こうえいフォーラム第１０号／２００２. １

STUDY ON SEISMIC RETROFIT PLANNING METHOD FOR SEWERAGE
FACILITIES ON THE BASIS OF SEISMIC RISK MANAGEMENT
下水道処理場の地震に対するリスク評価に関する検討

大角 恒雄*・山本 賢一**
Tsuneo OHSUMI* and Kenichi YAMAMOTO**

1995年兵庫県南部地震において，下水道施設も，甚大な被害を被った．特に下水道普及率が 9 割を越える神戸市を襲った施設の被害は，社会基盤としての重要性および環境への影響においても重要な課題となることを認識させた．現在，下水道施設を含む土木構造物の設計は，損傷を許容する変形性能照査型に移行されている．構造物の損傷が許容されるということは，その定量的評価が必要となる．下水道施設構造物の耐震診断及び補強方法の選定について地震リスクマネージメントの概念を導入し，「耐震設計基準をいかに強化してもその効果には限界がある」ということを前提に，地震時の機能分析に基づいた効果的な対策を検討する手法を提案するものである．
Key Words : sewage treatment plant, earthquake risk, aseismic reinforcement design,level 2 earthquake, non-linear analysis

１．INTRODUCTION

into establishing engineering methods. Introducing the concept

In Japan, sewerage facilities were heavily damaged by theof earthquake risk management put another way means, notHyogoken-Nanbu Earthquake, which hit in 1995. Severe

being overconfident in earthquake-resistant design. Taking into

damage to facilities in Kobe City, where the sewerage diffusion

account that, no matter how good the earthquake-resistantrate was more than 90%, made us realize the importance ofdesign, the fact must be recognized that we will never be able tosewerage as one of the infrastructures and consider its influence

make absolutely safe structures. It states that the most importanton the environment.

thing is the planning of effective measures and enhancing theThe Japan Society of Civil Engineers presented two proposals

necessity of the project from the viewpoint of risk management.

on the ideal earthquake-resistant design for infrastructures.

Disaster alleviation measures should be continuously considered

According to the second proposal (The Japan Society of Civilon the basis of the functional analysis of a stricken infrastructure

Engineers, 1996), input earthquake motion (Level II earthquakesystem. Risk evaluation takes the damage risk made by anmotion) is determined based on identification of active faults

earthquake as a prerequisite thereby defining the impossibility ofthat threaten an area and assumptions of source mechanism.

constructing absolutely safe structures. This has been the reason

Recntly, the concept of risk analysis would be applied to thethat it was not obviously evaluated in actual practices, such as

design of general infra strucure. One of trial methodology to bedesigning, planning, construction, and maintenance ofused id based on cost - benefit - analysis, decision - making

infrastructure facilities.

process of the Reinforcement of Slopes base.
However, it also states that considerable effort must be put

For establishment of the risk evaluation of sewerage facilities,
those need to be evaluated with: ① detailed risk evaluation ofsewerage facilities hit by an earthquake based on the analysis of＊
Research and Development Center, Nippon Koei Co., LTD.
＊＊ Japan Bank for International Cooperation, Development ProjectAnalysis Department

bed and texture as well as earthquake incidence rate; ②suggestion of its remedy; ③ calculation of the estimated

9STUDY ON SEISMIC RETROFIT PLANNING METHOD FOR SEWERAGE FACILITIES ON THE BASIS OF SEISMIC RISK MANAGEMENT

maximum damage for each seismic intensity; and ④ effectiveStart

distribution of the reclamation and insurance expenses.
For the damage risk by an earthquake having been considered① Selection of subject structure

as a prerequisite, this evaluation was not broadly evaluated in
actual practices, such as planning, designing, construction, andmaintenance. Since this clarifies that no absolutely safe structure
can be made, the importance lies in calculating the extent of
required additional budget for lessening the damage probabilityof structures and defining the relationship between the degree of② Calculation of occurrence probability ( Pi )･ Setting of the number of years of the probable occurrence(P1, P2, P3)･ Calculation of magnitude with earthquake risk oriented
analysis③ Setting of earthquake motion･ Calculation of earthquake movement under the seweragetreatment plant with the earthquake prediction program

damage and preventive measures when actual damage is done.
Considering these, the author et al. examined disasteralleviation measures based on the function analysis of a stricken④ Calculation of damage cost ( Ci )･ Calculation of damage cost for members with the non-linear
response seismic intensity method

sewerage. Specifically, we performed quantitative evaluation of
the effect of aseismic reinforcement acquired with risk analysis
after calculating the present state of an exemplification structureand degree of damage after calculation of aseismic
reinforcement, as well as costs for reinforcement and repair.

２．EARTHQUAKE RISK EVALUATION METHODThis method explains earthquake risk in order of calculationof annual risk, damage calculation method, selection of the mostsuitable reinforcement method. Aseismic reinforcement
selection method and exemplification as follow:（1）Aseismic reinforcement selection method⑤ Calculation of risk ( R )・ Calculation of damage cost of the treatment plant
R=Σ(Ci × Pi)⑥ Creation of alternate plan
・ Several reinforcement methods
・ Calculation of total cost for reinforcement⑦ Calculation of damage cost ( Ci )・ Same calculation as ④ for reinforced structures⑧ Calculation of risk ( R )・ Same calculation as ⑤ for reinforced structures⑨ Arrangement of the effect of alternate plans
・ Calculation of cost of damages of every reinforcement plan

The flow of selection for aseismic reinforcement is shown inFig. 1(Mizutani,1995). First, we calculated the intensities of⑩ Comparison
・ Selection of optimum reinforcement plan

earthquake motion on the basis of occurrence probability withthe earthquake motion prediction program. Also, the relationship

END

between the intensity of earthquake motion and the amount ofdamage is estimated with a method the author et al. invented, theFig. 1. Flow of selection for aseismic reinforcement method.

non-linear seismic coefficient method , which considers the nonlinear characteristics of ground and structures.

after reinforcement.

（2）Calculation of annual risk

（3）Damage calculation method.

The calculation process of the annual risk is shown in Fig. 2

The damage calculation method in the annual risk calculation

to 4. The annual risk is calculated by acquiring the amount ofis conducted in the following order:

damage on the size of several earthquake motions set for each

①Calculation of ductility factor (Φmax/Φy) from the analysis

occurrence probability. Three intensities of earthquake motion(L1, L2 and L3) for each occurrence probability was set. Then,the damage of both the present state (with no reinforcement) andresult using the response seismic intensity method.
②Setting the damage level for each member of framework using
the ductility factor using Fig. 4.

the state after reinforcement for each earthquake motion (Fig. 2)③Calculation of the damage amount by setting the repair costs

was calculated. Further calculation methods for more concrete

separately for each aseismic capacity shown in Table 1.

damage will be described in

How to calculate damage.

The damage level represents the load condition of Table 2.

From the above, the annual risk is calculated as the sum of each（4）Selection of the most suitable reinforcement method.

risk (Fig. 3), and the effect of a year is the difference in theHere, are comparisons of several possible aseismic

annual risks between the risk with no reinforcement and the risk

reinforcement plans. The effect of the aseismic reinforcement10こうえいフォーラム第１０号／２００２. １

Table 1 Relationship between earthquake-resistant performance and damage level of each member of framework.

Table 2 Standard for damage level of a member.

With rein forcement

Ｌ1
Ｌ2
Ｌ3
Level of earthquake motion
Fig. 2 Earthquake motion and total damage cost.

per year is acquired using the following formulas :Effect of aseismic reinforcement =Annual risk with noreinforcement - Annual risk after reinforcement.

Total damage cost

Annual risk density

Present state(with no reinforcement)
Present state(with noreinforcement)

With rein forcement

Ｌ1Ｌ2
Ｌ3
Level of earthquake motion
Fig. 3 Annual earthquake risk density.

（5）Exemplification
For an exemplification, the subject structure is a water
treatment plant that has a double structure where theThen, considering reinforcement costs (N: number of in-sedimentation pond is incorporated into buildings. Waveforms

service years), Effect of aseismic reinforcement - Costs ofof earthquake motion L1, L2, and L3 are decided as follows foraseismic reinforcement/N. The aseismic reinforcement plan thatthe prediction of earthquake motion. Earthquake risk oriented

makes the above value the greatest should be selected.

analysis is performed and the frequency of the target earthquake

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STUDY ON SEISMIC RETROFIT PLANNING METHOD FOR SEWERAGE FACILITIES ON THE BASIS OF SEISMIC RISK MANAGEMENTBending moment

Level 1

Level 2

Level 3

Level 4ＭuＭyＭc0φc φyφu

αφyCurvatureFig. 4 Damage level concept.

is set (occurrence probability: P) and the earthquake scale which
possibly hit the area concerned (magnitude: M) as P=30, 300,1000, and M= 7.0, 7.9, 8.3, respectively, by using the seismic
hazard analysis system (ERISA, shown in Fig. 5 to 6).
It uses data of the Minami-Kanto earthquake for earthquake

Fig. 5 Epicenter Distributions (ERISA)

motion waveforms and created an artificial waveform for each
earthquake motion using the Harada/Ohsumi method (Ohsumi etal, 1997). The maximum acceleration in earthquake-resistantbasements is 99 gal, 680 gal, and 800 gal for each (Fig. 7). In
regard to the damage, we performed non-linear seismic
coefficient method analysis (Yuasa et al., 2000) to judge thefracture mode for each member of framework and then acquired
the ductility factor.
The procedure to follow to perform cost calculations andsuggestions for aseismic reinforcement is shown in Fig. 8.
①Using seismic response analysis, calculation of repair costs
for no reinforcement and selection of members that need to bereinforced.
②Consideration of damage to the members and the analysis
distribution, followed by selection of countermeasureconstruction.
③Confirmation of damage and calculation of repair costs whenreinforcement based on the seismic response analysis isconducted (after aseismic reinforcement).
Regarding repair costs, the repair cost to be used per member

Fig. 6 Log Frequency of Magnitude (ERISA)

should be previously decided separately for each damage levels bending rate) separately, and each

Next, factor is the degree of damage to the members. Thes repair cost appropriate to the damage level should

number of members whose present ductility factor is more thanbe acquired using the ductility factor. The sum of these costs is

1 by the aseismic response analysis, and the repair costs are

the total of the repair costs.

shown in Table 3. It is clear that the present state will damage(defined by a membermember

12こうえいフォーラム第１０号／２００２. １

Note
Gain in concrete
Fig.9a Construction ① method.

Fig.7 Waveforms of earthquake motion L1, L2, and L3

Start

NoteSide wall
Buttress① Calculation of damage and repair cost
② Suggestion of aseismic reinforcement and calculation ofreinforcement cost

Fig.9b Construction ② method.

③ Calculation of damage after reinforcement and repair cost

reinforcement and the damage by an earthquake would be less.
（6）Risk evaluation.
END

The risk R of the present state (with no reinforcement) and of
reinforced structure are acquired by theFig. 8 Flow to acquire damage and cost.

following formula:more members and cost more. For the aseismic reinforcement（1）plan, two construction methods are selected, which can satisfy
the aseismic capacity aiming to improve the proof stress of theHere, Pi is the occurrence probability of the earthquake motion

whole structure (Fig. 9). Construction method ① is one thatLi (i=1, 2, 3).

places more concrete on columns and beams, and construction
method ② is one that uses side walls and buttresses. The
aseismic reinforcement costs are shown in Table 3. Also shownin Table 3 are the number of the members whose ductility

Ci is the total cost for earthquake motion Li with or without
reinforcement (i=1, 2, 3).
Ai is the area proportion, p is the probability of the aseismic
reinforcement (present: p=0, after reinforcement p=1).

factor is more than 1 and the repair cost amount acquired by

E is the cost for aseismic reinforcement. Pi x Ci is termed theperforming seismic response analysis on both sections ofannual risk density, which shows the risk at each scale ofconstruction methods ① and ②. The repair costs of earthquake

earthquake motion. In Fig.10, the relationship between themotion L1 is the value of less than the ductility factor 1 (crack).

annual risk density and the scale of earthquake motion is shown.

As it is obviously shown, construction method ① costs more forIn the case of earthquake motion L1, construction method ① is13

STUDY ON SEISMIC RETROFIT PLANNING METHOD FOR SEWERAGE FACILITIES ON THE BASIS OF SEISMIC RISK MANAGEMENT

more effective in aseismic reinforcement than method ②.
However, when the earthquake motion is more than L2, theeffect reverses; that is, this leads to the conclusion thatconstruction method ①is more brittle to the scale of earthquake
motion.
Table 4 shows the annual risk and the effect of aseismic
reinforcement. Assuming 10 years passed from the time ofconstruction, we set the in-service years N=40. This shows thatthe cheaper construction method ② is more effective thanmethod ①.
The sewerage of five cities and four river-basin sewerage
were damaged in Hyogoken-Nanbu earthquake. The averageamount of damage was 3.83 billion yen(Editorial Committee for
the Report on the Hanshin-Awaji Earthquake Disaster,1997),
Fig.10 Annual risk density.

which almost equaled to 3.84 billion yen, which is the damage atearthquake motion L2, calculated with this method. From this, itis safe to say that the validity of this method is verified.

Table 3 List of damage and cost.

Table 4 Annual risk and the effect of a seismic reinforcement14こうえいフォーラム第１０号／２００２. １

３．CONCLUSION

Ohtsu, H., Ohnishi, Y., Mizutani, M. and Itou, M. [2001], The proposal of themethodology associated with decision-making of reinforcement of slopes based

By applying earthquake risk management, we outlined theon cost-benefit-analysis, Journal of Environmental Systems andmethodology to select the optimum aseismic reinforcement

Engineering, No.679,/VI-51, 123-133, Japan Society of Civil Engineers, inmethod for the existing structures. The conventional evaluationof earthquake-resistant structures has been conducted with an
exemplification structure, only considering a specific earthquake

Japanese.
Tomatsu, Y., Katayama, T. [1985], An Online-graphic System for Earthquake
Risk Analysis <ERISA-G> - its Development and Dunctions, Journal ofEnvironmental Systems and Engineering, No.362,/I-4, 397-406, Japan
Society of Civil Engineers, in Japanese.

motion. In the meantime, the earthquake risk management

Japan Society of Civil Engineers[1997]. Proposals on Earthquake Resistancemethod enables the calculation of annual risks that are acquired

for Civil Engineering Structures, Japan society of civil engineers special

by adding up the risks separated for each earthquake scale andthe occurrence probability of an earthquake. This gives us theability to monastically compare the reinforcement plan, which
contributes the quantitative evaluation of the effect for aseismic

task committee of earthquake resistance of civil engineering, inJapanese.
Ohsumi,T., Darama,H. and Harada,T. [1997], Engineering simulation of groundmotions using a seismological model, Proc. of the 7th International
Conference on Structural Safety and Reliability, Structural Safety andReliability, Balkema, ISBN9054109785, Vol.3, pp.1471-1478.

reinforcement of existing structures.

REFERENCES
Editorial Committee for the Report on the Hanshin-Awaji Earthquake
Disaster[1997]. Report on the Hanshin-Awaji Earthquake DisasterDamage and Restoration of Lifeline Systems-, Japan Society of Civil
Engineers,pp.141-253,in Japanese.

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