Synopses of Master Papers

Bulletin of IISEE, 47, 73-78, 2013

EFFECT OF THE GEOLOGICAL IRREGULARITIES ON THE
INPUT MOTION AND SEISMIC RESPONSE OF AN 8
STOREYS STEEL REINFORCED CONCRETE BUILDINGCONSIDERING SOIL-STRUCTURE INTERACTION
Supervisor: Masayuki NAGANO**

José Roberto GUERRA CARBALLO*
MEE11613
ABSTRACTThis study investigates the effects of the geological irregularities on the input motion and seismic
response of an 8-storey steel reinforced concrete building considering the soil structure interaction.
The motivation regard on the seismic response characteristics observed in the North and South sites of
the Iwaki City Hall in a distance of just 130 m during the aftershock of the March 11 th, 2011 Great
East Japan Earthquake. 2D Finite Element Models (FEM) were used to simulate soil and structural
responses considering inclined engineering bedrock beneath the building. The 2D FEM represent theseismic response in good agreement with the observed records of the aftershocks. The dynamic
characteristics of the soil responses at the North and South site are different mainly due to the soil
condition beneath each site, though, the parametric study revealed that the effect of the inclined
interface of the engineering bedrock has a minor influence on the seismic soil response. Moreover, it
was found and equivalency between the inclined engineering bedrock model and other model
considering flat-soil layers, and the input motion and seismic response of the structure were similar.
Keywords: Geological Irregularities, Inclined Engineering Bedrock, Soil-Structure Interaction, Piles.
1. INTRODUCTION
It has been recognized that the dynamic behavior of a building structure during an earthquake isconsiderably affected by the geological formation and the properties of the soil medium. In particular,
the geological and topographical irregularities in the soil structure seem to have effect on thecharacteristics of the earthquake ground motion and also on the associated structural damage ofbuildings.
In this sense, in the seismic analysis of a structure founded on rock, the motion
experienced by the foundation is basically identical to that occurred in the same point before thestructure was built. However, for soft soil sites the incident waves produce two important
modifications, the former, the free-field motion at the site without structure is strongly affected; thelatter, the presence of the structure modifies the entire scenario of the seismic response. The structure
will interact with the surrounding soils which lead to a further change of the seismic motion at the base.
This scenario becomes interesting if the geological conditions change drastically as this study presents.
2. IWAKI GOVERNMENT BUILDING SITE OUTLINE AND STRONG MOTIONOBSERVATION
2.1. Strong Ground Motion Characteristics of the Target Site
With regards to the motivation of this study, it came from the observation of the strong ground motion
records of the aftershocks from the March 11th, 2011 Great East Japan Earthquake obtained in the
Iwaki Government Building site. Example of the response spectra observed is show in Figure 1. At the*

Urban Planning Office of the San Salvador Metropolitan Area (OPAMSS), El Salvador
Professor of Tokyo University of Science, Japan**

GL-N(NS)B1F(NS)

GL-S(NS)

GL-C(NS)

10

GL-N

North site the predominant period occurs
in the short period range (0.2s), while atthe South site the predominant peak isaround 0.9s; this phenomena happens in a
distance of just 130 meters.

N

pSv(cm/s)

Iwaki City Hall
B1F
Congress Hall

1

9FL

h=5%

130.00GL-C
Annex Bldg.

2.2.
Outline of the Target Building
and Aftershock Observation

Figure 2 presents the location of the
Iwaki Government Building; thisconstruction is located in Iwaki city,
Figure 1. Response Spectrum of the North-South Direction Fukushima prefecture, Japan. It consistsin the Iwaki Government Building Site due to theof three buildings: the congress hall, the2011/07/10 Aftershock in Off Sanriku
city hall and its annex. The target
building (the city hall) consists of 8 storeys, 1 basement and 2 penthouses. The foundations are madeof steel-concrete piles resting on an irregular shape of engineering bedrock.
0.1

0

0.1

10

30

1.0
10.0
Period (s)

50

S

GL-S

3. ANALYSIS OF THE SEISMIC RESPONSE OF
THE IWAKI CITY HALL CONSIDERING
SOIL-STRUCTURE INTERACTION
MODELS3.1. 2D Finite Element Model Process
The scope of this thesis is based on the 2D Finite
Element Model of the soil and building structure. The
commercial software named Finite Element Modelingand Postprocessing (FEMAP) version 10.0.2 was used,
as well as several non-commercial programs Figure 2. Location of the Iwaki Government
Building
developed by the supervisor of this thesis Dr.
Masayuki Nagano.
In order to create the model, it
was necessary to have the geological data
and the drawing plans, and furthermore, thegeological data used in the models was not
only from the Iwaki Government Buildingsite but also from the annex construction.
Figure 3 shows the depth andthe contour curves of the engineering
bedrock assumed in the modeling process;Figure 3. Depth of Engineering Bedrock Taken in
Table 1. Mechanical Properties
the Modeling Process and Stratum Ratioof the Soil Layers
the 8 red points represent the site where the Standard
Penetration Test (SPT) was conducted. Also, it is visible
Vs
ρ
ν
Layer
h(%) how in a short distance in the north-south direction the depth(m/s) (t/m3)of the engineering bedrock varies; being shallower in the
1
110
1.6
0.494 2.0
north area. This fact leads to take the N-S direction for study.
2
150
1.8
0.461 2.0
The next step was to take the target points and
3
110
1.8
0.494 2.0
setthecontrol
point as well; i.e. GL-N, B1F, 09F, GL-C and
4
310
1.8
0.474 2.0
GL-S
and
the
control
point was B1F. Also, with regard to the
5
410
1.6
0.491 2.0
thickness of the soil layers the assumption made was a
North Site

0.2D

0.5D

South Site

0.11D
0.09D
0.5D

0.3D

0.3D

D: Engineering Bedrock Depthrelation of 20%, 50% and 30% for the north side and 11%, 9%, 50% and 30% for the left side as
Figure 3 also shows. This assumption was taken accordingly to the SPT tests obtained in the Iwaki
Government site.
Regarding the frequency, it was considered for the model to be a range from 0.1 up to
10Hz, then, the finite element sizes were obtained in accordance with the shear wave velocity of thesoil layers. The shear waves velocity, soil density, Poisson ratio and damping for all the soil layers arepresented in Table 1.
3.2. Preliminary 2D FE Model of the Soil-Structure
The superstructure was modeled as lumpmass as Figure 4 shows, the unit area wastaken as 1,566 m2 and the weight per unit
area as 1.2 t/m2. The storey stiffness was
assumed linear according to Eq. ( 1).
Where, k: stiffness of the “i”
storey, N: Number of stories, m: Mass ofeach floor, ω: Natural frequency of the First
Mode, in this case T=1 then ω=2π.

A

B

C

Stiffness(ton/m)

Mass
(ton)
1879

D
PHR
3.50

PH1

5.93E+05

4.45

RFL
9FL

1879

3.50

8FL
3.50

1.11E+06

1879

7FL
3.50

1.56E+06

6FL

1879

3.50

5FL

1.93E+06

47.01

3.50

4FL

1879

3.60

3FL

2.23E+06

3.60

2FL

1879

2.45E+06

5.60

GL

1879
1
2.60E+06
𝑘𝑖 = 𝑁 𝑁 + 1 − 𝑖 𝑖 − 1 𝑚𝜔2 ( 1)1879
2
2.67E+06
The piles as the superstructure
masses were assumed as lumped in the four
Figure 4. Geometric Characteristics of theedges. Figure 7 presents the pile geometry.
Target Building in North-South Direction
The model consisted in a half
and Storey Lump Masses
space of 200m length per 100m depth. The
model was conceived with transmitting boundaries at both sides, while at the bottom viscous boundary.
Verticalpropagating
09F
35 m
40 m
56 m
33 m
waves
arose
from theSteel+Concrete
B1F
7m
GL-N
GL-C
GL-S
bottom to the top and the40 m
D = 467mm
transfer functions were
56 m
44.0
t = 9mm
GL-N
GL-S
B1F
obtained. It is important tomention that a linear
30 m
10m
100 m
Steel
analysis was carried out
since the aftershocks are
not larger compared withthe main shock. However,
Figure 7. Pilefor the preliminary model,
200 m
Figure 7. Preliminary 2D FE Model
Geometry
only the aftershock of July
10th, 2011 was considered since the agreement of
B1F
the observed record and the 2D FE Model are not as*
good as what was expected.
N, 09F, C, S
In this sense, transfer functions were
multiplied by the band passes filtered time histories
9F
from the basement (B1F) of 4 aftershocks; with thisstep the analytical time histories in the points GL-N,S
C
GL-C, 09F and GL-S were obtained. The following N
phase was plotted these function and comparedthose with the observed records. Figure 5
B1
summarized the methodology.
B1F

5.80

B1F

2.96

8.65

10.60

8.65

29.50

V2

13.2

9.0m

17.0m

Y

X

Transmitting
Boundary

Transmitting
Boundary

Viscous Boundary

10

Observed
Acceleration(gals)

5

B1F

0

-5

Time (s)-100Analytical Acceleration
Time History

20

40

60

80

20
15
10
5
0
-5
-10
-15

100

120

140

2D FEM
Acceleration(gals)

160

180

GL-N

Time (s)

0

20

40

60

80

N-OBS.

Comparison of
2D FE ModelAnd
Observed
Records

pSv(cm/s)

Z

6
5
4
3
2
1
0

100

120

140

N-2D FEM

h=5%

0.1

1.0
Period (s)Figure 5. Flow Chart of the Modeling
Process

10.0

160

180

3.3. Preliminary 2D FE Model Outputs
Figure 8 presents the
response spectra; in all sites
6
6
20
6
7
h=5
h=5
h=5
h=5
h=5
18
except for the GL-C the
6
5
5
5
16
shape agreements are not
5
14
4
4
4
12
good as expected, however,
4
3
3
10
3
3
in the long period range they
8
2
2
2
6
2
match. The peak in the North
4
1
1
1
1
site is not conservative since
2
0
0
0
0
0
the red line shows a
0.1
1.0
10.0
0.1
1.0
10.0
0.1
1.0
10.0
0.1
1.0
10.0
0.1
1.0
10.0
Period (s)Period (s)Period (s)Period (s)Period (s)predominant period of 0.4
Figure 8. Comparison of the Response Spectra from the Preliminary
and a large amplitude,
2D FE Model and the Observed Record, for the 2011/07/10
meanwhile the observed
Aftershock in Off Sanriku
records present a regularshape in the period less than 0.5s. In the South site the observed line presents a peak around 0.9s,
while the 2D FE Model shows a peak around 1.0s and two more peaks in the short period range, which
are not in agreement with the black one. The GL-C site presents good agreement in the whole spectra,
just in the period less than 0.2s the line goes beyond the observed one. For the 09F site thepredominant peak matches very well with the observed data, however in the period less than 0.5s the
spectra overestimates the response.
The decision to create a new model was a priority; however, the question was “whatshould be modified?” In this sense, considering the discrepancies of the North and South site,
Microtremor measurements were performed. The idea was to analyze the surface waves using the H/Vspectra, adjusting the thickness of the soil layers in the target points and computing the analytical H/Vanalysis, so that the both shapes match in the respective period.
GL-N 2D FEM

GL-N OBS.

B1F 2D FEM

B1F OBS.

09F 2D FEM

09F OBS.

GL-C 2D FEM

GL-C OBS.

GL-S 2D FEM

GL-S OBS.

%

3.4.
Modified 2D FE Model of the SoilStructure

GL-N

10.56

56 m
GL-C

B1F

33 m
GL-S

9.0m

35.20

17.0m

30 m

100 m

Transmitting
Boundary

Transmitting
Boundary
Viscous Boundary

200 m

Figure 9. Modified 2D FE Model
N-2D FEM

N-OBS.

B1F-2D FEM

6

B1F-OBS.

09F-2D FEM

6

20
h=5%

h=5%

5

4

4

3
2

0

0

0.1

1.0
Period (s)

10.0

12

10
8
6
4

h=5%

0.1

1.0
Period (s)

10.0

0.1

1.0
10.0
Period (s)

h=5%

6

5

4

3
2

0

S-OBS.

7

4
3
2

1

2
0

S-2D FEM

C-OBS.

5

14

2
1

C-2D FEM

6

16

3

1

09F-OBS.

h=5%

18

pSv(cm/s)

5

pSv(cm/s)

pSv(cm/s)

similar; else in the records of
the North site it was visiblethat the predominant period
was around 0.4 s, while in theSouth site it was around 0.9 s.
At the point “C” thepredominant period was 0.9
in both lines.

Figure 9 presents the new model which the
only parameter changed depth at the Northand South sites were decrease in 20%. Only
this modification was made.
The time history analysis of the
2D FE Model was performed considering 4
aftershocks. The response spectra of the July
10th, 2011 event presents concordance in thefour sites (Figure 10); even though the lines
did not exactly match, the tendencies werepSv(cm/s)

40 m

pSv(cm/s)

09F
35 m

%

pSv(cm/s)

pSv(cm/s)%

pSv(cm/s)%

pSv(cm/s)

pSv(cm/s)

%

1
0.1

1.0
Period (s)

10.0

0

0.1

1.0
Period (s)10.0

Figure 10. Comparison of the Response Spectra from the Modified
2D FE Model and the Observed Records, for the 2011/07/10
Aftershock in Off Sanriku

4. EFFECTS OF THE GEOLOGICAL IRREGULARITIES ON THE INPUT MOTION AND
SEISMIC RESPONSE OF THE IWAKI CITY HALL
4.1. Effects of the Geological Irregularities in the Soil Response
Figure 12 presents the diagram of the analysis followed to investigate the effects of the geologicalirregularities in the input motion and seismic response of the Iwaki city hall. Two models arepresented, i.e. the modified 2D FE Model and the Flat Soil Layer Model. Also, in the right side of the
figures the soil profile of the control point N, C and S are presented, aiming to compare how theground motion is affected by assuming that there is no inclination of engineering bedrock at thosepoints.
The results are presented in Figure 11, for the “N” and “S” site the geologicalirregularities modifies the seismic response, however the decreasing is not remarkable.
09F

N/2Eo Inclined E.B.

S

N

C

Fourier Spectra Ratio

Free Field Control Points
Assuming Thickness of
Inclined E.B.

B1F

Figure 12. Diagram of theSoil Response Analysis

Flat Layers Soils
2D ModelS/2Eo Flat Layers

3

B1F

Inclined Engineering
Bedrock 2D Model

S/2Eo Inclined E.B.

N/2Eo Flat Layers

S

4

2
1.5
1
0.5
0

4.2.
Effects of theGeological Irregularities
ontheStructural
Response

3.5

2.5

Fourier Spectra Ratio

C

N

3

2.5
2
1.5
1

Figure 13 shows the 2D
FE Model consideringflat-soil layers, in which
the varying parameter is
the
depth
of
the
engineering
bedrock.

0.5
0.1

1.0
10.0
Period (s)

0

0.1

1.0
10.0
Period (s)Figure 11. Comparison of
Two Models in “N” and“S” sites

Only 3 types of soil were considered, therefore
the ratio of the stratums thickness wasconsidered as 20%, 50% and 30%, also the soil
properties are same as the previous model
The charts of Figure 14 show the
Fourier spectra ratio of the basement for the 3models. The Model-A (D=22.5) presents adiphase of the natural period and a remarkable

09F
35 m

40 m
GL-N

56 m
GL-C

B1F

9.0m

D
17.0m

30 m

Transmitting
Boundary
B/2Eo Inclined E.B.

B/2Eo Inclined E.B.

B/2Eo Flat Layers

B/2Eo Flat Layers

2.5

2

1.5
1

0.5

Model
A
0.1

1.0
10.0
Period (s)

2
1.5
1
Model
B

0.5
0

0.1

1.0
10.0
Period (s)D=17.5 m

1.8

Fourier Spectra Ratio

2

D=20.0 m

Fourier Spectra Ratio

Fourier Spectra Ratio

B/2Eo Flat Layers

2.5
D=22.5 m

0

B/2Eo Inclined E.B.

1.6

1.4
1.2
1
0.8
0.6
0.4

Model
C

0.2
0

0.1

1.0
10.0
Period (s)Figure 14. Comparison of the Inclined
Engineering
Bedrock and Flat Soil Layers Models
0.6 s.

33 m
GL-S

100 m

Transmitting
Boundary
Viscous Boundary

200 m

Figure 13. 2D FE Model of the Considering Flat-Soillayers.
increase of the amplitude. For the Model-B, assumptionof the average depth of the basement sites was made,therefore the average depth between 22.5m and 17.5 m is
20m. Finally, the Model-C which represents the depth of
the right side of the basement boundary shows a good
agreement between the two models. The amplitude is thesame and the predominant period of the inclined bedrock
model is 0.5 meanwhile the flat-soil layers present it at

5. CONCLUSIONSThis study presented how the geological irregularities affects the input motion and seismic response of
8- storeys steel reinforced concrete building considering soil-structure interaction by a 2D FE Model.
The 2D FE Model used to compare the 4 aftershocks presented a good agreement with theobserved data, validating the assumption of soil strata properties and stratums thickness. Also, themicrotremor observation contributed in grand manner to establish the depth of the engineering
bedrock at the control points.
The comparison of the response spectra on the North and South sites have differences in
the predominant period, for the 4 aftershocks analyzed on the North site the natural period was around
0.4 s., while in the South around 0.9 s. As the observed records this trend was remarkable in every
aftershock.
The maximum peak ground acceleration were found in the North site with values in therange of 14.7 to 47.7 gals, while in the South site from 11.8 to 31.9 gals, the undergroundcharacteristics have remarkable influences in the observed acceleration. In this sense, it is worth notingthat the input motion of the building will be different and it cannot be directly compared with therecords at these sites. However, for the “C” site the peak ground acceleration was in the range of 10.3
to 36.8 gals, which can be related to those found in the South site, therefore the influence of theunderground characteristics of the South site have large influences in the superstructure.
This study demonstrated that the shape of the engineering bedrock can be equivalent to aflat-soil layers model considering the depth of the E.B. equal to 17.5m, which is the depth at the rightbasement site. This equivalency proved that the seismic response of the super-structure is not
drastically affected; in order to save time in the design process this assumption can be made, however
engineer judgment is vital.
The effects of the geological irregularities are remarkable in the soil response observed in
the control points, especially in the North and South sites. The one dimension vertical waves
experienced an amplification to the underground characteristics, but the interesting fact is thesecondary waves originated in the interface of the inclined engineering bedrock not only propagatedtoward horizontally, but the effects on the seismic response of the building was no so large.
ACKNOWLEDGEMENTS
From Building Research Institute and IISEE the author would like to acknowledge Dr. Koichi Morita,
Dr. Toshihide Kashima and Dr. Shin Koyama for the support on the Microtremor test and forproviding the data needed for the research.
REFERENCES
Akenori, Shibata, Dynamic Analysis of Earthquake Resistant Structures, Tohoku University Press,
2010, Sendai
Antes, H., and Estorff, O., 1988. Proceedings of Ninth World Conference on Earthquake Engineering,
August 2-9, Vol. III, pp. 411-415.
Ashford, S. A., Sitar, N., Lysmer, J., and Deng, Nan., 1997. Bulletin of the Seismological Society ofAmerica, June 1997, Vol. 87, No. 3, pp. 701-709.
Kashima, T., 2011. Building Research Institute, Japan, August 25th, pp. 1-15
Kobori, T. and Shinozaki, Y., 1978. Vol. 5, pp. 1769-1776.
Nagano, M., 2011-2012, Lecture Notes on Dynamic Soil-Structure Interaction, IISEE/BRI.
Tamura, C., and Konagai, K., 1988. Proceedings of Ninth World Conference on Earthquake
Engineering, August 2-9, Vol. III., pp. 373-378.
Web site: http://portal.cyberjapan.jp/
Westermo, B. D., and Wong, H. L., 1977. World Conference on Earthquake Engineering., Vol. 4., pp.
7-12
Wolf, J.P.,1985. Dynamic Soil-Structure Interaction. Prentice-Hall