Seismic Behavior and Failure Mechanisms Identification of Ancient Masonry Towers

Instituto Tecnológico y de Estudios Superiores de Occidente

2011-02

Seismic Behavior and Failure Mechanisms

Identification of Ancient Masonry Towers

Preciado, Adolfo; Budelmann, Harald; Sperbeck, Silvio T.; Bartoli, Gianni

Preciado, A. et al. (2011). Seismic Behavior and Failure Mechanisms Identification of Ancient Masonry Towers. 1st. Interquadrennial ICF Conference in Middle East and Africa

Enlace directo al documento: http://hdl.handle.net/11117/3437

Este documento obtenido del Repositorio Institucional del Instituto Tecnológico y de Estudios Superiores de Occidente se pone a disposición general bajo los términos y condiciones de la siguiente licencia:

http://quijote.biblio.iteso.mx/licencias/CC-BY-NC-2.5-MX.pdf

(El documento empieza en la siguiente página) Repositorio Institucional del ITESO rei.iteso.mx


)#) #65 5 ) 5

62 6 #
6#6#)5#

 003+"6  .+6.+-6.3( 6#+.(0306

+"3+ 3+"62. 3.(0

 256 ,#)6#6#) 62 5,#26

 
5#
66#5 
62#)6
/ )

* Technical University of Braunschweig, Faculty of Architecture, Civil Engineering and Environmental Sciences, iBMB Institute, Braunschweig, Germany

** Society for the Safety of Reactors and Nuclear Power Plants (GRS) mbh, Germany *** University of Florence, Faculty of Engineering, Dept. of Civil Eng., Florence, Italy

#
)#5

Strong damage or complete loss suffered by the cultural patrimony when subjected to considerable earthquake ground shaking has been occurring through the history of humanity. The occurrence of these unexpected and unavoidable events has demonstrated that ancient masonry towers are one of the most vulnerable structural types to suffer strong damage or collapse. There are certain relevant aspects determining the seismic vulnerability of towers in terms of behavior and failure mechanisms that differentiate these vertical structures from most of compact historical constructions. This is the first stage of an international scientific research between the University of Braunschweig and the University of Florence, aimed to develop a methodology to mitigate the seismic risk of ancient masonry towers located in seismic zones with the use of prestressing devices of smart materials. Therefore results quite important a deep understanding and identification of all the most important aspects that determine the seismic vulnerability of ancient masonry towers in terms of behavior and failure mechanisms. For achieving this, it is considered the relevant literature, observed damage after real earthquakes and mainly engineering experience.

Keywords: Old masonry towers, seismic vulnerability, behavior, failure mechanisms, prestressing devices.

ESLIA

Egyptian Society for Laser Industrial Applications

INTRODUCTION

AOL,OF:Q IFO)F, !IL$F Q ,$)LQ O,Q HO))F,Q II,)OL,OLQ IFHQ F,O$FLQ I)Q HOL,O*,)HQ ICCQ $?)Q ,)Q $CHQ IFHQ $FL,O,,)Q IQ )C)?IF,Q I,Q $Q ,)Q IO,),ICQ IFHQ C,ICQ )O,I:)Q $Q !IFO, #Q )L)Q O!$,IF,Q ?),OICQ L,,)LQ ))Q *OC,Q )O,)Q OL$CI,)HQ$Q$!!$FC QOFCH)HQOFQHO))F,Q!IFF)LQOFQ,)Q*IFQ$F,)A,QLQILQ*OC,QILQ I,Q$Q)LQIL,C)LQ!FOOICQ*OCHOF:LQIFHQO, QICCL#Q)CCQIFHQC$Q,$)LQICL$Q FI!)HQO?OQ,$)LQ))Q*OC,Q7O,)Q,ICCQO,Q,)QO!$,IF,Q$L)Q$QOF$!OF:Q)$C)Q ?OLICC Q IFHQ O,Q L$FHLQ * Q OF:OF:Q *)CCLQ IFHQ L,OOF:Q C$LQ I*$,Q ,O!)Q IFHQ )A,I$HOFI Q )?)F,LQ LQ ILQ O?OCQ H))F)Q $Q O)Q ICI!Q IFHQ !$)$?)Q ,$Q ICCQ ,)Q $!!FO, Q,$QL$OICQ!)),OF:L ;L))Q3O:)Q#Q

Figure 1 "IFQ)FIHOF$QQFHI!I:)HQIFHQ$*L)?)HQHI!I:)QI,)Q,)Q #Q

!I:FO,H)Q+<7OCIQ(,IC Q)I,7I)Q$FQ<OCQ ,Q866
QIILIFQ866


<F$,)QO!$,IF,Q)IL$FQ,I,QC)HQ,$Q,)Q$FL,,O$FQ$Q,ICCQO?OQ,$)LQ)L)OICC QOFQ,)Q !)HO)?ICQO,O)LQ$Q(,IC QILQ,I,Q,) Q))QL))FQILQIQL !*$CQ))L)F,OF:Q* Q,)Q)O:,Q IFHQIO,),)QL$OL,OI,O$FQ,)QOF)LLQIFHQ$)Q$Q,)Q:)I,QI!OCO)L#Q

SEISMIC VULNERABILITY OF ANCIENT MASONRY TOWERS

",$F:Q HI!I:)Q $Q $!C),)Q C$LLQ L))HQ * Q ,)Q C,ICQ I,O!$F Q )FQ L*0),)HQ ,$Q $FLOH)I*C)Q )I,7I)LQ ILQ *))FQ $OF:Q ,$:Q ,)Q OL,$ $Q !IFO, #Q )Q $)F)Q $Q ,)L)Q F)A),)HQ IFHQ FI?$OHI*C)Q )?)F,LQ ILQ H)!$FL,I,)HQ ,I,Q IFO)F,Q !IL$F Q,$)LQI)Q$F)Q$Q,)Q!$L,Q?CF)I*C)QL,,ICQ, )LQ,$QL)QL,$F:QHI!I:)Q$Q $CCIL)Q ILQ H)O,)HQ OFQ 3O:)Q # ))Q I)Q ),IOFQ )C)?IF,Q IL),LQ H),)!OFOF:Q ,)Q L)OL!OQ ?CF)I*OCO, Q $Q ,$)LQ OFQ ,)!LQ $Q *)I?O$Q IFHQ IOC)Q !)IFOL!LQ ,I,Q HO))F,OI,)Q,)!Q$!Q!$L,Q$Q $!I,QOL,$OICQ$FL,,O$FL# )L)Q)C)?IF,Q IL),LQ $)L$FHQ ,$Q O,LQ LC)FH)F)LLQ ,)Q )L)F)Q $Q IH0I)F,Q ICCLQ $Q I%IH)LQ O,Q HO))F,Q )O:,Q,IFQ,)Q,$)Q,)QCIQ$Q:$$HQ$FF),O$FQ*),))FQL,,ICQ)C)!)F,LQH)Q,$Q,)Q $$Q,)FLOC)Q L,)F:,Q $Q !IL$F Q,) )L)F)Q $Q CI:)Q$)FOF:LQ IFHQ *)CO)LQ C$F:E,)!Q )I? QC$IHLQIFHQH),)O$I,O$FQ$Q!IL$F Q,$:Q,)Q)F,O)LQ;$:)LLO?)QHI!I:)Q,I,Q $CHQ C)IHQ ,$Q IQ LHH)FQ $CCIL)Q * Q IFQ )A))HOF:Q $Q O,LQ $!)LLO?)Q L,)F:,Q C$ICQ LO,)Q

)),LQ IFHQ L$OCQ L,,)Q OF,)I,O$FQ H FI!OQ I,O$FLQ :)F)I,)HQ * Q *)CCLQ LOF:OF:Q IFHQ H)OFO,O?)C Q ,)Q !$L,Q O!$,IF,Q IL),Q ,)Q OFECIF)Q IFHQ $,E$ECIF)Q *)I?O$Q $Q ,)Q ,$)Q HOF:Q )I,7I)Q :$FHQ LIOF:# OLQ OLQ ,)Q OL,Q L,I:)Q $Q IF OF,)FI,O$FICQ LO)F,OOQ )L)IQ *),))FQ ,)Q 5FO?)LO, Q $Q IFL)O:Q IFHQ ,)Q 5FO?)LO, Q $Q 3C$)F)Q IO!)HQ ,$Q H)?)C$Q IQ !),$H$C$: Q ,$Q !O,O:I,)Q ,)Q L)OL!OQ OLQ $Q IFO)F,Q !IL$F Q ,$)LQ C$I,)HQ OFQ L)OL!OQ $F)LQ O,Q ,)Q L)Q $Q )L,)LLOF:Q H)?O)LQ $Q L!I,Q !I,)OICL# ))$)Q)LC,LQ7O,)QO!$,IF,QI H))QFH)L,IFHOF:QIFHQOH)F,OOI,O$FQ$QICCQ ,)Q!$L,QO!$,IF,QIL),LQ,I,QH),)!OF)Q,)QL)OL!OQ?CF)I*OCO, Q$Q$CH !IL$F Q,$)LQ OFQ ,)!LQ $Q *)I?O$Q IFHQ IOC)Q !)IFOL!L#Q 3$Q IO)?OF:Q ,OLQ O,Q OLQ $FLOH))HQ ,)Q )C)?IF,Q CO,)I,) $*L)?)HQ HI!I:)Q I,)Q )ICQ )I,7I)LQ IFHQ !IOFC Q )F:OF))OF:Q )A)O)F)#

Slenderness

1$*I*C Q ,)Q LOF:C)Q !$L,Q H)OLO?)Q I,$Q I),OF:Q ,)Q L)OL!OQ *)I?O$Q $Q IQ ICCQ OLQ O,LQ LC)FH)F)LLQ$!!$FC Q )A)LL)HQ OFQ,)!LQ $Q IL),Q I,O$Q;G#Q GO:Q LC)FH)F)LLQ ICCLQ ;GQBQ8QI)QII,)O)HQ* QIQH,OC)Q*)I?O$QIOCOF:QOFQIQ)H$!OFIF,QC)AICQ!$H)Q LO!OCIQ,$Q,I,Q$Q*)I!L#Q2O))F,Q,$Q,OLQOFQC$QLC)FH)F)LLQ$Q$!I,QICCLQ;GQQQ ,)QI,$QH$!OFI,OF:Q,)QL)OL!OQ)$!IF)QOLQL)IQ1)F)COLQIFHQ9I$L 

@#Q

Figure 2 3IOC)Q!$H)LQ$QLC)FH)Q!IL$F QL,,)L4 C)AO$FQL)IQ$OF:Q* Q*IL)Q

CO,OF:QIFHQ$OF:Q* Q$FHI,O$FQCO,OF:QIIFQIFHQN)COQ866

.P23Q 866MQ IFHQ IIFQ IFHQ N)COQ 866Q IO!Q ,I,Q ,)Q L)OL!OQ *)I?O$Q $Q ICCLQ HO)LQ O!$,IF,C Q H))FHOF:Q $Q ,)OQ LC)FH)F)LL#Q P$!I, ICCLQ ;GQ Q 8Q I)Q H$!OFI,)HQ !IOFC Q * Q L)IQ )),L#Q  Q ,)Q $,)Q IFHQ LC)FH)Q ICCLQ ;GQ BQ 8Q *)I?)Q !IOFC Q ILQ IF,OC)?)Q *)I!LQ O,Q :)F)ICC Q C$Q ?),OICQ C$IHOF:Q H$!OFI,OF:Q !IOFC Q ,)Q )),Q $Q C)AO$F#Q (Q GQ JQ MQ ,)Q L,,)Q $CHQ *)Q $FLOH))H ILQ )A)LLO?)C Q LC)FH)Q *)OF:Q,OLQ,)QIL)Q$Q!$L,Q$Q,)QOL,$OICQ!IL$F Q,$)LQ;3O:)QM#QOLQ$CHQIL)Q O,LQ IOC)Q * Q C)AO$FQ L)IQ $?),FOF:Q * Q OFL,I*OCO, Q IFHQ ,IFL!OLLO$FQ $Q )C)?I,)HQ ?),OICQC$IHLQ,$Q,)Q$FHI,O$FQIFHQL$OCQILQH)O,)HQOFQ3O:)Q8#

Boundary conditions

)Q$LO,O$FQ$QIQOL,$OICQ!IL$F Q,$)QOFQ,)Q*IFQ$F,)A,QOLQIQ?) QO!$,IF,QIL),Q ,I,Q OFC)F)LQ ,)Q ?CF)I*OCO, Q $Q ,)Q L,,)Q "))Q ),Q IC#Q 866#Q )L)Q *$FHI Q $FHO,O$FLQ ;L))Q 3O:)Q Q $CHQ L,$F:C Q !$HO Q O,LQ L)OL!OQ *)I?O$Q IFHQ ,$Q I?)Q *O:Q O!I,QOFQ,)Q:)F)I,O$FQ$QHO))F,QIOC)Q!$H)L#Q.$FEOL$CI,)HQ,$)LQ))Q$!!$FC Q *OC,QILQIQI,Q$QIQQ$QF)A,Q,$QIF$,)Q*OCHOF:#Q)Q)L)F)Q$QIH0I)F,QICCLQ$Q

façades with different height than the tower and the lack of connection between elements due to the poor tensile strength of masonry could generate during an earthquake a detachment of the different bodies, vibrating independently and hitting between them generating serious damages.

Figure 3 Position of the tower in the urban context [Sepe et al., 2008]

Curti et al. [2008] observed in 31 Italian bell towers (16 isolated and 15 with one or two sides shared with the church) damaged by the 1976 Friuli earthquakes (May M6.4 and September M6.1), that the presence of walls and façades adjacent to any tower at different heights are horizontal constraints that increase the seismic vulnerability of the tower by limiting its slenderness and by creating localised stiffening zones that could cause the concentration of important stresses.

Long term heavy loads

Historical masonry towers were built as most of the historical buildings to withstand mainly the vertical loading generated by their self weight. The walls thicknesses used to be determined following empirical rules transmitted from generation to generation by trial and error depending mainly of its height (in some cases taller than 60 m) and observed damages after earthquakes. This led to the construction of walls with enormous thicknesses, in some cases bigger than 2.0 m. The roof system of historical masonry towers was made commonly of the same material of the walls, even when reduced thicknesses were considered, the elevated mass of masonry generated problems of instability that could lead to its collapse even during the construction works. Due to this, and by architectonic aspects is very frequent to find especially in Italy masonry towers with a plane roof system integrated by wooden beams and fired-clay bricks. In Germany the masonry towers have commonly a triangular timber roof covered externally by thin plates made of metal (copper). By the other hand, in Mexico it was utilized frequently fired-clay bricks and in some cases to make lighter the roof system it was built with volcanic stones of low density and placed in some cases into the structural element a great number of artisanal clay vessels.

Figure 4 D)COIQ$Q,)Q$CCIL)HQ*)CCQ,$)Q$Q1OIIQ"IFQNI$=QOFQ)FO)Q(,IC

GOL,$OICQ!IL$F Q,$)LQI)QLC)FH)QL,,)LQFH)QO:Q?),OICQC$IHOF:#QOLQOLQH)Q ,$QO,LQ)O:,QICCQ,OF)LLQ,)Q)L)F)Q$QIQ$$QL L,)!Q,)QO:QH)FLO, Q$Q!IL$F Q IFHQ)I? Q*)CCLQC)IHOF:Q,$QIQ$F)F,I,O$FQ$QO:Q$!)LLO?)QL,)LL)LQI,Q,)OQ*IL)#Q<CCQ ,)L)QOLL)LQIFHQ!$)$?)Q,IOF:QOF,$QI$F,Q,)QH),)O$I,O$FQ$Q!IL$F Q,$:Q,)Q )F,O)LQ;$:)LLO?)QHI!I:)Q!I)Q,)QOL,$OICQ!IL$F Q,$)LQ)A,)!)C Q?CF)I*C)Q ,$QL)QIQLHH)FQ$CCIL)Q* QIFQ)A))HOF:Q$QO,LQ$!)LLO?)QL,)F:,Q$QOFQL$!)QIL)LQ ,)Q IOC)Q $Q ,)Q $FHI,O$FQ $Q ,)Q L$OC#Q )L)Q LHH)FQ $CCIL)LQ I?)Q *))FQ $OF:Q LOF)Q)F,O)LQI:$QOFQ,OLQ, )Q$QL,,)L#Q)Q!$L,QI!$LQIL)LQI)Q)$,)HQ)#Q:#QOFQ OFHIQ),QIC#Q

8QNIOQ

Q"Q

QIFHQOFHIQ866#Q) Q$)L$FHQ,$Q ,)QLHH)FQ$CCIL)LQ$Q,) *)CCQ,$)Q$Q1OIIQ"IFQNI$=Q)FO)QOFQ
68Q;IQ)COIQ ILQ*OC,QILQH)O,)HQOFQ3O:)QMQ,)QO?OQ,$)Q$Q1I?OIQOFQ

QIFHQ,)Q*)CCQ,$)Q$Q ,)QQ$Q",#QNIOIQNI:HIC)FI=QOFQ$Q)!IF QOFQ

8#

Local site effects and soil-structure interaction

")OL!OQ IIHQ II,)OL,OLQ IFHQ L$OCQ $FHO,O$FLQ $Q ,)Q LO,)Q I)Q O!$,IF,Q IL),LQ,I,Q H),)!OF)Q,)Q?CF)I*OCO, Q$QOL,$OICQ!IL$F Q,$)L#Q")OL!OQIIHQ$QIQ),IOFQLO,)Q OLQ ,)Q $*I*OCO, Q $Q $)F)Q $Q IFQ )I,7I)#Q OLQ H))FHLQ $FQ O,LQ $AO!O, Q ,$Q IQ L)OL!OQ L$)Q O,Q )?)F,LQ $Q )F$:Q !I:FO,H)Q ,$Q :)F)I,)Q LO:FOOIF,Q L)OL!OQ OF,)FLO,O)LQ I,Q ,)Q LO,)Q FH)Q L,H #Q )Q L$)Q $Q ,)Q )I,7I)LQ OLQ H)Q !IOFC Q ,$Q ,)Q )C)IL)HQ)F): Q:)F)I,)HQ* Q,)QI*,Q!$?)!)F,LQ$Q,)Q,),$FOQCI,)LQ$Q,)Q)I,/LQ L,Q)L)F,)HQOFQ,)Q$F)Q$Q$F,I,Q*),))FQCI,)LQ$QOFQ:)$C$:OICQIC,LQOFLOH)Q$QIQ CI,)#QI,7I)QLIOF:QH))FHLQL,$F:C Q$Q,)Q:)$,)FOICQ$FHO,O$FLQ$Q,)QLO,)QOFQ ,)!LQ$Q:)$C$: Q,$$:I QIFHQL$OC#Q)QO, Q$Q)F$,O,CIFQ;F$Q,)QOL,$OICQ)F,)Q $QN)AO$QPO, QILQ*OC,Q* Q,)Q<,)Q)!O)Q$FQIOL)HQOLC),LQOFQI)Q)A$$#Q2)Q ,$Q ,OLQ ,)Q L$OCQ )L)F,LQ *IHQ $FHO,O$FLQ OLQ ?) Q L$,Q IFHQ ,OLQ !$HOO)LQ ,)Q *ILOQ II,)OL,OLQ $Q ,)Q L)OL!OQ L$)Q * Q I!COOI,O$FQ $Q ,)Q :$FHQ !$,O$FQ ;O:)Q L)OL!OQOF,)FLO, Q))L)F,)HQ* QC$Q)7)FO)LQIFHQO:Q)O$HL#QOLQILQ,)QIL)Q$Q ,)Q )I,7I)Q $)HQ OFQ 
-Q ;!I:FO,H)Q #Q I,Q ,)Q 1IOOQ $IL,Q $Q NO$IIFQ N)AO$Q O,Q IL)HQ ,$LIFHLQ $Q H)I,LQ IFHQ L,$F: HI!I:)LQ ,$Q ,)Q *OC,Q )F?O$F!)F,Q

mainly in Mexico City that is located more than 350 km away from the epicenter. This low frequencies affect mainly slender structures like in the case of historical masonry towers that their fundamental vibration frequency is into the range of the earthquake frequency. The high vertical loading of the tower and its flexibility due to its slenderness generate that the structure presents during an earthquake important top displacements. By the other hand, high frequencies and low periods like those presented by an earthquake in hard soil, affect mainly compact buildings.

Figure 5 General view of the leaning tower of Pisa, Italy

Another geotechnical issue that depends on the local site effects and the seismic action corresponds to liquefaction and instability conditions by soil settlements. This is the case of the Metropolitan Cathedral of Mexico City that has been presenting since decades important settlements due to the soft soil conditions. The most famous case presented in historical masonry towers corresponds to the leaning tower of Pisa, Italy, as shown in Figure 2.14. It started to incline since its construction in the XII century due to the irregularities in the soil conditions, being with this quite vulnerable to overturning.

Seismic behavior and failure mechanisms

The excessive slenderness of historical masonry towers (H/L > 4) is characterized by a ductile behavior, failing in a predominant flexural mode similar to that of cantilever beams. Due to all these factors and its heavy mass, the lateral vibration at the top of the tower is considerably more amplified than the one of the base, generating with this important horizontal top displacements and inertia forces transmitted in-plane and out-of-plane as a consequence of the earthquake ground shaking. This behavior could cause as previously mentioned different failure mechanisms generated by flexion, shear, base and foundation uplifting due to the transmission of elevated vertical loads or poor soil conditions (see Figure 2). Moreover these poor conditions could generate an amplification of the ground shaking and an excitation very close to the natural frequency of the tower, leading to its failure by the resonance effect. Meli [1998] describes that during an earthquake historical masonry towers present important horizontal top displacements. The flexion generates horizontal cracks but rarely the overturning of the structure. This is due to the alternation of the direction of the movement that causes an

$)FOF:QIFHQC$LOF:Q)),Q$Q,)L)QILQHOLLOI,OF:QO,Q,OLQO!I,QIFQO!$,IF,QI,Q $Q,)Q)F): QOFH)HQ* Q,)Q)I,7I)#Q Q,)Q$,)QIFHQOFQ*)CCQ,$)LQ,)Q)L)F)Q $Q CI:)Q $)FOF:LQ I,Q ,)Q *)C Q $CHQ OF)IL)Q ,)Q ?CF)I*OCO, Q $Q ,)Q L,,)Q *)OF:Q !$)Q)7)F,QO,LQIOC)Q* QL)I#

Figure 6  OICQIOC)Q!)IFOL!LQ$Q*)CCQ,$)L4 ;), N)COQ

Q;DO:, ),LQ

$Q,)Q@# Q!I:FO,H)QP$CO!IQN)AO$ )I,7I) $FQIFI Q8L, 866

2)Q ,$Q ,)Q L,$F:Q HI!I:)Q ,)Q *)C Q $CHQ $CCIL)Q * Q OFL,I*OCO, Q )FHIF:)OF:Q ,)Q IH0I)F,Q*OCHOF:LQIFHQ!IOFC Q,)Q)$C)Q$Q$CHQ*)QOFLOH)Q$QOFQ,)QL$FHOF:LQ;L))Q 3O:)Q #Q P,OQ ),Q IC#Q 866Q $*L)?)HQ OFQ Q (,ICOIFQ *)CCQ ,$)LQ HI!I:)HQ * Q ,)Q 
@ Q 3OCOQ )I,7I)LQ ,I,Q ,)Q *)C Q OLQ ,)Q !$L,Q ?CF)I*C)Q I,Q $Q ,)Q ,$)Q H)Q ,$Q ,)Q )L)F)Q$QCI:)Q$)FOF:LQC)IHOF:Q,)QOCCILQ,$Q*)QLC)FH)QIFHQ* Q,)Q,$Q!ILL)L#QOLQ I!COO)LQ,)QL)OL!OQ!$,O$FQILOF:QO,OICQ)),LQOFQ,)QO:)QI,LQ$Q,)Q,$)#

Dynamic actions by bells swinging

(FQ !IL$F Q *)CCQ ,$)LQ )LC,LQ 7O,)Q $!!$FQ ,)Q )L)F)Q $Q CI:)Q IFHQ )I? Q *)CCLQ IF:OF:Q$!Q,)OQ)L),O?)QL$,QIFHQIF$)HQOFQHO))F,QCI)LQI,Q,)Q*)C #Q

Figure 7 )Q*)CCQ,$)Q$QNI,OCH)= OFQ1OLIQ(,IC 4 C$I,O$FQ$Q*)CCLQI,Q*)C QIFHQIQ

)QLOF:OF:Q$Q,)Q)I? Q*)CCLQOFH)LQH FI!OQI,O$FLQ,I,Q$CHQIL)QHI!I:)Q,$Q,)Q ,$)#Q Q,)Q$F)QIFHQ,OLQ!$,O$FQ:)F)I,)LQI,Q,)Q*)CC/LQL$,Q)C)?I,)HQ?),OICQIFHQ $O$F,ICQOF),OIQ$)LQ,I,QI)Q,IFL!O,,)HQ,$Q,)QL,,)#QP$FLOH)OF:Q,I,Q!$L,Q$Q,)Q ,$)LQ ))Q *OC,Q !IOFC Q ,$Q O,L,IFHQ ,)OQ ?),OICQ C$IHOF:Q )LC,LQ !$)Q O,OICQ ,)Q I,O$FQ$Q,)QOFH)HQ$O$F,ICQ$)LQ,I,Q$CHQ:)F)I,)QIOF:Q$Q,)QL)II,O$FQ$Q L,,ICQ )C)!)F,LQ H)Q ,$Q ,)Q C$Q ,)FLOC)Q L,)F:,Q $Q !IL$F Q ;3O:)Q@#Q  Q ,)Q $,)Q IFHQ ,)Q )AO,I,O$FQ OFH)HQ * Q ,)Q LOF:OF:Q $Q *)CCLQ $CHQ *)Q ?) Q C$L)Q ,$Q $F)Q $Q ,)Q FI,ICQ)7)FO)LQ$Q,)Q,$)QC)IHOF:Q,$QIQO:QH FI!OQI!COOI,O$FQ$Q,)QL,,ICQ )L$FL)Q * Q ,)Q )L$FIF)Q )),#Q 3$Q !$)Q H),IOC)HQ OF$!I,O$FQ I*$,Q ,)Q H FI!OQ I,O$FLQ* Q*)CCLQLOF:OF:Q,)Q)IH)QOLQ)))HQ,$Q)$FOFOQ),QIC#Q866Q)FFI,OQ),QIC#Q 8668QIFHQ(?$IQIFHQ1ICCI)LQ866 #

DISCUSSION

OLQI)Q)L)F,)HQIQ*O)QH)LO,O$FQ$ ICCQ,)Q!$L,QO!$,IF,QIL),LQ,I,QH),)!OF)Q ,)Q L)OL!OQ ?CF)I*OCO, Q $Q IFO)F,Q !IL$F Q ,$)LQ OFQ ,)!LQ $Q *)I?O$Q IFHQ IOC)Q !)IFOL!L# 3$Q IO)?OF:Q ,OL O,Q ILQ ,I)FQ OF,$Q I$F,Q ,)Q )C)?IF,Q CO,)I,)Q $*L)?)HQ HI!I:)Q I,)Q )ICQ )I,7I)LQ IFHQ !IOFC Q )F:OF))OF:Q )A)O)F)# <Q H))Q FH)L,IFHOF:Q $Q ICCQ ,)L)Q IL),LQ OCCQ ICC$Q LQ ,$Q H)?)C$Q IQ LO,I*C)Q !),$H$C$: Q ,$Q !O,O:I,)Q,)QL)OL!OQOLQ$QIFO)F,Q!IL$F Q,$)LQC$I,)HQOFQL)OL!OQ$F)LQO,Q,)QL)Q $Q)L,)LLOF:Q H)?O)LQ $Q L!I,Q !I,)OICL#Q OLQ )L)IQ $)L$FHLQ ,$Q IFQ OF,)FI,O$FICQ I,OOI,O$FQ *),))FQ ,)Q 5FO?)LO, Q $Q IFL)O:Q IFHQ ,)Q 5FO?)LO, Q $Q 3C$)F)#Q <,)IHLQ ILQ IQ L)$FHQ L,I:)Q ,)Q !),$H$C$: Q OCCQ *)Q ICO)HQ $FQ ,)$),OICQ IFHQ )ICQ IFO)F,Q !IL$F Q ,$)LQ * Q !)IFLQ $Q OF,)FLO?)Q F!)OICQ LO!CI,O$FQ ,$Q ILL)LLQ ,)OQ L)OL!OQ ?CF)I*OCO, Q OFQ L,I,OQ IFHQ H FI!OQ $FHO,O$FL#Q N$)$?)Q ,)Q ,$)L OCCQ *)Q ),$O,,)HQO,QHO))F,Q)L,)LLOF:QH)?O)LQ$QL!I,Q!I,)OICLQLQILQ3O*)QD)OF$)HQ 1$C !)LQ ;3D1LQ IFHQ "I)Q N)!$ Q <CC$ LQ ;"N<L OFQ $H)Q ,$Q ILL)LLQ 7IF,O,I,O?)C OQ $Q ,)Q )L,)LLOF:Q H)?O)LQ O!$?)LQ OFQ IQ *),,)Q I Q ,)Q L)OL!OQ )$!IF)Q $Q ,OLQ, )Q$QL,,)L#

REFERENCES

#Q#QIILIFQ(F?)L,O:I,O$FQ$QF$FEL,)F:,)F)HQIFHQ$L,E,)FLO$F)HQ!IL$F Q,$)LQ OFQ IL)Q $Q )I,7I)Q C$IHOF:Q * Q !)IFLQ $Q F!)OICQ LO!CI,O$FL= NIL,)Q ,)LOLQ )FOICQ5FO?)LO, Q$QIFL)O:Q)!IF Q866
#

8#Q#Q#Q1)F)COLQ<# #Q9I$LQI,7I)E)LOL,IF,Q$F),)QL,,)L= 1*COL)HQ* QQ IFHQ3.Q"$FQIFQO!OF,Q$QPI!IFQIFHQGICCQ$FH$FQ"QG.Q59Q

@#

#Q .P23Q P$!C)!)F,I Q )FOICQ .$!LQ $Q ,)Q N)AOIFQ P$FL,,O$FQ P$H)QQQQQQQQQQQQQQQQQQQQQ ;OFQ"IFOL= N)AO$Q866M#

M#Q#QIIFQD#QN)COQ")OL!OQH)LO:FQ$Q*OCHOF:LQ;OFQ"IFOL=QO!LIQN)AO$Q866# -#Q#Q"))Q#Q")IFIQ<#QOL$?OQ<Q!),$HQ$QCI:)ELIC)Q?CF)I*OCO, QILL)LL!)F,Q

$QOL,$OQ,$)L= ",,ICQ$F,$CQIFHQ)IC,Q!$FO,$OF: -4
EM- 866#

#Q#QP,OQ"# 1I$HOQ"#Q1$H)L,I "O!COO)HQ!$H)CLQ$QL)OL!OQ?CF)I*OCO, QIFIC LOLQ$Q *)CCQ,$)L= 1$))HOF:LQ$Q ,)Q ,Q (F,)FI,O$FICQP$F))F)Q $FQ",,ICQ <FIC LOLQ $QGOL,$OICQP$FL,,O$FLQ;"<GPQC Q8EMQI,Q59Q866#

@#Q#QOFHIQ#QI,,OQ#QNIF:IF$QP#Q1$::OQ#Q"IOEIFHOIFOQ)Q$CCIL)Q$Q,)Q O?OQ,$)Q$Q1I?OI4Q<QL?) Q$Q,)Q!I,)OICLQIFHQL,,)= NIL$F Q(F,)FI,O$FICQ E86Q

8#

#Q #Q NIOQ N$FO,$OF:Q !)HO)?ICQ L,,)LQ OFQ 1I?OI= ",,ICQ F:OF))OF: (F,)FI,O$FICQ(
Q

#
#Q" )FOICQ$OFO$FQI*$,Q,)Q$CCIL)Q$Q,)Q*)CCQ,$)Q$Q",#QNIOIQNI:HIC)FIQ OFQ$Q)!IF = IF,),QF:OF))OF:Q",HO$Q

# 6#Q#QOFHIQ)IFOF:Q$!QIOC)4Q$F:E,)!Q*)I?O$Q$Q)I? Q!IL$F QL,,)L= 1$CO,)FO$QHOQNOCIF$Q(,IC #Q1*COL)HQ* QK(Q)LLQQ866#

#Q D#Q N)COQ ",,ICQ )F:OF))OF:Q $Q ,)Q OL,$OICQ *OCHOF:LQ ;OFQ "IFOL= PO?OCQ F:OF))LQ<LL$OI,O$FQ;(P<Q3$FHI,O$FQ<#QP#QN)AO$Q

#

8#QN#Q#Q)$FOFOQ"#Q)FFI,OQK# "IC?I,$)Q",,ICQII,)OI,O$FQ$QIQ!)HO)?ICQ *)CCQ ,$)4Q 3OL,Q OL,$OICQ )A)O!)F,ICQ IFHQ F!)OICQ OF?)L,O:I,O$FL= 1$))HOF:LQ $Q ,)Q HQ (F,)FI,O$FICQ ")!OFIQ $FQ ",,ICQ <FIC LOLQ $Q GOL,$OICQ P$FL,,O$FLQ ;"<GPQO!II)LQ1$,:ICQ866#

#Q"#Q)FFI,OQK# "IC?I,$)Q# .IHOFOQN# 2)CCIQNI::O$IQ)CC/LQH FI!OQI,O$FQ$FQIQ OL,$OICQ !IL$F Q ,$)= 1$))HOF:LQ $Q ,)Q M,Q (F,)FI,O$FICQ P$F))F)Q $FQ ",,ICQ2 FI!OLQ;5D2 .QNFOQ)!IF Q8668#

M#Q "#Q (?$IQ 3#Q #Q 1ICCI)LQ 2 FI!OQ OF?)L,O:I,O$FLQ $FQ IQ !IL$F Q *)CCQ ,$)= F:OF))OF:QL,,)L 84Q 6E @Q866 #