Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, Japan

Lessons from Recent Earthquakes:
The Need for More Resilient Cities
Stephen Mahin11Professor, Department of Civil and Environmental Engineering, University of California,
Berkeley, USA, mahin@berkeley.edu

ABSTRACT: Safety of occupants is of paramount importance in the design of any
structure or system. However, in the aftermath of recent major earthquakes, worldwide
attention has increasingly focused on the need for resilient structures and lifelines that areable to return to service quickly following a major earthquake. In this paper, engineering
aspects of resilient communities are discussed, focusing on increasing the post-earthquake
operability of those structures and lifelines critical to a community’s needs in the aftermathof a major earthquake and the ability of occupants to “shelter-in-place” during repairs.
Key Words: design criteria, continued occupancy, buildings, bridges, seismic isolation,
protective structures, resiliency, resilient communities

INTRODUCTION
Engineers specializing in the field of earthquake engineering are constantly reminded of theimportance of their work by the worldwide occurrence of earthquakes, both great and small.
Earthquakes pose a threat to nearly every country in the world. While experimental and analyticalresearch and design studies provide important sources of information for continual improvement ofbuilding codes and construction practices, the largest impetus for change comes from the disparity
between the observed and expected behavior. Sometimes a region is inadequately prepared for anearthquake, and efforts are made quickly to improve the quality and enforcement of building codes.
In other cases, earthquake damages identify shortcomings in fundamental theory or detailing andanalysis requirements, and research is carried out to determine appropriate remedies for these
problems. In other cases, public expectations regarding the performance of the constructedenvironment have evolved, and design criteria should change in keeping with these expectations.
Architectural styles, construction types, building codes, quality of engineering and construction,
seismic hazard, and public expectations differ significantly from region to region around the world. As
such, the performance of structures and lifelines observed in one region may be acceptable to those
who live there, even though similar behavior in another region to a similar event might not be viewed
positively by those living in that region. Nonetheless, because of the substantial time between theoccurrence of design level or greater seismic events in any one region of the world, engineers andpublic officials can analyze and draw valuable lessons from the performance of structures and systems
that occur in other regions of the world. In many cases, these lessons are drawn from individual
structures or types of structures regarding fundamental issues of structural mechanics and dynamics.
However, there is increasing recognition that the impact of an earthquake on the occupants and ownerof a structure depends on more than the degree of structural damage to just that structure, and that andamaging earthquake can transform from a survivable disaster to a catastrophic event if the extent ofeven moderate damage to structures and lifelines results in the closing of a critical number of key
	
 

	
 

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businesses, services and housing units.
Moreover, in many industrialized countries, the public and government officials have come to
expect high levels of security in a citizen’s everyday life, whether this is while in the home, travelingand in the workplace. For examples, spalling and cracking associated with formation of plastic
hinges are pointed out as structural failures by public media, while they are expected according toductile design principles. As such, it appears that there is a growing discrepancy between the
public’s expectations of well being in the face of natural disasters, like earthquakes, and the focus bymany engineers on providing the minimum seismic design needed to protect life safety.
Recent earthquakes in China, Chile, New Zealand and Japan have been as large or larger than the
maximum levels of excitation considered in the design of typical structures. These events have tested
many structures to their limits. In most cases, modern structures have done well. However, it is clear
from all of these events that large earthquakes trigger other disasters that compound the disaster andmay elevate its status to that of a catastrophe. For example, earthquake ground shaking in China not
only leads to the collapse of many individual buildings, but also to an extraordinary number oflandslides that caused additional damages and hampered rescue and recovery operations. Both Chileand Japan suffered from the effects of tsunami following the main earthquake ground shaking. The
severity of ground shaking in New Zealand, Japan and Chile were large enough to trigger large-scaleliquefaction and differential ground displacement. While such issues are often considered in thedesign of an individual structure, the cascading effects of all of them occurring in a region as a resultof the same event and transforming a natural disaster to a catastrophic regional event have only
recently come to the forefront of discussions in earthquake engineering.
To accelerate development of cost-effective and robust procedures for seismic resistant design thattake into account the particular technological, economic and social conditions prevalent in various
regions susceptible to seismic hazards, it is necessary for earthquake engineering specialists fromaround the world to work together.
In this paper, observations from recent earthquakes worldwide
are reviewed to identify some new and some old lessons. From these, the need to improve theresilience of buildings, transportation structures and other lifelines is highlighted. For buildings,
effective approaches to minimize damage to structural and nonstructural components are stressed. In
the case of bridge structures, approaches are examined whereby highway and rail bridges can be used
immediately following a major earthquake to facilitate emergency response and promote rapid
recovery. Results of shaking table tests, nonlinear dynamic analyses and economic evaluations arepresented to illustrate the possible approaches to increase resiliency.
LESSONS LEARNED AND RELEARNED FROM RECENT EARTHQUAKES
Recent earthquakes in Chile, New Zealand and Japan reiterate many lessons learned in pastearthquakes. For example, recent earthquakes remind us of the substantial risk posed by existing
structures and lifelines constructed when earthquake engineering concepts were not as advanced asthey are today. Tremendous damage was observed in New Zealand in unreinforced masonry structures,and damage was observed in all three countries in reinforced concrete constructed prior to the adoptionof modern ductile detailing requirements. Other lessons re-learned include the need for providing acontinuous load path, avoiding discontinuities in the lateral-load resisting systems, designingconnections to develop the capacities of the elements framing into them, protecting systems frompotential failures of the supporting soil, and so on.
There are extraordinary opportunities to learn from these recent earthquakes. Substantial amountof instrumentation was installed throughout Japan, Chile and New Zealand to measure the movementof the ground, water and structures. Moreover, many efforts have been undertaken to document in highdetail direct damages and the economic, social, political, medical and other impacts of this damage.
Some newly emphasized lessons and areas for research are described below, with emphasis on
lessons from the Great Eastern Japan Earthquake of 2011.
Effects of ground motions on engineered facilities
Earthquakes in Chile, New Zealand and Japan are often noted as being larger than expected events.
While these events were indeed large, generally predictions of future earthquakes are probabilistic in
	
 

	
 

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nature. These events should serve as a warning that median probabilistic estimates are notdeterministic bounds on what may occur in future events.
Specific ground motion observations are provided below related to subduction zone events, theimportance of aftershocks and special issues raised regarding near and far filed ground shaking.
Subduction zone events
The subduction zone ruptures in Chile and Japan illustrate the potential for the damaging effects ofsuch earthquakes to be experienced over a vast geographic area. Strong motion records are often very
intense (<2.9 g) and very long (3 minutes). The motions in Japan are particularly complex, showing
strong evidence of multiple segments rupturing sequentially along the fault. There are several
thousands of records from these earthquake and their aftershocks, especially in Japan, including ones
at free field sites, down-hole arrays and in buildings. Efforts are underway to better characterize thesefeatures and their effects on structures and soils. The potential of subduction zone events to damage alarge numbers of buildings spread over a large geographic region may suggest need for design criteriathat mitigate the severity of this damage.
The occurrence of a larger than expected subduction zone event in Japan has prompted
considerable discussion of the potential of other large subduction zone events in Japan and otherlocations around the Pacific Ocean. The possibility of these “larger than expected events” has asubstantial influence on predictions of future tsunami, both locally in the region of a fault rupture, butalso in transoceanic tsunami waves from sources such as the Cascadia and Aleutian subduction zones
in North America, as well as similar zones in Asia and South America.
Near and far field motions
While subduction zone events in Chile and Japan provide new challenges to engineers, damages inChristchurch, New Zealand, and in many instances in the Wenchuan earthquake in China were due tothe intensity of shaking that occurred in the near field. Because Japan (and Chile) are susceptible tosuch near-field fault ruptures, as in the 1995 Hyogo-Ken Nambu earthquake, studies to better
understand the special features of off-shore subduction ground motions and on-shore fault rupturesthat lead to damage are needed.
In Japan and China, substantial evidence exists for tall buildings and other long period structures
being strongly excited by earthquake shaking more than 500 km from the fault. Thus, these types of
motions need to be carefully investigated as well.
Importance of aftershocks
Large events trigger large numbers of aftershocks. Better understanding of the number, severity andlocation of aftershocks is needed. The cumulative effects of multiple aftershocks following large
events on engineered structures and soil deposits need to be better understood and perhaps accounted
for in design of structures. While aftershocks are generally smaller than the main shock, they can belocated substantially closer to a particular structure or community than the main shock, and cause
substantially more damage, as observed in Christchurch, New Zealand. Even small accumulations ofdamage during aftershocks raise questions about the safety of rescue workers and construction crews
working on repairs or demolition. There is evidence in Japan (and elsewhere) that aftershocks
re-damaged structures where damage from the main shock had already been repaired.
Large tsunami accompany large earthquakes
The tsunami resulting from earthquakes in Japan and Chile caused severe widespread damage in thosecountries as well as in other countries around the Pacific Ocean. Tsunami waves, with heights
measuring up to 40 meters in Japan, were responsible for the majority of earthquake casualties there
and a large portion of the physical damage and loss. There is thus a need and opportunity tobenchmark and improve abilities to predict tsunami waves, their interaction with costal geometries andstructures, and the regions of expected flooding. The effectiveness of early warning systems andevacuation procedures should be studied. Similarly, there is a need to understand better and improvethe behavior of engineered structures to tsunami wave action, impact by debris and scouring. In
particular, the effectiveness of evacuation methods and the design of structures as vertical evacuation
shelters should be investigated.
	
 

	
 

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Liquefaction- or settlement-related damage to structures
Extensive liquefaction and related phenomena have been observed Chile, New Zealand and Japan inareas near rivers and coastlines. In Japan, permanent vertical and horizontal displacements of thesoils supporting a structure’s foundation were responsible for much of the damage to structures awayfrom the tsunami-affected zone. Liquefaction and differential settlement also contributed to heavily todamages observed in and near Christchurch, New Zealand. Such damages were seen over a large
geographic region, and occurred for a wide variety of soil types and ground motion characteristics. As
such, there is an opportunity and need to improve our understanding of the triggering of liquefaction
and/or lateral spreading, the deformations that occur, and the consequences of soil spreading andliquefaction on the behavior of supported structures. Methods for repair and restoration of structures
damaged by differential settlement need additional study as well.
Damage to lifelines, industrial facilities, such as nuclear power plants, can exacerbate disasters
There are a multitude of engineering issues raised by the response of the Fukushima Daiichi Nuclear
Power Plant to the initial shaking and aftershocks as well as to the tsunami. In addition, more than 20
other fossil fuel and nuclear power plants were taken off line immediately following the earthquake inJapan. Many of these remain off line. These outages have had a critical effect on Japanese businesses
and overall quality of life. Special issues related the behavior of nuclear power plants (and othercritical and hazardous industrial facilities) to earthquakes and tsunami, the effect of the inoperability ofcritical electric power lifeline facilities on a community, region and nation, and the special issues
related to radioactive or possible chemical/biological contamination, are high priority topics for futherinvestigation.
Disruption of Business and Social Systems
Wide spread economic and social disruption has resulted from the damage to housing, schools,
hospitals, commercial structures, factories and infrastructure systems in Japan and New Zealand. In
many cases, a facility’s structural system may not have been substantially damaged, but damage tononstructural elements and equipment as well as loss of lifelines (power, water, gas, transportation,communications, etc.) rendered it inoperable. Transportation was impaired since several national andlocal highways were closed due to ground shaking and landslides, and in the tsunami-affected region,
many highway and railway bridges were completely destroyed. In some areas, manufacturing andother critical facilities in the tsunami-affected zone suffered little structural damage, but were
inoperable due to water damage or the presence of debris. Thus, in addition to general economic,
business and related studies, investigations on improving the seismic resistance of the nonstructuralcomponents and equipment and operationally critical lifelines are needed.
The effect of the earthquake shaking on engineered facilities
Modern buildings and other structures are not designed to be damage free during rare earthquakes.
While there were substantial numbers of damaged structures in Japan, Chile and New Zealand, many
damaged buildings tended to be older with known deficiencies. In Chile, newer tall reinforced concretebuildings suffered damage in part due to extending the use of systems that had behaved well in the past
to structures having considerably greater height. In both Chile and New Zealand, newer tallerbuildings suffered structural damage and total or partial collapse due to discontinuous or irregularstructural systems. In many buildings in heavily shaken areas, substantial damage to nonstructuralelements caused expensive and disruptive delays in restoring use of a facility.
A particularly significant consequence of the vibration and damage to buildings in Japan is a lackof confidence in the safety of structures. The vibrations of tall buildings in particular, while notindicative of structural damage or danger, frightened many occupants so that they were reluctant tore-enter buildings and in some cases property values diminished greatly.

	
 

	
 

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RESILIENT STRUCTURES
As noted above, there are many fruitful topics for study to improve the assessment and design ofearthquake resistant structures. However, it appears that new issues related to the ability of abuilding to be repaired quickly following an earthquake, and for a community or region to recoverrapidly, are becoming of more concern. As such, many researchers and practitioners worldwide areworking to develop economical systems that can dependably permit engineered facilities and lifelines
to continue functioning even following a large seismic event.
Such solutions are critical in the case of facilities such as hospitals, emergency command centers,
and lifeline systems needed for emergency response and recovery. The types of facilities where
post-earthquake functionality should be maintained can be much larger, if consideration is given to
facilities needed to ensure rapid recovery of a community or organization. For example, the San
Francisco Planning and Urban Research Association (SPUR, 2009) has indicated that many schools,
residential buildings, and even retail and commercial buildings are necessary to provide the housing
and services that would enable a community to function following an earthquake. Similarly, thevitality and perhaps viability of a business depends on its ability to provide continuous service to its
customers.
As such, there is increasing recognition that the traditional approach of designing “ductile”
structures for collapse prevention while necessary, may not be sufficient in a modern society.
Engineering solutions that can with confidence preserve operations, or provide the promise ofminimizing the disruption and cost of repairs following major earthquakes, are needed.
To minimize post-earthquake disruption, a structure or system must be able to limit the damage
occurring in structural and nonstructural components, and should not exhibit permanent displacements.
Structures could simply be made stronger so that they remain essentially elastic for the level ofearthquake for which resilience is desired. However, this strength-based approach (1) requires morematerials that add to the cost and carbon footprint of the structure, and (2) results in higher
accelerations that necessitate special attention in the design of nonstructural components, equipment
and contents. Moreover, such strength approaches are only as good as the estimates of future shaking.
While many types of structures that behave nonlinearly are being explored to enhance resilience,
many share the following characteristics:
1. They are designed with an explicit nonlinear deformation mechanism that has only a modest
strength in order to reduce cost as well as in-structure accelerations,2. They incorporate highly durable and/or easily replaceable energy dissipation devices to reduceforces and displacements in structural and nonstructural elements not associated with thenonlinear deformation mechanism, and3. They exhibit self-centering characteristics that minimize permanent lateral and verticaldisplacements of the structure.
Systems considered include ones with beams, columns, walls, braces and systems that exhibit
origin-oriented hysteretic loops (Fig. 1a); this behavior is often achieved by use of a combination ofyielding or friction to provide hysteresis and use of gravity or post-tensioning to provide self-centering
capabilities. In some cases special materials, such as shape memory alloys, can be used alone toprovide these hysteretic characteristics.

Figure 1	 Hysteretic Loop Shapes for (a) Self- Centering Elements and(b) Traditional Isolation Devices	
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Another approach is to install viscous or similar damping devices. When installed in flexible
systems that remain essentially elastic during the response, viscous dampers can be quite effective in
reducing forces, accelerations and drifts in the structure. However, traditional design approaches in the
US do not focus on limiting drifts and accelerations to levels necessary to achieve resilience.
Viscous dampers and elements having self-centering characteristics are typically installed
pervasively throughout a structure. Where large nonlinear deformations are expected during the
dynamic response, such systems may limit forces and accelerations experienced by structural andnonstructural elements, but they may not be able to limit displacement-related damages.
An alternative approach is seismic isolation. While seismic isolation has been in use for several
decades, design codes and guidelines are often formulated to provide seismic performance comparable
to fixed base buildings. While it is presumed that isolated buildings perform better than traditional
structures, generally building codes permit substantial damage in the event of major earthquakes.
Isolated facilities
For resilience, it is convenient to consider a limit on peak story drifts and residual displacements. The
drift limit should prevent significant yielding of the structural elements that might result in damage
requiring inspection and repair, and permanent residual displacements. Drift should also be limited toprevent damage to nonstructural elements that are sensitive to relative displacements between stories.
It can be shown on the basis of first principles that steel braced frames will begin to buckle or yieldbracing elements at a story drift ratio of about 0.25% to 0.4% drift, where as moment frames in steel orconcrete will yield at a drift ratio around 0.9% to 1.25%. Many types of cladding and partitions begin
to display damage at a drift ration of about 0.3% with significant damage occurring by a drift ratio of
1%. Thus, in setting design criteria it is necessary to consider drift limits considering likely damage toboth structural and nonstructural elements. From the above example, it is technically feasible, andperhaps economical, to isolate a moment resisting frame, but it is likely that substantial damage will
occur in partitions, cladding, elevators and so on, before the structural system yields. Thus, dependingon the type of nonstructural elements utilized, braced frames (or shear walls or other stiff systems)
may be more consistent with the requirement of limiting damage to nonstructural components.
However, it may not be necessary to preclude all damage to nonstructural elements during theresilience targeted event, so long as continued functionality is preserved and the occupants are able to“shelter-in- place“ during repairs.
Many types of nonstructural elements, equipment and contents are sensitive to in-structure
accelerations. To prevent acceleration related damages, it is necessary to consider a limit of floor level
accelerations, or on the characteristics of pseudo-acceleration spectrum at each floor. For simplicity,
we might consider a design were in the floor level accelerations are limited to 0.4g, though othervalues may be appropriate for certain types of elements and equipment.
While near or completely elastic behavior in the portions of the facility away from the isolation
plane will eliminate the possibility of residual displacement in those locations. Some isolation systems
may tend to have residual displacements. The acceptability of residual displacement in the isolation
system following an intense earthquake should be carefully considered. If accounted for in the initialdesign with regards to accessibility and functionality of utilities, residual displacements in the isolators
may not impair post-earthquake functionality, and thus be acceptable. Re-centering an isolatedbuilding following an earthquake may be economical in certain cases, if needed. In other cases,
isolators can be designed to minimize residual displacements in the isolation plane.
For resilient designs, it is necessary to identify a level of shaking at which resilience is desired.
This may be expressed as a fraction of the maximum considered earthquake, or in terms of theconfidence with which resilience can be achieved over the life of the structure (i.e., 50% confidence
that the facility will remain operational following an earthquake having a 10% probability ofexceedence in 50 years, or a 90% confidence that the structure will be inoperable due to earthquakes
for less than 3 days during the life of the structure). It is expected that different facilities will likely
have different criteria, depending on their use and importance to the community or owner. For
example, some structures would be expected to be operable immediately, while others might beacceptable in terms of resiliency if their function is restored within 90 days. In all cases, a highconfidence of collapse resistance is assumed. For the purposes of this paper, resiliency for the
maximum considered event is targeted, consistent with important or critical facilities.
	
 

	
 

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Isolation systems
It has been noted that the design of isolated structures can be problematic due to the apparent over- or
under-conservativeness of some design provisions, and also because of challenges encountered when
designing isolated systems to resist intense near-field ground motions. To resist large near-fault groundshaking, relatively large and strong isolators are often necessitated to control displacements andmaintain isolator stability. In these cases, isolators may not act effectively, especially for small events,
and require relatively large design forces and trigger excessive accelerations in the superstructure[Morgan and Mahin, 2007]. Similarly, traditional designs have relied on isolation systems that haveessentially bilinear hysteretic loop shapes with effective periods in the 2-3 sec. range (Fig. 1b). This
type of isolator may develop large forces (and thereby drifts and accelerations) in the supported
structure due to the large lateral isolator forces that may develop in large earthquakes. Thus, efforts
are underway to improve isolation for small events, limit drifts for larger events, and reduce floor
accelerations.
Several approaches are being pursued at Berkeley and elsewhere to overcome these impediments
to resilience, including novel combinations of elastomeric isolators acting in series with sliding
bearings, and elastomeric bearings acting in parallel with nonlinear viscous dampers.
Another promising approach is the Triple Pendulum Slider [Earthquake Protection Systems, 2007].
This device has three independent pendulum mechanisms (Fig. 2). By strategically selecting friction
coefficients and the effective radius for each pendulum mechanism, hysteretic characteristics can be
optimized for occasional, rare and very rare events. Figure 3 shows hysteretic loops for different
combinations of friction coefficients and radii. Isolator displacements up to +/- 2 m are currently
feasible.

Figure 2

Triple Pendulum Bearing

Figure 3

Representative Hysteretic Loops

Isolated Building Tests and Analyses
Tests (Fig. 4) and analyses (Morgan and Mahin, 2011) demonstrate that the TP devices can be
designed to achieve about the same isolator displacements for a large event, but with smaller drifts andaccelerations in the superstructure and with a far greater degree of isolation during smaller events (Fig.
3) [Morgan and Mahin, 2011]. Numerical simulations were conducted of a three-story structure
similar to that shown in Fig. 4 subjected to the suite of SAC ground motions corresponding to a 2%
probability of exceedence at a site in metropolitan Los Angeles (Somerville, 1997). For TP bearings
with an effective period of 4 seconds and damping of 10% (see TP-4-10 plot in Fig. 3) the computed
median isolator displacement is 770 mm, but the peak median story drift is less than 0.4% and the peak
median floor level acceleration is less than about 20% g. Thus, for this type of isolation, highconfidence of resilience can be obtained for even MCE level excitations.
A current test program setup is shown in Fig. 5. This shows a simple shaking table capable ofone-dimensional motion. In this case, the test specimen is designed with replaceable plastic hinges,
made from clevises and replaceable steel rods. The rods are machined and located to produce a
	
 

	
 

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desired stiffness and moment capacity for the plastic hinge. In this case, various amplitudes anddistributions of strength are provided, based on US building codes as well as other alternatives.
These studies show that current US design provisions can result in significant yielding of thesuperstructure prior to the isolators reaching their ultimate displacement capacity and that the typical
distribution of strength used for design in the US results in weak story behavior. Superior behavior isachieved where the strength of the structure is increased to remain elastic until the ultimate
displacement capacity is reached and the distribution of story strength is consistent with that thatexisting when the bearing displacement capacity is reached.
This setup is also being used to as a hybrid shaking table, where the behavior of the isolated
structure to various soil conditions and structural pedestals is examined by including a numericalmodel of the supporting soil or substructure in the shaking table controller.

Figure 4

Test of 3-Story Isolated Building

Figure 5 Test of 3-Story Yielding Superstructure

A number of challenges remain in the design of seismically isolated structures. One of these is thedevelopment of structural systems and non-structural components that are complementary in theattainment of resilience. Some structural systems are too flexible and result in significant damage tononstructural systems. Similarly, many nonstructural elements are so fragile that they suffer
substantial damage even if drifts less than 0.25% are realized on average.
Moreover, current seismic isolation systems only provide protection for motions in the horizontal
direction. Vertical vibrations are not isolated, and in some cases may be amplified. Damage tononstructural elements can occur in such cases. While this damage maybe less than in fixed basesystems, damage may occur that will require special consideration when evaluating the resilience of asystem.
RESILIENT BUSINESSES
A business or service center may be located in a structure that is able to protect the occupants,
nonstructural components and structural system, but still not be able to continue operations
following an earthquake. As noted by the SPUR Resilient Communities initiative (SPUR,
2009), businesses need to have staffing, raw materials, a means of distributing products orservices, and various utilities and services in order to operate. As such, a growing number ofconsultants are providing services related to post-disaster emergency management andbusiness continuity.
	
 

	
 

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For staff to return to work (once the facility is restored to operability), they need housing,schools for their children, food, transportation, health care, and minimal utilities needed tolive. The business will also need utilities such as water, power, sewerage, telecommunications,Internet services, and so on. They need access to banking and other business related services.
Raw materials need to be shipped and delivered, and final products or services need to be sent
to customers. In many cases, raw materials, suppliers, packaging, and shipping services will
be located in the same region damaged by the earthquake and also not functioning. Thus,
the operation of modern industrial societies is highly interdependent and the success of aregion in carrying out business and industrial operations can be rapidly eroded by the failureof a few key services or lifelines.
Similarly, businesses and services well outside of the earthquake shaken area may bevulnerable to business interruption due to closure of businesses located in regions shaken by
the earthquake. Thus, careful attention to supply chains in business operations is needed toinsure adequate reserves of parts or redundancy in component suppliers.

RESILIENT COMMUNITIESFor a business to operate, it needs to be in a community that is functional following a large
earthquake. For small and moderate earthquakes, local neighborhoods and communities
can come to the aid of those who have suffered earthquake damage, and traffic can bererouted and other lifelines rapidly restored. However, as the area effected by the groundshaking grows, and the extent of damage increases, it is difficult for neighboring areas toprovide adequate relief. If housing is damaged moderately or severely so that many cannot
stay in their homes, they will need to find temporary housing or move to other areas. If a
factory or business is damaged, it may shift staff and operations to other locations. If workers
cannot get food, or other services, they cannot work in their normal jobs or help with repairs.
Thus, a critical balance is needed to maintain adequate housing, lifeline services, andbusiness for a community to operate and recover rapidly from a large earthquake. While thebalance of resilient facilities, lifelines, and services is a matter of public policy, realization ofthis balance and attaining a resilient society is a matter of engineering.
Resilient engineered facilities and businesses and services are critical as noted above. Also,
lifelines such as transportation, power, water, sewers and so on are also needed. To this end,
it is necessary to assess and design such infrastructure in terms of its criticality to
post-disaster functionality of a community. Thus, hardening of water and electrical
distribution systems may be needed.
Nodes such as bridges, power plants,
telecommunication centers, server farms, and so on may need to be designed with resilience
as a design criteria.
For example, bridges are a particularly vulnerable aspect of highway and railway
transportation systems.
Improved methods of designing bridges to withstand large
earthquakes without significant damage or additional cost are the subject of considerable
recent research. For example, various types of recentering bridge columns have beensuggested, along with bridge piers that are allowed to uplift during earthquakes. As withbuildings, several strategies for improving the resilience of bridges are being exploredassociated with advanced materials that do not exhibit damage like cracking or spalling until
substantially larger deformation demands compared to current conventional materials,
viscous and other supplemental energy dissipation devices, and so on A commonly usedstrategy to achieve post-earthquake functionality is seismic isolation.

Isolated bridges for improved resilience
Seismic isolation is especially easy to implement for bridges where decks are attached to the
supporting columns and abutments via elastomeric or other bearings. To develop improved
	
 

	
 

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understanding of the behavior of seismically isolated bridges and gain confidence in design methods, aseries of tests and analyses of isolated bridges has been recently completed at Berkeley (e.g., Anderson,2003). These studies included a variety of different isolator types, bridge dynamic properties, andearthquake characteristics. Three generic types of isolator bearings were considered, including leadrubber bearings, high damping bearings, and friction pendulum bearings. Extensive studies weremade to characterize bearings subjected to 3D loading, and to assess the effects of aging. Bridge
systems tested include one and two span bridges having substructure periods ranging from nearly zero
to more than 2 seconds. More than a dozen bridge configurations were tested to assess the effects ofmass eccentricity, stiffness eccentricity, bridge substructure period, bearing type, in-situ variations inbearing properties, yielding of supporting columns, and earthquake excitations. One of the bridge
models tested appears in Fig. 6.
Such experimental results were used to develop improved numerical models of the isolation
bearings, and to assess the sensitivity of system response to different design parameters, and to
uncertainties in these parameters. Various provisions of the AASHTO Guideline Specification [1999]
such as those related to treatment of bidirectional excitations were assessed in Anderson and Mahin[2003, 2004]. Overall, good performance was noted for all of the isolated systems studied.

(a)Figure 6 Shaking table test specimen of
simple supported bridge with different
substructure stiffness(b)

Figure 7 Interaction of segments in multi-segment
bridge system, (a) conventional system and (b)segmental displacement control

Segmental displacement control concept
A multi-segment isolated bridge system will inherently develop relative displacements between
adjacent bridge segments, thereby raising the possibility of damaging bridge decks, isolators andnon-structural components such as expansion joints. Therefore, seismically isolating a bridge mustalso incorporate a method to control the relative displacements between bridge decks while stillpreserving the desired seismic response of the bridge system. Segmental Displacement Control (SDC)
design was recently introduced by Earthquake Protection Systems of Vallejo, California, to achieve
this goal and provide a more resilient bridge system. This concept is the subject of intense
investigation.
Using seismic isolation concepts previously stated, the decoupled superstructure decks areconnected at the expansion joints by lock-up guide devices. This device contains user-definedproperties in accordance with the performance of the isolated bridge system and its fundamental
demands. However, the primary purpose of the lock-up guide is to resist the relative transverse
displacement between bridge decks, thereby allowing for protection of the non-structural components
and a more resilient structural system.
The concept of SDC is shown in Figure 7, where the fundamental behavior of an isolated bridgewith three deck segments is displayed in plan view. The dotted horizontal line references the centerlineof the bridge and is parallel to longitudinal motion. The middle bridge deck is fully supported by
	
 

	
 

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bearings that allow longitudinal and transverse displacement, shown as circles. However, the end
bridge decks are restrained in the transverse direction at the abutments by one-dimensional bearings(or shear keys), shown as rectangles. A real-life bridge would typically have more than three decks
in series and each segment may be continuous over one or more span. Thus, Fig. 7 is a simplificationof a test set up indicated in Fig. 8.
Another function of the lock-up guide is to allow a prescribed amount of longitudinal translation
between adjacent decks. This translation will allow thermal movement of the bridge, but during amajor event or when the prescribed longitudinal displacement capacity is exceeded, the connection“locks up”, providing a restraining stiffness that is characteristic of the lock-up guide dimensions andmaterial properties. Locking up the bridge system is not only beneficial for expansion joints and othernonstructural elements, but also for reducing the probability of bridge deck unseating.
To validate the effectiveness of SDC design for isolated bridges, an experimental program wasdesigned to impose a series of ground motions on a one-quarter scale isolated bridge model on ashaking table. Cases with and without lock up guides were tested.
For instance, imposing three simultaneous components of the 1994 Northridge, CA, Sylmar
ground motion record, the maximum transverse relative displacement between bridge decks wasapproximately 43 mm, as seen in Figure 9, correlating to 172 mm for a full-scale bridge. This amountof full-scale displacement will most likely damage the expansion joints. However, the experiment withthe lock-up guide installed shows relative transverse displacements only 30% of those measured
without the restraints. This is critical to prevent damage to non-structural components. It isnoteworthy to mention that the TPF bearings tend to minimize relative displacements between
segments, as their dynamic properties are independent of the amount and location of the supported
mass. In any event, the forces needed in isolated systems to limit these relative displacements is
relatively modest.
)
s
e
h
c
n
i
(

Transverse Relative Displacement Between Bridge Decks
2

No Lock-Up Guide - WC Location
No Lock-Up Guide - CE Location
Lock-Up Guide - WC Location
Lock-Up Guide - CE Location

1.5

1

t
n
e 0.5
m
e
c
0
a
l
p
s -0.5
i
D
-1

-1.5

-2
3

4

5

6

7

8

9

10

11

Time (sec)Figure 8 Three-segment segmental displacement
control test set up with slab rail system installedover bridge decks

Figure 9 Relative transverse displacement
between center and end segments with andwithout lock up guides (1 in = 25.4 mm)

Overall observations suggest segmental displacement control strategies have great potential forpreserving non-structural components and achieving a more resilient bridge system because of reduced
transverse relative displacement between bridge decks. Noteworthy is the fact that the centerline of thebridge remains continuous helping preserve driver safety and facilitating the use of this system for
railway bridges. Such segmental displacement control may be useful for other types of large isolated
structures.
CONCLUSIONSThe technology and design approaches described in this paper provide several alternative ways toachieve durable building and transportation structures that increase post-earthquake serviceability andreduce the need for repair. By permitting structures to undergo significant inelastic deformations
	
 

	
 

41

during seismic events, yet suffer little damage that would require post-earthquake repairs and impair
operability, structural engineers can achieve designs that are durable, dependable, and economical interms of initial construction cost and the potential losses that might occur in the event of a damaging
earthquake. As such, these approaches address the basic principles articulated by sustainable designand seismic resiliency.
Additional research is needed to refine these concepts, especially with regards to (1) reducing costand increasing speed of construction, (2) reducing the societal and ecological impacts of construction,(3) and refining concepts such as those presented herein and developing more generally applicable
design guidelines for resilient structures.

ACKNOWLEDGEMENTS
The author is appreciative of the superb assistance of his graduate students and post-docs
including Tracy Becker, Troy Morgan, Ken Ogorzalek, Brian Olsen and Andreas
Schellenberg. The financial support from the Transportation Systems Research Program of the
Pacific Earthquake Engineering Research Center is gratefully acknowledged. In addition, thispaper includes work supported by the National Science Foundation under Grant No.
CMMI-0724208. Any opinions, findings, and conclusions expressed here are those of theauthors and do not necessarily reflect the views of the sponsors.
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