Shaking Table Tests for Brick Masonry Houses

Transcription

1 Shaking Table Tests for Brick Masonry Houses Minowa, C. National Research Institute for Earth Science and Disaster Prevention ( Ali, Q. NWFP University of Engineering and Technology Peshawar, Pakistan ( Narafu, T. Building Research Institute ( Imai, H. Building Research Institute ( Hanazato, T. Mie University ( Nakagawa, T. Building Research Institute Abstract In under-developing countries, many devastating earthquake damages occurred. In order to mitigate the damage of under-developing countries, Japanese research group has initiated cooperative studies with Asian countries, such as Pakistan, Indonesia, and so on. In under-developing countries, there are many masonry structures. The structures were damaged in earthquakes. In order to provide the study and dissemination materials for countermeasure against earthquake damages, the dynamic collapse test of a masonry house was conducted. The tests were carried three times. The tests of first two times were conducted by using one horizontal direction shaking table of NIED in Tsukuba, third test was conducted by using one horizontal direction shaking table of PUCP in Peru. The house models used in the three tests were almost same box type masonry houses with four brick walls. Two walls had open area for window. One wall had open area for door. The open areas were supported with RC lintels. Dimension of house was 3m x 3m x 3m. The roof weights of house models were 2ton for PUCP test, almost zero for NIED tests. In the test at PUCP in Peru, reinforced masonry houses were tested. On NIED shaking table, ordinary masonry houses were tested. The Pakistan Made bricks were used in tests at NIED, and the low quality Peru Made Bricks were used in the test at PUCP. House models were very strong in every test. After more than 1G shock excitations, the cracks were found in walls. In two tests at NIED, the house models ware shaken until collapsing, by using the 25

2 modified strong motion records. In PUCP test, three house models were tested..one was a confined masonry model without reinforcement, another had a continual lintel reinforced concrete beams and reinforce steel bars between walls and columns, the other had an external steel wire mesh covering the surface of the walls, and additionally, a mortar cover was placed in one in-plane wall. the shaking numbers of PUCP tests were limited. Therefore, there were no collapses. Only many cracks were observed, and the house models were reusable. However, the effects of reinforcements for masonry houses were verified. The videos of collapsing tests gave the impression of importance of seismic countermeasure. Keywords: Brick masonry house, Shaking table, Collapse test 26

3 1. Introduction In recent years, there were many devastating earthquake occurred all over the world, especial in developing area of earthquake-prone zone. Most damaged houses were built in non-engineering; low quality masonry construction of adobe, stone and brick, etc. Improvements of awareness to earthquake proof construction and workmanship are required to mitigate seismic disaster. Also, simple reinforcement methods have been proposed by various specialists. Dissemination of earthquake proof construction to residents is considered to be important. Demonstration using a shaking table would be a good method for dissemination of the earthquake proof construction. Considering the surrounding situation of earthquake disaster and countermeasure, the project of Research and Development of Feasible and Affordable Seismic Construction for developing Countries was initiated in cooperation with Building Research Institute and Mie University. In the project, brick houses of mountainous region in Pakistan and confined masonry brick houses in Indonesia were considered. Shaking table tests were conducted three times. First test was for Pakistan mountainous region brick house. Second test was for Indonesia confined masonry brick house. Third test was also for Indonesia confined masonry brick houses with reinforcements. Many shaking table tests of masonry structures were already conducted in Portugal, Italy, Greece, Macedonia, US, and useful results were already provided. Main objectives of the shaking table tests were to verify the effects of reinforcements for developing country masonry houses by using shaking tables and to understand dynamic damage characteristics of masonry houses. This paper describes the results of shaking table tests. 2. First test 2.1 House model The house models of first shaking table test was designed by Mie University and NWFP University of Peshawar, Pakistan. A house model had roof of steel folded-plates with wood beams, a plan of 3m by 3m, and height of 3m, approximately. East wall, south wall and north wall have opening, but west wall without an opening. The walls were built up with English bond brickwork. Bricks of 230mm x 110mm x 70mm and 2.92kg were imported from Pakistan. Before brickwork, bricks soaked in water. Mortar of compounding ratio; early-strength cement 1 and sand 8, early-strength cement 1 and water 1, was used as bond for bricks. A thickness of joint mortal was 15mm. A brick wall had 32 layers. The compressive strengths in three piece average were 14.7MPa for bricks, 9.2MPa for cubic mortar test pieces, approximately. The young modulus was estimated 7.7GPa for brick, and 1.1GPa for mortal in material tests. Lintels were installed on openings. Weight of house model was about 10.23ton (bricks: 7.74ton, mortar: 1.79ton, lintels: 0.37ton, roof: 0.25ton). In the test, there is no test weight. Base of house model was assembled by H steel (H300 x 300 x 15 x 10) at the centre of one- 27

4 direction horizontal shaking table 14.5m x 15m of hydraulic electric control, and, on the trench of H steel base, house model was built up in a week. It took two weeks to get mortar strength. In Figure 1, the outline of house model is shown. In order to estimate a rigidity of the house model, a fundamental natural frequency was calculated in Rayleigh-Ritz method. In the calculation, simple supported edge lines were assumed, and four walls were considered as homogeneous. An equivalent young modulus of walls, which were composed with brick and mortal, was taken as 7GPa by using axial compression condition. The fundamental natural frequency of the house model was calculated 30.5Hz. Figure 1. House Model of First Shaking Table Test 2.2 Shaking table NIED has two shaking tables. One is E-Defence which completed in The other is one-direction horizontal Tsukuba Shaking Table which completed in 1970 and improved in Tsukuba Shaking Table was used for the test. Tsukuba table has the performances of displacement 22cm (44cm of stroke), velocity 100cm/s, exciting force 3.6MN. Test weight is 500ton, and table dimension is 14.5m by 15m. 2.2 Input motions The shaking table were excited by sinusoidal waves, rectangular waves, and strong earthquake records. Two strong earthquake motions were prepared. One was a L component wave which was observed at Bam Governor s Building in Iran Earthquake on Dec. 26, 2003; called as Bam. The other 28

5 was a NS component wave which observed at JMA (Japan Meteorological Agency) Kobe Observatory in 1995 Hansin Great Disaster; called as JMA KOBE. NIED Tsukuba Shaking capacity is less than amplitudes of L (EW) direction component in Bam Strong Motion Record. Model test concepts were in mind. Time scale of input wave changed to Moreover, the amplitude of input wave reduced to 22cm so as to produce the maximum shaking table velocity of 100cm/s and the maximum amplitude of Bam L direction acceleration 799 cm/s/s. Because, NIED Tsukuba Shaking Table has enough capacity to make the 110% amplified JMA KOBE, 900cm/s/s, 100cm/s, 40cm p-p, there were no modifications for shaking table inputs. Figure 2 shows Bam and JMA Kobe waves. Figure 2. Bam and JMA Kobe waves reproduced on shaking table 2.3 Test description and change of dominant frequency At first, in order to check the vibration characteristics of the house, amplitude 1mm rectangular wave of period 10sec were inputted into the shaking table. There was no natural frequency in the region of less than 20Hz. However, a spectra ratio of noise response records indicated dominant frequency 30.76Hz, as shown in Figure 3. In Bam and JMA KOBE excitations, no changes occurred in a model. The actual amplitude of time-scale (TS) reduced Bam excitation 94cm/s was less than the expecting amplitude 100cm/s. However, the actual amplitude of JMA KOBE 104cm/s was stronger than the expecting; 100cm/s. The response accelerations of a house model at the roof amplified to about 110%. Amplification ratio was very little as shown in Figure 2. A house model of brick masonry under controlled construction was rigid and strong to ever recorded strong earthquake motions. Aim of the shaking table test is to obtain the data on the collapse process of brick masonry structures in developing countries. Therefore, a house model must be collapsed in the shaking table test. However, No cracks occurred in the planned excitations. So, the excitations for making cracks in masonry walls were carried out additionally. Two sinusoidal excitations of No.4, No.5 were tried. However, no cracks found. Next, amplitude 20mm rectangular waves of No.6, No.7, No.8 with interval period 10sec. were inputted. In the shaking steps of No.6, No.7, No.8, velocity pulse shocks 29

6 of 40cm/s and 1.7G occurred and applied to a house model. By the shocks, distinct racks appeared in walls. By crack appearance, the test of aiming collapse would be possible. After No.6, No.7, No.8, a spectra ratio indicated a decrease of dominant frequency to 9.47Hz, as shown in Figure 3, and a Bam motion of No.9 applied. Cracks grew large and dominant frequency decreased to 3.61Hz. For last excitation, JMA KOBE motion of No.10 applied. A house model collapsed. Shaking list is shown as follows. Table1 Outline of first test No Iran Bam Eq. L (EW) TS= cm/s No damage No Iran Bam Eq. L (EW) TS= cm/s No damage No JMA KOBE NS100cm/s(110%) No damage No.4. Sin 15Hz 1G 50second No damage No.5. Sin 1Hz 0.4G 20second No damage No.6. Pulse Shock1 40cm/s Cracks occurred No.7. Pulse Shock2-40cm/s Cracks devrlopped No.8. Pulse Shock3 30cm/s No Cracks development No Iran Bam Eq. L(EW) TS= cm/s Crack development No JMA KOBE NS 100cm/s(110%) Collapsed Before No.1 After No.8 Before No.10 Figure 3 Change of dominant frequency in accordance with the test progress 2.4 Cracks by Pulse Shocks The house model collapsed finally. Walls of the house model were considered to fracture along the existing cracks. Cracks initiated in Pulse shocks No.6,7, acceleration 1.7G and velocity 40cm/s. Figure4,5 show the cracks and response accelerations in Pulse shocks excitations No.6,7,8. The cracks were traced by the use of video analysis. The initiating position of cracks close to opening in the south wall, agreed with FEM analysis of SAP 2000, by one of authors, as shown in Figure 6. In FEM analysis, homogeneous shell elements were used, and the length, width and height of the house model were 10 ft, 10ft and 9 ft respectively. Material properties, Young modulus 15.2GPa, density = 1.9gram/cm 3 and 230mm thick shell section were assigned to the shell elements. Figures 6 shows 30

7 shear stresses in south wall of the house model subjected to horizontal excitation (0.799G) as applied to the physical model. The stress concentration around the openings is evident in the figure. The maximum stress indicated is around 0.21MPa at the corners of openings. Stresses in the range of 0.14 to 0.17MPa were observed in piers and centre of the lower spandrel. The shear capacity of the masonry at the relevant locations would be correspond to the crack initiation of masonry at those locations. The dislocations of bricks above lintels in No. 7 excitation were observed. Cracks of walls occurred along masonry joint lines. South and North walls; in-plane walls toward the excite direction displayed many cracks. The shear stress of initial cracks would be estimated less than 0.43MPa. Figure 4. Cracks in No.6 Figure 5. Cracks in No.7 Figure 6 Stress by SAP Collapse Process In Bam excitation No.9, the growth and opening of cracks was observed. The accelerations 1.5G 2G were measured at roof position. The deformation about 30mm 50mm was observes by image processing. Deformation angle was about 1/100-1/50. The next excitation was JMA KOBE No.10. In less than 10 second, the house model collapsed. Cracks were similar to typical crack patterns of shear walls. Figure 7 of continual video copies of west and south walls with interval time of around 2 second, presented the collapse process of house model; a typical masonry structure. Figure 8 showed the scattering of debris. Debris of the test differed from the collapse photo of the developing country. The collapse pattern of the test was similar to the collapse of Japanese brick building heritage in 2007 Niigata Off Chuetsu Earthquake. Adhesion Tests indicated that the bond of cement mortar had the adhesive strength of 0.32MPa. This shows good agreement to FEM analysis 1.5se. 3.5sec. 5.5sec. Figure 8. Collapsed house Figure 7. Video analusis 31

8 3. Second test Second shaking tests was planned for confined masonry houses commonly using in Indonesia. The test was conducted also at NIED Shaking Table in Tsukub 3.1 House model The house model of second test was designed by imaging the half brick work confined masonry houses of Indonesia. There was no roof of house model with a plan of 3m by 3m, and height of 3m approximately, same as first test. East, south and west walls were made of Pakistan brick. The Pakistan brick walls had 32 layers. North wall was made of Japan Brick. The Japanese brick wall had 36 layers. Walls were built up with half bond brickwork. Japan bricks have dimension of 210mm 100mm 60mm. The thickness of mortal joint was around 15mm for Pakistan brick walls and Japanese brick wall. The compressive strengths were 29.8MPa for Japanese bricks, 2.58MPa for mortar test pieces, approximately. The young modulus was estimated 8.3GPa for Japanese brick, and 2.5GPa for mortal in material tests. Total weight was about 5tonf. Frame columns and beams have sections of 120mm x 120mm with steel bars of D10 x 4 and steel The house model was built by no skill men, without soaking, with wood lintels. In Figure 9, the outline of model house is shown. Improved Brick house in Figure 9 is out of consideration in the study. Figure 9. House Model of Second Shaking Table Test 3.2 Input motions JMA Kobe and Ica record in Pisco Earthquake of Peru on August 16, 2007 were used as input motions to the shaking table. Original Ica record has amplitudes of 0.33G, 62cm/s, 24cm and dominant period of about 3second, as shown Figure 10. In the shaking table test, Ica record time scale was reduced by the reason of shaking table limitations. 32

9 Figure 10. Ica record in Pisco Earthquake of Peru on August 16, Test description and results By velocity pulse responses, the dominant frequency of a full wall was perpendicular to the shaking direction was observed about 15Hz, and the dominant frequency of walls parallel to the shaking direction was observed about 20Hz. There were no damages as same as previous first test before strong motion inputs. By the input of Ica record of time scale 0.1, amplitude 30mm, one frame column was damaged, and separation cracks occurred between the damaged column and west wall. The Ica record of time scale 0.1 and amplitude 30mm, have to present maximum velocity 72cm/s, maximum acceleration 4G. However, by the dynamic response characteristics of hydraulic shaking tables, relative high frequency components of inputs were reduced, and maximum velocity and acceleration decreased to 57cm/s, about 2.2G. After the shaking of Ica record of time scale 0.1, the dominant frequency of out-of-plane wall from 15Hz decreased to 2.5Hz, and in-plane walls decreased from 20Hz to 5Hz for Pakistan brick walls. The dominant frequency of Japanese brick wall deceased from 20Hz to 17Hz, as shown in Figure 11. After the damages were occurred, the house model was shaken by Ica record of time scale 0.6, and velocity 62cm/s, amplitude 140mm. However, damages would not progress largely in the house model. Next, JMA Kobe 110% of velocity 100cm/s was inputted. By JMA Kobe 110% shaking, the test house was collapsed, as shown as Figure 12. However, Japan brick wall remained. NoCrack No Crack No Crack Damaged Damaged Damaged West Wall South Wall North Wall Figure 11 Change of dominant frequency by Ica record of time scale 0.1, amplitude 30mm 33

10 Figure 12. Video Records Collapse by JMA Kobe 110% Shaking. 4. Third test Third shaking table test took place at Ponteficia Universidad Catolica Peru(PUCP) In this test, three house models of confined masonry walls, with material specifications similar to those of Indonesia, were investigated for seismic resistance characteristics 4.1 House models A total amount of three house models, named Model A, B, C were built at PUCP s Structures Laboratory and tested on the one-horizontal shaking table. The overall dimensions of all models were similar: 3000 mm square plan and 3000mm maximum height. Each model was built over a reinforced concrete beam of adequate strength and stiffness for transportation and anchorage to the shaking table platform, as shown in Figure 13. The walls consisted of fired clay solid bricks of round dimension of 210mm x 105 mm x 65mm. The compressive strength of solid bricks was 7MPa. The walls of height 2740mm had 29 layers. The thickness of joint mortal was around 25mm. The compressive strength of joint mortal was around 5MPa. Each model had reinforced concrete confinement square columns (cross section 150mm x 150mm). The top beams (200mm x 600mm) had a cross section wider than the thickness of the wall to accommodate extra weight 2ton. The models had no roof. The three models were composed each of 4 masonry walls, named North, South, East and West walls, according to their geographic position. East and West Walls were parallel to the shaking direction and featured a window opening. North and South Walls were perpendicular to the shaking direction, with a door opening in South wall, North wall was a full wall. Figure 13 is the photo of Model A on the shaking table at PUCP laboratory. Model A had no added reinforcement. Model B had a continual lintel reinforced concrete beam over the door and windows openings, and reinforce steel bars between walls and columns at three 34

11 positions (Figure 14). Model C had an external steel wire mesh covering the surface of the walls, and additionally, a mortar cover was placed in East Wall. Over the upper ring beam, sand bags were placed to simulate extra roof weight. The added weight was calculated so that the total load over the platform was nearly less than 15ton, the limit load for PUCP shaking table Figure 13. Model A on the PUCP shaking table Figure 14. Outline of East wall of Model B 4.2 PUCP Shaking Table The shaking table has a single horizontal movement. The original design and specifications are given by Dutch expert Jeunink (1985). The stroke is 300 mm. Other design specifications are the maximum velocity of 0.5 m/s, the maximum acceleration of 1.0G, and the maximum force of actuator of 500 kn. 4.3 Input motions The test of any model combines the use of strong motion records to observe the seismic behavior. The strong motion records come from real earthquakes; ICA from earthquake in Peru August 15, 2007, KOBE from earthquake in Japan January 17, 1995, LIMA from Earthquake in Peru May 31, Before and after each strong motion record shakings, pulse records were used to study the free vibration characteristics. The pulse record consists on a rectangular wave. Each input is provided as a displacement data for the actuator and it can be amplified so that the peak value corresponds to a specified value. 4.4 Test description and Results Model A was subjected to 7 shaking steps. The outline of test results was described in table 2. The cracks started to appear in the East and West walls in both diagonal directions, during shaking step No.4. Separations between walls and columns occurred. Figure 15 displays spectra ratios of upper ring beam and platform. According to Figure 15, dominant frequencies decrease with crack 35

12 developments. Table 2 Outline of test for Model A Model B was subjected to 4 shaking steps. The outline of test results was described in Table 3. Step L1 showed elastic behavior, without cracks. After step L2, thin cracks of 0.3mm width were observed in East and West walls. These cracks and other developed during step L3, reaching 0.6 mm width. After step L4, the cracks continue to widen, reaching 1.2 mm width. According to spectra ratios of Figure 16, dominant frequency decreases with crack development as same as Model A. Crack developments were stopped at lintel positions, and there were no separations between walls and columns. Table 3 Outline of test for Model B For Model C, the test had two parts. The first part consisted in a slip tests or base isolation test, using a special resin sheet. The sheet was placed over the platform slab, so Model C rested upon the sheet, without anchoring it to the platform. In the second part of test for Model C, it was previously anchored to the shaking table platform. It consisted in shaking steps M2 through M4, similar to those used in model B, and the stronger inputs used in model A. Table 4 Outline of test for Model C 36

13 F2 F4 F5 F6 F7 Figure 15. The changes of Spectra Ratios of Model A and platform with shaking steps L1 L2 L3 L4 Figure 16. The changes of Spectra Ratios of Model B and platform with shaking steps. M1 M2 M3 M4 Figure 17. The changes of Spectra Ratios of Model C and platform with shaking steps No distinct cracks appeared during the test. Dominant frequency slightly decreased, as shown in Figure 17. Figure 18 displays the cracks of East Walls at the end of the tests of Model A, B, C. Shear Stress for Model B was calculated by SAP2000, shown in Figure

14 Model A Model B Figure 18. The cracks of East Walls at the end of the tests. Figure 19. Shear stress calculation for Model B The curve between base shear force and deformation would be obtained, using the actuator force, solid the platform mass, the platform acceleration record and the horizontal deformation of model. Figure 20 is the curve for the shaking step F6 of Model A test. Rigidity degradation would be recognized in Figure 20. Figure 20. The curve between base shear force and deformation Test results would be summarized as bellows. 5. Result List 38

15 First Test Second Test Third Test Structure Brick Confined Masonry Confined Masonry with Rigid Roof Girder Wall Thickness 230mm 110mm (100mm) 105mm Added Reinforcement Input Max. Acce Input Max. Velo. Results 1.7G 1m/s No No No Lintel Beam Wire Mesh and Collapse 2G 1m/s Collapse 2G 0.5m/s HeavyCracks 2.5G 0.5m/s Cracks Mortal Finishing 2.5G 0.5m No Damage 6. Conclusions If brick masonry houses would be constructed properly, brick houses will resist severe earthquakes. Collapses of brick masonry houses would be brought by the improper construction and careless workmanship. Judging from the results of third tests, the finishing and wire mesh will give additional strength to masonry houses, and continual lintel will make wall strong. The damage result of second test indicated that the out-of-plane motion should be consider for the walls with low thickness. Further study is necessary to develop affordable and feasible construction. Acknowledgement The authors express our special sincere thanks to the people of Ponteficia Universidad Catolica Peru, especial to Director Marcial Blondet, Ing. Gladys Villa Garcia M., and Professor Daniel Quiun for conducting the third test in Peru. 39