PROBABILISTIC SEISMIC HAZARD ASSESSMENT...

International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 14-15, 2008

PROBABILISTIC SEISMIC HAZARD ASSESSMENT AND STRUCTURAL PERFORMANCE AGAINST SEISMIC ACTIONS

Adang Surahman1, I Wayan Sengara2 and Widiadnyana Merati1

1Faculty of Civil and Environmental Engineering, Institut Teknologi Bandung Jl. Ganesha 10, Bandung-40132, Indonesia, email: [email protected]; [email protected]

2Center for Disaster Mitigation, Institut Teknologi Bandung, Jl. Ganesha 10, Bandung-40132, Indonesia, email: [email protected]

ABSTRACT: This paper describes various studies and research activities in the field of earthquake engineering carried out in Indonesia. These researches address some efforts on probabilistic seismic hazard analysis (PSHA) to estimate peak ground acceleration at baserock, site effect, response spectra, and earthquake impacts on some structural details. Summary on some results of the current PSHA with respect to the recent advances in seismic hazard and ground motion analysis is presented. These studies have been carried out for input to revision of the Indonesian seismic code for buildings. The results of the researches and studies show some basic agreements. More studies are needed before compiling these studies into new code. Complementary studies and researches are carried out on the seismic design methodology. The design concept is based on the energy approach. Since this method is far from being used world wide, further studies are still needed before being adopted into any seismic design code.

1. INTRODUCTION

Indonesia is one of the most earthquake prone countries in the world. It is crossed by three earthquake belts, i.e., the Eurasian, Indo-Australian and Pacific belts. Almost ten percent of earthquakes occur in Indonesia and its vicinities. The depths of earthquake foci are in many cases less than 60 km which entitle them to be qualified as shallow earthquakes, spread all over Indonesia. These types of earthquakes have been reported to have caused damages and losses of lives and properties.

Indonesia's annual budget is 60% allocated for infrastructures. Most of them are located in highly seismic areas and thus have to be protected. In recent years, major earthquakes have occurred in Flores (1992 and 1996), Kerinci (Sumatra Island, 1995), Banyuwangi (Java Island, 1994), Liwa (1994), Halmahera (1995), Biak and Irian Jaya (1996), Bengkulu (2000). All of this earthquakes have caused many fatalities and damages. The most recent large earthquakes in the last four years are Aceh December 2004 (Mw=9.3) earthquake with tsunami, Nias March 2005 earthquake (Mw=8.7), Yogyakarta May 2006 earthquake (Mw=6.3), Pangandaran July 2006 earthquake (Mw=7.2) with tsunami, and lastly Bengkulu September 2007 earthquake (Mw=8.4). All of these earthquakes have caused disasters with many casualties, buildings, residential houses, and infrastructures damages.

Before the year of 2002, the Indonesian Seismic Code (Department of Public Works, 1989a and 1989b) is officially used for design purposes. The seismic zonation adopted in the seismic code was based on level of probability of earthquake equivalent to 200 years earthquake return period (10% probability of being exceeded in 25 years) based on results of seismic hazard analysis performed by Beca Carter Hollings & Ferner (1979).

The seismic zonation map in Figure 1 was generated from earthquake data obtained before 1962 and has been evaluated in the mid eighties which results in the recent codes (Department of Public Works, 1989a and 1989b). Since then, the data procurement systems have been significantly improved and more accurate empirical formulas have been introduced. Some earthquakes prior to 1997 show that there is a need for revising the 1989 Indonesian earthquake zoning map by using more appropriate formulas (Merati et al., 1996 and 1997).

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Figure 1 Indonesian earthquake zonation map before 2002 (Department of Public Works, 1989a).

The Indonesian seismic zonation was then revised and officially released in the year 2002 (SNI-03-1726-2002), and it is currently used for design code of new construction of buildings. Seismic zonation in the current Indonesian building seismic code is based on the probability level equivalent to 475 years earthquake return period (10% earthquake probability of being exceeded in 50 years) that corresponds to many international seismic building codes including Uniform Building Code 1997, although new development in the seismic building codes such as International Building Code (International Code Council, 2006), adopts more strict criteria, that is using probability level equivalent to 2475 years earthquake return period (2% probability of exceedance in 50 years). In principle, both level of probabilities are developed from the basic concept that earthquake resistant design of buildings or infrastructures is intended to prevent loss of human life and personal injury and to minimize damage to property with vital services are maintained in the earthquake event.

2. PREVIOUS RESEARCHES IN SEISMIC HAZARD ANALYSIS

There are some essential geologic, seismic, and engineering input parameters that need to be considered in improvement of seismic zonation and seismic ground motion. The geologic input such as slip-rate and maximum magnitude, seismo-tectonic setting for seismic source zoning, earthquake history for recurrence modeling are among parameters that need to be updated considering new information available that was not considered and lacking in the previous analysis. In addition, attenuation functions to be used in the PSHA would significantly affect the seismic ground motions and response spectra. Then, the decision on the probability level to be adopted in the seismic building codes would determine the level of ground motion under various site-class. Some previous seismic hazard researches in the process to the development of the current Indonesian seismic zonation as well as the most resent researches post Great Sumatra earthquake towards improvement of the seismic zonation are summarized in the following paragraphs.

Study by Merati et al. (1997) have put some efforts in improving the 1989 Indonesian seismic zonation by adopting some new seismic data. Three approaches were used. The first is by attenuation function proposed by Donovan (1974) through PSHA, chosen from many attenuation formulas used to determine the peak ground acceleration at a point located at a certain distance from the hypo center. At the time, the formula proposed by Donovan was widely used by many Indonesian structural engineers. The attenuation formula used was

a = 1080e0.5M(R+25)-1.32 (1)

where a = peak horizontal ground acceleration at the base rock [cm/s2]R = distance from the hypo center [km] M = earthquake magnitude in Richter scale



As shown by Eq. (1), the peak ground acceleration obtained by using the attenuation formula proposed by Donovan appears to be very simplified to represent the complex nature of earthquake and to define the characteristic value for 200 year return period was derived by using earthquake data from slightly more than 30 years. To be used for design code purposes, the validity of the results needs to be tested by using other approaches.

The second approach uses the geological conditions as a starting point to differentiate the various locations according to their similarities such as fault type and focal depth. From these analyses, as many as 164 earthquake sources have been identified. Using the earthquake data from 1900, the recurrent relationship using Gutenberg-Richter formula was determined:

Log N= A – B M (2)

M = earthquake magnitude in Richter scale N = number of earthquake occurrences with magnitude equal to M, or larger A, B = constants

By using regression analysis from the data, the constants A and B have been found for all earthquake sources. The analysis also produced results for the maximum, minimum and average values for M for each earthquake source. The analysis to obtain the peak ground acceleration was carried out by using EQRISK program (McGuire, 1976). Additional information was needed for using this program, i.e., the probability of exceedance and the attenuation formula used. The probability of exceedance was defined as the function of the return period (200 years) as follows:

1/T=1-exp(-P) (3)

where,

P = probability of exceedance T = return period (taken as 200 years)

This second approach suggested that two different attenuation formulas must be used. The first is by Joyner and Boore (1981) which suggested an attenuation formula best fitting earthquake due to strike fault mechanism such as Sumatra, Maluku (including Halmahera), and Irian Jaya islands.

log(a) = 0.43 + 0.23(M-6) - log(R) - 0.0027R +0.28 (4)

The second attenuation formula best fits earthquakes due to subduction mechanism. This was used to predict the peak accelerations for Java, Nusa Tenggara (including Flores), Kalimantan, and Sulawesi islands. This formula was proposed by Crouse (1991).

ln(a) = 6.36 +1.76M- 2.73 ln[R+exp(0.608M)] + 0.0091 H (5)

where

H = focal depth [km]

The attenuation formula is applied on the sites assuming the earthquake source to be rectangular (Merati et al., 1996, and 1997).

The third approach was by using Eq. (5) which assumed that the earthquakes occurred due to subduction mechanisms. This involved further studies in the geophysical discipline which determined the widths and lengths of subduction zones. This attenuation formula was applied assuming that earthquake sources were line sources.



90.00 95.00 100.00 105.00 110.00 115.00 120.00 125.00 130.00 135.00 140.00-15.00

-10.00

-5.00

0.00

5.00

Figure 2 Peak ground acceleration contour map of Indonesia (Merati et al., 1997).

The maximum probable earthquake magnitude M is found by using Eq. (4), setting the annual occurrence of 10-5. By using Kanamori equation (Merati et al., 1996 and 1997), the peak ground acceleration is found.

M = log (a) + 4 (6)

The results of this research were the peak ground acceleration map for Indonesia (200 year return period) as shown in Figure 2. It can be seen that in some places, potentials for zoning alterations have been developed. To be applicable, the map in Figure 2 should be translated into equivalent static load zoning map, as was the case for the present zoning map as shown in Figure 1. For this purpose, researches are being carried out to generate response spectra for various soil conditions and locations throughout Indonesia (Department of Public Works, 1997; Mangkoesobroto, 1998; and Surahman and Moestopo, 2000).

Some additional seismic hazard analyses toward development of the current Indonesia seismic code, consisting several important cities in Indonesia have been performed by researchers, among others are Kertapati (1987 and 1999), Najoan (1996 and 1999), Shah and Boen (1996), Mangkoesoebroto (1998), Sengara et al. (1998 and 1999, 2001, and 2002), Firmansjah and Irsyam (1999). Peak ground accelerations for several cities have been proposed based on updated seismic data and some adopted attenuation functions.

Research by Firmansjah and Irsyam (1999) and Sengara et al. (1999, 2001, and 2002) adopted a methodology based on total probability theorem through PSHA. Seismic hazard analysis is conducted to estimate the PBA. Total probability theorem assumes earthquake magnitude (M), hypocenter distance (r) as continuous independent random variable that affected the intensity (I). in this case PBA, with reference geological material with shear wave velocity higher than 760m/s. This theorem can be expressed as:

miI[P]iI[Pr p

and dmdr)r(f)m(f]r RM (7)

where:

P [I i m and r] is conditional probability for earthquake occurrence that cause local ground movement of intensity I i provided any point in the seismic source with a known magnitude (M) and a source distance (r), fM(m) is magnitude probability density function for seismic source area, and fR(r) is source distance probability density function for seismic source area.

This total probability theorem is embedded in the computer program EQRISK (McGuire, 1976). The program was modified to update the attenuation functions appropriate for the Indonesian earthquake mechanisms. In this analysis earthquake sources were divided into area and point



sources based on their characteristics. Joyner and Boore (1981) and Boore et al. (1997) attenuation functions are used for strike slip type mechanism, whereas Crouse (1991) and Youngs et al. (1997) attenuation functions are used for subduction type mechanism.

The methodology could be employed to develop seismic macrozonation of Indonesia as has been conducted by Firmansjah and Irsyam (1999). The methodology has been applied to obtain PBA for city of Jakarta and Bandung (Sengara et al., 1999 and 2001), and Denpasar (Sengara and Susila, 2002). Seismic source zones involving geological and seismo-tectonic input for the PSHA are mainly provided by (Kertapati, 1987, 1999). Recurrence models used in this analysis are exponential model that consist of Guttenberg-Richter and maximum entropy principle (Dong etal., 1984). All the PSHA using this methodology were conducted for probability level of 500 years return period, a probability hazard criteria proposed in the current Indonesian seismic code.

Development of Indonesian seismic zonation map is finalized in 2002 and the map was based on the input of the many pertinent experts of earthquake engineering as described above. The resulting map that integrates results of those researches on seismic hazard analysis is presented in Figure 3. This map is officially included in the Indonesian seismic building code (SNI-03-1726-2002) released in 2002.

Figure 3 Indonesian seismic zonation for reference rock (Site-Class-B), currently adopted in the Indonesian seismic building code (SNI-03-1726-2002).

3. RECENT RESEARCHES IN SEISMIC HAZARD ANALYSIS

More recent researches toward improvement of Indonesian seismic zonation are conducted by Irsyam et al. (2005 and 2007a), Sengara et al. (2007 and 2008) and Paristhuta (2007). Recent PSHA conducted by Irsyam et al. (2007b and 2007c) and Sengara et al. (2007 and 2008) consider 3-Dimensional source zones model, with more detailed seismic source zoning and more recent recurrence models. The PSHA has considered the tectonic setting, regional geology, and seismicity of the site of interest employing more recent geological and seismological input data. In this analysis, seismic source characterization of both subduction sources in the south and shallow crustal faults within radius of 500km from site or grid of interest has been made. The subduction earthquake source zone characterization consists of separate identification of megathrust and benioff geometry, each zone representing potential seismic source that would provide probability contribution.

Appropriate attenuation functions have been adopted in the PSHA. Youngs et al. (1997) attenuation function has been adopted to represent the subduction earthquake sources. While, combination of Sadigh et al. (1997) and Boore et al. (1997) attenuation functions have been adopted for shallow crustal seismic sources. In addition, both exponential and characteristic recurrence models have been adopted. Some uncertainties are minimized through logic-tree methodology. The PSHA use EZ-FRISK computer program (Risk Engineering, 2004). The PSHA



would provide result of hazard curve and uniform hazard spectra (UHS) for particular level of probability of interest.

Sengara et al. (2007, 2008) has applied the above methodology for PSHA for city of Banda Aceh and Sumatra Island as a whole, post Great Sumatra earthquake of December 2004, as well as for city of Jakarta (2007). Result of PSHA for Jakarta suggests that PBA is 0.2g and 0.28g, for earthquake return period of 475 and 2475 years return period, respectively. This result is close agreement with some of other PSHA such as by Seed et al. (1996) and Irsyam et al. (2000). As comparison, the current Indonesian seismic code applied PBA for city of Jakarta is 0.15g corresponding to 475 years return period earthquake. For seismic zonation of Sumatra island, seismo-tectonic setting and geological input of subduction and Sumatra Fault Zone (SFZ) was provided by Natawidjaja (2002) and Sieh and Natawidjaja (2000), in addition to the previous source zone from Kertapati (1999). The characteristic model is adopted to represent high magnitude recurrence, meanwhile the exponential model is applied to represent to the low magnitude recurrence. The analysis was conducted at a grid spacing of 0.5 degree and seismic hazard map for 10% and 2% probability of exceedance ground motions has been developed for 3 periods of interest, that is for PBA, short period (T=0.2 sec), and long period (T = 1.0 sec) spectral value, respectively. As an example, Figure 4a shows seismic hazard map of Sumatra Island for 10% PE in 50 years with reference to T=PBA, whereas, Figure 4b shows the hazard map for 2% PE in 50 years with reference to T=0.2 sec. As can be seen from Figure 4a, the seismic hazard map resulted from this study shows some noticeable differences compared to the current seismic hazard map, particularly in the area along the SFZ.

Further analysis to recommend design spectra for various site-class was also conducted. The analysis is based on UHS resulted from the PSHA. De-aggregation analysis has been conducted to identify the controlling earthquake for particular ground motion period of interest. The UHS was then used as a basis for formulating seismic input motion for some site-specific response analyses for various site-class. Seismic zonation map for Sumatra Island and design spectra development is elaborated in Sengara et al. (2007 and 2008). The PSHA, mapping, and seismic ground motion analysis that has been developed for Sumatra Island is considered as a pilot study that could be applied further for whole Indonesia in the effort towards improvement of the current Indonesian seismic zonation map.

Figure 4a New seismic zonation map for Sumatra island correspond to 10% PE in 50 years for T= 0 (PBA) spectral value (Sengara et al., 2008).



Figure 4b Seismic zonation map for Sumatra island correspond to 2% PE in 50 years and for T=0.2 sec spectral value (Sengara et al., 2008).

Irsyam et al. (2007) have also applied the aforementioned methodology for PSHA for improvement of current seismic hazard map. Some improvements in seismic hazard analysis were implemented by considering the recent seismic activities in Indonesia. The seismic hazard analysis was carried out using the latest research works regarding the tectonic setting of Indonesia. Two hazard levels are also analysed for representing 10% and 2% probability of exeedance in 50 years ground motions, and also represents short period (0.2sec) and long-periods (1.0sec) spectral values at the base-rock.

The recent research work towards improvement of Indonesian seismic zonation is currently still in progress with some issues of input verification such as on seismic data completeness, seismo-tectonic setting, maximum magnitudes, slip-rates, seismic recurrence parameters for both subduction and shallow crustals. Other issues among others are identification of shallow crustal faults and background earthquake source characterization, and near source consideration. In addition, the need on use of appropriate attenuation functions is a challenge. Recent work on development of attenuation function based on Indonesian earthquake data considering deep source event has been initiated by Parithusta (2007). There has been still limited earthquake strong-motion data monitoring for development of this attenuation function. Current efforts to install some numbers of accelerometers for earthquake strong-motion recording and some collocated with seismometer for tsunami early warning system is in progress. This effort is coordinated by Indonesian Ministry of Research and Technology and Indonesian Meteorological and Geophysical Agency in cooperation with some engineering universities in other related institutions.

Characteristic of ground motions and site effect for the design spectra recommendation due to both subduction and shallow crustal seismic source zone, remain a challenging research topic. In this particular problem, the design spectra have to consider various level of peak base accelerations, earthquake input motions corresponding to their spectral values at short and long-period, and site-class. It is expected that, with more advanced probabilistic seismic hazard analysis and with more strong-motion records available, characteristics of Indonesian earthquake ground motions could be quantified more accurately, in the efforts to improve Indonesian seismic zonation and building code.



4. DESIGN APPLICATION

For practitioners, the use of probabilistic terminology was felt to be cumbersome. Despite its probabilistic nature, the structural design has been carried out deterministically. To facilitate this, the Indonesian earthquake loading code uses the characteristic value for 200 (prior to the year 2002) and 500 year return period earthquakes to be used as earthquake loading for buildings with 50 year life time.

For zoning purposes, the knowledge of peak ground acceleration at the base rock is sufficient. This ensures the uniqueness of the zoning map. In present practice Earthquake loading used in building design is construed as lateral loading obtained either by dynamic or static analysis (Department of Public Work, 1989b). This lateral loading is proportional to the building mass and the maximum horizontal ground acceleration (also called peak ground acceleration) given by the zoning map. The complete analysis to obtain the equivalent static earthquake loading to the building requires the response spectra calculation which depends on the structural stiffness and soil conditions. This, however, can be carried out with ease, once the peak ground acceleration at the base rock is known (Surahman and Merati, 1998).

For regular buildings and buildings with a height of no more than 40 m, the earthquake load may be considered as a static equivalent load. Based on this code, the Base Shear V may be computed from the seismic coefficient C obtained from the response spectra given for each of the 6 seismic zones of Indonesia, as shown in Figure 3. Empirical formulas are given to assess the fundamental period of the structure, but this must be verified by comparing its value with that obtained from other methods such as simplified modal analysis. The Base Shear is then distributed along the height of the building, following a triangular (first mode) distribution.

The base shear V is defined in as

RTIWCV tv (8)

where Cv is a coefficient depending on the soil conditions at the site and seismicity of the region, I is the importance factor, Wt is the total building weight, R the ductility and overstrength factor, and T is the fundamental period of the structure. For specific conditions V is limited not to be less or need not be more than a certain value (International Code Council, 1997). The equivalent static earthquake horizontal forces are then defined for each segment along the building height as follows:

)(

1

tn

jjj

iii FV

hW

hWF (9)

where Fi is the lateral force acting on the i-th storey, Wi is the weight of the i-th floor, hi is the i-th storey elevation from the ground, Ft is the additional force applied at the top of the building to account for higher mode contributions, and n is the number of stories.

This loading formula is valid only for ordinary buildings with design life times of 50 years, allowing ductility of 4 to occur during extreme earthquake load with a return period of 200 years. For irregular buildings and buildings of more than 40 m height, the distribution of the lateral load along the height of the building must be verified by comparing it with the actual distribution obtained from a response spectrum modal dynamic analysis, using the same response spectra given for each seismic zone.

To enhance the existing design practice, alternate earthquake load representations have been explored from researches and analyses on response spectra suitable to Indonesian conditions such as carried out by Department Of Public Works (1997), Merati et al. (1996) and Surahman and Moestopo (2000). Various earthquake records have been analyzed. Based on this approach, a response spectra diagram for the elastic response have been obtained and can be expressed in a



non-dimensional representation such as in the form of dynamic magnification factor. To take the inelastic response into account, the earthquake loading is reduced by factors depending on the structural ductility (Department Of Public Works, 1997, and Wangsadinata, 1997). This is to ease and generalise the design formulations involving various earthquake prone regions. Similar approach has been carried out by Surahman (1992 and 1996) and Surahman and Moestopo (2000), resulting in a response spectra based on energy approach.

00.20.40.60.8

11.2

0 2 4 6 8 10

Normalised Displacement

Nor

mal

ised

Loa

d

-2

-1

0

1

2

-10 -5 0 5 10

Normalised Displacement

Nor

mal

ised

Loa

d

Figure 5 Failure due to monotonic and cyclic loading.

This static method has been found satisfactory for structures with simple and regular configurations. Moreover, in recent years the above formulations have been gradually refined by incorporating new findings in earthquake engineering (Surahman, 2007). By using the LRFD procedures to handle the probabilistic aspects of the design, deterministic structural calculations can be implemented by using stochastically determined site specific spectra. Some of the changes in the design concepts have been fundamental such as those used in the Displacement-Based Design and Performance-Based Design methods.

The approach however, does not reflect the actual structural behavior since earthquake is actually a combination of inertia and displacement induced loads due to random vibrations. Figure 5 shows that the failure due to static actions differs significantly from that due to dynamic actions. Monotonic loading caused by static action results in one time maximum deformation induced failure whereas cyclic loading caused by dynamic actions caused cumulative low cycle fatigue type of failure.

Recognizing that there are significant differences between the static approach and the actual structural response under seismic actions, more researches are carried out on structural components under seismic actions. The differences are clearly visible in the failure mechanism shown in Figure 5 and energy absorbed before failure (Surahman and Moestopo, 2000; and Surahman, 2007).

Researches on structural components subjected to cyclic loading involves energy calculations as defined by



d d d dt t t t

dtytFdtyyFdtyCdtyyM0 0 0 0

2 )()( (10)

where M is the mass, y is the acceleration, C is the structural damping, y is the velocity, F(y) is the restoring force as a function of the displacement y and F(t) is the earthquake load at a certain time t, and td is the earthquake duration. The energy inputted to the structure EI is equal to the terms in both sides of Eq. (10).

It has been found that sections under cyclic loads can absorbed as much as four time energy, or more, as compared to those under monotonic loading (Surahman and Moestopo, 2000 and Surahman et al., 2003). Sections that exhibit high strength may not necessarily sufficiently ductile to resist cyclic loading. This is related to the use of high strength but less ductile material (Surahman and Merati, 1996). There have been researches carried out to investigate structural response due to reversed cyclic loading, such as on partially pre-stressed concrete beam connections (Budiono, 1996), exterior beam column joints with variable column stirrups and longitudinal beam reinforcements (Merati and Widagdo, 1996), fully ductile beams with variable shear reinforcements (Mangkoesoebroto, 1998). Experimental research carried out in Surahman and Moestopo (2000) was to verify analytical computations carried out by Surahman (1992 and 1996).

Results of Merati and Widagdo (1996) showed that in the beam-column joint tests, the beams exhibit bending failures, whereas the joint exhibit shear failures. Beams with compressive to tensile reinforcement ratios of 0.50, 0.75, and 1.00 have almost the same energy dissipation capacities. The increase of compressive to tensile reinforcement ratio increases the stirrup required in the joint. Research by Imran et al. (2006) shows that it is important to use materials complying with seismic code requirements. Concrete section using noncompliant steel reinforcement, with actual to nominal yield strength ratio larger than 1.35 and ultimate to yield strength ratio less than 1.25, despite its higher strength capacity it is less ductile than the section using compliant reinforcement. This conclusion is very clear when energy criteria is used as shown in Table 1. Lauw (2005) showed that the presence of plate increases the energy dissipation capacity of the beam-column sub assemblage.

Results of Surahman and Moestopo (2000) and Surahman et al. (2003) showed that there is a positive correlation between energy absorbed and damage suffered by a structural component. The energy absorbed by a structural component due to cyclic loading is far larger than that due to monotonic loading before reaching failure. This energy approach can be used to assess building damage based on the severity of the load (Surahman, 2000). When damage is expressed in the cumulative ductility ratio rather than the maximum ductility . Figure 6 shows as the sum of bold lined (plastic) non-dimensional deformations. This value can be used as an indication of cumulative damage. For elastic response the value of is equal to zero. With this approach the inelastic response is expressed by

12e

iRR (11)

where Re and Ri are the elastic and inelastic response respectively. This equation shows that the structural strength can be reduced when damage is allowed to occur.



Figure 6 Elastic-perfectly-plastic load-deformation relationships.

As shown and proposed by Surahman (2007) that earthquake energy per unit mass is a stable quantity and therefore the earthquake resistant structural design can also be based on the amount of the total energy can be sustained by the structural components in the building. The fact that the energy per unit mass is a stable parameter is also shown by Fajfar (1996).

dt

g

gI dttaav

ME

0

2)(85.0 (12)

where vg/ag is the peak ground velocity to peak ground acceleration ratio and a(t) is the ground acceleration during earthquake.

0

2000

4000

6000

8000

0 2000 4000 6000 8000

Integration, [mm/sec]

Empi

rical

For

mul

a,

[mm

/sec

] CaliforniaMexicoJapanTurkeyIndonesiaOthers

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4

Period [sec]

Nor

mal

ised

Equ

iv. V

el.

[sec

]

HachinoheEl Centro and TaftParkfield

Figure 7 a) Input energy b) Input energy per unit mass.

Figure 7a shows input energy of various earthquake occurrences obtained by using integration equation of Eq. (10) and using empirical formula of Eq. (12) expressed in equivalent velocity whereas Figure 7b shows the energy demand per unit mass of the building for some well known earthquakes expressed in terms of equivalent velocities normalized with respect to the peak ground acceleration. The equivalent velocity is defined as

MEV I

E2

(13)

where EI/M is the energy per unit mass absorbed by the building. This non-dimensional formulation is to ensure that a certain site is defined by a single curve.

The energy absorption capacity of individual member EC can be estimated by

)5.0( uYYC FE (14)



where is the cumulative plastic ductility capacity of the structural component, Y is the member yield displacement, Fy is the member yield strength, and is the energy coefficient representing the structural detailing quality of the component.

The value of is equal to unity for perfectly elastic-plastic behavior as shown in Figure 6, larger than unity if strength reserve effects including strain hardening is dominant as shown by Figure 8b and smaller than unity if strength degradation effects, including pinching, are dominant, as shown by Figure 8a.

Figure 8 Hysteresis loop of a) Concrete beam and b) Steel beam.

It is also useful to compare results obtained from static experiments (monotonic loading) and pseudo-dynamic (reversed cyclic loading) experiments. When is the ductility ratio obtained from the monotonic loading then the cumulative plastic ductility capacity can be computed from the

/( -1) ratio, that is the cumulative plastic ductility capacity divided by the monotonic plastic ductility. Table 1 shows the non-dimensionalized energy capacity of various members.

Table 1 Energy capacity for various components.

Component /( -1) Fy [kN] k [kN/mm]

Concrete Beam1 90.00 – 125. 00 8.40 – 11.67 0.50 – 0.60 39.5 2.60

Concrete Beam2 102.23 14.6 0.8 59.5 3.33

Concrete Beam3 146.12 20.7 0.9 59.5 3.33

Concrete Beam4 23.14 5.78 0.43 79.0 2.54

Concrete Beam5 9.69 3 0.35 118.0 4.54

Steel Beam6 41.92 9.32 1.46 142.0 3.59

1) Surahman and Moestopo, 2000 2) Mangkoesoebroto et al., 2003 3) Mangkoesoebroto et al., 2003 diagonally reinforced 4) Imran et al., 2006 5) Imran et al., 2006, noncompliant reinforcement 6) Surahman et al., 2003

Based on this information for future design the load can be based on the energy absorbed by the building which depends on the site, building mass, and the peak ground acceleration, instead on based on the lateral force. The member capacity will be determined by , and Fy Y. Except for concrete beam with noncompliant reinforcement, the values of cumulative plastic ductility exceed four times of those obtained from static experiments. Knowing that Y = Fy/k where k is the translational stiffness of the member, the term Fy Y can be replaced by Fy

2/k. For flexural members, the forces are measured in terms of moments and deformations in terms of rotations. Hence, the term Fy Y becomes MY Y and the corresponding member stiffness would be rotational. The design philosophy is then to make sure that the sum of member energy capacity in the building is larger than the energy to be absorbed by the building. The member stiffness shown on



Table is based on the experimental setup. In actual rigid building columns, the member stiffness depends on member size and the story height, whereas the rotational stiffness of flexural members depend on the member size only.

5. CONCLUSIONS

The paper shows that there have been many researches carried out in the discipline of earthquake engineering, starting from seismological researches up to applied structural components designed against earthquake loads. The first part of the paper has presented summary on development of Indonesian seismic zonation to the current stage. Some previous research on probabilistic seismic hazard analysis that support to the 2002 Indonesian seismic zonation is presented. Furthermore, recent researches on probabilistic seismic hazard analysis employing new methodology with improvement in seismic source zoning, recurrence models, and attenuation functions, as well as new approach on the spectral values covering short and long-period motions are presented. All these efforts on seismic hazard and spectral analysis by pertinent experts in earthquake engineering are expected to provide significant improvement on the current Indonesian seismic zonation. Seismic strong-motion monitoring data would be an essential input to verify some of the assumptions adopted in the analysis. Further researches in understanding the characteristics of seismic ground motions from strong-motion record data and development of attenuation functions representing subduction and shallow crustal earthquakes, more detailed investigation on seismic source zone characterization, and its corresponding recurrence modeling would still be necessary.

The second part of the paper shows the researches carried out on structural performance of components under cyclic load. For energy based design purposes, the knowledge of energy absorbing capacity of structural components is necessary. The energy absorbing capacity can be expressed as non-dimensional cumulative ductility. Conservatively it can be assumed to be four times the plastic ductility obtained from static loading. Since energy is a stable parameter, it can be used as an alternate design approach which eventually may replace the force or displacement design method. To achieve this state, further researches are needed.

6. REFERENCES

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