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11/25/2013


-

1

-

S
EISMIC
C
ONSIDERATIONS
FOR
P
RE
-
F
ABRICATED

A
CCELERATED
B
RIDGE
C
ONSTRUCTION



CONTENTS


1.

Introduction


2.

Scope and Objectives


3.

Precast Concrete Components and Systems

in ABC

3.1

Precast Concrete Bridge System

3.2

Geotechnical Co
nsideration

3.3

Precast Concrete Deck Panel
s

3.3.1

Precast Deck Panels

3.3.2

Panel
-
to
-
Panel Connections

3.3.3

Panel
-
to
-
Girder Connections

3.3.4

NCHRP 12
-
65 System

3.3.5

Applicability of Precast Deck Panel in ABC

3.4

Superstructure

3.4.1

Precast

Concrete Girders

3.4.1.1

Types of Girders

3.4.1.2

Techniques to Increase Sp
an Length

3.4.1.3

On
-
Going Researches

3.4.1.4

Applicability of Precast Beams in ABC

3.4.2

Spliced Girders

3.4.2.1

Types of Girders

3.4.2.2

Construction Detail

3.4.2.3

Construction Issues

3.4.2.4

Connection Detail

3.4.2.5

Application of Spliced Girder in Seismic Regions

3.4.3

Precast Segmental Box Girders

3.4.3.1

Structural Concept
s

3.4.3.2

Construction Issues

3.4.3.3

Seismic Consideration

3.5

Substructure

3.5.1

Precast Bent Cap

3.5.2

Integral Piers

3.5.3

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



APPENDICES

A.

Literature Review

B.

Precast Concrete Segmental Columns

C.

Analysis To
ols

D.

Experimental Data

E.

Design Guidelines and Commentary

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3

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1.

I
NTRODUCTION


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

This monograph discusses seismic analysis and design of bridges buil
t 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.
S
COPE AND
O
BJECTIVES


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

construction.
The area
of interest corresponds to the intersection of three different aspects as in
F
igure

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 in
high seismic regions, for
example, by improving scheduling and financing procedures.



ABC
P
r
e-
F
a
b
S
eismi
c


FIGURE
2
-
1
The Area of Interest


For the past
two
decade
s
,
the benefits of
combination of ABC with pre
-
fabricated construction

have

been shown in many cases
(Shahawy 2003)
.
Also pre
-
fabricated construction is envisioned
as a tool to accomplish public’s needs in the future
(Bhide, Culm
o et al. 2006)
. However,
employing those developed techniques in the moderate and high seismic regions is del
ayed
mainly because of uncertainties of

behavior that those systems will experience under seismic
loadings.


In the development of ABC scheme, pr
ecast concrete components has been widely applied
in
deck, superstructure, and substructure
(FHWA 2006)
. For steel members, the application evolves
into

fabricating large blocks of superstructure and placing with SPMT
(Self
-
Propelled Modular
Transporter)
vehicle

(Figure 2
-
2)
. The advantage of precast concrete systems over steel systems
is the possibility of standardization of smaller components, which w
ill 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 desi
gn.


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5

-



FIGURE 2
-
2. Wells Street Bridge Construction, Chicago


In the application of precast concrete components, the
proper connection between components is
the
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 or
nonlinear behavior that is not expected in conventional cast
-
in
-
place constru
ction.
Recently,
several studies, including NCHRP 12
-
74, focused on seismic behavio
r of pre
-
fabricated
components and their connections.
The present study not only collects information from related
studies but also identifies and investigates issues that n
eeds 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 philo
sophy 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 method
ologies 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 Techn
ology Transfer) workshop. The ACTT concept was originated
by 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. At
the workshop, skill sets provide counsel on innovative ways to accelerate construction, reduce
project costs, and minimize impacts. The following i
s the list of skill sets
(FHWA 2005)
.




Innovative Contracting / Financing

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ROW / Utilities / Railroad Coordination



Geotechnical / Materials / Accelerated Testing



Traffic Engineering / Safety / Intelligent Trans
portation 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, b
ut they are not the only factors that should be considered. For
example, conclusions from ACTT workshops indicated that the holistic approach to the entire
project was needed to come up with the most appropriate way to realize ABC. Mostly, global
organizat
ion and financing take more important roles in the planning state, unless structural
consideration 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)
. F
igure 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 increase
s. 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 planned
operation 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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8

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3.
P
RECAST
C
ONCRETE
C
OMPONENTS AND
S
YSTEMS IN
ABC


Bridge construction utilizing pr
ecast components is one of the widely implemented techniques in
accelerated bridge construction. Since its introduction in the
1950’s
, the number of applications
of precast components has been increased.



The recent development of precast concrete bas
ed 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


summar
y 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 prove
to 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
replac
ement 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 the
applicable span length.



Short span: Precast Prestressed Beam (Texas Type)



Medium span : Spliced Girders



Lon
g 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 o
f simply supported
spans is quite different from the continuous spans, and those of bridge system with integral piers
are different from responses of bridge systems where superstructure is separated from the
substructures.
As cast
-
in
-
place construction, th
e determination of structural system for the
seismic aspects is important, but there are additional restrictions that originated from precast
construction. The technical difficulties for providin
g continuity in structures need

to be identified
when the str
uctural 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 in
spection 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
investigatio
n, 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 disrupt
ion. 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. After
a
dequate 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 c
onstruction quality can be
improved, 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 on
the 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 products
from the factory to the construction site and the hardware associated with the connections. Th
e
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 the
connection 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 implement
the designed geometry as a part of roadway. The bridge deck should be constr
ucted 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 construction
has 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
-
resi
sting 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 co
nstructed. They include precast concrete stay
-
in
-
place panels,
full
-
depth precast concrete panels, metal grid decks, and orthotropic steel (aluminum) decks. As
metal grid decks, open grid floors, filled and partially filled grid decks, and unfilled grid de
cks
composite 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, Osegueda
et 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 F
ull
-
Depth Precast Deck Panel
(PCI 2003)


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12

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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 by
leveling bolts or shims. Also the grouting was used to construct haunches which were formed by
dams with various materials.
The followings are the typical con
struction 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



Transve
rse 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 p
ocket 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 c
onnection 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 harde
ned 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 t
he depth of the slab,
excluding any provision for gr
inding, 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 spans
the full width. The design generally follows the conventional cast
-
in
-
place concrete deck d
esign
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
-
tensioning
can produce thinner panels with better crack control, which can re
duce damages during
transportation and erection
(Yamane, Tadros et al. 1998)
.


When post
-
tensioning is introduced in the longitudinal direction, the minimum average effective
pre
stress shall not be less than
25
0 p
si.
The longitudinal po
st
-
tensioning is a good solution for
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-

providing flexural continuity. The post
-
tensioning ducts should be located at the center of the
slab cross
-
section. Block
-
outs should be provided in the joints to permit the splicing of post
-
tensioning ducts. Panels sho
uld be placed on the girders without mortar or adhesives to permit
their movement relative to the girders during prestressing.
Congestion problems can occur as a
result 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 grout
having 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 the
width 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 of
the 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 construct
ed in cast
-
in
-
place concrete whereas the
interior parts are constructed with precast deck panels. For this case, longitudinal connections are
required. For both connections, the reinforcement splice or connection detail and grouting detail
are the major is
sues to transfer forces between panels and panel to CIP concrete.


Transverse connections should transfer shear and moment from live load. Most of the joints used
have been female
-
to
-
female shear key type connections.
The shear keys were generally provide
d
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 induced
forces can be resisted by the whole deck systems not by the indi
vidual 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, wer
e
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 found
to have poor performance
(Kropp, Milinski et al. 1975)
.



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


The grouted female
-
to
-
female connections have been more common details in the tran
sverse
connections of panels. Figure 3
-
3 shows some of detail used in the bridges. The bottom of the
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-

openings can be detailed by polyethylene backer rods or wood forming. The bond between the
grout 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 reinfor
cement 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 e
xtended 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 maint
ain 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
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

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15

-


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 rebar
can be highly maintained (Figure 3
-
5).


FIGURE 3
-
5 Spiral Confinement Lo
ngitudinal Reinforcement Splice


Post
-
Tensioning

Post
-
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 i
s typically provided after the transverse panel
-
to
-
panel joints are grouted and
cured, 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 comp
ression under
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. Pra
ctically, high strength threaded rods or strands can be used for post
-
tensioning (Figure 3
-
6 and 3
-
7).

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16

-


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




FIGURE 3
-
7 Post
-
tensioning provided by Strands (the Skyline Drive Bridge in Omah
a,
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 fro
m 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 po
cket is cast in the precast
panel

during fabrication. In some instances, the location
s

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 protr
uding from the girder web are
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17

-

generally 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 Connections

The 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 co
nnection 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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18

-



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 pocke
ts 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 Project
used 48 in. spacing of clustered studs.



3.3.4 N
CHRP 12
-
65 System


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


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19

-



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 grout
ed 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 layers
of reinforcement rebar. The other panel does not include any pre
-

and post
-
tensionin
g 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 ft long,
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 Hollow
Structural Steel (HSS) tubes. Research proposed

two longitudinal connection details.



The first detail requires threading a No.6 reinforcing bar, which extends about 7½in.
outside the panel to be installed, into the old panel, which results in a 6 in. bar
embedment length.




The second detail allows ve
rtical 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
11/25/2013


-

20

-

installed, a 24 in. No.6 long splice bar is dropped through a vertical slot, which results in
an 11 in. splice le
ngth.




Panel
-
to
-
Girder Connection

For precast concrete girders, a new connection detail is provided where clusters of three double
head 1¼ in. studs are used. Also the clusters are spaced at 48 in., which is much wider than the
current AASHTO Specifica
tion. Additional reinforcement was shown to be needed in the web of
the 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

A new con
nection detail is proposed, where clusters of eight 1¼ 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.


11/25/2013


-

21

-



FIGURE Proposed Connection Detail between panel a
nd 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
-
place
construction. They can be used on various geometric girder configurations, including skewe
d 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:




T
his system generally requires large amount of time
-
consuming gr
outing 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 add
itional 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
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 i
ssues as the
capability of developing diaphragm action in the deck and effects from vertical accelerations
(Hieber, Wacker et al. 2005)
.

11/25/2013


-

22

-

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


Bridge

State

Panel

P
-
to
-
G
Connection

P
-
to
-
P Connection

I
-
80 overpass in Oakland

California

14'
-
2" wide

shear pocket



(replace outside lane only)



61/2"~7" thi
ck

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

P
T
-
L (90 psi)

(pony truss)









Woodrow Wilson Memorial
Bridge

Maryland

46'
-
7
¼
" wide

studs

PT
-
T





10'
-
12" long

hold
-
down bolts

PT
-
L





8" thick





Suspension bridge over
Rondout Creek

NY

9' wide

stud bolts

PT
-
T





24' long



V
-
shaped M
-
F shear

keys





6"~7" thick





Bridge over the Delaware River

NY, Penn

7
½
" thick

single
stud/pocket

epoxy shear keys

Bridge over Cattarougus Creek

NY

7
½
" thick

single
threaded/pocket



Batchellerville Bridge

NY

8
½
" thick

stud

welded steel PL
(long.)

QEW
-
We
lland River Bridge

Ontario

43'
-
6" long

studs

NO PT
-
T





7'
-
11" wide

leveling bolts

PT
-
L (435 psi at the
int. pier)





8 7/8" thick





Dalton Highway Bridge

Alaska

27'
-
5 3/8"
wide

studs

PT
-
T (2 layers)





4'
-
10" long



F shear keys





7
½
" thick



NO PT
-
L









Discont. Long rebar

Pedro Creek Bridge

Alaska

7
½
" thick

studs

NO PT
-
L & PT
-
T







leveling bolts



Kouwegok Slough Bridge

Alaska

6.9"~9.8"
thick

studs

NO PT
-
T & PT
-
L





7500mm
wide

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





7'
-
7" long





Bridge
-
4 on R
oute 75

Illinois

7.7" thick

3 studs/pocket

PT
-
L





7'
-
10" wide

leveling bolts

F shear keys





9
½
" thick





Lake Koocanusa Bridge

Montana

15'
-
7" or 20'
-
studs & steel
PT
-
L

11/25/2013


-

23

-

7" wide

channel





8' long

leveling bolts

bolted conn. In Long.
Direction





7"~10" thick





Skyline Drive Bridge

Nebraska

5.9" thick



PT
-
T & PT
-
L

(NUDECK, 25
º

skewed)



7' long



V
-
shaped F shear
keys

A through truss bridge

New
Hampshire

8' long

4 studs/pocket

F shear keys





3 3/4" ~ 5
3/4" thick



NO PT
-
L









rebar
at tran. Conn.

I
-
287 Westchester

NY

9" thick

9 studs

PT
-
T & PT
-
L

(curved with 32
º

skewed)



41'
-
9" wide

plastic shim
packs

F shear keys

US59 Tied Arch Bridge

TX

60' wide



PT
-
T & PT
-
L





7' long



no shear keys

a bridge in the country road

Utah

8
¼
"
thick

3 studs/pocket



(45
º

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
¾
" thick



PT
-
T & PT
-
L

(30
º

skewed)









N
CHRP 12
-
41



7'
-
10" wide

threaded stud

PT
-
T & PT
-
L (200psi)





4
½
" thick



F shear keys

NCHRP 12
-
65












3.4

Superstructure


comparison with steel girders



3.
4
.1 Precast
Concrete

Girder
s


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 the
competition 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
a
ccelerated bridge construction, but recent consideration of accelerated bridge construction takes
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/PCI
11/25/2013


-

24

-

Standard, New England Bulb
-
Tees, a
nd 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, the
possible span l
ength is increased by using high
-
strength concrete.

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-63
BT-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
W85G
W74G
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
8ksi
PCI
7
7
7
7
7
7 ksi
12ksi
7
7
AASHTO
7
7
12
12
7
7
Washington DOT
6ksi
6ksi
8.5
8.5
8.5


FIGURE Comparison of Span Ranges of Precast Concrete Girders


11/25/2013


-

25

-



F
or

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

are demo
nstrated in the following figure
(PCI 2003)
.






(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 their
development.



11/25/2013


-

26

-



FIGURE Typical Cross Section of New England Bulb Tee Girders


Girders in W
ashington DOT can be categorized into tub girders, I
-
Girders, and Bulb
-
Tees as
shown in Figure

(WSDOT 2006)
.





11/25/2013


-

27

-




FIGURE Precast Concrete Girder Types from Washington DOT


One of the important issue for long girders is the transportation issue. St
ates generally restrict
oversize and overweighted vehicles on their roadway network. The allowable girder weights
vary 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 p
recast 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 Le
n
g
th

In
the research report of
NCHRP 12
-
59, “Extending Span Ranges o
f Precast Prestressed
Concrete Girders,” available techniques for extending spans are summarized
(Castrodale and
W
hite 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 Concre
te
. Application of High Strength Concrete (HSC) helps to extend span
length by providing higher compressive and tensile stress limits at long
-
term and at transfer
stages.
As summarized in the table for several girder sections, the concrete strength varies
from
5000 psi to 12000 psi.

Specifically, beams made with HSC exhibit the following structural
benefits
(PCI 2003)
:

11/25/2013


-

28

-



Permit the use of high levels of prestress and therefore a greater capacity to carry gravity
loads. HS
C 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
-
t
erm 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 require
d 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 the
efficiency of pre
-
tensioned girders. The ap
plication 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 shapes
because each 0
.6
-
in
-
dia. Strand provides 40 percent more pretension force for only a 20 percent
increase 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 Sect
ion
. 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 the
girder, 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 us
ed in girders combined with pre
-
tensioning and/or in the deck over internal piers. Also staged post
-
tensioning can be scheduled to
introduce 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 beam
can span and the greater the capacity to resist positive moment. It is recogn
ized 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 flange
is preferred when using HSC.



Allowable stresses are increased when using HS
C. 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
strength of 0.6
-
in.
-
dia strands is nearly 40 percent greater than the capacity of 0.5
-
in.
-
dia
strands.


11/25/2013


-

29

-

Higher concrete c
ompressive 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 the
effect 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 is
compared 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
-
72
girder section. The following table summarizes the design variation.

11/25/2013


-

30

-


In this comparative design study, the resulting conclusions are:



Th
e 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, whe
re 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 a
effective 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 the
applicable methods to increase span length.
Abstrac
tly, 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 is
developed, and the add
ed 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 t
he internal piers as demonstrated in Figure
(PCI
2003)
. The similar design approach may be used for bridges adopting precast full
-
depth deck
panels.
The precast panels are post
-
tensioned over the piers or projecting rein
forcement are
spliced to provide negative moment capacity at the section.


11/25/2013


-

31

-




FIGURE Continuity Developed with Post
-
Tensioning



FIGURE Continuity Developed with Conventional Deck Reinforcement


11/25/2013


-

32

-


FIGURE Methods to Establish Continuity

for Precast Deck P
anels




3.4.1.3

On
-
Going Researches


One of the recent areas of research is the allowable design release stress limits for pretensioned
concrete girders. The reasons for limiting stresses in the current provisions are to prevent
cracking and excessive def
lection or camber
(Castro, Kreger et al. 2004)
. Additionally, the
extreme fiber compressive stress limit is an indirect design check to prevent concrete crushing by
the applied prestressing forces. The following is the current provisions for limiting stresses

at
prestressing transfer.



11/25/2013


-

33

-



Along with the primary purpose of estimating realistic stress limits for the related studies, t
he
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,
0.60

f’
ci
,
0.
7
0

f’
ci
, and
0.
8
0

f’
ci
,
of sustained compressive stressed are applied.
Compressive strength of the specimens do not
show 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

<

f’
ci
, whereas the strain limitation is controlled in the nonlinear analysis
,


< 0.003
.
This study showed the inherent conservatism of the linear analysis. For e
xample, the member
designed to have
f’
ci

in linear method reaches only
0.
9
0

f’
ci
, when the nonlinear analysis is used.
As one of conclusions, the possibility of using
0.
75

f’
ci

was proposed.


In the following study on strength design of pretensioned flexu
ral concrete members,
Noppakunwijai et al.

(Noppakunwijai, Tadros et al. 2001)

investigated the appropriateness of
strength design approach instead of allowable stress design at prestressing transfer. The strength
reduction factors for the nominal axial cap
acity and the flexural capacity were proposed as 0.7.
Also the corresponding loads factors were proposed as in Table.

11/25/2013


-

34

-



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
s
ections
is larger than those for the NU inverted
-
tee sections. However, those values are always larger
than
0.
60

f’
ci
.

In the experimental investigation, it was also shown that
there are no adverse
effects in m
easured camber and concrete strains with the elevated compressive strength which is
measured
0.
79

f’
ci

and
0.
84

f’
ci
.

This study was concluded with emphasizes on the possibility of
substitution 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 stress
design approach, but addresses the possible problem to increase the tensile stress limit by
observing crack
s designed for
'
7.1
ci
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.
75

f’
c

In the analytical and exp
erimental 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 elevated
compressive stress levels at prestress release a
s 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.






11/25/2013


-

35

-


3.4.1.4

Applicabil
ity of Precast Beams in ABC


For

the application in accelerated bridge construction scheme, the deck bulb
-
tee type precast
concrete 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 of
superstructure was studied as a prefabricated precast concrete bridge system for the state of
Alabama
(Fouad, Rizk et al. 2006)

as in the following figu
re.




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 between
girders, has been identified in th
e overlay on many bridges of this type
(El
-
Remaily, Tadr
os 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 stresses
across the joints. The followin
g is the standard mechanical connection configuration used by
Washington State DOT.


11/25/2013


-

36

-



FIGURE. Standard Mechanical Connection Detail in Washington State


Deck bulb
-
tee superstructure can be a viable option for accelerated bridge construction. The
main a
dvantage 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 of
the bridge, the partial
-
depth precast deck system can be consid
ered in the application.



3.4
.2
Spliced

Girders


Spliced girders

are a type of precast prestressed concrete girders, which spans over 160 ft by
utilizing 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 temporary
towers to span the full distance between supports. Then the segments are post
-
ten
sioned together,
they lift off the temporary falsework and span between their permanent pier and abutment
supports
(PCI 2003)
.
The following figure shows spliced girders used in simply
-
supported and
continuous 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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37

-



(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 recommended
that the AASHTO LRFD Specifications should be revised to address the spliced girder clearly
and appro
priately 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 trap
ezoidal box beams, and box beams.
Sometimes, the combination of them, hybrid section, may be taken. The most popular shape 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.



TABLE
.

Types of Spliced Girders Sections

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38

-

Agency
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
U66PTG5
U66PTG6
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 t
hese 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 leng
th is based on the
researched 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 blo
ck system.


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39

-



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 and
negative 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
a
ccommodate 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
gr
ound 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 the
girder, because workers can easily access any part of the girder. Howeve
r, 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 doe
s 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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40

-

falsework needs to be h
igh 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 spliced
girder
(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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-

41

-



Stage 3. The deck slab is constructed. T
he splice joints are cast, tendons inserted in ducts
and post
-
tensioning introduced,

which

com
plet
e the assembly of the girder
. Before the
splice joints are cast, the end elevations of the segments need to be carefully positioned to
allow 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 carr
ied 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

S
pans


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




FIGURE. Construction Sequence.

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42

-



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 tower
s and a
connection is made to the pier. Ideally the pier connection should be one that allows for
horizontal 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 al
low for horizontal
movement during
post
-
tensioning operations.
Placement of the first end segment creates
moments 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, t
he 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
th
e deck.
After the concrete in the splice has achieved the specified compressive strength
and 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 (following
figure (a)). Or the standard shaped girders can be combined with a separate precast haunch block
as in figur
e (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 shown
in 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 finall
y.



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-

43

-



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 u
sually located at internal regions due to high
moments and shear forces. In order to accommodate these large forces, the sections are generally
deepened 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



Add
ing 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 we
ight.
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 size
webs 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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44

-

in the early 1990s, a 6.9
-
in. web wa
s 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) ½
-
in.
-
dia. Strands per tendon.




FIGURE Web Configuration for
Washington State
I
-
Beam



Most of the segmental I
-
beam bri
dges built using post
-
tensioning over the past four decades have
not 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 Fabric
ation Details


Wet
-
cast splice are the standard practice in the beam splices. The ends of the beams at splices
should formed shear keys, if required. Ducts for post
-
tensioning should be made of semi
-
rigid
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45

-

galvanized metal, high density polyethylene or poly
propylene. 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 to
cor
rosion 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
wi
despread problem and showed a very low frequency of occurrence
(Castrodale and White
2004)
. As the causes of thes
e problems, including those from the precast segmental box
construction,
were identified as; poor design details; low
-
quality materials; and improper
grouting procedures combined with inadequate inspection practices.
The recent developments in
the Post
-
Ten
sioning Institute
(PTI 2001)
, the American Segmental Bridge Institute
(ASBI 2000)
,
and several DOTs address this issue from vario
us 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 anal
ysis and possibly a complicated temporary support
scheme until the new deck is in place. However, if properly analyzed and the economics are
verified, there is no reason this approach should not be considered. Some states have avoided this
issue by requiri
ng 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 benefi
ts to
multistage post
-
tensioning in terms of s
tru
ctural 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 sec
tion. 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 need
replacement. It may be desirabl
e 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 beam
system where the top flange is an integral part of the cross
-
section when the post
-
tensioning
t
endons 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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46

-

Post
-
tensioning anchorages require th
e 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 LRFD
Article 5.1
0.8.1, 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 the
same 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


Anchor
age zones are designed to accommodate anchorage hardware with its associated special
reinforcement and to provide adequate space for the reinforcement needed to distribute the
highly concentrated post
-
tensioning force. Detail guidance for the design of an
chorage 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 mostly
related to the post
-
tensioning and its losses such as
(PCI 2003)
:




Losse
s in post
-
tensioning tendons. Additional sources of prestress losses must be
considered such as friction and anchor losses

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47

-



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



Time
-
dependent analysis. This method of analysis s
hould 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 mome
nts



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 were
based on the related study on pre