SPECIAL REPORT

Observations on the Performance of Structures in the Earthquake of January 17, 1995

From February 14 to 20, Dr. S. K. Ghosh, along with a select group of investigators, inspected the site of the Kobe earthquake in . This article presents an overview of the author's assessment of the damage to various types of structures caused by the earthquake. Although there was a relatively small number of precast, prestressed concrete buildings in the Kobe area, the structures withstood the earthquake remarkably well. S. K. Ghosh, Ph.D. Director Engineering Services, Codes t the break of dawn on January fires, thus adding more devastation and and Standards Portland Cement Association 17, 1995, a devastating earth­ casualties to an already tragic scene. Skokie, Illinois A quake hit the Japanese port of Property damage has been esti­ Kobe, wreaking havoc along a narrow mated at over $100 billion. To put corridor extending as far east as the this figure in perspective, the January city of . The initial shock wave 17 , 1994, Northridge earthquake in had a duration of 20 seconds and reg­ California is estimated to have caused istered a 7.2 JMA (Japan Meteorologi­ $20 billion in damage. Therefore, in cal Agency) magnitude (a moment economic terms, the Kobe earthquake magnitude of 6.9). The earthquake, caused about five times as much dam­ called the Hanshin-Awaji earthquake age as the Northridge earthquake. and the Hyogo-Ken Nanbu earth­ This article is a sequel to the brief, quake, will be referred to as the Kobe but excellent report on the Kobe earth­ earthquake in this article for brevity. quake that appeared in the January­ With a population of 11h million, February 1995 PCI JOURNAL. ' It Kobe is Japan's second largest port. As presents an overview of the geotechni­ of February 15, an estimated 5373 peo­ cal aspects of the location, Japanese ple had died, 34,568 had suffered seri­ building code issues, soil characteris­ ous injuries, and 320,298 were left tics, and the performance of structures homeless as a result of the earthquake. during the earthquake. In particular, To aggravate matters, leaking gas and the behavior of precast, prestressed broken powerlines caused hundreds of concrete structures is reported.

14 PCI JOURNAL Fig. 1. Epicenter of foreshocks and aftershocks of the Great Hanshin (Kobe) Earthquake, January 16-23, 1995 (Ref. 3).

onal to the fault rupture (buildings J.OMON COAST LINE ( 8000 YEARS AGO) failing approximately in the north­ SEA SHORE south direction) predominated. LOWLAND AREA

~CLAY Japanese Code e:~3 SILT The document loosely referred to as 1;_;.~ . '·\ J SAND the Japanese Code is the Building Standard Law, including the Enforce­ ~HARD SOIL ~(GRAVEL) ment Order of the Building Standard Law. The Enforcement Order contains Fig. 2. Soil profile from the mountains north of Kobe to on the south. requirements enabling implementation of the Building Standard Law, which is brief and contains very few details. Near-Fault Effects before and after the January 17 quake. While the Building Standard Law This plot also represents an approxi­ (including its Enforcement Order) The epicenter of the earthquake was mate trace of fault rupture in the Jan­ specifies only loads and allowable located at a depth of 6 miles (1 0 km) uary 17 event. stresses, and certain minimal require­ and at a distance of about 12 miles (20 Significantly, a vast majority of the ments for detailing of members, de­ km) southwest of downtown Kobe be­ damage was concentrated within a rel­ tails of structural design (such as tween the northeast tip of Awaji Island atively narrow band centered on the methods of structural analysis and the and the main island of Honshu. 2 Fig. 1 trace of fault rupture, indicating that proportioning of members) are speci­ from Ref. 3 is a plot of epicenters of the so-called near-fault effects were fied in the Structural Standards issued foreshocks and aftershocks recorded extremely important. Damage orthog- by the Architectural Institute of Japan

March-April 1995 15 times during the life of a building. Such earthquake motions may be considered as having an intensity of 5 or 6 on the Modified Mercalli Intensity (MMI) scale, with expected peak ground accel­ erations of 0.08g to 0.10g. This design objective is assumed to be achieved by adoption of the traditional level of seis­ mic force and the traditional method of allowable stress design. The newly added second phase de­ sign is intended to ensure safety against an earthquake that could occur once in the lifetime of a building. Such an earthquake motion may be as strong as the 1923 Kanto earthquake was in Tokyo, with an MM intensity of 7 to 10, and peak ground accelera­ Fig. 3. Collapse of timber houses with heavy roofs and inadequate walls. tions of 0.3g to 0.4g. Traditional seis­ mic design assumed that buildings would survive severe earthquakes as a had a profound impact on seismic de­ result of built-in overstrength and duc­ sign practice in Japan. The Building tility. Whether the structure possessed Standard Law underwent a partial re­ adequate levels of overstrength and vision. More importantly, a large­ sufficient ductility did not expressly scale revision of AIJ Standards en­ require confirmation. sued, incorporating ultimate shear Post-1981 structures designed by strength design of reinforced concrete the two-phase procedure described beams and columns, including much above performed much better in the more stringent shear reinforcement re­ Kobe earthquake than pre-1981 struc­ quirements. These changes were com­ tures. Severe damage in such modern parable in many ways to important structures was relatively rare. changes made to the Uniform Build­ ing Code in 1973, following the San Soil Characteristics Fernando earthquake of 1971. Post- 1971 concrete structures performed Fig. 2 shows a soil profile of the significantly better in the Kobe earth­ quake-affected area from the moun­ quake than their pre-1971 counter­ tains to the north to the sea on the parts, primarily because of the im­ south. Higher ground accelerations proved shear design of columns. were generally recorded on softer The Miyagiken-oki earthquake of soils. However, as far as structural Fig. 4. Failure of a nonductile concrete 1978 was another milestone in the damage was concerned, near-fault ef­ column. evolution of Japanese seismic codes. fects appear to have been more impor­ Damage was as severe as in the 1968 tant than soil characteristics. Also im­ Tokachi-oki earthquake, largely cen­ portant were the age of the structure (AIJ). These Standards, prepared sepa­ tered in the city of Sendai. The earth­ and its foundation type. rately for each structural material, quake led directly to a revision of the Newer structures supported by deep serve as supplements to the Law. The Enforcement Order of the Building foundations performed quite well, AIJ Standard for reinforced concrete Standard Law that began to be en­ even when soil conditions were very bears roughly the same relationship to forced, together with supplementary poor. The man-made Port Island just the Building Standard Law as the ACI documents, from June 1981. The revi­ off Kobe, which is entirely made of 318 Building Code Requirements for sion introduced a requirement of two­ fill, has a large number of modern Reinforced Concrete does to the Uni­ phase design for most buildings.• multistory residential, commercial and form Building Code requirements and The purpose of the first phase design, other buildings, mostly built of rein­ the other two U.S. model codes. which is essentially the same as seismic forced concrete and predominantly The 1968 Tokachi-oki earthquake design prescribed by the previous supported by deep pile foundations. caused significant damage to modern Building Standard Law, is to protect All these structures performed satis­ buildings designed in accordance with buildings against loss of function in factorily in spite of ground displace­ building regulations then in force and earthquakes that can occur several ments of several feet in some places.

16 PCI JOURNAL Fig. 5. Failure of a segment of the elevated system.

Damage to Dwellings parts of wood, without the use of nails provides shear strength and confines or other positive (non-wood) connec­ the concrete in compression. Failures Most deaths and injuries occurred in tors. "Thus, during severe shaking, of these columns (see Fig. 4) were re­ one- or two-story houses accommo­ beams frequently pulled out of their sponsible for the total or partial col­ dating one or a small number of fami­ socketed supports in columns or other lapse of buildings (discussed in the lies. Such houses in Japan are almost beams, leading to the immediate cav­ next section) and railway and highway invariably built of timber post-and­ ing-in of the supported story."5 Tradi­ bridges. beam construction (see Fig. 3). The tional houses built in recent years have The Hanshin expressway system roofing consists of ceramic tiles that, had to comply with specifications re­ consists of elevated roadways, sup­ in older houses, are set in an insulating quiring extensive nailing, the use of ported mostly on single column bents, layer of clay or mud. Walls between steel connectors, and provision of that convey traffic above surface the posts consist of either bamboo walls as a percentage of the total floor streets throughout the Osaka-Kobe thatch (traditional Shinkabe construc­ area. These and other newer houses, area. "The superstructure constantly tion) or a lattice-work of thin wood some of "2 by 4" or stud wall con­ changes from simple span steel strips (more modern Okabe construc­ struction (imported from the United stringers to continuous steel box gird­ tion), with a thin layer of plaster on States) performed well. ers to concrete T-girders with drop-in the outside. These types of walls obvi­ sections. The columns also varied ously have a very poor resistance to Nonductile Columns and from concrete or steel circular sections lateral loads. The weight of the clay Transportation Structures to concrete or steel rectangular sec­ and ceramic tiles on the roof, com­ tions of varying heights and widths."2 bined with the inadequate walls, con­ After residential construction, non­ A stretch of the Hanshin Express­ tributed to the total or partial collapse ductile concrete columns were the sec­ way approximately 2000 ft (600 m) of many of these houses. ond biggest factor in Kobe's devasta­ long in , east of Kobe, Also contributing significantly to tion. Concrete columns built before suffered what is probably the most such failures was the traditional 1971 , when Japan changed its seismic spectacular failure caused by the Japanese practice of constructing design codes, were nonductile, i.e., de­ earthquake (see Fig. 5). The super­ wooden houses using interlocking ficient in transverse reinforcement that structure had changed from steel to

March-April 1995 17 columns in a bent supporting the same line was observed. Portions of the Shinkansen (bullet train) line linking Tokyo with the Osaka-Kobe area suffered significant damage because of joint failure and/or shear failure in the upper or lower columns of one-bay, two-story bents that support the railway, except at river crossings where single column bents are used. The single columns also suffered flexure-shear damage. A fairly common feature of bridge column failure was the popping of "gas pressure welded" splices, which represent fairly common Japanese practice (see Fig. 6). These splices failed frequently in older bridges, less Fig . 6. Failure of "gas-pressure welded" splices in a bridge pier. frequently in older buildings, and oc­ casionally in new construction.

Structural Irregularity An inordinate number of low- to mid-rise (2- to 15-story height range) reinforced concrete and steel rein­ forced concrete (SRC) frame build­ ings, without significant shear walls that continued down to the foundation level, lost one or more stories. This type of "pancaking" or soft-story fail­ ure occurs due to a concentration of inelastic deformations in a particular story or stories. Such excessive defor­ mations, coupled with non-ductile de­ tailing of columns and joints that ren­ dered them deficient in shear strength, caused column failures. Steel reinforced concrete, or SRC, is a type of composite construction that uses structural steel frames encased in Fig. 7. Loss of the first story of a non­ Fig . 8. Failure of nonductile corner concrete.' This composite construction ductile concrete frame school building. column of building depicted in Fig. 7. has a long history in Japan, probably dating back to the 1920s, when build­ ings were constructed with exterior concrete in this stretch, thus increasing age. Damage in concrete columns steel frames encased in brick masonry the mass considerably. The lower ends appeared to be caused by a combina­ and interior steel frames encased in of the single column supports failed tion of shear and flexure. Damage, concrete. SRC is generally believed to and allowed rotation of the deck until however, was not limited to concrete be more ductile and, hence, more the edge was resting on the roadway columns. Local buckling of steel earthquake resistant than ordinary re­ below. The columns failed in shear at columns was common; complete fail­ inforced concrete. For buildings be­ a location where flexural hinging had ure due to member buckling also oc­ yond seven stories and up to about 20 apparently occurred because of prema­ curred in a number of cases. stories, or even higher in some cases, ture discontinuation of a significant Several elevated portions of the SRC is the most common type of con­ portion of the longitudinal steel at a Hankyu railway line, which links Osaka struction in Japan. For economy in particular section. with Kobe and runs through Kobe, older SRC buildings, the steel frame Throughout the remainder of the ex­ collapsed because of nonductile con­ was used only over the lower stories. pressway system, a large number crete column failure. In downtown Many of the story failures occurred of columns suffered significant dam- Kobe, the brittle fracture of steel in the first story above ground. This is

18 PCI JOURNAL Fig . 10. Pancaking of floor in Kobe City Hall building.

Fig. 9. Leaning wood-frame building with soft first story.

quite often a soft story because it is usually taller than the other stories and also because shear walls are quite often discontinued below this level to have open space for stores or ball­ rooms or other column free spaces. Sometimes, the building leaned with­ out collapsing. Other times it col­ lapsed (see Fig. 7). The nonductile columns not only lacked transverse reinforcement, but quite often contained smooth, rather than deformed, longitudinal bars (see Fig. 8), which apparently continued to be used in Japan after their use was discontinued in the United States. Also, soft-story failures were not lim­ ited to concrete buildings. Fig. 9 Fig. 11 . One of many collapsed steel buildings in Ashiya. shows a wooden building that is lean­ ing because of soft-story failure. Soft-story failures also occurred at an upper level, which contained meet­ primarily on vertical irregularity, plan various locations along the heights of ing rooms requiring long column-free irregularities also caused damage, as multistory buildings. Almost invari­ spans. expected. For instance, corner build­ ably, there was a stiffness discontinu­ Importantly, to the best of the au­ ings with shear walls on the back and ity at the level that failed. The reasons thor's knowledge as of this writing, all side face, and with more open framing could be discontinuation of shear story collapses have been observed in on the front and the other side face, walls, column section changes that older buildings designed before the performed quite poorly. used to be frequent in older designs, 1971 code change introduced what can No building relying on reinforced geometric offsets (setbacks) and other be considered ductility details for rein­ concrete shear walls for lateral load factors. Fig. I 0 shows the Kobe City forced concrete beams and columns. resistance collapsed. This was true Hall, which pancaked in the first level Although newer RC and SRC build­ even though many ductile and nomi­ above the location where SRC ings suffered structural damage in nally ductile shear walls suffered a columns ended and RC columns some instances, they did not totally or considerable amount of damage, began. Across a corner from the City partially collapse. which was often quite extensive in Hall was a building that pancaked at While the above discussion focused older buildings.

March-April1995 19 Fig. 13. Satisfactorily performing low-rise precast, prestressed apartment building in the Kobe area.

Fig. 12. Severely damaged steel braced frame building in Sannomiya district of Kobe. bly be attributed to failure of the welds. buildings utilize precast concrete wall At the time of this writing, some or panel units. Donald R. Logan of new steel buildings were known to Stresscon Corporation, who along Steel Buildings have suffered significant damage. with Mark Kluver of the Portland Ce­ "While most of the damage observed ment Association (PCA) accompanied Many older steel buildings dating in these new steel buildings has been the author on his visit to Kobe, sup­ back to the early 1960s, which were de­ ductile deformation, expected as per plied the following observations con­ signed and constructed without much the design philosophy, some brittle cerning these buildings: regard for seismic considerations, suf­ failures of steel sections and welds fered a considerable amount of damage. "The typical construction con­ have been observed."' "The potential ductility of these build­ sisted of generally stiff, mid-rise A major residential complex at ings is severely limited as the cold­ shear wall structures utilizing Ashiyahama (Ashiya Beach) used formed steel sections used for columns NMB Splice Sleeve connections steel "superframes" consisting of large will typically develop local buckling to tie the shear walls to the foun­ built-up columns and horizontal prior to attainment of their plastic mo­ dation system, and to connect the trusses to support precast concrete ment capacity."' Many such buildings stacked shear walls to each other. dwelling units. Many of the columns collapsed in Ashiya (east of Kobe) and This system permits ductility to suffered brittle fractures not only at other places (see Fig. 11). Others that occur, if required, in the grouted the location of the welds, but also survived often shed their cladding. reinforcing bars encased by the along their height. "While specula­ A large number of braced frame NMB Splice Sleeve. The group tions abound as to the cause of these buildings suffered damage, some­ inspected three of these structures failures, more elaborate studies are un­ times to the point of being rendered (see Figs. 13 to 15) and found no derway to provide definitive answers useless (see Fig. 12). Generally, damage in the precast concrete on these matters."' Some of the truss frames with slender braces had the structures themselves, and only members in the Ashiyahama complex braces rupture in tension or buckle minor spalling or cracking in the also buckled in compression. in compression; these braced steel cast-in-place concrete at the loca­ frames performed quite poorly. tion of the splice to the founda­ Frames with more substantial braces tion, in a few areas. These struc­ Precast, Prestressed also experienced brace buckling, gus­ tures did not suffer any functional Concrete Structures set buckling and fractures. damage and were ready for con­ In moment frame construction, it ap­ One bright spot amidst all the dev­ tinued occupancy immediately peared common practice to shop-weld astation in Kobe was the performance after the earthquake, except in beam studs to columns prior to erection of precast, prestressed concrete struc­ certain cases where soil subsi­ and to bolt beams to beam studs during tures. Apartment buildings in Japan in dence adjacent to the structure erection. A few moment frames suf­ the two- to five-story height range are may have cut off some of the util­ fered damage in their beam-to-column usually of reinforced concrete bearing ity services connected to the connections; such damage could possi- wall construction. Some of these building."

20 PCI JOURNAL Taller precast concrete structures are increasingly being built in Japan. These are likely to use frames, rather than bearing walls, at least in one di­ rection. To the best of the author's knowledge, the area that bore the major impact of the earthquake did not contain any of these buildings.

Fire Damage According to a report just issued by the Earthquake Engineering Research Institute (EERI)/ approximately 100 fires broke out within minutes of the earthquake. These fires occurred pri­ marily in densely built-up, low-rise areas of the central city which house mixed residential-commercial occu­ pancies, predominantly of wood con­ Fig. 14. A second undamaged low-rise precast, prestressed apartment building in struction. The total number of fires the Kobe area. that occurred on January 17 was 142. The fire spread by radiant heat and flame impingement, building to build­ ing in the densely built-up areas. For­ tunately, the wind was calm so the fire advanced relatively slowly. Final burnt areas in Kobe are estimated to 2 be 10 million sq ft (1 million m ), with 50 percent of this area in Nagata-ku. One impressive performance by a structure was the survival of the Taka­ hashi Hospital in Nagata-ku (see Fig. 16). This five-story concrete shear wall structure not only survived the strong earthquake with no apparent damage, but also survived the intense fire that engulfed virtually everything around it. The hospital reportedly con­ tinued with normal operations imme­ diately following the earthquake. Fig. 15. A third low-rise precast, prestressed apartment building exhibiting good performance in the Kobe earthquake. Statistical Data The Disaster Prevention Research Institute of Kyoto University has is­ clearly shows that the strongest corre­ age to structures is the fact that a very sued some revealing statistics, which lation of damage was with the age of severe earthquake occurred almost di­ are presented in Table 1.5 While the the structure. Significant changes in rectly below a highly populated area. total population of engineered con­ the Japanese Building Code in 1971 The epicenter was located at a depth crete buildings and that of engineered and 1981 are major factors in this of only 6 miles (I 0 km) below the is­ steel buildings in Chuo-ku are not correlation. land of Awaji. known, the total number of concrete 2. Older buildings and dwellings, buildings is undoubtedly much larger especially those built prior to the Conclusions than that of steel buildings. The fig­ adoption of the 1971 and 1981 ures in Table 1 indicate that the per­ Based on the author's on-site obser­ Japanese Building Code changes, suf­ centage of collapsed or severely dam­ vations of earthquake damaged struc­ fered the heaviest damage. aged buildings in the concrete tures, the fol lowing conclusions can 3. Most of the casualties and col­ category is lower than that in the steel be made: lapses occurred in the old timber category. 1. The major reason for the large dwellings that were poorly constructed. Fig. 17 , also adapted from Ref. 5, number of casualties and heavy dam- 4. Damage to reinforced concrete

March-April 1995 21 and steel reinforced concrete struc­ tures was understandable and pre­ dictable. Structural irregularities, com­ bined with nonductile detailing of columns and beam-to-column joints, accounted for the vast majority of the cases of serious damage or collapse. 5. While the damage to older steel buildings was also understandable and predictable, significant damage in a number of new steel buildings is more troublesome. Causes of such damage are at this point unclear. 6. Statistical data from Kyoto Uni­ versity show that the percentage of col­ lapsed or damaged concrete buildings is lower than that of steel buildings. 7. Low-rise apartment buildings of Fig . 16. Undamaged reinforced concrete shear wall hospital building amidst precast, prestressed concrete, designed widespread fire damage at Nagata-ku. and built emulating monolithic con­ struction, performed remarkably well. 8. It is believed that the soil condi­ tions underlying the earthquake area did not play a decisive role in the dam­ age to structures. Predictably, dwellings and buildings on soft soils suffered se­ Table 1. Statistics on damaged concrete and steel buildings in downtown Kobe.* vere damage. On the other hand, struc­ Number of damaged buildings in Concrete 1 . Steel tures with pile foundations going down Chuo-ku (downtown) area of Kobe City buildings +--buildings to bed-rock performed well. Total 1638 1032 Percent experiencing col lapse 4.89 5.33 Percent experiencing severe damage 7.69 9.01 References Percent experiencing moderate damage 10.56 17 .15 1. Nasser, George D., "Devastating Earth­ Percent experiencing minor damage 20.57 28.59 quake Hits Japan," PCI JOURNAL, Percent experiencing slight damage 56.29 39.92 V. 40, No. I, January-February 1995, p. 120. *Data extracted from Ref. 5. -· 2. The Hyogo-Ken Nanbu Earthquake January 17, 1995, Preliminary Recon­ naissance Report, Earthquake Engineer­ ing Research Institute, Oakland, Cali­ fornia, February 1995. 3. Japan - The Great Hanshin Earth­ quake January 17, 1995, Event Report, Risk Management Solutions, Inc. and DAMAGE LEVEL Fai lure Analysis Associates, Inc., Menlo Park, California, January 1995. 4. Aoyama, H. , "Outline of Earthquake 0 SLIG HT OR NONE Provisions in the Recently Revised [J MINOR Japanese Building Code," Preprint, First Meeting of the U.S.-Japan Joint ~MODERATE Technical Coordinating Commjttee on • SEVERE (COLLAPSE) Precast Seismic Structural. Systems (JTCC-PRESSS), San Diego, Califor­ nia, November-December 1990. 5. Fujiwara, T., Suzukj, Y., Nakashima, - '65 '66-'7o ' 11-'75 '76-'ao 's1 -·a5 's6J9o '91-'95 (25) (10) (13) (10) (12) (9) (4) M. , lwai, S. , Kitahara, A., and Brun eau, M ., "Overview of Building Damage YEAR OF CONSTRUCTION from the 1995 Great Hanshin Earth­ quake," Newsletter No. 2, Disaster Pre­ Fig . 17. Correlation between degree of building damage and year of construction vention Research Institute, Kyoto Uni­ (Ref. 5). versity, Kyoto, Japan, February 1995.

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