Ssi Effects On Rc Structures Resting On Geosynthetic Reinforced Soil

DYNAMIC SOIL STRUCTURE INTERACTION EFFECTS ON RC STRUCTURES RESTING ON GEOSYNTHETIC REINFORCED SOIL B.R. Jayalekshmi Faculty, Department of Civil Engineering , NITK , Surathkal, email:[email protected]

Deepthi Poojary V.G., R.Shivashankar, Katta Venkataramana

Abstract Structures are generally assumed to be fixed at their bases in the process of analysis and design under dynamic loading. But the consideration of actual support flexibility reduces the overall stiffness of the structure and increases the period of the system. Considerable change in spectral acceleration with natural period is observed from the response spectrum curve. Thus the change in natural period may alter the seismic response of the structure considerably. In this study the dynamic characteristics of the three dimensional structurefoundation-soil system of a building model is studied by time history analysis using Elcentro ground motion record. The 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 FEM software. The soil is characterized by its stiffness, mass density, and poisson’s ratio and the geogrids by stiffness, mass density and thickness. 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 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, geogrid INTRODUCTION The dynamic characteristics of a structural system gets modified when the supporting medium of soil is also considered as an integral part of the structure rather compared to those with the conventional completely restrained supports. 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 soft soil condition and to determine the deformations and seismic response quantities under seismic loading as compared with the fixed base condition. IDEALIZATION OF THE SYSTEM 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 called BEAM4.The Slabs are modeled with four nodded plate element with 6 DOF at each node, called SHELL 63. The foundation, which supports the superstructure, is also discretized as 4 nodded plate – bending element, SHELL 63. The element has bending and membrane capabilities, both inplane 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 taken as 22.36 GPa, ν is taken as 0.15 and density of concrete as 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 soil with and without reinforcement. Idealization of soil The structures are assumed to be resting on soft soil designated as soil20 with E value of 20000 kN/m2, and a poisson’s ratio of 0.3 is considered. 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 solid 45 with 3 DOF at each node. 5% of the critical damping is considered for the whole system. 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 are 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 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, SHELL 63, with bending and

membrane capabilities and modeled using two parameters, the modulus of elasticity E =2065000 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). METHODOLOGY Three-dimensional finite element modeling of the whole structure –foundation –soil system is generated using the software ANSYS and shown in fig 2.

Fig2. 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, ξ i , for a particular

mode of vibration i, based on Rayleigh Damping [1], such that the critical damping is taken as 5%. RESULTS AND DISCUSSIONS 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, base shear and corner column base bending moment for soft soil with and without geogrids are tabulated in table1 and plotted in fig 3 to fig 6, the time histories of the same are presented are fig 7 to fig 11 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 Table1.Variation of Structural response quantities for Elcentro Earthquake Frame type 1x1x1

Parameters

i Natural Period (sec) ii Displacement at roof (mm) iii BaseShear (kN) iv Column moment(kNm) 2x2x1 i Natural Period (sec) ii Displacement at roof (mm) iii BaseShear (kN) iv Column moment(kNm)

Support condition

Fixed Reinforced soil20 0.37 0.52

% Variation Soil20 Reinforced soil20 Soil20 0.53 41.4 43.81

28.24 352.48 146.17 0.43

130.604 1266.89 512.99 0.54

141.88 1376.52 552.44 0.55

362.478 259.42 250.97 25.55

402.42 290.53 277.95 26.64

42.38 1028.5 214.3

185 4728.06 792.93

195.34 4952.16 832.55

336.53 359.73 270.02

360.92 381.52 288.51

Variation in Structural Response It is seen from the three dimensional transient analysis that the incorporation of flexibility of soil increases the structural column moment to more than 277% and base shear to three to four times. 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.

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

Fig 3. Variation of Natural period for 1x1x1 and 2x2x2 building models 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 4. Variation of roof displacement for 1x1x1 and 2x2x2 building models 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 5 . Variation of Base shear for 1x1x1 and 2x2x2 building models

Column moment(kNm)

1000.00 800.00 600.00

1X1X1

400.00

2X2X1

200.00 0.00 Fixed

soil w ith geogrid

soil w ithout geogrid

Fig 6 . Variation of Column bending moment for 1x1x1 and 2x2x2 building models

Fig 7. Variation of roof displacement for 1x1x1 building model

Fig 8. Variation of roof displacement for 2x2x2 building model

Fig 9. Variation of Base Shear for 1x1x1 building model

Fig 10. Variation of Base Shear for 2x2x2building model

Fig 11. Variation of Corner column bending moment for 1x1x1 building model

Fig 12. Variation of Corner column bending moment for 2x2x2 building models

CONCLUSIONS It is concluded that the analysis of the integrated soil- foundation - structure system reports considerable increase in the displacement, base shear and column moment in 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, 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 Hills, 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.