Kajima Open Paper - J. Wallace 19 October 1999 1THE İZMİT (KOCAELİ), TURKEY EARTHQUAKE OF 17 AUGUST 1999 PRELIMINARY RECONNAISSANCE REPORT JOHN W. WALLACE, Ph.D., PE Pacific Earthquake Engineering Research Center Department of Civil and Environmental Engineering, 5731 Boelter Hall University of California, Los Angeles, CA 90095-1593 INTRODUCTION At 3:02 am on 17 August 1999, a magnitude Mw 7.4 earthquake occurred along the North Anatolian fault in the province of Kocaeli. The epicenter was located southwest of the city of İzmit, approximately 75 km southeast of İstanbul. The region affected is densely populated and includes the industrial heartland of Turkey. Approximately 120,000 residential buildings were heavily damaged or collapsed, and 15,000 deaths were reported. Monetary loss estimates range from 10 to 40 billion USD, or approximately 10 to 15% of the GNP of Turkey. SEISMOLOGICAL, GROUND MOTIONS, AND CODE BACKGROUND The North Anatolian Fault Zone and Observed Fault Rupture The earthquake was produced by rupture along a branch of the 1300 km-long North Anatolian Fault (NAF) system. The right-lateral strike-slip NAF has been very active in recent years, with seven earthquakes since 1939 exceeding MS 7.0 (Fig. 1). The fault rupture between 1939 and 1999 has generally progressed from west to east (Fig. 1). Studies prior to the 17 August 1999 earthquake estimated that there was a 12% in 30-year probability of a large earthquake near İzmit (Stein, Barka, and Dieterich, 1997). During the 17 August 1999 earthquake, surface faulting extended 110 km east of Gölcük to nearly Düzce. The distribution of aftershocks suggests that faulting extended west of Gölcük towards Yalova (Fig. 2) for another 50 to 60 km, for a total length of rupture of 150 to 200 km. Ground motion records as well as observations by residents indicate that the fault may have ruptured in stages. The fault offset is predominantly right-lateral strike slip with offsets generally in the range of 2 to 4 meters (Fig. 3). A short dip-slip segment of the fault east Gölcük exhibited vertical offsets of 2 (Fig. 3), inundating the coastal area of Gölcük. Engineering Characteristics of the Measured Earthquake Ground Motions Peak ground accelerations measured at 13 stations (Fig. 4) varied between 0.04g to 0.41g. The measured ground motions at the YPT station are plotted in Fig. 5. Acceleration response spectra for the east-west and north-south components of the ground motion records obtained at five stations are plotted in Fig. 6. Spectral amplitudes for the east-west (fault parallel) direction plotted in Fig. 6 are approximately equal to those for the north-south direction (not plotted). Equivalent code spectra for the Turkish Standards (1975 and 1996) and the UBC (1994 and 1997) are also plotted on Fig. 6. The unreduced code spectra for the Turkish Code and the UBCKajima Open Paper - J. Wallace 19 October 1999 2are not substantially different. The relations plotted in Fig. 6 reveal that the spectral demands for the 17 August 1999 earthquake are generally less than those for the unreduced code spectra. In addition, the design level forces for the 1975 Turkish Standard are somewhat less than those for the other cases plotted (no near-field factors or redundancy factor were considered for UBC 97). For buildings with adequate detailing and designed for the code required forces, acceptable performance would be expected (collapse prevention). It is also noted that the 1996 Turkish code uses a variable force reduction factor for low periods. Turkish Code Requirements for Detailing of RC Buildings The 1975 Turkish Code for reinforced concrete contains detailing provisions and design concepts that are consistent the 1976 version of the UBC. For example, column hoops and crossties with 135-degree hooks, reduced spacing of beam and column transverse reinforcement at member ends, transverse reinforcement within the joint, and adherence to a strong-column, weak-beam design concept. Since the region affected by the earthquake lies within the highest seismic zone in Turkey, the observed performance is not consistent with expectations based on code requirements. Improved details are contained in the 1996 code. Observed damage indicates that the code provisions are not followed and that improvements in construction practice (as well as seismic retrofit) are needed if improved performance is to be achieved in future earthquakes. Construction Practice and Materials A majority of the residential construction in Turkey consists of three to seven story reinforced concrete frames with hollow clay tile infill walls. Rectangular column cross-sections (25 cm by 60 cm) with 12-15 mm diameter smooth vertical reinforcing bars are common (ρ = 0.014). In Fig. 1 North Anatolian Fault Activity: 1939 to 1999Kajima Open Paper - J. Wallace 19 October 1999 3some of the newer buildings, square columns (40 cm by 40 cm) with 8 vertical 15 mm diameter deformed reinforcing bars are used (ρ = 0.009), typically for corner columns. Column splices are usually located just above the floor slab, consisting of a straight bar extension from below and a hooked bar from above. Transverse reinforcement typically consists of 6 or 8 mm smooth reinforcement spaced at 20 to 25 cm on center. Based on observations from damaged buildings by the reconnaissance team, crossties and joint transverse reinforcement are rare. In one building where joint transverse reinforcement was observed, it did not appear to be used consistently throughout the joints in the building. Concrete quality appeared to vary widely and in many cases the concrete was poorly consolidated. Hollow clay tiles are used predominantly for infill walls. The use of lightweight white “foam” block has become more common in some newer buildings. In general, the infill walls are placed directly against the narrow side of the column so that the column is contained within the partition wall. This practice results in an irregular layout and orientation of columns; however, a majority of columns are commonly oriented with the long side parallel to the sides of the building. It was common to see about 75% of the columns with the same orientation in residential buildings, whereas a uniform orientation was observed in some commercial/residential construction along main streets and in buildings with parking on the ground level. Therefore, the lateral strength of most