2007 AASHTO BRIDGE COMMITTEE AGENDA ITEM: 7 SUBJECT: LRFD Bridge Design Specifications: Sections 1, 3, 4, 5, 10 and 11, Various Articles and AASHTO Guide Specifications for LRFD Seismic Bridge Design (Two Documents) TECHNICAL COMMITTEE: T-3 Seismic Design REVISION ADDITION NEW DOCUMENT DESIGN SPEC CONSTRUCTION SPEC MOVABLE SPEC LRFR MANUAL OTHER DATE PREPARED: 4/5/07 DATE REVISED: AGENDA ITEM: Item #1 Attachment A – Updated Seismic Provisions in Sections 1, 3, 4, 5, 10 and 11 of AASHTO LRFD Bridge Design Specifications. Item #2 Attachment B – AASHTO Guide Specifications for LRFD Seismic Bridge Design. OTHER AFFECTED ARTICLES: None BACKGROUND: Item #1 UPDATED SEISMIC PROVISIONS IN LRFD SPECIFICATIONS These updates to the seismic provisions in the 2007 edition of the LRFD Specifications are of two kinds: (1) those related to changing the return period of the design earthquake from 500-years to 1000-years, and (2) those related to keeping the Specifications up to date and inline with recent developments in the seismic design of bridges. The change in return period for characterizing the seismic hazard necessitates changing the 1988 USGS maps in the current Specifications to new maps developed at the request of the AASHTO T-3 Committee by USGS. These new maps not only give peak ground acceleration (PGA) but also values of the spectral acceleration at 0.2 seconds (S S ) and 1.0 second (S 1 ) allowing an improved spectral shape to be used for defining the seismic response coefficient. Consequential changes include new zone boundaries, new soil factors, new minimum design forces and displacements, introduction of P-requirements, and a revised -factor for flexural resistance. 1.0 REVISIONS RELATED TO CHANGING THE HAZARD MAPS 1.1 Art 1.3.2. -factor Zone 2 Columns. Art 1.3.2.1 has been updated to note that Zone 2 bridge columns also 88 88
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2007 AASHTO BRIDGE COMMITTEE AGENDA ITEM: 7
SUBJECT: LRFD Bridge Design Specifications: Sections 1, 3, 4, 5, 10 and 11, Various
Articles and AASHTO Guide Specifications for LRFD Seismic Bridge Design (Two
Documents)
TECHNICAL COMMITTEE: T-3 Seismic Design
REVISION ADDITION NEW DOCUMENT
DESIGN SPEC CONSTRUCTION SPEC MOVABLE SPEC
LRFR MANUAL OTHER
DATE PREPARED: 4/5/07
DATE REVISED:
AGENDA ITEM:
Item #1
Attachment A – Updated Seismic Provisions in Sections 1, 3, 4, 5, 10 and 11 of AASHTO LRFD Bridge Design
Specifications.
Item #2
Attachment B – AASHTO Guide Specifications for LRFD Seismic Bridge Design.
OTHER AFFECTED ARTICLES:
None
BACKGROUND:
Item #1
UPDATED SEISMIC PROVISIONS IN LRFD SPECIFICATIONS
These updates to the seismic provisions in the 2007 edition of the LRFD Specifications are of two kinds: (1) those
related to changing the return period of the design earthquake from 500-years to 1000-years, and (2) those related
to keeping the Specifications up to date and inline with recent developments in the seismic design of bridges.
The change in return period for characterizing the seismic hazard necessitates changing the 1988 USGS maps in the
current Specifications to new maps developed at the request of the AASHTO T-3 Committee by USGS. These new
maps not only give peak ground acceleration (PGA) but also values of the spectral acceleration at 0.2 seconds (SS)
and 1.0 second (S1) allowing an improved spectral shape to be used for defining the seismic response coefficient.
Consequential changes include new zone boundaries, new soil factors, new minimum design forces and
displacements, introduction of P- requirements, and a revised !-factor for flexural resistance.
1.0 REVISIONS RELATED TO CHANGING THE HAZARD MAPS
1.1 Art 1.3.2. !-factor Zone 2 Columns. Art 1.3.2.1 has been updated to note that Zone 2 bridge columns also
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have flexural !- factors less than 1.0.
1.2 Art 3.10.2. USGS Maps. Twenty-seven new hazard maps are given in Figs 1 to 21 of Art 3.10.2.1. These
include 1000-year maps for PGA, SS and S1 for the conterminous U.S., Regions 1, 2, 3 and 4, Alaska, Hawaii,
Puerto Rico and USVI.
1.3 Art. 3.10.3 Site Effects. The current soil factors have been replaced with Fa and Fv, the so-called NEHRP
factors used in the NEHRP Building Provisions, the NCHRP 12-49 and 20-7(193) Provisions, and the
FHWA Bridge Retrofit Manual (2006). Site Class Definitions are defined in Table 3.10.3.1-1. Site Factors
are given in Tables 3.10.3.2-1, -2, and -3.
1.4 Art 3.10.4. Seismic Hazard Characterization. The design response spectrum is changed to be inversely
proportional to period T, instead of T2/3 in the medium-long period range (Fig.3.10.4.1-1). The spectrum is
now constructed by a three-point method using PGA, SS and S1. A note has been added to the Commentary
that the new shape is conservative beyond periods of about 3 seconds where new data shows the shape tends
to be inversely proportional to T2 rather than T.
1.5 Art 3.10.6. Seismic Performance Zones. New zone boundaries are defined in Table 3.10.6-1. These are
higher than specified in the current Specifications and reflect the increase in the return period of the design
earthquake from 500 to 1000 years.
1.6 Art 3.10.9.2 Calculation of Design Forces, Seismic Zone 1. Minimum connection forces are increased to
0.15 times reaction force when AS is less than 0.05, and 0.25 times reaction force when AS is greater than or
equal to 0.05.
1.7 Appendix A3. Two boxes in this flow chart need editing to be consistent with the revised Articles.
1.8 Art 4.7.4.4 Minimum Support Length Requirements. Minimum support lengths for bridges in Zones 1
and 2 are increased as a consequence of the change in zone boundaries (See Item 1.5 above). These are
shown in Table 4.7.4.4-1. Other changes regarding support lengths are noted below under Item 2.8.
1.9 Art. 4.7.4.5 P- Requirements. An explicit P- check has been introduced because two conservative
provisions in the current LRFD Specifications have been relaxed in this revision. These are:
(a) As noted in Item 1.4 above, the shape of the response spectrum (Figure 3.10.4.1-1) is changed from being
proportional to 1/T2/3 to 1/T. One of the principal reasons for the 1/T2/3 provision was to give conservative
estimates of force and displacement in bridges with longer periods which, in an indirect way, provided for
such effects as P- . With the change of the spectrum to 1/T, it is prudent to have an explicit check for P- .
(b) As noted in Item 1.14 below, the flexural resistance factor, !" for column design is increased from a
minimum value of 0.5 for columns with high axial load to 0.9. The low resistance factor in the current
edition provides additional strength in heavily loaded columns that can be used to offset reductions due to
P- . With a higher value for ! now permitted, it is prudent to have an explicit check for P- .
1.10 Art 5.5.4.2.1 Resistance Factors
Art 5.5.4.2.3 Special Requirements for Seismic Zones 2, 3 and 4.
Art 5.7.4.2 Limits for Reinforcement.
Art 5.10.6.2 Spirals
Above Articles require bridges in the revised Zone 2 to be covered by the Special Requirements for bridges
in Zones 3 and 4. This is another consequence of the change in zone boundaries (See Item 1.5 above).
1.11 Art 5.10.11.2. Provisions for Seismic Design – Seismic Zone 1. For bridges in Seismic Zone 1 where the
response coefficient SD1 is greater than or equal to 0.10, the transverse steel requirements in the columns
shall be the same as for bridges in the Zone 2.
1.12 Art 5.10.11.3. Provisions for Seismic Design – Seismic Zone 2. All of the requirements for bridges in
Zones 3 and 4 shall apply to bridges in Zone 2 with one exception: the area of longitudinal reinforcement
shall not exceed 0.06 Ag.
1.13 Art 5.10.11.4.1a Longitudinal Reinforcement. The upper limit for the area of longitudinal reinforcement is
reduced from 0.06 Ag to 0.04 Ag.
1.14 Art 5.10.11.4.1b Flexural Resistance. In the current Specifications, the !-factor for flexural resistance
varies from 0.5 to 0.9 according to axial load on the column. In this revision, ! is taken as 0.9 regardless of
the axial load. It is noted that the Guide Specifications, and other ‘displacement-based’ specifications, use !
= 1.0. But since the LRFD Specifications are ‘force-based’ and do not calculate ductility demand explicitly,
as is done in these other specifications, limiting the factor to 0.9 is considered prudent.
1.15 Art 11.6.5. Abutments and Conventional Retaining Walls - Seismic Design. The coefficient A is replaced
by AS in the expression for kh in Equation C11.6.5-1, and in the commentary below this equation.
1.16 Art C11.8.6 Nongravity Cantilevered Walls – Seismic Design. The coefficient A is replaced by AS in the
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expression for kh.
1.17 Art 11.10.7.1 Mechanically Stabilized Earth Walls - Seismic Design. The coefficient A is replaced by AS in
the expression for Am in Equation 11.10.7.1-1.
1.18 Appendix Section 11, Art A11.1.1.2 Design for Displacement. The coefficient A is replaced by AS in the
expression for kh, and the allowable outward displacement of the abutment. Bridge abutments in Zone 2 are
required to undergo more detailed consideration for inertial forces as for bridges in Zones 3 and 4.
2.0 ADDITIONAL REVISIONS
2.1 Art 1.1 General. Specific reference is made in this Article to the Guide Specifications as an acceptable set of
alternative provisions for the seismic design of bridges to give these Specifications necessary legal standing.
2.2 Art 3.10.1 Performance Objective. A statement has been added in this Article to clarify the no-collapse
performance objective implicit in these Specifications. The Commentary has been rearranged to further clarify
this objective. No new philosophy is introduced. Existing material has been restated and re-ordered to
highlight the performance objective of this Specification.
2.3 Art C3.10.1 Commentary. A note has been added explaining the difference between force- and
displacement-based procedures. It recommends that designers use a ‘displacement-based’ procedure, such as
the Guide Specifications, to check the displacement capacity of bridges designed by the LRFD Specifications,
particularly those in high seismic zones.
2.4 Art C3.10.2.1 Alternate Maps. A new section has been added to the Commentary to this Article indicating
how state hazard maps should be developed if they are to be approved as an alternate to the national (USGS)
hazard maps.
2.5 Art 3.10.2.2 Site-Specific Procedure. A new Article and Commentary has been added to describe minimum
requirements for the development of site-specific design response spectrum.
2.6 Art C4.7.1.3 Stiffness/Cracked Sections. New Commentary has been added to explain the need to consider
the effect of inelastic deformation and section cracking when modeling element stiffness. Some of this
material has been relocated from the Commentary to the previous section.
2.7 Art 4.7.4.3.4a Acceleration Time Histories. A new Article and Commentary has been added to describe
selection and development of time histories for use in both linear and nonlinear time- history analyses. A
number of related references have also been added.
2.8 Art 4.7.4.4 Min. Support Length. All references to ‘minimum seat width’ (N) have been changed to
‘minimum support length’ and a figure has been added to the commentary to define dimension ‘N’. A note
about cover concrete has been added to the Commentary.
2.9 Art 10.5.4.1 Liquefaction. A new Article and Commentary has been added to describe design requirements
for liquefaction for bridges in Seismic Zone 4, and supplements existing material in Appendix A10.
Item #2
GUIDE SPECIFICATIONS FOR LRFD SEISMIC BRIDGE DESIGN
These Guide Specifications are an alternate, stand-alone, set of provisions for the seismic design of highway
bridges. The major difference between these provisions and those in the Updated LRFD Specifications (above) is
the methodology used for determining design forces. Elastic methods of analysis are used to calculate earthquake
demands, but if these demands exceed the elastic strength (implicit capacity) of the columns, a nonlinear static
analysis (a ‘pushover’ analysis) must be used, as a minimum, to determine design forces. The pushover analysis
method explicitly models various displacement limit states and calculates member and component forces at each
limit state, including collapse if required. Design forces at the earthquake displacement, calculated using the design
response spectrum, can be found using the same pushover curve. Accordingly the R-factors in the current LRFD
Specifications are not used. Since the methodology focuses on displacements, it is often referred to as
‘displacement-based’. By contrast, the LRFD Specifications are ‘force-based’.
Displacement-based procedures are widely believed to lead to more efficient designs against collapse in large
earthquakes and are strongly recommended for bridges in high seismic zones. The 2006 edition of the FHWA
Seismic Retrofitting Manual for Highway Structures: Part 1- Bridges also recommends the use of these procedures
in high seismic zones. In this Manual, and elsewhere, these procedures are called ‘capacity-spectrum methods’.
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ANTICIPATED EFFECT ON BRIDGES:
Improved performance of bridges during small and large earthquakes.
REFERENCES: Item #2
Imbsen, R.A., Recommended LRFD Guidelines for the Seismic Design of Highway Bridges, NCHRP Report 20-7,
Task 193, May 2006.
OTHER: None
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1
ATTACHMENT A - 2007 AGENDA ITEM 7 – T-3
SECTION 1: INTRODUCTION
TABLE OF CONTENTS
1.1 SCOPE OF THE SPECIFICATIONS......................................................................................................................... 1.2 DEFINITIONS............................................................................................................................................................ 1.3 DESIGN PHILOSOPHY ............................................................................................................................................ 1.3.1 GENERAL .................................................................................................................................................................. 1.3.2 LIMIT STATES............................................................................................................................................................
1.3.2.1 General........................................................................................................................................................... 1.3.2.2 Service Limit State......................................................................................................................................... 1.3.2.3 Fatigue and Fracture Limit State.................................................................................................................... 1.3.2.4 Strength Limit State ....................................................................................................................................... 1.3.2.5 Extreme Event Limit States ...........................................................................................................................
3.10.2.1 General Procedure..................................................................................................................................
3.10.4 3.10.6 Seismic Performance Zones ................................................................................................................. 3.10.5 Site Effects .....................................................................................................................................................
3.10.5.1 General.................................................................................................................................................. 3.10.5.2 Soil Profile Type I................................................................................................................................. 3.10.5.3 Soil Profile Type II ............................................................................................................................... 3.10.5.4 Soil Profile Type III .............................................................................................................................. 3.10.5.5 Soil Profile Type IV..............................................................................................................................
3.10.8 Combination of Seismic Force Effects............................................................................................................ 3.10.9 Calculation of Design Forces ..........................................................................................................................
3.10.9.1 General.................................................................................................................................................. 3.10.9.2 Seismic Zone 1...................................................................................................................................... 3.10.9.3 Seismic Zone 2...................................................................................................................................... 3.10.9.4 Seismic Zones 3 and 4 ..........................................................................................................................
3.10.9.4.3a General............................................................................................................................... 3.10.9.4.3b Single Columns and Piers .................................................................................................. 3.10.9.4.3c Piers with Two or More Columns...................................................................................... 3.10.9.4.3d Column and Pile Bent Design Forces ................................................................................ 3.10.9.4.3e Pier Design Forces ............................................................................................................. 3.10.9.4.3f Foundation Design Forces..................................................................................................
Seismic loads shall be assumed to act in any lateral
direction.
The appropriate R-factor shall be used for both
orthogonal axes of the substructure.
C3.10.7.2
Usually the orthogonal axes will be the longitudinal
and transverse axes of the bridge. In the case of a curved
bridge, the longitudinal axis may be the chord joining
the two abutments.
A wall-type concrete pier may be analyzed as a
single column in the weak direction if all the provisions
for columns, as specified in Section 5, are satisfied.
Wall-type piers may be treated as wide columns in
the strong direction, provided the appropriate R-factor in
this direction is used.
3.10.8 Combination of Seismic Force Effects
The elastic seismic force effects on each of the
principal axes of a component resulting from analyses in
the two perpendicular directions shall be combined to
form two load cases as follows:
100 percent of the absolute value of the force
effects in one of the perpendicular directions
combined with 30 percent of the absolute
value of the force effects in the second
perpendicular direction, and
100 percent of the absolute value of the force
effects in the second perpendicular direction
combined with 30 percent of the absolute
value of the force effects in the first
perpendicular direction.
C3.10.8
Where foundation and/or column connection forces
are determined from plastic hinging of the columns
specified in Article 3.10.9.4.3, the resulting force effects
may be determined without consideration of combined
The exception to these load combinations indicated
at the end of this section should also apply to bridges in
Zone 2 where foundation forces are determined from
plastic hinging of the columns.
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load cases specified herein. For the purpose of this
provision, “column connection forces” shall be taken as
the shear and moment, computed on the basis of plastic
hinging. The axial load shall be taken as that resulting
from the appropriate load combination with the axial
load, if any, associated with plastic hinging taken as EQ.
If a pier is designed as a column as specified in
Article 3.10.7.2, this exception shall be taken to apply
for the weak direction of the pier where force effects
resulting from plastic hinging are used; the combination
load cases specified must be used for the strong
direction of the pier.
3.10.9 Calculation of Design Forces
3.10.9.1 General
For single-span bridges, regardless of seismic zone,
the minimum design connection force effect in the
restrained direction between the superstructure and the
substructure shall not be less than the product of the site
coefficient, the acceleration coefficient, AS, specified in
Eq. 3.10.4.2-2, and the tributary permanent load.
Seat widths Minimum support lengths at expansion
bearings of multispan bridges shall either comply with
Article 4.7.4.4 or STUs, and dampers shall be provided.
C3.10.9.1
This Article refers to superstructure effects carried
into substructure. Abutments on multispan bridges, but
not single-span bridges, and retaining walls are subject
to acceleration-augmented soil pressures as specified in
Articles 3.11.4 and 11.6.5. Wingwalls on single-span
structures are not fully covered at this time, and the
Engineer should use judgment in this area.
3.10.9.2 Seismic Zone 1
For bridges on sites in Zone 1 where the
acceleration coefficient, AS, as specified in Eq. 3.10.4.2-
2, is less than 0.025 0.05, and the soil profile is either
Type I or Type II, the horizontal design connection force
in the restrained directions shall not be taken to be less
than 0.1 0.15 times the vertical reaction due to the
tributary permanent load and the tributary live loads
assumed to exist during an earthquake.
For all other sites in Zone 1, the horizontal design
connection force in the restrained directions shall not be
taken to be less than 0.2 0.25 times the vertical reaction
due to the tributary permanent load and the tributary live
loads assumed to exist during an earthquake.
For each uninterrupted segment of a superstructure,
the tributary permanent load at the line of fixed
bearings, used to determine the longitudinal connection
design force, shall be the total permanent load of the
segment.
If each bearing supporting an uninterrupted segment
or simply supported span is restrained in the transverse
direction, the tributary permanent load used to determine
the connection design force shall be the permanent load
reaction at that bearing.
Each elastomeric bearing and its connection to the
masonry and sole plates shall be designed to resist the
horizontal seismic design forces transmitted through the
bearing. For all bridges in Seismic Zone 1 and all single-
span bridges, these seismic shear forces shall not be less
than the connection force specified herein.
C3.10.9.2
These provisions arise because, as specified in
Article 4.7.4, seismic analysis for bridges in Zone 1 is
not generally required. These default values are used as
minimum design forces in lieu of rigorous analysis. The
division of Zone 1 at a value for the acceleration
coefficient, AS, 0.025 for sites with favorable soil
condition of 0.05 is an arbitrary expedience intended to
provide some relief to recognizes that, in parts of the
country with very low seismicity, seismic forces on
connections are very small.
If each bearing supporting a continuous segment or
simply supported span is an elastomeric bearing, there
are no restrained directions due to the flexibility of the
bearings.
The magnitude of live load assumed to exist at the
time of the earthquake should be consistent with the
value of !eq used in conjunction with Table 3.4.1-1.
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3.10.9.5 Longitudinal Restrainers
Friction shall not be considered to be an effective
restrainer.
Restrainers shall be designed for a force calculated
as the acceleration coefficient, AS, as specified in Eq.
3.10.4.2-2, times the permanent load of the lighter of the
two adjoining spans or parts of the structure.
If the restrainer is at a point where relative
displacement of the sections of superstructure is
designed to occur during seismic motions, sufficient
slack shall be allowed in the restrainer so that the
restrainer does not start to act until the design
displacement is exceeded.
Where a restrainer is to be provided at columns or
piers, the restrainer of each span may be attached to the
column or pier rather than to interconnecting adjacent
spans.
In lieu of restrainers, STUs may be used and
designed for either the elastic force calculated in Article
4.7 or the maximum force effects generated by inelastic
hinging of the substructure as specified in Article
3.10.7.1.
3.10.9.6 Hold-Down Devices
For Seismic Zones 2, 3, and 4, hold-down devices
shall be provided at supports and at hinges in continuous
structures where the vertical seismic force due to the
longitudinal seismic load opposes and exceeds 50
percent, but is less than 100 percent, of the reaction due
to permanent loads. In this case, the net uplift force for
the design of the hold-down device shall be taken as 10
percent of the reaction due to permanent loads that
would be exerted if the span were simply supported.
If the vertical seismic forces result in net uplift, the
hold-down device shall be designed to resist the larger
of either:
120 percent of the difference between the
vertical seismic force and the reaction due to
permanent loads, or
10 percent of the reaction due to permanent
loads.
3.10.10 Requirements for Temporary Bridges and
Stage Construction
Any bridge or partially constructed bridge that is
expected to be temporary for more than five years shall
be designed using the requirements for permanent
structures and shall not use the provisions of this Article.
The requirement that an earthquake shall not cause
collapse of all or part of a bridge, as stated in
C3.10.10
The option to use a reduced response coefficient
and a reduced ground acceleration coefficient is
provided to reflects the limited exposure period for a
temporary bridge.
NO CHANGES TO ARTICLES
3.10.9.3 AND 3.10.9.4
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53
Article 3.10.1, shall apply to temporary bridges expected
to carry traffic. It shall also apply to those bridges that
are constructed in stages and expected to carry traffic
and/or pass over routes that carry traffic. The
acceleration coefficient given in Article 3.10.2 elastic
seismic response coefficient and the ground acceleration
coefficient given in Article 3.10.4.2 may be reduced by
a factor of not more than 2 in order to calculate the
component elastic forces and displacements. Response
and acceleration coefficients for construction sites that
are close to active faults shall be the subject of special
study. The response modification factors given in
Article 3.10.7 may be increased by a factor of not more
than 1.5 in order to calculate the design forces. This
factor shall not be applied to connections as defined in
Table 3.10.7.1-2.
The minimum seat width support length provisions
of Article 4.7.4.4 shall apply to all temporary bridges
and staged construction.
REFERENCES
Add the following new reference:
AASHTO. 200x. AASHTO Guide Specifications for LRFD Seismic Bridge Design, American Association of State
Highway and Transportation Officials, Washington, DC.
FHWA. 2006. Seismic Retrofitting Manual for Highway Structures, Part 1 – Bridges, FHWA Publication No. FHWA-
HRT-06-032, Federal Highway Administration, Washington DC.
MCEER/ATC. 2003. Recommended LRFD Guidelines for the Seismic Design of Highway Bridges. Special
Publication No. MCEER-03-SP03, Multidisciplinary Center for Earthquake Engineering Research, Buffalo NY.
NCHRP. 2002. Comprehensive Specification for the Seismic Design of Bridges. NCHRP Report 472, Transportation
Research Board, Washington DC.
NCHRP 2006, Recommended LRFD Guidelines for the Seismic Design of Highway Bridges. Draft Report NCHRP
Project 20-07, Task 193, TRC Imbsen & Associates, Sacramento CA.
Somerville, P.G. 1997. “The Characteristics and Quantification of Near Fault Ground Motion,” Proceedings of the
FHWA/NCEER Workshop on the National Representation of Seismic Ground Motion for New and Existing Highway Facilities, Center for Earthquake engineering Research, Buffalo, New York, Technical Report 97-0010, State
University of New York at Buffalo, pp. 1293-1318.
Somerville, P. G., Smith, N. G., Graves, R. W., and Abrahamson, N. A. 1997. “Modification of Empirical Strong
Ground Motion Attenuation Relations to Include the Amplitude and Duration Effects of Rupture Directivity,”
Seismological Research Letters, Vol. 68, pp. 199-222.
144
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APPENDIX A3 – SEISMIC DESIGN FLOWCHARTS
DETERMINE- Spectral Acceleration Coefficients- Site Factors- Ground Acceleration Coefficient- Seismic Performance Zone
Articles 3.10.2, 3.10.4, 3.10.6
DETERMINE- Bridge Importance Category
Article 3.10.5
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146
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SECTION 4: STRUCTURAL ANALYSIS AND EVALUATION
PARTIAL TABLE OF CONTENTS – ANALYSIS FOR EARTHQUAKE LOADS
4.7.4 Analysis for Earthquake Loads ................................................................................................................................ 4.7.4.1 General ........................................................................................................................................................... 4.7.4.2 Single-Span Bridges....................................................................................................................................... 4.7.4.3 Multispan Bridges ..........................................................................................................................................
4.7.4.3.1 Selection of Method............................................................................................................................. 4.7.4.3.2 Single-Mode Methods of Analysis.......................................................................................................
United States Army Corp of Engineers. 2003. Time History Dynamic Analysis of Concrete Hydraulic Structures,
USACE Engineering Circular EC1110-2-6051.
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SECTION 5: CONCRETE STRUCTURES
PARIAL TABLE OF CONTENTS –
STRENGTH LIMIT STATE, COMPRESSION MEMBERS (part),
SECTIONAL DESIGN (part), TRANSVERSE REINFORCEMENT FOR COMPRESSION
MEMBERS, PROVISIONS FOR SEISMIC DESIGN
5.5.4 Strength Limit State ................................................................................................................................................. 5.5.4.1 General........................................................................................................................................................... 5.5.4.2 Resistance Factors..........................................................................................................................................
5.5.4.2.1 Conventional Construction .................................................................................................................. 5.5.4.2.2 Segmental Construction....................................................................................................................... 5.5.4.2.3 Special Requirements for Seismic Zones 2, 3 and 4............................................................................
5.5.4.3 Stability.......................................................................................................................................................... 5.7.4 Compression Members (part)...................................................................................................................................
5.7.4.1 General........................................................................................................................................................... 5.7.4.2 Limits for Reinforcement...............................................................................................................................
5.8.3 Sectional Design Model (part) .................................................................................................................................
5.10.6 Transverse Reinforcement for Compression Members.......................................................................................... 5.10.6.1 General......................................................................................................................................................... 5.10.6.2 Spirals .......................................................................................................................................................... 5.10.6.3 Ties ..............................................................................................................................................................
5.10.11 Provisions for Seismic Design ............................................................................................................................. 5.10.11.1 General....................................................................................................................................................... 5.10.11.2 Seismic Zone 1........................................................................................................................................... 5.10.11.3 Seismic Zone 2........................................................................................................................................... 5.10.11.4 Seismic Zones 3 and 4 ...............................................................................................................................
5.10.11.4.1 Column Requirements ..................................................................................................................... 5.10.11.4.1a Longitudinal Reinforcement ................................................................................................... 5.10.11.4.1b Flexural Resistance................................................................................................................. 5.10.11.4.1c Column Shear and Transverse Reinforcement........................................................................ 5.10.11.4.1d Transverse Reinforcement for Confinement at Plastic Hinges ............................................... 5.10.11.4.1e Spacing of Transverse Reinforcement for Confinement......................................................... 5.10.11.4.1f Splices .....................................................................................................................................
5.10.11.4.2 Requirements for Wall-Type Piers .................................................................................................. 5.10.11.4.3 Column Connections ....................................................................................................................... 5.10.11.4.4 Construction Joints in Piers and Columns ........................................................................................
" For tension in steel in anchorage zones ...... 1.00
" For resistance during pile driving ............... 1.00
For sections in which the net tensile strain in the
extreme tension steel at nominal resistance is between
the limits for compression-controlled and tension-
controlled sections, ! may be linearly increased from
0.75 to that for tension-controlled sections as the net
tensile strain in the extreme tension steel increases from
the compression-controlled strain limit to 0.005.
This variation in, !, !4may be computed for
prestressed members such that:
0.75 0.583 0.25 1 1.0- .7 & + / 70 12 3
td
c (5.5.4.2.1-1)
and for nonprestressed members such that:
0.75 0.65 0.15 1 0.9! "# $ % & #' () *
td
c (5.5.4.2.1-2)
where:
c = distance from the extreme compression fiber to
the neutral axis (in.)
dt = distance from the extreme compression fiber to
the centroid of the extreme tension steel
element (in.)
The -factor of 0.8 for normal weight concrete
reflects the importance of the anchorage zone, the brittle
failure mode for compression struts in the anchorage
zone, and the relatively wide scatter of results of
experimental anchorage zone studies. The -factor of
0.65 for lightweight concrete reflects its often lower
tensile strength and is based on the multipliers used in
ACI 318-89, Section 11.2.1.2.
The design of intermediate anchorages, anchorages,
diaphragms, and multiple slab anchorages are addressed
in Breen et al. (1994).
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For tension-controlled partially prestressed
components in flexure, the values of may be taken as:
0.90 0.10( )PPR $ % (5.5.4.2.1-3)
in which:
ps py
ps py s y
A fPPR =
A f A f% (5.5.4.2.1-4)
where:
PPR = partial prestress ratio
As = area of nonprestressed tension
reinforcement (in.2)
Aps = area of prestressing steel (in.2)
fy = specified yield strength of reinforcing bars
(ksi)
fpy = yield strength of prestressing steel (ksi)
Resistance factors shall not be applied to the
development and splice lengths of reinforcement as
specified in Article 5.11.
5.5.4.2.2 Segmental Construction
Resistance factors for the strength limit state shall
be taken as provided in Table 1 for the conditions
indicated and in Article 5.5.4.2.1 for conditions not
covered in Table 1.
In selecting resistance factors for flexure, f, and
shear and torsion, v, the degree of bonding of the post-
tensioning system shall be considered. In order for a
tendon to be considered as fully bonded at a section, it
should be fully developed at that section for a
development length not less than that required by
Article 5.11.4. Shorter embedment lengths may be
permitted if demonstrated by full-size tests and approved
by the Engineer.
Where the post-tensioning is a combination of fully
bonded tendons and unbonded or partially bonded
tendons, the resistance factor at any section shall be
based upon the bonding conditions for the tendons
providing the majority of the prestressing force at the
section.
Joints between precast units shall be either cast-in-
place closures or match cast and epoxied joints.
C5.5.4.2.2
Comprehensive tests of a large continuous
three-span model of a twin-cell box girder bridge built
from precast segments with fully bonded internal
tendons and epoxy joints indicated that cracking was
well distributed through the segment lengths. No epoxy
joint opened at failure, and the load deflection curve was
identical to that calculated for a monolithic specimen.
The complete ultimate strength of the tendons was
developed at failure. The model had substantial ductility
and full development of calculated deflection at failure.
Flexural cracking concentrated at joints and final failure
came when a central joint opened widely and crushing
occurred at the top of the joint. Based on the observation
of this limited test data, a maximum of 0.95 was
selected.
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Table 5.5.4.2.2-1 Resistance Factor for Joints in Segmental
Construction.
f
Flexure
v
Shear
Normal Weight Concrete
Fully Bonded Tendons
Unbonded or Partially
Bonded Tendons
0.95
0.90
0.90
0.85
Sand-Lightweight Concrete
Fully Bonded Tendons
Unbonded or Partially
Bonded Tendons
0.90
0.85
0.70
0.65
5.5.4.2.3 Special Requirements for Seismic Zones 2, 3 and 4
A reduced modified resistance factor for columns in
Seismic Zones 2, 3 and 4 shall be taken as specified in
Articles 5.10.11.3 and 5.10.11.4.1b.
5.5.4.3 Stability
The structure as a whole and its components shall
be designed to resist sliding, overturning, uplift and
buckling. Effects of eccentricity of loads shall be
considered in the analysis and design.
Buckling of precast members during handling,
transportation, and erection shall be investigated.
5.7.4 Compression Members
5.7.4.1 General
Unless otherwise permitted, compression members
shall be analyzed with consideration of the effects of:
+ Eccentricity,
+ Axial loads,
+ Variable moments of inertia,
+ Degree of end fixity,
+ Deflections,
+ Duration of loads, and
+ Prestressing.
In lieu of a refined procedure, nonprestressed
columns with the slenderness ratio, K u/r < 100, may be
designed by the approximate procedure specified in
Article 5.7.4.3.
C5.7.4.1
Compression members are usually prestressed only
where they are subjected to a high level of flexure or
when they are subjected to driving stresses, as is the case
with prestressed concrete piles.
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where:
K = effective length factor specified in
Article 4.6.2.5
u = unbraced length (in.)
r = radius of gyration (in.)
The requirements of this Article shall be
supplemented and modified for structures in Seismic
Zones 2, 3, and 4, as specified in Article 5.10.11.
Provisions shall be made to transfer all force effects
from compression components, adjusted for second-
order moment magnification, to adjacent components.
Where the connection to an adjacent component is
by a concrete hinge, longitudinal reinforcement shall be
centralized within the hinge to minimize flexural
resistance and shall be developed on both sides of the
hinge.
5.7.4.2 Limits for Reinforcement
Additional limits on reinforcement for compression
members in Seismic Zones 2, 3 and 4 shall be
considered as specified in Articles 5.10.11.3 and
5.10.11.4.1a.
The maximum area of prestressed and
nonprestressed longitudinal reinforcement for
noncomposite compression components shall be such
that:
C5.7.4.2
0.08ps pus
g g y
A fA +
A A f# (5.7.4.2-1)
and
0.30ps pe
g c
A f
A f#
, (5.7.4.2-2)
The minimum area of prestressed and
nonprestressed longitudinal reinforcement for
noncomposite compression components shall be such
that:
0.135ps pus y
g c g c
A fA f +
A f A f-
, , (5.7.4.2-3)
where:
As = area of nonprestressed tension steel (in.2)
Ag = gross area of section (in.2)
Aps = area of prestressing steel (in.2)
fpu = specified tensile strength of prestressing steel
According to current ACI codes, the area of
longitudinal reinforcement for nonprestressed
noncomposite compression components should be not
less than 0.01 Ag. Because the dimensioning of columns
is primarily controlled by bending, this limitation does
not account for the influence of the concrete
compressive strength. To account for the compressive
strength of concrete, the minimum reinforcement in
flexural members is shown to be proportional to f,c/fy in
Article 5.7.3.3.2. This approach is also reflected in the
first term of Eq. 3. For fully prestressed members,
current codes specify a minimum average prestress of
0.225 ksi. Here also the influence of compressive
strength is not accounted for. A compressive strength of
5.0 ksi has been used as a basis for these provisions, and
a weighted averaging procedure was used to arrive at the
equation.
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(ksi)
fy = specified yield strength of reinforcing bars (ksi)
f,c = specified compressive strength of concrete (ksi)
fpe = effective prestress (ksi)
The minimum number of longitudinal reinforcing
bars in the body of a column shall be six in a circular
arrangement and four in a rectangular arrangement. The
minimum size of bar shall be No. 5.
Where columns are pinned to their foundations, a
small number of central bars have sometimes been used
as a connection between footing and column.
For bridges in Seismic Zones 1 and 2, a reduced
effective area may be used when the cross-section is
larger than that required to resist the applied loading.
The minimum percentage of total (prestressed and
nonprestressed) longitudinal reinforcement of the
reduced effective area is to be the greater of 1 percent or
the value obtained from Eq. 3. Both the reduced
effective area and the gross area must be capable of
resisting all applicable load combinations from
Table 3.4.1-1.
For low risk seismic zones, the 1 percent reduced
effective area rule, which has been used successfully
since 1957 in the Standard Specifications, is
implemented, but modified to account for the
dependency of the minimum reinforcement on the ratio
of f,c /fy.
For columns subjected to high, permanent axial
compressive stresses where significant concrete creep is
likely, using an amount of longitudinal reinforcement
less than that given by Eq. 3 is not recommended
because of the potential for significant transfer of load
from the concrete to the reinforcement as discussed in
the report of ACI Committee 105.
5.8.3 Sectional Design Model
5.8.3.1 General
The sectional design model may be used for shear
design where permitted in accordance with the
provisions of Article 5.8.1
C5.8.3.1
In the sectional design approach, the component is
investigated by comparing the factored shear force and
the factored shear resistance at a number of sections
along its length. Usually this check is made at the tenth
points of the span and at locations near the supports.
See Articles 5.10.11.3 and 5.10.11.4.1c for
additional requirements for Seismic Zones 2, 3 and 4.
In lieu of the methods specified herein, the
resistance of members in shear or in shear combined
with torsion may be determined by satisfying the
conditions of equilibrium and compatibility of strains
and by using experimentally verified stress-strain
relationships for reinforcement and for diagonally
cracked concrete. Where consideration of simultaneous
shear in a second direction is warranted, investigation
shall be based either on the principles outlined above or
on a three-dimensional strut-and-tie model.
An appropriate nonlinear finite element analysis or
a detailed sectional analysis would satisfy the
requirements of this Article. More information on
appropriate procedures and a computer program that
satisfies these requirements are given by Collins and
Mitchell (1991). One possible approach to the analysis
of biaxial shear and other complex loadings on concrete
members is outlined in Rabbat and Collins (1978), and a
corresponding computer-aided solution is presented in
Rabbat and Collins (1976). A discussion of the effect of
biaxial shear on the design of reinforced concrete beam-
to-column joints can be found in Pauley Paulay and
Priestley (1992).
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76
5.10.6 Transverse Reinforcement for Compression
Members
5.10.6.1 General
The provisions of Article 5.10.11 shall also apply to
design and detailing in Seismic Zones 2, 3, and 4.
Transverse reinforcement for compression members
may consist of either spirals or ties.
C5.10.6.1
Article 5.10.11.2 applies to Seismic Zone 2 1 but
has no additional requirements for transverse
reinforcement for compression members.
5.10.6.2 Spirals
Spiral reinforcement for compression members
other than piles shall consist of one or more evenly
spaced continuous spirals of either deformed or plain bar
or wire with a minimum diameter of 0.375 in. The
reinforcement shall be arranged so that all primary
longitudinal reinforcement is contained on the inside of,
and in contact with, the spirals.
The clear spacing between the bars of the spiral
shall not be less than either 1.0 in. or 1.33 times the
maximum size of the aggregate. The center-to-center
spacing shall not exceed 6.0 times the diameter of the
longitudinal bars or 6.0 in.
Except as specified in Articles 5.10.11.3 and
5.10.11.4.1 for Seismic Zones 2, 3 and 4, spiral
reinforcement shall extend from the footing or other
support to the level of the lowest horizontal
reinforcement of the supported members.
Anchorage of spiral reinforcement shall be provided
by 1.5 extra turns of spiral bar or wire at each end of the
spiral unit. For Seismic Zones 2, 3 and 4, the extension
of transverse reinforcement into connecting members
shall meet the requirements of Article 5.10.11.4.3.
Splices in spiral reinforcement may be one of the
following:
+ Lap splices of 48.0 uncoated bar diameters,
72.0 coated bar diameters, or 48.0 wire
diameters;
+ Approved mechanical connectors; or
+ Approved welded splices.
5.10.6.3 Ties
In tied compression members, all longitudinal bars
shall be enclosed by lateral ties that shall be equivalent
to:
+ No. 3 bars for No. 10 or smaller bars,
+ No. 4 bars for No. 11 or larger bars, and
+ No. 4 bars for bundled bars.
The spacing of ties along the longitudinal axis of
the compression member shall not exceed the least
dimension of the compression member or 12.0 in.
C5.10.6.3
Figure C1 illustrates the placement of restraining
ties in compression members which are not designed for
plastic hinging.
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77
Where two or more bars larger than No. 10 are bundled
together, the spacing shall not exceed half the least
dimension of the member or 6.0 in.
Deformed wire or welded wire fabric of equivalent
area may be used instead of bars.
Figure C5.10.6.3-1 Acceptable Tie Arrangements.
No longitudinal bar shall be more than 24.0 in.,
measured along the tie, from a restrained bar. A
restrained bar is one which has lateral support provided
by the corner of a tie having an included angle of not
more than 135°. Where the column design is based on
plastic hinging capability, no longitudinal bar shall be
farther than 6.0 in. clear on each side along the tie from
such a laterally supported bar and the tie reinforcement
shall meet the requirements of Articles 5.10.11.4.1d
through 5.10.11.4.1f. Where the bars are located around
the periphery of a circle, a complete circular tie may be
used if the splices in the ties are staggered.
Ties shall be located vertically not more than half a
tie spacing above the footing or other support and not
more than half a tie spacing below the lowest horizontal
reinforcement in the supported member.
Columns in Seismic Zones 2, 3, and 4 are designed
for plastic hinging. The plastic hinge zone is defined in
Article 5.10.11.4.1c. Additional requirements for
transverse reinforcement for bridges in Seismic Zones 2,
3 and 4 are specified in Articles 5.10.11.3 and
5.10.11.4.1. Plastic hinging may be used as a design
strategy for other extreme events, such as ship collision.
5.10.11 Provisions for Seismic Design
5.10.11.1 General
The provisions of these Articles shall apply only to
the extreme event limit state.
In addition to the other requirements specified in
Article 5.10, reinforcing steel shall also conform to the
seismic resistance provisions specified herein.
Displacement requirements specified in
Article 4.7.4.4 or longitudinal restrainers specified in
Article 3.10.9.5 shall apply.
C5.10.11.1
These Specifications are based on the work by the
Applied Technology Council in 1979–1980. The Loma
Prieta earthquake of 1989 provided new insights into the
behavior of concrete details under seismic loads. The
California Department of Transportation initiated a
number of research projects that are currently producing
have produced information that is useful for both the
design of new structures and the retrofitting of existing
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78
Bridges located in Seismic Zone 2 shall satisfy the
requirements in Article 5.10.11.3. Bridges located in
Seismic Zones 3 and 4 shall satisfy both the
requirements specified in Article 5.10.11.3 for Seismic
Zone 2 and the requirements specified in Article
5.10.11.4 for Seismic Zones 3 and 4.
structures. Much of this information has formed the
basis of recent provisions published by NCHRP (2002,2006), MCEER/ATC (2003) and FHWA (2006).
This new information relates to all facets of seismic
engineering, including design spectra, analytical
techniques, and design details. Unfortunately, much of
this information is still evolving, making its codification
premature. Bridge Ddesigners working in Seismic Zones
2, 3 and 4 are encouraged to avail themselves of current
research reports and other literature to augment these
Specifications.
The Loma Prieta earthquake confirmed the
vulnerability of columns with inadequate core
confinement and inadequate anchorage of longitudinal
reinforcement. New areas of concern that emerged
include:
+ Lack of adequate reinforcement for positive
moments that may occur in the superstructure
over monolithic supports when the structure is
subjected to longitudinal dynamic loads;
+ Lack of adequate strength in joints between
columns and bent caps under transverse
dynamic loads; and
+ Inadequate reinforcement for torsion,
particularly in outrigger-type bent caps.
The purpose of the additional design requirements
of this Article is to increase the probability that the
design of the components of a bridge are consistent with
the overall design philosophy of ATC 6, especially for
bridges located in Seismic Zones 2, 3 and 4, and that the
potential for failures observed in past earthquakes is
minimized. The additional column design requirements
of this Article for bridges located in Seismic Zones 2, 3
and 4 are to ensure that a column is provided with
reasonable ductility and is forced to yield in flexure and
that the potential for a shear, compression, or loss of
anchorage mode of failure is minimized. The additional
design requirements for piers provide for some inelastic
resistance; however, the R-factor specified for piers in
Section 4 is to ensure that the anticipated inelastic
resistance is significantly less than that of columns.
The actual ductility demand on a column or pier is a
complex function of a number of variables, including:
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79
+ Earthquake characteristics,
+ Design force level,
+ Periods of vibration of the bridge,
+ Shape of the inelastic hysteresis loop of the
columns,
+ Elastic damping coefficient,
+ Contributions of foundation and soil conditions
to structural flexibility, and
+ Plastic hinge length of the column.
The damage potential of a column is also related to the
ratio of the duration of strong motion shaking to the
natural period of vibration of the bridge. This ratio will
be an indicator of the number of yield excursions and
hence of the cumulative ductility demand.
5.10.11.2 Seismic Zone 1
For bridges in Seismic Zone 1 where the response
acceleration coefficient, SD1, specified in Article
3.10.4.2, is less than 0.10, no consideration of seismic
forces shall be required for the design of structural
components, except for that the design of the connection
of the superstructure to the substructure shall be as
specified in Article 3.10.9.2.
For bridges in Seismic Zone 1 where the response
acceleration coefficient, SD1, is greater than or equal to
0.10 but less than or equal to 0.15, no consideration of
seismic forces shall be required for the design of
structural components, except that:
+ The design of the connection of the
superstructure to the substructure shall be as
specified in Article 3.10.9.2.
+ The transverse reinforcement requirements at
the top and bottom of a column shall be as
specified in Articles 5.10.11.4.1d and
5.10.11.4.1e.
C5.10.11.2
These requirements for Zone 1 are a departure from
those in the previous edition of these Specifications.
These changes are necessary because the return period
of the design event has been increased from 500 to 1000
years, and the Zone Boundaries (Table 3.10.6-1) have
been increased accordingly. The high end of the new
Zone 1 (0.10 < SD1 < 0.15) overlaps with the low end of
the previous Zone 2. Since performance expectations
have not changed with increasing return period, the
minimum requirements for bridges in the high end of
Zone 1 should therefore be the same as those for the
previous Zone 2. Requirements for the remainder of
Zone 1 (SD1 < 0.10) are unchanged.
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80
5.10.11.3 Seismic Zone 2
The transverse reinforcement requirements at the
top and bottom of a column shall be as specified in
Articles 5.10.11.4.1(d) and 5.10.11.4.1(e).
The requirements of Article 5.10.11.4 shall be taken
to apply to bridges in Seismic Zone 2 except that the
area of longitudinal reinforcement shall not be less than
0.01 or more than 0.06 times the gross cross-section area
Ag.
C5.10.11.3
Bridges in Seismic Zone 2 have a reasonable
probability of being subjected to seismic forces that will
cause yielding of the columns. Thus, it is deemed
necessary that columns have some ductility capacity,
although it is recognized that the ductility demand will
not be as great as for columns of bridges in Seismic
Zones 3 and 4. The most important provision to ensure
some level of ductility is the transverse reinforcement
requirement, which is expected to prevent buckling of
the longitudinal steel and provide confinement for the
core of the column. Nevertheless all of the requirements
for Zones 3 and 4 shall apply to bridges in Zone 2, with
exception of the upper limit on reinforcing steel. This is
a departure from the requirements in the previous
edition of these Specifications where selected
requirements in Zones 3 and 4 were required for Zone 2.
Satisfying all of the requirements, with one exception, is
deemed necessary because the upper boundary for Zone
2 in the current edition is significantly higher than in the
previous edition due to the increase in the return period
for the design earthquake from 500 to 1000 years.
5.10.11.4 Seismic Zones 3 and 4
5.10.11.4.1 Column Requirements
For the purpose of this Article, a vertical support
shall be considered to be a column if the ratio of the
clear height to the maximum plan dimensions of the
support is not less than 2.5. For a flared column, the
maximum plan dimension shall be taken at the minimum
section of the flare. For supports with a ratio less than
2.5, the provisions for piers of Article 5.10.11.4.2 shall
apply.
A pier may be designed as a pier in its strong
direction and a column in its weak direction.
C5.10.11.4.1
The definition of a column in this Article is
provided as a guideline to differentiate between the
additional design requirements for a wall-type pier and
the requirements for a column. If a column or pier is
above or below the recommended criterion, it may be
considered to be a column or a pier, provided that the
appropriate R-Factor of Article 3.10.7.1 and the
appropriate requirements of either Articles 5.10.11.4.1
or 5.10.11.4.2 are used. For columns with an aspect ratio
less than 2.5, the forces resulting from plastic hinging
will generally exceed the elastic design forces;
consequently, the forces of Article 5.10.11.4.2 would
not be applicable.
5.10.11.4.1a Longitudinal Reinforcement
The area of longitudinal reinforcement shall not be
less than 0.01 or more than 0.06 0.04 times the gross
cross-section area Ag.
C5.10.11.4.1a
This requirement is intended to apply to the full
section of the columns. The lower limit on the column
reinforcement reflects the traditional concern for the
effect of time-dependent deformations as well as the
desire to avoid a sizable difference between the flexural
cracking and yield moments. Columns with less than 1
percent steel have also not exhibited good ductility
(Halvorsen 1987). The 6 4 percent maximum ratio is to
avoid congestion and extensive shrinkage cracking and
to permit anchorage of the longitudinal steel. During
development of these requirements, the ATC-6 Project
Engineering Panel gave serious consideration to
reducing the upper limit to 4 percent and recommended
that a lower value be used where feasible. The previous
edition of these Specifications limited this ratio to 6
percent but this cap is lowered in this edition because
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81
the boundaries for Zones 3 and 4 are significantly higher
than in the previous edition, due to the increase in the
return period for the design earthquake from 500 to 1000
years. The 4 percent figure is consistent with that
recommended in recent publications by the NCHRP 12-
49 and 20-7(193) provisions. NCHRP (2002, 2006) and
MCEER/ATC (2003).
5.10.11.4.1b Flexural Resistance
The biaxial strength of columns shall not be less
than that required for flexure, as specified in
Article 3.10.9.4. The column shall be investigated for
both extreme load cases, as specified in Article 3.10.8, at
the extreme event limit state. The resistance factors of
Article 5.5.4.2 shall be replaced for columns with either
spiral or tie reinforcement by the value of 0.50 where the
extreme factored axial load for the column exceeds
0.20f cAg. For compression members with flexure, the
resistance factor may be increased linearly from 0.50 to
the value of specified in Article 5.5.4.2 for flexure
with no axial load when the extreme factored axial load
is between 0.20f ,c Ag and 0.0. 0.9.
C5.10.11.4.1b
Columns are required to be designed biaxially and
to be investigated for both the minimum and maximum
axial forces. The previous edition of these Specifications
reduced the flexural resistance factor from 0.9 to 0.5 as
the axial load increased from 0 to 0.20 f,c Ag., because of
the trend toward a reduction in ductility capacity as the
axial load increases. This requirement is relaxed in this
edition but a P-. requirement has been added (Article
4.7.4.5) to limit the demand on ductility capacity due to
excessive deflection. Also the R-factors have been
maintained at their previous levels (Article 3.10.7) even
though the return period of the design earthquake has
been increased from 500 to 100 years. In both the
NCHRP 12-49 and 20-7(193) provisions, the
recommend flexural resistance factor is 1.0. But since
the current Specifications are force-based and do not
explicitly calculate the ductility demand, as in both 12-
49 and 20-7(193) provisions, limiting the factor to 0.9 is
considered justified in lieu of more rigorous analysis.
For columns with a maximum axial stress exceeding
0.20 f,c Ag, the resistance factor, , is reduced to 0.50 as
shown in Figure C1. This requirement was added
because of the trend toward a reduction in ductility
capacity as the axial load increases. Implicit in this
requirement is the recommendation that design axial
force be less than 0.20 f,c Ag. Columns with axial forces
greater than this value are permitted, but they are
designed for higher force levels, i.e., lower factor, in
lieu of the lower ductility capacity.
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82
On the y-axis of Figure C1, the origins of the solid
lines are the values of/ //specified in Article 5.5.4.2.1
for tension-controlled prestressed and nonprestressed
members. The value of to be used on the y-axis is
determined by the strain condition at a cross-section at
nominal flexural strength with no axial load. If the
analysis indicates a tension-controlled member in pure
flexure, the equations shown for the solid lines in
Figure C1 may be used to calculate the value of to be
used in the column design based on the maximum
extreme factored axial load. If the cross-section
analyzed for pure flexure is compression-controlled, or
is in the transition region between tension-controlled and
compression-controlled members, the appropriate value
of at an axial load of 0.0 may be calculated by Eqs.
5.5.4.2.1-1 or 5.5.4.2.1-2. The value of /to be used in
the column design is then interpolated between this
value of /at 0.0 axial load and 0.50 at an axial load of
0.20f !cAg, as indicated by the dashed line in Figure C1.
Figure C5.10.11.4.1b-1 Variation of Resistance Factor in Seismic Zones 3 and 4.
REFERENCES
Add the following new reference:
FHWA. 2006. Seismic Retrofitting Manual for Highway Structures, Part 1 – Bridges, FHWA Publication No. FHWA-
HRT-06-032, Federal Highway Administration, Washington DC.
MCEER/ATC. 2003. Recommended LRFD Guidelines for the Seismic Design of Highway Bridges. Special
Publication No. MCEER-03-SP03, Multidisciplinary Center for Earthquake Engineering Research, Buffalo NY.
NCHRP. 2002. Comprehensive Specification for the Seismic Design of Bridges. NCHRP Report 472, Transportation
Research Board, Washington DC.
NCHRP 2006, Recommended LRFD Guidelines for the Seismic Design of Highway Bridges. Draft Report NCHRP
Project 20-07, Task 193, TRC Imbsen & Associates, Sacramento CA.
NO FURTHER CHANGES TO SECTION 5
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SECTION 10: FOUNDATIONS
PARTIAL TABLE OF CONTENTS
10.5 LIMIT STATES AND RESISTANCE FACTORS ................................................................................................... 10.5.1 GENERAL ................................................................................................................................................................ 10.5.2 SERVICE LIMIT STATES ...........................................................................................................................................
10.5.2.1 General......................................................................................................................................................... 10.5.2.2 Tolerable Movements and Movement Criteria ............................................................................................ 10.5.2.3 Overall Stability........................................................................................................................................... 10.5.2.4 Abutment Transitions ..................................................................................................................................
10.5.5 RESISTANCE FACTORS ............................................................................................................................................ 10.5.5.1 Service Limit States ..................................................................................................................................... 10.5.5.2 Strength Limit States ...................................................................................................................................
10.5.5.3 Extreme Limit States ................................................................................................................................... 10.5.5.3.1 General............................................................................................................................................... 10.5.5.3.2 Scour.................................................................................................................................................. 10.5.5.3.3 Other Extreme Limit States ...............................................................................................................
APPENDIX A10................................................................................................................................................................ A10.1 Investigation........................................................................................................................................................... A10.2 Foundation Design ................................................................................................................................................. A10.3 Special Pile requirements....................................................................................................................................... REFERENCES ..................................................................................................................................................................
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10.5.4 Extreme Events Limit States
Foundations shall be designed for extreme events as
applicable.
10.5.4.1 Liquefaction Design Requirements
Liquefaction assessment shall be required for
bridges in Seismic Zone 4. If it is determined that
liquefaction can occur at a bridge site then the
bridge shall be supported on deep foundations or
the ground improved so that liquefaction does not
occur. For liquefied sites subject to lateral flow, the
Engineer shall consider the use of large diameter
shafts in lieu of the conventional pile cap
foundation type in order to minimize lateral flow
demands on the bridge foundation. If liquefaction
occurs then the bridge shall be designed and
analyzed in two configurations as follows:
+ Nonliquefied Configuration. The structure shall
be analyzed and designed, assuming no
liquefaction occurs using the ground response
spectrum appropriate for the site soil
conditions.
+ Liquefied Configuration. The structure as
designed in nonliquefied configuration above
shall be reanalyzed and redesigned, if
necessary, assuming that the layer has liquefied
and the liquefied soil provides the appropriate
residual resistance (i.e., P-y curves or modulus
of subgrade reaction values for lateral pile
response analyses consistent with liquefied soil
conditions). The design spectra shall be the
same as that used in nonliquefied configuration
unless a site-specific response spectra has been
developed using nonlinear, effective stress
methods that properly account for the buildup
in pore-water pressure and stiffness degradation
in liquefiable layers. The reduced response
spectra resulting from the site-specific
nonlinear, effective stress analyses shall not be
less than 2/3 of that used in the nonliquefied
configuration.
The Designer shall cover explicit detailing of plastic
hinging zones for both cases specified above since
it is likely that locations of plastic hinges for the
Liquefied Configuration are different than locations
of plastic hinges for the Nonliquefied
Configuration. Design requirements of Zone 4
including shear reinforcement shall be met for the
liquefied and nonliquefied configuration.
C10.5.4
Extreme events include the check flood for scour,
vessel and vehicle collision, seismic loading, and other
site-specific situations that the Engineer determines
should be included. Appendix A10 gives additional
guidance regarding seismic analysis and design.
C10.5.4.1
Liquefaction below a spread footing foundation can
result in three conditions that lead to damage or failure
of a bridge:
+ Loss in bearing support which causes large
vertical downward movement,
+ Imposition of horizontal forces on the footing
from lateral flow or lateral spreading of the
soil, and
+ Settlements of the soil as pore water pressures
in the liquefied layers dissipate.
Most liquefaction-related damage during past
earthquakes has been related to lateral flow or spreading
of the soil. In these cases ground movements could be 3
ft. or more. If the spread footing foundation is located
above the water table, as is often the case, it will be very
difficult to prevent the footing from being displaced
with the moving ground. This could result in severe
column distortion and eventual loss of supporting
capacity.
In some underwater locations, it is possible that the
lateral flow could move past the footing without causing
excessive loading; however, these cases will be limited.
If liquefaction with no lateral flow occurs for
Seismic Zone 4 bridges, then the only additional design
requirements are those reinforcement requirements
specified for the piles. Additional analyses are not
required, although for essential or critical bridges
additional analyses may be considered in order to assess
the impact on the substructures above the foundation.
If liquefaction and lateral flow are predicted to occur
for Seismic Zone 4, a detailed evaluation of the effects
of lateral flow on the foundation should be performed.
Lateral flow is one of the more difficult issues to address
because of the uncertainty in the movements that may
occur. Ultimate plastic rotation of the piles is permitted. This
plastic rotation does imply that the piles and possibly other
parts of the bridge will need to be replaced if these levels of
deformation do occur. Design options range from (a) an
acceptance of the movements with significant damage to
the piles and columns if the movements are large, to (b)
designing the piles to resist the forces generated by lateral
spreading. Between these options are a range of mitigation
measures to limit the amount of movement to tolerable
levels for the desired performance objective. Pile group
effects are not significant for liquefied soil.
175
85
SECTION 11: ABUTMENTS, PIERS AND WALLS
PARTIAL TABLE OF CONTENTS – SEISMIC DESIGN
11.6 ABUTMENTS AND CONVENTIONAL RETAINING WALLS……………………………………………..
A11.1.1.2 Design For Displacement..........................................................................................................................