SPECIAL REPORT Lessons Learned from the Kobe Earthquake A Japanese Perspective Hiroshi Muguruma This report presents an overview of the Ph.D. performance of reinforced and precast, Professor Emeritus Department of prestressed concrete buildings during the Architectural Engineering Kyoto University Hyogoken-Nanbu earthquake (also known as Kyoto, Japan the Great Hanshin earthquake) of January 17, 1995, situated in and around the city of Kobe, Japan. The performance of pile foundations is also examined. Highway bridges, rapid transit structures, and other special structures are covered elsewhere. The assessment of damage is related to the Minehiro Nishiyama evolution of design code provisions for Ph.D. concrete building structures in Japan. Department of Preliminary reports indicate that precast, Architectural Engineering Kyoto University prestressed concrete structures performed Kyoto, Japan remarkably well during the earthquake, especially those designed with recent seismic code provisions. The probable causes of the damage are examined, although it should be emphasized that several investigations are currently being carried out to determine more comprehensive causes of structural failures Fumio Watanabe, Ph.D. by many researchers, engineers, the Professor Architectural Institute of Japan (AIJ), the Department of Architectural Engineering Japan Prestressed Concrete Engineering Kyoto University Kyoto, Japan Association (JPCEA), and other organizations. 28 PCI JOURNAL t precisely 5:46 a.m. in the N early morning of January 17, A 1995, a devastating earthquake struck Japan, imparting a trail of de­ ® ~ Severely damaged area struction across a narrow band extend­ ing from northern Awaji Island through the cities of Kobe, Ashiya, Nishinomiya and Takarazuka (see Fig. 1). The 7.2 Richter magnitude registered was one of the strongest earthquakes ever recorded in Japan. Initially, Sumoto City on Awaji Is­ land and Kobe City were assigned a Shindo 6 intensity. However, later the Japan Meteorological Agency (JMA) revised the Shindo intensity level from 6 to 7 for parts of the cities of Kobe, (1 gal= 1 em/sis) Ashiya, Nishinomiya and Takarazuka, J. .I and parts of northern Awaji Island. Skm The Shindo intensities 1 to 7 corre­ spond to the Modified Mercali Inten­ sity Scale of I to II, II to IV, IV to V, Fig. 1. Area map of severe earthquake damage and recorded accelerations. V to VII, VII to Vill, Vill to IX, and IX to XII, respectively. The Hyogoken-Nanbu earthquake GROUND MOTIONS (also called the Great Hanshin earth­ The focal depth of the earthquake quake) will hereafter be referred to as was approximately 14 km (8.6 miles). the Kobe earthquake. The epicenter was located at 34 o 36.4' The earthquake resulted in 5502 north latitude, 135° 2.6' east longitude. deaths. More than 24,000 people were Three faults are believed to have rup­ injured in the Hyogo Prefecture alone. tured during the main shock. A hori­ As of June l, about 40,000 people still zontal displacement of up to 1.6 m live in temporary shelters. The esti­ (5.25 ft) was found at the Nojima Fault mated property damage ranges from on Awaji Island. The severely dam­ $95 to $140 billion. aged area consists of a narrow band The earthquake caused significant from the northern part of Awaji Island damage not only to old buildings de­ to the city of Takarazuka. The cities of Period (sec) signed according to former design Kobe, Ashiya, and Nishinomiya are in­ codes but also to modern buildings cluded in this region (see Fig. 1). Fig. 2. Velocity spectrum of that conformed to current design codes The Preliminary Reconnaissance earthquake recorded by Kobe Maritime Meteorological Observatory. and regulations. The performance of Report of the 1995 Hyogoken-Nanbu building structures during the earth­ Earthquake published by the Archi­ quake has been studied by numerous tectural Institute of Japan (AIJ)' states around 0.25 to 0.4 seconds. researchers, engineers, and organiza­ that the characteristics of the ground 5. Ground motions were affected by tions. At this time, several clues to the motions recorded may be summarized local soil conditions and topography. causes of the devastating damage have as follows: The preliminary reconnaissance report 5 been found. '· 1. Peak ground accelerations were of AIJ states that ground motions were In this report, the damage to rein­ large in both the horizontal and verti­ most likely amplified in the plains forced concrete buildings and precast, cal directions. The peak accelerations near the mountains between the cities prestressed concrete buildings is as­ observed at several sites are summa­ of Kobe and Nishinomiya. sessed. Several possible causes for typi­ rized in Table 1 and in Fig. L. cal damage are presented. Also, the 2. Fig. 2 shows the velocity spec­ performance of precast, prestressed trum recorded by the Kobe Maritime GEOLOGICAL ASPECTS concrete piles is discussed. This report Meteorological Observatory. Because the geological aspects of is limited to building structures. Special 3. The duration of strong shaking the region are described elsewhere by structures, such as highway bridges and was 10 to 15 seconds. experts in the field, this section will rapid transit structures, are covered 4. The predominant period was 0.8 only briefly mention the highlights of elsewhere by other researchers or engi­ to 1.5 seconds. A second predominant the AIJ report. 1 The area of Shindo 7 neers in the civil engineering field. period was, at times, observed to be intensity is approximately 20 km (12.3 July-August 1995 29 Table 1. Peak accelerations and soil conditions (Ref. 1). Peak ground acceleration (gal) Soil Measured North- East- Up- Recorded point Location condition level south west down JMA- Kobe Chou Ward, Kobe City Diluvial IF 818 617 332 JMA - Osaka Chou Ward, Osaka City Oil uvial B3F 81 66 65 - ~ Rock foundation 272 265 232 MTRC Kita Ward, Kobe City - Rock GL-15 m 208 213 11 6 - A Building Chuo Ward, Kobe City Diluvial B3F 223 208 292 B Building Kita Ward, Osaka City Alluvial GL ' 1:1 182 267 302 C Building Kita Ward, Osaka City Diluvial B4F 155 157 193 -- Oil uvi al GL >52 49 46 SiteT Minamikawachi District Diluvial GL-IOOm 23 - 16 -- Diluvial GL .. 43 50 49 Site Y Kita Ward, Osaka City - Oil uvial GL-60m 24 49 I - - Obayashi building Chou Ward, Osaka City Diluvial B2F X: 139 Y: 87 Z: 210 • c, M apartment Miyakojima Ward Alluvial IF X: 60 Y: 86 Z:42 house Osaka City p Osaka mechanical Taisyo Ward '"""· Alluvial GL 195 140 122 material center Osaka City ~ .. ~ I Abiko apartment Sumiyoshi Ward f, Alluvial IF I· 107 115 92 house Osaka City ' Takami Tall Konohana Ward Alluvial IF 156 178 176 • residence Osaka City Alluvial GL .. · 222 267 255 Fill-in GL f3 11 7 85 53 Takatsuki Campus Reizenjicho ground ~ of Kansai University Takatsuki City Sandstone GL 67 61 36 Sandstone GL-13 m 66 49 39 ~ - Abeno Ward I Point A Oil uvial GL-3 m 76 - 26 Osaka City I Alluvial IF 129 I 103 91 1: PointD Asahi Ward, Osaka City Alluvial GL 189 155 126 ,,, Oil uvial GL-25 m 129 113 8 1 ·~· ·~ Note: I m = 3.28 ft; I gal = I em/sis; I em = 0.39 m. miles) long and reaches from Kobe to Mount Rokko consists primarily of earthquake in 1923. The seismic de­ Nishinorniya. Mount Rokko lies north granite and is crossed by many faults. sign procedures in Japan have been re­ of Kobe, extending in an east-west di­ The southern side of the mountain has vised every time a significant earth­ rection. The plains are within a narrow step-like slopes, consistent with down­ quake occurs and causes severe band of land between Osaka Bay and ward displacements at the fault scarps damage. The evolution of the seismic the mountains. relative to the north. Near the ground design codes is described below. surface, the granite has weathered into decomposed granite soil. A simplified Historical Review of 1. SOkrn 0.35krn profile of the ground cross section in the north-south direction in Kobe City Seismic Design Provisions is illustrated in Fig. 3. for Reinforced Concrete Buildings in Japan EVOLUTION OF In the seismic provisions of the Building Standards Law of 1950, the DESIGN CODES seismic design load applied to each The damage to buildings caused by floor, Vj, was calculated by the follow­ the Kobe earthquake is closely related ing equation: to the design methods adopted. The Vj = [0.2 + O.O l(H; - 16)/4]w; (1) Fig. 3. Simplified profile of ground first seismic design provisions were cross section in Kobe City. established just after the Great Kanto where 30 PCI JOURNAL ...,._ 0.26 ...,. 0.25 Subsoil II/ (Flexible) 1(; =0.8 0.24 rf: ...,. ....: ...,. 0.23 c: 0.22 - ~ ...,. ~ 0.21 Q) ...,. 0 0.20 (.) ~ E (3 <D Q) c% c: ~~ ~ /. -~ (I) 0 Fig. 4. Seismic design load specified in ~ 0 1.0 2.0 3.0 4.0 the Building Standards Law of 1950. Period, T (seconds) 1 (T<T.:) 2 wi = weight of ith story R1 1-0.2(T/T., -1) (7;, STS2T.,) 1.67;, IT (27;, S T) Hi = height of ith story from ground 1 level in meters Fig. 5. Design spectral coefficient, Rr If Hi S 16, then Hi = 16 m (52 ft) and therefore V; = 0.2wi. The seismic design load is illustrated in Fig. 4 as an example for a ten-story building. Center of Center of The allowable stress design (work­ Y mass ing stress method) was conducted for rigidity design stresses calculated by linear elastic analysis. The combination of design stresses was D + L + E, where D, L, and E are stresses resulting from dead load, live load and seismic de­ L--------rl--1 • sign load specified by Eq.