DRAFT
Caltrans Earthquake Committee
MTD 20-20
Column and PC Girder Seismic Connections
Disclaimer:
The opinions presented herein are those of
the Earthquake Committee (EQC) and do not
necessarily reflect official Caltrans policy. In
no event shall the authors be liable for
incidental or consequential damages arising
out of the use of this document.
DRAFT
MEMO
TO
DESIGNERS 20-20
●
MARCH 2019
NEW DRAFT MEMO – 3/25/2019
20-20
Column and PC Girder Seismic Connections
General
This memorandum provides requirements and guidelines for seismic bent cap connections of
precast concrete (PC) elements. Cast-in-place (CIP) and precast bridges are expected to meet
the seismic performance requirements found in Caltrans’ Seismic Design Criteria (SDC)
regardless of method of construction (ABC or conventional).
PC elements can be assembled with other PC members or in combination with CIP components
(see Figure 1). For example, PC girders on PC bent caps, PC girders on CIP bent caps, or PC
columns on CIP footings/shafts are combinations that do not negate the need to meet all
applicable performance requirements. This memorandum focuses primarily on the seismic
aspects of component connections.
Figure 1: Integral connection of 154 foot PC Girders with CIP bent cap and columns
Riego Road OC, Bridge 18-0050 (2014)
PC Girder Continuity at Bents
PC girder continuity through bent caps improves the seismic redundancy of traditional PC
connections, and when combined with top of column integral connections allows for pin-atbase column designs that can reduce foundation costs. This newly developed framing
technology enables PC to be more competitive with CIP construction.
Background
The need for improving girder continuity through bent caps surfaced after the 1989 Loma
Prieta earthquake where evidence of seismic damage at the girder end region was discovered
20-20 COLUMN AND PC GIRDER SEISMIC CONNECTIONS
©2019 California Department of Transportation ALL RIGHTS reserved.
1
DRAFT
MEMO
TO
DESIGNERS 20-20
●
MARCH 2019
(see Figure 2). The integrity of spans were at risk when transverse girder web (stab) bars pulled
away from diaphragms. This behavior does not meet SDC requirements which state that
superstructures, joints, and bent caps (capacity protected members) are to remain essentially
elastic.
Figure 2: Napa River Bridge (23-0064) diaphragm damage
Loma Prieta earthquake, 1989
This example of non-elastic behavior shows the need for improving this area of practice when
considering the number of PC girder structures being built and the variability in the
magnitude (probabilistic nature) of seismic demands. Ensuring the soundness of girder-togirder continuity through research, prescriptive requirements, and detailed analysis greatly
reduces the risk of structure seismic damage in capacity protected members.
Continuity of superstructures and adequate framing is critical to the seismic performance of
bridge systems. Caltrans’ PC superstructures have traditionally been constructed on drop-caps
or inverted ‘T’ bent caps and coupled by bent cap diaphragm pours. The diaphragm pours
have encapsulated transverse ‘stab bars’ placed through preformed holes near the girder ends
to help facilitate longitudinal continuity through the support bent. This was followed by the
deck pour. Continuity provided by the CIP reinforced concrete deck provides a large negative
longitudinal bending moment capacity through the bent cap. However, lack of continuous
steel at the bottom of girders through the bent cap has traditionally been a shortcoming of PC
bridges when compared to CIP structures.
Expected Performance of PC Girder Continuity Details
Caltrans SDC specifies that columns, as seismic critical members, be designed and detailed
to deform inelastically while superstructures remain essentially elastic. No exceptions are
allowed for PC superstructures. To investigate this topic, Caltrans sponsored two research
projects at Iowa State University (2011, 2015) to investigate PC girder continuity
20-20 COLUMN AND PC GIRDER SEISMIC CONNECTIONS
©2019 California Department of Transportation ALL RIGHTS reserved.
2
DRAFT
MEMO
TO
DESIGNERS 20-20
●
MARCH 2019
performance. The first project (see Figure 3) revealed that the service level performance of
traditional detailing is satisfactory, but the seismic performance degrades at large ductility
demand levels. The study resulted in design recommendations to improve seismic
characteristics through the joint, especially for integral bents designed structurally fixed at
top of column. This was accomplished by providing positive moment continuity steel in the
form of strand through the bent cap and bottom flanges of PC girders. Reference to strand
herein denotes various diameters of low relaxation prestress steel having fu=270,000 psi.
Figure 3: Testing of existing connection detail (right) and newer
integral strand connection (left) under high ductility (2011)
The second research project was more elaborate, and tested six girder continuity connection
alternatives with integral column-to-cap details. All specimens were designed to resist the
column overstrength moment per SDC and were exercised to failure. Plastic hinging was
shown to have occurred in the column while superstructure elements remained essentially
elastic. Test results and details for these integral connections were subsequently reviewed by
the EQC and language for PC girder connections was added to the SDC. Requirements for
girder-to-girder continuity using pin-at-top and fix-at-top columns were added to the SDC as
improvements to seismic design practice for PC construction. The six connection details for
girder-to-girder continuity across bent caps are shown in Figure 4.
Though all six PC connections performed to seismic test requirements, designers must consider
the complexity, construction difficulty, estimated cost, and aesthetic ramifications for each
project site.
20-20 COLUMN AND PC GIRDER SEISMIC CONNECTIONS
©2019 California Department of Transportation ALL RIGHTS reserved.
3
DRAFT
MEMO
TO
DESIGNERS 20-20
●
MARCH 2019
Figure 4: Six ISU Tested connection details using steel PS strands (2015)
Connections are separated into four fixity categories; Type A, B, C, and D (SDC 7.2.1.2). Type
A features a substructure that is integral with the superstructure, with positive and negative
moment continuity across the bent cap. Girder embedment into the diaphragm must be a
minimum of 18 inches, with 12 inch minimum seats for dapped end girders. Type B features a
pin (or isolated) top of column connection and superstructure positive and negative moment
continuity across the bent cap. Girder embedment and seat requirements are the same as for
Type A. Type C (allowed only for PGA < 0.6g) features a top of column pin connection and
only negative moment superstructure continuity across the bent cap. Girder embedments and
seats must again be 18 inches. Type D connections where the column is fully fixed into the
superstructure with only negative moment continuity should no longer be practiced.
20-20 COLUMN AND PC GIRDER SEISMIC CONNECTIONS
©2019 California Department of Transportation ALL RIGHTS reserved.
4
DRAFT
MEMO
TO
DESIGNERS 20-20
●
MARCH 2019
Analysis Considerations
Dead load (DL) moment distribution for precast girder systems is not typically the same as that
for CIP box construction. CIP box construction features falsework that supports all dead load
until stressing and curing is complete. Once released, DL is allowed to induce stress into the
cap region. In contrast to this, PC systems typically have girders placed as simply supported
beams prior to concrete being poured in bent cap regions, and this construction method
difference creates a different DL moment demand when compared to CIP construction (see
Figure 5). The relatively small PC negative DL moment does not help offset the positive
earthquake moment in the joint in this case.
Figure 5: CIP vs PC Superstructure Dead Load Moment Envelope
Comparison at time of construction; bent cap poured last
Subsequent placement of deck concrete with longitudinal deck reinforcement on PC girders
and over the bent cap provides negative moment capacity, but no additional positive moment
capacity. For any column overstrength moment (Mo), the superstructure must be capacity
protected in both negative and positive bending at the joint. For longitudinal earthquake
moment demands on PC systems, the demand quickly overcomes the small DL moment in the
joint. Therefore, the net effect is a positive earthquake moment appearing in the joint. If no
positive steel is provided, the PC girder may pull away from the joint.
The designer must analyze the structure using seismic push analysis to determine the amount
of positive and negative reinforcement required to force plastic hinging into the top of column
when designing an integral connection. Negative reinforcement must be provided in the form
of deck reinforcement or top of girder reinforcement. Positive reinforcement shall be provided
by extended test-approved strand configurations. Alternatively, mild reinforcement made
20-20 COLUMN AND PC GIRDER SEISMIC CONNECTIONS
©2019 California Department of Transportation ALL RIGHTS reserved.
5
DRAFT
MEMO
TO
DESIGNERS 20-20
●
MARCH 2019
continuous between girders shall be used as in Figure 6 where #36 (#11) bars with ultimate
splice couplers provided positive moment capacity through the bent cap. Note that this
reinforcement is in addition to girder end hair-pins wrapped around transverse stab bars.
Girder continuity design examples with details are provided as attachments B and C to this
memorandum. Research and finite element analysis has shown that the total positive and
negative moment capacities are greatly influenced by an interaction of reinforcement and shear
friction resistance by way of clamping action along the sides of PC girders. The design
examples accounts for this shear friction by conservatively reducing the positive moment
demand by 20% for select continuity strand configurations.
Figure 6: Mild Steel Continuity – San Mateo Bridge (Widen), Br. 35-0054, 1999
Girder Connection Detailing
Positive continuity reinforcement not less than the equivalent of four PS strands of 0.5 inch
diameter or four #8 mild reinforcement bars (two from each side of each girder) must be
anchored into the cap. When strand systems are used, the radius of bends must be shown on
the plans. See XS1-123-1 for strand detail examples on PC/Pretensioned Wide Flange Girder.
Continuity research has shown that PS wires may break if bent at 90 degree angles.
Manufacturers have indicated that strand can be bent to a minimum radius of 15 strand
diameters before significantly reducing its tensile load capacity. Girders using harped strand
will require completely separate bottom of girder anchorage. Note that some continuity
systems using strand must feature end anchorage, be grouted in tubes with sufficient
development length, or demonstrate adequate development length. Caltrans sponsored
research at Iowa State University (2015) indicates that approximately 128 strand diameters are
required to develop straight strand in a grouted tube (alternative C1, Figure 4).
20-20 COLUMN AND PC GIRDER SEISMIC CONNECTIONS
©2019 California Department of Transportation ALL RIGHTS reserved.
6
DRAFT
MEMO
TO
DESIGNERS 20-20
●
MARCH 2019
Where possible, continuity steel is to be spaced such that it is surrounded by concrete (not in
contact with other steel) as it exits the girder. Attention must be paid to checking and detailing
tolerances between components for proper fit up and to reduce potential construction delays
and claims. Columns should ideally be placed between PC girder lines to reduce congestion.
Girder Connection Construction Considerations
If PC girders are anchored to the CIP diaphragm before wet deck concrete is poured, the
additional DL and girder end rotation may produce excessive negative moment demand in and
near the joint. Girder end damage may result from these construction moments, even if post
tensioning is planned. This can be accounted for in the design by providing additional negative
moment reinforcement in the top of PC girders, or by showing a deck pour sequence in the
project plans. The situation is more pronounced on longer spans where girders will tend to
have more end rotation. Designers typically include an intermediate diaphragm for PC girders
longer than 80 feet, and multiple diaphragms for girders over 120 feet (MTD 11-8) to provide
better load distribution and construction stability, but contractors often desire to stabilize the
girder ends as well. Pouring the entire bent cap for end stability before placing deck concrete
may contribute to the negative moment demand before deck reinforcement can resist this
moment.
Designers should calculate expected end rotation and be prepared to consider alternatives for
girder end stability that do not adversely affect the structure as required. One such method that
has been used is to pour only the lower portion of the bent cap for girder flange stability,
followed by the deck pours and lastly the remainder of the bent cap. This process also provides
less locked in joint stresses for future earthquake demands.
In general, projects with longer spans and/or narrower bent caps should require more time
between girder fabrication and bent cap diaphragm pour. This is to help account for any
shrinkage effects that may create additional tension in the continuity steel once anchored.
Girder creep and shrinkage can have a significant impact on continuity strand tension if
anchorage pours are completed early in the age of girders. Provisions are to be made to account
for restraint moments per AASHTO LRFD 5.14.1.4.4 and 5.14.1.4.2. The current target girder
age at time of continuity pour for optimal moment reduction is approximately 90 days.
Integral Connections to Columns
For integral connections where the column-to-cap and girder-to-girder connections are
designed to result in a fixed condition, plastic hinging is expected to form just below the
superstructure. Per SDC, the total nominal superstructure capacity (Mne) shall be at least 20%
greater than the column plastic moment capacity. Location of plastic hinge formation is to be
verified by longitudinal pushover analysis as shown in the attached PC girder continuity design
examples. Design modifications to meet this requirement shall not jeopardize AASHTO LRFD
service load design requirements.
20-20 COLUMN AND PC GIRDER SEISMIC CONNECTIONS
©2019 California Department of Transportation ALL RIGHTS reserved.
7
DRAFT
MEMO
TO
DESIGNERS 20-20
●
MARCH 2019
General
For CIP columns, the main longitudinal reinforcement must be anchored into the bent cap to
ensure a fixed connection. Standard practice is to anchor the main longitudinal reinforcement
by extending to the bottom of deck reinforcement. This assists in meeting joint shear
requirements.
For PC columns, the main longitudinal reinforcement and confinement steel must be cast in
the outer hollow shell which is then filled after column placement. Columns must be filled,
and Self Consolidating Concrete (SCC) may be used to avoid vibrating. Alternatively, solid
columns may be cast with reinforcement placed similar to CIP construction with main bars
extending from column end(s). Size and weight are the main factors in considering hollow
filled or solid PC columns. The method of anchorage into the bent cap and/or footing shall
result in at least one fixed (moment resisting) end.
Fixed End PC Column Connections
80
350
70
300
60
250
50
200
40
CIP
HCPP
GCPP
PNC
30
20
10
0
0
1
2
3
4 5 6 7
Drift (%)
HCNP
GCNP
GCDP
HCS
8
9 10 11
150
100
50
Lateral Force (kN)
Lateral Force (kips)
Formation of plastic hinges outside the superstructure and/or footing are critical to the seismic
performance of bridge systems. Caltrans has completed extensive testing to evaluate numerous
filled PC shell connection alternatives. These have included connections with and without
couplers, and coupler specimens with and without pedestals in an attempt to elevate couplers
away from the plastic hinge zone. Specimens with ultimate splice couplers, service splice
couplers, and continuous bars were tested at the University of Nevada at Reno (2014). A
comparison of test results is shown in Figure 7.
0
Figure 7: Comparative test results (UNR 2014) for cyclic monotonic tests
of filled PC column shell connections [Ref. 1]
20-20 COLUMN AND PC GIRDER SEISMIC CONNECTIONS
©2019 California Department of Transportation ALL RIGHTS reserved.
8
DRAFT
MEMO
TO
DESIGNERS 20-20
●
MARCH 2019
Upon consideration of all relevant factors, Caltrans accepted use of the Plunged-No-Coupler
(PNC) connection. This connection provides superior strength and ductility to near 9% drift,
is far simpler to construct, is less expensive, provides more construction tolerance, and is in
keeping with Caltrans’ SDC which stipulates couplers shall not be used in plastic hinge zones.
As tested, the connection involves plunging (in the case of footings) or placing concrete over
(in the case of PC bent cap) extended column bars into steel ducts which are filled with high
strength grout or Ultra High Performance Concrete (UHPC). Duct diameter requirements and
thicknesses are provided in Attachment A. Figure 8 shows plunging of extended PC shell
column bars into footing ducts filled with UHPC. It is important to include a partial debond
length of 8db on main column bars starting at the interface of materials when using UHPC.
This compensates for bar strain loss when embedded in the much stronger UHPC material.
Figure 8: Construction of filled PC shell-to-footing connection with no
couplers (PNC) using continuous bars with partial debonding
Non-Integral PC Column Connections
Columns designed as pinned at one end have the advantage of transmitting little or no moment
to capacity protected members. Pinned connections can feature solid or filled pipe sections
with cover pipes to ensure no moment transmittal, or reduced diameter reinforcement cages
that transmit minimal moment. Pin connections transmitting any column moment into adjacent
members shall account for the moment in the design of the adjacent member.
Structure isolation may also be considered as a connection at top of columns. For details, see
MTD 20-22.
20-20 COLUMN AND PC GIRDER SEISMIC CONNECTIONS
©2019 California Department of Transportation ALL RIGHTS reserved.
9
DRAFT
MEMO
TO
DESIGNERS 20-20
●
MARCH 2019
PC Column and Shaft Connections
Details for PC columns used in Type II Shafts must be approved by Caltrans. Minimum
features shall include a joint region with overlap of column and shaft reinforcement cages per
SDC, a dry pour construction condition, and feature a roughened exterior contact surface with
3 ½ inch (minimum) clearance to shaft main reinforcement. Joint region lap length is based
on Type II column/shaft tests (UCSD 2015). Figure 9 is a detail approved by the Caltrans
Substructure Committee.
20-20 COLUMN AND PC GIRDER SEISMIC CONNECTIONS
©2019 California Department of Transportation ALL RIGHTS reserved.
10
DRAFT
MEMO
TO
DESIGNERS 20-20
●
MARCH 2019
Figure 9: Approved details for connecting PC Column/Shell in Type II Shaft
20-20 COLUMN AND PC GIRDER SEISMIC CONNECTIONS
©2019 California Department of Transportation ALL RIGHTS reserved.
11
DRAFT
MEMO
TO
DESIGNERS 20-20
●
MARCH 2019
References:
1. Tazarv, Mostafa, Saiidi, M. Saiid, (2014) Next Generation of Bridge Columns for
Accelerated Bridge Construction in High Seismic Zones, Report No. CA 14-2176,
CCEER 14-06, Center for Civil Engineering Earthquake Research, University of
Nevada Reno, Department of Civil and Environmental Engineering, Reno NV
2. Vander Werff, Justin, Peggar, Robert, Cheng, Zhao and Sritharan, Sri, (2015) Seismic
Performance of Precast Girder-to-Cap Connections for Accelerated Bridge
Construction of Integral Bridges, Report No. CA16-2265, Iowa State University of
Science and Technology, Department of Civil, Construction and Environmental
Engineering
3. R. Snyder, J. VanderWerff, Z. Thiemann, S. Sritharan, J. Holombo (2011) Seismic
Performance of an I-Girder to Inverted-T Bent Cap Connection, Report No. 05-0160,
Iowa State University of Science and Technology, Department of Civil, Construction
end Environmental Engineering
4. US Army Corps of Engineers, Engineering Research and Development Center
(September, 2009): High Strain-Rate Testing of Mechanical Couplers, ERDC TR09-8
5. Caltrans Contract 59A0710 (July, 2013) Required Embedment Length of Column
Reinforcement Extended into Type II Shafts. SSRP-13/05
6. State of Washington Department of Transportation girder continuity documents (July
2012) http://www.wsdot.wa.gov/eesc/bridge/designmemos/08-2012.htm
7. Precast/Prestressed Concrete Institute (1999), Manual for Quality Control For Plants
and Production of Structural Precast Concrete Products, PCI MNL 116-99, 4th Ed.
8. Precast/Prestressed Concrete Institute (2008), Tolerance Manual for Precast and
Prestressed Concrete Construction, PCI MNL 135-00
9. AASHTO, LRFD Bridge Design Specifications with California Amendments, Sixth
Edition, 2007, 2012
10. University of Missouri (1974), End Connections of Pre-tensioned I-Beam Bridges,
Study Number 72-2. In cooperation with U.S. Department of Transportation Federal
Highway Administration, November
11. National Cooperative Highway Research Program, (2004), Connection of SimpleSpan Precast Concrete Girders for Continuity, Report 519 TRB
12. National Cooperative Highway Research Program, (2010), Development of Precast
Bent Cap Systems for Seismic Regions, Project No. 12-74
_______________________________
Thomas A. Ostrom
State Bridge Engineer
Deputy Division Chief, Structure Policy & Innovation
Division of Engineering Services
20-20 COLUMN AND PC GIRDER SEISMIC CONNECTIONS
©2019 California Department of Transportation ALL RIGHTS reserved.
12