Time History Analysis Of Reinforced Concrete Framed Buildings On Geosynthetic Reinforced Soil

TIME HISTORY ANALYSIS OF REINFORCED CONCRETE FRAMED BUILDINGS 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 Engineering National 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-foundationsoil 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

shear. Like other construction materials with limited strength, soil can be reinforced with foreign material

It is observed from the earthquake affected areas that

to form a composite material that has increased shear

the major destruction is caused by the collapse of

strength and some apparent tensile strength [3].

multistoreyed buildings. Studies on the seismic

Reinforced soil is a construction technique that

behavior of these buildings reveal that the dynamic

consists of soil that has been strengthened by tensile

response is greatly affected by the local site

elements such as metal strips, geotextiles, or geogrids

conditions. The soil on which a structure is

[3]. These geosynthetics placed under foundations

constructed may interact dynamically with the

can absorb seismic energy, and hence transmit

structure during earthquakes . This is reflected as the

smaller ground motions to an overlying structure.

significant modification of stress components and

Documented

deflections in the structural system from the expected

performance of reinforced soil structures showed that

behavior of the system on a rigid supporting

reinforced soil slopes and walls tend to perform well

foundation. This is termed as the interaction of soil

under

with the structure that it supports and generally called

reports point out a lack of monitoring in practice,

as dynamic soil structure interaction [7]. Soil is

making it difficult to validate seismic design

capable

in

assumptions. The main objective of this study is to

compression, but virtually no strength in tension [3].

evaluate the dynamic soil structure interaction effects

In civil engineering applications, soil usually fails in

of reinforced soil for very soft soil condition and to

of

providing

very

high

strength

case

earthquake

histories

of

loading[8,9].

seismic

However,

field

field

determine the deformations and seismic response quantities of the structure under seismic loading as

Modeling of soil media

compared with the fixed base condition. The structures are assumed to be resting on very soft Model of structure - foundation - soil interacting system

soil designated as soil20 with E value of 20000 kN/m2, and a poisson’s ratio of 0.3. The bearing

Finite element analysis of the soil –foundation – structure system with and without geogrid

capacity and density of the soil are taken as 200 kN/m2 and 18 kN/m3. The soil is assumed to be

reinforcement is performed.

linear, elastic and isotropic material. Width of soil mass beyond the outermost footing is considered as 4

Structural idealization

B and depth as 8B, where B is the width of isolated

The building frame elements have been idealized as three dimensional space frames consisting of two

footing [2]. Soil is discretized using 8 nodded brick element with 3 DOF at each node.

nodded 3D beam elements with 6 DOF at each node.The Slabs are modeled with four nodded plate

Geometric parameters and Idealization of geogrid

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.

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 3. Acceleration Time History of the Elcentro Ground motion METHODOLOGY Three-dimensional finite element modeling of the whole structure –foundation –soil system is generated Fig.1 Foundation on geogrid reinforced soil

using the software ANSYS and shown in fig 4.

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

Fig 4. Finite element Model of a 2x2x1 RC frame –

motion is 0.6827 sec. It is seen from fig 2. that the

foundation - soil system with geogrids

major portion of the frequency content for this motion lies in the range of 1.16 Hz to 3.79 Hz. Fig

The seismic analysis of the building frames is carried

3.represents

out with transient dynamic analysis using mode

the acceleration

time history the

considered input motion.

superposition method. For the mode superposition type of transient analysis, Alpha and Beta damping are calculated from modal damping ratios, ξ i , for a particular mode of vibration i, based on Rayleigh Damping [1], such that the critical damping is taken as 5%.

Fig 2. Acceleration Fourier Spectrum of the Elcentro motion

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

Natural 2x2x1 i Period (sec) 0.43 Displacemen ii t

0.54

at roof (mm) 42.38 185 Base Shear iii (kN) 1028.5 4728.06

from the analysis of a fixed base structure.

0.55

25.55

26.64

195.34

336.53

360.92

4952.16

359.73

381.52

Variation in Natural Period

It is also observed that, when the soil is stiffened with

The analysis of the effect of dynamic SSI on the

geogrids,

natural period of the system shows an elongation of

quantities is reduced by 20% to 30%. It may be

natural period by 43% for one bay structure and 26 %

interpreted that by properly reinforcing the soil the

for a two bay structure. The variation in the natural

structural response can be reduced nearer to a fixed

period due to the effect of soil stiffening is studied on

base condition.

the two building models and a slight reduction in the

The Fourier spectra represent the frequency content

natural period is observed as compared to non-

of

reinforced soil

frequency of the input motion considered is 1.467 Hz

the

the

increase

response

in

quantities.

structural

The

response

predominant

and the frequency content of the displacement of 1x1x1 structure lies in the range of 1.5 Hz to 2.7 Hz

Effect of increase in number of bays

and that of 2x2x1 is in the range of 1.4 Hz to 2.6 Hz. 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.

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

Variation in Structural Response

observed for the structural base shear also.

It is seen from the three dimensional transient

CONCLUSIONS

analysis that the incorporation of flexibility of soil

It is concluded that the analysis of the integrated soil-

increases base shear to three to four times.

foundation - structure system reports considerable increase in the displacement and base shear in

Table 1.Variation of Structural response quantities for Elcentro Earthquake

Frame type

Parameters

Natural period(sec)

0.60 0.50 0.40

1X1X1

0.30

2X2X1

0.20 0.10 0.00 Fixed

soil w ith geogrid

soil w ithout geogrid

Support condition % Variation Reinforce Reinforce Fixed d soil20 Soil20 d soil20 Soil20

Natural 1x1x1 i Period (sec) 0.37 Displacemen ii t

0.52

at roof (mm) 28.24 130.604 Base Shear iii (kN) 352.48 1266.89

0.53

41.4

43.81

141.88

362.478 402.42

1376.52

259.42

290.53

Fig 5. Variation of Natural period for 1x1x1 and 2x2x1 building

Displacement(mm)

250.00 200.00 150.00

1X1X1

100.00

2X2X1

50.00 0.00 Fixed

soil w ith geogrid

soil w ithout geogrid

Fig 6. Variation of roof displacement for 1x1x1 and 2x2x1 building

Fig 10. Variation of roof displacement for 2x2x1 building

Baseshear kN

6000.00 5000.00 4000.00

1X1X1

3000.00

2X2X1

2000.00 1000.00 0.00 Fixed

soil with geogrid

soil without geogrid

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 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

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. Fig 14. Variation of Base Shear for 2x2x1 building

[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.

Fig 15. Fourier spectra of Base Shear for 2x2x1 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.

[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 Fullscale 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.