3. Survey of Reinforced Concrete Buildings and Minarets. 3.1 Reinforced Concrete Buildings Damage and deficient detailing

Transcription

1 3. Survey of Reinforced Concrete Buildings and Minarets 3.1 Reinforced Concrete Buildings Damage and deficient detailing Extensive damage was observed due to the captive column condition. A large number of buildings in the region visited have partial underground first stories used for storage. The partial restrain of the soils and retaining walls led to a dramatic increase in the possible maximum shear that may have acted on the columns. A common consequence was brittle shear failure (Figure 9). Figure 9. Captive column The amount of partition walls (typically made of clay brick) in the first story of many buildings was observed to be much lower than in higher stories (Figure 10). These soft stories seemed to have absorbed most of the induced motion and in many cases this caused their collapse. 16

2 Figure 10. Soft story related failure It was observed that infill masonry walls may have influenced the behavior of the reinforced concrete structure in other ways too: Figure 11 shows how the failure of the corners of infill panels caused the columns in between them to act as captive ones. Deficient detailing (inadequate anchorage of stirrups, insufficient shear reinforcement) was also observed to be associated with many structural failures. Generalized use of plain bars and lap splices in regions of maximum moment was also noticed. In general, the quality of the concrete was poor. Concrete compressive strength estimates (from Schmidt hammer measurements) of about 2000 psi were common. 17

3 Figure 11. Captive column shear failure due to partial infill masonry walls 3. 2 Correlation of damage level and shear Areas Ninety-three reinforced concrete buildings were surveyed in Duzce, Bolu and Kaynasli after the 12 November event. Most of these buildings were observed to have very irregular floor plans. Dimensions of typical columns and walls, and floor area were recorded for each building. Following the format proposed by Hassan (1997), these data were organized using two indexes: Column Index: Half the cross-sectional area of columns at base level divided by the product of the area of a typical story times the number of stories above ground level. Wall Index: Ratio of the sum of the total cross-sectional area of reinforced concrete walls and a tenth of the area of infill masonry walls in a given horizontal direction to the product of the area of a typical story times the number of stories above ground level. Each building surveyed was also ranked using the following damage scales: STRUCTURAL ELEMENTS MASONRY None None Light : Hairline inclined or flexural cracks Light : Hairline cracks. Flaking of plaster. Moderate : Concrete spalling Moderate : Cracks in walls and joints between panels. Flaking of large pieces of plaster. 18

4 Severe : Local structural failure Severe : Wide and through cracks. Collapse Collapse Figures 12 and 13 show the correlation observed between damage level and column and wall indexes respectively. Table 2 contains the data collected. The following notation is used in Table 2: Acw: area of concrete walls Amw: area of masonry walls Acol: area of columns WI: wall index CI: column index RC: reinforced concrete M: masonry Despite the scatter observed in both figures, it can be seen that most of the cases in which severe structural damage, masonry collapse, or collapse of the structure was observed correspond to combinations of wall and column indexes under the light-damage line(ld). Observe that in the majority of buildings surveyed the damage of the masonry was classified as either severe or as collapse RC Damage Duzce/Bolu/Kaynasli, Turkey 1999 None Light Moderate Severe Collapse LD Wall Index % Column Index % Figure 12. Correlation between damage level to reinforced concrete elements and the column and wall indexes as defined by Hasan and Sozen,

5 MASONRY Damage Duzce/Bolu/Kaynasli, Turkey 1999 None Light Moderate Severe Collapse LD Wall Index % Column Index % Figure 13. Correlation between damage level to masonry walls and the column and wall indexes as defined by Hasan and Sozen,

6 Table 2. Building Data Location Shape No. of Stories. Floor Area Direction Acw Amw Direction Acw Amw Acol Minimum WI CI Damage m 2 m 2 m 2 m 2 m 2 m 2 % % RC M Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S L S Duzce Rectangular E-W N-S S S Duzce Rectangular E-W N-S M --- Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S L S Duzce Rectangular 5 84 E-W N-S M S Duzce Rectangular E-W N-S M C Duzce Rectangular E-W N-S C C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S M C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S M -- Duzce Rectangular E-W N-S M L Duzce Rectangular E-W N-S M C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S S S Duzce Rectangular E-W N-S M C Duzce Rectangular E-W N-S M C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S M C Duzce Rectangular E-W N-S S S Duzce Rectangular E-W 7.8 N-S S S 21

7 Duzce Rectangular E-W 4.8 N-S S S Duzce Rectangular E-W N-S Duzce Rectangular E-W N-S S Duzce Skewed E-W N-S L Rec. Duzce Rectangular E-W N-S N Duzce Rectangular E-W N-S S S Duzce Rectangular E-W N-S M M Duzce Rectangular 5 90 E-W N-S M S Duzce Rectangular E-W N-S L C Duzce Rectangular E-W N-S L C Duzce Rectangular E-W N-S M C Duzce Rectangular 3 78 E-W N-S N N Duzce Rectangular E-W N-S L L Duzce Rectangular E-W N-S S S Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S M C Duzce Rectangular E-W N-S S C Bolu Rectangular E-W N-S N L Bolu Rectangular E-W N-S N C Bolu Rectangular E-W N-S S C Bolu Rectangular E-W N-S S C Duzce Rectangular E-W 7.8 N-S S S Duzce Rectangular E-W 4.2 N-S S C Duzce Rectangular E-W 3.4 N-S S C Duzce Rectangular E-W 0.0 N-S L S Duzce Rectangular E-W 5.3 N-S S S Duzce Rectangular E-W 0.0 N-S M --- Duzce Rectangular E-W 4.8 N-S S S Duzce Rectangular E-W N-S Duzce Rectangular E-W N-S S Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S S C Duzce Rectangular E-W N-S S C 22

8 Duzce Rectangular E-W N-S L S Duzce Rectangular E-W N-S M C Duzce Rectangular E-W N-S S S Duzce Rectangular E-W N-S N N Duzce Rectangular 4 E-W N-S C C Duzce Rectangular E-W N-S S S Duzce Rectangular E-W N-S N L Duzce Rectangular 4 99 E-W N-S N N Duzce Rectangular E-W 3.4 N-S M S Duzce Rectangular E-W 3.9 N-S M S Duzce Rectangular E-W 6.5 N-S N M Duzce Irregular E-W N-S S S Duzce Rectangular E-W N-S L L Duzce Trapezoidal E-W N-S S Duzce Rectangular E-W N-S C Duzce Rectangular E-W 1.9 N-S S C Duzce Rectangular E-W 2.9 N-S L S Duzce Rectangular E-W N-S S S Duzce Rectangular E-W 1.3 N-S S C Duzce Rectangular E-W N-S N S Duzce Irregular E-W 3.1 N-S N Duzce Irregular E-W N-S S C Duzce Rectangular E-W N-S L S Duzce Rectangular E-W N-S L L Duzce Irregular E-W N-S L S Duzce Trapezoidal E-W 1.0 N-S L Kaynasli Rectangular E-W N-S C C Kaynasli Rectangular E-W N-S S C Kaynasli Rectangular E-W N-S C C Kaynasli Rectangular E-W N-S S C Kaynasli Rectangular E-W N-S N L Kaynasli Rectangular E-W N-S C C Kaynasli Rectangular E-W N-S L S C: Collapse, S: Severe, M: Moderate, L: Light, N: None 23

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10 3.3 Survey of Damage to Monuments Motivation of Survey The findings in this section are the result of two different surveys. The 1 st survey was carried out in Yalova, Duzce, Adapazari, Izmit, Istanbul after the August 17, 1999 earthquake and the 2 nd one in Duzce, Kaynasli, Adapazari, Izmit and Bolu after the November 12, 1999 earthquake. The aim of both of the surveys is to document the damage suffered by the minarets. The motivation is that the analysis of a minaret - a simple and a tall structure - should provide information to correlate the mode of failure with the characteristics of the ground motion. The types of the damage observed are documented together with the material properties, dimensions and the locations with respect to the trace of the fault. An additional goal is to propose an improved design procedure for the minarets to avoid some of the observed structural deficiencies. Only the data regarding Duzce, Kaynasli and Bolu is within the scope of this report Results of the survey in Duzce The common mode of failure occurred at the base of the minaret. The range of dimensions of the minarets in Duzce observed was from 20m. to 30m. for the single balcony minarets, and 25m. to 30m for the double balcony ones (Figure 14). The minarets are placed either on a 9m. high separate footing or on top of the mosque building. The diameter of the minaret body was 1.7m. (1.4m. inner diameter + 2x0.15m., wall thickness) to 2.0m. (1.7m.+ 2x0.15m. wall thickness) Survey after August 17 Nine minarets were surveyed in the city of Duzce after the 17 August 1999 earthquake. Most of the minarets were reinforced concrete (R/C) and suffered little damage. According to official records only 40% of the minarets were assumed to perform well. As a consequence the rest are scheduled for demolition or strengthening to prevent additional damage in the vicinity of the minaret. In our survey, one out of the nine surveyed minarets (11%) was masonry (Figure 14). In this instance the mosque had twin minarets with double balconies. The minarets had their own footings and suffered only the slight dislocation of a few blocks near base. Figure 14. Masonry minarets survived after 17 August 1999 in Duzce 25

11 Figure 15. Collapsed masonry minarets after the 12 November 1999 event in Duzce After the 12 November event, both minarets in the same mosque collapsed (Figure 15). The remaining eight mosques had reinforced concrete minarets. Half of those mosques had single minarets and double balconies, of the remaining four, three were single minarets with single balconies. The last mosque surveyed had lightweight concrete twin minarets with double balconies. In this mosque, the direction of fall was different for each of the minarets; one collapsed in the southwest direction while the other one collapsed in the southeast direction. Three of the R/C minarets collapsed on 17 August. Two of these three failed at the base, and one failed somewhere up in the body of the minaret and the direction of fall was northwest. One of the eight minarets had severe cracks at base and was demolished immediately after the earthquake. Two of the eight reinforced concrete minarets had slight shear cracks at base. Two of the eight surveyed suffered no visible damage Survey after 12 November Thirteen minarets were surveyed in the city of Duzce after the 12 November 1999 earthquake. Most of the minarets damaged during the 17 August earthquake, collapsed on November 12. Two of the thirteen surveyed minarets were masonry. Both of those minarets failed at the base and collapsed. Eleven of the thirteen minarets were reinforced concrete (R/C). Four of those failed at the base and collapsed. One failed at mid-height and collapsed. Four showed cracking and spalling at the base. One minaret had cracks at midheight and one was under construction and did not suffer any damage Results of the survey in Kaynasli The information regarding Kaynasli addresses the damage suffered by minarets on 12 November Kaynasli, is a very small city when compared with Duzce and Bolu. However, it was of importance due to its proximity to the fault trace. In Kaynasli, two mosques were surveyed after November 12. Both of them had reinforced concrete minarets that failed at the base and collapsed Results of the survey in Bolu The information presented in this section illustrates the damage suffered by minarets on 12 November Little damage was noted in the downtown area. Most of the damage seemed to be located towards the southern part of the city. In Bolu, thirteen minarets were surveyed following the 12 November 1999 earthquake. Six of the thirteen minarets were masonry. Three of the six masonry minarets collapsed. Two of the three collapsed minarets failed at base. The remaining masonry minaret failed above the 2 nd balcony 26

12 (near the top). One of the six masonry minarets showed cracks at the base. Two of the six minarets suffered no damage at all. The remaining seven minarets were R/C. None of those collapsed, and only two of the seven surveyed had cracks at the base. 3.4 Summary of Observed Damage Figures 16 and 17 summarize the results of the survey of monuments. Types of Minarets BOLU KAYNASLI DUZCE 0% 20% 40% 60% 80% 100% Masonry R/C Figure 16. Summary of surveyed minarets by type Level of Damage with Respect to Location BOLU KAYNASLI DUZCE 0% 20% 40% 60% 80% 100% Collapse Cracks None Figure 17. Level of Damage The damage suffered by the minarets was quite extensive in the surveyed cities. There were fewer collapses of reinforced concrete minarets than masonry minarets. In the masonry minarets, reinforcing rods (φ8-10mm. S220, smooth bars) are used to help positioning the stone blocks. In the reinforced concrete minarets, the most common application was the use of φ12-18mm. smooth or deformed bars. The common failure noted occurred at the base. This observation seems to indicate that the anchorage system was inadequate to attach a tall, slim structure to the more rigid concrete footing or to the mosque building. More damage of the minarets was observed in Duzce than in Bolu. This observation correlates well with the fact that Duzce was closer to the epicenter of the 12 November event. 27