CSRN Tonji UBC Symposium August 19, 2013 W.D Liam Finn and - - PowerPoint PPT Presentation

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T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A AMPLIFICATION EFFECTS OF THIN SOFT SURFACE LAYERS: A STUDY FOR NBCC 2015 W.D. Liam Finn The University of British Columbia, Vancouver, BC, Canada Francisco Ruz R y V

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T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A

AMPLIFICATION EFFECTS OF THIN SOFT SURFACE LAYERS: A STUDY FOR NBCC 2015

W.D. Liam Finn

The University of British Columbia, Vancouver, BC, Canada

Francisco Ruz

R y V Ingenieros, Santiago, Chile

CSRN – Tonji – UBC Symposium

August 19, 2013

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MOTIVATION FOR STUDY

NBCC 2010 recommends that the classification of shallow surface layers less

than 30m thick for amplification potential be based on the properties of the surface layer, in particular the average shear wave velocity, Vs.

There was little data to support this recommendation. The extensive data

from the 2011 Tohoku earthquake provides the opportunity to evaluate this recommendation

Prior to NBCC 2010 there was no specific recommendation but practice was to

base the classification on Vs30 even though this meant including thicknesses of rock or stiff soil to make up the 30m. There was one restriction the surface layer could not be classified higher than Site Class C ( Vs30 in range 360m/s to 760m/s.

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THEORETICAL BASIS OF AMPLIFICATION ar

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ANALYSES OF TOHOKU BOREHOLE SITES

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ANALYSES OF TOHOKU BOREHOLE SITES

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ANALYSES OF TOHOKU BOREHOLE SITES

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ANALYSES OF TOHOKU BOREHOLE SITES

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MEAN SPECTRAL AMPLIFICATION

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MEAN SPECTRAL AMPLIFICATION

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MAXIMUM SPECTRAL AMPLIFICATION

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TYPICAL SCATTER OF FIELD RESPONSE DATA

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MEAN SPECTRAL AMPLIFICATION

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BORCHERDT (1994) AMPLIFICATION FACTORS

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FINN-RUZ RESULTS

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FOURIER SPECTRA AMPLIFICATION FACTORS

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FUNDAMENTAL SITE FREQUENCY VS MEAN SHEAR WAVE VELOCITY (Vs30)

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REGIONAL SITE AMPLIFICATION FACTORS

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CONCLUDING REMRKS

Average shear wave velocity in a soft surface layer significantly less than 30m

thick is not a reliable indicator of amplification potential.

A strong correlation was found between Vs30 and average amplification

factors for the thin, soft surface layers.

Very strong correlation between the period of the surface layer and Vs30.

Ghofani et al (2012) found a similar correlation with the entire borehole period.

For the Tohoku borehole sites, the average Fourier amplification factors in the

short period range T = 0.1- 0.5s are much higher than the longer period amplification factors in contrast to the findings by Borcherdt (1994) for Loma Prieta data which were recorded on much thicker deposits.