,and scheduling. The adoption of pre-fabricated components and systems has been known a good

example of ABC, but this does not mean every project using pre-fabricated components is ABC.

Based on the same reasoning, ABC can be acquired without using pre-fabricated components inhigh seismic regions, for example, by improving scheduling and financing procedures.

ABC

Pre-Fab Seismic

FIGURE 2-1 The Area of Interest

For the past two decades, the benefits of combination of ABC with pre-fabricated constructionhave been shown in many cases (Shahawy 2003). Also pre-fabricated construction is envisioned

as a tool to accomplish publics needs in the future (Bhide, Culmo et al. 2006). However,

employing those developed techniques in the moderate and high seismic regions is delayed

mainly because of uncertainties of behavior that those systems will experience under seismicloadings.

In the development of ABC scheme, precast concrete components has been widely applied indeck, superstructure, and substructure (FHWA 2006). For steel members, the application evolves

into fabricating large blocks of superstructure and placing with SPMT (Self-Propelled ModularTransporter) vehicle (Figure 2-2). The advantage of precast concrete systems over steel systemsis the possibility of standardization of smaller components, which will reduce the initial cost of

construction. So the present study focuses on precast concrete components not including those

from steel and composite. Also substructure is generally made from concrete material, which is

the main concern in the seismic design.

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FIGURE 2-2. Wells Street Bridge Construction, Chicago

In the application of precast concrete components, the proper connection between components isthe most critical consideration. Under seismic loadings, the conventional connection methods

and detail may not be enough to transmit excessive moment and shear forces. Also bridge

systems consist of precast components and connections may be experience different inelastic ornonlinear behavior that is not expected in conventional cast-in-place construction. Recently,

several studies, including NCHRP 12-74, focused on seismic behavior of pre-fabricated

components and their connections. The present study not only collects information from relatedstudies but also identifies and investigates issues that needs to be addressed in order to provide a

big picture for reliable application of ABC techniques in high seismic regions. The main tasks

include:

Collecting information from other related research and projects Proposing appropriate seismic design philosophy and procedure

Selecting bridge systems for in-depth investigation

Performing trial seismic design for the bridge set

Identifying critical seismic design issues and performing analytical and experimental

investigations

Providing seismic design methodologies and wording as a form of guidelines

Proposing bridge information system to facilitate ABC with proposed seismic design

methodologies

Currently, the appropriate way to plan and to implement ABC is to go through ACTT(Accelerated Construction Technology Transfer) workshop. The ACTT concept was originatedby the Transportation Research Board (TRB) in conjunction with FHWA and the Technology

Implementation Group (TIG) of the AASHTO (FHWA 2007). The ACTT program helps owner

agencies achieve ABC goals by bringing national transportation experts to the planning stage. Atthe workshop, skill sets provide counsel on innovative ways to accelerate construction, reduce

project costs, and minimize impacts. The following is the list of skill sets (FHWA 2005).

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Innovative Contracting / Financing

ROW / Utilities / Railroad Coordination

Geotechnical / Materials / Accelerated Testing

Traffic Engineering / Safety / Intelligent Transportation System (ITS)

Structures

Roadway / Geometric Design

Long Life Pavements / Maintenance

Construction

Environment

Public Relations

Innovative techniques in design and construction of structures are one of the important

components to implement ABC, but they are not the only factors that should be considered. For

example, conclusions from ACTT workshops indicated that the holistic approach to the entireproject was needed to come up with the most appropriate way to realize ABC. Mostly, global

organization and financing take more important roles in the planning state, unless structuralconsideration is critical for directing the project. A flowchart and determining factors for pre-fabricated ABC selection are proposed in one of recent studies (FHWA 2006). Figure 2-3 shows

the decision making flowchart.

The necessity of organized planning in ABC has been emphasized in its development. As the on-

site construction time is reduced, the time and efforts for in-house works increases. The expected

and possible construction problems should be addressed in advance. Also management of

information and data from different sectors is one of the critical issues to implement plannedoperation smoothly. The present study, therefore, includes the discussion on bridge information

system that can provide such operation and management of information.

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FIGURE 2-3. Pre-fabricated ABC Decision Making Flowchart

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3. PRECAST CONCRETE COMPONENTSAND SYSTEMSIN ABC

Bridge construction utilizing precast components is one of the widely implemented techniques inaccelerated bridge construction. Since its introduction in the 1950s, the number of applications

of precast components has been increased.

The recent development of precast concrete based bridge system for ABC comes up to many

practical publications such as;

Conferences on the same topic

PCI, Guidelines for Accelerated Bridge Construction (PCI 2006)

FHWA, Decision-Making Framework for Prefabricated Bridge Elements and Systems

(PBES) (FHWA 2006)

The main focus is the applicability of these components and bridge systems in high- and

moderate- seismic regions from the conventional and innovative seismic design point of view.

The goal of most bridge projects taking advantage of precast concrete components is to build a

bridge system that responds as close as conventional

summary existing projects based on components used

Recently, PCI addressed this issue more systematic way by summarizing issues as a format of

guidelines (PCI 2006).

Basic Philosophy of precast bridge structure??

A prefabricated system is designed using the same design approach as cast-in-place concrete structures.

Designers should refer to the PCI Tolerance Manual MNL 135-00 for guidance on setting appropriate

tolerances for each component.

Designers should refer to the ACI 550.1R-01,Emulating Cast-in-Place Detailing in Precast Concrete

Structures for specifications on emulation design.

Round columns are difficult to fabricate. These will likely have to be poured vertically which may proveto be difficult in a precast plant. This will likely result in higher component prices

Design Guidelines for the use of Full Depth Precast Deck Slabs used for new construction or for

replacement of existing decks on bridges.

The following articles should be included

(Bhide, Culmo et al. 2006)

Texas report should be addressed for durability of post-tensioned substructure.

Seismic design concern

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3.1 Precast Concrete Bridge System

In the present study, three precast concrete superstructures are considered based on theapplicable span length.

Short span: Precast Prestressed Beam (Texas Type)

Medium span : Spliced Girders Long span : Box Girders (SFOB or Otay River Bridge)

Compared to the CIP construction,

Precast concrete bridge systems can be categorized by the continuity of superstructures and

continuity between superstructure and substructure. The dynamic response of simply supported

spans is quite different from the continuous spans, and those of bridge system with integral piersare different from responses of bridge systems where superstructure is separated from the

substructures. As cast-in-place construction, the determination of structural system for the

seismic aspects is important, but there are additional restrictions that originated from precastconstruction. The technical difficulties for providing continuity in structures need to be identified

when the structural system is determined.

In the seismic design of bridges, the reliable fuse mechanism that limit strength demands of

components takes a major role in the modern design philosophy. The location of fuse is limited

to the location that permits easy inspection and rehabilitation after earthquake. The focused areas

are pier columns to implement this fuse mechanism through plastic hinging behavior.

Continuous girders provide

Over the past few years, growing attention has been paid to the investigation, development and

application of precast concrete bridge elements and systems to highway bridges. Traditional cast-in-place concrete bridge construction activity normally causes lane closures and traffic detour,

thus causing the problem of traffic disruption. The cost of the traffic disruption to road users can

be very high in busy urban areas. Precast concrete bridge elements and systems can offer a viable

solution to the problem. It shifts most of construction activities into the precast factory. Afteradequate concrete strength is obtained, the precast products are then transported to the

construction site. Thus, the on-site construction activities are greatly reduced. Reducing the on-site construction activities also means the work zone safety and the construction quality can beimproved, because the working environment in a precast factory is safer and easier for the

workers to perform their skills in terms of formwork, reinforcing ironwork, concreting,

compacting and curing. Besides, the environmental impact can be reduced, since the demand onthe land for construction purpose around the construction site is decreased.

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There are disadvantages to use precast construction as well, such as the high initial cost and the

concerns regarding the performance of the connection or joint which connects precast products

to the structure. The high initial cost is largely attributed to the cost of transporting the productsfrom the factory to the construction site and the hardware associated with the connections. The

higher initial cost as opposed to conventional cast-in-place construction may become less

important if the many benefits the precast production can bring are appropriately weighed.However, if a good behavior of the precast connection can not be ensured, it will surely prevent

the engineers from using precast construction. As a result, the development of any new precast

system will require the rigorous research on the design of the connections to ensure theconnection can perform as expected.

3.2 Geotechnical Consideration

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3.3 Precast Concrete Deck Panel Systems

The basic role of bridge deck system is to provide the smooth surface to traffic and to implementthe designed geometry as a part of roadway. The bridge deck should be constructed to satisfy

designed elevation, longitudinal slope, skew, cross slope, etc. Compared to cast-in-place

construction that modification of geometry can be improvised at the site, the precast constructionhas limited construction tolerance. Additionally, AASHTO Specifications specify the following

issues as the implicit philosophy for deck construction (AASHTO 2004).

Jointless, continuous decks

Deck systems to improve weather and corrosion-resisting effects of the whole bridge

Reduce inspection efforts and maintenance costs

Increase structural effectiveness and redundancy

In the development of prefabricated bridge elements and systems, various deck systems have

been studied, implemented, and constructed. They include precast concrete stay-in-place panels,

full-depth precast concrete panels, metal grid decks, and orthotropic steel (aluminum) decks. Asmetal grid decks, open grid floors, filled and partially filled grid decks, and unfilled grid deckscomposite with reinforced concrete slabs are examples included in AASHTO LRFD

Specifications.

The discussion in this section is limited to the full-depth precast deck panels that can be

incorporated with other prefabricated bridge elements to be used in accelerated bridge

construction schemes. Full-depth panels have been used since the early 1960s (Biswas, Oseguedaet al. 1984). The first application of full-depth panels for composite construction was in 1973

(Biswas 1986). The historical background of their development can be found at (PCI 2003;

Hieber, Wacker et al. 2005; Badie, Tadros et al. 2006). Figure 3-1 shows one of the developed

full-depth precast deck panels at the University of Nebraska.

FIGURE 3-1. Typical Full-Depth Precast Deck Panel (PCI 2003)

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This composite action was accomplished by shear connection through shear studs or steel

channels grouted into the pockets of the decks. Alternatively, steel channels may be welded on

the top flange or bolted connections may be implemented. The deck elevations were adjusted byleveling bolts or shims. Also the grouting was used to construct haunches which were formed by

dams with various materials. The followings are the typical construction procedures for full-

depth precast deck panel systems (Hieber, Wacker et al. 2005).

Girders are cleaned and variations in the elevation are corrected with shims

Panels are lifted and placed onto the girders

Panels are leveled using leveling bolts or shims

Transverse joints between panels are filled with grout and allowed to reach the required

strength

When longitudinal post-tensioning is included, tendons are fed through ducts in the

panels and stressed

Shear connectors are connected to the girders inside shear pocket openings in the panels

The shear pockets, the haunch between the girders and panels, and post-tensioning ducts

are filled with grout and allowed to cure

If required, an overlay or wearing surface is applied

The grouting is used in panel-to-girder connection and panel-to-panel connection. As commonly

required properties of the grout material, the followings can be listed.

Relatively high strength (2000 to 4000 psi) at the early stage (1 to 24 hours)

Small shrinkage deformation

Bonded well with hardened concrete surfaces

Low permeability for durability.

Issa et al. compares the performance of several commercial products (Issa and al 2003).

In the discussion of the full-depth precast concrete deck panel systems, the focuses are on deck

panels, panel-to-panel connections, and panel-to-girder connections.

3.3.1 Precast Deck Panels

AASHTO LRFD Design Specifications (AASHTO 2004) specifies the depth of the slab,

excluding any provision for grinding, grooving, and sacrificial surface, shall not be less than 7.0

in. Precast deck panels are approximately 8 inches thick and the width of them typically spansthe full width. The design generally follows the conventional cast-in-place concrete deck design

procedures. In the transverse direction, two layers of reinforcement are designed. The

reinforcement can be either pre-tensioning steel or mild steel reinforcement. The pre-tensioningcan produce thinner panels with better crack control, which can reduce damages during

transportation and erection (Yamane, Tadros et al. 1998).

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When post-tensioning is introduced in the longitudinal direction, the minimum average effective

prestress shall not be less than 250 psi. The longitudinal post-tensioning is a good solution for

providing flexural continuity. The post-tensioning ducts should be located at the center of theslab cross-section. Block-outs should be provided in the joints to permit the splicing of post-

tensioning ducts. Panels should be placed on the girders without mortar or adhesives to permit

their movement relative to the girders during prestressing. Congestion problems can occur as aresult of mild steel, prestressing steel, block-out formwork, and post-tensioning ducts.

The transverse joint and the block-outs shall be specified to be filled with a nonshrink grouthaving a minimum compressive strength of 5.0 ksi at 24 hours. Block-outs shall be provided in

the slab around the shear connectors and shall be filled with the same grout upon completion of

post-tensioning.

3.3.2 Panel-to-Panel Connections

The panels can be connected in transverse direction and in longitudinal direction. When thewidth of panel is narrower than the width of the bridge, longitudinal connections are needed.

Longitudinal connections are frequently used for the staged construction, which soma parts ofthe old deck should be maintained for traffic during the construction of the other part of the deck.

Also, the exterior part of the deck can be constructed in cast-in-place concrete whereas the

interior parts are constructed with precast deck panels. For this case, longitudinal connections arerequired. For both connections, the reinforcement splice or connection detail and grouting detail

are the major issues to transfer forces between panels and panel to CIP concrete.

Transverse connections should transfer shear and moment from live load. Most of the joints usedhave been female-to-female shear key type connections. The shear keys were generally provided

in the transverse connections between adjacent panels. The shear keys were designed to make the

panels to behave as a continuous structure so that the vertical movements and traffic inducedforces can be resisted by the whole deck systems not by the individual panels. There are two

typical connection details used in the transverse directions: the non-grouted match-cast shear key

and grouted female-to-female joints.

Figure 3-2 shows one of the match-cast examples. The match cast connections, however, were

found that to be difficult to provide perfectly matching connections because of the construction

tolerance and required elevations of the decks. Male-to-female shear key connections are foundto have poor performance (Kropp, Milinski et al. 1975).

FIGURE 3-2. Non-Grouted Match-Case Shear Key

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The grouted female-to-female connections have been more common details in the transverse

connections of panels. Figure 3-3 shows some of detail used in the bridges. The bottom of the

openings can be detailed by polyethylene backer rods or wood forming. The bond between thegrout and the shear key surface has been found to be important, in particular, when there is no

longitudinal post-tensioning between panels. The surface can be roughened by several methods.

FIGURE 3-3 Grouted Female-to-Female connections

In the full-depth precast concrete panels, splicing the longitudinal reinforcement at the transverse

joints is one of designers challenges. The followings are major reasons for these difficulties(Badie, Tadros et al. 2006)

The panels are relatively narrow, 8 to 10 ft. Therefore, a wide concrete closure joint (2 to

3 ft) is needed if the longitudinal reinforcement splices were to be lapped. This would

require forming under the panels and extended period of time for curing.

The longitudinal reinforcement is spliced at the transverse grouted-joint between panels

that is considered the weakest link in the system. Therefore, great care has to be taken in

detailing the splice connection to maintain the construction feasibility and avoid leakage

at the joint during the service life of the deck.

Splicing the longitudinal reinforcement requires a high level of quality control during

fabrication to guarantee that the spliced bars will match within a small tolerance.

Splicing the longitudinal reinforcement requires creating pockets and/or modifying the

side form of the panels, which increase the fabrication cost.

For some simply-supported bridges, the longitudinal reinforcement may be intentionally

discontinued. At the positive moment sections, the longitudinal reinforcement does not actively

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involve in the design calculation, except for the creep and shrinkage controls. When the

longitudinal reinforcement is utilized, the following methods have been implemented.

Lap splice

A wide concrete closure is expected, however, the conventional methods of design and

construction can be utilized (Figure 3-4).

FIGURE 3-4. Lap Splice Connection of Longitudinal Reinforcement

Spiral Confinement

This detail has been developed to reduce the lap splice length. The spiral confinement provides

lateral stressed to the concrete in the spliced area so that the development stress between rebarcan be highly maintained (Figure 3-5).

FIGURE 3-5 Spiral Confinement Longitudinal Reinforcement Splice

Post-TensioningPost-tensioning in the longitudinal direction pushes the stress at the joints into compression

under service condition. The chances of developing cracks at the joints decrease. Longitudinal

post-tensioning is typically provided after the transverse panel-to-panel joints are grouted andcured, but before the deck-to-girder connections are constructed. For simply-supported spans, a

minimum prestress level in 150 to 200 psi is required to keep the joints in compression under

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service loads. For continuous spans, a minimum prestress level in 300 to 450 psi is needed for

the same purpose (Issa, Idriss et al. 1995). AASHTO requires a minimum of 250 psi prestress

throughout the joint. Practically, high strength threaded rods or strands can be used for post-tensioning (Figure 3-6 and 3-7).

FIGURE 3-6 Post-tensioning provided by high strength threaded rods

FIGURE 3-7 Post-tensioning provided by Strands (the Skyline Drive Bridge in Omaha,

Nebraska)

3.3.3 Panel-to-Girder Connections

Early applications were not designed for composite action. These simple connections clamped

the panels to the beams with a bolt and plate system. This ensured only that the panels would not

be dislodged from the beams during subsequent construction operations. More recently,connections have been designed to transfer horizontal shear between the beams and slabs to

make use of the efficiency of composite action. In most cases, a pocket is cast in the precast

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panel during fabrication. In some instances, the locations of these pockets are coordinated with

those of shear connectors attached to the beams. For steel girders, grouped shear studs are used

for composite action. For concrete girders, the stirrups protruding from the girder web aregenerally used, but dowels can be replaced with stirrups. Followings are the types of connection

used.

Shear Pocket Connections

This connection emulates a cast-in-place slab-to-girder connection. Full composite action can be

developed without the need for an excessive number of connectors.

FIGURE Shear Pocket Connection (Issa, Idriss et al. 1995)

Bolted ConnectionsThe ducts for bolts are cast into the panels, and they align with the holes in the flanges of the

steel girders. After the gap between the girder flange and panel has been grouted, bolts are

connected through ducts.

FIGURE Bolted Connection (Issa, Idriss et al. 1995)

Tie-down Connections

This connection consists of mechanical clamps to attach panels to the girders. This connection

does not provide full composite action, and experiences poor performance during earthquakes.

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FIGURE Tie-Down Connection (Issa, Idriss et al. 1995)

Other connections

In some studies, combined connections were introduced. As one of examples, headless studs are

proposed with tie-down devices (Yamane, Tadros et al. 1998).

FIGURE Combined Connection (Yamane, Tadros et al. 1998)

Currently, the maximum permitted longitudinal spacing of studs is 24 inches (AASHTO 2004).

For precast deck panels, it can be uneconomical when many numbers of pockets are required to

make composite action. So extending this spacing needs to come up with economical panels. As

one of the examples that utilize larger stud spacing, US Interstate 39/90, Door Greek Projectused 48 in. spacing of clustered studs.

3.3.4 NCHRP 12-65 System

In NCHRP 12-65 study, researchers studies new types of full-depth precast deck panel systems(Badie, Tadros et al. 2006). The proposed systems have the following characteristics.

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Longitudinal post-tensioning is not needed

No needs for proprietary products

The precast panels can be fabricated off the construction site or at a precast yard

The grouted areas are minimized and kept hidden as possible

No overlay is required

Two types of precast deck panels were studied. One has the transverse pretension with two layersof reinforcement rebar. The other panel does not include any pre- and post-tensioning systems

and the design follows the Empirical Design Method. Both panels have only single layer of

longitudinal rebar which is located in the mid section. The panels studied are 44 ft wide, 8 ftlong, and 8 inches thick. Figure shows the sectional view of panels.

(a) Panel with Pre-tensioning

(b) Panel with Reinforcement rebar only

FIGURE NCHRP 12-65 Panels

In longitudinal direction, the splice length for reinforcement rebar is reduced by using HollowStructural Steel (HSS) tubes. Research proposed two longitudinal connection details.

The first detail requires threading a No.6 reinforcing bar, which extends about 7in.

outside the panel to be installed, into the old panel, which results in a 6 in. bar

embedment length.

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The second detail allows vertical installation of the new panels, where a NO.6 bar is

embedded 11 in. in the HSS tube, in each of the mating joints. After a new panel is

installed, a 24 in. No.6 long splice bar is dropped through a vertical slot, which results inan 11 in. splice length.

Panel-to-Girder ConnectionFor precast concrete girders, a new connection detail is provided where clusters of three double

head 1 in. studs are used. Also the clusters are spaced at 48 in., which is much wider than thecurrent AASHTO Specification. Additional reinforcement was shown to be needed in the web ofthe girder to help reach the stud capacity and distribute the concentrated stud stresses.

FIGURE Grouped Stud connection to Concrete Girder

Panel-to-Steel Girder Connection

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A new connection detail is proposed, where clusters of eight 1 in. studs at 48 in. spacing are

used. HSS tubes or individual closed ties were shown to be effective in confining the grout

surrounding the studs.

FIGURE Proposed Connection Detail between panel and Steel Girder.

3.3.5 Applicability of Precast Deck Panel in ABC

Full-depth precast slabs provide the potential for significant time-savings over cast-in-placeconstruction. They can be used on various geometric girder configurations, including skewed and

curved girders. Table 3-1 summarizes the applications of precast deck panel for comparison.

However, several disadvantages of full-depth precast deck have been discussed as followings:

This system generally requires large amount of time-consuming grouting works

Splice of longitudinal reinforcement needs complicated operation with significant time

for construction Post-tensioning in the longitudinal direction may introduce congested reinforcement

detail in the panel and complication at the site

If additional cast-in-place concrete overlay is required, the whole construction schedule

cannot save much time compared to the conventional construction method

The above problems were addressed in the NCHRP 12-65 research. The proposed systems from

this study are efficient to be used in the ABC scheme. In the following study on seismic design

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of precast deck panel systems, therefore, panel geometry and connection details of this system

will be adopted. The seismic study of precast deck panel will include such issues as the

capability of developing diaphragm action in the deck and effects from vertical accelerations(Hieber, Wacker et al. 2005).

TABLE 3-1. Applications of Full-Depth Precast Deck Panel (Badie, Tadros et al. 2006)

Bridge State PanelP-to-G

ConnectionP-to-P Connection

I-80 overpass in Oakland California 14'-2" wide shear pocket(replace outside lane only) 61/2"~7" thick 4 studs/pocket

leveling bolts

I-84-Conn. Route 8 inter. Connecticut 26'-8" wide shear pocket PT-T(vertical 7% grade) 8' long leveling bolts PT-L (150 psi)

8" thick grouted ocket

Bloomington Bridge Indiana 4' wide tie-down clips PT-L (90 psi)(pony truss)

Woodrow Wilson MemorialBridge

Maryland 46'-7" wide studs PT-T

10'-12" long hold-down bolts PT-L8" thick

Suspension bridge overRondout Creek

NY 9' wide stud bolts PT-T

24' longV-shaped M-F shearkeys

6"~7" thick

Bridge over the Delaware River NY, Penn 7" thicksinglestud/pocket

epoxy shear keys

Bridge over Cattarougus Creek NY 7" thicksinglethreaded/pocket

Batchellerville Bridge NY 8" thick studwelded steel PL(long.)

QEW-Welland River Bridge Ontario 43'-6" long studs NO PT-T7'-11" wide leveling bolts

PT-L (435 psi at theint. pier)

8 7/8" thick

Dalton Highway Bridge Alaska27'-5 3/8"wide

studs PT-T (2 layers)

4'-10" long F shear keys 7" thick NO PT-L

Discont. Long rebar

Pedro Creek Bridge Alaska 7" thick studs NO PT-L & PT-Tleveling bolts

Kouwegok Slough Bridge Alaska6.9"~9.8"thick

studs NO PT-T & PT-L

7500mmwide leveling bolts F-F shear keys

1485mm long

Castlewood Canyon Bridge Colorado 5'-4" wide PT-L

16'-4" ~ 38'-4" long

15" ~ 18"thick

Dead Run Structure DC 7.9" thick leveling bolts PT-T & PT-L(curve, 3.37% long. Slope) 35' wide F shear keys

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7'-7" long

Bridge-4 on Route 75 Illinois 7.7" thick 3 studs/pocket PT-L7'-10" wide leveling bolts F shear keys

9" thick

Lake Koocanusa Bridge Montana15'-7" or 20'-7" wide

studs & steelchannel

PT-L

8' long leveling bolts bolted conn. In Long.Direction7"~10" thick

Skyline Drive Bridge Nebraska 5.9" thick PT-T & PT-L

(NUDECK, 25 skewed) 7' longV-shaped F shearkeys

A through truss bridgeNew

Hampshire8' long 4 studs/pocket F shear keys

3 3/4" ~ 53/4" thick

NO PT-L

rebar at tran. Conn.

I-287 Westchester NY 9" thick 9 studs PT-T & PT-L

(curved with 32 skewed) 41'-9" wideplastic shimpacks

F shear keys

US59 Tied Arch Bridge TX 60' wide PT-T & PT-L7' long no shear keys

a bridge in the country road Utah 8 " thick 3 studs/pocket(45 skewed) 38'-3" long leveling bolts

15'-11 3/4"wide

Route 7 over Route 50 Virginia leveling bolts PT-L (200psi)F shear keys

Door Creek Bridge Wisconsin 8" thick PT-T & PT-L(30 skewed)

NCHRP 12-41 7'-10" wide threaded stud PT-T & PT-L (200psi) 4" thick F shear keys

NCHRP 12-65

3.4 Superstructure

comparison with steel girders

3.4.1 Precast Concrete Girders

Since its introduction in the 1950s, the precast concrete beams have been used mainly for short-

and medium span bridge construction. However, the conceived span length limitation of 160 ft of

precast concrete beams restricts their applications to various bridge projects requires longer

spans. Recently, several techniques provides the possibility to extend the span length so that thecompetition with steel girders provides many design options with improved aesthetics and

reduced construction cost. The longer span is not directly related to the accomplishment of

accelerated bridge construction, but recent consideration of accelerated bridge construction takes

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advantage of prefabricated bridge members and systems where precast concrete beams will stand

out by their successful applications in past bridge construction practice.

3.4.1.1 Types of Girders

For girders that can span more than 100 ft, AASHTO and other agencies have developed

standard girder sections. The girder types listed in the present study are limited to AASHTO/PCIStandard, New England Bulb-Tees, and girders developed at Washington DOT. The cross-

sectional shapes can be categorized into box, I-Beam, Bulb-Tee, Tub, and deck bulb-tee sections.

In the following figure, the span lengths of girder types are compared. For some girders, thepossible span length is increased by using high-strength concrete.

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Agency Span (ft) 100 110 120 130 140 150 160 170 180 190 200

BII-36

BIII-36

BIV-36

BIII-48

BIV-48

BT-54

BT-63BT-72

I-Beam (III) 7

I-Beam (IV) 12

I-Beam (V)

I-Beam (VI) 12

Deck BT-35 7

Deck BT-53

Deck BT-65

NEBT1200 6 8

NEBT1400

NEBT1600

NEBT1800

W50G

W85GW74G

WF42G

WF50G

WF58G

WF74G

W83G

W95G

WBT62G

U54G4

U54G5

U54G6

U66G4

U66G5

U66G6

U78G4

U78G5

U78G6

UF60G4

UF60G5

UF60G6 8.5

UF72G4

UF72G5

U72G6 8.5

UF84G4

UF84G5 8.5

UF84G6 8.5

W41DG 6

W53DG

W65DG

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5

8.5ksi

6 8

6 8

12

12

7

6ksi 8ksiPCI

7

7

7

7

7

7 ksi 12ksi

7

7

AASHTO

7

7

12

1277

Washington DOT

6ksi

6ksi

8.5

8.5

8.5

FIGURE Comparison of Span Ranges of Precast Concrete Girders

For AASHTO Standard products, the sectional shapes of box beams, I-Beams, Bulb-Tees, and

Deck bulb-tee are demonstrated in the following figure (PCI 2003).

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(a) Box Beams (b) AASHTO-PCI Bulb-Tee

(c) AASHTO I-Beams (d) Deck Bulb-Tee

The cross-sectional shape of New England Bulb-Tee (NEBT) is as follows (PCI 2003). Bardow

et al.(Bardow, Seraderian et al. 1997) summarizes the historical background of theirdevelopment.

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FIGURE Typical Cross Section of New England Bulb Tee Girders

Girders in Washington DOT can be categorized into tub girders, I-Girders, and Bulb-Tees asshown in Figure (WSDOT 2006).

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FIGURE Precast Concrete Girder Types from Washington DOT

One of the important issue for long girders is the transportation issue. States generally restrict

oversize and overweighted vehicles on their roadway network. The allowable girder weightsvary from 120 kips to 200 kips. As extreme cases, 209 ft long and 260 kip heavy NU 2800

girders were transported in British Colombia, and 14 precast girders, 170 ft long and 190 kip

heavy for each, were transported in Washington State (Castrodale and White 2004). However,the size and weight limitation on the roadway network from the fabrication site to the

construction site are generally limited.

3.4.1.2 Techniques to Increase Span Length

In the research report of NCHRP 12-59, Extending Span Ranges of Precast Prestressed

Concrete Girders, available techniques for extending spans are summarized (Castrodale and

White 2004) They are categorized into four groups such as:

Material-related options

Design enhancements

Methods utilizing post-tensioning

Spliced girder construction

Among them, the followings are restated here for further discussion.

High-Strength Concrete. Application of High Strength Concrete (HSC) helps to extend span

length by providing higher compressive and tensile stress limits at long-term and at transferstages. As summarized in the table for several girder sections, the concrete strength varies from

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5000 psi to 12000 psi. Specifically, beams made with HSC exhibit the following structural

benefits (PCI 2003):

Permit the use of high levels of prestress and therefore a greater capacity to carry gravity

loads. HSC allows the use of (1) fewer beans lines for the same width of bridge (2) longer

spans for the same beam depth and spacing and (3) shallower beams for a given span

For the same level of initial prestress, reduced axial shortening and short-term and long-term deflections

For the same level of initial prestress, reduced creep and shrinkage result in lower

prestress losses, which can be beneficial for reducing the required number of strands.

Higher tensile strength results in a slight reduction in the required prestressing force if the

tensile stress limit controls the design.

Strand transfer and development lengths are reduced

Increased Strand Size. The use of a larger strand at the same strand spacing improves theefficiency of pre-tensioned girders. The application of 0.6 inch-diameter strands provides longer

span compared to the 0.5 inch diameter strands in the same section. Designers are rapidly

implementing the use of 0.6-in.-dia strands. This will improve the efficiency of all beam shapesbecause each 0.6-in-dia. Strand provides 40 percent more pretension force for only a 20 percentincrease in diameter. The AASHTO specifications allow the same center-to-center spacing for

0.6-in.-dia strand as for 0.5-in-dia strand.

Modification of Standard Girder Section. This option includes increasing size of some part of the

girder such as the depth of the bottom flange, the total depth of the girder, and the width of the

top flange. The latter can be adopted to reduce deck forming, improve lateral stability of thegirder, and increase section properties.

Modification of Strand Pattern. This includes reducing strand spacing, bundling strands at drape

points, and debonding strands to improve the performance.

Methods using Post-Tensioning. Post-tensioning can be used in girders combined with pre-

tensioning and/or in the deck over internal piers. Also staged post-tensioning can be scheduled tointroduce compressive stress at the deck. In order to implement post-tensioning, the fabrication

and construction become more complicated because of post-tensioning work, grouting, and

additional reinforcing works with anchorage blocks.

Two conclusions can be made regarding effective utilization of beams with high strength

concrete (PCI 2003):

The effective ness of HSC is largely dependent on the number of strands that the bottom

flange can hold. The more strands contained in the bottom flange, the farther the beamcan span and the greater the capacity to resist positive moment. It is recognized that

designers do not always have a large number of choices of available beam sections.

Nonetheless a beam that provides for the greater number of strands in the bottom flangeis preferred when using HSC.

Allowable stresses are increased when using HSC. If these limiting stresses cannot be

fully utilized with 0.5-in.-dia strands, then 0.6-in.-dia strands should be used. The tensile

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strength of 0.6-in.-dia strands is nearly 40 percent greater than the capacity of 0.5-in.-dia

strands.

Higher concrete compressive strength at transfer allows a beam to contain more strands and

increases the capability of the beam to resist design loads. To achieve the largest span for a given

beam size, designers should use concrete with the compressive strength needed to resist theeffect of the maximum number of strands that can be accommodated in the bottom flange.

In NCHRP12-59 study, the influence of each option to lengthening bridge span length iscompared based on comparative design of a simply supported span (Castrodale and White 2004).

The elevation view and cross-sectional view of the bridge are as follows:

FIGURE Design Comparison

For this given configuration, several options are used in the design of girder based on PCI BT-72girder section. The following table summarizes the design variation.

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In this comparative design study, the resulting conclusions are:

The greatest increase in maximum span length was obtained by casting the deck with the

girder (case 13) and by adding post-tensioning to a pre-tensioned girder (case 15), with

increases in maximum spans of 33.1 and 24.6 percent, respectively.

The next most effective strategy for increasing the maximum span length was the

combination of increased strand size with high-strength concrete (Case 12), with an

increase in the maximum span of 16.9 percent. A significant finding was the increase shown in Case 12, where two strategies were

combined to produce a much higher increase in maximum span than either strategy alone.

In the comparative design among PCI BY-72, NEBT 1800, and AASHTO Type VI, it is also

shown that the combination of high-strength concrete with 0.6-inch diameter strands is aeffective method to increase the span length regardless of girder types.

As discussed, providing continuity of precast concrete girders over the internal piers is one of theapplicable methods to increase span length. Abstractly, two groups of techniques can be

considered; post-tensioning and nonprestressed reinforcement. When post-tensioning is utilized,

the precast girders are pre-tensioned only for the dead-load applied before continuity of girders isdeveloped, and the added post-tensioning resists the other part of dead load and live load. One of

the examples is shown in Figure (PCI 2003). Also the continuity can be provided by longitudinal

nonprestressed reinforcement in the deck over the internal piers as demonstrated in Figure (PCI

2003). The similar design approach may be used for bridges adopting precast full-depth deckpanels. The precast panels are post-tensioned over the piers or projecting reinforcement are

spliced to provide negative moment capacity at the section.

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FIGURE Continuity Developed with Post-Tensioning

FIGURE Continuity Developed with Conventional Deck Reinforcement

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FIGURE Methods to Establish Continuity for Precast Deck Panels

3.4.1.3 On-Going Researches

One of the recent areas of research is the allowable design release stress limits for pretensionedconcrete girders. The reasons for limiting stresses in the current provisions are to prevent

cracking and excessive deflection or camber (Castro, Kreger et al. 2004). Additionally, the

extreme fiber compressive stress limit is an indirect design check to prevent concrete crushing bythe applied prestressing forces. The following is the current provisions for limiting stresses at

prestressing transfer.

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Along with the primary purpose of estimating realistic stress limits for the related studies, the

increased stress limits may help to increase span length of the precast concrete girders.

Pang and Russell (Pang 1996) investigated the change of compressive strength of concrete

cylinders subjected to sustained loads. Specimens were made from the high-early strength

concrete mix and steam-cured. After curing, three different levels, 0.60fci, 0.70fci, and 0.80fci,of sustained compressive stressed are applied. Compressive strength of the specimens do notshow any detrimental effects due to sustained loads. Huo and Tadros (Huo and Tadros 1997)

studied the allowable compressive strength of precast concrete beams, and compared results

from linear and nonlinear analyses. In the linear analysis, the compressive stress limits are

controlled,f

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This study indicated the required concrete compressive strength differs with the different

sectional shapes. For example, the required compressive strength for the PCI double-tee sections

is larger than those for the NU inverted-tee sections. However, those values are always largerthan 0.60fci. In the experimental investigation, it was also shown that there are no adverse

effects in measured camber and concrete strains with the elevated compressive strength which is

measured 0.79fci and 0.84fci. This study was concluded with emphasizes on the possibility ofsubstitution of the compressive stress limit requirement with the strength design approach at

transfer.

In order to understand the impact of elevated concrete stresses in pretensioned concrete beams at

prestress transfer, analytical and experimental studies were conducted for the girder sections

used in Texas DOT (Castro, Kreger et al. 2004). This study confirms some major conclusions

derived from (Noppakunwijai, Tadros et al. 2001) including the limitation of allowable stressdesign approach, but addresses the possible problem to increase the tensile stress limit by

observing cracks designed for '7.1ci

f . In this study, the beams were designed based on

nonlinear analysis, which result in higher compressive stress at transfer. The compressive

stresses were equal to or higher than 0.75fc In the analytical and experimental investigation, the

camber in short-term and long-term did not show any sign of failure. As one of conclusions,

therefore, it is pointed that pretensioned concrete beams can be subjected to elevatedcompressive stress levels at prestress release as long as long-term camber response is adequately

predicted and values are accepted to the engineer. Also it was shown that the allowable stress

design method typically overestimates extreme fiber compressive stresses at transfer.

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3.4.1.4 Applicability of Precast Beams in ABC

For the application in accelerated bridge construction scheme, the deck bulb-tee type precastconcrete girders have been studied because the construction of girders and deck can be done

without cast-in-place concrete works. The girders are connected longitudinally by grout-filled

shear keys, mechanical fasteners, and/or transverse post-tensioning. For example, type type ofsuperstructure was studied as a prefabricated precast concrete bridge system for the state of

Alabama (Fouad, Rizk et al. 2006) as in the following figure.

FIGURE. UAB Precast Bridge System

However, its use in heavy traffic areas has been cautioned because of the durability issues at the

longitudinal joints (Hieber, Wacker et al. 2005). Cracking, over the longitudinal joints betweengirders, has been identified in the overlay on many bridges of this type (El-Remaily, Tadros et al.

1996; Badie, Kamel et al. 1999).

The longitudinal joints should be designed for out-of-plane shear caused by wheel loads and in-

plane tension due to shrinkage of the slab (Stanton and Mattock 1986). So the design needs to

consider not only shear keys but also possible mechanical connection to carry tensile stressesacross the joints. The following is the standard mechanical connection configuration used by

Washington State DOT.

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FIGURE. Standard Mechanical Connection Detail in Washington State

Deck bulb-tee superstructure can be a viable option for accelerated bridge construction. The

main advantage of this system is that girders also serve as deck. However, the poor performance

of longitudinal joints limits its use for low traffic areas. In order to improve the riding quality ofthe bridge, the partial-depth precast deck system can be considered in the application.

3.4.2 Spliced Girders

Spliced girders are a type of precast prestressed concrete girders, which spans over 160 ft byutilizing post-tensioning to the precast prestressed girder segments. Precast pretensioned beam

segments are usually post-tensioned together at or near the project site and lifted as one piece

onto final supports. In most cases, however, the precast segments are erected on temporarytowers to span the full distance between supports. Then the segments are post-tensioned together,

they lift off the temporary falsework and span between their permanent pier and abutmentsupports (PCI 2003). The following figure shows spliced girders used in simply-supported andcontinuous spans (Castrodale and White 2004). Castrodale and White (Castrodale and White

2004) summarized spliced girder bridges with their comparable cost-information.

(a) Simply-Supported Span

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(b) 2-Span Continuous Girder

(c) Three-Span Continuous Girder

FIGURE Examples of Spliced Girders

Spliced girders have several similarities with segmental box girders, and two constructions share

the basic idea of making the entire span with smaller precast segments combined by post-tensioning and grouting. In spliced girder construction, however, segments are much longer,

connections between girder segments are generally cast-in-placed, and I-shape, bulb-tee, and U-

beams are preferred than closed box shapes (Castrodale and White 2004). It was recommendedthat the AASHTO LRFD Specifications should be revised to address the spliced girder clearly

and appropriately not to confuse designers with segmental box girder construction.

3.4.2.1 Types of Girders

The beam segments used in a spliced girder can be pretensioned to resist self-weight. The typical

spliced girder cross-sectional shapes are I-beam, open top trapezoidal box beams, and boxbeams. Sometimes, the combination of them, hybrid section, may be taken. The most popularshape is the I-beams because of their moderate self-weight, ease of fabrication, and ready

availability (PCI 2003). In the following table, girders used for a spliced girder configuration in

Washington State DOT (WSDOT 2006) and Nebraska (Tadros, Girgis et al. 2003) are

summarized.

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TABLE. Types of Spliced Girders SectionsAgency Span (ft) 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340

WF74PTG

W83PTG

W95PTG

U66PTG4

U66PTG5U66PTG6 9

U78PTG4

U78PTG5 9

U78PTG6 9

NU1100

NU1350

NU1600

NU1800

NU2000

Nebraska

9 ksi

9

9

Washington

DOT

9

9

9

8 ksi

8

8

8

8

The following figure shows the cross sectional shapes of precast post-tensioned spliced girders in

Washington State. It is assumed that these types of girders are used with pier segments of which

depth increases at internal pier section as shown in the previous figure.

FIGURE. Spliced Girders in Washington State

For Nebraska University (NU) girders in the table, the possible span length is based on theresearched configuration called haunch block system. In this system, the same section is used

along the entire span length, but the haunch block is introduced at internal pier section. The

following figure shows the concept of haunch block system.

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FIGURE. Haunch Block System in Nebraska

As the trend continues toward continuous superstructures, the need becomes evident for

optimum I-beam sections. The I-beam geometry should perform well in both the positive andnegative moment regions. This is clearly a different goal from shapes that were developed

specifically for simple spans. Simple-span beams generally have inadequate sections for negative

moment resistance and have webs too thin for post-tensioning ducts. A minimum web width to

accommodate the post-tensioning tendon ducts and shear reinforcement is required (PCI 2003).

3.4.2.2 Construction Detail

By the way of connecting segments at the site, two methods can be classified: splicing on the

ground and in-place. As the main advantage of the splicing on the ground method, the major

falsework is not necessary. This saves the cost of the falsework, and increases quality of thegirder, because workers can easily access any part of the girder. However, this method requires

large areas of fabrication next to the site. Generally, it is not easy to find or to prepare such space

without much increase of cost, splicing in-place method has advantage on this aspect.Additionally, in-place splicing method does not require large capacity of transportation and

lifting equipment. In-place splicing does require falsework to support segments temporatily

during the operation of camber control and post-tensioning. So the axial stiffness of the

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falsework needs to be high enough to support the camber operation done by shimming or skrew

jacking between the girders and falsework.

Construction Sequence of Single spans

The following figure shows one of the possible construction sequences of single span splicedgirder (Castrodale and White 2004). Single span girders are made of two or more segments.

FIGURE Construction Sequence

Stage 1. The temporary and permanent supports are constructed.

Stage 2. The segments are placed on supports and braced.

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Stage 3. The deck slab is constructed. The splice joints are cast, tendons inserted in ducts

and post-tensioning introduced, which complete the assembly of the girder. Before the

splice joints are cast, the end elevations of the segments need to be carefully positioned toallow for calculated long-term deflection. This also impacts the aesthetic appearance of

the profile due to camber in the beam. These elevations also determine the amount of

concrete needed for the haunches. When the post-tensioning is applied, the full span,spliced beam cambers upwards and lifts up away from the temporary towers. The beam

reactions that were being carried by the temporary towers are now carried by the spliced

girders, so they must be considered in the analysis.

Stage 4. Placing additional components and eliminate the temporary supports.

Construction Sequence of Multi Spans

For continuous spans, the following figure shows one of the construction methods.

FIGURE. Construction Sequence.

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Stage 1. Construction of temporary and permanent supports, and placement of girder

segments on supports. The pier segment is installed on the pier and adjacent towers and a

connection is made to the pier. Ideally the pier connection should be one that allows forhorizontal displacement of the beam at the time of post-tensioning. However, a fully

integral joint can be utilized as long as the supports at the abutment allow for horizontal

movement during post-tensioning operations. Placement of the first end segment createsmoments in the pier segment and overturning effects on the tower and pier that must be

evaluated. When an end segment is erected on the second span, the temporary

overturning effect is eliminated.

Stage 2. The splices are cast.

Stage 3. A part of tendons are post-tensioned.

Stage 4. The deck slab is constructed.

Stage 5. The rest of tendons are post-tensioned, which introduces compressive stress on

the deck. After the concrete in the splice has achieved the specified compressive strengthand the post-tensioning tendons are stressed, the tower reactions must be applied as loads

to the continuous two-span system as the beam lifted from the towers.

Stage 6. Additional parts are constructed.

Vertical Splicing for Continuous Spans (Tadros, Girgis et al. 2003)

For continuous spans, large negative moments and large shear forces at the negative moment

sections are supported by haunched girder section of which web depth is increased (followingfigure (a)). Or the standard shaped girders can be combined with a separate precast haunch block

as in figure (b).

(a) Haunched Pier Segment (b) Vertial Splicing of Pier Segment

FIGURE. Comparison of Pier Segments

The configuration of this girder system can be found in the previous figure. The connection

between haunched block and I-girder is provided by 8 inch spaced shear connection. As shownin Figure ??, treaded rods from the bottom of the girder and from the top of the haunched block

are arranged to provide shear transfer between two blocks. This space will be filled with a

flowable concrete finally.

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FIGURE Connection Detail of Vertically Spliced Girder

The advantages of this system can be summarized as:

Lengthening the spanning length. Also increase the span-to-depth ratio for the cirder

It is possible to completely eliminate the falsework, which results in lower cost.

From aesthetic point of view, the improved span-to-depth ratio allows more pleasing

appearance and clearance below the girder.

3.4.2.3 Construction Issues

System Optimization

For continuous spans, the critical section is usually located at internal regions due to high

moments and shear forces. In order to accommodate these large forces, the sections are generallydeepened as discussed in the previous section. These heavier segments need more careful design,

transportation, erection, and construction planning. The designers can utilize other options such

as (PCI 2003):

Placement of a cast-in-place bottom slab

Gradual widening of a member toward the support

Using higher concrete strength

Adding compression reinforcement in the bottom flange

The use of a hybrid system

The use of a composite steel plate in the bottom of the bottom flange

Minimum Web Width

Web width should be as small as possible to optimize cross-section shape and minimize weight.Yet it should be large enough to accommodate a post-tensioning duct, auxiliary reinforcement

and minimum cover for corrosion protection. LRFD Article 5.4.6.2 states that the duct cannot be

larger than 40 percent of the web width. This requirement has been traditionally used to sizewebs for internal ducts in segmental bridge construction. Historically, this requirement has not

existed and has not been observed for segmental I-beams. When the NU I-Beam was developed

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in the early 1990s, a 6.9-in. web was selected to provide approximately 1-in. cover on each side

plus two #5 vertical bars plus a 3.75-in dia. Pos-tensioning duct.

FIGURE Web Configuration for NU I-Beam

The Washington DOT chose a web width of 7.87 in. for their new series of beams. The 4.33-in.

duct can accommodate commercially available post-tensioning systems of up to (19) 0.6-in.-dia.Strands per tendon, or (29) -in.-dia. Strands per tendon.

FIGURE Web Configuration for Washington State I-Beam

Most of the segmental I-beam bridges built using post-tensioning over the past four decades havenot met the limit of duct diameter and web width. However, there has been no problems in the

application to spliced girders (PCI 2003).

Design and Fabrication Details

Wet-cast splice are the standard practice in the beam splices. The ends of the beams at splicesshould formed shear keys, if required. Ducts for post-tensioning should be made of semi-rigid

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galvanized metal, high density polyethylene or polypropylene. They must be adequately

supported within the beam during casting to maintain alignment and minimize friction losses.

Grouting of Post-tensioning Ducts

Grouting is mainly used to prevent corrosion of the post-tensioning tendons. Compared tocorrosion issues of pre-stressing steel, the post-tensioning tendons are more vulnerable from the

following point of view:

Strands are exposed within ducts for several days prior to grouting.

Ducts are grouted after the tendons are stressed, but the quality of grouting cannot be

determined along the whole length of the tendon.

Anchorages are encased after grouting, but may be subject to infiltration by water.

Even though there have been several occasions of corrosion, it is discussed that this is not the

widespread problem and showed a very low frequency of occurrence (Castrodale and White

2004). As the causes of these problems, including those from the precast segmental boxconstruction, were identified as; poor design details; low-quality materials; and improper

grouting procedures combined with inadequate inspection practices. The recent developments in

the Post-Tensioning Institute (PTI 2001), the American Segmental Bridge Institute (ASBI 2000),and several DOTs address this issue from various points.

Deck Removal

When the deck is in place when the beams are post-tensioned, it becomes an integral part of the

resistance system. Removal of the deck for replacement may temporarily overstress the bare

beam. This would require an elaborate analysis and possibly a complicated temporary support

scheme until the new deck is in place. However, if properly analyzed and the economics areverified, there is no reason this approach should not be considered. Some states have avoided this

issue by requiring designers to apply the post-tensioning in its entirety before the deck is placed(Nebraska 2001). An additional benefit of this single-stage post-tensioning is simplified

scheduling and coordination of construction. However, there are significant benefits to

multistage post-tensioning in terms of structural efficiency, compared with single-state post-tensioning. A convenient option is to divide the post-tensioning into thirds: two-thirds applied to

the bare beam and one-third applied to the composite section. This is demonstrated in the

example of section 11.8. There are a number of benefits to this division. The deck is subject to

compression that controls transverse cracking and extends its first life before it might needreplacement. It may be desirable to apply all of the post-tensioning after the deck becomes part

of the composite section. This case would be similar to the conditions of a segmental box beamsystem where the top flange is an integral part of the cross-section when the post-tensioningtendons are stressed. This solution in the US and abroad has proven to provide a deck surface of

excellent durability, perhaps not requiring any provisions for deck removal and replacement.

Post-tensioning Anchorages

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Post-tensioning anchorages require the use of end blocks, which are thickened webs for a short

length at the anchorages. End blocks can increase production costs of beams considerably due to

the need for special forms and forming anchorages during production. According to the LRFDArticle 5.10.8.1, the end block length should be at least equal to the beam depth and its width at

least equal to the smaller of the widths of the two flanges. The following figure shows one

example of the end block. The anchorage zone is typically detailed using an end block that is thesame width as the bottom flange and extends for a distance from the end of the beam of at least

one beam height before a tapered section returns the cross-section to the width of the web.

FIGURE Post-Tensioning End Block

Anchorage zones are designed to accommodate anchorage hardware with its associated special

reinforcement and to provide adequate space for the reinforcement needed to distribute thehighly concentrated post-tensioning force. Detail guidance for the design of anchorage zones is

given in the PTI publication, Anchorage Zone Design (2000). Some research has indicated that a

much smaller anchorage zone may be adequate. A research project by Tadros and Khalifa (1998)(Tadros and Khalifa 1998) The new details have been adopted and used on several projects in

Nebraska and other area such as project shown in Figure 11.7-3. A paper by Ma, et al (1999)

(Ma, Saleh et al. 1999), Breen, et al (1994) (Breen, Burdet et al. 1994).

Post-Tensioning Losses

Because of post-tensioning used in spliced girders, additional issues become active mostlyrelated to the post-tensioning and its losses such as (PCI 2003):

Losses in post-tensioning tendons. Additional sources of prestress losses must be

considered such as friction and anchor losses

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The interaction of losses between pre-tensioned strands and post-tensioned tendons

Time-dependent analysis. This method of analysis should take into account the effects of

creep and shrinkage of concrete and the relaxation of pre-stressing steel

The effect of post-tensioning to continuous beams. The method of analysis should

properly account for post-tensioning, including secondary moments

The effect of post-tensioning ducts on shear capacity

The method to evaluate the post-tensioning including its losses can be found in (PCI 2003) and(Collins and Mitchell 1997). For the interactive pre-stressing losses between pre-tensioning and

post-tensioning, detailed and approximate methods were proposed considering the use of high-

performance concrete for girder fabrication (Tadros, Girgis et al. 2003). These methods werebased on the related study on pre-stressing losses in high strength concrete precast concrete

girders (Tadros, Al-Omaishi et al. 2003). The study focused the possible changes on material

properties of high strength concrete used in precast concrete girder as follows:

prediction of modulus of elasticity, shrinkage, and creep of concrete, especially as they

relate to the high-strength concrete

methods for estimating prestress losses that would account for the effects of differential

creep and shrinkage between precast concrete girder and cast-in-place concrete deck andfor relatively high prestress levels and low creep and shrinkage in high strength concrete

as indicated in the following figure.

FIGURE Stress versus time in the strands in a pretensioned concrete girder (Tadros, Al-

Omaishi et al. 2003)

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3.4.2.4 Connection Detail

A wide variety of joint details have been used for splicing between beams. The following figureshows some of the beam splice configurations used for I-beams. Most precast concrete beam

splices are cast-in-place as shown in Figure a, b, and c. Cast-in-place splices give the designer

more construction tolerances. These details use a sap width of from 6 to 18 or even 24 in. Thespace is filled with high-early-strength concrete. Detail a is not recommended, even when the

end of the beam is roughened, because the high vertical interface shear generally requires a more

positive shear key system. Detail d is discouraged because of the difficulty in adequatelymatching two pretensioned beam ends, especially when the beams are of different lengths and

with different pretensioning levels. Detail e is used with continuous post-tensioning but is

sometimes used when the designer desires to have an expansion joint in the bridge.

FIGURE I-Beam Splice Configuration

Cast-in-Place Splice

Cast-in-place, post-tensioned splices are most commonly used because of their simplicity and

their ability to accommodate fabrication and construction tolerances. The segments are erected

on flasework, the ducts are coupled at the same time as the concrete for the splice, or the deckslab may be placed at the same time as the concrete for the splice, or the deck concrete may be

placed after the splice and following the first stage of post-tensioning. The following figure

shows details of a cast-in-place, post-tensioned splice.

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FIGURE Cast-in-Place Post-Tensioned Splice

Stitched Splice

Stitched splice is used for connecting two girders by post-tensioning at the end parts only. The

following figure shows one of detail of this kind of details. In this type of cast-in-place splice,

the precast, pretionsioned degments are post-tensioned across the splice using short tendon orthreaded bars. It should be noted that precise alignment of the post-tensioning ducts is essential

for the effectiveness of the post-tensioning. If proper alignment is not achieved, considerable

frictional losses can result. In addition, because of the short length of the tendons, anchor seating

losses could be unacceptably large. End blocks are required at the spliced ends of the beams in

order to house the post-tensioning hardware and provide the end zone reinforcement to resistconcentrated stresses due to the anchorages. This type of splice may be suitable for long bridges

where continuous tendon post-tensioning over the full length produces excessive friction losses.

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FIGURE Stitched Splice

Match Cast Splice

Match-cast segments were used in early applications of spliced beam bridges to eliminate the

time and expense of cast-in-place joints. They are seldom used today. Match-casting of I- orother beam sections has significant challenges. Beams that are pretensioned and cast on a long-

line system, as most are, have continuous pretensioning strands that must be cut before these

products are removed from the form. That operation is usually facilitated by the use of headersthat form the ends of beams. The space between headers is used to cut the strands. Emulated

match-casting has been used where a machined steel header provides precisely formed concrete

surfaces. The header is precision-made in a machine shop to exacting tolerances. Installed in the

casting bed, it has stubs to accurately position the ends of the post-tensioning ducts and accessports to allow cutting the strands that have been threaded through it. Other necessary details to

consider include: (1) the coupling of post-tensioning ducts. This requires the forming of small

recesses around the duct where it meets the header. (2) sealing of the coupling zone against

leakage of post-tensioning grout (3) camber in the pretensioned beams that causes the ends torotate. The rotation must be accounted for during fit-up of the beams at the joints.

FIGURE Match Cast Splice

3.4.2.5 Application of Spliced Girder in Seismic Regions

The spliced girders provide better structural systems for seismic design aspect because theygenerally consist of continuous spans, and because an integral connection between superstructure

and substructure is usually established (Castrodale and White 2004). This aspect will be

discussed in the substructure section, in particular, integral pier section.

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3.4.3 Segmental Box Girders

Segmental concrete box construction is a method to construct bridge by adding segmentsutilizing post-tensioning techniques. Segments can be fabricated in cast-in-place method or can

be pre-fabricated. The post-tensioning can be internal or external. For internal cases, ducts are

embedded in the box. For external cases, ducts are usually located inside of the box. Recently,external types are preferred for the convenience in construction and management. The ducts can

be grouted or not. If they are not grouted, i.e. unbonded, special cares need for corrosion

protection. These applications can be found in the stay cables in cable-stayed bridges.

In the early development of segmental concrete box girder construction, cast-in-place segmental

method was used in Germany whereas precast segmental method was developed in France.

Eugne Freyssinet in France built precast segmental bridges over the Marne River in the 1940sas the original form of the following precast segmental box constructions (Sauvageot 2000).

Precast segments construction were introduced in U.S. by Jean Muller and further developed by

him and Eugene Figg. As the balanced cantilever method has been modified and revised,

progressive and span-by-span construction methods were developed and used widely.

The vision that Jean Muller had was to create an industrialized construction system to build anytype of bridge with standard modules, assembled with post-tensioning, without any cast-in-place

concrete. So he developed the concept of match-cast joints, which allows the transverse slicing

of concrete box girders and the assembly of such slices the segments in the same order asthey were produced, without any need for additional in situ concrete to complete the bridge deck

(Sauvageot 2000). In 1978, through the design of the Long Key Bridge in Florida, internal post-

tensioning was replaced by external post-tensioning. A number of other concepts invented by

Jean Muller allowed further development of the modular construction concept: span-by-spanassembly method (Long Key Bridge), progressive placing (Linn Cove Bridge), precast segmental

construction of the piers, D6 cable-stayed segments (Sunshine Skyway Bridge), delta frames

(James River Bridge, C&D Canal Bridge), etc. The other historically important development andcurrent trend of the precast segmental constructions, including segmental box columns, can be

found in (Figg and Pate 2004). As was stressed by the authors, precast concrete construction is a

representative example of accelerated bridge construction.

3.4.3.1 Structural Concepts

In precast segmental construction, the precast segments are fabricated in the precasting yard,

where the good quality of products can be continuously maintained. Fabrication of segments

usually starts with the foundation construction at the site so that the time-dependentcharacteristics such as shrinkage and creep will not affect significantly at the construction stage

at the site. Also the speed of assembling segments is fast enough that backlog of segments is

required. This method is, however, more suitable for large project because of significant amountof initial investment for precasting yard, molds, lifting equipment, transportation, and erection

equipment.

Segments

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The span-to-depth ratio can differ from the construction method (Sauvageot 2000).

Span-by-span method 1:25 (1:20 more preferable)

Balanced Cantilever method 1:18 (constant depth) and 1:40 (varying-depth)

Single-cell box girders are preferred for efficient casting and fabrication. The following figure

shows the typical sections of a varying-depth box girder.

FIGURE. Typical Cross Section of a Varying-Depth Box Girder (Span length : 93 m)

Match-casting technique is one of key ideas that make the precast box girder construction bepossible and applicable. The essential feature of match casting is that successive segments are

cast against the adjoining segment in the correct relative orientation with each other starting from

the first segment (Sauvageot 2000). The segments are subsequently erected in the same order,and hence no adjustments are necessary between segments during assembly. The joints are

wither left dry or made of a very thin layer of epoxy resin. Post-tensioning proceeds as early as

practicable, since there is no need for joints to be cured. The strength of epoxy is not consideredin the shear capacity calculation. Shear keys are usually used to support shear forces in

combination with longitudinal post-tensioning.

The post-tensioning can be internal or external. The size and the number of tendons depend onthe dimension of the box cross section. For cantilever construction, two groups of tendons are

used: cantilever tendons and continuity tendons. Cantilever tendons are used to process the

balanced cantilever erection, whereas continuity tendons are used after the span are madecontinuous. The following figure shows the typical example of internal post-tensioning layouts.

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FIGURE Typical Post-tensioning Layout for Internal Tendons

3.4.3.2 Construction Issues

Construction Methods

In Balanced Cantilever methods, the erection of the segments starts from piers that has beenconstructed by cast-in-place or precast construction. The erection continues to the mid span from

the each side in a balanced manner so not to introduce significant construction load to thepiers. The final closure joints at the mid span connects cantilever from adjacent piers. The

following figure shows the main concept of this construction method.

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FIGURE Balanced Cantilever Construction Method

Theprogressive construction is derived from cantilever construction, where segments are placedin a successive cantilever fashion (Sauvageot 2000). The excessive high moments will bedeveloped in this method, and temporary supports are needs to compensate these moments. As a

example, the following figure shows the case where a temporary tower and stay-cable are used.

FIGURE. Frburge Viaduct, France

The Linn Cove Viaduct on the Blue Ridge Parkway in North Carolina is an excellent example toshow the advantage of this construction method.

In span-by-span construction, segments are placed and adjusted on a steel erection girderspanning from pier to pier, then they are post-tensioned together in one operation. This

construction is efficient for projects having many relatively short spans.

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Corrosion problems

Whereas in the case of pre-tensioning conditions to ensure durability are relatively easily met,

in the case of post-tensioning the problems are more complex. In pre-tensioned beams the

prestressing steels are almost always straight and tensioned between thee anchorages in aprestressing bed. Passivation of the steel to resist corrosion is provided by concrete poured

directly around the prestresing steel in factory-controlled conditions, thus ensuring that they are

surrounded by well compacted durable concrete. By contrast the steel tendons in post-tensionedconcrete are in bundles located in internal or external ducts whose profile must be carefully

controlled during construction. When the structural members have been cast, the steel strands are

threaded through the ducts and tensioned by means of jacks reacting against the concrete, which

is compressed in the process. When the required stress level is achieved the stresses anddeformations are locked within the steel and concrete by mechanical anchorages. The steel

tendon is then usually protected by injecting the duct with cementitious grout which also

provides protection at the anchorage. During this demanding operation it is difficult to ensure

that ducts are filled with grout, and problems may occur. This difficulty, in combination withconstruction joints and the poorer quality control of in-situ construction result in the protection of

the tendons being less well controlled than in pretensioned concrete. This has been the root causeof many other problems. (HA, SETRA et al. 1999)

3.4.3.3 Seismic Consideration

For seismic design and related considerations, the AASHTO GuideSpecifications for Design and Construction of Segmental Concrete Bridges(AASHTO 1999) allows precast segmental construction without continuous

reinforcement steel across the joints. But the following requirements areadded:

For Seismic Zones C and D, either cast-in-place or epoxy joints arerequired

At least 50% of the prestress force should be provided by internal

tendons

The internal tendons alone should be able to carry 130% of the deadload

For other seismic design and detailing issues, design literature provided by

the California Department of Transportation (CALTRANS), for cast-in-placeconcrete box girders, can be referred (Sauvageot 2000).

Compared to the bearing supported piers, integral piers have provided betterseismic design. The integral pier can also be implemented, in particular,when the balanced cantilever methods are used. Following the practicesused for cast-in-place structures in CALTRANS, the integral pier in thefollowing