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.