2011
AASHTO Guide Specifications for
LRFD Seismic
Bridge Design
2015 Interim Revisions
2nd Edition
Published by the
American Association of State Highway and Transportation Officials
ISBN: 978-56051-621-7 Publicaton Code: LRFDSEIS-2-I3
© 2014 by the American Association of State Highway and Transportation Officials.
All rights reserved. Duplication is a violation of applicable law.
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© 2013 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is
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ISBN: 978-1-56051-621-7 Pub Code: LRFDSEIS-2-I3
© 2014 by the American Association of State Highway and Transportation Officials.
All rights reserved. Duplication is a violation of applicable law.
2015 INTERIM REVISIONS TO AASHTO GUIDE
INSTRUCTIONS AND INFORMATION SPECIFICATIONS FOR LRFD SEISMIC BRIDGE DESIGN
INSTRUCTIONS AND INFORMATION
General
AASHTO has issued interim revisions to AASHTO Guide Specifications for LRFD Seismic Bridge Design, Second
Edition (2011). This packet contains the revised pages. They are not designed to replace the corresponding pages in the
book but rather to be kept with the book for quick reference.
Affected Articles
Underlined text indicates revisions that were approved in 2014 by the AASHTO Highways Subcommittee on Bridges
and Structures. Strikethrough text indicates any deletions that were likewise approved by the Subcommittee. A list of
affected articles is included below.
All interim pages are printed on pink paper to make the changes stand out when inserted in the second edition binder.
They also have a page header displaying the section number affected and the interim publication year. Please note that
these pages may also contain nontechnical (e.g. editorial) changes made by AASHTO publications staff; any changes of
this type will not be marked in any way so as not to distract the reader from the technical changes.
Please note that in response to user concerns, page breaks are now being added within sections between
noncontiguous articles. This change makes it an option to insert the changes closer to the affected articles.
20145 Changed Articles
SECTION 1: INTRODUCTION
1.3
SECTION 4: ANALYSIS AND DESIGN REQUIREMENTS
4.6
iii
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2015 INTERIM REVISIONS TO AASHTO GUIDE
SECTION 1 SPECIFICATIONS FOR LRFD SEISMIC BRIDGE DESIGN
SECTION 1: INTRODUCTION
1.3—Flowcharts
Revise Figure 1.3-1 as follows:
PRELIMINARY DESIGN BRIDGE SDC A
TYPE SELECTION AND DESIGN AND SINGLE
FOR SERVICE LOADS SPAN BRIDGES
APPLICABILITY OF SPECIFICATIONS ESTABLISH BRIDGE
ARTICLE 3.1 GEOMETRY AND
SUPPORT
ARTICULATIONS
YES TEMPORARY
ARTICLE 3.6
BRIDGE
DETERMINE
DESIGN FORCES
NO
ARTICLE 4.6/4.5
PERFORMANCE CRITERIA
ARTICLE 3.2
DETERMINE MINIMUM
FOUNDATION INVESTIGATION SUPPORT LENGTH
ARTICLE 6.2 ARTICLE 4.12
LIQUEFACTION EVALUATION COLUMN DETAILING FOR
ARTICLE 6.8 SDC A IF S D1 ≥ 0.1
ARTICLE 8.2
EARTHQUAKE RESISTING SYSTEMS
(ERS) REQUIREMENTS FOR SDC C & D
FOUNDATION DESIGN FOR
ARTICLE 3.3 FOR SINGLE-SPAN BRIDGES
FIGURE1.3-5
DETERMINE DESIGN RESPONSE SPECTRUM
ARTICLE 3.4
COMPLETE
DETERMINE SEISMIC DESIGN CATEGORY (SDC)
ARTICLE 3.5
Figure 1.3-1—Applicability of the Guide Specifications and the Seismic Design Procedure for Bridges in SDC A
and Single-Span Bridges
1
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2015 INTERIM REVISIONS TO AASHTO GUIDE
SECTION 1 SPECIFICATIONS FOR LRFD SEISMIC BRIDGE DESIGN
2
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2015 INTERIM REVISIONS TO AASHTO GUIDE
SECTION 4 SPECIFICATIONS FOR LRFD SEISMIC BRIDGE DESIGN
SECTION 4: ANALYSIS AND DESIGN REQUIREMENTS
Please revise article 4.6 as follows:
4.6—DESIGN REQUIREMENTS FOR SEISMIC C4.6
DESIGN CATEGORY A
These provisions arise because, as specified in
Articles 4.1 and 4.2, seismic analysis for bridges in SDC
For bridges in SDC A, where the acceleration
A is not generally required. These default values are used
coefficient, As, as specified in Article 3.4, is <0.05, the
as The minimum connection design forces of this Article
horizontal design connection force in the restrained
are used in lieu of determining such forces through
directions shall not be less than 0.15 times the vertical
rigorous analysis. The division of SDC A at an
reaction due to the tributary permanent load.
acceleration coefficient of 0.05 recognizes that, in parts
For all other sites in SDC A, the horizontal design
of the country with very low seismicity, seismic forces
connection force in the restrained directions shall not be
on connections are very relatively small. However, as
<0.25 times the vertical reaction due to the tributary
outlined below, the intent of this Article is to prevent
permanent load and the tributary live loads, if
connections from becoming unintended weak links in the
applicable, assumed to exist during an earthquake.
seismic lateral load path. Accordingly, the minimum
The horizontal design connection force shall be
connection forces specified in this Article are intended to
addressed from the point of application through the
be sufficiently conservative to prevent premature failure
substructure and into the foundation elements.
and are not intended to precisely reflect the expected
For each uninterrupted segment of a superstructure, the
dynamic seismic forces. Connections that transfer forces
tributary permanent load at the line of fixed bearings, used to
from one part of a structure to another include, but are
determine the longitudinal connection design force, shall be
not limited to, fixed bearings, expansion bearings with
the total permanent load of the segment.
restrainer devices, STUs or dampers, and shear keys.
If each bearing supporting an uninterrupted segment
Note that a connection, as considered in this Article, may
or simply supported span is restrained in the transverse
be an element that simply restrains a member and may
direction, the tributary permanent load used to determine
not physically connect to that member, such as transverse
the connection design force shall be the permanent load
shear keys. Additionally, anchorage detailing for
reaction at that bearing.
connections should be extended far enough into the
Each elastomeric bearing and its connection to the
adjacent member to ensure that premature or
masonry and sole plates shall be designed to resist the
unintentional local failure is prevented. Similarly, the
horizontal seismic design forces transmitted through the
design of a girder support pedestal should consider the
bearing. For all bridges in SDC A and all single-span bridges,
connection forces specified in this Article, since failure
these seismic shear forces shall not be less than the
of a pedestal located above the pier cap could potentially
connection force specified herein.
lead to loss of span support.
The minimum support length for bridges in SDC A
In SDC A, the prevention of superstructure collapse
shall be as specified in Article 4.12.
due to unseating of spans is the primary objective behind
the provisions for minimum connection forces in
restrained directions, as covered by this Article, and for
minimum support lengths for unrestrained directions
(e.g., expansion bearings), as covered by Article 4.12.
The minimum connection forces specified in this Article
are not intended to be minimum design forces for the
substructure or foundation because the main elements of
a bridge in SDC A should generally be capable of
resisting the expected lateral seismic forces by virtue of
satisfying the non-seismic design requirements.
However, this presumed structural resistance is
predicated on providing sufficient integrity and
connectivity within the structure to mobilize the lateral
resistance of the main structural elements (e.g., columns,
pier caps, superstructure, abutments, and foundations).
3
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2015 INTERIM REVISIONS TO AASHTO GUIDE
SECTION 4 SPECIFICATIONS FOR LRFD SEISMIC BRIDGE DESIGN
Accordingly, the design forces for connections need
only be considered for those elements that directly
prevent loss of span support or prevent system
instability. Connections that fall into this category
include, but are not limited to, those elements restraining
the superstructure at in-span hinges and at substructure
support locations. Other connections in this category
include connections between substructure elements if
failure of such connections could lead to loss of span
support. For example, failure of the connections between
steel piles and a precast concrete bent cap could lead to
loss of support for both the cap and superstructure and,
therefore, such connections should meet the requirements
of this Article.
If the minimum connection forces are deemed
unreasonably large, the design may be completed using
the requirements of a higher seismic zone. The minimum
requirements of this Article require adequate connection
strength for restrained directions and adequate support
length in unrestrained directions. In many cases, it is
feasible, conservative, and economical to provide both
sufficient connection force capacity and support length
and both should be considered. In situations where load
sharing of connections may be uncertain, adequate
support length, in addition to the required connection
force capacity, should be considered. An example is the
case of bearings that may not take up load equally, thus
leading to the possibility of “unzipping” of the lateral
restraint elements. In cases where support length is
needed in the transverse direction, the designer is
cautioned that the minimum support length equations for
N were developed empirically considering longitudinal
response. Thus adequate support in the transverse
direction should be based on engineering judgment to
prevent loss of superstructure support.
Lateral connection forces are transferred from the
superstructure into the foundation elements through the
substructure. The force effects in this load path from
seismic and other lateral loads should be addressed in the
design. If each bearing supporting a continuous segment
or simply supported span is an elastomeric bearing, there
may be no fully restrained directions due to flexibility of
the bearings. However, the forces transmitted through
these bearings to substructure and foundation elements
should be determined in accordance with this Article and
Article 14.6.3 of AASHTO LRFD Bridge Design
Specifications. If positive connection capable of
transferring the minimum force is not provided, then the
minimum support length requirements for expansion
bearings of Article 4.12 should be followed. For this
Article, friction is not considered a positive connection
due to uncertainty resulting from vertical effects.
The designer is cautioned that in some geographic
locations for certain site conditions, spectral
accelerations may exceed the minimum connection
forces of this Article. Typically, such a condition may
occur for structures with fundamental vibration periods
at or near the short-period plateau of the response spectra
(e.g., stiff structures, such as those with wall piers).
When this occurs, the designer should consider the
4
© 2014 by the American Association of State Highway and Transportation Officials.
All rights reserved. Duplication is a violation of applicable law.
2015 INTERIM REVISIONS TO AASHTO GUIDE
SECTION 4 SPECIFICATIONS FOR LRFD SEISMIC BRIDGE DESIGN
effects due to potential connection failure and should
consider providing the minimum support lengths of
Article 4.12.
5
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