workshop questions (nov. 7, 2003)

03/24/04 NGA Workshop: Fling 1


Workshop Questions (Nov. 7, 2003)

• For the few near-fault records that include fling response, should these effects be explicitly removed before developing the initial ground motion models?

YES

• How should fling effects be removed?

WORK IN PROGRESS


20 40 60 80

-50

0

50

Time (sec)

Vel

ocity

(cm

/sec

)

TCU129, E-W

t1

t2=30 sec

t2=50 sec

t2=70 sec

File

:C

:\m

etu_

03\r

ec_p

roc\

129e

vfit_

colo

r.dr

aw;

Dat

e:20

03-0

9-10

;Ti

me:

14:2

5:09

To model fling we need to accurately recover the residual displacement. Baseline correction of recorded motion is important step.


0 20 40 60 80 100-200

0

200

400

Time (sec)

Dis

pla

cem

en

t(c

m)

TCU129, E-W

remove mean only

t2 = 30 sec

t2 = 50 sec

t2 = 70 sec

quadratic fit to velocity

low-cut filter only, fc=0.02 Hz

GPS, station AF11

File

:C:\

me

tu_

03

\re

c_p

roc\

12

9e

d4

pp

_co

lor.

dra

w;

Da

te:2

00

3-0

9-1

0;T

ime

:21

:50

:10

Although the results look physically plausible, the residual displacements can

be sensitive to t1, t2

Quadratic fit to velocity gives best fit to GPS residual displacement in this case


TCU068 displacement record baseline corrected using Boore’s method (modified Iwan) with three choices of t2. For this record there is not much sensitivity to the choice of t2.

t1: acc > 50 cm/sec2

t2(1): Boore v0

t2(2): acc < 50 cm/sec2

t2(3): 2 x t2(2) – t1


Sine-pulse fits to the three corrected displacement records. The pulse start times are all very close (33.24, 33.36, 33.42) as are the pulse periods (4.39, 4.12, 3.83). The main difference are the final displacements (617, 570, 528).


Spectral acceleration for the three tcu068 records with the sine-pulse removed, compared with SA for the original (I.e., with fling) record (magenta line).

1) There is some sensitivity to t2 between 2 and 5 sec period (difference btw red, green and black lines). Max. difference is about 60%.

2) There is a large difference compared to original record, with the fling-removed records actually being much larger than the original record up to about 6-7 sec period.


Similar analysis for tcu052 record. The sensitivity of baseline correction to t2 is greater than for tcu068. However, the sensitivity in SA for the fling-removed records is less.


Conclusions

• Baseline correction of raw record must be done with care.

• Sine-pulse model for fling is attractive because it provides a time domain parameterization and can be related to the underlying physical process.

• Response spectra for sine-pulse corrected records can show sensitivity to choice of processing parameters-

Is this acceptable? (9 m disp. at TCU068)

workshop questions (nov. 7, 2003)

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