Influence of the construction quality in the seismic intensity.pdf

8/10/2019 Influence of the construction quality in the seismic intensity.pdf

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INFLUENCE OF THE QUALITY OF CONSTRUCTION ON THE ESTIMATED SEISMIC

INTENSITY

S. Dimova1, P. Negro2

1Central Laboratory for Seismic Mechanics and Earthquake Engineering, Bulgarian Academy ofSciences, bl.3, Acad. G. Bonchev str., Sofia 1113, Bulgaria

Currently: European Laboratory for Structural Assessment, Institute for the Protection andSecurity of the Citizen, Joint Research Centre of the European Commission, T.P. 480, I-21020Ispra (VA), Italy, Tel.: +39 0332 78 5368, Fax.: 39 0332 78 9049 e-mail: [email protected]

2European Laboratory for Structural Assessment, Institute for the Protection and Security of theCitizen, Joint Research Centre of the European Commission, T.P. 480, I-21020 Ispra (VA), Italy

ABSTRACT

The damage observations after most recent earthquakes refer to cases of poor quality ofconstruction, inadequate detailing of reinforcement and absence of capacity design principles.Having in mind the importance of the damage-based assessment of the macroseismic intensity,the proper accounting for the quality of construction directly reflects the realistic estimation of theseismic hazard.

The paper presents an experiment-based assessment of the influence of the quality ofconstruction on the seismic vulnerability of single-storey industrial reinforced concrete framedesigned according to Eurocodes. The influence of the quality of construction is estimated byconsideration of two models of the experimental prototype: structure erected under strictmeasures for control of the execution and structure erected with normal measures for control ofthe execution, which resulted into significant deficiencies in the practical arrangement of thereinforcement. The vulnerability of the structures is estimated by fragility analysis based on fittingthe numerical models of the structural response to the experimental data for different seismicintensity levels. Global damage indices, such as the interstorey drift and the modified Park &Ang overall structural damage index, are related to the homogenized reinforced concrete

damage scale index and in this way the calculated damage states are associated with theobservational damage. On this basis, the damage states of the structures are related to theexpected European Macroseismic Scale (EMS) intensity. The results show that the deficienciesin the construction caused only by the poor execution of the reinforcement increased the EMS-defined damage state by one degree. Accordingly, the appropriate vulnerability class of thestructure with deficiencies in the construction is D instead of E. It is concluded that the quality ofconstruction considerably influences the estimated seismic intensity and should therefore betaken properly into account when applying the macroseismic intensity scales.

1. INTRODUCTION

The building quality is one of the main factors that affect the seismic vulnerability of structures,

both engineered and non-engineered. Since after most earthquakes the insufficient quality isreported as one of the main reasons for the extent of damage, the quantitative assessment ofthe effects of construction quality is a problem of paramount importance, which has possibly notbeen sufficiently investigated.

Having in mind the importance of the damage-based assessment of the macroseismicintensity, the proper accounting for the quality of construction directly reflects the realisticestimation of the seismic hazard. The European Macroseismic Scale (EMS-98) defines buildingclasses by type of construction as an attempt to express the vulnerability of buildings. Whenassessing the vulnerability class of a structure or a group of structures, the quality ofconstruction could be taken into account in the decision of the vulnerability class in the range of


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peak ground acceleration (PGA) of 5% g, 32% g, 64% gand 80% g, as well as an additionaldisplacement-controlled cyclic excitation [1,3], where g is the acceleration of gravity. Theexperimental programme of Structure B encompassed pseudodynamic tests with PGA of 5% g,32% gand 64% g, as well as a repeated test with PGA of 64% g.

As it was mentioned in the introduction, no special survey measures were taken during theconstruction other than acceptance procedures for the materials and check of the quantities ofrebars (and not of their positioning), relying on what could have been intended as standard

construction practice. As a result, a number of inaccuracies and mistakes in the practicalarrangement of the rebars took place, the effect of which is the main subject of this paper. Thefollowing inaccuracies in the execution of Structure B were observed after removing the concretecover at the end of the test programme:

inaccurate spacing of the stirrups. In the critical regions of the structure the design projectenvisaged spacing of the stirrups at 50 mm [4]. The actual spacing measured after thetesting was 90-150 mm, as shown in Figure 3a for the bottom of the North-East column. It isworth mentioning that this was solely due to the movements of the stirrups during thecompaction of the concrete, since it was reported that all stirrups were in their exact positionbefore casting;

Figure 3a. Structure B -spacing of stirrups at thebottom of the North-Eastcolumn

Figure 3b. Structure B -

anchoring of longitudinalreinforcement in the beam-column joint of the North-Eastcolumn

Figure 3c. Structure B - lack

of stirrups in the beam-column joint (the South-East column)

wrong anchoring of the longitudinal reinforcement of the columns into the beams, which,together with the insufficient thickness of the concrete cover, caused the prematureseparation of the longitudinal reinforcement from the concrete during the tests. In Figure 3bthe disposition of the straight lead embedment in the beam-column joint of the North-Eastcolumn is shown. The design envisaged their positioning within the rebars of the beam, andthe mistake originated either from a wrong interpretation of the construction drawings, orfrom the sought ease in assembling the pre-arranged rebars of columns and beams;

inaccurate placing or lack of stirrups in the beam-column joints. In the beam-column joint of

the South-East column shown in Figure 3c the stirrups are missing. Positioning of the stirrupsinside the joint was omitted by the construction workmanship, since the stirrups would havemade it impossible to connect the pre-assembled rebars of columns and beams.

3. MODELLING OF THE SEISMIC BEHAVIOUR

The seismic behaviour of the structures was modelled by means of the computer code IDARC5.5 [5], taking into account the P-delta effects. The hysteretic behaviour of the elements wasmodelled by using the smooth hysteretic model, which offers better possibilities to model the


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elastic-yield transition, the shape of unloading and the slip in comparison with the tri-linearhysteretic model. Separate hysteretic models for the columns were defined for each test in orderto describe as better as possible the seismic behaviour of the structure.

The deficiencies in the construction of Structure B were taken into account in the numericalmodelling as follows:

the increased strength of concrete due to confinement was not taken into account whenestimating the moment-curvature relationships of the structural members. For comparison, incase of correct spacing of stirrups at 50 mm the strength of concrete increased by 25 %, asconsidered in the modelling of Structure A;

the beam-column joints were considered as semi-rigid. Based on the experimental data forthe absolute rotations of the top parts of the columns, the curvatures corresponding to thecracking and yielding moments were determined, as described in [1];

the inelastic buckling of the longitudinal reinforcement was considered as a possible failurecriterion;

having in mind the observed inaccuracies in the construction of Structure B, the depth of theconcrete cover din central North column was taken as d= 20 mm. For the other columns thedesign thickness of the concrete cover d= 30 mm was considered.

4. STRUCTURAL CAPACITY AND VULNERABILITY

In Figure 4 the experimental capacity curves of Structure A and Structure B are compared.

0 100 200 300 400 500

displacement, mm

0

50

100

150

200

250

base-shear,kN

Structure B

experimental envelope curve

Structure Aexperimental envelope curve

yield force Structure B

design base-shear

yield fo rce Structure A

xuStructure B xuStructure A

Figure 4. Capacity curves of Structure A andStructure B

0 0.2 0.4 0.6 0.8 1

PGA/g

0

100

200

300

400

500

stoydisplaceme

nt,mm

Structure A

Structure B

quality zone

Figure 5. Maximum storey displacements ofStructure A and Structure B over PGA

The experimental data conclusively show that the deficiencies of the construction caused bythe inaccurate execution of the reinforcement decreased the seismic capacity of the structure asfollows:

the first yield displacement by 27% and the corresponding base shear force by 12%;

the ultimate storey displacement by 34 %;

the maximum base-shear force by 22%;

the behaviour factor supply by 35%.


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The fragility is defined as the probability of attaining a limit state, conditioned on a particularvalue of a random demand [6]. The maximum interstorey drift (in the particular case equal to themaximum storey displacement) was considered as a seismic response parameter, since it isdirectly related to the ultimate limit state defined by the ultimate storey displacement xu (seeFigure 4). The maximum interstorey drift was calculated for twenty seismic excitationscompatible with the response spectrum of the experimental accelerogram, but having longereffective duration than the experimental one, in order to obtain more conservative estimation of

structural vulnerability, as motivated in [1].The peak ground acceleration (PGA) was chosen as the seismic intensity parameter,

because the fragility of the structures was estimated for spectrum compatible seismic excitations[7]. Since in the present study the ultimate displacements of the structures were obtained fromexperimental data, no uncertainty in the median value of the capacity due to limitations in dataand approximation in modeling was to be taken into account in the fragility estimation. Similarly,since the characteristics of the materials were estimated experimentally, the randomness of theseismic capacitywas also set to be zero. In this way, in the present experiment-based study onlythe randomness in the seismic response parameter affected by the randomness of the seismicexcitation was considered. The response of the experimental structures was calculated fordifferent levels of seismic intensity corresponding to the PGA of the experimental tests.

In Figure 5 the functional relationships between the maximum storey displacement and PGAfor Structure A and Structure B are compared. As it can be seen from this figure, for theconsidered case the quality of construction does not influence substantially the maximumstructural response. Up to the PGA for which Structure B reaches the ultimate limit state, thelargest difference between the responses of the two structures is in the order of 5%. However,there is a significant difference between the capacities of the two structures to sustain highintensity seismic excitations. The enhanced ductility of Structure A supplies much larger ultimatestorey displacement, and in this way makes it possible to sustain 40% larger PGA.

In Figure 6 the obtained fragility curves of Structure A and Structure B are compared.

0 0.4 0.8 1.2 1.6

PGA/g

0

0.2

0.4

0.6

0.8

1

probabilityoffailure

Structure A

Structure B

Figure 6. Fragility of Structure A and Structure B

Structure A demonstrates very reliable seismic behaviour, since the conditional probability offailure is less than 1% for PGA < 0.65 g. At PGA of 1 gStructure A will reach the ultimate limitstate with provability of 95%. The conditional probability of failure of Structure B is less than 1%for PGA < 0.43g. Structure B will reach the ultimate limit state with probability of 95% at PGA of0.76 g. The comparison of the fragility curves of Structure A and Structure B shows that thedeficiency in the construction increased considerably the vulnerability of the structure, since thePGA corresponding to equal probability of failure decreased approximately by 0.25 g.


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5. DAMAGE INDICES AND DAMAGE GRADE

The experimental tests and numerical simulations of the seismic response of the two structuresmade it possible to examine the influence of the quality of construction on the damage state ofthe structures described by global damage indices, as well as by the observed degree ofdamage. The overall structural damage index (OSDI) based on the modified Park & Angdamage model [5,8,9] and the maximum interstorey drift expressed in percentage of the storey

height (ISD%) were considered as global damage indices. The overall structural damage indexbased on the modified Park & Ang damage model was obtained during the numericalsimulations by IDARC for both the experimental and generated accelerograms. According to theprocedure implemented in such computer code, the global damage was obtained as a weightedaverage of the local damage at the ends of each element, with the dissipated energy as aweighting factor.

A new damage scale named the homogenized reinforced concrete damage scale (HRCscale) was proposed and used to generate vulnerability curves [10] in connection with thederivation of vulnerability functions for European-type RC structures based on observationaldata. The HRC scale comprises seven damage states, each of which is defined in terms of thetypical expected structural and non-structural damage, as shown in Appendix A for ductilemoment resisting frames (MRF). The HRC-damage index (DIhrc) provides a numerical referencescale for calibration of the damage states of RC structures. According to the damage observedduring the experimental tests of Structure A, the values of 0, 30, 50 and 70 were assigned toDIhrc for the tests with PGA of 5% g, 32% g, 64% g and 80% g, respectively. In the case ofStructure B, values of 5, 40 and 80 were assigned to DIhrcfor the tests with PGA of 5% g, 32% gand 64% g, respectively. In Figure 7a,b these values of DIhrc are related to ISD% measuredduring the corresponding tests of Structure A and Structure B. Furthermore, these fits were usedto transform the predictive equation for ISD over PGA to DIhrc over PGA obtained from thegenerated accelerograms and in this way to relate the calculated damage indices to the index,based on observational data.

0 2 4 6 8

ISD, %of story high

0

20

40

60

80

DIhrc

Structure A

Figure 7a. Relation between DIhrcand ISD% forStructure A

0 2 4 6

ISD, % of story high

0

20

40

60

80

DIhrc

Structure B

Figure 7b. Relation between DIhrcand ISD% forStructure B

In Table 1 the relation of the rounded-off values of the studied damage indices is shown,along with probability of failure Pf, PGA and structural damage grade. The description of thedamage grade is the same as in [10] (see Appendix A).


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Table 1. Structural damage grade in terms of damage indices and fragility

Pf PGA/g OSDI ISD% DIhrcDamage

gradeEMS

EMSintensity

Structure A, vulnerability class E

< 10-6 < 0.5 < 0.3 < 4.5 < 50 Light Grade 1 IX

10-6- 10-1 0.5 0.7 0.3 0.5 4.5 6.5 50 - 70 Moderate Grade 2 X

0.1 0.95 0.7 1.0 0.5 0.95 6.5 - 9 70 - 90 Extensive Grade 3 XI

> 0.95 > 1.0 > 0.95 > 9 > 90 Partialcollapse

Grade 4 > XI

Structure B, vulnerability class D

< 10-4 < 0.35 < 0.3 < 3.2 < 50 Light Grade 1 VIII

10-4- 10-1 0.35 0.5 0.3 0.6 3.2 4.6 50 - 70 Moderate Grade 2 IX

0.1 0.8 0.50.67 0.6 1.0 4.6 6.6 70 - 90 Extensive Grade 3 X

> 0.80 > 0.67 > 1.0 > 6.6 > 90Partial

collapseGrade 4 > X

For the considered structure with good quality of construction (Structure A) the light andmoderate damage grades are associated with practically no probability of failure. The extensivedamage corresponds to the ambiguous part of the fragility curve and the partial collapse would

take place at very high probability of failure (Pf> 0.95). For the structure with deficiencies in theconstruction (Structure B) the light and moderate damage levels are associated with very lowprobability of failure (Pf< 10-4). The extensive damage corresponds to the ambiguous part of thefragility curve and the partial collapse would take place at high probability of failure, namely Pf>0.8. This value is smaller than the probability of failure delimiting the partial collapse state ofStructure A (Pf> 0.95). Since the fragility was estimated taking the interstorey drift as seismicresponse parameter, this result could be attributed to the larger values of OSDI and DIhrc forStructure B for one and the same ISD%. The relations of both OSDI and DI hrc, with the storeydrift calculated for the generated accelerograms presented in [1] show that the deficiencies inthe construction correspond to considerably larger values of OSDI and DIhrc.

The values of OSDI associated with the different damage grades of the two structures agreevery well with the definitions of the damage state in terms of OSDI based on experimentalobservations [11,12,13]. The values of OSDI delimiting the light, moderate, extensive and partial

collapse damage states shown in Table 1 fully coincide with the values defined in [11,12,13]:OSDI < 0.3 for minor damage, 0.3


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0 0.4 0.8 1.2

OSDI

0

0.2

0.4

0.6

0.8

1

DIh

rc/100

Structure A

Structure B

Figure 8. Relations between DIhrcand OSDI

The good coincidence between the relationships for the two structures gives reasons for a

wider implementation of the calculated values of OSDI for the prediction of the eventualobservational damage.

6. QUALITY OF CONSTRUCTION AND MACROSEISMIC INTENSITY ASSESSMENT

According to the European Macroseismic Scale (EMS) [13], the most likely vulnerability class forthe studied industrial structure (frame structure with high level of earthquake resistant design) isthe vulnerability class E. This vulnerability class is assigned to Structure A due to its good qualityof construction. The obtained damage grades in function of PGA for Structure A and Structure Bare compared in Figure 9.

Figure 9. Comparison of the damage grades of Structure A and Structure B


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It can be seen from Figure 9 that the deficiencies in the construction increased the EMS-defined damage state of Structure B by one degree. In this way the most appropriatevulnerability class for Structure B is the vulnerability class D. It should be mentioned that thedeficiencies in the construction of Structure B were caused only by the poor execution of thereinforcement. The concrete and the steel were of good quality, as reported in [1]. In case ofcombination of bad quality of both, the materials and the execution of the reinforcement, anincrease of the damage grade of Structure B of more than one degree might be expected.

Consequently, the quality of construction does influence considerably the estimated seismicintensity and should be properly taken into account when applying the macroseismic intensityscales.

In Table 1 the expected EMS intensity is associated to the probability of failure and thedamage indices. The structure with good quality of construction (Structure A) will suffer lightdamage from earthquakes with EMS intensity IX, corresponding to PGA/g< 0.5. The extensivedamage would take place only during devastating earthquake with EMS intensity of XI. On theother hand, the structure with deficiencies in the construction (Structure B) would become lightlydamaged from earthquakes with EMS intensity VIII, corresponding to PGA/g < 0.35. Thepartial collapse would take place during devastating earthquake with EMS intensity of XI and thecorresponding PGA/g> 0.67.

5. CONCLUSIONS

1. In the considered case, the quality of construction did not substantially influence themaximum interstorey drift corresponding to a predefined value of the peak ground acceleration.Up to the value of the peak ground acceleration for which the structure with the constructiondeficiencies reached the ultimate limit state, the larger difference with respect to the responsesof the structure with good quality of construction was in the order of 5%. However, the enhancedductility of the structure with good quality of construction supplied much larger ultimate storeydisplacement and therefore made it possible to sustain 40% larger peak ground acceleration.

2. The deficiencies in the construction considerably increased the vulnerability of thestructure, since the peak ground acceleration corresponding to equal probability of failure wasdecreased by approximately 0.25 g. The structure with poor quality of construction will become

lightly damaged by earthquakes intensity

VIII, corresponding to PGA/g< 0.35, whereas thewell constructed structure will become lightly damage from earthquakes with intensity IX,corresponding to PGA < 50% g. The partial collapse of the structure with deficiencies in theconstruction would take place during devastating earthquakes with EMS intensity of XI and therespective peak ground acceleration larger than 0.67 g, while the well constructed structurewould become extensively damaged during devastating earthquakes with EuropeanMacroseismic Scale intensity of XI.

3. The construction deficiencies increased the damage grade of Structure B defined by theEuropean Macroseismic Scale by one degree. Accordingly, the appropriate vulnerability class isD instead of E. Consequently, the quality of construction considerably influences the estimatedseismic intensity and should be properly taken into account when applying the macroseismicintensity scales.

4. For both the structure with good quality of construction and the structure with deficiencies

in the execution, strong correlation was proved between the overall structural damage indexbased on the modified Park & Ang damage model and the homogenized reinforced concretedamage scale damage index. This gives reasons for further implementation of the calculatedvalues of the above overall structural damage index for the prediction of eventual observationaldamage.

ACKNOWLEDGEMENTS

Some of the test results mentioned in the present paper, namely the tests on the structureerected under strict measures for control of the quality of execution, were obtained as a part of


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the research project Seismic Behaviour of Reinforced Concrete Industrial Buildings, funded inthe V Framework Programme under the contract ECOLEADER-JRC (contract HPRICT199900059). The project was coordinated by Prof. G. Toniolo from the Technical University of Milan,with the participation of the University of Ljubljana, the Italian Precast Industry Association(ASSOBETON), the Spanish Precast Concrete Association (ANDECE) and the PortugueseConcrete Products Association (ANIPC), and the collaboration of Prof. F. Karadogan from theTechnical University of Istanbul.

In producing the work described in the present paper, the authors received much assistanceand information from Prof. Toniolo, as well from many partners of the ECOLEADER - JRCproject, in particular from Dr. A. Colombo, Dr. L. Ferrara, Mr. C. Bonfanti and Mr. P. Kante.

The tests were performed at the European Laboratory for Structural Assessment. Theenthusiasm and dedication of the whole ELSA staff has made possible the preparation andexecution of the tests.

REFERENCES

1. Dimova S.L., Negro P. Influence of the quality of construction on the seismic vulnerabilityof structures.Report EUR 21009 EN, European Commission, Joint research Centre,IPSC, 2004.

2. Biondini F., Toniolo G. Seismic behaviour of concrete frames: experimental andanalytical verification of Eurocode 8 design rules. Proc. FIB 2003 Symposium, Athens,2003.

3. Ferrara L., Negro P. Seismic behaviour of reinforced concrete structures: test on thecast-in-situ prototype. Report EUR 21097 EN, European Commission, Joint researchCentre, IPSC, 2004.

4. Ferrara L. Design calculations. ECOLEADER research project: Seismic behaviour ofreinforced concrete industrial buildings. Politecnico di Milano, 2002.

5. Valles R.E,. Reinhorn A.M., Kunnath S.K., Li C., Madan A. IDARC 2D. A computerprogram for the inelastic damage analysis of buildings. Technical report NCEER-96-0010, University of New York at Buffalo, 1996.

6. Ellingwood B.R. Earthquake risk assessment of building structures. Reliability

Engineering and System Safety2002;74:251-262.7. Dimova S.L., Hirata K. Simplified seismic fragility analysis of structures with two typesfriction devices.Earthquake Engineering and Structural Dynamics2000; 29:1153-1175.

8. Park Y.J., Ang A.H. Mechanistic seismic damage model for reinforced concrete. Journalof Structural Engineering1985;111:722-739.

9. Bracci J.M., Reinhorn A.M., Mander J.B., Kunnath S.K. Deterministic model for seismicdamage evaluation of reinforced concrete structures. Report NCEER-89-0033, StateUniversity of New York at Buffalo; 1989.

10. Rossetto T., Elnashai A. Derivation of vulnerability functions for European-type RCstructures based on observational data. Engineering Structures2003;25:1241-1263.

11. Hatamoto H., Chung Y.S., Shinozuka M. Seismic capacity enhancement of RC frames bymeans of damage control design. Proceedings of the 4th U.S. National Conference onEarthquake Engineering, EERI, Oakland California 1990:279-288.

12. Gunturi S.K.V., Shah H.C. Building specific damage evaluation. Proc. 10th WorldConference on Earthquake Engineering Madrid 1992;10:6001-6006.

13. Conseil de lEurope. European macroseismic scale 1998 (EMS-98). Cahier du CentreEuropen de Godynamique et de Sismologie, G. Gruenthal, editor. Luxemburg; 15,1998.

14. Negro P., Magonette G.E. Experimental methods in structural dynamics. EuropeanEarthquake Engineering 1998: 12(1):29-39.


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APPENDIX A. The HRC-Scale

Typical damage expected in ductile moment resisting frames (MRF) according to the HRC-scale:

DIhrc Damage state Ductile MRF

0 None No damage

10 Slight

Fine cracks in plaster of partitions/infills

20

30

40

Light

Start of structural damage

Hairline cracking in beams andcolumns near joints (< 1 mm)

50

60

70

Moderate

Cracking in most beams & columns

Some yielding in a limited number

Larger flexural cracks & start ofconcrete spalling

80

90Extensive

Ultimate capacity reached in someelements large flexural cracking,concrete spalling & rebar buckling

100 Partial collapse

Collapse of a few columns, a buildingwing or single upper floor

Collapse

Complete or impending buildingcollapse

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Influence of the construction quality in the seismic intensity.pdf