DYNAMIC RESPONSE OF BRIDGES TO NEAR-FAULT,
FORWARD DIRECTIVITY GROUND MOTIONS
Cole C. McDaniel, Ph.D., P.E.
Assistant Professor
Department of Civil & Environmental Engineering
and
Adrian Rodriguez-Marek, Ph.D.
Assistant Professor
Department of Civil & Environmental Engineering
Washington State University (WSU)
Transportation Research Center (TRAC)
August 2, 2004
Final
TABLE OF CONTENTS
Section
Page
Problem Statement
2
Background
2
Objectives
4
Benefits
4
Products
5
Implementation
5
Work Plan
5

Task 1
5

Task 2
5

Task 3
6

Task 4
6

Task 5
7

Task 6
7

Task 7
7
Staffing Plan
7
Level of Effort
8
Facilities Available
8
Supporting Data
8
Work/Time Schedule
8
References
9
Budget Estimate
11
1
PROBLEM STATEMENT
Research over the last decade has shown that pulse-type earthquake ground motions that
result from forward-directivity effects can result in significant damage to structures. Both
experimental evidence (Makley 2001) and observations in recent earthquakes (e.g. the 1994
Northridge earthquake and the 1995 Kobe earthquake, Alavi and Krawinkler 2000) support this
assessment. Furthermore, analytical models (e.g. Krawinkler and Alavi 1998) indicate that
traditional analysis methods are insufficient to capture the full effects of pulse-type ground
motions. Fortunately, the recent increase in the number of recorded ground motions has
allowed a better characterization of this near fault, forward-directivity ground motions (FDGMs;
Mavroeidis and Papageorgiou 2003, Bray and Rodriguez-Marek 2004). The objective of this
research is to use the wealth of recent ground motion data to improve the understanding of the
response of typical reinforced concrete and precast concrete bridges to pulse-type ground
motions that result from forward-directivity effects. Increased clarity concerning FDGMs and the
structural response to this type of ground motion will result in direct benefits to communities
across the United States exposed to nearby faults thus resulting in reduced seismic risk as well
as the opportunity for improved resource allocation.
BACKGROUND
In the near-fault region, ground motions at a particular site are significantly influenced by the
rupture mechanism and the rupture direction relative to the site, as well as the permanent
ground displacement at the site resulting from tectonic movement. Depending on the first two
factors, ground motions in the near-fault zone can exhibit the dynamic consequences of
“forward-directivity,” “neutral-directivity,” or “backward-directivity.” Depending on the last factor,
ground motions close to the rupture surface may contain a significant permanent static
displacement, which is termed “fling-step” (Bray and Rodriguez-Marek 2004). The estimation of
ground motions for a project site close to an active fault should account for these special
aspects of near-fault ground motions. The "fling-step" usually induces only limited inertial
demands on structures due to the long-period nature of the static displacement. On the other
hand, ground motions that are influenced by forward-directivity effects can be very damaging to
structures. Forward-directivity effects are seen when the rupture direction is aligned with the
direction of slip, and the rupture front moves towards a given site (Bray and Rodriguez-Marek
2004). These conditions occur readily in strike-slip earthquakes when the rupture propagates
horizontally towards a given site. Forward-directivity conditions are also met for dip-slip faulting
at sites that are located close to the surface projection of the fault. Whereas in a strike slip
earthquake forward-directivity effects can be observed at all locations along the fault away from
the hypocenter, in dip-slip earthquakes forward directivity effects are concentrated in a limited
region up-dip from the hypocenter (Somerville 2003). FDGMs typically contain very few long
period, high intensity ground motion pulses that are best observed in velocity time histories
(Figure 1). Due to the radiation pattern of the fault, these pulses are typically aligned with the
fault normal direction. However, strong pulses may be present in the fault parallel direction as
well (Bray and Rodriguez-Marek 2004). These motions typically have a short duration with
amplitudes larger than those of generic motions, and with a strong preferential fault-normal
orientation.
The effects of FDGMs on structures were first recognized in the 1970’s (Bertero 1976),
however, engineers largely ignored FDGMs in structural design until after the 1994 Northridge
earthquake. Since then, a number of studies have been directed at the effect of near-fault
ground motions on structural response, prompting revision of design codes. In current practice,
2
rupture directivity effects are generally taken into account by modifications to the elastic
acceleration response spectrum at 5% damping (Somerville et al. 1997, Somerville 2003).
Velocity (cm/s)
100
Erzincan
50
0
-50
Soil
-100
0
5
10
15
20
Velocity (cm/s)
150
Lucerne
75
0
-75
Rock
-150
0
5
10
15
20
Velocity (cm/s)
50
Kobe University
25
0
-25
Rock
-50
0
5
10
15
20
Velocity (cm/s)
50
Gebze
25
0
-25
Rock
-50
0
5
10
15
20
Period (s)
Figure 1. Typical forward-directivity motions recorded in various earthquakes: a) 1992 Erzincan
Earthquake, b) 1992 Landers Earthquake, c) 1995 Kobe Earthquake, d) 1999 Kocaeli
Earthquake.
However, recent research has found that a time-domain representation of FDGMs is preferable
over frequency-domain representations (Krawinkler and Alavi 1998). This is because traditional
response spectrum representations of ground motions do not adequately represent the demand
for a high rate of energy absorption presented by near-fault pulses. More specifically, when the
high intensity levels of these motions drive structures into the nonlinear range, the linear-elastic
assumption underlying the response spectrum concept is invalidated (Somerville 2003).
Recent near-fault ground motion research with respect to structures includes work by Makris
and Black (2003) on dimensional analysis of structures subjected to near-fault ground motions,
Iwan (1995) on specification of near-fault ground motions, Yang and Agrawal (2002) on the use
of passive and semi-active control systems for near fault applications, Filiatrault and Trembley
(1998) on the use of passive dampers in near field applications, Symans et al. (2003) on the use
of passive dampers in wood structures subject to near-fault ground motions, Chopra and
Chintanapakdee (2001) on the use of drift spectrum versus modal analysis for structural
response to near-fault ground motions, and Krawinkler and Alavi (1998) on improving design
procedures for near-fault ground motions. Although there has been an increase in research on
near-fault ground motions, significant work is still needed to provide an improved understanding
3
of the response of structures to FDGMs and to develop appropriate design provisions (Alavi and
Krawinkler 2000; Milonakis and Reinhorn 2001; Zhang and Iwan 2002). There is still significant
uncertainty in how to properly account for FDGMs, as illustrated by the latest changes to the
design for FDGMs in building codes and the current lack of recognition of the effect of the nearfault pulse period on the response of structures. Research is needed in the area of soilstructure interaction in near-fault ground motions as well to determine the influence of soil type
on the FDGMs and the corresponding structural response. Rodriguez-Marek (2000), based on
an empirical analysis of recorded FDGMs and site response simulations, showed that a) the
pulse period and pulse amplitude of FDGMs can be extremely high, b) site response can play
an important role in both the pulse period and the pulse amplitude, and c) the fault-parallel
component of forward-directivity motions, traditionally ignored, can also have significant
amplitudes.
Although FDGMs pose a significant threat to structures, this threat is not equal for all structures.
For example, coincidence of the structure and pulse period intuitively leads to the largest
structural response for a given earthquake. However, the period of the structure and the pulse
period can vary significantly. The FDGM pulse period is proportional to the earthquake
magnitude, lengthening as the earthquake magnitude increases. As a result, damage due to
smaller magnitude earthquakes can be more significant for short period structures than damage
due to larger magnitude earthquakes, since the near-fault pulse period is closer to the
fundamental period of the structure in the smaller magnitude earthquake. This contradicts
conventional engineering intuition that directly correlates damage potential with earthquake
magnitude, thus highlighting the need for a unique way to accurately assess the potential for
structural damage due to FDGMs. The near-fault pulse can impose an additional damage
variable on structures: large residual deformations. Although consisting only of a few cycles,
the pulses can impose large inelastic drift on structures, resulting in significant permanent
deformations. Not only are conventional damage indices such as maximum displacement and
energy absorbed important for assessing the response of structures, alternatives including
residual displacement are necessary as well (Priestley, 2003).
OBJECTIVES
The technical objectives of this research include:
 Compile an updated database of near-fault, FDGMs.
 Determine influence of FDGMs on structural response.
 Determine influence of site response and soil-structure interaction on the seismic
demand to structures subject to FDGMs.
 Provide FHWA and WSDOT with design and assessment recommendations for
bridges likely to be affected by near fault, FDGMs.
BENEFITS
This research will benefit the profession by reducing the uncertainty associated with near-fault
ground motions and the resulting structural response. Many structures are founded in close
proximity to faults and must account for this hazard. However, current methods do not properly
consider FDGMs. This is partly due to the lack of recorded near-fault ground motions and the
difficulty in characterizing the near-fault ground motions for sites without recorded time history
4
records. This research will provide a more accurate prediction of FDGMs including the nearfault pulses, which will be used to assess the response of structures to FDGMs.
This research will have direct benefits to society by providing adequate methods of risk analysis
that will allow for better resource allocation and life protection. Near-fault ground motions, and
in particular FDGMs, pose a significant risk to long period structures, and consequently bridges
in particular, in large urban centers in the United States. Since cities such as Los Angeles, San
Francisco, and Seattle, as well as population centers near the New Madrid seismic zone, are
underlain by faults capable of producing significant ground motions enhanced by forwarddirectivity effects, the result of this research will directly benefit these communities.
PRODUCTS
The following products will be provided to the research sponsor:
 Quarterly progress reports
 Draft and final (camera-ready) project report
 One-page project summary
 Conference papers or refereed journal articles
IMPLEMENTATION
The results of the proposed research will enable WSDOT and FHWA to improve their
assessment of bridge vulnerability to FDGMs and thereby improve new design and allocate
funding for bridge retrofit. Research results will be disseminated through presentations at
national conferences, publication in journals and/or conference proceedings, and delivery of
final reports to FHWA and WSDOT.
WORK PLAN
The research proposed is divided into seven tasks that are interdependent. Close interaction
between all participants of the research is expected. The different tasks are described below,
including the allocation of responsibility.
Task 1: Literature review of current FDGM research and design practice
A literature review will be made of research on FDGMs and structural response. Current nearfault seismic design provisions for bridges will be assessed as well, focusing on understanding
the basis in the current provisions for both the ground motion demand and the bridge capacity
under FDGMs.
TASK 2: Selection of forward-directivity ground motions
The characterization of FDGMs has been the subject of a number of recent studies (Somerville
et al. 1997, Krawinkler and Alavi 1998, Somerville 1998, Rodriguez-Marek 2000, Bray and
Rodriguez-Marek 2004). A frequency-domain characterization (e.g. Somerville et al. 1997) is
commonly used in seismic hazard analysis (Abrahamson 2002) and is common in current
engineering practice. A time-domain characterization, however, is preferable to traditional
frequency-domain characterizations due to the highly non-stationary nature of pulse-type
motions. Moreover, as indicated before, researchers have shown that the spectral analyses
prescribed in current codes cannot fully capture the effect of pulse-type motions on structural
5
response (e.g. Krawinkler and Alavi 1998). In this research, an updated database of near-fault,
FDGMs will be compiled. This database will be an expansion of a database compiled by one of
the co-PIs (Rodriguez-Marek 2000).
Based on the updated database of FDGMs, ground motions will be chosen for the bridge
analyses. Vertical ground motions, which are often ignored in analysis, will be included in
ground motion selection to study the influence of vertical ground motions on the bridge
response to FDGMs. Ground motions will be selected based on matching the seismo-tectonic
factors that control forward-directivity to those expected at the selected bridge sites. Both a
time-history base selection (e.g. based on time-domain parameters of the ground motions) and
a response-spectrum based selection procedure will be evaluated. Whenever appropriate,
ground motions will be modified to fit a target 5% damped acceleration spectrum using the
program RSPMATCH (Abrahamson 1998). The acceleration spectrum will be generated using
appropriate attenuation relationships that account for the effect of forward-directivity in the
response spectrum (Somerville et al. 1997). RSPMATCH alters the frequency content of a
ground motion by adding pulses of motion in the form of tapered cosine waves. The end result
is a ground motion of the desired frequency content and peak ground acceleration without
significantly altering the time signature of the original ground motion
TASK 3: Selection of bridges for analysis and development of bridge models
Typical reinforced concrete and precast concrete bridges in western Washington State will be
selected based on interaction with WSDOT and a review of maps of active faults. Current
bridge assessment methodologies and bridges deemed to be vulnerable will be discussed as
well. Three-dimensional nonlinear finite element models of the bridges will be developed to
study the response of the structures subject to FDGMs. The bridges will be assessed in depth
including the monitoring of bridge column inelastic demand/capacity ratios and key bridge
details such as the connection of prestressed bridge girders at bridge bents and abutments. In
addition, a variety of boundary conditions and soil types will be investigated to determine the
influence of soil-structure interaction on the response of bridges subject to FDGMs.
TASK 4: Perform nonlinear time history analyses of the selected bridges
Damage to structures in near-fault ground motions can be significant as was illustrated in the
1994 Northridge earthquake and 1995 Kobe earthquake (Alavi and Krawinkler, 2000) and must
be accounted for directly by engineers. Currently, there is uncertainty in the engineering
profession with how to design structures to adequately resist near-fault ground motions. Key
factors to structural response under FDGMs include the ratio of the structure period to the nearfault pulse period, as well as the variance of the pulse period and amplitude with earthquake
magnitude. As the earthquake magnitude increases, the pulse period increases as well,
thereby uniquely affecting structures for a given earthquake magnitude. The ratio of the demand
in the fault normal and fault parallel directions needs study as well to provide guidance for
practicing engineers designing for FDGMs.
The nonlinear bridge models will be used to explore these issues and others leading to a better
understanding of both near-fault ground motion input and structural response. The selected
bridges will be analyzed using dynamic direct displacement-based assessment principles
(Priestley, 2003) incorporating soil-structure interaction. To determine the bridge demands and
capacities, three-dimensional nonlinear time history analyses will be conducted using the
ground motions developed in Task 2. Key performance parameters will include member flexural
and shear force demands, member inelastic rotation demands, bridge deck connection
demands, bridge abutment demands, and overall system drift demands. In addition to the
conventional damage indices, residual displacement will be included in the damage assessment
6
as well (Priestley, 2003). Since site response can play an important role in both the FDGM
pulse period and the pulse amplitude, the influence of site response will be bound by modeling
the range of soil conditions expected in bridge sites in western Washington State. Soil-structure
interaction effects on FDGMs will also be evaluated for selected site conditions. Depending on
the sensitivity of the structural response to the soil property assumptions, additional research
will be performed to give guidance for how to properly model soil-structure interaction. Typical
WSDOT response spectra analyses will also be performed on the selected bridges in order to
compare the results to nonlinear time history analyses to illustrate the advantage of using a
time-domain characterization of FDGMs for an accurate assessment of bridge vulnerability.
TASK 5: Design and assessment conclusions and recommendations for bridges subject to
FDGMs
Conclusions will be made regarding bridge vulnerability to FDGMs. Recommendations for
improved analytical procedures based on this research and recommendations for bridge design
details to resist FDGMs will be developed. Engineers are open to improved design and analysis
techniques as long as the principles are based on sound research and the implementation of
the techniques is straightforward and efficient. Federal and state departments of transportation
will be engaged to ensure that the research results can be beneficial to industry. Examples of the
bridge seismic assessments will be clearly documented as well in order to encourage the
implementation of the recommendations from this research.
TASK 6: Presentation to FHWA and WSDOT
A presentation will be made to appropriate FHWA and WSDOT bridge personnel on the major
conclusions and implications resulting from this research.
TASK 7: Project Report and Publications
STAFFING PLAN
Each of the PIs is well qualified to perform this research with both their Ph.D. research and
current research activities focused on FDGMs.
Dr. McDaniel, Assistant Professor of Civil Engineering (WSU) is a civil engineer (structural
earthquake engineering emphasis). He will take a leading role in the structural portion of this
research, focusing on the response of structures subject to FDGMs. Dr. McDaniel’s Ph.D. work
(McDaniel, 2002) focused on the seismic response of the new San Francisco-Oakland Bay
Bridge (SFOBB) East Span, which is located on a near-fault site. The near-fault pulse had a
large impact on the bridge design. Current research by Dr. McDaniel is focused on near-fault
motions as well, implementing passive dampers to mitigate the effects of the near-fault ground
motions on SDOF systems and the new SFOBB East Span.
Dr. Rodriguez-Marek, Assistant Professor of Civil Engineering (WSU), is a civil engineer
(geotechnical earthquake engineering emphasis). Dr. Rodriguez-Marek has done extensive
research on earthquake engineering with a particular focus on site response and ground motion
characterization. Dr. Rodriguez-Marek has been among the first to work on time-domain
characterization of forward-directivity ground motions. Dr. Rodriguez-Marek’s current research
is focused on site response and soil-structure interaction for forward-directivity ground motions.
Dr. McDaniel and Dr. Rodriguez-Marek teach classes in structural and geotechnical earthquake
engineering, respectively.
Each of these classes is attended both by structural and
7
geotechnical graduate students. The collaborative nature of this research will undoubtedly
result in a broader view of earthquake engineering for each of the PIs. This, in turn, should
result in classes that better address issues where communication between structural and
geotechnical engineers is key. The PIs intend to immediately introduce the results of this
research into the content of the classes, including lecture notes, homework, and projects.
The PIs are requesting funding for two M.S. students over an 18-month period, and a 24-month
period, respectively. The M.S. students will work jointly with the PI’s in all stages of the project.
LEVEL OF EFFORT
Level of Effort (Hours)
Personnel
Task 1
C. McDaniel
40
Task 2
20
Task 3
80
Task 4
160
Task 5
80
Task 6
40
Task 7
120
Total
600
A. Rodriguez-Marek
40
120
60
40
40
40
120
340
M.S. studentStructural Eng.
M.S. studentGeotechnical Eng.
160
40
600
600
360
0
240
1800
160
500
300
100
100
0
120
1300
FACILITIES AVAILABLE
The analyses for this project will be performed on PC’s obtained for this project. RUAUMOKO
3D (Carr 2004), a three-dimensional nonlinear finite element analysis program, will be used for
the bridge modeling and analysis. Ground motions will be modified to fit a target 5% damped
acceleration spectrum using the program RSPMATCH (Abrahamson 1998).
SUPPORTING DATA
The experience and capabilities of the principal investigators are summarized in the STAFFING
PLAN section of this proposal. Academic resumes for the investigators are on file with WSDOT.
WORK TIME SCHEDULE
Task 1
Task 2
Task 3
Task 4
Task 5
Task 6
Task 7
2004
Fall
X
X
X
2005
Spring Summer Fall
X
X
X
2006
Spring Summer Fall
2007
Spring Summer
X
X
X
X
X
X
X
8
X
X
X
X
X
X
X
X
X
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BUDGET ESTIMATE
See attached page.
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