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C-1
APPENDIX C
PROPOSED REVISIONS TO THE AASHTO LRFD BRIDGE DESIGN
SPECIFICATIONS
INTRODUCTION of the AASHTO Subcommittee on Bridges and Structures
and any subsequent minor revisions.
A major goal for this research project was to develop pro-
posed revisions to the AASHTO LRFD Bridge Design Spec-
ifications addressing the design of simple-span precast gird-
ers made continuous. Existing Article 5.14.1.2.7 specifically PRESENTATION OF REVISIONS
addresses this type of construction. The proposed revisions
incorporate many of the existing provisions and provide addi- The proposed revisions are presented in a format as simi-
tional requirements to more fully address design issues and lar to the actual specifications (including the commentary) as
to implement the findings of research. possible. Since the existing Article 5.14.1.2.7 is brief and the
proposed revisions are much longer, the proposed revisions
are presented in their final format without typographical con-
VERSION OF SPECIFICATIONS USED
AS BASIS FOR REVISIONS
ventions identifying the additions and deletions from the cur-
rent article.
The base text for the proposed revisions to Section 5
includes revisions approved at the 2001 and 2002 meetings
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C-2
PROPOSED REVISIONS
SECTION 5--CONTENTS
Replace the current headings for the subarticles of Article 5.14.1.2.7 with the following:
5.14.1.2.7 Bridges Composed of Simple Span Precast Girders Made Continuous
5.14.1.2.7a General
5.14.1.2.7b Restraint Moments
5.14.1.2.7c Material Properties
5.14.1.2.7d Age of Girder when Continuity is Established
5.14.1.2.7e Degree of Continuity at Various Limit States
5.14.1.2.7f Service Limit State
5.14.1.2.7g Strength Limit State
5.14.1.2.7h Negative Moment Connections
5.14.1.2.7i Positive Moment Connections
5.14.1.2.7j Continuity Diaphragms
5.3 NOTATION
Revise or add the following definitions:
Ac = area of core of spirally reinforced compression member measured to the outside diameter of the spiral (IN2);
gross area of concrete deck slab (IN2) (5.7.4.6) (C5.14.1.2.7c)
As = area of nonprestressed tension reinforcement (IN2); total area of longitudinal deck reinforcement (IN2)
(5.5.4.2.1) (C5.14.1.2.7c)
Atr = area of concrete deck slab with transformed longitudinal deck reinforcement (IN2) (C5.14.1.2.7c)
Ec deck = modulus of elasticity of deck concrete (KSI) (C5.14.1.2.7c)
fpsl = stress in the strand at the SERVICE limit state. Cracked section shall be assumed. (KSI) (C5.14.1.2.7i)
fpul = stress in the strand at the STRENGTH limit state. (KSI) (C5.14.1.2.7i)
dsh = total length of extended strand (IN) (C5.14.1.2.7i)
n = modular ratio between deck concrete and reinforcement (C5.14.1.2.7c)
effective = effective concrete shrinkage strain (IN/IN) (C5.14.1.2.7c)
sh = concrete shrinkage strain at a given time (IN/IN); unrestrained shrinkage strain for deck concrete (IN/IN)
(5.4.2.3.3) (C5.14.1.2.7c)
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SPECIFICATIONS COMMENTARY
Replace the current subarticles of Article 5.14.1.2.7 with
the following:
5.14 PROVISIONS FOR STRUCTURE TYPES
5.14.1 Beams and Girders
5.14.1.2 PRECAST BEAMS
5.14.1.2.7 Bridges Composed of Simple-Span Precast
Girders Made Continuous
5.14.1.2.7a General C5.14.1.2.7a
When the requirements of Article 5.14.1.2.7 are This type of bridge is generally constructed with a
satisfied, multi-span bridges composed of simple-span composite deck slab. However, with proper design and
precast girders with continuity diaphragms cast between detailing, precast members used without a composite deck
ends of girders at interior supports may be considered may also be made continuous for loads applied after
continuous for loads placed on the bridge after the continuity is established. Details of this type of construction
continuity diaphragms are installed and have cured. are discussed in Miller et al. (2004).
Continuity diaphragms shall fill the gap between ends of
precast girders and shall connect adjacent lines of girders.
The connection between girders at the continuity
diaphragm shall be designed for all effects that cause
moment at the connection, including restraint moments
from time-dependent effects, except as allowed in Article
5.14.1.2.7.
The requirements specified in Article 5.14.1.2.7
supplement the requirements of other sections of these
Specifications for fully prestressed concrete components
that are not segmentally constructed.
Multi-span bridges composed of precast girders with The designer may choose to design a multi-span
continuity diaphragms at interior supports that are designed bridge as a series of simple spans but detail it as continuous
as a series of simple spans are not required to satisfy the with continuity diaphragms to eliminate expansion joints in
requirements of Article 5.14.1.2.7. the deck slab. This approach has been used successfully
in several parts of the country.
Where this approach is used, the designer should
consider adding reinforcement in the deck adjacent to the
interior supports to control cracking that may occur from
the continuous action of the structure.
Positive moment connections improve the structural
integrity of a bridge, increasing its ability to resist extreme
events and unanticipated loadings. These connections
also control cracking that may occur in the continuity
diaphragm. Therefore, it is recommended that positive
moment connections be provided in all bridges detailed
as continuous for live load.
If a negative moment connection is provided between See Article 5.14.1.2.7h.
precast girders and the continuity diaphragm is installed
prior to placement of the deck slab, the girders may be
considered to be continuous for the dead load of the deck
slab and for any other applied loads placed on the
noncomposite girder after continuity is established.
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SPECIFICATIONS COMMENTARY
5.14.1.2.7b Restraint Moments C5.14.1.2.7b
The bridge shall be designed for restraint moments Deformations that occur after continuity is established
that may develop because of time-dependent or other from time-dependent effects such as creep, shrinkage,
deformations, except as allowed in Article 5.14.1.2.7d. and temperature variation cause restraint moments.
Restraint moments are computed at interior supports
of continuous bridges but affect the design moments at all
locations on the bridge (Mirmiran et al., 2001 a, b).
800
Restraint Moment (k-ft)
600
400
200
0
-200 28 Days
-400 90 days
-600
0 2000 4000 6000 8000
Girder Age (Days)
Figure C5.14.1.2.7b-1--Development of Restraint
Moments at Interior Support for Two-Span Continuous
AASHTO Type III Girder Bridge (Miller et al., 2004)
Figure C-1 shows the predicted development of
restraint moments at the interior support for a typical
bridge with continuity established at a girder age of 28 and
90 days. The figure demonstrates that the age of the girder
when continuity is established has a significant influence
on the development of restraint moments.
Positive restraint moments have the most significant
effect on bridges of this type because they may cause
cracking at the bottom of the continuity diaphragm and
they may cause stresses to exceed the stress limits at
critical locations in the span. Analysis predicts that positive
restraint moments do not develop for all precast girders
made continuous.
Analysis also predicts that precast girder bridges
with composite deck slabs that are made continuous
will develop negative restraint moments because of
differential shrinkage between the girders and the deck.
The older the girders are at the time the deck is cast, the
more severe the predicted negative moment development.
However, the consequences of negative restraint moments
on these bridges are not generally as significant as for
positive restraint moments. Data from various projects
(Miller et al., 2004 ; Russell et al., 2003) does not show the
effects of differential shrinkage of the decks. Therefore, it is
questionable whether negative moments due to differential
shrinkage form to the extent predicted by analysis. Since
field observations of significant negative moment distress
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SPECIFICATIONS COMMENTARY
have not been reported, negative moments caused by
differential shrinkage are often ignored in design.
Even if the negative moment formation in Figure C1 is
ignored, the curves still show that the later continuity is
formed, the lower the predicted values of positive moment
that will form. Therefore, waiting as long as possible after
the girders are cast to establish continuity and cast the
deck appears to be beneficial.
Several methods have been published for computing
restraint moments (Oesterle et al., 1989; Mirmiran et al.,
2001 a, b). While these methods may be useful in estimating
restraint moments, designers should be aware that these
methods may overestimate the restraint moments--both
positive and negative. Existing structures do not show the
distress that would be expected from the moments
computed by some analysis methods.
Restraint moments shall not be included when Since estimated restraint moments are highly
computing design quantities that are reduced when dependent on actual material properties and project
combined with the restraint moment. schedules, the computed moment may never develop.
Therefore, a critical design moment must not be reduced
by a restraint moment in case the restraint moment does
not develop.
5.14.1.2.7c Material Properties C5.14.1.2.7c
Creep and shrinkage properties of the girder concrete The development of restraint moments is highly
and the shrinkage properties of the deck slab concrete dependent on the creep and shrinkage properties of the
shall be determined from either: girder and deck concrete. Since these properties can vary
widely, measured properties should be used when
· Tests of concrete using the same proportions and available to obtain the most accurate analysis. However,
materials that will be used in the girders and deck these properties are rarely available during design.
slab. Measurements shall include the time-dependent Therefore, the provisions of Article 5.4.2.3 may be used
rate of change of these properties. to estimate these properties.
· The provisions of Article 5.4.2.3.
The restraining effect of reinforcement on concrete Because longitudinal reinforcement in the deck slab
shrinkage may be considered. restrains the shrinkage of the deck concrete, the apparent
shrinkage is less than the free shrinkage of the deck
concrete. This effect may be estimated using an effective
concrete shrinkage strain, effective, which may be taken as:
effective = sh (Ac /Atr) (C5.14.1.2.7c-1)
where:
sh = unrestrained shrinkage strain for deck
concrete (IN/IN)
Ac = gross area of concrete deck slab (IN2)
Atr = area of concrete deck slab with transformed
longitudinal deck reinforcement (IN2)
= Ac + As(n - 1)
As = total area of longitudinal deck reinforcement
(IN2)
n = modular ratio between deck concrete and
reinforcement
= Es / Ec deck
Ec deck = modulus of elasticity of deck concrete (KSI)
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SPECIFICATIONS COMMENTARY
Equation C1 is based on simple mechanics (Abdalla
et al., 1993). If the amount of longitudinal reinforcement
varies along the length of the slab, the average area of
longitudinal reinforcement may be used to calculate the
transformed area.
5.14.1.2.7d Age of Girder when Continuity Is Established C5.14.1.2.7d
The minimum age of the precast girder when continuity Analytical studies show that the age of the precast
is established shall be specified in the contract documents. girder when continuity is established is an important factor
This age shall be used for calculating restraint moments in the development of restraint moments (Mirmiran et al.,
due to creep and shrinkage. If no age is specified, the 2001 a, b). These studies suggest that delaying continuity
girders shall be assumed to be 7 days old at the time reduces or eliminates time-dependent positive restraint
continuity is established. moments (see Figure C5.14.1.2.7b-1). The formation of
positive restraint moments is largely the result of creep
and shrinkage in the prestressed girder. The creep and
shrinkage begin as soon as the prestressing force is
applied, which may be less than 1 day after casting. If
continuity is delayed, a larger fraction of the creep and
shrinkage in the girder will occur prior to establishing
continuity, so a lower positive moment will develop in
the continuity diaphragm. In addition, allowing the girder
to shrink before establishing continuity increases the
differential shrinkage between the girder and deck. The
differential shrinkage counteracts the formation of positive
restraint moments.
According to analysis, establishing continuity when
girders are young causes larger positive moments to
develop. Therefore, if no minimum girder age for continuity
is specified, the earliest reasonable age must be used.
Results from surveys of practice (Oesterle et al., 1989;
Miller et al., 2004) show a wide variation in girder ages at
which continuity is established. An age of 7 days was
reported to be a realistic minimum. However, the use of
7 days as the age of girders when continuity is established
results in a large positive restraint moment. Therefore, a
specified minimum girder age at continuity of at least
28 days is strongly recommended.
The following simplification may be applied if the owner If girders are 90 days or older when continuity is
will permit this simplification to be used and if the contract established, the provisions of Section 5.4.2.3 predict that
documents require a minimum girder age of at least approximately 60% of the creep and 70% of the shrinkage
90 days when continuity is established: in the girders, which could cause positive moments, has
· Positive restraint moments caused by girder creep and already occurred prior to establishing continuity. Since most
shrinkage and deck slab shrinkage may be taken to be of the creep and shrinkage in the girder has already
as 0. occurred before continuity is established, the potential
· Computation of restraint moments shall not be development of time-dependent positive moments is limited.
required. Differential shrinkage between the deck and the girders, to
· A positive moment connection shall be provided with a the extent to which it actually occurs (see C5.14.1.2.7b)
factored resistance, fMn, not less than 1.2 Mcr, as would also tend to limit positive moment development.
discussed in Article 5.14.1.2.7i.
Even if the girders are 90 days old or older when continuity
is established, some positive moment may develop at the
connection and some cracking may occur. Research (Miller
et al., 2004) has shown that if the connection is designed
with a capacity of 1.2 Mcr , the connection can tolerate this
cracking without appreciable loss of continuity.
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SPECIFICATIONS COMMENTARY
This provision provides a simplified approach to design
of precast-girder bridges made continuous that eliminates
the need to evaluate restraint moments. Some states allow
design methods where restraint moments are not evaluated
when continuity is established when girders are older than
a specified age. These design methods have been used
for many years with good success. However, an owner
may require the computation of restraint moments for all
girder ages.
5.14.1.2.7e Degree of Continuity at Various Limit States C5.14.1.2.7e
The connection between precast girders at a continuity A fully effective joint at a continuity diaphragm is a joint
diaphragm shall be considered fully effective if either of the that is capable of full moment transfer between spans,
following are satisfied: resulting in the structure behaving as a continuous structure.
In some cases, especially when continuity is
· The calculated stress at the bottom of the continuity established at an early girder age, continuing upward
diaphragm for the combination of superimposed cambering of the girders due to creep may cause cracking
permanent loads, settlement, creep, shrinkage, 50% at the bottom of the continuity diaphragm (Mirmiran et al.,
live load and temperature gradient, if applicable, is 2001 a, b). Analysis and tests indicate that such cracking
compressive. may cause the structure to act as a series of simply
supported spans when resisting some portion of the
· The contract documents require that the age of the permanent or live loads applied after continuity is
precast girders shall be at least 90 days when established; however, this condition only occurs when the
continuity is established and the positive restraint cracking is severe and the positive moment connection is
moments are assumed to be 0 as allowed in Article near failure (Miller et al., 2004). Where this occurs, the
5.14.1.2.7d. connections at the continuity diaphragm are partially
effective.
Theoretically, the portion of the permanent or live
If the connection between precast girders at a loads required to close the cracks would be applied to a
continuity diaphragm does not satisfy these requirements, simply supported span, neglecting continuity. The remainder
the joint shall be considered partially effective. of the load would then be applied to the continuous span,
assuming full continuity. However, in cases where the
portion of the live load required to close the crack is less
than 50% of the live load, placing part of the load on simple
spans and placing the remainder on the continuous bridge
results in only a small change in total stresses at critical
sections due to all loads. Tests have shown that the
connections can tolerate some positive moment cracking
and remain continuous (Miller et. al., 2004). Therefore, if
the conditions of the first bullet point are satisfied, it is
reasonable to design the member as continuous for the
entire load placed on the structure after continuity is
established.
The second bullet follows from the requirements of
Article 5.14.1.2.7d, where restraint moments may be
neglected if continuity is established when the age of the
precast girder is at least 90 days. Without positive moment,
the potential cracks in the continuity diaphragm would not
form and the connection would be fully effective.
Superstructures with fully effective connections at
interior supports may be designed as fully continuous
structures at all limit states for loads applied after continuity
is established.
Superstructures with partially effective connections at Partially effective construction joints are designed by
interior supports shall be designed as continuous structures applying the portion of the permanent and live loads
for loads applied after continuity is established for strength applied after continuity is established required to close the
and extreme event limit states only. assumed cracks to a simple span (neglecting continuity).
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SPECIFICATIONS COMMENTARY
Only the portion of the loads required to close the
assumed cracks are applied. The remainder of the
permanent and live loads would then be applied to the
continuous span. The load required to close the crack can
be taken as the load causing zero tension at the bottom of
the continuity diaphragm. Such analysis may be avoided
if the contract documents require the age of the girder at
continuity to be at least 90 days.
Possible cracking of the deck due to negative moments
may be neglected in the analysis, as specified in Article
4.5.2.2.
If the negative moment resistance of the section at an
interior support is less than the total amount required, the
positive design moments in the adjacent spans shall be
increased appropriately for each limit state investigated.
5.14.1.2.7f Service Limit State C5.14.1.2.7f
Simple-span precast girders made continuous shall be
designed to satisfy service limit state stress limits given in
Article 5.9.4. For service load combinations that involve
traffic loading, tensile stresses in prestressed members
shall be investigated using the Service III load combination
specified in Table 3.4.1-1.
At the service limit state after losses, when tensile Tensile stresses under service limit state loadings may
stresses develop at the top of the girders near interior occur at the top of the girder near interior supports. This
supports, the tensile stress limits specified in Table region of the girder is not a precompressed tensile zone,
5.9.4.1.2-1 for other than segmentally constructed bridges so there is not an applicable tensile stress limit in Table
shall apply. The specified compressive strength of the 5.9.4.2.2-1. Furthermore, the tensile zone is close to the
concrete, f' c, shall be substituted for the compressive end of the girder, so adding or debonding pretensioned
strength of concrete at the time of prestressing, f' ci, in the strands has little effect in reducing the tensile stresses.
stress limit equations. The Service III load combination Therefore, the limits specified for temporary stresses before
shall be used to compute tensile stresses for these losses have been used to address this condition, with
locations. modification to use the specified concrete strength. This
provision provides some relief for the potentially high
tensile stresses that may develop at the ends of girders
because of negative service load moments.
Alternatively, the top of the precast girders at interior This option allows the top of the girder at the interior
supports may be designed as reinforced concrete support to be designed as a reinforced concrete element
members at the strength limit state. In this case, the stress using the strength limit state rather than a prestressed
limits for the service limit state shall not apply to this region concrete element using the service limit state.
of the precast girder.
A cast-in-place composite deck slab shall not be The deck slab is not a prestressed element. Therefore,
subject to the tensile stress limits for the service limit state the tensile stress limits do not apply. It has been customary
after losses specified in Table 5.9.4.2.2-1. to apply the compressive stress limits to the deck slab.
5.14.1.2.7g Strength Limit State C5.14.1.2.7g
The connections between precast girders and a The continuity diaphragm is not prestressed concrete,
continuity diaphragm shall be designed for the strength so the stress limits for the service limit state do not apply.
limit state. Connections to it are therefore designed using provisions
The reinforcement in the deck slab shall be for reinforced concrete elements.
proportioned to resist negative design moments at the
strength limit state.
5.14.1.2.7h Negative Moment Connections C5.14.1.2.7h
The reinforcement in a cast-in-place, composite deck Research at PCA (Kaar et al., 1961) and years of
slab in a multispan precast girder bridge made continuous experience show that the reinforcement in a composite
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SPECIFICATIONS COMMENTARY
shall be proportioned to resist negative design moments deck slab can be proportioned to resist negative design
at the strength limit state. Requirements of Article 5.7.3 moments in a continuous bridge.
applicable to the strength limit state shall be satisfied.
Longitudinal reinforcement used for the negative Limited tests on continuous model and full-size structural
moment connection over an interior pier shall be anchored components indicate that, unless the reinforcement is
in regions of the slab that are in compression at strength anchored in a compressive zone, its effectiveness becomes
limit states and shall satisfy the requirements of Article questionable at the strength limit state (Priestly and Tao,
5.11.1.2.3. The termination of this reinforcement shall be 1993). The termination of the longitudinal deck slab
staggered. All longitudinal reinforcement in the deck slab reinforcement is staggered to minimize potential deck
may be used for the negative moment connection. cracking by distributing local force effects.
Negative moment connections between precast girders A negative moment connection between precast
into or across the continuity diaphragm shall satisfy the girders and the continuity diaphragm is not typically
requirements of Article 5.11.5. These connections shall be provided because the deck slab reinforcement is usually
permitted when the bridge is designed with a composite proportioned to resist the negative design moments.
deck slab and shall be required when the bridge is designed However, research (Ma et al., 1998) suggests that
without a composite deck slab. Additional connection mechanical connections between the tops of girders may
details shall be permitted if the strength and performance also be used for negative moment connections, especially
of these connections is verified by analysis or testing. when continuity is established prior to placement of the
deck slab. If a composite deck slab is not used on the
bridge, a negative moment connection between girders is
required to obtain continuity. Mechanical reinforcement
splices have been successfully used to provide a negative
moment connection between box beam bridges that do
not have a composite deck slab.
The requirements of Article 5.7.3.4 shall apply to the
reinforcement in the deck slab and at negative moment
connections at continuity diaphragms.
5.14.1.2.7i Positive Moment Connections C5.14.1.2.7i
Positive moment connections at continuity diaphragms Positive moment connections improve the structural
shall be made with reinforcement developed into both the integrity of a bridge, increasing its ability to resist extreme
girder and continuity diaphragm. Two types of connections events and unanticipated loadings. Therefore, it is
shall be permitted: recommended that positive moment connections be
provided in all bridges detailed as continuous for live load.
· Mild reinforcement embedded in the precast girders Both embedded bar and extended strand connections
and developed into the continuity diaphragm. have been used successfully to provide positive moment
resistance. Test results (Miller et al., 2004) indicate that
· Pretensioning strands extended beyond the end of the connections using the two types of reinforcement perform
girder and anchored into the continuity diaphragm. similarly under both static and fatigue loads and both have
These strands shall not be debonded at the end of the adequate strength to resist the applied moments.
girder.
Additional requirements for connections made using
each type of reinforcement are given in subsequent articles.
The critical section for the development of positive
moment reinforcement into the continuity diaphragm shall
be taken at the face of the girder. The critical section for
the development of positive moment reinforcement into
the precast girder shall consider conditions in the girder as
specified in this article for the type of reinforcement used.
Reinforcement for the positive moment connection Analytical studies (Mirmiran et al., 2001 a, b) suggest
shall be proportioned to resist the calculated positive that a minimum amount of reinforcement, corresponding
moment, except that the positive moment connection shall to a capacity of 0.6 Mcr, is needed to develop adequate
be proportioned to provide a factored capacity (fMn) of at resistance to positive restraint moments. These same
least 0.6 Mcr. studies show that a positive moment connection with a
capacity greater than 1.2 Mcr provides only minor
improvement in continuity behavior over a connection with
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SPECIFICATIONS COMMENTARY
a capacity of 1.2 Mcr. Therefore, it is recommended that
the positive moment capacity of the connection not
exceed 1.2 Mcr. If the computed positive moment exceeds
1.2 Mcr, the section should be modified or steps should be
taken to reduce the positive moment.
The cracking moment, Mcr, is computed using Equation The cracking moment, Mcr, is the moment that causes
5.7.3.6.2-2 with the gross composite section properties for cracking in the continuity diaphragm. Since the continuity
the girder and the effective width of composite deck slab, diaphragm is not a prestressed concrete section, the
if any, and the material properties of the concrete in the equation for computing the cracking moment for a
continuity diaphragm. reinforced section is used. The diaphragm is generally
cast with the deck concrete, so the section properties are
computed using uniform concrete properties and the deck
width is not transformed.
Article 5.7.3.3.2 specifies a minimum capacity for all
flexural sections. This is to prevent sudden collapse at the
formation of the first crack. However, the positive moment
connection that is being discussed here is not intended to
resist applied live loads. Even if the positive moment
connection were to fail completely, the system may, at
worst, become a series of simple spans. Therefore, the
minimum reinforcement requirement of Article 5.7.3.3.2
does not apply. Allowing a positive moment connection with
lower quantities of reinforcement will relieve congestion in
continuity diaphragms.
The precast girders must be designed for any positive
restraint moments that are used in design. Near the ends
of girders, the reduced effect of prestress within the transfer
length shall be considered.
Additional positive moment connection details shall
be permitted if the strength and performance of these
connections are verified by analysis or testing.
The requirements of Article 5.7.3.4 shall apply to the
reinforcement at positive moment connections at continuity
diaphragms.
· Positive Moment Connection Using Mild
Reinforcement
The positive moment connection is designed to utilize
The anchorage of mild reinforcement used for positive the yield strength of the reinforcement. Therefore, the
moment connections shall satisfy the requirements of connection must be detailed to provide full development of
Article 5.11 and the additional requirements of this article. the reinforcement. If the reinforcement cannot be detailed
Where positive moment reinforcement is added between for full development, the connection may be designed
pretensioned strands, consolidation of concrete and bond using a reduced stress in the reinforcement.
of reinforcement shall be considered. Potential cracks are more likely to form in the precast
The critical section for the development of positive girder at the inside edge of the bearing area and at
moment reinforcement into the precast girder shall consider locations of termination of debonding. Since cracking within
conditions in the girder. The reinforcement shall be the development length reduces the effectiveness of the
developed beyond the inside edge of the bearing area. The development, the reinforcement should be detailed to avoid
reinforcement shall also be detailed so that debonding of this condition. It is recommended that reinforcement be
strands does not terminate within the development length. developed beyond the location where a crack radiating from
the inside edge of the bearing may cross the reinforcement.
The termination of the positive moment reinforcement
Where multiple bars are used for a positive moment is staggered to reduce the potential for cracking at the ends
connection, the termination of the reinforcement shall be of the bars.
staggered.
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SPECIFICATIONS COMMENTARY
· Positive Moment Connection Using Prestressing
Strand
Pretensioning strands that are not debonded at the end Strands that are debonded or shielded at the end of a
of the girder may be extended into the continuity diaphragm member may not be used as reinforcement for the positive
as positive moment reinforcement. The extended strands moment connection. There are no requirements for
shall be anchored into the diaphragm by bending the development of the strand into the girder because the
strands into a 90o hook or by providing a development strands run continuously through the precast girder.
length as specified in Article 5.11.4. The following equations for the development length
of hooked 0.5 IN diameter prestressing strands were
developed by Salmons (1974, 1980), Salmons and May
(1974), and Salmons and McCrate (1973). Strands used
for a positive moment connection shall project at least 8 IN
from the face of the girder before they are bent. The stress
in the strands used for design, as a function of the total
length of extended strand, shall not exceed:
fpsl = (dsh 8)/0.228 (5.14.1.2.7i-1)
fpul = (dsh 8)/0.163 (5.14.1.2.7i-2)
where:
dsh = total length of extended strand (IN)
fpsl = stress in the strand at the SERVICE limit state.
Cracked section shall be assumed. (KSI)
fpul = stress in the strand at the STRENGTH limit state.
(KSI)
Other equations are available to estimate stress in
nonprestressed bent strands (Noppakunwijai et al., 2002).
· Details of Positive Moment Connection
Positive moment reinforcement shall be placed in a Tests (Miller et al., 2004) suggest that reinforcement
pattern that is symmetrical, or as nearly symmetrical as patterns containing significant asymmetry may result in
possible, about the centerline of the cross section. unequal bar stresses that can be detrimental to the
performance of the positive moment connection.
Fabrication and erection issues shall be considered in With some girder shapes, it may not be possible to
the detailing of positive moment reinforcement in the install prebent hooked bars without the hook tails
continuity diaphragm. Reinforcement from opposing girders interfering with the formwork. In such cases, a straight bar
shall be detailed to mesh during erection without may be embedded and then bent after the girder is
significant conflicts. Reinforcement shall be detailed to fabricated. Such bending is generally accomplished without
enable placement of anchor bars and other reinforcement heating, and the bend must be smooth with a minimum
in the continuity diaphragm. bend diameter conforming to the requirements of Table
5.10.2.3-1. If the engineer allows the reinforcement to be
bent after the girder is fabricated, the contract documents
shall indicate that field bending is permissible and shall
provide requirements for such bending. Since requirements
regarding field bending may vary, the preferences of the
owner should be considered.
Hairpin bars (a bar with an 180o bend with both legs
developed into the precast girder) have been used for
positive moment connections to eliminate the need for
post-fabrication bending of the reinforcement and to reduce
congestion in the continuity diaphragm.
5.14.1.2.7j Continuity Diaphragms C5.14.1.2.7j
The design of continuity diaphragms at interior The use of the increased concrete strength is permitted
supports may be based on the strength of the concrete in because the continuity diaphragm concrete between girder
the precast girders. ends is confined by the girders and by the continuity
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SPECIFICATIONS COMMENTARY
diaphragm extending beyond the girders. It is recommended
that this provision be applied only to conditions where the
portion of the continuity diaphragm that is in compression
is confined between ends of precast girders.
The continuity diaphragm shall be detailed to provide The width of the continuity diaphragm must be large
the required anchorage of reinforcement extending from enough to provide the required embedment for the
the precast girders into the continuity diaphragm. development of the positive moment reinforcement into
the diaphragm. An anchor bar with a diameter equal to
or greater than the diameter of the positive moment
reinforcement may be placed in the corner of a 90o hook
or inside the loop of a 180o hook bar to improve the
effectiveness of the anchorage of the reinforcement.
The sequence for concrete placement in the continuity Several construction sequences have been
diaphragms and deck slab shall be shown in the contract successfully used for the construction of bridges with
documents. precast girders made continuous. When determining the
construction sequence, the engineer should consider the
effect of girder rotations and restraint as the deck slab
concrete is being placed.
Precast girders may be embedded into continuity Test results (Miller et al., 2004) have shown that
diaphragms. embedding precast girders 6 IN into continuity diaphragms
improves the performance of positive moment connections.
The observed stresses in the positive moment
reinforcement in the continuity diaphragm were reduced
compared with connections without girder embedment.
If horizontal diaphragm reinforcement is passed The connection between precast girders and the
through holes in the precast beam or is attached to the continuity diaphragm may be enhanced by passing
precast element using mechanical connectors, the end horizontal reinforcement through holes in the precast
precast element shall be designed to resist positive beam or attaching the reinforcement to the beam by
moments caused by superimposed dead loads, live loads, embedded connectors. Test results (Miller et al., 2004;
creep and shrinkage of the girders, shrinkage of the deck Salmons, 1974, 1980; Salmons and May, 1974; and
slab, and temperature effects. Design of the end of the Salmons and McCrate, 1973) show that such reinforcement
girder shall account for the reduced effect of prestress stiffens the connection. The use of such mechanical
within the transfer length. connections requires that the end of the girder be
embedded into the continuity diaphragm. Tests of
continuity diaphragms without mechanical connections
between the girder and diaphragm show the failure of
connection occurs by the beam end pulling out of the
diaphragm with all of the damage occurring in the
diaphragm. Tests of connections with horizontal bars
show that cracks may form in the end of the precast girder
outside the continuity diaphragm if the connection is
subjected to a significant positive moment. Such cracking
in the end region of the girder may not be desirable.
Where ends of girders are not directly opposite each A method such as given in Article 5.6.3 may be used
other across a continuity diaphragm, the diaphragm must to design a continuity diaphragm for these conditions.
be designed to transfer forces between girders. Continuity
diaphragms shall also be designed for situations where an
angle change occurs between opposing girders.
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REFERENCES
Add the following references to the list of references currently appearing at the end of Section 5:
Abdalla, O.A., Ramirez, J.A. and, Lee, R.H., "Strand Debonding in Pretensioned Beams: Precast Prestressed
Concrete Bridge Girders with Debonded Strands--Continuity Issues", Joint Highway Research Project, Indiana
Department of Transportation/Purdue University, FHWA/INDOT/JHRP-92-94, June 1993, 235 p.
Hanson, N. W., "Precast-prestressed Concrete Bridges, 2. Horizontal Shear Connections", Journal of the PCA
Research and Development Laboratories, Vol. 2, No. 2, May 1960, pp. 3858. Also reprinted as PCA Bulletin D35.
Kaar, P.H., Kriz, L.B. and Hognestad, E., "Precast-prestressed Concrete Bridges, 1. Pilot Tests of Continuous
Girders", Journal of the PCA Research and Development Laboratories, Vol. 2, No. 2, May 1960, pp. 2137. Also
reprinted as PCA Bulletin D34.
Kaar, P.H., Kriz, L.B. and Hognestad, E., "Precast-prestressed Concrete Bridges, 6. Test of Half-Scale Highway
Bridge Continuous over Two Spans", Journal of the PCA Research and Development Laboratories, Vol. 3, No. 3, Sept
1961, pp. 3070. Also reprinted as PCA Bulletin D51.
Ma, Z., Huo, X., Tadros, M.K. and Baishya, M., "Restraint Moments in Precast/Prestressed Concrete Continuous
Bridges," PCI Journal, Vol. 43, No. 6, Nov/Dec 1998, pp. 4056.
Mattock, A.H and Kaar, P.H., "Precast-prestressed Concrete Bridges, 3. Further Tests of Continuous Girders", Journal
of the PCA Research and Development Laboratories, Vol. 2, No. 3, Sept 1960, pp. 5178. Also reprinted as PCA
Bulletin D43.
Mattock, A.H. and Kaar, P.H, "Precast-prestressed Concrete Bridges, 4. Shear Tests of Continuous Girders", Journal
of the PCA Research and Development Laboratories, Vol. 3, No. 1, Jan 1961, pp. 1946. Also reprinted as PCA
Bulletin D45.
Mattock, A.H., "Precast-prestressed Concrete Bridges, 5. Creep and Shrinkage Studies", Journal of the PCA Research
and Development Laboratories, Vol. 3, No. 2, May 1961, pp. 3266. Also reprinted as PCA Bulletin D46.
Miller, R.A., Castrodale, R., Mirmiran, A. and Hastak, M., NCHRP Report 519: Connection of Simple-Span Precast
Concrete Girders for Continuity, National Cooperative Highway Research Program Report, Transportation Research
Board, National Research Council, Washington, DC, 2004.
Mirmiran, A., Kulkarni, S., Castrodale, R., Miller, R. and Hastak, M., "Nonlinear Continuity Analysis of Precast,
Prestressed Concrete Girders with Cast-in-Place Decks and Diaphragms", PCI Journal, Vol. 46, No. 5,
SeptemberOctober 2001, pp. 6080.
Mirmiran, A., Kulkarni, S., Miller R.A., Hastak, M., Shahrooz, B. and Castrodale, R.C., "Positive Moment Cracking in
Diaphragms of Simple Span Prestressed Girders Made Continuous, SP 204, American Concrete Institute, August
2001.
Noppakunwijai, P., Jongpitakseel, N., Ma, Z. (John), Yehia, S.A., and Tadros, M.K., "Pullout Capacity of Non-
Prestressed Bent Strands for Prestressed Concrete Girders," PCI Journal, Vol. 47, No. 4, JulyAugust 2002, pp.
90103.
Oesterle, R.G., Glikin, J.D. and Larson, S.C., NCHRP Report 322: Design of Precast-Prestressed Bridge Girders Made
Continuous, Transportation Research Board, National Research Council, Washington, DC, November 1989, 97 p.
Priestley, M.J.N. and Tao, J.R., "Seismic Response of Precast Prestressed Concrete Frames with Partially Debonded
Tendons", PCI Journal, Vol. 38, No. 1, JanuaryFebruary 1993, pp. 5869.
Russell, H., Ozyildirim, C., Tadros, M. and Miller, R., Compilation and Evaluation of Results from High Performance
Concrete Bridge Projects, Project DTFH61-00-C-00009 Compact Disc, Federal Highway Administration, Washington,
DC, 2003.
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Salmons, J.R., Behavior of Untensioned-Bonded Prestressing Strand, Final Report 77-1, Missouri Cooperative
Highway Research Program, Missouri State Highway Department, June 1980, 73 p.
Salmons, J.R., End Connections of Pretensioned I-Beam Bridges, Final Report 73-5C, Missouri Cooperative Highway
Research Program, Missouri State Highway Department, Nov. 1974, 51 p.
Salmons, J.R. and May, G.W., Strand Reinforcing for End Connection of Pretensioned I-Beam Bridges, Interim Report
73-5B, Missouri Cooperative Highway Research Program, Missouri State Highway Department, May 1974, 142 p.
Salmons, J.R. and McCrate, T.E., Bond of Untensioned Prestress Strand, Interim Report 73-5A, Missouri Cooperative
Highway Research Program, Missouri State Highway Department, Aug. 1973, 108 p.
