v38-163

Study on Indonesian Seismic Code SNI 03-1726-2002 and Seismic Impact to High-rise Buildings in Jakarta, Indonesia

C.M. Yang

Jakarta is the capital city of Indonesia with more and more highrise buildings, of which seismic resistance capacity is most concerned. The seismo-tectonic of Jakarta Region is complicated, and the Author considers the PGA specified in SNI 03-1726-2002 is inadequate. Hence the Author queries and discusses it here-in, and wishes to lead for a reasonable solution for the seismic resistance design.

Abstract --- The Author studies the basis of Indonesian Seismic Code SNI 03-1726-2002 and proposes an updated Seismic Hazard Map with Peak Ground Acceleration (PGA) for Indonesia in the paper. The ground motion considered in the Code is 10% probability of being exceeded in a 50-year reference period. On this basis, the relevant design criteria, model of structural analysis and seismic reduction factor are discussed. The Author concluded that in the yielding process of structural system, forming of plastic hinge at the base of columns and shear walls should not be allowed. An adequate factor of safety should be applied to prevent from reaching the threshold of collapse. The Author also studies the seismo-tectonic of Jakarta region, discuss the relevant updated PGA and the structural design requirement. It is considered that the PGA = 0.15g desinated for Jakarta in SNI 03-1726-2002 is inadequate and proposed the Authority to review the structural design of highrise buildings in Jakarta, and take appropriate action as necessary.

Keywords --- Seismic design Code, PGA and hazard map, ground motion, seismic reduction factor, highrise buildings in Jakarta.

I. INTRODUCTION

II. RECENT EARTHQUAKE AND HAZARD MAP

The Earthquake Hazards Program of United States Geological Survey (USGS) [3] and Meteorology and Geophysics Agency of Indonesia (BMG) [4] have provided Indonesian earthquake records for many years. Based on the records, the Author summarizes the events in Figure 1, which shows the main earthquakes occurred in year 2002 to 2008 in Indonesia, historic major earthquakes are also shown.

A moderate earthquake of M=5.1 in Kalimantan is included in Fig.1, where the source depth is 10 km only. In 2007, there were one M=6.0 and one M=6.20earthquake occurred near this one, but the source depth is not known.

Indonesia is located in one of the world’s most active seismic zone. Following the development of technology, the Public Works Department of Indonesia published an Indonesian National Standard SNI 03-1726-2002 “Tata Cara Perencanaan Ketahanan Gempa Untuk Bangunan Gedung” [1], i.e. “Seismic Resistance Design for Building Structures”. The Code has similar basic concept as U.S. Code 2000 IBC [2], such as Equivalent lateral force procedure, seismic response modification factor in 2000 IBC and seismic reduction factor in SNI 03-1726-2002. However, the basic analysis for seismic response in SNI 03-1726-2002 is not same as the U.S. Code.

Peak Ground Acceleration (PGA) is an important factor in seismic analysis. The Author has proposed his equation for evaluation of PGA and the acceleration attenuation for Indonesia as follows [5],

ln (pga) = 1.486 M – 2.386 ln (R1 + 1.782e0.554M) (1)

where pga is the peak ground acceleration in g (g = 9.81m/s/s). M is the magnitude of the earthquake and R1 is the closest distance to the source/hypocenter of the earthquake in kilometer.

R1 = (Z2 + 2)0.5 (2)

Z is the depth of the source, in kilometer. is the closest distance to the epicenter in kilometer.

It can be seen that, for pga, magnitude of the earthquake M is important, depth Z of the source is also an important factor.

C.M. Yang is a consultant with Mannings (Asia) Consultants Ltd., Hong Kong (Telephone: 852-3168 2028; Fax: 852-3168 2022; e-mail: [email protected])

PROCEEDINGS OF WORLD ACADEMY OF SCIENCE, ENGINEERING AND TECHNOLOGY VOLUME 38 FEBRUARY 2009 ISSN: 2070-3740

PWASET VOLUME 38 FEBRUARY 2009 ISSN 2070-3740 961 © 2009 WASET.ORG

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M 7.42008 Feb.20

M 7.62002 Nov.02

M 9.12004 Dec.26

M 8.72005 Mar.28

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Fig. 4. M 9.1 Earthquake on 2004 Dec. 26

M 7.51938 May.19

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M 7.81915 Jan .22

M 7.81939 Dec.21

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Fig. 5 Major Earthquake in Northern Sulawesi



Fig.6 The Destructive/Major Earthquakes occurred in Irian Jaya Region – For 6.0 to 6.5, source depth 15km shown; For 6.6 to 6.9, 30km shown; For 7.0 to 8.5, 35km shown.

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M 6.91995 Mar.19

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M 6.81955 Jan.27

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M 6.51994 May.25

M 7.11936 Feb.15

M 6.31978 Apr.03

M 7.11964 Apr.23

M 6.42001 Apr.04

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Based on the events, using equation (1), the Author refers to the USGS and GSHAP Seismic Hazard Map for Indonesia [2] & [6], and herein proposes his Seismic Hazard Map with PGA for Indonesia as shown in Figure 2. The PGA is in 10% probability of being exceeded in a 50 year reference period. This map is much different with the Hazard Map specified in SNI 03-1726-2002.

In the analysis of the structural response, SNI 03-1726-2002 stresses the ductility of building structures. In the yielding process of structural system, SNI 03-1726-2002 stresses forming of the plastic hinge at both ends of all the beams, forming of plastic hinge at the base of all columns and all shear walls simultaneously, until reaching the threshold of collapse – Clause 3.1.3.1, Clause 3.1.4, Clause 4.5, Clause 9.1.3 and Clause A.4.5.1 of SNI 03-1726-2002 refers. Then, the base shear is analyzed by equivalent lateral force procedure.

The PGA is based on the rock site. When the site is soft soil, the PGA could be amplified as shown in Table 1. The amplification factor taken is NEHRP (1994) Correction Factor [7]. This amplification is gradually dismished when ground motion increases to 0.5g or above due to yielding of soil under it.

May the Author call this state – at the threshold of collapse, as “collapse limit state”.

The query is;

TABLE 1 PGA FOR BASE ROCK AND SOIL IN g

PGA for base rock

PGA for soil in g

in g Firm soil Medium soil

Soft soil

smsv /750 smsv /350 sv175 <350 sv 175

0.08 0.10 0.13 0.20

0.16 0.19 0.24 0.32

0.24 0.28 0.32 0.36

0.32 0.32 0.38 0.38

0.40 0.40 0.44 0.44

0.48 0.48 0.48 0.48

The distribution of the earthquakes in Fig.1 and the Hazard Map in Figure 2 are somewhat complicated. However, referring to Figure 3, it is generally in line with the tectonic setting of Indonesia, and agreed with the seismicity Records in Indonesia. It is reminded that, in 2000 IBC definition, a fault for which there is an average historic slip rate of 1 mm per year or more is am active fault. As shown in Figure 3, the motion between Tectonic Plate in Indonesia could be 65mm/year to 111mm/year. Hence, the complication of seismicity in Indonesia is normal.

III. GROUND MOTION ANALYSIS AND STRUCTURAL MODEL FOR DESIGN

A. Ground Motion Analysis

a. Shear wall is a cantilever structure to resist lateral load to the building.

As the plastic hinge forms at the base of the shear wall, the hinged “shear wall” would not function as shear wall again, could the structure still stand? Or, any further ground motion or lateral force would cause the structure collapse immediately?

b. The equivalent lateral force procedure for base shear analysis, the column and shear wall is considered to be fixed at the base, not hinged, in 2000 IBC [2] and FEMA 450 [8].

Please note that for any strong earthquake, there will be many aftershocks follow up. Just after the 26 Dec. 2004 Sumatra M9.1 Earthquake, there were more than hundred of aftershocks within 26 Dec. 2004 to 10 Jan. 2005, including 12 Nos. of M6.0 – M6.9, 91 Nos. of M5.0-M5.9. 3 years after an M7.4 earthquake occurred on 20 Feb. 2008 in Simeulue, just 65 km to the M9.1 one. As shown in Figure 4, it was a M7.6 Earthquake occurring nearby.

In the M8.5 and M7.9 Southern Sumatra Earthquakes of 12 & 13 September 2007, there were 6 Nos. and 17 Nos. of aftershocks with M6.0 to M6.9respectively.

It means that, the aftershocks of a strong earthquake should not be neglected. Besides aftershocks, there often have some earthquakes occurred near a Major Earthquake in few years. As shown in Figure 5, in Northern Sulawesi Region, there was a M7.3 Earthquake of 2008 Nov. 16 (M7.7in BMG records) in Minahasa Peninsula. There were a M7.6 and M7.5 Earthquakes in 1990 and 1991 respectively closed together nearby. In Irian Jaya Region, as shown in Figure 6, there were more earthquakes closed together -- The motion between the Tectonic Plate is 111mm/year here, made it the most active seismic zone. The question is:

Even the design structures do not collapse at the “collapse limit state”, could it be repaired within 15 days or 3 years to resist the aftershocks or the next earthquake?

In SNI 03-1726-2002, the design seismic load is based on the earthquake ground motion with 10% probability of exceedance in a 50-year reference period, i.e. with 475-year or 500-year of return period.



B. Ground Motion Considered and Design Criteria

In 2000 IBC and FEMA 450, the Ground Motion Considered is the earthquake with 2% probability of being exceeded in a 50-year reference period, i.e. a return period of 2475 years, not 475 years or 500 years. The corresponding Peak Ground Acceleration (PGA) considered and the earthquake spectral response acceleration Sa considered are much different with those in SNI 03-1726-2002.

Refer to Figure 10 of “Probabilistic Seismic Hazard Assessment for the state of California” [9], we can compare the PGA for 2% and 10% probabilities of exceedance in 50 years, and get the ratio between them is about 2.3 (Los Angeles) to 2.5 (San Francisco). The FEMA 450 Part 2 indicates in coastal California, the ratio between the 0.2 second spectral acceleration for 2% and 10% probabilities of exceedance in 50 years is about 1.5 whereas, in other parts of United States, the ratio varies from 2.0 to 5.0.

Fig. 7 Structural System

on model 2. Even this is not specified in the Code. The same, the analysis of structural systems for

aftershocks and next earthquake, should also be based on model 2.

This is the case in the U.S. However, the Author considers that similar case would be in Indonesia.

IV. SEISMIC IMPACT TO HIGHRISE BUILDINGS IN JAKARTA

U.S. is a highly developed, very rich country. The maximum considered earthquake ground motion in the U.S. Code is 2% probability of exceedance in 50 years. This is a high and good standard to be adopted. On the other hand, Indonesia is a developing country. The considered ground motion is 10% probability of exceedance in 50 years. This is reasonable. As the basis is different, the structural design criteria should be different. The Author considers that the building structures should in no case be allowed to reach the threshold of collapse in Indonesia. An adequate factor of safety should be specified and applied.

A. Seismo-tectonic and PGA of Jakarta Region

In Figure 3 of SNI 03-1726-2002, Seismic Zoning Map for base-rock, Jakarta is located in Zone 3 with a Peak Base-rock Acceleration of 0.15g, with a return period of 500 years. That means PGA = 0.15gfor Jakarta with 10% probability of exceedance in a 50-year reference period.

Petersen et al. have analyzed the probabilistic seismic hazard for Southeast Asia [10]. The research indicates the seismic sources in Southeast Asia and presents the seismic hazard map for Southeast Asia. Where, as Author understanding, the PGA for Jakarta is 0.25g.

C. Structural Model for Design Equivalent lateral force procedure is applied to

analysis the seismic base shear V. Then the internal force due to seismic load is combined with the internal force due to dead load, live load, etc. As plastic hinge is developed in seismic analysis, the Author considers that the structure model is developed from model 1 to model 2 as shown in Figure 7. Please note that the structures, including all shear walls and columns, are fixed at the base, even in model 2.

Sindku Rudianto et al. have analyzed the seismo-tectonic of Jakarta region [11]. The research indicates that the seismic sources of Jakarta include:

a. Subduction zone earthquake, the recorded maximum is M = 8.1 in year 1903

b. Four large shallow crustal faults within 300 km of Jakarta, which are the great Semangka, Krakatau, Sukabumi-Padalarang, and Cilacap -Kuningan faults.

Normally, the structural system is analyzed with model 1. But in seismic analysis, the Codes allow the progressive forming of plastic hinges. The structural systems will change to model 2. Hence, in this case, all the internal forces due to dead load, live load, wind load and seismic load should be analyzed based

c. To accommodate earthquakes on unknown faults, they indicate a background seismic zone, which has a radius of 25 km from Jakarta, where the maximum earthquake magnitude is estimated to be 6.5.

As the result of their probabilistic seismic hazard analysis (PSHA), they adopted PGA = 0.23g for Jakarta [11].

In fact, there were some earthquakes occurring in Jakarta Region. As shown in Figure 8, they already



Fig.8 Earthquakes in Jakarta Region, with 7.0M

M7.0 1949 June 24

M7.1 1943 Apr. 01

M7.3 1913 Aug. 13 M7.0

1883

M7.5 2007 Aug. 08

M7.0 1934

M8.1 1903 Feb. 27

108°107°106°105�

6

7

8



have reached magnitude of M7.0 and higher. This could not be ignored.

In the USGS Earthquake Hazards Program [3] and GSHAP [6] Seismic Hazard Map for Indonesia, Jakarta is located in the zone with PGA = 2.40 to 3.20 m/s2 = 0.245g to 0.326g, with 10% probability of exceedance in 50 years. The Author prefers to take PGA = 0.28g for Jakarta Region. This is higher than PGA = 0.15g specified in SNI 03-1726-2002.

In fact, PGA = 0.28g is corresponding to moderate earthquake. As shown in Figure 9, assume an earthquake of M = 6.0 occur in Jakarta at a depth of 20km. Then using equation (1), PGA = 0.30g at epicenter, and major of Jakarta is in the area with PGA = 0.25g to 0.30g. It is not corresponding to strong earthquake of M 7.0.

EPICENTER0.30 = PG A g

= , = 5km 0.29PGA g

= , = 10km 0.27PG A g

= = 15km, 0.25PG A g

0 10 20 30 40 50 km

N

Fig. 9 PGA in Jakarta, assuming an earthquake of M = 6.0 with source depth 20km in Jakarta

If the earthquake is M = 5.0, occurs at a depth of 20km. Then using equation (1), PGA = 0.16g at epicenter. This figure is closed to the PGA = 0.15gfor Jakarta specified by SNI 03-1726-2002.

B. Structural System and Seismic Reduction Factor

The structural systems for highrise buildings in Jakarta commonly are the combination of reinforced concrete open frames and reinforced concrete core wall/shear walls. The seismic base shear is analyzed by equivalent lateral force procedure. In this analysis, the seismic reduction factor R (the seismic response modification coefficient R in 2000 IBC) is applied. In dynamic analysis procedures, including model response spectra analysis and time-history analysis, this reduction factor R is also applied. In SNI 03-1726-2002, the maximum R may be taken as 8.5 for dual systems with special reinforced concrete

shear wall. Wayan Sengara et al. in their seismic design of a 48-storey tower building in Jakarta [12], consider seismic ground motion with 2% probability of exceedance in 50 years and adopt R = 8.5. “This put a high demand on the ductility of all structural elements”. – Yes, how should we achieve this demand?

In fact, this seismic reduction factor R would dramatically reduce the design seismic base shear. As shown in Figure 10, the design response spectrum with R = 8.5 and the elastic response spectrum (R =1) are much different. As a structural engineer, one must be clearly aware of it.

R=8.5

R=1.0

Fig. 10 Spectral response acceleration with R : RSa

In the Table 1617.6 of 2000 IBC, for dual systems with special moment frames and special reinforced concrete shear walls, response modification coefficient R = 8, system over-strength factor o = 2.50 and deflection amplification factor Cd = 6.50.

In fact, the elastic design lateral force could be significantly reduced by R provided suitable structural systems are selected and detailed with appropriate levels of ductility, regularity, and continuity, so as complete plastification could be performed. As indicated in Part 2 of FEMA 450:

R = Rd x o (3) where Rd is system ductility reduction factor.

For this structural system, R = 8, o = 2.50, then Rd = 8/2.50 = 3.20.

As described in FEMA 450, there are three basic components to system over-strength factor o. These are the design over-strength ( D), the material over-strength ( M) and the system over-strength ( s). Usually, the shear wall will resist most of the lateral load, so as the systems are strength controlled. As most designers will seek to optimize their designs and provide a strength that is close to the minimum requirement, the design over-strength coefficient D



could be as low as 1.0. Material over-strength ( M) results from the fact

that the design value used to be conservative lower bound of the structural materials and is lower than their effective strengths in the as-constructed structure. Historically a factor of M = 1.25 is used for concrete and steel.

System over-strength ( S) is dependent on the amount of redundancy contained in the structure. In the dual system with reinforced concrete shear walls, the shear wall will resist most of the lateral force in the structural system. So as, the system will be strength controlled and the system over-strength ( S)would be not more than 1.10.

Then, the system over-strength factor O = 1.0 x 1.25 x 1.10 = 1.38, say O = 1.40. This gives R = Rd

x O = 3.20 x 1.40 = 4.48, say R = 4.50, not R = 8.0 or R = 8.5. Hence, it has to be studied further.

Back to the structural system for the highrise building in Jakarta, we considered that the seismic reduction factor R taken should not be more than R = 4.50 and correspondingly suitable factor of safety should be taken in design. The structural details should be properly designed to ensure that the development of sequential plastic hinging conforms to the design requirement.

In fact, concrete itself is brittle material, yielding of reinforcement allows the forming of plastic hinge and inelastic behavior. The modification effect of R is the development of plastic yielding. In this process, the structural systems dissipate the earthquake energy by significant increase of displacement. This will also damage the structure itself. Hence, the greater levels of inelastic behavior correspond to increased structural damage. The larger of reduction factor R, the larger of structural damage. If R is too large and over ductility of the structure, the structure will collapse. Hence, the value of R must be chosen and used with careful judgment. The limitation of R = 4.50 for highrise buildings in Jakarta is also a kind of limitation to structural damage.

Concerning the performance of building, 2000 IBC in story drift determination specifies that, the deflection amplification factor Cd should be multiplied to the deflection determined by an elastic analysis of the seismic-force-resisting system, Cd = 6.50 here. If it is ignored, it could substantially underestimate the analyzed deflection and is obviously inadequate!

C. Structure of Existing Highrise Buildings in Jakarta

Before year 2002, there have been lots of highrise buildings in Jakarta. The seismic design code was

revised, from SNI 03-1726-1987 to SNI 03-1726-2002, the basic analysis was developed, the PGA specified for Jakarta was changed: PGA = 0.05g in year1987 and PGA = 0.15g in 2002. Now the Author proposes PGA = 0.28g for Jakarta, someone may propose other figure.

Please note that, increase of one grade of earthquake magnitude, means increase of 10 times of energy released in the earthquake, such as:

Magnitude (Richter scale) Approx. energy released

(ergs)

5.0-5.9 3-55 x 1019

6.0-6.9 8-150 x 1020

7.0-7.9 2-42 x 1022

BMG record about 50 events of M 5.0 each month in Indonesia [4].

Fortunately, there was no strong earthquake occurred in Jakarta Region in the past 50 years. However, there are still probabilities of moderate /strong earthquake occurred in Jakarta Region. Hence, it is worth reviewing the design of new/old highrise buildings in Jakarta.

V. CONCLUSIONS

The publication of SNI 03-1726-2002 was over six years. While three events of great earthquake (M

8.0) and 16 events of major earthquake (M = 7.0-7.9) occurred in Indonesia. Hence, the Author considers this is the time to review and update the Code now.

In this process, the Author wishes the followings could be considered:

a. update the Seismic hazard map with PGA

b. clarify the plastic yielding process and seismic design criteria

c. clarify the model of structural system to be analyzed

d. selection of seismic reduction factor R The Author hereby proposes the Authority, the

owner and the relevant engineer to review the structural design and details of existing highrise buildings in Jakarta, and takes appropriate action as necessary.

VI. REFERENCE

[1] SNI 03-1726-2002 “Tata Cara Perencanaan Ketahanan Gempa Untuk Bangunan Gedung”,



i.e. Indonesian National Standard SNI 03-1726-2002 “ Seismic Resistance Design for Building Structures”, Indonesian Public Works Department, Jakarta, 2002

[2] 2000 IBC – International Building Code (2000), International Code Council, Inc., U.S.A, 2000

[3] U.S. Geological Survey, “Earthquake Hazard Program”, USGS Website, 2007 & 2008

[4] BMG, Badan Meteorologi & Geofisika, Indonesia, i.e. Indonesian Meteorology & Geophysics Agency, “Indonesia Earthquake Records”, BMG Website, 2007 & 2008

[5] C.M.Yang, “Study on Indonesian Seismic Impact and Hazard Map for Design”, Proceedings of International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 14-15, 2008, P.308-314

[6] GSHAP – “Global Seismic Hazard Assessment Program (GSHAP)”, United Nations International Decade for Natural Disaster Reduction (UN/IDNDR), (1990-2000), GSHAP Website

[7] Tianqing Cao, W.A.Bryant, B.Rowshandel, D.Branum, and C.J.Wills, “The Revised 2002 California Probabilistic Seismic Hazard Maps, June 2003”, CGS Website

[8] FEMA 450 – Federal Emergency Management Agency, 2003, “NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (FEMA 450), 2003 Edition, Part 1, Provisions, Part 2 Commentary”, Washington, D.E. 2004

[9] California Geological Survey, “Probabilistic Seismic Hazard Assessment for the state of California” 1996, CGS Website

[10] M.D.Petersen, S.Harmsen, C.Mueller, K.Haller, J.Dewey, N.Luco, A.Crone, K.Rukstales and D.Lidke, “Probabilistic Seismic Hazard for Southeast Asia”, Proceedings of International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 14-15, 2008, P.126-136

[11] Sindhu Rudianto, E.Rathje and B.Soedjono, “Site-specific Seismic Analysis for Deep Alluvial Jakarta Site, Indonesia”, Proceedings of International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 14-15, 2008, P.381-388

[12] I.W.Sengara, D.Sukamta and P.Sumiartha, “Site Response Analysis for Seismic Design of a 48-storey Tower Building in Jakarta”, Proceedings of International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 14-15, 2008, P.399-406

C.M. Yang, the Author, is a senior professional in Hong Kong. He graduated in the civil engineering in 1960 in China. He then attended Concrete Structures and Technology Course in Imperial Collage, University of London, UK in 1978 and was awarded MSc and DIC in 1979. He is the Fellow of The Institution of Structural Engineers, U.K. and Class 1 Registered Structural Engineer of PRC. C. M. has over 40 years experience in civil, structural and building engineering. He has extensive experience in the design, contract administration and construction supervision of various engineering projects, including foundation, structural, geotechnical and civil engineering works. He now leads the structural design team of Mannings (Asia) Consultants Ltd responsible for a wide range of permanent works designs, temporary works designs and alternative designs both in Hong Kong and International.



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