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CHAPTER 3
Interpretation, Appraisal, and Applications
3.1 Overview (SDCs A, B, C, and D). Construction specifications are pro-
vided as a new proposed article--Article 8.13.8, "Special
This chapter provides interpretation, evaluation, and appli- Requirements for Precast Bent Cap Connections"--to be
cations of the findings of Chapter 2 in developing research added to the AASHTO LRFD Bridge Construction Specifications
deliverables for the precast bent cap systems investigated. In (LRFD BCS) (35).
particular, this chapter presents design specifications, design
examples, and design flow charts developed using specimen test
results and related references. Design methodologies for emu- 3.2 Development of Design
lative connections generally follow existing CIP methodologies, Specifications
but changes are incorporated into new or revised design speci-
This section presents the basis for the provisions proposed
fications. Presented construction specifications were developed
for incorporation into the LRFD 2009 SGS (1). For simplic-
using specifications previously developed together with results
ity, the following sections generally use the same outline as
from test specimen fabrication and assembly (8, 21, 22, 23, 26).
that found in the 2009 LRFD SGS. Proposed specifications are
All research deliverables are also presented as attachments to
given below. References to articles within this section refer to
this report, grouped in the following categories: proposed
the LRFD SGS (2009 edition or proposed specifications).
design specifications (new and revised), design flow charts,
Proposed design specifications have been prepared in the
design examples, construction specifications, and example
format and language of the 2009 LRFD SGS with detailed com-
connection details. In addition, an implementation plan is
mentary (1). In addition, detailed drawings are incorporated
provided. The attachments provide a detailed list of these
into the design specifications, including labeling of precast bent
deliverables (attachments available at www.trb.org/Main/
cap features and joint shear reinforcement. Many sections of
Blurbs/164089.aspx).
this chapter are directly incorporated into the proposed design
Design specifications for the SDCs--SDCs C and D, SDC B,
specifications, but not all sections of the specifications are
and SDC A--are given in appropriate format for incorpora-
shown herein. It is recommended that the accompanying
tion into a future edition of the AASHTO Guide Specifications
attachments be reviewed together with this section.
for LRFD Seismic Bridge Design (LRFD SGS) (1). A major
Chapter 3 refers extensively to 2009 LRFD SGS (1) provi-
proposed change is to revise Article 8.13, "Joint Design for
sions, which adopt the AASHTO convention of using units of
SDCs C and D" of the 2009 LRFD SGS to include precast bent
. This practice results in different coefficients than
ksi for f c
cap connections (grouted duct and cap pocket). However, to
address all seismic design categories, two new articles are also those presented formerly using units of psi. For example, terms
required. Therefore, current Article 8.13, "Joint Design for such as 3.5 fc psi (likely joint cracking) appear as 0.11 fc ksi
SDCs C and D" is renumbered as Article 8.15, "Joint Design for the same provision (likely joint cracking). This chapter
for SDCs C and D." This allows two new articles to be added: adds clarifying units as needed.
Article 8.13, "Joint Design for SDC A" and Article 8.14, "Joint
Design for SDC B."
3.2.1 Overview
Design flow charts and design examples are presented to
illustrate the proper use of design specifications for both Based on specimen test results and analysis, the design spec-
grouted duct and cap pocket connections at all SDC levels ifications presented in the following sections differ in some
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respects from the 2009 LRFD SGS provisions for nonintegral seismic response that has inherently less energy dissipation
CIP bent caps (1). Precast bent cap connections conservatively capacity when considering the hysteretic response. The series
require that the joint principal tensile stress be calculated to of nonlinear time history analyses conducted on hybrid sys-
determine the additional joint shear reinforcement require- tems described in Chapter 2 indicate that the experienced seis-
ment not only for SDCs C and D (as required for CIP design), mic demands for hybrid systems designed in accordance with
but also for SDC B. Where the joint principal tensile stress, pt, the provisions described herein are of similar magnitude to a
indicates likely joint cracking (0.11 fc ksi or larger), grouted CIP system. Therefore, the current provisions as specified in
duct design specifications for joint shear reinforcement closely Article 4.3.3 can be implemented for hybrid systems.
match CIP specifications. Cap pocket specifications, however,
account for use of a single corrugated pipe that replaces trans-
3.2.3 Vertical Ground Motion
verse joint reinforcement, require a supplementary hoop at
Design Requirements
each end of the pipe and a smaller area of vertical joint stir-
rups, and do not specify horizontal J-bars. Where the joint The jointed nature of discontinuous integral precast super-
principal tensile stress indicates joint cracking is not expected structures with vertical joints at the bent cap face requires spe-
(less than 0.11 fc ), precast bent cap connections in SDCs B, cial attention when considering potential flexural and shear
C, and D still require minimum transverse reinforcement and demands. The basic design philosophy for lateral loading is to
vertical stirrups within the joint. All precast bent cap connec- use capacity design procedures to ensure the elastic response
tions require bedding layer reinforcement, and specifications of the superstructure. However, vertical seismic loading cannot
ensure proper design and placement of the column top hoop. be handled with the same capacity design procedures because
SDC A joints also prescribe minimum transverse reinforce- there is not a well-defined mechanism for inelastic response.
ment and vertical joint stirrups within the joint. The effects of vertical excitation can impose significant flex-
For hybrid bent cap connections, the experimental results ural and shear demands on the superstructure at the interface
indicated that many of the existing joint detailing provisions between the bent cap and girder whether it is a precast or CIP
are reasonable for implementation. Therefore, the underly- system. Therefore, seismic demands generated from vertical
ing joint transfer mechanism for CIP and emulative bent caps motions must be considered in seismic design. Additionally,
is employed for hybrid bent caps. New provisions were added seismically induced foundation movements such as relative
to the LRFD SGS for the design of hybrid systems to ensure settlement, lateral spreading, and liquefaction can induce sub-
that the response characteristics of a hybrid system are stantial demands on the superstructure. This topic is covered
achieved. in more detail in the discussion of superstructure design pro-
In the 2009 LRFD SGS, there are some disparities between visions. For precast systems, the potential implications of ver-
the joint design provisions for nonintegral systems and for tical excitation are greater than comparable CIP systems due
transverse design of integral systems (1). The general mecha- to possible concentrated joint rotations and the reliance on
nism for transverse response of a multicolumn nonintegral or shear friction mechanisms to resist vertical shear demands
integral structure is essentially the same. Therefore, recom- across the joint. It is recommended that more refined vertical
mended modifications are presented for integral systems to seismic demand provisions be developed for all bridge sys-
develop a consistent design specification. Furthermore, a tems; however, at a minimum, the following provision is rec-
variety of design and detailing provisions is recommended for ommended for inclusion in Article 4.7.2 of the LRFD SGS for
integral precast systems in order to ensure that reliable and precast systems:
safe seismic response is achieved.
For integral precast bridge superstructures with pri-
mary members that are discontinuous at the face of the
bent cap (i.e., precast segmental, integral spliced girder
3.2.2 Displacement Magnification systems, etc.), vertical seismic demand shall be explicitly
for Short Period Structures considered in superstructure design for both moment
and shear using equivalent static, response spectrum,
It is essential to consider the impacts on expected seismic or time history analysis. Demands from vertical ground
demands when utilizing a system that has a significantly differ- motion shall be combined with horizontal seismic
ent mode of seismic response. The nonintegral emulative and demands based on plastic hinging forces developed in
integral details have been shown to perform in a similar man- accordance with Article 4.11.2.
Seismic demands shall be combined considering 100%
ner to CIP structures. Therefore, these systems can be imple-
of the demand in the vertical direction added with 30%
mented using the same lateral seismic demand procedures that of the seismic demand resulting from flexural hinging in
are currently employed in the LRFD SGS. However, the hybrid one of the horizontal perpendicular directions (longitu-
details investigated are aimed at providing a different mode of dinal) and 30% of the seismic demand resulting from
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flexural hinging in the second perpendicular horizontal 3.2.6 Plastic Moment Capacity
direction (transverse). for SDC B, C, and D
A major obstacle that must be overcome is the development The current provisions for the determination of plastic
of improved provisions for the development of vertical seismic moment capacities of ductile concrete members are suf-
loadings. Current provisions in the 2009 LRFD SGS admit that ficient for CIP and precast emulative systems. However,
there are shortcomings in the design requirements for vertical the intentional debonding of post-tensioning and reinforce-
excitation that must be resolved for all bridge systems (1). ment within a hybrid system creates complications in the
application of the existing provisions. With discrete joint
rotations and distributed straining of steel elements, moment-
3.2.4 Analytical Plastic Hinge Length curvature analysis cannot be directly implemented for hybrid
For integral bridge systems, it is desirable to have an under- concrete members. Therefore, additional provisions are
standing of the expected rotation capacity of the superstructure required.
when considering demands associated with vertical loading or Debonded elements and associated distributed straining can
potential seismically induced relative settlement. Similar to be accounted for using moment-rotation analyses. The prem-
column systems, the use of moment-curvature analysis and ise of moment-rotation analysis is similar to that of moment-
an analytical plastic hinge length can provide an easy-to- curvature analysis where strain compatibility is used to
implement method for the estimation of the inelastic response perform sectional analysis of the member. In a moment-
of a superstructure joint and its ultimate rotation capacity. curvature analysis, the strain distribution is considered linear
Moment-curvature analysis for capacity protected superstruc- and identical for both steel and concrete elements at the same
ture elements is already required for SDC C and D structures location. Moment-rotation analysis makes a similar plain sec-
per the 2009 LRFD SGS Article 8.10 (1). The only obstacle in tion assumption, but allows for varying strain at a given section
the determination of the inelastic flexural response is a reason- by considering a fixed length over which an element accumu-
able estimate of the analytical plastic hinge length. For ele- lates strain. To account for the analysis procedure required
ments that are flexurally dominated, the analytical plastic for hybrid members, a new Article 8.5.2 is recommended for
hinge length can be reasonably approximated as one-half of addition to the 2009 LRFD SGS (1):
the element depth in the direction of loading. Therefore, the
following is a recommended addition to the 2009 LRFD SGS For hybrid concrete members, the plastic moment
as Article 4.11.6.2: capacity shall be calculated using a moment-rotation
(M-) analysis based on the expected material proper-
Lps = 0.5Ds Proposed LRFD SGS Eq. 4.11.6.2-1 ties. The moment-rotation analysis shall include the axial
forces due to dead load together with the axial forces
where: due to overturning as given in Article 4.11.4.
The M- curve can be idealized with an elastic per-
Lps = analytical plastic hinge length for integral concrete fectly plastic response to estimate the plastic moment
superstructures (in) capacity of a member's cross section. The elastic portion
Ds = total depth of superstructure (in) of the idealized curve passes through the point marking
the first reinforcing bar yield. The idealized plastic
moment capacity is obtained by equating the areas
3.2.5 Reinforcing Steel Modeling between the actual and the idealized M- curves beyond
the first reinforcing bar yield point similar to as shown in
Localized joint rotations associated with hybrid systems Figure 1.
can cause increased straining in reinforcing bars due to geo-
metric compatibility. As the joint opens, the rotation must be
In the execution of a moment-rotation analysis, the follow-
accommodated in the reinforcing bar with the bar being fixed
ing are the recommended strain lengths for specific elements
at both ends. These additional strain demands caused by geo-
in the section. The concrete compressive strain length can be
metric loading can be accounted for by a conservative reduc-
approximated as the neutral axis depth. In performing any
tion in the ultimate tensile strain considered. The following
strain-compatibility sectional analysis, the neutral axis depth
recommended addition to Article 8.4.2 of the 2009 LRFD SGS
is calculated to determine the cross-sectional deformation
accounts for this geometric loading in combination with the
distribution. This depth can be used to define the region over
traditional reduced ultimate tensile strain (1):
which the concrete strain is approximately constant. The rein-
For hybrid connections, the reduced ultimate tensile
forcement strain length can be approximated as the length
strain, su
R
, shall equal one-half the ultimate tensile over which the reinforcement is intentionally debonded. The
strain, su. post-tensioning strain length can be approximated as the dis-
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tance between anchorages as the tendons are debonded for where:
their full length. c = distance from extreme compression fiber to the
neutral axis at the reference yield point (in)
Dc = column diameter or smallest dimension in the direc-
3.2.7 Hybrid Performance Requirements tion of loading (in)
To ensure that hybrid systems exhibit the desired lateral
The next recommended modification is to Article 8.8.2 of
response characteristics of self-centering behavior and lim-
the LRFD SGS. This provision specifies a minimum flexural
ited damage, a variety of provisions are recommended for
contribution of mild reinforcement for hybrid systems to
inclusion in the LRFD SGS. These provisions are intended to
ensure that stable and predictable lateral response is achieved.
limit various design parameters within specific target ranges
The traditional minimum reinforcement requirements are
to produce the intended mode of lateral response. not applicable to hybrid systems and therefore a new provi-
The aim of the first set of provisions is to ensure that the sion is added. For hybrid systems, the minimum amount of
contribution of reinforcement is such that the system will be reinforcement ensures that the response predictions for ref-
capable of a reduction in the residual deformations as com- erence yield are reasonable and the overall seismic demands
pared to traditional bridge systems. A series of limits is rec- as modified by Article 4.3.3 are valid. The recommended
ommended for inclusion in Article 8.8.1 of the LRFD SGS as addition to Article 8.8.2 is the following:
outlined below. The first of the three equations ensures that
the effective axial load acting on the column following a seis- The minimum area of longitudinal reinforcement for
mic event is large enough to force the column reinforcement hybrid compression members shall satisfy:
back to a zero strain state, thereby aiding in the self-centering
response. The second equation limits the contribution of the Ms
0.25 Proposed LRFD SGS Eq. 8.8.2-5
reinforcement on the overall flexural capacity in order to limit My
the potential residual deformations associated with traditional
where:
bridge construction. Increases over this limit will produce
lateral response that is similar to traditional CIP bridges with Ms = flexural moment capacity provided by longitudinal
reinforcement at reference yield moment (kip-ft)
more noticeable damage and residual deformation. The third My = reference yield moment (kip-ft)
equation is a limit on the neutral axis depth that is intended to
limit the magnitude of strain in the concrete compression toe To prevent the premature fracture of column reinforcement
due to joint opening. in hybrid systems, the reinforcement must be intentionally
debonded to accommodate the localized joint opening at the
The maximum longitudinal reinforcement for hybrid
compression members shall be proportioned to satisfy ultimate displacement capacity. A provision is recommended
Equations 2 through 4: for inclusion in the LRFD SGS as Article 8.8.14 to explicitly
enforce this requirement:
0.9PD + Ppse
> 1.0 Proposed LRFD SGS Eq. 8.8.1-2
Ts Longitudinal reinforcement in hybrid columns shall
be intentionally debonded from the surrounding con-
where: crete at hybrid column end connections. The minimum
debonded length shall be such to ensure that the strain
PD = dead load axial load action on column (kip)
in the longitudinal reinforcement does not exceed the
Ppse = effective force in post-tensioning tendon at end
reduced ultimate tensile strain specified in Article 8.4.2
of service life (kip)
at the column ultimate rotation capacity.
Ts = resultant column reinforcement tension force
associated with ultimate moment capacity (kip)
As was previously discussed, the current provisions for
Ms short period displacement amplification are acceptable for use
0.33 Proposed LRFD SGS Eq. 8.8.1-3
My with hybrid systems within the bounds of the provisions pre-
sented in the LRFD SGS and herein. However, the influence
where:
of joint opening at the reference yield point must also be
Ms = flexural moment capacity provided by longitudinal accounted for in the mathematical modeling of hybrid con-
reinforcement at reference yield moment (kip-ft)
My = reference yield moment (kip-ft)
crete members to ensure that the added flexibility is consid-
ered. The moment-rotation analysis performed in accordance
c with the recommended Article 8.5 modifications provides a
0.25 Proposed LRFD SGS Eq. 8.8.1-4
Dc means to approximate the added flexibility for equivalent
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elastic analysis. To account for the added flexibility, the effec- demand, the superstructure to bent cap connection should be
tive moment of inertia can be modified based on the effective capable of experiencing a defined level of rotation demand.
section properties calculated using moment-rotation analysis. The intent of the following recommended Article 8.10.3 is to
The recommended Article 5.6.6 of the LRFD SGS provides ensure that the superstructure can resist a limited amount of
this requirement: inelastic action:
The effective moment of inertia for calculation of The superstructure to bent-cap connection shall have
elastic flexural deformations for hybrid bridge columns plastic rotation capacity equal to or greater than 0.01
can be taken equal to the gross moment of inertia. For radians. The plastic rotation capacity shall be calcu-
mathematical modeling, the increase in flexibility at lated using the moment-curvature analysis required per
reference yield due to joint opening shall be considered. Article 8.9 and the analytical plastic hinge length for
The influence of joint rotation shall be determined in superstructures as defined in Article 4.11.6.
accordance with the provisions of Article 8.5 using
moment-rotation analysis. For equivalent elastic analy-
sis, Ieff shall be decreased to account for the additional Torsional Design for Open Soffit Superstructures
flexibility due to joint rotation.
CIP integral bridge systems traditionally have a top deck
slab and bottom soffit slab that provide reliable distribution
3.2.8 Superstructure Capacity for of column overstrength demands into the superstructure.
Longitudinal Direction, SDCs C and D However, precast systems without a soffit slab cannot trans-
fer the seismic demands through the same mechanism. The
Superstructure Demand
column flexural overstrength demands must be transferred
As discussed in relation to the recommended modifica- into the superstructure by way of torsional response of the bent
tions to the 2009 LRFD SGS, vertical seismic demands can cap. Commonly used torsional mechanisms cannot develop
play a significant role in the performance of integral precast over the short distance between the face of the column and
bridge systems (1). Therefore, additional recommendations girder. Instead, a modified torsional response must be con-
were specified for the development and consideration of ver- sidered. The following new Article 8.10.4 requires the explicit
tical ground motions in the design of integral precast bridges. consideration of a torsional transfer mechanism for open
For longitudinal response, seismic actions are distributed soffit systems:
into the superstructure based on an effective width calculated
in accordance with Article 8.10. However, for vertical demands, The transfer of column overstrength moment, Mpo,
the seismic loading can be distributed across the entire width and associated shear and axial load via torsional mecha-
nisms must be explicitly considered in the superstructure
of the bridge. To account for this, the recommended addition design for open soffit structures.
to Article 8.10 is the following:
Vertical seismic demands determined in accordance Shear Design for Integral Precast Superstructures
with Article 4.7.2 shall be distributed to the entire width
of the superstructure. The demands associated with the The potential for inelastic superstructure response due to
column overstrength moment, Mpo, shall be considered vertical motion and seismically induced settlement was men-
concurrently with vertical seismic demands as specified tioned in the discussion of the minimum superstructure rota-
in Article 4.7.2. tion capacity. The bottom flanges of precast girders in the
superstructure should be detailed to accommodate the poten-
Minimum Superstructure Rotation Capacity tial inelastic actions without degradation of the compression
zone. Therefore, the use of closed hoops is recommended as
The use of capacity design procedures cannot ensure that a a means to enhance the integrity of the girder flanges in the
superstructure system does not experience loads in excess of event of inelastic loading. The recommended addition to the
the superstructure capacity when considering actual vertical 2009 LRFD SGS (1) is the following:
seismic motions and seismically induced foundation move-
ments. The potential for seismically induced relative settle- The bottom flange of integral precast girders shall be
ments may induce substantial geometrically driven demands reinforced with closed hoops within the region from
on a bridge superstructure system. These mechanisms can the face of the bent cap equal to a distance equal to the
superstructure depth. These hoops shall be spaced with
result in loadings in the superstructure that may cause inelas- the girder shear reinforcement, with spacing not to
tic superstructure action. To ensure that the superstructure exceed 8 in. The hoops shall be the same size as the girder
can accommodate a limited amount of inelastic rotation shear reinforcement, with a minimum bar size of No. 4.
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Experimental results highlighted in Chapter 2 and described 3.2.10 Joint Performance
in detail in the attachments, indicate the importance of a
SDCs C and D
well-developed shear transfer mechanism at the girder to
bent cap connection. The potential for concentrated joint The joint performance for SDCs C and D is stated as follows:
opening during seismic loading will result in a significant
decrease in the effective shear depth across the joint that must Moment-resisting connections shall be designed to
be considered in design. The shear reinforcement can be transmit the maximum forces produced when the column
has reached its overstrength capacity, Mpo.
distributed in the superstructure based on an assumed strut
mechanism with a 30-deg maximum compression strut angle.
Most importantly, the shear reinforcement must be extended This matches the existing provision for SDCs C and D in the
as close to the top of the deck as possible while still satisfying 2009 LRFD SGS (1).
concrete cover requirements. The recommended shear detail-
ing uses headed reinforcement to ensure the sufficient anchor- SDC B
age of the reinforcement within the short distance allocated.
The joint performance for SDC B is stated as follows:
The following is a recommended addition to the 2009 LRFD
SGS (1) as Article 8.10.5: Moment-resisting connections shall be designed to
transmit the unreduced elastic seismic forces in columns
For integral precast superstructures with girders dis- where the column moment does not reach the plastic
continuous at the face of the bent cap, headed shear moment, Mp, and shall be designed to transmit the col-
reinforcement shall be placed within a distance from umn forces associated with the column overstrength
the face of the bent cap equal to 1.75 times the neutral capacity, Mpo, where the plastic moment, Mp , is reached.
axis depth at nominal capacity as determined in accor-
dance with Article 8.9. The headed shear reinforcement
within this distance shall be capable of resisting the fac- Based on Article 8.3.2 of the 2009 LRFD SGS, this provi-
tored shear demand including effects of vertical seismic sion requires that connections be designed to transmit the
loading in accordance with Article 4.7.2. The shear lesser of the forces produced by Mpo or the unreduced elastic
demand shall be calculated considering the direction of seismic forces (1). However, when the elastic seismic moment
loading and shears generated during positive flexural
loading of the superstructure. This reinforcement shall
reaches the plastic moment, Mp, significant plastic hinging
extend as close to the top of the deck as possible while may develop. Therefore, it is conservatively required in such
maintaining required concrete cover dimensions. cases that connections be designed to transmit the forces pro-
duced by Mpo. For SDC B, the column section may be designed
and governed by load cases other than seismic.
3.2.9 Joint Definition This proposed article is an application of Articles 8.3.2 and
Specimen test results and related research provide a suffi- C8.3.2 of the 2009 LRFD SGS, which recognize that SDC B
cient basis for safe, constructible, durable, and economical bridges may be subjected to seismic forces that can cause
design of nonintegral emulative precast bent cap systems yielding of the columns and limited plastic hinging, as they are
using grouted duct or cap pocket connections and hybrid designed and detailed to achieve a displacement ductility, µD,
connections in all SDC levels. However, as shown in Chapter of at least 2.0 (1). According to Article C4.8.1, SDC B columns
2, testing was limited to interior joints of multicolumn bent are targeted for a drift capacity corresponding to concrete
caps. Therefore, proposed provisions for all SDCs follow the spalling. Article 4.8.1 provides an approximate equation for
precedent for CIP joints found in Article 8.13.4.1 of the 2009 local displacement capacity, providing an approach that lim-
LRFD SGS (1) in limiting specifications to interior joints of its the required seismic analysis (i.e., expands the extent of a
multicolumn bent caps: "No Analysis" zone). Thus, based on Article 4.11.1, joint shear
checks and full capacity design using plastic overstrength
Interior joints of multicolumn bents shall be consid- forces are not required. This more liberal practice, as stated
ered "T" joints for joint shear analysis. Exterior joints in Article C4.11.1, may be adopted for CIP joints. However,
shall be considered knee joints and require special owners may also choose to implement the more conservative
analysis and detailing that are not addressed herein,
unless special analysis determines that "T" joint analysis
capacity-protection requirements given in Article 8.9.
is appropriate for an exterior joint based on the actual Full and limited ductility specimen tests indicated initial
bent configuration. concrete spalling of the column at drift ratios ranging from
0.9% to 1.8% (µ1.5 to µ3). Specimens used a moderate amount
Specifications for knee joints in CIP and precast bent cap of column longitudinal reinforcement--1.58%. For this case,
systems should be developed. joint shear cracks developed at loads less than effective yield
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(µ1) for all specimens at principal tensile stresses that ranged 15.0 in from the face of the column connection into the
from 2.95 fc psi to 4.3 fc psi, close to the 3.5 fc psi assumed adjoining member (29).
by the 2009 LRFD SGS (1). At µ2, all specimens had significantly According to these alternative provisions, when SD1 is greater
than or equal to 0.10 but less than 0.15, minimum transverse
exceeded 3.5 fc and reached forces that were 88% to 100%
reinforcement is required for CIP joints. When SD1 is less
of the maximum overall force induced in the joint during
than 0.10, transverse shear reinforcement is not required.
testing. Furthermore, the CPLD specimen, designed accord-
For all values of SD1 in SDC A, the designer may choose to con-
ing to SDC B detailing requirements (i.e., no joint reinforce-
servatively provide joint reinforcement as specified for SDC B,
ment other than the steel pipe), exhibited extensive joint shear
although SDC A is typically considered a "No Analysis" region
cracking. As reported in Matsumoto 2009 (26), the absence of
for which seismic analysis is not required (2009 LRFD SGS,
joint stirrups--in accordance with SDC B design--was the
Article C4.6) (1).
main cause of the development and growth of joint shear
Precast bent cap connections for SDCs B, C, and D are
cracks. This indicates that SDC B joint design for precast
designed and detailed to provide sufficient reinforcement for
connections should be based on a check of principal tensile
force transfer through the joint and bent cap. The precast
stresses and that all SDC B joints should include at least min-
bent cap design provisions for SDC A, including minimum
imum joint shear reinforcement, defined as transverse rein-
provisions, are more liberal than those for precast bent caps
forcement and joint stirrups.
for SDC B and may be considered "No Analysis" requirements
The proposed LRFD SGS adopts the more conservative
for SDC A precast bent cap systems. They are deemed appro-
provisions that principal tensile stresses be checked for SDC B
priate for all values of SD1 in SDC A.
and that joint design depend on this check. These provisions
When SD1 is less than 0.10, alternative precast bent cap
help ensure that the precast bent cap connections accommo-
connections developed for nonseismic regions may be used.
date forces in an essentially elastic manner and do not become
Figure 3.1 and Figure 3.2 show several nonintegral precast bent
a weak link in the earthquake resisting system (Articles 4.11.1
cap details developed by Matsumoto et al. (8) for grouted duct,
and C4.11.1, 2009 LRFD SGS) (1).
grout pocket, and bolted connections (7). Other references
such as Brenes et al. (36) provide additional recommendations
SDC A for detailing nonintegral precast bent cap connections using
grouted ducts. It is recommended that minimum vertical stir-
The joint performance for SDC A is stated as the following:
rups within the joint be used, as required for SDC A details.
In addition, column longitudinal reinforcement should be
Moment-resisting connections shall be designed to
transmit the unreduced elastic seismic forces in columns. extended into the connection as close as practically possible to
the opposite face of the bent cap.
According to the 2009 LRFD SGS, bridges designed for
SDC A are expected to be subjected to only minor seismic dis- 3.2.11 Joint Proportioning
placements and forces; therefore, a force-based approach is
Two provisions should be satisfied in proportioning bent cap
specified to determine unreduced elastic seismic forces, in lieu
joints: (1) provide cross-sectional dimensions to satisfy limits
of a more rigorous displacement-based analysis (Articles 4.1
on principal tensile and compression stresses and (2) provide
and 4.2, 2009 LRFD SGS) (1).
sufficient anchorage length to develop column longitudinal
However, some SDC A bridges may be exposed to seismic
reinforcement in the bent cap joint under seismic demand.
forces that may induce limited inelasticity, particularly in the
columns. For this reason, Article 8.2 states that when SD1 is
greater than or equal to 0.10 but less than 0.15, minimum Principal Stress Requirements
column shear reinforcement shall be provided in accordance
SDCs C and D. Principal tensile and compression should
with Article 8.6.5 for SDC B, subject to Article 8.8.9 for the be checked for SDCs C and D as required by Article 8.13.2 of
length over which this reinforcement is to extend. Although the 2009 LRFD SGS for CIP connections (1).
Article 8.8.9 does not specify placement of transverse col-
umn reinforcement into the joint, Articles 5.10.11.4.1e and SDC B. As mentioned previously, to ensure that SDC B
5.10.11.4.3 of AASHTO LRFD Bridge Design Specifications structures using precast bent caps are designed and detailed
(4th edition) with 2008 and 2009 Interims, referenced by the to achieve a displacement ductility, µD, of at least 2.0 (Article
alternative provisions in Articles 8.2 and 8.8.9, specify place- C8.3.2, 2009 LRFD SGS) (1), the proposed provisions conser-
ment of transverse reinforcement into the joint for a distance vatively require that SDC B joints be proportioned based on
not less than one-half the maximum column dimension or a check of principal stress levels. The provisions of Article
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(b) Bolted
(a) Grouted Duct
Figure 3.1. Alternative precast bent cap connections for SDC A (SD1 < 0.10) (7).
8.13.2 of the 2009 LRFD SGS are thus used for joint propor- beams as close as practically possible to the opposite face of
tioning, except that the design moment used in determina- the cap beam and that for seismic loads, the anchorage length
tion of principal stresses should be the lesser of Mpo or the into the cap beam satisfy the following (1):
unreduced elastic seismic column moment.
0.79dbl fye
lac 2009 LRFD SGS Eq. 8.8.4-1
SDC A. Check of principal stresses is not required for fc
SDC A.
where:
lac = anchored length of longitudinal column reinforcing
Minimum Anchorage Length bars into cap beam (in)
dbl = diameter of longitudinal column reinforcement (in)
Column longitudinal bars should be extended into joints a
fye = expected yield stress of longitudinal column rein-
sufficient depth to ensure that the bars can achieve approxi- forcement (ksi)
mately 1.4 times the expected yield strength of the reinforce- fc = nominal compressive strength of bent cap concrete
ment, i.e., a level associated with extensive plastic hinging and (ksi)
strain hardening up to the expected tensile strength. For
SDCs C and D, Article 8.8.4 of the 2009 LRFD SGS requires Prior research by Matsumoto et al. (7, 8) and Mislinski (9)
that column longitudinal reinforcement be extended into cap on anchorage of reinforcing bars in grouted ducts--confirmed
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70
(a) Grouted Pocket (Double Line)
(b) Grouted Pocket (Single Line)
Figure 3.2. Alternative precast bent cap connections for SDC A (SD1 < 0.10) (7).
by the NCHRP Project 12-74 grouted duct specimen (22)-- The maximum grout compressive strength used in Eq.
indicates that the following equation can be conservatively 8.15.2.2.2-1 should be limited to 7,000 psi, even where the
used for seismic applications: specified grout compressive strength (based on 2-in cubes)
exceeds 7,000 psi. In addition, this equation applies #11 col-
2dbl fye umn reinforcing bars or smaller ones.
lac Proposed LRFD SGS Eq. 8.15.2.2.2-
-1
fcg Anchorage of reinforcing bars in cap pocket connections can
be based on prior precast bent cap research on grout pocket
where: connections using trapezoidal prism-shaped pockets without
lac = anchored length of longitudinal column reinforc- a stay-in-place form (7, 8). Anchorage equations were modi-
ing bar into grouted duct (in) fied by removing a 0.75 factor that accounted for extensive
dbl = diameter of longitudinal column reinforcement (in)
splitting cracks at reentrant corners of grout pockets. Such
fye = expected yield stress of longitudinal column rein-
forcement (ksi) cracking did not develop for the cylindrical-shaped cap pocket
fcg = nominal compressive strength of grout (cube connections for CPFD and CPLD that used steel pipes as stay-
strength) (ksi) in-place forms. The following equation--confirmed by CPFD
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and CPLD results--can be used for cap pocket connections than 7% to fixed end rotation, and bar slip was comparable
(7, 8, 23, 26): to that of the CIP specimen (21, 23). However, significant bar
slip was observed in the CPLD specimen, as summarized in
2.3dbl fye Chapter 2 and detailed in Matsumoto 2009 (26). The level of
lac Proposed LRFD SGS Eq. 8.15.2.2.2
2-2
fc bar slip observed is attributed to significant shear cracking in
the joint that developed due to the lack of joint reinforce-
where:
ment, especially vertical stirrups. The proposed LRFD SGS
lac = anchored length of longitudinal column reinforc- requires at least minimum joint reinforcement (both trans-
ing bars into cap pocket (in) verse confinement and vertical stirrups) for all SDC levels.
dbl = diameter of longitudinal column reinforcement (in)
fye = expected yield stress of longitudinal column re-
In addition, the embedment depth of the CPLD column
inforcement (ksi) bars into the cap pocket was 26% less than that required by
fc = nominal compressive strength of cap pocket con- Eq. 8.15.2.2.2-2, due to the relatively low compressive strength
crete fill (ksi) of the concrete fill. Article 8.13.8.3 of the proposed LRFD
Bridge Construction Specifications (4) requires a minimum
The anchored length includes the length of bar within the 500-psi margin between the compressive strength of the
steel pipe and within the portion of the bent cap between the bent cap and precast connection concrete fill (or grout). This
bottom of the steel pipe and the bent cap soffit. margin accounts for the likelihood that the actual bent cap
Maximum compressive strength for the concrete fill used in compressive strength will exceed its specified strength and
Eq. 8.15.2.2.2-2 should be limited to 7,000 psi, even where the the possibility of a low compressive strength of the grout or
specified concrete fill compressive strength exceeds 7,000 psi. concrete fill. This provision is intended to ensure that the
In addition, this equation applies to #11 column reinforcing connection does not become a weak link in the system and
bars or smaller ones. helps limit bar slip.
As for CIP connections, the proposed specifications for
grouted duct and cap pocket connections require that col- Comparison of Anchorage Length Equations. Figure 3.3
umn longitudinal reinforcement be extended into precast compares seismic anchorage (or development) length require-
bent caps as close as practically possible to the opposite face ments for anchoring column longitudinal reinforcement into
of the bent cap. bent cap joints, based on the equations given in Table 3.1. For
Only minor slip of column longitudinal bars was observed simplicity, anchorage length, lac, is used herein for both
in the full ductility test specimens (CIP, GD, and CPFD). For anchorage and development lengths (lac and ld) applied to
example, for the CPFD specimen, bar slip contributed less anchorage of column bars in a joint. Provisions in the 2009
60
2009 LRFD SGS
AASHTO LRFD *1.25
UT Matsumoto, CP (GP*0.75)
UT/CSUS Matsumoto, GD
Anchorage Length/Bar Diameter (lac /db)
50 UT Brenes, GD (=.75, =1)
UT Brenes, GD (=.75, =1.3)
UW Steuck *1.5, GD
40
30
20
10
0
4000 5000 6000 7000 8000
Compressive Strength of Concrete or Grout (psi)
Figure 3.3. Anchorage length versus compressive strength--comparison of
equations.
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Table 3.1. Comparison of anchorage length equations.
/ or , /
Reference Anchorage Length, (#11; or ,
/
6000 psi)
2009 AASHTO LRFD SGS
30.9 1.00
[15]
AASHTO LRFD BDS1
59.8 1.94
[27]
UT Matsumoto,
Cap Pocket2 36.0 1.17
[3, 4]
UT/CSUS Matsumoto,
Grouted Duct 32.0 1.04
[3, 4]
UT Brenes, 36.7 ( ) 1.19 ( )
Grouted Duct3
[34]
47.7 ( ) 1.54 ( )
UW Steuck,
Grouted Duct4 14.9 0.48
[35]
1
Includes 1.25 seismic factor
2
Embedment includes 0.75 factor
3
fs,cr taken as fye.; ; for galvanized steel; for plastic duct
4
Includes 1.5 seismic factor; taken as 1.5 in.
LRFD SGS (Article 8.8.4) and LRFD BDS (Article 5.10.11.4.3) Brenes et al. (36) extended the grouted duct research of
apply to CIP connections (29, 1). The grouted duct and cap Matsumoto et al. (8), examining group effects ( factor) and
pocket column bar anchorage equations (Eq. 8.15.2.2.2-1 and plastic ducts ( factor), among other variables. Values of
Eq. 8.15.2.2.2-2) are recommended for use in precast bent range from 0.45 to 0.9 for typical configurations of bars in a
cap connections. Additional equations for grouted duct con- grouted duct connection. The case of = 0.75 and = 1.0
nections based on recent research are also provided (28, 37). shown in Figure 3.3 represents bar anchorage that accounts for
Table 3.1 also compares the ratio of anchorage length to group effects based on a moderate number of grouted column
bar diameter (lac/db) for #11 rebar and compressive strength bars simultaneously subjected to tension under the design load
of 6,000 psi (grout or concrete) as an example. In addition, the combinations ( = 0.75) as well as galvanized steel duct mater-
lac/db ratio for each equation is compared to that of the 2009 ial ( = 1.0). The equation in Brenes et al. for grouted ducts
LRFD SGS (Eq. 8.8.4-1), which is taken as a reference (1). is slightly more conservative than that of Matsumoto et al. for
Figure 3.3 and Table 3.1 indicate that the LRFD BDS equation the assumed values of and . Significantly, Brenes et al.
is extremely conservative, requiring nearly twice the anchorage found that the required anchorage length increased by 30%
length required by the 2009 LRFD SGS. The proposed grouted when polyethylene or polypropylene (plastic) ducts are used
duct and cap pocket (CP) equations are slightly more conser- instead of steel. Tension cyclic tests were not conducted.
vative (4% and 17%, respectively) than the 2009 LRFD SGS The University of Washington equation, which is multiplied
equation for the example, although Figure 3.3 shows the by the recommended 1.5 seismic factor (37), results in an
change in anchorage length with compressive strength. The exceptionally short development length and is not recom-
proposed grouted duct equation is based on both tension cyclic mended for use in precast bent cap design.
and monotonic tension tests and includes a factor of safety of at SDCs B, C, and D. Based on the foregoing development,
least 2.0. In addition, this equation can be conservatively used Eq. 8.15.2.2.2-1 and Eq. 8.15.2.2.2-2 are proposed for anchorage
for epoxy-coated bars. The use of f cg rather than fcg
in the of column bars in grouted duct and cap pocket connections,
denominator is explained in Matsumoto et al. (7). respectively.
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SDC A. SDC A incorporates the same requirements as the larger of Eq. 8.13.3-1 and Eq. 8.13.3-2 be used because the
those for SDCs B, C, and D except that the nominal yield transverse reinforcement requirement of Eq. 8.13.3-2 can
stress of the column longitudinal reinforcement may be used become less than that of Eq. 8.13.3-1 in some cases, as shown
in lieu of the expected yield stress. This allows for a slightly in this research.
reduced safety margin due to the significantly lower seismic
demand and limited inelasticity in the columns. SDC B. The proposed provisions for precast connections
in SDC B require the same check of principal tensile stress to
3.2.12 Minimum Joint Shear Reinforcement determine transverse reinforcement in the joint as is required
for SDCs C and D.
SDCs C and D. Minimum joint shear reinforcement
refers to transverse reinforcement within the joint region in However, the 2009 LRFD SGS does not include provisions
the form of column reinforcement, spirals, hoops, intersect- for minimum transverse reinforcement for CIP structures in
ing spirals or hoops, or column transverse or exterior trans- SDC B (1). The SDC B design requirement for CIP would then
verse reinforcement continued into the bent cap. For precast default to Article 5.10.11.3 of the 2009 LRFD BDS for Seismic
connections, minimum transverse joint reinforcement is Zone 2, which refers the designer to Article 5.10.11.4.3 (Seismic
required to help ensure that the connection does not become Zones 3 and 4). Article 5.10.11.4.3 requires the following:
a weak link in a precast bent cap system. Transverse reinforce-
ment for a grouted duct connection is the same as that for a Column transverse reinforcement, as specified in
CIP connection. However, for a cap pocket connection, the Article 5.10.11.4.1d, shall be continued for a distance
steel pipe serves as the transverse reinforcement. not less than one-half the maximum column dimension
For SDCs C and D, the minimum joint shear reinforce- or 15.0 in from the face of the column connection into
the adjoining member.
ment for precast and hybrid connections is determined using
essentially the same basis as that used for CIP connections. If
the nominal principal tensile stress in the joint, pt, is less than It is judged that this reinforcement is not adequate for CIP
limited ductility connections. It is therefore recommended that
0.11 fc , then the transverse reinforcement in the joint, s, minimum joint transverse reinforcement requirements also be
must satisfy the following equation and no additional rein- established for CIP bridges in SDC B. For limited or simplified
forcement within the joint is required: seismic analysis (i.e., a "No Analysis"-type approach), mini-
mum reinforcement satisfying Eq. 8.13.3-1 of the 2009 LRFD
0.11 fc SGS is recommended (1).
s 2009 LRFD SGS Eq. 8.13.3-1
fyh
SDC A. For SDC A, principal stresses are not checked,
where: but minimum joint shear reinforcement is proposed for pre-
fyh = nominal yield stress of transverse reinforcing (ksi) cast connections. This is simple, yet conservative and should
f c = nominal compressive strength of concrete (ksi) be considered good detailing practice. Such reinforcement is
s = volumetric reinforcement ratio of transverse rein-
not required for CIP joints per the 2009 LRFD SGS, but is rec-
forcing provided within the cap
ommended (1).
Where the principal tensile stress in the joint, pt , is
greater than or equal to 0.11 fc , then transverse re- Grouted Duct Connections
inforcement in the joint, s, must satisfy both Eq. 8.13.3-1
and the following equation: SDCs B, C, and D. Grouted duct connections use the same
basis as CIP connections in SDCs C and D, with the additional
Ast requirement that spacing of transverse reinforcement not
s 0.40 2009 LRFD SGS Eq. 8.13.3-2
lac
2 exceed 0.3Ds nor 12 in. This is intended to provide a reason-
able number of hoops within the joint when the minimum
where: requirement governs.
Ast = total area of column longitudinal reinforcement
anchored in the joint (in2) SDC A. Joint transverse reinforcement provisions con-
lac = length of column longitudinal reinforcement servatively match minimum requirements for SDC B.
embedded into the bent cap (in)
Cap Pocket Connections
For this case, additional joint reinforcement is also required.
The 2009 LRFD SGS requires only Eq. 8.13.3-1 to be satis- SDCs B, C, and D--Basic Equation for Pipe Thickness.
fied (1). However, the proposed specifications require that For cap pocket connections, the thickness of the corrugated
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steel pipe, tpipe, is based on providing shear resistance to the The maximum spacing requirements of 0.3Ds and 12 in do
joint that is approximately the same as that provided by the not apply to the determination of nh.
hoops required for CIP joints: The minimum thickness of the steel pipe, tpipe, of 0.060 in
corresponds to 16-gage steel pipe, which was used for the 18-in
Cap pocket connections shall use a helical, lock-seam, nominal diameter pipe in the cap pocket specimens (with a
corrugated steel pipe conforming to ASTM A760 to 20-in diameter column). As shown in Table 3.2, this is the
form the bent cap pocket. A minimum thickness of cor-
rugated steel pipe shall be used to satisfy the transverse
thinnest gage typically available off the shelf for corrugated
reinforcement ratio requirements specified in Article steel pipe. Other pipe thicknesses (nominal and tolerance
8.15.3.1. The thickness of the steel pipe, tpipe, shall not range) are shown in Table 3.2, with specified and minimum
be taken less than that determined by Eq. 1: values for coated steel sheet per ASTM A929 (25). Thicker
pipes (gages 8, 7, and 5) are usually available through special
FH
order. Material costs increase roughly according to the weight
tpipe max Hpfyp cos
0.060 in. shown in the last column of Table 3.2.
Proposed LRFD SGS Eq. 8.15.3.2.2-1 SDCs B, C, and D--Alternative Equation for Pipe
Thickness. The following simplified equations, Eq. C8.15.
In which:
3.2.2-1 and Eq. C8.15.3.2.2-2, may be used to conservatively
FH nh Aspfyh Proposed LRFD SGS Eq. 8.15.3.2.2-2 determine pipe thickness, tpipe, in lieu of calculating the num-
ber of hoops in an equivalent CIP joint, nh, as the basis for
where: determining pipe thickness. This avoids iteration in design cal-
FH =
nominal confining hoop force in the joint (kips) culations, but may result in thicker gage pipe used in design.
Hp =
height of steel pipe (in) Where the principal tensile stress in the joint, pt, specified
fyp =
nominal yield stress of steel pipe (ksi)
=
angle between horizontal axis of bent cap and
in Article 8.15.2.1, is less than 0.11 fc , the thickness of the
pipe helical corrugation or lock seam (deg) steel pipe, tpipe, may be determined from the following:
nh = number of transverse hoops in equivalent CIP joint
Asp = area of one hoop reinforcing bar (in2) fcDcp
tpipe 0.04 Proposed LRFD SGS Eq. C 8.15.3.2.2-1
fyh = nominal yield stress of transverse reinforcement fyp cos
(ksi)
where:
The derivation of this equation is provided in the CPT fc = nominal compressive strength of the bent cap
Attachment. As shown in the design examples provided in the concrete (ksi)
attachments, the spacing of transverse joint hoops can be = average diameter of confined cap pocket fill
Dcp
between corrugated pipe walls (in)
directly related to the number of hoops, nh, by the volumet- fyp = nominal yield stress of steel pipe (ksi)
ric reinforcement ratio for transverse joint hoops, s, using = angle between horizontal axis of bent cap and
Eq. 8.6.2-7 of the 2009 LRFD SGS (1). pipe helical corrugation or lock seam (deg)
Table 3.2. Steel corrugated pipe thicknesses.
Thickness (in)
Gage Pounds per
Number Square Foot
Nominal Tolerance Range Specified Minimum
16 0.0598 0.0648 to 0.0548 0.064 0.057 2.439
14 0.0747 0.0797 to 0.0697 0.079 0.072 3.047
12 0.1046 0.1106 to 0.0986 0.109 0.101 4.267
10 0.1345 0.1405 to 0.1285 0.138 0.129 5.486
8* 0.1644 0.1742 to 0.1564 0.168 0.159 6.875
7* 0.1838 0.1883 to 0.1703 No value No value 7.500
5* 0.2092 0.2162 to 0.2022 No value No value 8.750
*Nonstandard size available by special order
Values refer to coated steel sheet thicknesses per ASTM A929 (25)
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Where the principal tensile stress in the joint, pt, is greater the thinnest required pipe; (2) using the approximate equa-
than or equal to 0.11 fc , the thickness of the steel pipe, tpipe, tions (larger of the two equations, where principal tensile
may be determined from the larger of Eq. C8.15.3.2.2-1 and stress is greater than or equal to 0.11 fc ) usually results in
the following equation: a pipe thickness one gage size larger than that required by
the general equation, using the gage sizes given in Table 3.2;
fyh
Ast Dcp
tpipe 0.14 (3) a reasonable pipe thickness results in all cases; and (4) Eq.
lac 2fyp cos
C8.15.3.2.2-1 governs over Eq. C8.15.3.2.2-2 for all but the
Proposed LRFD SGS Eq. C 8.15.3.2.2-2 largest column diameter (60 in).
Figure 3.5 compares the pipe thicknesses for column lon-
where:
gitudinal steel ratios, Ast/Acol of 0.010, 0.015, and 0.020. This
Ast = total area of column longitudinal reinforcement figure shows the expected significant impact of Ast/Acol on
anchored in the joint (in2)
Dcp = average diameter of confined cap pocket fill required pipe thickness. It also shows that Eq. C8.15.3.2.2-2
between corrugated pipe walls (in) results in thick gage pipes for larger columns, indicating that
fyh = nominal yield stress of transverse reinforcing (ksi) the designer may prefer to use the general equation in such
lac = anchored length of longitudinal column reinforc- conditions to minimize the required pipe thickness.
ing bars into precast bent cap (in)
fyp = nominal yield stress of steel pipe (ksi)
The CPT Attachment provides additional plots that show
= angle between horizontal axis of bent cap and the effect of f c on pipe thickness for 4,000 psi, 6,000 psi, and
pipe helical corrugation or lock seam (deg) 8,000 psi bent cap concrete. The required pipe thickness
increases approximately 10% to 30% with f c based on
The derivations of these equations are provided in an attach- Eqs. C8.15.3.2.2-1 and C8.15.3.2.2-2. For example, for a
ment together with a comparison of the influence of different 36-in diameter column with #6 hoops (Ast/Acol = 0.015), the pipe
variables in these equations. For example, Figure 3.4 com- thickness increases 18% as f c increases from 4,000 psi to 8,000
pares the pipe thicknesses required by Eq. 8.15.3.2.2-1, Eq. psi. Eq. C8.15.3.2.2-1 results in a larger increase of 41% (pro-
C8.15.3.2.2-1, and Eq. C8.15.3.2.2-2. For comparison, column portional to fc ). Eq. C8.15.3.2.2-2 is not dependent on f c.
diameters range from 24 in to 60 in; equivalent hoop sizes vary
according to the column diameter; the column is assumed to SDC A. Cap pocket pipe thickness for SDC A is based on
have a longitudinal steel ratio, Ast/Acol, of 0.015; and the bent cap the minimum provision for transverse reinforcement, as
compressive strength is assumed to be 6,000 psi. Figure 3.4 given in Eq. 8.15.3.2.2-1. Eq. C8.13.3.2.2-1 may be alterna-
reveals that (1) using the general (refined) equation results in tively used.
0.30
Eq. 8.15.3.2.2-1
Eq. C8.15.3.2.2-1
0.25 Eq. C8.15.3.2.2-2
Gage 5
0.20
Pipe Thickness (in)
Gage 7
Gage 8
0.15
Gage 10
Gage 12
0.10
Gage 14
Gage 16
0.05
0.00
#3 #4 #4 #5 #6 #5 #6 #8 #6 #8
Dc = 24 in Dc = 36 in Dc = 48 in Dc = 60 in
Figure 3.4. Pipe thickness versus column diameter (Dc ) and equivalent hoop size
(Ast /Acol = 0.015, f c = 6,000 psi for bent cap).
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76
0.30
Eq. 8.15.3.2.2-1
Eq. C8.15.3.2.2-1
0.25 Eq. C8.15.3.2.2-2
Gage 5
0.20
Pipe Thickness (in) Gage 7
Gage 8
0.15
Gage 10
Gage 12
0.10
Gage 14
Gage 16
0.05
0.00
0.010 0.015 0.020 0.010 0.015 0.020 0.010 0.015 0.020
Ast /Acol
Dc = 36 in Dc = 48 in Dc = 60 in
Figure 3.5. Pipe thickness versus column diameter (Dc ) and column flexural
reinforcement ratio (#6 hoop, f c
= 6,000 psi for bent cap).
3.2.13 Integral Bent Cap Joint Shear Design This requirement is based on an assumed longitudinal force
transfer mechanism and is appropriate for use in both CIP
Per 2009 LRFD SGS, joint shear design is required for SDCs
and precast integral bridge connections. In accordance with
C and D, but not SDC B (1). Where the principal tensile
Figure 8.13.4.2.1-1, for multicolumn bent caps additional
stress, pt, is greater than or equal to 0.11 fc , additional joint reinforcement is required on both sides of the column based
shear reinforcement is required. The 2009 LRFD SGS requires on an assumed transverse force transfer mechanism. These
placement of joint shear reinforcement based on assumed force requirements vary from those presented for nonintegral bent
transfer mechanisms in the longitudinal and transverse direc- caps and must be updated for consistency.
tions. Based on a review of the testing presented in Chapter 2 The recommended modifications to this provision elimi-
and other previous research on precast integral bridge systems, nate the second portion of Figure 8.13.4.2.1-1 for multicolumn
the assumed force transfer mechanism for longitudinal load- bent caps. Instead, this article should reference the recom-
ing is adequate for the system considered. mended provisions of LRFD SGS Article 8.15.5 for vertical stir-
However, the requirements presented in the 2009 LRFD rups inside and outside of the joint. The transverse provisions
SGS for transverse response vary from the requirements for for nonintegral bent caps are described in more detail in sub-
nonintegral transverse response (1). The mechanism for trans- sequent sections of this report.
verse response is the same whether an integral or nonintegral
connection is used. Therefore, there are recommended mod- Horizontal Stirrups
ifications to the integral design provisions to account for these
differences. The 2009 LRFD SGS requires the placement of horizontal
stirrups around the vertical stirrups within the bent cap (1).
Vertical Stirrups These provisions are adequate for integral systems in the longi-
tudinal direction but must also be updated to be in agreement
Requirements specified in the 2009 LRFD SGS for vertical with the nonintegral transverse requirements. The provisions of
stirrups in integral bridge systems are based on longitudinal Article 8.13.4.2.2 of the 2009 LRFD SGS provide a minimum
and transverse loadings. Per Figure 8.13.4.2.1-1 of the 2009 quantity of horizontal stirrups required in addition to spacing
LRFD SGS, for single column bent caps only vertical stirrups requirements. The provisions for nonintegral bent caps specify
are required along both faces of the bent cap extending one- only spacing and size requirements. The recommended modi-
half the column dimension on both sides of the column (1). fication for integral bent caps is the addition of a provision to
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ensure that the horizontal stirrups for multicolumn bent caps The proposed joint shear design approach for nonintegral
also satisfy the nonintegral provisions. precast bent caps and integral bent caps in the transverse direc-
tion follows the same approach, but conservatively requires the
Additional Longitudinal Cap Beam Reinforcement principal tensile stress check for SDC B as well as SDCs C and
D. In addition, the additional joint shear reinforcement differs
Provisions for nonintegral bent caps per the 2009 LRFD in certain regards from CIP requirements. Although vertical
SGS require the placement of additional longitudinal rein- joint stirrups, horizontal J-bars, and additional longitudinal
forcement within the cap beam (1). There is currently no bent cap reinforcement are addressed, other provisions, such as
requirement for the placement of this reinforcement for multi- bedding layer reinforcement and supplementary hoops, are
column integral bent caps. Therefore, it is recommended that included. In addition, minimum joint shear reinforcement--
a provision for integral bent caps that requires the placement both transverse joint reinforcement and vertical joint stir-
of additional longitudinal cap beam reinforcement for multi- rups inside the joint--is conservatively required for all SDC
column bent caps be added. The adequacy of this requirement levels.
for transverse response of nonintegral bent caps is discussed The proposed joint shear reinforcement provisions are
in more detail in a subsequent section. based on precast bent cap specimen response reported in the
work of Matsumoto (21, 22, 23, 26) as well as additional
3.2.14 Nonintegral Bent Cap analysis presented herein. Table 3.3 compares the joint rein-
Joint Shear Design forcement used in the full ductility specimen design to that
Per the 2009 LRFD SGS, joint shear design is required for required by the 2006 LRFD RSGS, which was the original
SDCs C and D, but not SDC B (1). For SDCs C and D, where design basis of specimens, and the 2009 LRFD SGS, the current
AASHTO seismic guide specifications (2, 1). The proposed
the principal tensile stress, pt, is less than 0.11 fc , minimum LRFD SGS for precast bent caps for SDCs C and D is also listed
joint shear (transverse) reinforcement is required. Where pt and compared to the test specimens (Proposed Specification/
is greater than or equal to 0.11 fc , the additional joint shear Test Specimen) and to the 2009 LRFD SGS (Proposed
reinforcement (Asjvi, Asjvo, Asjl, and horizontal J-bars) is required. Specification/2009 LRFD SGS).
Table 3.3. Comparison of joint reinforcement for various seismic guide specifications--SDCs C and D.
Proposed Proposed
Specimen 2006 LRFD 2009 LRFD Proposed Guide Specification Specification
Reinforcement Type Term
Quantity RSGS SGS Specification
Test Specimen 2009 LRFD SGS
max of
Transverse Hoop - A
-A, B 1.00B
0.27 0.20C 0.175 0.175 0.65D 1.00
Vertical Joint
GD: CPFD: GD: CPFD: GD: CPFD:
0.089E - 0.135
0.135 0.12 1.52 1.35 1.00 0.89
Additional Bent
0.0 0.0 0.245 0.245 -F 1.00
Cap Longitudinal
Every other
Every other GD: CPFD:
Horizontal J-bar 0.13G 0.10G intersection -G
intersection in joint 1.00 -H
in joint
Reinforcement per
Bedding Layer Hoop - - - - -I -I
specification
Notes:
A
GD test specimen used hoops close to minimum per 2006 LRFD RSGS, and CPFD used a steel corrugated pipe thickness based on 2006 LRFD RSGS.
B
Typically this will be 1.0, except that the proposed specification requires the larger of 2009 LRFD SGS Eq. 8.13.3-1 and Eq. 8.13.3-2 be used.
C
Placed transversely within Dc from either side of column center line per 2006 LRFD RSGS. Placement was adjacent to joint.
D
Difference was due to change in design requirements and rounding of bar sizes in specimen.
E
Specimen used construction stirrups; 2006 LRFD RSGS did not require Asjvi .
F
Not used because 2006 LRFD RSGS did not require Asjl .
G
Proposed Guide Specification and 2009 LRFD SGS require Asjh in joint for GD; GD specimen used Asjh adjacent to joint per 2006 LRFD RSGS.
H
Horizontal J-bars are not used for cap pocket connections.
I
Specimen did not use hoop in scaled 1- in bedding layer. Bedding layer hoop applies only to precast connections.
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From Table 3.3, it is evident that the joint design require- The full ductility specimens used the more liberal (and
ments became considerably more conservative from the 2006 constructible) placement of joint stirrups outside the joint as
LRFD RSGS to 2009 LRFD SGS and that the differences the more severe condition for investigating joint response,
between the test specimens and the 2009 LRFD SGS reflect permissible by the 2006 LRFD RSGS (2). Rounding stirrup
this (2, 1). As a whole, the proposed joint reinforcement is bar diameters to practical sizes for the test specimens resulted
conservative compared to that used in the test specimens. In in a larger area of vertical stirrups outside the joint region
addition, there is considerable consistency (i.e., a ratio of 1.00 than required. However, two 2-leg construction stirrups with
for Proposed Specification/2009 LRFD SGS) between the a total area of 0.089Ast were included within the joint region,
proposed specifications and 2009 LRFD SGS for SDCs C and as mentioned in Matsumoto (21). As shown in Table 3.3, this
D where principal tensile stress, pt, is greater than or equal to resulted in an area 66% of that required by the 2009 LRFD
0.11 fc . However, there are still considerable differences in SGS (0.135Ast) (1).
design specifications between nonintegral CIP and precast As shown in the strain profiles of Figure 2.48 (CIP), Figure
bent caps, as shown in the following sections. 2.54 (GD), and Figure 2.59 (CPFD), vertical joint stirrups
were highly effective for the CIP and GD specimens for which
maximum joint crack widths were 0.025 in and 0.040 in,
Limits on Bent Cap Depth respectively. This confirms the importance of such stirrups in
Nonintegral precast bent caps are subject to the same bent achieving emulative response. Smaller joint stirrup strains were
cap depth limitations and the alternative design basis required evident for the CPFD specimen, which exhibited much smaller
by Article 8.13.5 of the 2009 LRFD SGS (for CIP bent caps) (1). crack widths and a crack pattern that differed from the CIP and
The proposed provision is the following: GD specimens. In contrast, the CPLD specimen exhibited
severe joint cracking, which is attributed to the absence of joint
Cast-in-place, emulative precast and hybrid bent cap reinforcement, especially joint stirrups. Figure 2.46 and Fig-
beams satisfying Eq. 1 shall be reinforced in accordance ure 2.64 portray the significant effect of the CPLD joint shear
with the requirements of Articles 8.15.5.1 and 8.15.5.2. cracking on joint shear stiffness and system displacement, even
Bent cap beams not satisfying Eq. 1 shall be designed on though the specimen achieved an exceptionally large drift of
the basis of the strut and tie provisions of the AASHTO
5.0% in the presence of cracks up to 0.080 in wide. Outside the
LRFD Bridge Design Specifications and as approved by
the Owner. joint, the GD and CPLD stirrup strains were the largest, 68%
of yield and 61% of yield, respectively.
Dc d 1.25Dc Proposed LRFD SGS Eq. 8.15.5-1 Based on a comparison of GD and CIP results and the
overall GD emulative response, the proposed specification
where: requires full ductility grouted duct connections to use the
Dc = column diameter (in) same joint stirrups inside the joint as required by the 2009
d = total depth of the bent cap beam (in) LRFD SGS (1).
Based on a comparison of the CPFD and CIP response, it is
Vertical Stirrups Inside and Outside the Joint deemed reasonably conservative to require 0.12Ast--12% less
than the CIP requirement (0.135Ast) but 35% more than that
SDCs C and D--Principal Tensile Stress, pt, 0.11 fc or used in the CPFD specimen (0.089Ast), which exhibited min-
Larger. The 2006 LRFD RSGS used for the design of the imal joint distress and exceptional joint performance:
prototype bridge and full ductility test specimens did not dis-
tinguish between integral and nonintegral bent cap systems, Vertical stirrups inside the joint with a total area, Asjvi,
nor between the design of vertical stirrups inside the joint spaced evenly over a length, Dc , through the joint shall
region and outside (i.e., adjacent to) the joint region (2). satisfy:
Based on the work of Sritharan (38), the 2009 LRFD SGS (1)
Asjvi 0.12 Ast Proposed LRFD SGS Eq. 8.15.5.2.3a-1
1
for nonintegral bent caps increased the required total area of
vertical joint stirrups (inside and outside the joint) approxi- Vertical stirrups inside the joint shall consist of dou-
mately 21% over the 2006 LRFD RSGS requirement. In addi- ble leg stirrups or ties of a bar size no smaller than that
tion, Articles 8.13.5.1.1 and 8.13.5.1.2 of the 2009 LRFD SGS of the bent cap transverse reinforcement. A minimum
require placement of 0.175Ast outside the joint (adjacent to of two stirrups or equivalent ties shall be used.
each side of the column) as well as 0.135Ast inside the joint.
These are major changes in joint stirrup requirements over the Due to the presence of the pipe, overlapping double-leg
2006 LRFD RSGS provisions. These provisions should also be vertical stirrups are not practical.
required for the design of integral bent cap systems in the trans- Figure 3.6 shows the minimum number of two-leg vertical
verse direction as the response mechanism is the same. joint stirrups required for values of Ajvi
s /Ast ranging from 0.08
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79
Ast /A g = 0.01 Dc = 36 in Ast /Ag = 0.015 Dc= 36 in Ast /Ag = 0.02 Dc= 36 in
8 8 8
Minimum No. of 2-leg Stirrups
Minimum No. of 2-leg Stirrups
Minimum No. of 2-leg Stirrups
(6.11) (6.87)
(5.15) (5.09)
6 6 6
(4.58) (4.07) (4.43)
(3.05) (3.44) (3.32) (3.28) (3.94) (3.12)
4 4 4
(2.04) (2.54) (2.22) (3.05) (3.82) (2.96) (2.08) (2.34) (2.63) (2.31) (2.78)
(
(11.3
31)
1) (
(11.64)
64) (
(11.97)
97) (
(00.93)
93) (1
(1.16)
16) (1
(1.39) (1.56)
39) (1 56) (1
( 1.97) (
(22.46)
46) (1
( 39)
1.3 (1
9) ( 1.74)
4) (1 85)
(1.85)
2 2 2
0 0 0
#4 #5 #6 #4 #5 #6 #4 #5 #6
Rebar Size (No.) Rebar Size (No.) Rebar Size (No.)
0.08 Asjvi/Ast 0.10 Asjvi/Ast 0.12 Asjvi/Ast 0.135 Asjvi/Ast 8% Asjvi/Ast 10% Asjvi/Ast 0.12 Asjvi/Ast 0.135 Asjvi/Ast 0.08 Asjvi/Ast 0.10 Asjvi/Ast 0.12 Asjvi/Ast 0.135 Asjvi/Ast
Ast /A g = 0.01 Dc= 48 in Ast /A g = 0.015 Dc= 48 in Ast /A g = 0.02 Dc= 48 in
(7.88)
Minimum No. of 2-leg Stirrups
Minimum No. of 2-leg Stirrups
Minimum No. of 2-leg Stirrups
8 8 8
(7.00)
(5.25) (5.91) (5.84) (5.55)
6 6 6
(4.38) (4.16) (4.67) (4.11) (4.94)
(3.50) (3.94) (3.50) (3.08) (3.70) (3.29) (3.09)
4 4 4
(
(2.33)
2.33) (2.92) (2.06) (2.47) (2.78) (2.47) (2.06) (2.32) (2.29) (2.75)
(1.65) (0.92) (1.15) (1.37) (1.55) (1.37) (1.72) (1.83)
2 2 2
0 0 0
#5 #6 #8 #5 #6 #8 #5 #6 #8
Rebar Size (No.) Rebar Size (No.) Rebar Size (No.)
0.08 Asjvi/Ast 0.10 Asjvi/Ast 0.12 Asjvi/Ast 0.135 Asjvi/Ast 0.08 Asjvi/Ast 0.10 Asjvi/Ast 0.12 Asjvi/Ast 0.135 Asjvi/Ast 0.08 Asjvi/Ast 0.10 Asjvi/Ast 0.12 Asjvi/Ast 0.135 Asjvi/Ast
Figure 3.6. Minimum number of 2-leg stirrups inside the joint--36-in and 48-in diameter columns.
to 0.135, based on several stirrup sizes. Results are shown for symmetrically. Three sets of stirrups can be placed symmetri-
36-in and 48-in diameter columns and for column longitudi- cally when column bars are rotated a half turn (to avoid con-
nal reinforcement ratios, Ast/Ag, of 0.01, 0.015, and 0.02. The flict); however, conflict between bent cap longitudinal bars and
height of the bar indicates the number of required stirrups, column bars and/or corrugated ducts should also be avoided.
subject to the proposed specification minimum of two stir- Based on a comparison of CIP results with GD and CPFD
rups. Additionally, the number in parentheses at the top of response and the overall GD and CPFD emulative response,
the bar indicates the calculated stirrup requirement (i.e., the proposed specification requires grouted duct and cap
without rounding or the 2-stirrup minimum). Figure 3.6 pocket full ductility connections to use the same joint-related
shows that both the 0.135Ast requirement for grouted ducts vertical stirrup area outside the joint (Ajvo
s ) as required by the
and the 0.12Ast requirement for cap pocket connections pro- 2009 LRFD SGS (1).
duce a number of stirrups that can be reasonably satisfied in
design and construction. SDCs C and D--Principal Tensile Stress, pt, Less than
Although the design requirement can become significant 0.11 f c . Per the 2009 LRFD SGS, where the principal
for larger percentages of column longitudinal reinforcement
(Ast), potential congestion can be alleviated by the use of larger tensile stress, pt, for a CIP connection is less than 0.11 fc ,
stirrup bar sizes. Grouted duct connections require a larger only minimum joint transverse (hoop) reinforcement is
required; vertical joint stirrups are not required (1). As men-
area of vertical stirrups than cap pocket connections, and as
tioned previously, additional joint shear reinforcement for
shown in Figure 3.6, this sometimes results in a larger number
precast bent caps should be based on principal tensile stress
of stirrups; however, the area requirement can be satisfied by
using overlapping two-leg stirrups. Cap pocket connections exceeding 0.11 fc (i.e., likely joint shear cracking). How-
accommodate only two-leg stirrups; therefore, it is important ever, given the inherent variability in actual bridge fabrica-
that the designer carefully consider using larger stirrup sizes and tion, assembly, and seismic response and the potentially
possibly bundling stirrups, especially when a larger column severe impact of joint shear cracking on intended emulative
reinforcement ratio is used. In addition, care should be taken in response, the proposed specifications conservatively require
design to ensure that the number and placement of stirrups a minimal area of vertical joint stirrups in grouted duct and
over the top opening of the pocket does not unduly interfere cap pocket connections in SDCs C and D, even where the
with concrete placement in the pocket during the assembly principal tensile stress, pt, is less than 0.11 fc . This provision
operation. For both connection types, stirrups should be placed is expected to be more commonly associated with SDC B and
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is therefore addressed in the next section (SDC B). Vertical cap reinforcement, which is prescribed in the 2009 LRFD SGS
stirrups outside the joint are not required. (1) for nonintegral bent caps as follows:
SDC B. Where the principal tensile stress, pt, for an SDC B Longitudinal reinforcement, As jl, in both the top and
bottom faces of the cap beam shall be provided in addi-
bridge using a precast bent cap connection is 0.11 fc or tion to that required to resist other loads. The addi-
larger, the joint shear design provisions for SDCs C and D are tional area of the longitudinal steel shall satisfy:
conservatively required. Where the principal tensile stress is
Asjl 0.245 Ast Proposed LRFD SGS Eq. 8.13.5.1.3-1
less than 0.11 fc , the following minimum provision for ver-
tical stirrups in the joint must be satisfied for grouted duct Maximum bent cap longitudinal bar strains for the speci-
connections: mens were limited to 46% of yield for CIP and 53% of yield for
GD, but exceeded yield for CPFD and CPLD. The area of the
Vertical stirrups with a total area, A jvi
s , spaced evenly CPLD bent cap longitudinal reinforcement was reduced 30%
over a length, Dc, through the joint shall satisfy: from the CPFD, which contributed to the extent of yielding, as
discussed in Matsumoto (26).
Asjvi 0.10 Ast Proposed LRFD SGS Eq. 8.14.5.2.2a-1
1
Based on specimen response, the proposed specifications
Vertical stirrups inside the joint shall consist of dou- do not modify the 2009 LRFD SGS (1). Therefore, this addi-
ble leg stirrups or ties of a bar size no smaller than that tional reinforcement is required where the principal tensile
of the bent cap transverse reinforcement. A minimum
stress, pt, for a precast bent cap connection is 0.11 fc or
of two stirrups or equivalent ties shall be used.
larger.
As shown in the example connection details for cap pocket
This limited provision, which still results in a highly con- connections for SDCs B, C, and D provided in the attachments,
structible joint region, provides reinforcement to restrict joint inverted U-bars or hairpins may be placed within the pocket to
shear effects in the event of joint shear cracking. As shown in help restrain potential splitting cracks and buckling of top bent
Figure 3.6, joints will typically require only two to three 2-leg cap flexural bars within the joint. This additional conservative
stirrups. Vertical stirrups outside the joint are not required. measure is optional but recommended where the principal
The Ajvi
s requirement for cap pocket connections is identi-
cal to that for the grouted duct connections. tensile stress, pt, is 0.11 fc or larger. It is not required for
grouted duct connections where overlapping vertical stirrups
SDC A. For SDC A, the principal tensile stress, pt, is not within the joint can serve the same purpose.
calculated. However, the following minimum provision for
SDC B. As for other additional joint shear reinforcement,
vertical stirrups in the joint conservatively applies for grouted
the additional longitudinal bent cap reinforcement stipulated in
duct connections:
the previous section for SDCs C and D is conservatively
Vertical stirrups with a total area, Asjvi, spaced evenly
required for SDC B where the principal tensile stress, pt, for a
over a length, Dc, through the joint shall satisfy: precast bent cap connection is 0.11 fc or larger.
Asjvi 0.08 Ast Proposed LRFD SGS Eq. 8.13.4.2.2a-1
1 SDC A. For SDC A, additional longitudinal cap beam
reinforcement is not required.
Vertical stirrups inside the joint shall consist of dou-
ble leg stirrups or ties of a bar size no smaller than that
of the bent cap transverse reinforcement. A minimum
Horizontal J-Bars
of two stirrups or equivalent ties shall be used.
SDCs C and D. In accordance with the 2006 LRFD RSGS
As shown in Figure 3.6, the minimum 2-stirrup require- (2), horizontal J-bars with an area, Asjk, of at least 0.10Ast was
ment is expected to govern. used in the CIP and GD specimens together with Asjv re-
The provision for cap pocket connections is identical to inforcement adjacent to the joint (within Dc/2 of column face).
that for the grouted duct connections. The 2009 LRFD SGS for nonintegral bent caps modified this
requirement as follows (1):
Additional Longitudinal Cap Beam Reinforcement Horizontal J-bars hooked around the longitudinal
reinforcement on each face of the cap beam shall be
SDCs C and D. The 2006 LRFD RSGS (2) used in the provided as shown in Figure 8.15.5.1.1-1. At a mini-
design of the prototype bridge and emulative test specimens mum, horizontal J-bars shall be located at every other
did not include the significant additional longitudinal bent vertical-to-longitudinal bar intersection within the joint.
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The J-dowel reinforcement bar shall be at least a #4 modate fabrication and placement tolerances. Transverse
size bar. reinforcement around the column bars within the bedding
layer provides confinement and reduces the unsupported
This provision is included in the proposed LRFD SGS for length of column bars, thereby reducing the potential for buck-
grouted duct connections, when the principal tensile stress, pt, ling during plastic hinging of the column. Accurate place-
for a precast bent cap connection is 0.11 fc or larger. How- ment of reinforcement is, therefore, essential to achieving the
ever, based on specimen response, cap pocket connections expected system ductility capacity. Reinforcement should nor-
were shown not to require horizontal J-bars. mally be placed evenly through the depth of the bedding layer.
However, in some cases, an uneven bedding layer (e.g., a slop-
SDC B. Where the principal tensile stress, pt, for a precast ing bent cap on top of a large diameter column) or a bedding
bent cap connection is 0.11 fc or larger, J-bars stipulated for layer of an unusual shape may be encountered, requiring place-
SDCs C and D are similarly required for grouted duct con- ment of bedding layer reinforcement that is not uniformly dis-
nections in SDC B. tributed, to minimize the unsupported length of column bars.
In all cases, plan sheets should show the intended placement of
SDC A. For SDC A, horizontal J-bars are not required. the bedding layer reinforcement. The associated requirement
for shop drawings is addressed in proposed Article 8.13.8.4.4 of
Supplementary Hoops for Cap Pocket Connections the AASHTO LRFD Bridge Construction Specifications (LRFD
BCS) (35). Adequate flowability of the concrete fill or grout
SDCs C and D. The CPFD specimen response demon- should not be prevented by the size and placement of the bed-
strated the effectiveness of a supplementary hoop placed at ding layer reinforcement. Matsumoto et al. (8) present an alter-
each end of the steel pipe to limit dilation and potential native approach to accommodating tolerances and enhancing
unraveling. This reinforcement, which matched the column durability by embedding the column or pile into the bent cap.
hoop bar size, reached up to 52% of yield during the test,
indicating its contribution to joint performance. Therefore, The proposed design specification is as follows:
where the principal tensile stress, pt, is 0.11 fc or larger, Bedding layers between columns and precast bent
cap pocket connections in SDCs C and D require the use of caps shall be reinforced with transverse reinforcement,
supplementary hoops: as shown in Figure 8.15.5.2.2-1 and Figure 8.15.5.2.3-1.
Bedding layer reinforcement shall match the size and
A supplementary hoop shall be placed one inch from type of transverse reinforcement required for the col-
each end of the corrugated pipe. The bar size of the umn plastic hinging region and shall be placed evenly
hoop shall match the size of the bedding layer rein- through the depth of the bedding layer.
forcement required by Article 8.15.5.2.1. Grout bedding layer heights shall not exceed 3 in.
The hoop area meeting the requirement of the bedding layer For seismic loading scenarios, the bedding layer thickness is
reinforcement (and column hoop) is considered sufficient. limited to 3 in when constructed of a cementitious grout
Where the principal tensile stress, pt, is less than 0.11 fc , hoops material. Grout properties do not show the same improve-
are not required but may be conservatively included. ment with lateral confinement that concrete materials show.
Increasingly large grout bedding layer thicknesses can result in
SDC B. As for the additional joint shear reinforcement, the development of poor lateral response due to the degrada-
supplementary hoops stipulated in the previous section for tion of the bedding layer. Therefore, the use of grout materials
SDCs C and D are conservatively required for SDC B where the is limited to joints 3 in or less in dimension.
principal tensile stress, pt, for a precast bent cap connection is Lateral Reinforcement Requirement for Columns
0.11 fc or larger. Where the principal tensile stress, pt, is less Connecting to a Precast Bent Cap. Uniform spacing between
than 0.11 fc , supplementary hoops may be optionally in- hoops at the top of the column and the bedding layer is crit-
cluded as a simple, inexpensive, and conservative measure. ical to ensuring that system ductility is not compromised.
Hoop spacing is addressed in Article 8.8.14 of the proposed
SDC A. For SDC A, supplementary hoops are not required. LRFD SGS and Article 8.13.8.4.4 of the proposed LRFD BCS.
A smaller cover than that used for typical column applications
Reinforcement at the Bedding Layer is permitted for the top hoop because the placement of the
bedding layer concrete or grout will provide additional cover
and Top of Column
after the precast bent cap is set. Plan sheets and shop draw-
Bedding Layer Reinforcement. A bedding layer between ings are required to show the intended placement of the first
the bent cap soffit and the top of column is used to accom- hoop at the top of the column.
