Structural damage observation and case study of 2006 Yogyakarta earthquake

Ika Bali, Wu-Wei Kuo, Cindrawaty Lesmana, Budi Suswanto, Keng-Ta Lin & Cheng-Cheng Chen Department of Construction Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, ROC

Jen-Wen Ko & Shyh-Jiann Hwang Division of Building Structures, National Center for Research on Earthquake Engineering, Taipei, Taiwan, ROC

ABSTRACT: This paper mainly presents the observation results of structural damages in the affected regionof 2006 Yogyakarta earthquake. The observed buildings consist of public buildings, residential buildings, and historical buildings. In case of the public buildings, the observation shows that most of low rise RC buildings were damaged due to a common failure mode of weak-column and strong-beam. Common draw-backs of public buildings include the lack of integrity at roof, the nonductile reinforcing details of columns, and a possible insufficient concrete strength. Based on this result, a two story RC classroom building with slight damages and an adjacent three story RC mosque building with heavy damages are evaluated by the simplified pushover analysis that commonly used in seismic evaluation of existing low rise shear buildings in Taiwan. The evaluation results verify that the simplified pushover analysis can be used in seismic evaluation of existing low rise shear buildings in the affected region.

Keywords: structure, damage, earthquake, simplified pushover, low rise, reinforced concrete, shear building

1 INTRODUCTION

The observation results presented on this paper are a part of Taiwan reconnaissance team report on 2006 Yogyakarta earthquake. This paper includes observation results of structural damages in the affected region and case study of a two story RC classroom building with slight damages and an adjacent three story RC mosque building with heavy damages. In this case study, the buildings are evaluated by the simplified pushover analysis (JBDPA 1990, Sheu et al. 2002, Lee & Hwang 2005, Tu et al. 2005) that is commonly used in seismic evaluation of existing low rise shear buildings in Taiwan.

2 OBSERVATION OF STRUCTURAL DAMAGES

The observations of structural damages in the affected region consist of public buildings (i.e. RC and brick buildings), residential buildings (i.e. unreinforced and reinforced brick buildings), and historical buildings (i.e. Borobudur and Prambanan temples). 2.1 Public buildings It was reported that there were lots of damages found in public buildings. For example school build-

ings were reported that 32% of school buildings are categorized as severe damages, 34% and 29% of school buildings with medium and minor damages, respectively (UGM 2006).

There are two types of public buildings com-monly used in the affected area such as RC and brick buildings. The most common damage found for both building types is roof damages. The dam-ages of roof system (the traditional roof system with steep triangle shape is named “Joglo”) due to loosen roof tiles (Fig. 1), the poor integrity between roof structure and underneath structure (Fig. 2), and no diaphragm action on the top of brick wall (Fig. 3). The poor integrity between roof structure and un-derneath structure was observed such as weak con-nection between wooden truss and top of brick wall or wooden truss just sits on brick wall. For the case of no diaphragm action, there was no ties element to hold the top of brick wall in position or insufficient of wooden brace to hold the wooden truss.

Figure 1. Loosen roof tiles (SMK Muhammadiyah Imogiri) Figure 2. Poor integrity of roof structure (SD Bantul Timur) F igure 3. No diaphragm action (SMK Muhammadiyah Imogiri)

2.1.1 RC buildings

Most of the damages for the RC buildings are due to a common failure mechanism of weak-column and strong-beam. This failure mechanism was typically caused by the nonductile reinforcing details of col-umn. The nonductile detailing of RC column was found such as transverse reinforcement without 135° hook, no cross ties, widely spaced and small size di-ameter of 6 mm of hoops (Fig. 4). This insufficient amount of hoops is only 10% of requirement of ACI code (ACI 2005). The other drawback is the lap splice of main reinforcement in critical region (Fig. 5) or the lap splice location was not controlled to the middle of column. In addition to detailing problem, the RC column probably has insufficient concrete strength (Fig. 6). This insufficient concrete strength was indicated by the concrete was easily crushed by hand. Figure 4. Nonductile detailing of two RC columns (SMK Mu-

ammadiyah 3 Yogyakarta) h

Figure 5. Improper lap splice in column (BPKP Yogyakarta) Figure 6. Probably insufficient concrete strength (SMK Mu-hammadiyah 3 Yogyakarta) The common low rise RC buildings failure mecha-nism of weak-column and strong-beam in the af-fected region is comparable to the case of the low rise RC buildings damaged by 1999 Chi-Chi earth-quake in Taiwan. Figure 7 shows a compulsory school building that was damaged by 1999 Chi-Chi earthquake due to the failure mechanism of weak-column and strong-beam. This failure mechanism was subjected to the nonductile reinforcing details of column (Fig. 8). Figure 9 shows the illustration of nonductile reinforcing details of column which has transverse reinforcement without 135° hook, no cross ties, widely spaced of hoops, and lap splice in critical region. Figure 7. Collapsed school building due to 1999 Chi-Chi earth-

uake q Figure 8. Two damaged RC columns due to 1999 Chi-Chi earthquake

No cross tie

Without 135 hook

Widely spaced of hoop

Lap splice in critical region

F igure 9. Illustration of nonductile column

2.1.2 Brick buildings

For brick buildings, total collapse or damage of the buildings was mostly due to lack of transverse integ-rity of the walls, no diaphragm action and insuffi-cient reinforcement of RC ties. This lack of trans-verse integrity of the walls was observed such as the split of transverse wall as seen in Figure 10. For the case of no diaphragm action, Figure 11 shows that there was no ties element to hold the top of column or brick wall in position. Therefore, the column or brick wall acted as a cantilever and these vertical elements were easily broken by flexure at bases. The other drawback is the column or vertical tie has in-sufficient reinforcement and small size of 10 cm × 10 cm (Fig. 12). Figure 10. Lack of transverse integrity due to the split of trans-verse wall (SD Bantul Timur) Figure 11. No diaphragm action at the top of column (SD Ban-tul Timur) Figure 12. Insufficient reinforcement of RC ties (SD Imogiri 2)

2.2 Residential buildings

A significant number of casualties and injures in the rural areas were associated with the total collapse of residential buildings. The major problem of residen-tial buildings is roofing problem such as roof total collapse or roof tiles falling down (Fig. 13). Most residential buildings in the earthquake affected re-gion are unreinforced and reinforced brick buildings.

2.2.1 Unreinforced brick buildings

The collapse or damage of unreinforced brick build-ings were mostly associated to poor integrity be-tween wall and roof (Fig. 14), brick wall have no in-tegrity in transverse direction, no diaphragm action at top of brick wall, and the mortar is too weak (Fig. 15). Figure 13. Roofing problem (a house next to Ahmad Dahlan University) Figure 14. Poor integrity of roof structure (a house next to Ahmad Dahlan University) Figure 15. Insufficient mortar strength (a house next to Ahmad Dahlan University)

2.2.2 Reinforced brick buildings

Drawbacks for the reinforced brick buildings were found to be insufficient size of tie column of 10 cm × 10 cm (Fig. 16) and insufficient amount of tie col-umn (Fig. 17). Figure 18 shows an example of rein-forced brick building with complete tie elements from wall base to top of the roof. In this case, the tie elements are able to hold the wall and the roof. This example of RC tie frame of brick wall could be ac-cepted according to the 1993 Indonesia brick build-ing standard (MoPW 1993), as shown in Figure 19.

Figure 16. Insufficient size of tie columns (a house in Bantul) Figure 17. Insufficient amount of tie columns (a house in Ban-tul) Figure 18. Proper tie elements (a house in Bantul)

Figure 19. 1993 Indonesia brick building standard (MoPW, In-donesia)

2.3 Historical buildings

Two historical buildings were observed namely Borobudur temple and Prambanan temple. The Borobudur temple (Fig. 20) was observed slight damage but the Prambanan temple has severe dam-ages (Fig. 21). Since the location of the Borobudur temple from epicenter is relatively farther than the Prambanan temple, the earthquake intensity at Boro-budur temple area is probably not big compared with the Prambanan temple. Therefore, it is possible the damage of the Borobudur temple structure was very slight compared with Prambanan temple. The other reason is the height to base ratio of Borubudur tem-ple is much lower than the height to base ratio of Prambanan temple. Common drawback of these temples is the interface between stones is quite weak. This interface should be improved in order to in-crease the structure integrity. Since the temples are priceless, it will be better to retrofit these kinds of

buildings in advance in order to preserve the original creation of the temples.

Figure 20. View of Borobudur temple Figure 21. Heavy damages of Prambanan temple

3 CASE STUDY OF SCHOOL BUILDINGS horizontal tie bracing wall tie

tie

horizontal

column/vertical tie

tie beam

anchor

anchor

ring beam

RC TIE FRAME OF BRICK BUILDING ( MoPW,Indonesia)

horizontal tie bracing wall tie

tie

horizontal

column/vertical tie

tie beam

anchor

anchor

ring beam

RC TIE FRAME OF BRICK BUILDING (MoPW,Indonesia)

A two story RC classroom building with slight damages and an adjacent three story RC mosque building with heavy damages were selected in this case study. The objective of this case study is to identify the reason behind the damages of these two low rise RC buildings and to verify the proposed seismic evaluation.

Figure 22. View of the classroom (SMK Muhammadiyah 3 Yogyakarta)

10 φ19φ 6 - 20 cm

4 φ19φ 6 - 20 cm

C1 C2

First Floor PlanC1 C1 C1 C1 C1 C1

C2 C2 C2 C2 C2 C2 C2

21 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

A

B

C1 C1 C1 C1 C1 C1C1C1C1C1C1x

y

Assumed reinforcing details

Total weight = 332 tonf EQx

C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1C1C1C1C1C1

50

30

25

25

Figure 23. The first floor plan of the classroom and assumed reinforcing detail of columns (SMK Muhammadiyah 3 Yogya-karta) Figure 24. View of the mosque (SMK Muhammadiyah 3 Yogyakarta)

Figure 25. The first floor plan of the mosque and reinforcing detail of columns (SMK Muhammadiyah 3 Yogyakarta)

Figure 22 shows the front view of the two stories RC classroom building. The first floor plan and col-umn dimension (Fig. 23) are drawn based on meas-urement of the building, and the reinforcing details are assumed to be the same as the adjacent three sto-ries RC mosque building. The direction of earth-quake load is selected as x-direction or along corri-dor direction, which is the weak axis of building.

The view of adjacent three stories RC mosque building with heavy damages can be seen in Figure 24 and the first floor plan and the reinforcing details of columns is shown in Figure 25. For the resistance of building due to earthquake, it is checked both in x-and y-direction.

Both buildings are assumed has same material properties of concrete strength c = 200 kgf/cmf ′ 2, steel yield strength yv = 3200 kgf/cmf 2 for longitu-dinal reinforcement and = 2400 kgf/cmyhf 2 for transverse reinforcement.

In order to evaluate more accurately the capacity of existing RC buildings, it is necessary to have the real data of concrete strength c , steel yield strength , and crack pattern of RC buildings.

Therefore, it is suggested to collect these data before demolish the severe damaged buildings in the earth-quake affected region.

f ′yf

3.1 Seismic Evaluation of the Existing Shear Buildings

In the case of weak-column and strong-beam or col-umn sway mechanism of low rise RC buildings, the buildings could be assumed as shear buildings (JBDPA 1990, Sheu et al. 2002). In this paper, sim-plified pushover analysis is proposed to evaluate the capacity of existing low rise shear buildings.

The proposed method simply estimates the capac-ity of an existing building by superposing the load-displacement response of vertical members in each story (Fig. 26). In this proposed method, the load-displacement response of vertical members is classi-fied into shear, flexural-shear and flexural failures (Chen 2006) as shown in Figure 27.

0 i ii 1 2 3 4 5

A

B

C

D

6

First floor plan

EQx

EQy

C1

C1

C1

C1

C2 C2

C2

C2

C3

C3 C3 C3

C4

C4

C2C2

C2

C2

C2

C4

C4

C3

C2

C2

C2C3

C3 C3

16 φ19φ6 - 20 cm

8 φ19φ6 - 20 cm

4 φ19φ6 - 20 cm

C1

C3

12 φ19φ6 - 20 cm

C2

C4

Total weight = 860 tonf

x

y

Reinforcing details

30

5040

25

2540

3050

500

500

500

500500 500

1500

30021012

0360 270

2840200

In order to get the peak ground acceleration (PGA) curve, the ATC-40 capacity spectrum analysis (ATC 1996) is used. Generally, this method transforms the pushover load-deflection curve to the capacity earth-quake spectrum that suggested by ATC, then based on the capacity earthquake spectrum to calculate the ground acceleration. Finally, the analysis results are the PGA curve and the maximum PGA point.

Q

Δ

Member B

Member A

Member C

Q

Δ

Late

ral f

orce

Lateral displacement Lateral displacement

Late

ral f

orce

QC

+ QCQA

+ + QCQA QB

QA QCQB Σ Q = + + QCQA QB

Q3Q3

Q2Q2

Q1Q1

Fig-ure 26. Simpli-fied pushover analysis of shear build-ngs

sV

0 ∆

V

a∆Shear failure

0y∆u∆

Flexural-shear failure

yVmnV

a∆s∆

V

V

N

N

V

V

N

N

0

Flexural failure

V

V

N

N

V V

mnV

yV

u∆u∆ y∆∆ ∆

i

Figure 27. Load-deflection responses used in simplified push-over analysis of shear buildings

3.2 Results and discussion Load-displacement response and PGA curve of the two stories RC classroom building are shown in Fig-ure 28 and Figure 29, respectively. The maximum base shear and roof displacement are 304 tonf and 2.8 cm, respectively. While the maximum PGA is 0.53 g. Figure 28. Load-deflection response of the two stories RC classroom building Figure 29. PGA curve of the two stories RC classroom build-ing For the three stories RC mosque building, the maxi-mum base shear and roof displacement in x-direction of earthquake loading are 284 tonf and 8.1 cm, re-spectively (Fig. 30). The maximum PGA is 0.32 g (Fig. 31). In y-direction of earthquake loading, the maximum base shear and roof displacement are 265

tonf and 8.8 cm (Fig. 32), respectively, and the maximum PGA is 0.34 g (Fig. 33).

0 2 4 6 8 10 12 14 16 18 200

50

100

150

200

250

300

Roof displacement (cm)

Base

she

ar (t

onf)

(8.1 cm, 284 tonf)

Figure 30. Load-deflection response of the three stories RC mosque building (x-direction)

0 2 4 6 8 10 12 14 16 18 200

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Roof displacement (cm)

PG

A (g

)

Max PGA = 0.32

0 2 4 6 8 10 12 14 16 180

50

100

150

200

250

300

350

Roof Displacement (cm)

Bas

e S

hear

(ton

f)

(2.8 cm, 304 tonf) Figure 31. PGA curve of the three stories RC mosque building (x-direction) Comparison of these two buildings shows that the two stories RC classroom building has larger strength and smaller weight as compared with the adjacent three stories RC mosque building. This re-sults lead to significant different of maximum PGA. In this case, the two stories RC classroom building has about 50% higher in maximum PGA.

0 2 4 6 8 10 12 14 16 180

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Roof displacement (cm)

PG

A (g

)

Max PGA = 0.53The above results show that the two stories RC

classroom building with higher maximum PGA has larger earthquake resistance as compared with the adjacent three stories RC mosque building and give the reason behind the slight damages of classroom building and the heavy damages of mosque building. As a note in this case study, there is no torsional ef-fect was considered in the calculation.

Roof displacement (cm)

Base

she

ar (t

onf)

(8.8 cm, 265 tonf)

0

50

100

150

200

250

300

0 2 4 6 8 10 12 14 16 18 20

Figure 32. Load-deflection response of the three stories RC mosque building (y-direction) Figure 33. PGA curve of the three stories RC mosque building (y-direction)

4 CONCLUSIONS

The observations consisted of public buildings, resi-dential buildings, and historical buildings in the af-fected region are presented. Also, the cases of low rise RC school buildings were studied. The observa-tion and the case study give the following conclu-sions:

1. Common drawbacks of public buildings include

the lack of integrity at roof, the nonductile rein-forcing details of column, and a possible insuf-ficient concrete strength.

2. The low rise RC buildings were damaged due to a common failure mode of weak-column and strong-beam.

3. For residential building damages mostly due to the lack of integrity at roof, insufficient of RC tie frame of brick wall, and a possible insuffi-cient concrete strength.

4. Common drawback of the historical buildings is the weak interface between stones. This in-terface should be improved in order to increase the structure integrity.

5. The seismic evaluation results verify that the simplified pushover analysis can be used in seismic evaluation of existing low rise shear buildings in the affected region.

6. It is suggested that before demolish the severe damaged buildings in the affected region, it is necessary to collect data of concrete strength c , steel yield strength y , and crack pattern of RC buildings for earthquake engi-neering research purpose such as research on seismic evaluation of building capacity.

f ′ f

ACKNOWLEDGEMENT

The authors gratefully acknowledge the supports of Minister of Education of Republic of China, and In-donesian Economic and Trade Office to Taipei; the funding supports of the National Taiwan University of Science and Technology, and the National Center for Research on Earthquake Engineering of the Re-public of China; as well as the valuable contribu-tions of Mr. H. Idham. S. of Bantul Regent, Dr. H. K. Bashori of Rector of Muhammadiyah University of Yogyakarta (UMY), Dr. Nizam of Gadjah Mada University, Prof. P. Suprobo of Institute of Technol-ogy Sepuluh Nopember, Mr. As’at Pujianto of UMY lecturer, and Mr. Suro of education bureau officer of Bantul regency.

Max PGA = 0.34

0 2 4 6 8 10 12 14 16 18 20

Roof displacement (cm)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

PGA

(g)

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American Concrete Institute (ACI) Committee 318 2005. Building Code Requirements for Structural Concrete. ACI 318-05, Farmington Hills, Michigan.

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