EVALUATION OF INPUT SEISMIC MOLTION LOSSAND STRUCTURE RESPONSE USING STRONGGROUND MOTION RECORDS
Samuel E. Sevilla Cano ∗MEE09194

Supervisor:

Toshihide KASHIMA ∗∗ABSTRACTA new technique to evaluate the real input motion called the soil structure interaction (SSI) appeared;
this new tool on structural analysis has clearly shown that a reduction of the motion occurs when abuilding is on a specific study site compared with free field surface questioning the fixed base design.
Now, the discussion is, if this reduction is so important or it could be neglected in structure
design, although the fixed base design have been considered as an overestimation on the seismic design
which assure the structure compared with SSI.
The input motion reduction has economic meaning because it means a reduction of base shear
force and overturning moment acting on the structures reducing the structural elements size.
This study presents a verification of the input motion reduction on the foundation using strong
ground motion records and also presents how the input motion reduction changes the building responsecompared with that considered in the fixed base design. The effect of the foundation depth was also
evaluated by using a numerical method which considers the frequency domain soil response on a
massless system. The reduction of the motion was observed and it becomes more important as thefoundation depth increases.
Keywords: Input motion, Spectral ratio, Frequency domain, Soil Structure Interaction

1. INTRODUCTION
Nowadays Soil Structure Interaction (SSI), has become one of the most important concept to evaluate
the real behavior not only of new structure but also existing ones, and it has generated new issues ofresearching related to seismic design, also have awaken the interest of researchers on this new field ofstructural design.
One of the last issue in this field is the evaluation of seismic input motion on structures; thegeneral discussion presented in some researches is, if the input motion decrease or increase related tothe traditional input motion used in structural design described in many seismic codes and, if the input
motion evaluated in soil structure interaction is beneficial or not to the structural design. Actually thetechnique used to evaluate directly this phenomenology is through geotechnical instrumentation and it
consists of installing devices of seismic motion record on the buildings, beneath and around it.
Strong ground motion records devices are used for evaluating soil structure interaction, they give
results of the building response including the site effect which is referred to the wave propagation in
the immediate vicinity of the site; and propagation effect which is related to the complete path fromsource to receivers. Strong ground motion itself is the final product of source, path and site effects.

∗

Centro de Investigaciones Goescientificas CIGEO/UNAN-Managua, Nicaragua.
Research Engineer, Building Research Institute, Tsukuba, Japan.

∗∗

12. PREVIOUS RESEARCH AND AWARNESS OF THE ISSUE
The input seismic motion has been one of most important issues treated recently in earthquake
engineering. In many researches, the results have shown that reduction on the input motion occurs andit have been considered as a benefit to the building design because the force and overturning moment
acting on the structures are reduced. However for other ones it is not, but rather increases the
responses of the buildings when soil structure interaction is considered.
Some researchers consider that SSI may indeed hold for a large class of structures and seismic
environments. However, it is not always because there is evidence documented in numerous historiccases that the perceived beneficial role of it is an unjust over simplification that may lead to an unsafe
design of both the superstructure and the foundation.
The design spectra presented in most seismic codes around the world are based on a statisticalanalysis of the response spectra for the ensemble of ground motions. The problem is that it does notresemble the spectra recorded on deep soft soil which increase the fundamental period when SSI istaking into account. This effect also leads to an increased response which contradicts the expectation
incited by the conventional design spectra.
The traditional design spectra hold that an increment in natural period of the building (anddamping) due to soil deformability leads almost invariably to smaller accelerations and stresses in the
structure and its foundation.
Other discussing point is the traditional design demand spectrum. It differs from those whensoil structure interaction is considered. For getting the demand spectrum a series of oscillators are used
and they are forced using the same input motion, but when ground motion is calculated or determinedin the middle of foundation the resulting demand spectrum concern only to a specific structuraldynamic characteristics. Only one point of the curve is correct which corresponds to the effective
structural period, so another structure with different demand characteristics will have a different
demand spectra curve. To vary the input motion for every oscillator in order to consider the realcondition of the structures is the most important action to solve this problem.
The analysis point of the previous researches turns around to the importance of evaluating the
input motion as an indicator of safe buildings design.

3. TARGET BUILDINGIn this research, the annex building at Building Research Institute (BRI) was chosen. It is located in
Tsukuba City, Japan, about 50 km northeast of Tokyo. The building lies on diluvial heights
between the Sakuragawa River flowing into the Kasumigaura Lake and the Kokaigawa River.
The annex building is a steel reinforced concrete structure with nine stories, eight stories
above the ground level and one story under ground. It is supported by a mat foundation in 8m depth ona sandy clayey and clay layer, with a floor area of approximately 546 m2 on every story and total highof approximately 43.05 m (see Fig. 1).

Figure 1. Target building

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The monitoring system consist of 11
sensors on annex building, 7 insurrounding ground (see Fig. 2) and 4 in
the main building of BRI.

The building lies on diluvial heights between the
Sakuragawa River flowing into the Kasumigaura
Lake and the Kokaigawa River. The subsoil
properties beneath the building were identified
until 88m depth where a relatively hard gravel
layer was found (see table.1).

8FE

8FN

8FS

5FW

5FE

2FW
1FE

2FE

Table 1. Soil profile beneath the target building
BFE

BFN
N14

20 m

Loam
Sandy Clay &Clayey Sand
BFS Sandy Clay
& ClayFine Sand &Clayey Sand

50 m
A01

100 m
B01

C01

A14

Sandy Clay
& ClayA43

GravelSandy Clay
& ClayThe target building was instrumented since 1998
with a detailed strong ground motion observation
system on, around and beneath it, just whenconstruction of the building finished.

Gravel &Fine Sand
Sandy Clay
& ClayGravelA89

Figure 2. Monitoring System on the target
building

4. DATAy

Six hundred fifty eight (658) strong groundmotions record were used. They were recorded
since 1st of June, 1998 up to 7th of May, 2010
with magnitude range between 2.6 and 8.2, andpeak ground acceleration (PGA) range
between 0.4 and 74.3.
The earthquakes were divided using
the Japan Meteorological Agency Intensity(IJMA) and earthquakes with intensity III wereselected to evaluate the input seismic motion
loss whose magnitudes vary from 4.6 to 7.1
and peak ground acceleration (PGA) from 7.9
to 74.3. These earthquakes represent the 5% of
the all records (see Fig. 3).

1%

5%
16%

36%

42%
0

I

II

III

IV

Figure 3. Strong ground motion record divided
by JMA Intensity Scale

5. THEORY AND METHODOLOGYOne of most common form to explain the loss of the input seismic motion on the structures iscomparing the motion on free field surface again that motion occurred when the building is on it.
In a general sense, decrease in the motion is expected when the building lie on the ground.
There are some parameters which quantify this reduction considering the ground and building
characteristics of a specific site.
Light structures lying on stiff soil will transmit little energy and the motion considered on
response calculation due to earthquake will be same as that experiment on free field surface and also
the motion of rigid soil will not interact with the dynamic response of building. This consideration isused on fix base design described in many seismic codes.

3Different behavior is observed when a heavy structure lies on soft soil, in this caseconsiderable energy is transferred by the building to soil, so the motion to consider on building design
will be different from that one observed on free field.
In this study the input motion was firstly evaluated considering the peak ground acceleration,
peak ground velocity and seismic intensity got in the sensors installed on foundation and free fieldsurface. Two case were considered, first one correspond to earthquakes (52 records) occurred atepicenter distances between 7 and 370km, and hypocenter distances between 2 and 20km. The second
one correspond to earthquakes (29 records) occurred at epicenter distances between 26 and 476km, andhypocenter distances between 80 and 100km.
The input motion was also evaluated through the spectral ratio between motions recorded on
foundation and free field ground. Finally the input motion on foundation was calculated using the finite
element method which also included the effect of foundation embedment in the input loss. A computer
program based on numerical analysis called Finite element calculation for irregularly layered Soil in
2D analysis (FEIS2D) was used. It was developed by Professor Dr. Masayuki Nagano from Tokyo
Science Institute; which is based on linear equivalent site response of wave propagation.

6. RESULTS AND DISCUSSION
6.1 Peak Ground Acceleration (PGA)
Peak ground acceleration is used sometimes to characterize the earthquakes and it give an idea how
strong they are, but must be considered that light or important reduction of the input seismic motion
will depend on the characteristics of the ground motion. In the first case, the 90% of the records
showed decrease of PGA values between 10% and 50% compared with those recorded on free fieldsurface, and for second case the 86% showed reduction of PGA values between 40% and 60%.
6.2 Peak Ground Velocity (PGV)
PGV is a better parameter to characterize strong ground motion than PGA because it is less affected by
the frequencies contents produced by strong shaking. It indicates the capacity of ground and building
for dissipating energy produced in high frequencies. In the first case the 63% of the records showeddecrease values between 1% and 20%. Because suddenly shaking, building can not dissipate enoughenergy on high frequencies contents, but in the second case the 69% of the records show decrease of
PGV values between 20 and 40%.
6.3 Seismic Intensity
Unlike of the seismic intensity used around theworld so far, Japan Meteorological Agency
Intensity (IJMA) is a parameter used as aquantitative measure of earthquakes and it isbetter than PGA and PGV because alreadyinclude the distribution of all peak amplitude
on the frequency domain.
In the first case the 64% of the data
showed decreasing of IJMA between 1 and 20%.
In the second case only the 69% of records
showed decrease of IJMA between 20 and 100%(see Fig. 4); all parameters taken into accountto evaluate input motion loss showed that
reduction of seismic motion occurs. It becomes

more notable as epicenter and hypocenter
distance and earthquake intensity increase.

Figure 4. Decreasing of seismic intensity4

6.4 Spectral Ratio
Motion reduction was quantified through the spectral ratio between the motions on the foundation andfree field surface. It was observed beginning on 1.3 Hz approximately on both directions (see Fig. 5a).
This reduction occur when building begin to vibrate in its fundamental period. The input motion
reduction is very important in short frequencies because most building have its fundamental vibration
frequency less than 1.5Hz.
Important reduction can be observed on high frequencies higher than 2Hz which are not only
directly related to the fundamental frequency of the building but also with the different vibration
modes of the structure. The first three fixed base vibration frequencies of the target building wereconsidered on the motion reduction. Decreasing of input motion for these frequencies are 12, 36 and
46% on N – S direction, and 6, 47 and 49% on E – W direction. Their influences in the total response
considering the modal analysis were 6, 4 and 3% in the N – S direction (see Fig. 5b) and 4, 6 and 5%
in the E – W direction (see Fig 5c).
Spectral ratio BFS/A01

80

100

80

N-S

N-S

70

70

60

60

50

50

Factor

Factor (%)

10

1

Factor (%)

E-W

40
30

40
30

20

20

10

10

0

0

0

0.1
0

1

2

3 4 5 6 7
Frequency (Hz)

8

E-W

1

2

9 10

3

4 5
Modes

Fix Base Modes

6

7

8

9

0

1

SSI Modes

2

3

4 5
Modes

6

Fix Base Modes

7

8

9

SSI Modesa)b)c)Figure 5. Input motion reduction and its influence in building total response
As is observed on Figures 5b and 5c, the influence of the motion reduction on the total
response is not large when theses vibration frequencies are considered in the modal analysis.
6.5 Effect of foundation embedment depth on input motion loss
case of 12m it appeared early on 1.8Hz
between 26 and 75% (see Fig. 6). Generally
speaking, a decreasing of the input motion isobserved as embedment foundation depth
increase.

There are various parameters whose influenceson the building response, which are layering of
the site, the flexibility of the base andfoundation embedment. The latter was
evaluated considering different foundation
depth 0m, 2m, 4m and 12m in order toevaluate its influence on the input motion.
Decrease of the motion was notobserved on foundation in case of embedment
depth is 0m. It was due to decreasing of thedamping because only the bottom area offoundation reflex the wave that come on it,
which mean the contact area with grounddecrease.
In case of 2m important reduction
between 23 and 60% in frequencies higher
than 5Hz was observed, in case of 4m, it
appears since 3Hz between 25 and 70%. In

Input Motion on Foundation
100

0m/A 01
2m/A 01
4m/A 01
12m/A 01

Factor

10

1

0.1
0

1

2

3 4 5 6 7
Frequency (Hz)

8

9 10

Figure 6. Effect of embedment depth in the
motion reduction

57. CONCLUSIONS
Input seismic motion loss on the foundation was verified using strong motion records from themonitoring system of the building. A decrease of the motion occurs in the higher frequency than 1.3Hz
which is the fundamental frequency of the target building.
A decrease of the motion with increase of the foundation depth was observed. This reduction
can be attributed to an increment of the contact area between the foundation and the ground, so
increment of the radiation damping is generated.

8. RECOMMENDATION
Clear evidence of reduction of the motion on foundation was observed in this research. To generalize
the obtained results, the following subjects should be considered:i.
ii.

Evaluation of the foundation input seismic motion loss on buildings supported by different
foundation systems to identify in more detail its effect on reduction of the motion.
Development of two dimensional dynamic response analysis of soil or site response in time
domain, in order to take into account the irregularity of the boundary layers. Strong motion
records were used in this research and no good agreement was found with linear equivalentfrequency domain response because the supposition of the constant shear modulus anddamping values, so soil nonlinearity must be taken in to account.

AKNOWLEDGEMENT
I would like to express my sincere gratitude to my advisor and supervisor Dr. T. Kashima of BRI for
their continuous support, valuable suggestion and guidance during my study and to Professor M.
Nagano from Tokyo Science Institute for his collaboration in this work.

REFERENCES
Boore, D. M. (1983). Bulletin of the Seismological Society of America, Vol 73, No 6, pp. 1865-1894,
Chopra, A. K. (1995). New Jersey: Prentice Hall.
Chowdhury, I., & Dasgupta, S. P. (2009). London: CRC Press/Balkema.
Clough, R. W., & Penzien, J. (2003). Berkeley, USA: Computers & Structures, Inc.
Data, T. K. (2010. India: John Wiley and Sons Pte Ltd.
Dunnicliff J. (1988). New York: John Wiley & Sons, Inc.
Elghazouli, A. Y. (2009). London and New York: Taylor & Francis e-Library.
Elnashai A. S. & Di Sarno, L. (2008). United Kingdom: John Wiley & Sons, Ltd.
Gazetas, G. (2006). Paper presented at ECEES, Geneva, Switzerland
Ishihara, K. (1996). New York: Oxford University Press Inc.
Kashima, T., & Kitagawa, Y. (2006). Paper presented at ECEES, Geneva, Switzerland.
Katsanos, E. I., Sextos, A. G., & Manolis, G. D. (2010). Vol 30, issue 4, pp. 157-169.
Kramer, S. L. (1996). New Jersey: Prentice Hall.
Kuwata, Y., & Takada, S. (2002). Japan: Kobe University.
Lee, J. W. (2009). Michigan: University of Michigan, Library.
Madenci, E. & Guven, I. (2006). Springer, United State.
Park, D., (2004). National Science Foundation Mid America Earthquake Center, Illinois.
Pitilakis, K., Pitilakis, D. & Karatzetzou, A. (2010). Paper 7CUEE & 5ICEE, Tokyo, Japan.
Towhata, I. (2008). Berlin: Springer- Verlag Berlin Heidelberg.
Wolf, J. P. (1985). New Jersey, USA : Prentice Hall.

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