EERI SPECIAL EARTHQUAKE REPORT -NOVEMBER 1998

Up to the first week of September,around 510 aftershocks were de-tected by the seismological networkof the Geophysics Institute at EPNin Quito. The biggest aftershock oc-curred at 01 :51 local time on August7, with a magnitude of mb=4.6.

Preliminary location of the after-shocks defines an BO-km rupturesemi-parallel to the trench, movingalong the subduction plane, awayfrom the main and premonitoryshocks (see Figure 2).

Structural Aspects

There is no strong-motion record ofthe earthquake in Bahra. As can beseen in the isoseismal map of theevent shown in Figure 1, MM in-tensities between VII and VIII wereestimated for Bahra. These levels ofintensity were obtained from dam-age observation of reinforced con-crete and wooden structures which,in general, showed lack of main-tenance.

E~

The EPN Structural Department hascarried out a detailed evaluation ofthe performance of six differenttypes of reinforced concrete struc-tures based on the architectural andstructural information provided byinsurance companies. Analyticalevaluation of their response wasperformed using SAP-90 andETABS. Comparing this responsewith the observed damage, the ac-celeration could be estimated to be15% to 18% g. No information onthe frequency content is availabledue to the lack of accelerograms inthe near field.

-82.00 -81.50 -81.00 -80.50 -80.00

Figure 2 -Ecuadoran Coastal Zone 1998 Seismicity

79.50 79.00

Most of the observed damage to re-inforced concrete structures can besummarized as follows:

1) It appears that there is extremeflexibility in most of the buildings.The calculated inter-story drift ratiofor the sample of six buildings isusually greater than 0.004, and Insome cases reaches values of0.008. The infill walls are able toaccept values of 0.001 to 0.002

only.

c) The slab is usually the so-called"flat slab," so the beam in theframe has the same height asthe slab--35 to 40 cm--with thebase of the beams rangingbetween 60 and 80 cm.

d) The infill walls are made of fra-gile hollow bricks, made out ofpumice, with no anchorage tothe frames.

Local effects should also be con-sidered since most of the new re-infor'ced concrete structures werebuilt on marine sands deposited onthe north side of the city by sea cur-rents during the last 50 years. It isclear that structures built on top ofthe new marine deposits sufferedmore damage than others.

2) There are very long spans be-tween columns or resisting walls toprovide wide open spaces for win-dows or swimming pools; in somecases 8 to 11 m.

3) Large balconies resulted ininadequate cantilever systems.

4) Soft first stories performed

badly.

a) The seismic-resisting system formost of these buildings isformed by space frames, withbeams and columns.

b) Few of them have shear walls,but, they are not long enough tocarry most of the lateral loads.

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EERI SPECIAL EARTHQUAKE REPORT -NOVEMBER 1998

the walls are used to hide electricaland sanitary installations, so theconsequences to this type of in-stallation were observed to be verysevere.

Infrastructure Damage

The first aftershock was strongenough to prompt the evacuation ofthe Bahia de Car13quez RegionalHospital, which is a structureformed by four blocks separated byconstruction joints. Two of theseblocks have L and T in-planshapes. The buildings also have asoft first story , and some exteriorcolumns are short due to the pres-ence of incomplete in-filled walls(see Figure 4). Since at least sixadditional hospitals in the countrywere built using the same architec-tural and structural designs, it isnecessary to start a retroffittingprogram for them.

Figure 3 -Total collapse of a six-story reinforced concrete buildingNear the epicentral area, the brittlefailure of walls and roofs in schoolbuildings was seen (see Figure 5).Fortunately, there were no childrenin class at the time of the earth-quake. The vulnerability of some

damage to all the walls of the en-tire building. In most cases, walldestruction was conspicuous in thefirst three to four stories. Usually,

5) Short columns were vulnerable.

6) Torsional effects especially af-fected corner buildings which hadtwo facades with open spaces forwindows and two with completeinfill walls.

7) Excessive mass unnecessarilyadded to the structures throughpartition walls and thick plasters toproduce architectural details likerounded walls. In most of the sixevaluated buildings, the loads dueto walls and plasters range be-tween 250 to 400 Kg/m2.

From a sample of around 30 build-ings, three suffered severe struc-tural damage and one totally col-lapsed (see Figure 3). Most ofthem will need to be repaired andstrengthened in order to withstandthe strong ground motion that willaffect the region in the future.

Nonstructural Damage

The above-mentioned structural de-ficiencies resulted in severe dam-age to the exterior and partitionwalls. In some cases, there was Figure 4 -Damage to short columns

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EERI SPECIAL EARTHQUAKE REPORT -NOVEMBER 1998

typical schools across the countrywas pointed out in a previous reportby the EPN and GeoHazards Inter-national: Investing in Quito 's Fu-ture: The Schools Seismic SafetyProject. The most common type isa one-story steel structure proto-type, very light, with brick or pumiceblocks infill walls not attached to theconfining steel frames.

Electrical and telephone serviceswere disrupted mainly due to polecollapses and short-circuits; in thecase of telephones, there was ex-tensive damage to the building sys-tems. Water supply was alreadydisrupted due to El Nirio, but therewas no clear evidence of damage tothe water tanks and pipes. Minorlandslides partially blocked the roadbetween Bahia de Caraquez andCanoa.

Concluding Remarks

Figure 5- Collapse of the infill walls in the steel frames of a typical schoolExcess flexibility was the maincause of the damage observed,especially related to the partitionand external walls. Many buildingsin Bahia de Caraquez will need tobe retrofitted, and the cost will bevery high. It is more cost-effectiveto implement a seismic qualitycontrol program during design andconstruction.

One tool that will help most to re-duce the high vulnerability of struc-tures in the country is a revised andupdated building code, especially ifit is enforced. The current Ecua-doran construction code was pub-lished in 1977. It was copied fromSEAOC, UBC and ACI standardsof that time, but it has never beenrevised, and local conditions werenever taken into account.

One of the most powerful tools formitigation could be earthquakeinsurance, but only a few buildingsin Bahia were insured, maybe dueto the high premiums. It would bedesirable to involve the insuranceindustry in the mitigation process. Figure 6 -Damage to traditional wood frame construction

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