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International Conferences on Recent Advances 2001 - Fourth International Conference on in Geotechnical Earthquake Engineering and Recent Advances in Geotechnical Earthquake Soil Dynamics Engineering and Soil Dynamics

30 Mar 2001, 1:30 pm - 3:30 pm

Site Dependent Spectra for the Umbro-Marchigiano Earthquake () of September-October 1997

Paolo Carrubba Università di Padova, Italy

Michele Maugeri Università di Catania, Italy

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Recommended Citation Carrubba, Paolo and Maugeri, Michele, "Site Dependent Spectra for the Umbro-Marchigiano Earthquake (Italy) of September-October 1997" (2001). International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. 3. https://scholarsmine.mst.edu/icrageesd/04icrageesd/session03/3

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This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. SITE DEPENDENT SPECTRA FOR THE UMBRO-MARCHIGIANO EARTHQUAKE (ITALY) OF SEPTEMBER-OCTOBER 1997

Paolo Carrubba Michele Maugeri Dipartimento di Ingegneria Idraulica, Marittima e Geotecnica, Dipartimento di Ingegneria Civile ed Ambientale, Universita di Padova, Padova, Italy. Universita di Catania, Catania, Italy.

ABSTRACT

Free-field ground motion during earthquake can be conveniently represented by response spectrum for a given level of damping and ductility. Typical response spectra have been established in several seismic codes, as the Eurocode EC8, but for real seismic events some disagreements may be still encountered. The disagreements come from the presence of soil formations or local site conditions which can differ G-om those provided by seismic codes. During the recent Umbro-Marchigiano earthquake, happened in Italy in the September-October 1997, 198 records ofhorizontal acceleration have been collected from more than 30 accelerometric stations. Soil conditions have been known in detail for 90 records so allowing their classification into four categories: hardrock (28 records), weak rock (28 records), stiff soil (20 records) and soft soils (14 records). Depending on soil conditions, elastic and inelastic response spectra of acceleration, velocity and displacement have been evaluated for discrete values of damping ratio and ductility factor. Comparisons have shown that for sofi soil the Umbro-Marchigiano earthquake gives an elastic acceleration spectrum and a displacement spectrum which are less conservative than those provided by EC8.

INTRODUCTION RECORDS AVAILABLE FROM THE UMBRO- MARCHIGIANO EARTHQUAKE Earthquake hazard in urban sites has two principal aspects: the hrst is related to the vulnerability of building to the seismic forces, The available records from the Umbro-Marchigiano earthquake while the second concerns the site behaviour under shaking, such have been grouped together in four groups, depending on the as amplification, landslide, settlement and liquefaction. general soil conditions of the site. The groups are identified In the last decades, amplification efIects have been predicted in according to these general soil conditions which are: hard rock, many countries by means ofnumerical modelling of the dynamical weathered rock, stiff soil and soft soil (Cavallaro and Maugeri response of the sub-soil (Seed et al. 1976, Maugeri and Carrubba 2000, Crespellani et al 2000). 1997) or by means of site dependent spectra which describe the The main information for the four groups of records are shown in effect related to real seismic events (EC8 1996). Tab. I (hard rock), Tab. II (weathered rock), Tab. III (stiffsoil) and In the period between the 3’d of September and the 14”’ of October Tab. IV (soft soil). In these tables are reported the locality in which 1997, seven main seismic events occurred in the regionsofumbria the time history was recorded, the day, the time, the magnitude, the and Marche in Italy (Fig. 1). The events have been recorded by focus depth, the duration and the maximum ground accelerations more than 30 stations of the SERVIZIO SISMICO NAZIONALE for the E-W and the N-S components. (S.S.N. 1999) locatedin this area. The amount ofavailablerecords Soft soil is referred to loose deposits of clays and sand with shear was 198 but the geotechnical soil conditions have been known in wave velocities less than 200 m/s and depth less than 30 m, somedetails only for 90 records. overlaying a stiff bedrock. In this case14 recordsare available. The records have been normalised with respect to the peak ground StiK soil is referred to old deposits with shear wave velocity not acceleration; therefore, elastic and inelastic response spectra of exceeding 400 m/s; for this case 20 records are available. acceleration, velocity and displacement have been evaluated for Weathered rock is referred to sites in which the geological surveys different levels of damping ratio, ductility factor and general soil have indicated the presence of shallower crushed rock with shear conditions of the site. wave velocity not exceeding 600 m/s; for this case 28 records are Comparisons with the normalised response spectra of EC8 have available. shown some differences which must be considered in the case of Finally, hard rock is referred to intact rock formations with shear moderate intensity seismic events as those related to the Italian wave velocities exceeding 800 m/s; also in this case 28 records are Umbro-Marchigiano earthquake. available.

Paper No. 3.03 Locality ( , Locality

] Senigallia ]26/09/97]09 40 5,8 78,98 27,Ol 0,039 0,047 25 5,4 18,86 28,94 0,053 0,038 (Piana) ]26/09/97]09 40]5,8] 38,01]106,2]0,096]0,094 25 5,4 31,98 28,Ol 0,024 0,033 Pietralunaa ]26/09/97]09 4015.8 154.92 128.55 1 0.07 10.044 23 5,5 19,08 39,13 0,136 0,099 Matelica (14/10/97]15 23]5,5(38,18]27,33]0,021]0,025 Bevagna ]14/10/97] 15 23]5,5 ]25,93 ]40,37]0,038]0,046

Tab. III. Available records on stlf soil.

Spoleto Monteluco 14/10/97 15 23 5,5 25,49 39,33 0,049 0,033 Time M Depth t ::F ;y Locality 14/10/97 15 23 5,5 22,44 35,99 0,063 0,053 Day U-4 WI ISI [ ] [ ] Tab. I. Available records on hard rock.

Time Locality Day PI NoceraUmbra 03/09/97 22 07 - - 26,99 0,164 0,309

Castelnuovo 14/10/97 15 23 55 29,45 29,76 0,033 0,055 () Borgo Ottomila 14/10/97 15 23 5,5 - 97,27 0,012 0,008 126/09/97109 4015.8 1 10.72 Tab. IV. Available records on soft soil.

Nocera Umbra [06/10/97]23 25]5,4] 13,08 37,20 0,311 0,527 Forca Canapine ]06/10/97]23 25]5,4 41,07 19,21 0,038 0,035 Total n of buildings \Jocera Umbra 2]06/10/97]23 2515.4 13.08 28.99 0.257 0.426 a t50 0 ( 50 NoceraUmbra ]14/10/97]15 23]5,5]25,29]32,79]0,138(0,133 Survey perfoneci on. Forca Canapine ]l4/10/97]15 23]5,5]28,38]28,40]0,069]0,084

Tab. II. Available records on wealhered rock.

ANALYSISOF RECORDS

The selected 90 time-histories have been analysed to obtain the elastic response spectra for discrete values of damping ratios (c=O%, E=l%, 5=2%, 5=5% and ~=lO%). The analysis has been carried out by means of the code GEODIN (Frenna and Maugeri 1995) lvhich performs the superposition of effects for a linear system by means of the convolution integral (Newmark and Rosenblueth 197 1). The spectra of acceleration, velocity and displacement have been normalised by scaling the time-history respect to the maximum Fig. 1. Epicenlres of the main shocks of [he Ilalian Unbro- ground acceleration achieved in the record. Marchigiano earlhquake.

Paper No. 3.03 2 IO 4

8 -Hard Rock

- Sttif Soil

-Soft Soil 4 -Weathered Rock

0 02 0.4 06 OR 1 I.2 IA 1.6 IS 2 22 2.4 U 28 3

Fig. 3. Average site dependent elastic spectra of accelerationfor 1 WeatheredRock ] damping ratro 5~5%.

The mean spectra have been evaluated for a single category ofsoil,, by averaging the spectral values related to a same category. The normalised mean acceleration response spectra for a single soil category are shown in Fig. 2 for various damping ratios. A comparison of the amplification effects related to the soil nature is found in Fig. 3 where the normalised average site dependent

0 02 OA 06 0.R I 12 IA 16 IB 2 22 24 26 2.R 3 spectra are compared for the same level of damping ratio of 5%. Tn ISI For periods of vibration lower than 0.3 s amplification of about 1 SO-3.25 times the maximum ground acceleration canbe expected

R for both hard and weathered rocks; for periods of vibration greater than 0.3 s, amplification of about 1.50-2.25 can be expected only for soft soils. 6 Inelastic response spectra have been derived from elastic spectra by means of the statistical-analytical procedure outlined by Newmark (Newmark and Hall 1 Y 82, Clough and Penzien 1 Y 93, Chopra 1995). In Fig. 4 the normalised average site dependent inelastic spectra of acceleration are shown for a damping ratio of 5% and for discrete values ofthe ductility factor @=l .OO, u=2.50, u=3.75 and p=S.OO). As ductility increases the spectral

0 acceleration decreases, but it can be noticed that very large values 0 02 0.4 06 0.8 I 12 ,A lb 1.8 2 22 24 26 2R 3 of ductility are not necessary to attenuate the induced accelerations: Tn [s] for periods greater 0.3 s a ductility of about 2.50 is sufficient to reduce the spectral acceleration below the maximum ground x acceleration. For periods lower than 0.3 s the same value of 7 ductility is able to limit amplification to a range not exceeding 1.50 -0 times the maximum ground acceleration. The Eurocode EC8 gives the normalised elastic spectra ot - 1% acceleration in relation to three categories of soil (soil A, B and C). -2% The soil A is referred to very stiff soil with shear wave velocities - 5% not smaller than 400 m/s, or rock with shear wave velocities not

2 - w/o smaller than 800 m/s. The soil E? is referred to loose deposits with shear wave velocities between 200 m/s and 3 50 m/s; finally, soil C is referred to sott soil with shear wave velocities not exceeding 200 :L- n ' ' m/s. Starting from elastic spectra, the EC8 allows to evaluate the 0 02 04 06 OS I I.2 1.4 16 1.8 2 22 21 2b 28 3 design spectra for a given value of ductility -l-n ISI The comparison between the EC8 elastic spectra and those related to the Umbro-Marchigiano earthquake is shown in Fig. 5. Due to Fig. 2. Average normalised elastic response spectra OJ the wide range of stiffness of soils involved by the Umbro- acceleration for various soil conditions (Damping ratios eO%, Marchigiano earthquake, such a comparison can be performed &l%, (=2%, <=S% and (=lO%). easily.

Paper No. 3.03 3 -1 -Ha% Razk -2.5 -EY3(WA) - 3.75 -5

-1 -25 - 3.75 -5

-1 -2.5 - 3.75 -5

-1 -2.5 - 3.75 Fig. 5. Comparisons between the normalised elastic response spectra ofEC8 and those obtainedfor the Gnbro-Marchigiano -5 earthquakefor different soil conditions and dumping ratio @5%.

A general agreement can be observed for hard and stiff soil; conversely, for soft soil the EC8 becomes less conservative than the Umbro-Marchigiano earthquake, especially when vibration periods greater than 1 .O s are involved. Fig. 4. Average site dependent inelastic spectra of acceleration Comparisons between inelastic spectra, evaluated for p=2.50 (Fig. for values of ductili(y ~=l.OO, ,u=2.50, ,~=3.75, ~=5.00 and 6), indicate that for all soil conditions EC8 becomes less, damping rafioc=5%. conservative only for periods lower than 0.3 s.

Paper No. 3.03 4 The authors have concluded that EC8 gives greater values of displacements and proposed the corrections shown in fig. 7. The main modification is to maintain constant the spectral displacements for periods of vibration exceeding 2.0 s. -Hard Rock A comparison between displacement spectra obtained for the -FJEa (Soil .4) Umbro-Marchigiano earthquake and those considered byEC8 and by Tolis and Faccioli (1998) is shown in Fig. 8. The results of this work indicate that for moderate intensity earthquakes, as the Umbro-Marchigiano earthquake, no correction are necessary for spectral displacements in the range of periods of vibration lower than 1.5 s. For soft soil the EC8 displacement spectrum is less conservative than that related to the Umbro-Marchigiano earthquake, especially for periods ofvibration greater than 1.5 s. In the case of hard rock the spectra ot’ EC8 and of the Umbro- Marchigiano earthquake are comparable, while for soiis of - Weathered intermediate stifiess, like weathered rock and stir’ soil, the EC8 Rock displacement spectra IS more conservative. -IX8 (Soil A)

- EC8 (Sod A) - EC8 (MB) - EC8 (Sal q

0 02 08 06 08 I I: IA 16 IX 2 22 2A 26 28 3 Tn[sl

Fig. 6. Comparisons between the normalised inelastic response Fig. 7. Proposal oj”modiJicufion for EC8 by To/is and Fuccioli speclra ofEC8 and those obtainedjbr the Umbra-Marchigiano (1998): a) Displacement spectra, b) acceleration spectra. earthquake for d&fferent soil condilions (ductility factor oj‘2.50 and damping ratio of 5%). CONCLUSION

Tolis and Faccioli ( 1 Y Y 8) have performed a study on displacement Many horizontal acceleration records have been collected during spectra related to real seismic events and have comparedthe results the recent Italian Umbro-Marchigiano earthquake, happened inthe with the displacements spectra of EC8. September-October 1997.

Paper No. 3.03 5 Site conditions have beenknown in detail for 90 over 198 records,, so allowing a classification of the records into four categories: hard rock, weak rock, stiff soil and soft soils. Elastic and inelastic response spectra for acceleration, velocity and - EC8 (Sod A) displacement have been evaluated in relation to soil conditions, damping ratios and ductility coefficients. --Tok & FaccOl~ (lS8) Comparisons with the Eurocode EC8 have shown that for vibration periods greater than 1 second the Umbro-Marchigiano earthquake gives an elastic acceleration spectrum for soft soil that is less conservative than that of EC8; moreover, for periods of vibration greater than 1.5 s also displacement spectrum for son soil is less conservative than that ofEC8.

REFERENCES

Cavallaro, A. and M. Maugeri. [ ZOOO], Dynamic characterisation of soils at for seismic microzonation. Italian Geotechnical - EC8 (Soil A) Journal, to be published. -Tok &Faccidi (1998) Chopra, A.K. [ 19951. Dynamics oJ’ Structures ~ Theory and -L,kTtmMarctigiam Applications to Earthquake Engineering. Prentice Hall.

Clough, R.W. and J. Penzien. [ 19931. Dynamics of Strucfures. McGraw-Hill, Inc, Second Edition.

Crespellani, T., C. Madiai, G. Simoni and G. Vannucchi. [2000]. Dynamic geotechnical testing and seismic response analyses in two sites of the commune ofNocere Umbra, Italy. Italian Geotechnical .Joumal, to be published.

-EC8 (SIB) ECX. [ 19961. Indicazioni progettuali per la resistenza sismica delle strutture. ENV 1998-l-2, Ottobre 1994. - Tolk & Faccioii (1998) Frenna, S.M. and M. Maugeri. [ 19951. 11 codice di calcolo GEODIN per l’analisi della risposta dinamica dei terreni, IX Convegno Italian0 di Meccanica Computazionale, Catania, 20-22 Giugno 1995.

Maugeri, M. and P. Carrubba. [ 19971. Microzonation for Ground Motion During the 1980 Irpinia Earthquake at Calabritto, Italy. Proc. 14thI.C.S.M.F.E.,Hamburg.

Newmark, N.M. and W.J. Hall. [ 19821. Earthquake Spectra and Design. Earth. Eng. Res. Inst., Berkeley, California.

- EC8 (Soil C) Newmark, N.M. and E. Rosenblueth. [ 197 11. FoundamentaO oj’

-T& & Faccdi (1998) Earthquake Engineering. Prentice Hall, Inc.

Seed, H.B., C. Ugas and J. Lysmer. [ 19761. Site Dependent Spectra for Earthquake Resistant Design. Bullettin of the Seismological Society of America, Vol. 66, No 1.

0 0.5 I I5 2 25 1 S.S.N. [ 19991. Registrazioni accelerometriche de1 terremoto 7” I4 Umbro-Marchigiano de1 Settembre-Ottobre 1997. Servizio Sismico Nazionale, Roma, Italy. Fig. 8. Comparisons between the displacementspectraofUrnbro- Marchigiano earthquake, Eurocode EC8, and Tolis and Faccioli Tolis, S.V. and E. Faccioli. [ 19981. Displacement Design Spectra. (1998). Dipart. di Ing. Strutt. Politecnico di Milano, Milano, Italy.

Paper No. 3.03 6