first impressions of earthquake damage in san … journal...special report first impressions of...

Special Report

First Impressions ofEarthquake Damage in

San Francisco Areaby

James K. Iverson, PhD, P.E.PrincipalThe Consulting Engineers Group, Inc.Napa, California

O n October 17, 1989, at 5:04 p.m.,during rush hour, the San Fran-

cisco area was hit with a major earth-quake. The entire nation witnessed thedramatic event on television as the twolocal baseball teams prepared for thethird game of the World Series.

The epicenter of the earthquake waslocated southeast of San Francisco, nearSanta Cruz, and occurred on the SanAndreas fault system. Seismologists nowcall this the Loma Prieta earthquake andits intensity has been set at 7.1 on theRichter scale.

Fig. 1 shows a portion of a map pre-pared by the California Division of

Mines and Geology. The map shows thelocation and extent of the aftershockarea as a stippled line and locations ofground response stations are shown asdots. As can be seen, one station is al-most directly on the aftershock zone andthis Corralitos station acceleration recordis given in Fig. 2. Maximum horizontalaccelerations of 0.64g were measured.

The California report also notesmaximum horizontal ground accelera-tions in San Francisco that ranged from0.06g in Pacific Heights to 0.21g at thePresidio station. Across the bay in Oak-land, values of 0.18g to 0.29g were re-corded and south in San Jose values of

108

RICHMOND-SAN J AFAEL BRIDGE

SAN FRANCISCO BAY BRIDGE

Oakland

NCISCO-4-STORY HOSPITALD4NTERNA ZONAL AIRPORT

\ IAYWARD1 MUIR SCHOOL—SAf4 MATED RIDGE

TER\CITY7

San Jose3AN JOSE-SANTA CLARACOUNTY BUILDING

EPICENTER

CORRALITOSfQ `•GILROY NO.3

LA U(ZONE OF

T

AFTERSHOCKS,NVILLE 'ITONE BLDG.SAN JUAN BAUTISTA • HOLLI S T E R101 /156 OVERPASS

0 10 20 MILES

0 10 20 30 KILOMETERS

•:SEISMIC STATION ( Monterey

Fig. 1. Map of San Francisco area showing places affected by earthquake.

PCI JOURNAL/November-December 1989 109

C

Corralitos (CSMIP Station 57007) Record 570074809-89291.00

Max._ — — '— -- — — — -- — — — — — —

-----------_ Accel.

90 0.50 g

UP 0.47 g

0 ^^^ 0,64 g

0 1 2 3 4 5 10 15 20 Sec.

Fig. 2. Corralitos accelerogram, recorded almost directly above the fault, showed a horizontal peak acceleration of 0.64 g.

Fig. 3. Collapsed portion of 1-880.

about 0.12g were reported. The currentUBC seismic design code is stated to bepredicated on assumed maximumground acceleration of from 0.5g to 0.6g.

The rupture zone for this earthquakeis estimated at about 30 miles (48 km) inlength. In comparison, the 1906 SanFrancisco earthquake was estimated tohave had a 300 mile (484 km) rupturelength.

Damage was widespread, and currentdollar estimates have climbed to over $7billion. It is estimated that more than100,000 homes have been evacuated.Thousands of buildings were damagedor destroyed.

The largest loss of life occurred on anelevated section of Highway 1-880which collapsed. The half mile (0.8 km)long section of two-level structure ransouth through Oakland from the inter-section of 1-80 and I-580 at the BayBridge. By the first week of November ithad yielded 42 casualties. Total deaths

attributed to the earthquake were listedat 67 at this time.

The overpass collapse has beenwidely reported in the media. Fig. 3shows typical collapsed column sectionsand the sandwiched deck sections. Ini-tial reports have noted that the mid-1950s standard bridge design and de-tailing of the columns and pier bentsmay have contributed to the collapse.Several of the other elevated highwaystructures accessing the Bay Bridge onthe San Francisco side, also experienceddamage and have been closed.

The Bay Bridge which carries I-80from Oakland to San Francisco and pro-vides the main access to the city, experi-enced damage to two of its deck sectionsas shown in Fig. 4, which is taken fromthe earthquake report in ENR.* By thefirst week in November replacementunits were already under construction

*Engineering News-Record, October 26, 1989.


• 'U. -t. rap leu uecK section of Bay Bridge (from Engineering News-Record report).

and the contractor targeted completionof the work by the Thanksgiving holi-day. Basalt Precast, located in Napa,working around the clock, finishedcasting 42 stay-in-place prestressed con-crete deck panels for the repair. Thepanels were in place on the bridge byNovember 6. The Bay Bridge was re-opened to full service on November 18.

Heavy localized damage was noted inseveral areas of Oakland and San Fran-cisco and is attributed to poor soil con-ditions. An example of this was the dev-astation of the Marina residential area,which was also widely reported on tele-vision. One such view is shown in Fig.5. Only blocks away from these heavilydamaged homes, identical structuresexperienced only minor cracking andmany are occupied today.

This paper presents a preliminary re-view of the performance of precast andprestressed concrete construction in theLoma Prieta earthquake. The primaryprecast product in this region is ar-

chitectural concrete cladding panels.Many of the structures utilizing thesepanels are high rise offices in thedowntown areas. Several of thesebuildings were reviewed. There are alsomany structural applications of precastand prestressed concrete in the affectedregion including parking garages andother types of buildings. Again, anumber of these structures were in-spected.

The inspections were intended to bepreliminary and many of the buildingswere viewed only from the outside. Thereview emphasized the downtown re-gions of San Francisco and Oakland,both of which received significantearthquake damage and contain numer-ous structures utilizing precast concrete.

A telephone survey of about elevenprecast manufacturers that supply prod-ucts in this area was also made. The in-tent of this survey was to summarize thedamage reports that producers had re-ceived as a means to provide an early

112

Fig. 5. Damage to homes and pavement inthe Marina area of San Francisco.

estimate of the amount and types ofproblems that precast concrete productshad experienced in the earthquake.

PERFORMANCE OFPRECAST CONCRETE

This section of the article discussesthe performance of the architecturalprecast concrete and the precast pre-stressed concrete structures.

Architectural Precast ConcreteThe overall response of precast clad-

ding to the earthquake was excellent.

Most of the structures in downtown SanFrancisco were undamaged and only afew buildings suffered minor cracking.About half of the producers in the areareported no damage calls and the othersreceived only a few inconsequentialmessages. The typical problem con-cerned minor problems existing beforethe earthquake. In one case, in down-town San Francisco, an older adjacentbuilding pounded against a newly con-structed hotel. Two phone calls in-volved minor cracking in cast-in-placeconcrete at panel supports of a high risebuilding. After a preliminary inspection,the cracking was believed to have beencaused by insufficient design allowancefor story drift.

The results of the visual inspection ofbuildings were very good. In aboutthirty clad structures that were in-spected, only two showed any signs ofsignificant cladding cracking or move-ment. Various views of typical structuresin both San Francisco and Oakland areshown in Figs. 6a through 61.

Several of these structures are ap-proximately forty stories tall. Theircladding performance was judged to beexcellent. Many of these structures fea-ture unusual geometry in both layoutand panel configuration and numerouscutouts in the structure at higher stories.These conditions always present prob-lems in connection detailing and layoutand apparently were well handled in theaffected area.

The two structures with cracking arein Oakland, and both contain telephonefunctions. One is a telephone exchangebuilding located in downtown Oaklandonly slightly over a mile (1.62 km) fromthe collapsed I-880 strucure. It is re-ported that the telephone exchangeequipment has operated continuouslythroughout the earthquake and the re-covery period. A view of the building isshown in Fig. 7.

The cracking is apparently limited tocolumn covers on the lower level steelcolumns. No damage was observed in

PCI JOURNAUNovember-December 1989 113

Fig. 6a. Hyatt Regency Hotel, San Francisco.

Fig. 6b. Stevenson Place, San Francisco.

114

Fig. 6c. Federal Reserve Bank, San Francisco.

Fig. 6d. Federal Reserve Bank, San Francisco.


Fig. 6g. Standard Oil Building, San Francisco.

Fig. 6h. Kaiser Permanente Parking, Oakland.


mm CD

C)

0)

om 3 cr

C) CD 0 0 0 CD co

C_ sQ

cfl rn K CD N 0 P.

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Fig. 7. Telephone Exchange Building, Oakland.

the upper level panels. The columncovers are already under repair and thecorner column is shown in Fig. 8. Notethat these column covers were concretefilled, which leaves little allowance forstory drift between the steel columnsand concrete cladding.

A concrete shear wall at the first levelin this structure also experiencedcracking and can be seen in the back-ground of Fig. 8. This dual systemstructure functioned as expected,with the stiffer concrete shear wallstaking loads at lower displacements andthe more flexible steel frame takinggreater loads at larger movements whenthe shear wall has cracked.

The second building experiencingcracking in the cladding is a few blocksfrom the first structure and is called theTrans Pacific Centre. It contains officesof several companies including AT&T.This steel structure is clad with glassfiber reinforced concrete panels. A view Fig. 8. Corner column under repair.


Fig. 9. Trans Pacific Centre, Oakland.

Fig. 10. Panel cracking. Fig. 11. Opened seismic joint.

120

of the building is shown in Fig. 9. Thecloseup in Fig. 10 shows cracking alongthe edges of panels near a corner of thebuilding and Fig. 11 shows an openedseismic joint at a column cover.

All of the cladding damage is appar-ently associated with large movementsat seismic separation joints in the struc-ture. At the locations where cracking oc-curred there are dual columns and thestructure is divided into independentportions which react separately toearthquake motions. Movement be-tween these independent building sec-tions caused most of the cracking.

Precast Prestressed StructuresSeveral precast prestressed concrete

structures were reviewed. Garages inOakland, Emeryville and Berkeley, allwithin a few miles of the collapsedI-880 bridge, were unaffected by theseismic motions. The structures weregenerally constructed with topped,double-tee diaphragms and had rein-forced cast-in-place concrete shear wallsand frames.

Minor slab cracking adjacent to ashear wall was noted in one garage andin general there was topping crackingover the tee joints. It was difficult to as-sess whether the cracking was the resultof previous volume change effects ordue to the earthquake, and in any casethe cracks were minor. In Emeryvillenear an office structure adjacent to thegarage, clear indications of about aninch (2.54 cm) of ground subsidencebetween a sidewalk and stairs could benoted.

A two-story garage in Berkeley nearthe University of California campus wasan exception, with a precast single-teeframe and post-tensioned slab, withdiaphragm ties to adjacent retainingwalls. Fig. 12 shows a view of the con-crete ties. Only minor cracking in theties was noted.

All of the inspected structures were inuse. One garage, owned by the City of

Fig. 12. Diaphragm ties between garageand retaining wall in Berkeley.

Oakland and located on Telegraph St.,had several earlier repairs from previouscracking at the beam seats at thedouble-tee bearings and to increase lat-eral bracing. This garage is shown inFig. 13. It survived the earthquake ex-ceptionally well.

A circular garage utilizing pie-shapeddouble tees is shown in Fig. 14. Thisunusual structure had no apparent dam-age and was practically full of automo-biles at the time of review.

Within about a half mile (0.8 km) ofthe I-880 collapse, a housing develop-ment project utilizing reinforced con-crete block and prestressed hollow-coreslab construction was occupied andshowed no signs of cracking. Fig. 15shows two of the 12-story structuresfrom this housing development and acloseup of the exterior is shown in Fig.16.

One structure of note is the SantaCruz County Administration Building,located in Santa Cruz only a few miles


Fig. 13. Parking garage in Oakland.

Fig. 14. Circular parking garage in Oakland.

122

Fig. 15. Housing buildings.

Fig. 16. Closeup of hollow-core balcony.


from the epicenter. This unusual struc-ture was constructed using Vierendeeltrusses, consisting of precast concretesegments, post-tensioned together.Time did not allow review of this struc-ture but other investigators have re-ported that this building performed verywell, with no significant cracking.

CONCLUDING REMARKS,The Loma Prieta earthquake caused

considerable damage and grief. TheCalifornia Transportation Department iscarefully examining the seismic designcriteria of its bridges in the aftermath ofa collapse that was extremely costly inhuman lives.

In retrospect, the overwhelmingly pos-itive evaluation of the precast concretethat was reviewed is most gratifying.The precast and prestressed concretecomponents withstood the earthquake

with practically insignificant problems.In the very few cases where crackingdid occur, poor design or inappropriatedetailing appear to be at fault.

The general conclusion is that thestructures designed according to thelatest codes of practice and sound engi-neering principles performed exceed-ingly well.

ACKNOWLEDGMENTThe author wishes to thank several

people who contributed generously tothis article. The Northern Californiaprecasters were most cooperative andhelpful in their response to telephonecalls.

In particular, Mr. Ray McCann wasextremely helpful with directions andlocations and accompanied the writer onone of the inspection trips.

Many thanks to all!

NOTE: Discussion of this paper is invited. Please submityour comments to PCI Headquarters by August 1, 1990.

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