The Hyogoken-Nanbu earthquake severely damaged many civil engineering structures and
was caused by the activity of an inland fault which was, unfortunately, near a large urban center.
Earthquake motions in near field of an active fault with a magnitude of 7, however, has not been
incorporated into conventional earthquake-resistant design standards. The very strong
earthquake motions of the Hyogoken-Nanbu earthquake, which had a maximum acceleration of
about 8 m/s2, a maximum velocity of about 1 m/s, and a maximum displacement of about 30-50
cm, were widely observed near the fault, the first time such observations have been made in
Japan. The severity of the damage can be attributed to the extremely strong earthquake
motion-forces beyond the design criterion-that directly struck above-ground structures built
before the introduction of elasto-plastic design, as well as underground structures which had
been considered relatively safe. Many structures built with the latest earthquake-resistant
technologies, however, were not severely damaged, an indication that strong earthquake
motions near a fault can be overcome through engineering.
The return period of an active fault is thought to be about 1,000 years, so through the course
of history it has been rare for active faults to directly strike major urban centers and cause
severe damage. Expressed in a time frame more relevant to human life, the likelihood of such
a disaster occurring over a period of 50 years is roughly 5%. Since the level of risk is low,
strategic judgments must be made in order to maintain the capacity of civil engineering
structures to withstand earthquakes. However, there have been quite a few instances in which
serious damage has resulted from inland earthquakes with a magnitude of 7 or more.
Therefore, even though the risk level is low, it is still possible for strong earthquakes of this type
to strike somewhere in Japan, so their potential for disaster should not be ignored. To take
full advantage of the bitter experience of the Hyogoken-Nanbu Earthquake, therefore, it is
necessary to incorporate the effects of earthquake motions in near field of inland faults into
earthquake-resistant design considerations.
1.2 Ground motion in earthquake-resistant design
Two types of earthquake motions should be considered in assessing the aseismic capacity of civil engineering structures. The first type is likely to strike a structure once or twice while it is in service. The second type is very unlikely to strike a structure during the structure's life time, but when it does, it is extremely strong. The second type ground motion includes those generated by interplate earthquakes in the ocean and those generated by earthquakes by inland faults. The concepts behind these two types of motion have been incorporated into the existing earthquake-resistant design of some structures, and these two types of the ground motions are called "Level I earthquake motions" and "Level II earthquake motions." Objectives for and characteristics of these earthquake motions in earthquake-resistant design are as follows:
1.3 Level II earthquake motions
The following concepts are used to determine Level II earthquake motions.
(1) Level II earthquake motions generated by active inland faults are determined based on indentification of active faults that threaten an area and assumptions of source mechanism, through comprehensive examination of geological information on active faults, geodetic information on diastrophism, and seismological information on earthquake activity. To be able to do this, considerable effort must be put into establishing engineering methods
. (2) Since the Hyogoken-Nanbu earthquake, research in Japan on the above points has been advancing. However, the accuracy of methods for forecasting earthquake return periods and magnitudes, as well as the characteristics of the motions of earthquakes caused by active inland faults is still insufficient for establishing a basis for earthquake-resistant design. Therefore, when earthquake motions cannot be specified directly using information on an active fault, strong motion records caused by near field earthquakes, such as the Hyogoken-Nanbu earthquake, should be used to create a Standard Level II earthquake motions.
(3) It is thought that earthquake motions that are generated in the near field by a large interplate earthquake occurring near land have different characteristics from earthquake motions generated through the movement of an inland fault. Since there are no records on very strong earthquake motions of this type, there are a lot of unknowns about the characteristics of these earthquake motions. More research needs to be done on very strong earthquakes generated by earthquake motions near interplates.
1.4 How Level II earthquake motions are expressed
Below is a discussion of how Level II earthquake motions are expressed.
(1) Level II earthquake motions are basically used for earthquake-resistant design based on damage control concepts. Therefore, the dynamic characteristics of earthquake motions should be expressed concisely, such as in the response spectrum or time history waveforms.
(2) Ground levels where earthquake motions are given
Effects of vertical motions: A lot of attention has been paid to the three-dimensional effects of the motions of the Hyogoken-Nanbu earthquake, particularly the vertical motions, on damage to and destruction of structures. Considerable effort has been made to clarify these effects. Thus far it has not been proven that the vertical motions were the primary cause of the destruction of major civil engineering structures. It is important to continue with detailed research on the effects of the three-dimensional characteristics of earthquake motions on the destruction of structures.
In this section, the expected aseismic performance of civil engineering structures against
Level I and II earthquake motions is discussed, and design methods for achieving this
performance are proposed. Civil engineering structures are of many different types, but they
may be categorized as follows. 1) Above-ground structures such as bridges, tanks, dams,
towers, etc.; 2) in-ground structures such as subways, buried pipelines, tunnels, etc.; and 3)
various types of foundation such as piles, caissons, etc. and soil structures such as dikes,
retaining walls, etc.
It is quite difficult to define a unified aseismic performance level for these different types of
civil engineering structures. Hence, in this chapter, aseismic performance and design methods
are proposed separately for each category.
2.2 Required aseismic capacity and earthquake-resistant design of above-ground structures
(1) Earthquake resistance to Level I earthquakes
In principle, no damage should occur to any structure when earthquake motion of Level I
occurs. Accordingly, the dynamic response during motion of this level should not exceed the
elastic limit.
(2) Earthquake resistance to Level II earthquakes
Important structures and structures requiring immediate restoration in the event of an
earthquake should, in principle, be designed to be relatively easily repairable; even if damage is
suffered in the inelastic range. Accordingly, the maximum earthquake response of such
structures must not exceed the allowable plastic deformation or the limit of ultimate strength.
For other structures, complete collapse should not occur even if damage is beyond repair.
Accordingly, deformation during an earthquake of this level should not exceed the ultimate
deformation.
The degree of importance of structures can be determined based on the following
factors:
2.3 Required aseismic capacity and earthquake-resistant design of underground structures
The basis of earthquake-resistant design for underground structures is the stability and deformation behavior of the ground when subjected to earthquake inputs. Knowledge of three-dimensional displacement behavior, including depth-wise movements, is critical to the earthquake-resistant design of large tunnels, whether of shield or cut-and-cover type. Ground displacements along the structure axis are important in the case of extended structures of small cross-section, such as buried pipes. This means that the earthquake response of the near- surface ground should be thoroughly investigated. Since ground liquefaction and resulting ground displacement have a great influence on the earthquake resistance of underground structures, the stability of the ground under earthquake excitation should be studied in adequate detail.
(1) Retained earthquake resistance of structures
The function of structures should be retained after a Level I earthquake. In the case of a
Level II earthquake, the damage should be limited such that there is no fatal damage to the
structure's functions and functions can be restored within a short period.
(2) Use of flexible structures
To ensure that structures retain earthquake resistance after Level II earthquakes, it is highly
recommended that structures and materials with good flexibility be used. Further, total
collapse of a structure due to the collapse of a single member should be prevented by designing
structural details so as to ensure brittle failure does not occur.
(3) Plans for lifeline systems
In designing trunk lines for lifeline systems such as water, sewerage, electricity, gas, and
telecommunications designs best able to maintain functionality after a Level II earthquake
should be chosen, taking into account the topography, ground conditions, and the city layout in
the vicinity. If this is difficult for economic reasons or because of ground conditions,
continued functionality (or rapid restoration) after a disaster should be ensured by selecting the
most appropriate route, adopting a multi-route system, using a block system, or implementing
some alternative measure.
(4) Underground structures straddling faults
When the location of an active fault is well identified, such measures as increasing the
flexibilities of structures, duplicating lines, and isolation of line systems from the casing
structure may be considered. However, if such measures are technically difficult to implement,
operational measures including the provision of alternative systems should be considered.
2.4 Required aseismic capacity and earthquake-resistant design of foundation and soil structures
(1) Seismic stability of foundation structures
In the case of a Level I earthquake, the objective of earthquake-resistant design for a
foundation structure is to maintain the original engineering function of the superstructure which
the foundation supports. One principle of design is, wherever possible, to prevent soil
liquefaction in ground with a high liquefaction potential by implementing suitable ground
improvements.
In cases where it is judged that ground improvements would be difficult, however, the
function of the superstructure should be maintained by proper design and/or reinforcement of
the foundation structure and/or the superstructure itself.
In the case of Level II earthquakes, the objective of earthquake-resistant design for a
foundation structure is to ensure that no serious damage occurs to the superstructure supported
by the foundation. Where it would be difficult to implement ground improvements, the
foundation structure should be reinforced or the whole structural system should be re-evaluated,
or both, to minimize displacement of the foundation due to seismic response and lateral ground
displacement, thus preventing serious damage to the superstructure.
(2) Seismic stability of quay walls, dikes, and embankments
There may be no need for seismic stability along the entire length of this type of structure
from an economic viewpoint, since quay walls, dikes, embankments, retaining walls, and
similar structures are long, continuous structures which can be easily repaired when slight
damage occurs. It is recommended that segments of relatively high importance be isolated
and designed for high seismic stability.
For Level I earthquakes, the original functions of relatively important sections of quay walls,
dikes, retaining walls, and embankments should not deteriorate, maintaining the original design
requirement after the earthquake. Slight damage to other less-important sections is allowable
unless it would have a detrimental effect on adjacent structures. The objective of
earthquake-resistant design is, however, to ensure that damage can be repaired within a short
period and the whole system returned to functionality.
For Level II earthquakes, the objective of earthquake-resistant design in the case of important
sections of quay walls, dikes, retaining walls, and embankments is that the damage should not
seriously affect the structures they support and adjacent facilities, even if some degree of
damage occurs. In the case of important structures which form an essential part of an
emergency transportation route, the aim is to ensure that original functions are maintained.
For ordinary sections, it is necessary to ensure that, even if damaged, there are no detrimental
effects on adjacent areas, such as by secondary damage.
(3) Important issues in the earthquake-resistant design of ground improvements, foundations,
quay walls, dikes, retaining walls, and embankments, and related research and
development topics
If a soil mass that includes a large amount of gravel also has some sandy matrix, it may liquefy
depending on its density, fine-material content, hydraulic conductivity, etc. Accordingly,
present design standards and codes should be re-evaluated and, if necessary, revised to include
evaluation of the possibility of liquefaction for Holocene soil deposits and reclaimed fill with a
gravel content.
Recently, detailed evaluations of the liquefaction potential of relatively dense sand have been
described. These recent investigations revealed that, at blow counts above about twenty as
measured by standard penetration tests, resistance to liquefaction increases rapidly with rising
blow count. It was also revealed that the amplitude of cyclic shear stresses required to cause
soil liquefaction rapidly increases as the number of loading cycles involved increases. This
recent information suggests that the present design standards and codes should be re-evaluated
and, if necessary, revised to properly take into account the liquefaction potential of dense sand,
particularly in the case of the high stress amplitude and relatively small number of loading
cycles in a near field earthquake.
It is also necessary to improve understanding of the mechanism of liquefaction-related large
ground displacement and to develop methods of predicting it.
The behavior of piles, caissons, buried structures, and other similar structures in a liquefied
soil mass undergoing lateral displacement is poorly understood. It is highly important to foster
research into design methods for foundations and buried structures exposed to this situation.
The seismic behavior of quay walls, dikes, embankments, and retaining walls is also poorly
understood. Accordingly, there is a great need to foster studies on the development of
methods for evaluating the settlement and displacement of ground, and also the dynamic earth
pressure caused by an earthquake. Methods are also needed for increasing the seismic stability
of ground. This requires relevant field observations, model tests, etc.
(1) Basic policies on aseismic diagnosis
Earthquake resistance diagnosis of existing civil engineering structures is in two stages:
primary diagnosis using approximate methods and secondary diagnosis using detailed methods.
Primary diagnosis should be based on damage to civil engineering structures caused by the
Hyogoken-Nanbu earthquake. After ground conditions and the ages, design standards, and
outlines of the structural characteristics are examined, structures requiring aseismic
reinforcement and those requiring a detailed aseismic capacity examination by secondary
diagnosis are selected. In primary diagnosis, the following five factors are taken into the
consideration: 1. effect on human life when a structure is damaged; 2. effect on evacuation,
rescue, emergency medical services, and activities for preventing a secondary disaster; 3. effect
on provision of basic requirements for daily life and economic activities of the area; 4.
substitution of system function by providing another structure; and 5. changes in design
conditions after construction.
Objects for secondary diagnosis, which is based on drawings and specifications and ground
conditions, are structures judged in primary diagnosis to require a detailed examination of
aseismic capacity. Secondary diagnosis is used to judge whether a structure has the required
aseismic capacity to withstand Level I and Level II earthquake motions, and to select structures
for reinforcement. In secondary diagnosis, the bottom line in judging the aseismic capacity of
a structure is that it does not collapse even when damaged beyond repair. In secondary
diagnosis, on-site measurements and testing, and surveys on the ground conditions should be
conducted, and the aseismic capacity of the structure to withstand the earthquake motions
through redesigning and/or numerical analysis.
(2) Establishing data bases for aseismic diagnosis
For the smooth implementation of primary diagnosis, it is urgent that data bases (design
standards and age of the structure) for existing civil engineering structures be established.
If the structure is old and adequate data on it cannot be obtained, primary diagnosis should be
done in a strict manner and the site surveys and tests required for secondary diagnosis should be
conducted.
(3) Aseismic capacity of an overall structure
In selecting parts of a structure for aseismic reinforcement, it is necessary to thoroughly take
into consideration the effects of reinforcement on the aseismic capacity of the overall
structure.
(4) Earthquake disaster prevention as a system
In selecting structures for aseismic reinforcement, it is necessary to attempt to effectively
improve the earthquake disaster prevention capacity of the overall system which consist of
structures.
3.2 Aseismic reinforcement
(1) Basic policies of aseismic reinforcement
In aseismic reinforcement of an existing civil engineering structure, as with a new structure,
both Level I and Level II earthquake motions must be taken into consideration. The in-
service period of the structure should be considered the same as that of a new structure.
The target aseismic capacity of a structure for reinforcement should also be the same as that
of a new structure. In short, as with a new structure, the importance of the structure and the
risk of Level I or Level II earthquake motions are taken into consideration when the target
aseismic capacity of the structure is established.
With some existing civil engineering structures, increasing the aseismic capacity to the level
of a new structure is problematic because of difficulties with construction methods or because
of financial constraints. In such cases, the importance of the structure should be carefully
examined, and alternative measures, such as the establishment of a quick restoration system
after an earthquake, should be adopted. In addition, issues pertaining to demolition and
reconstruction should also be examined.
(2) Determining a priority for reinforcement
Determination of which structures have priority for aseismic reinforcement is based on the
importance of the structure, as well as the risk of an earthquake in the area. It is also necessary
to examine economic factors and the potential effects of reinforcement on the earthquake
disaster prevention capacity of the overall system which consist of structures.
Clarification of the reasoning behind the process for determining which structures have
priority for aseismic reinforcement is required.
(3) Aseismic reinforcement methods
Feasibility, safety, economic factors, and the effects of aseismic reinforcement on the
surrounding environment must all be carefully examined when selecting an aseismic
reinforcement method. Therefore, new construction methods and new materials appropriate
for the structural characteristics of the structure and the environment of the site should be
developed and applied.
(4) Evaluating the aseismic capacity of a reinforced structure
The aseismic capacity of a reinforced structure is evaluated with quantitative methods. This
requires verification of the validity of the evaluation methods by, if necessary, conducting tests
with full-size models, numerical analysis, and earthquake observations of reinforced structures.
A thorough verification of evaluation methods for determining aseismic capacity is needed
when a new construction method or new materials are used.
It is vital not only to evaluate the aseismic capacity of reinforced parts of a structure; it is also
necessary to evaluate the aseismic capacity of the overall structure and to assess the safety
against other loads such as winds and floods.
Further, it is necessary to evaluate how the earthquake disaster prevention capacity of the
system consisting of reinforced structures is improved.
(5) Maintenance and management, and repair
As with new structures, reinforced structures require thorough periodic inspections. It may
be necessary to conduct earthquake observations and various measurements in order to check
whether the target aseismic capacity is being maintained.
3.3 Issues for future research on and development of aseismic diagno-sis and aseismic reinforcement
(1) Development of aseismic diagnosis techniques based on structural characteristics
There are many different types of civil engineering structure, and the aseismic diagnosis
method used for a particular structure must be appropriate for its structural characteristics. It
is necessary to establish through research and development rational and appropriate aseismic
diagnosis methods for each type of civil engineering structure.
(2) Development of aseismic reinforcement techniques
There exists large number of civil engineering structures that require aseismic reinforcement.
In many cases, aseismic reinforcement work must be done while a structure is being used,
which necessitates strict limitations on work periods and spaces as well as restrictions related to
the surrounding environment, such as vibration and noise. It is therefore urgent to develop
proper aseismic reinforcement techniques that satisfy these conditions based on the
characteristics of each type of civil engineering structure.
(3) Construction of data base for design documents
The construction of data base for design documents is essential for conducting appropriate
and reasonable aseismic diagnosis and reinforcement, as well as for restoring earthquake-
damaged structures. Each organization responsible for a civil engineering structure should
put considerable effort into research on and development of construction of data bases.
(1) Need for a seismic hazard assessment system
In Japan, open spaces formed by streets, roads, and parks are lacking in most urban areas, a
result of inadequate effort to plan public facilities. Further, certain areas are densely packed
with houses on small lots that do not meet present building and earthquake resistance codes.
Such urban communities are less resilient to disasters as well as less comfortable to live in than
those in other advanced countries.
Fundamental improvement of the urban environment is one of the most serious issues facing
Japan. Since improvements are by no means possible within a couple of years, efforts must be
initiated to attain them as early as possible. A "regional seismic hazard assessment system" is
one key element to be taken into consideration in such efforts. This is explained below.
(2) Review and revision of urban/regional plans and infrastructure planning guidelines
An important element in urban/regional plans has always been safety in times of disaster.
However, plans have not always been well coordinated with urban/regional disaster
plans.
The urban infrastructure usually consists of a hierarchy of systems, each with different size
and coverage. For example, the street system consists of arterial roads, collector-distributor
streets, and local streets. In the case of Japan, however, this infrastructural hierarchy has not
been well established, and it is lacking in both quality and quantity. As has been discussed for
years, it is necessary to improve and extend Japanese planning standards.
Further, not all the minimum requirements for public facilities-such as evacuation/rescue
routes and open spaces useful in case of emergency-have yet been established. To increase
the seismic safety of society, an urgent review and revision of planning standards for such
facilities is needed. These standards will be also useful in the assessment system described
above.
4.2 Emergency management system for disaster mitigation
Delays in rescue operations and fire-fighting aggravated the Hyogoken-Nanbu earthquake disaster, and revealed the inadequacy of current emergency management systems in Japan. Measures for disaster mitigation include several that can be implemented both pre- and post- disaster. Among them, the following require urgent consideration:
(1) Integrated use of various disaster information systems: Various disaster information systems are being constructed by both the public and private sector. However, none are intended or designed to be linked to each other. It is desirable to develop a technology for integrating these independent systems, and to carry out repeated drills prior to a disaster so as to master the integration functions.
(2) Preparing disaster management strategies: Disaster management involves serious decision- making issues such as whether evacuation vehicles or rescue vehicles should have priority, and whether use of water-dropping helicopters is appropriate in urban fire-fighting. Certain strategies for emergency management may be quite different from those used in normal situations, and may at first be considered unacceptable to the community. Through in-depth discussions prior to a real disaster, mitigation strategies that have community-wide consensus should be prepared for various disaster situations.
(3) Drill improvement: A large-scale earthquake disaster is likely to require efforts beyond the capacity of public emergency management agencies, so local communities should be asked to organize effective disaster drills that go beyond the conventional focus on evacuation and early fire fighting. These drills should be more comprehensive, encouraging people to think about what they themselves can do in such a disaster. Drill methods should be changed from the prepared-scenario type to an improvisational type aimed at improving adaptability in an emergency.
(4) Cultivation of disaster managers: Since large-scale disasters are rare, the lessons of past disasters tend to be lost without experts such as disaster managers, and disaster preparedness programs are tend to lack consistency and continuity. However, varied duties and Japan's tradition of periodic transfers tends to inhibit the cultivation of such trained experts. Disaster managers, including high-level decision makers, need to be cultivated to facilitate the early establishment of efficient disaster management systems in Japan.
4.3 Cost sharing for reinforcement and reconstruction
The earthquake resistance of the infrastructure, schedules for seismic reinforcement of existing structures, and plans for post-disaster reconstruction are closely related to cost and the cost burden. Besides cost-benefit evaluations usually performed before determining the cost burden, a number of other cost-related issues arise, as follows.
(1) As with the Hyogoken-Nanbu earthquake, the cost burden sometimes exceeds the ability of the affected community to pay. Moreover, the occurrence of such a disaster in particular community is quite low in probability.
(2) Disaster-related damage extends not only to the economic sphere, but also to human life and even mental health.
(3) Increased investment to secure a safe community may result in a lower budget for new
projects. Hence, the trade-off between the two needs to be evaluated from a socio-economic
point of view.
A quantitative evaluation of the hazard mitigation achieved by disaster-related investment is
important; it must be done together with the assessment mentioned in 4.1. The evaluated
socio-economic effects will form the basis of planning standards for disaster
preparedness.
Various legislative and financial relief measures were adopted after the Hyogoken-Nanbu
earthquake without in-depth discussion. Some of these gave inconsistent relief to the various
types of facilities, and left much room for improvement as regards rule standardization. Rules
for financing reinforcement of existing facilities, post-disaster recovery, and reconstruction
should be established, especially as regards placing an appropriate cost burden on the various
regions and generations. These should take into account some of the important factors listed
below.
2) National consensus on appropriate investment for disaster mitigation: The greater the safety we require of our infrastructure, the more we have to pay for its construction and maintenance. Thus, through a process of in-depth discussion among taxpayers, a national consensus on the degree of seismic safety we require of our infrastructure should be developed; this is especially critical as we approach the 21st century and the greater cost burden of the so-called "advanced-age society" becomes an issue.
3) Cost burden rules for reinforcing existing facilities: Much discussion among the private and public sectors, and the development of a consensus, is also required as regards the issue of the cost burden of seismic reinforcement work. The cost burden rules which apply to the reinforcement of a facility should not be the same as those for its construction. This is partly because the latter is usually determined without considering devastating disasters like the Hyogoken-Nanbu earthquake, and partly because a high cost burden may cause a dangerous delay in the reinforcement work. A national consensus is also needed for cost burdens of this kind.