DAMPING MODIFICATION FACTOR FOR STRUCTURES SUBJECTED TO NEAR-FAULT GROUND MOTIONS

Transcription

1 DAMPING MODIFICATION FACTOR FOR STRUCTURES SUBJECTED TO NEAR-FAULT GROUND MOTIONS Wuchuan PU, Wuhan University of Technology ( 蒲武川, 武汉理工大学 ) University, Shanghai

2 Spv/PGV The 6 th Kwang-Hua Forum on Innovations and Implementations in Earthquake Engineering Research, , Shanghai,China 1. Problem Statement 1.1. Definition of damping modification factor DMF(T, h 0, h eq ) = S(T, h eq S(T, h 0 (= 5%)) h=30% h=5% Some representative formulas DMF = ( 5 + h 10 ) EuroCode 8 (2004) DMF = DMF = 1/( Ln(h) ) 1/( Ln(h) ) 1/( Ln(h) ) Newmark & Hall (1982) (T + 1) 0.65 Lin & Chang (T + 1) 0.65 ( Lnh)T0.3 (2003) T(s) DMF = 1 + αh eq 1 + αh 0 Kasai (2003)

3 1. Problem Statement 1.2. Existing problem Most of the DMFs regulated in codes or proposed by researchers were derived from far-fault ground motions. Near-fault ground motions (forward directivity) exhibit typically long period velocity pulse, short duration, high energy (large velocity), narrow band spectra. Damping reduction effect is likely to be overestimated in case that DMFs derived from far-fault ground motions are inappropriately used in design related to near-fault ground motions, as demonstrated by researches showing that response of structures subjected to near-fault ground motions are insensitive to damping. How does DMF of near-fault ground motions differ from that of far-fault ground motions? How effective is it to increase damping to reduce response of structures under excitation of near-fault ground motions?

4 1. Problem Statement S S1 Acc Vel Fault breaking direction Fault S2 is station where pulse like motions are likely to happen due to that waves propagated from different breaking points arrive S2 at almost the same time. Route of investigation Selecting pulselike ground motions Response spectra calculation Damping modification factor calculation Investigation on influence of magnitude, pulse period Developing new DMF formulas, comparison with other DMFs

5 Vel. (cm/s) The 6 th Kwang-Hua Forum on Innovations and Implementations in Earthquake Engineering Research, , Shanghai,China 2. Selection of Near-fault Pulselike Ground Motions Pulse Indicator (PI)= 0<PI<0.85 non-pulselike; 0.85 PI 1 pulselike 1 1+e PGV ratio +20.5(Energy ratio) (Baker 2007) PGV ratio and energy ratio are ratios between residual record and original record Original record Dominant Pulse t(s) A set of (50) ground motions which are identified as pulselike are selected from 18 earthquake events with magnitude between Mw

6 2. Selection of Near-fault Pulselike Ground Motions No. Event (Year) Mag Station Comp. Tp(s) PGV(cm/s) PGA(g) PI Distance (km) 1 Northridge (1994) 6.7 Sylmar - Converter Sta SCS Northridge (1994) 6.7 Sylmar - Converter Sta East SCE Northridge (1994) 6.7 Rinaldi Receiving Sta RRS Northridge (1994) 6.7 Jensen Filter Plant JEN Northridge (1994) 6.7 Pacoima Kagel Canyon PKC Northridge (1994) 6.7 Pacoima Dam (upper left) PUL Northridge (1994) 6.7 Newhall - Fire Sta NWH Northridge (1994) 6.7 Sylmar - Olive View Med FF SYL Northridge (1994) 6.7 Newhall - W. Pico Canyon Rd. STC Loma Prieta (1989) 7.0 Gilroy - Gavilan Coll. GIL Loma Prieta (1989) 7.0 Gilroy Array #2 G Loma Prieta (1989) 7.0 Gilroy Array #3 G Loma Prieta (1989) 7.0 Gilroy - Historic Bldg. GOF Loma Prieta (1989) 7.0 Saratoga - Aloha Ave STG Loma Prieta (1989) 7.0 Saratoga - W Valley Coll. WVC Kocaeli (1999) 7.4 Arcelik ARC Kocaeli (1999) 7.4 Duzce DZC Kocaeli (1999) 7.4 Gebze GBZ Chi-Chi (1999) 7.6 TCU052 TCU052-W Chi-Chi (1999) 7.6 TCU075 TCU075-W

7 Log Tp The 6 th Kwang-Hua Forum on Innovations and Implementations in Earthquake Engineering Research, , Shanghai,China 2. Selection of Near-fault Pulselike Ground Motions LogT p =0.4104M w LogTp= M (Mavroeidis & Papageorgiou 2003) data points for distance <10km data points for distance >10km Fitted line for distance<10km Fitted line for all data Mw Log Tp vs. Magnitude

8 3. Damping Modification Factor

9 3. Damping Modification Factor: effect of magnitude To investigate effect of magnitude, ground motions are categorized into 3 groups: Moderate earthquake: Mw 6.3 Moderate to large earthquake: 6.4 Mw 6.7 Large earthquake: Mw 6.8

10 3. Damping Modification Factor: effect of magnitude To investigate effect of magnitude, ground motions are categorized into 3 groups: Moderate earthquake: Mw 6.3 Moderate to large earthquake: 6.4 Mw 6.7 Large earthquake: Mw 6.8

11 3. Damping Modification Factor: effect of magnitude To investigate effect of magnitude, ground motions are categorized into 3 groups: Moderate earthquake: Mw 6.3 Moderate to large earthquake: 6.4 Mw 6.7 Large earthquake: Mw 6.8

12 DMF exhibits dependency on earthquake magnitude. If directly use totally averaged DMF, DMF of moderate earthquake ground motions may be underestimated in long period range (overestimation of damping effect), which yields nonconservative design. DMF of large earthquake ground motions may be overestimated for longperiod structures (underestimation of damping effect), and the error is probably large. Since magnitude is approximately linearly related with pulse period Tp, dependency of DMF on earthquake magnitude is probably reflection of underlying effect of pulse period.

13 3. Damping Modification Factor: effect of Tp

14 1 When T 0.6T p, DMF d = 1 ( T T p ) ( T T p ) 3 1 ( T T p ) (h eq )( T T p ) 3 2 When 0.01T p <T<0.6T p, linear interpolation between 0.01<logT/T p <0.6 3 When T 0.01T p, DMF d =1

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16 In short period range, DMFv<DMFd; in long period range, DMFv>DMFd. As damping ratio increases, the difference between DMFv and DMFd becomes large. When T>0.6T p, DMF v = 1 ( T T p ) ( T T p ) 3 1 ( T T p ) (h eq )( T T p ) 3 When 0.01T p <T<0.1T p, DMFv=DMFv(T/Tp=0.6) When 0.01T p <T<0.1T p, linear interpolation between 0.01<logT/T p <0.1 When T<0.01T p, DMF d =1

17 The 6th Kwang-Hua Forum on Innovations and Implementations in Earthquake Engineering Research , Shanghai,China

18 When damping ratio is small, DMFd DMFa, since Sa Spa=w 2 Sd As damping ratio increases, DMFa becomes larger than DMFd, particularly in long period. DMFa/DMFd changes very smoothly with respect to T/Tp, and is very similar to DMF ratio of structures under harmonic excitations. DMF a = DMF d 1+4h 2 eq ( T Tp +0.2) ( T Tp +0.2)2

19 4. Conclusions Damping modification factor of near-fault pulselike ground motions shows great dependency on both structural period T and dominant pulse period Tp. When T/Tp>0.6, damping modification factor is affected mainly by the dominant pulse, and it is very close to that derived from response of structure under harmonic excitations. When T/Tp<0.6, damping response reduction is still relatively large but shows larger dispersion due to effect of high frequency components. The damping modification factor derived from far fault ground motions generally are usable only if T approaches Tp. Otherwise, actual damping reduction effect will be likely overestimated. Estimation formulas were developed for displacement, velocity and acceleration spectra, respectively, based on the mean of computed DMFs.

20 THANK YOU! QUESTION & COMMENTS