TIME HISTORY ANALYSIS OF REINFORCED CONCRETE FRAMED BUILDINGS ON GEOSYNTHETIC REINFORCED SOIL
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Transcript of TIME HISTORY ANALYSIS OF REINFORCED CONCRETE FRAMED BUILDINGS ON GEOSYNTHETIC REINFORCED SOIL
TIME HISTORY ANALYSIS OF REINFORCED CONCRETE FRAMED BUILD-INGS ON GEOSYNTHETIC REINFORCED SOIL
B R Jayalekshmi 1, Deepthi Poojary V.G.2, R.Shivashankar3, Katta Venkataramana3
1 Senior Lecturer, 2 P.G.Scholar, 3 Professor, Department of Civil EngineeringNational Institute of Technology Karnataka, Surathkal 575025
ABSTRACT
The interaction among structures, their foundations and the soil medium below the foundations alter the actual behavior of the structure considerably than what is obtained from the consideration of the structure alone. Thus the flexibility of the support reduces the stiffness of the structure and increases the period of the system. In the present study the dynamic characteristics of the three-dimensional structure-foundation-soil system of a building model is studied by time history analysis using modified Elcentro ground motion record. The very soft soil and soil reinforced with ‘Tensar’ geogrids placed in three layers below the foundation is considered. Finite element analysis of the integrated system is carried out using finite element software. The change in the dynamic characteristics of the structure due to the incorporation of the effect of flexibility of soil and the effect of reinforced soil is noted. The time histories and Fourier spectra of displacement and base shear are presented and the variation in structural seismic response for various parameters is compared to that of a fixed base structure.
Key words: Dynamic soil structure interaction, time history analysis, Fourier spectra, geogrid.
INTRODUCTION
It is observed from the earthquake affected areas that
the major destruction is caused by the collapse of
multistoreyed buildings. Studies on the seismic
behavior of these buildings reveal that the dynamic
response is greatly affected by the local site
conditions. The soil on which a structure is
constructed may interact dynamically with the
structure during earthquakes . This is reflected as the
significant modification of stress components and
deflections in the structural system from the expected
behavior of the system on a rigid supporting
foundation. This is termed as the interaction of soil
with the structure that it supports and generally called
as dynamic soil structure interaction [7]. Soil is
capable of providing very high strength in
compression, but virtually no strength in tension [3].
In civil engineering applications, soil usually fails in
shear. Like other construction materials with limited
strength, soil can be reinforced with foreign material
to form a composite material that has increased shear
strength and some apparent tensile strength [3].
Reinforced soil is a construction technique that
consists of soil that has been strengthened by tensile
elements such as metal strips, geotextiles, or geogrids
[3]. These geosynthetics placed under foundations
can absorb seismic energy, and hence transmit
smaller ground motions to an overlying structure.
Documented case histories of seismic field
performance of reinforced soil structures showed that
reinforced soil slopes and walls tend to perform well
under earthquake loading[8,9]. However, field
reports point out a lack of monitoring in practice,
making it difficult to validate seismic design
assumptions. The main objective of this study is to
evaluate the dynamic soil structure interaction effects
of reinforced soil for very soft soil condition and to
determine the deformations and seismic response
quantities of the structure under seismic loading as
compared with the fixed base condition.
Model of structure - foundation - soil interacting
system
Finite element analysis of the soil –foundation –
structure system with and without geogrid
reinforcement is performed.
Structural idealization
The building frame elements have been idealized as
three dimensional space frames consisting of two
nodded 3D beam elements with 6 DOF at each
node.The Slabs are modeled with four nodded plate
element with 6 DOF at each node. The foundation,
which supports the superstructure, is also discretized
as 4 nodded plate – bending element. The element
has bending and membrane capabilities, both in-plane
and normal loads are permitted. The behavior of
superstructure and foundation is assumed as elastic
and is modeled using two parameters, the modulus of
elasticity E and poisson’s ratio ν. Structural members
are considered to be reinforced concrete of grade
M20.Value of E is 22.36 GPa, ν is 0.15 and density
of concrete is 25 kN/m3. The bay length of the
building is taken as 4.0 m and height as 3 m for all
the cases. Sizes of beams and columns as 230mm x
400 mm. Thickness of slab is taken as 150mm and
wall as 230mm with density of 20 kN/m3.The
geometric sizes and loadings on the frames have been
arrived on the basis of general requirement
confirming to design code [4,5, 6].The live load is
taken as 3 kN/m2. Square footing of size 2m x 2m
with 500mm thickness is considered for all
structures. The frames considered here are one bay
and two bay structures with one storey designated as
1x1x1 and 2x2x1 with fixed base and resting on very
soft soil with and without reinforcement.
Modeling of soil media
The structures are assumed to be resting on very soft
soil designated as soil20 with E value of 20000
kN/m2, and a poisson’s ratio of 0.3. The bearing
capacity and density of the soil are taken as 200
kN/m2 and 18 kN/m3. The soil is assumed to be
linear, elastic and isotropic material. Width of soil
mass beyond the outermost footing is considered as 4
B and depth as 8B, where B is the width of isolated
footing [2]. Soil is discretized using 8 nodded brick
element with 3 DOF at each node.
Geometric parameters and Idealization of geogrid
In this study, the soil is reinforced with 3 layers of
geogrid designated as reinforced soil20 with the
vertical spacing between the consecutive geogrid
layers as h equal to 0.5 m. The top layer of geogrid is
located at a depth u equal to 0.5 m measured from the
bottom of the foundation. The width of the geogrid
reinforcements under the foundation is calculated as
b equal to the total footing area and extending a
distance of B i.e. width of footing, beyond the
outermost footing . The depth of reinforcement, d,
below the bottom of the foundation can be given as d
= u + (N-1) B where N is the number of layers of
geogrid [3] as shown in the fig1. The specification of
the geogrid considered is ‘Tensar’ SR2. Its tensile
strength is taken as 150 kN/m with 2% strain and
thickness of 1.2 mm with weight of 0.85 kg/m2 . The
geogrid elements have been idealized as 4 nodded
plate element, with bending and membrane
capabilities and modeled using two parameters, the
modulus of elasticity E =2.065 x 107 and poisson’s
ratio ν= 0.2.
Fig.1 Foundation on geogrid reinforced soil
Ground Motions considered
The effect of dynamic soil structure interaction of
reinforced and non reinforced soft soil on the
building frames is studied under the modified
acceleration time history that correspond to a peak
ground acceleration of 0.5 g of the earthquake ground
motion of Imperial Valley Earthquake, Station
Elcentro (1940).The Predominant period of this
motion is 0.6827 sec. It is seen from fig 2. that the
major portion of the frequency content for this
motion lies in the range of 1.16 Hz to 3.79 Hz. Fig
3.represents the acceleration time history the
considered input motion.
Fig 2. Acceleration Fourier Spectrum of the
Elcentro motion
Fig 3. Acceleration Time History of the Elcentro
Ground motion
METHODOLOGY
Three-dimensional finite element modeling of the
whole structure –foundation –soil system is generated
using the software ANSYS and shown in fig 4.
Fig 4. Finite element Model of a 2x2x1 RC frame –
foundation - soil system with geogrids
The seismic analysis of the building frames is carried
out with transient dynamic analysis using mode
superposition method. For the mode superposition
type of transient analysis, Alpha and Beta damping
are calculated from modal damping ratios, i , for a
particular mode of vibration i, based on Rayleigh
Damping [1], such that the critical damping is taken
as 5%.
RESULTS AND DISCUSSION
The seismic structural response of 1x1x1 and 2x2x1
building for Elcentro motion with and without
geogrids is studied. The variation of natural period
and structural response for various parameters like
roof displacements and base shear for very soft soil
with and without geogrids are tabulated in table 1 and
plotted in fig 5 to fig 7, the time histories and the
Fourier spectra of the same are presented in fig 8 to
fig 15 and comparisons are made with those obtained
from the analysis of a fixed base structure.
Variation in Natural Period
The analysis of the effect of dynamic SSI on the
natural period of the system shows an elongation of
natural period by 43% for one bay structure and 26 %
for a two bay structure. The variation in the natural
period due to the effect of soil stiffening is studied on
the two building models and a slight reduction in the
natural period is observed as compared to non-
reinforced soil
Effect of increase in number of bays
It is observed here that, natural period increases as
the number of bays increases and the percentage
variation of natural period decreases with increase in
number of bays for the building models.
Variation in Structural Response
It is seen from the three dimensional transient
analysis that the incorporation of flexibility of soil
increases base shear to three to four times.
Table 1.Variation of Structural response quantities for Elcentro Earthquake
Frame Parameters Support condition % Variation
type FixedReinforced soil20 Soil20
Reinforced soil20 Soil20
1x1x1 i Natural Period (sec) 0.37 0.52 0.53 41.4 43.81
iiDisplacement
at roof (mm) 28.24 130.604 141.88 362.478 402.42
iiiBase Shear (kN) 352.48 1266.89 1376.52 259.42 290.53
2x2x1 iNatural Period (sec) 0.43 0.54 0.55 25.55 26.64
iiDisplacement
at roof (mm) 42.38 185 195.34 336.53 360.92
iiiBase Shear (kN) 1028.5 4728.06 4952.16 359.73 381.52
It is also observed that, when the soil is stiffened with
geogrids, the increase in structural response
quantities is reduced by 20% to 30%. It may be
interpreted that by properly reinforcing the soil the
structural response can be reduced nearer to a fixed
base condition.
The Fourier spectra represent the frequency content
of the response quantities. The predominant
frequency of the input motion considered is 1.467 Hz
and the frequency content of the displacement of
1x1x1 structure lies in the range of 1.5 Hz to 2.7 Hz
and that of 2x2x1 is in the range of 1.4 Hz to 2.6 Hz.
It is observed that the structure on very soft soil
undergoes considerable displacement in this
frequency range and the addition of geogrid reduces
this response by 40 % for one bay structure and 24 %
for the two bay structures. Similar variation is
observed for the structural base shear also.
CONCLUSIONS
It is concluded that the analysis of the integrated soil-
foundation - structure system reports considerable
increase in the displacement and base shear in
0.00
0.10
0.20
0.30
0.40
0.50
0.60
Fixed soil w ithgeogrid
soil w ithoutgeogrid
Nat
ura
l per
iod
(sec
)
1X1X1
2X2X1
Fig 5. Variation of Natural period for 1x1x1 and 2x2x1 building
0.00
50.00
100.00
150.00
200.00
250.00
Fixed soil w ithgeogrid
soil w ithoutgeogrid
Dis
pla
cem
ent(
mm
)
1X1X1
2X2X1
Fig 6. Variation of roof displacement for 1x1x1 and 2x2x1 building
0.00
1000.00
2000.00
3000.00
4000.00
5000.00
6000.00
Fixed soil w ithgeogrid
soil w ithoutgeogrid
Basesh
ear
kN
1X1X1
2X2X1
Fig 7. Variation of Base shear for 1x1x1 and 2x2x1 building
Fig 8. Variation of roof displacement for 1x1x1 building
Fig 9. Fourier spectra of roof displacement for 1x1x1 building
Fig 10. Variation of roof displacement for 2x2x1 building
Fig 11. Fourier spectra of roof displacement for 2x2x1 building
Fig 12. Variation of Base Shear for 1x1x1 building
Fig 13. Fourier spectra of Base Shear for 1x1x1 building
Fig 14. Variation of Base Shear for 2x2x1 building
Fig 15. Fourier spectra of Base Shear for2x2x1 building
comparison with the fixed base assumption. Transient
analysis of reinforced soil-foundation-structure
system suggests that, due to placement of geogrids on
soft soil beds with appropriate number of layers,
positioning and stiffening properties of geosynthetics,
the seismic response quantities can be reduced closer
to the fixed base condition.
REFERENCES
[1] Anil, K. Chopra (2003) “ Dynamics of structures “ Theory and application to Earthquake Engineering , Prentice hall , New Delhi.
[2] Bowles, J.E. (1998).”Foundation Analysis and design”, McGraw Hill, New York.
[3] Braja M. Das (1999) “Shallow Foundations, Bearing capacity and settlement”, CRC press, New York.
[4] IS 1893 (Part I): 2002 Criteria for Earthquake Resistant Design of Structures - General provisions and Buildings, Bureau of Indian Standards, New Delhi.
[5] IS 456:2000 Plain and Reinforced Concrete – Code of Practice, Bureau of Indian standards, New Delhi.
[6] IS 875 : 1987 (Part I & Part II ) Code of practice for design Loads ( Other than Earthquake ) for buildings and structures , Bureau of Indian Standards , New Delhi.
[7] John P. Wolf (1985) , “ Dynamic Soil-Structure Interaction” , Prentice- Hall, Inc , Englewood Cliffs, New Jersey
[8] Christopher Burke , Hoe I.ling and Huabei Liu(2004),” Seismic Response Analysis of a Full-scale reinforced soil retaining wall”,17 th ASCE Engineering Mechanics conference, newmark,DE.
[9] C.R.Patra , B.M.Das and C. Atalar (2005),” Bearing Capacity of embedded strip foundation on geogrid-reinforced sand” , Geotextiles and Geomembranes, vol 23 , 454-462.