Combined Effect of Soil Structure Interaction and Infill Wall Stiffness on Building – A Review

Combined Effect of Soil Structure Interaction and Infill Wall Stiffness on Building – A Review

Prof. Wakchaure M. R a* Khatal Prakash B.b**

aDean & Asso. Professor, Dept.of Civil Engineering, bP.G.Student, Dept.of Civil Engineering

Amrutvahini College of Engineering, Amrutvahini College of Engineering,

Sangamner-422608,Maharashtra,India Sangamner-422608,Maharashtra,India

Abstract–The present study makes an attempt to show the effect of flexibility and rigidity of foundation in earthquake analysis of structure by the considering the combined effect of soil structure interaction and infill wall stiffness of building. For superstructure G+3 simple rectangular building is considered for seismic analysis. The infill wall is replaced by equivalent diagonal member. The total work is divided into two parts. In first part the manual analysis is done in X direction and in second part the analysis is done with ANSYS software. The more emphasis has given on manual earthquake analysis by using model superposition response method as per IS 1893-2002 (part-II) the stiffness of soil is calculated by the formulae as per the FEMA

1. After studying this behavior it is found that base shear decreases in seismic analysis of superstructure by considering the effect of soil structure interaction in X direction.

   Keywords: soil structure interaction, infill wall, base shear, soil stiffness

   1. INTRODUCTION

      The soil response analyses is one of the most important aspects of earthquake engineering, as it will determine the ground motion that will be experienced at the top of soil without the presence of a structure or the so- called free field response. The analysis involves estimation of the seismologic characteristics of the region, and determination and modeling of the soil profile and its dynamic characteristics. Further, it accounts for the multiple reflections and refractions that will occur at the soil layer interfaces as the seismic waves propagate though the soil deposits. Although special purpose computer programs exist for this purpose, the validity of the results depend greatly on how accurate dynamic soil properties are estimated, which in spite the improvements in the in situ testing, is still a challenging task. In the present study, no soil amplification analysis was performed; rather, they considered accelerograms were used directly to excite the structure

      The analysis of fundamental SSI effects is well

      established and some computer programs can be used for SSI analysis of even complicated models. It should also be pointed out that while the analysis of SSI effects has been focused mainly on the investigation in terms of

      deformation and force the SSI effects may have a significant influence on the structural performance, the performance-based design procedures currently in use are still inadequate to account for these effects on the inelastic structural response. A number of damage models have been developed for evaluation of the structural performance,but usually assuming the structure as rigidly supported.

   2. NOTATION

      B Damping coefficient

      D Depth of foundation

      d Depth of footing at outer edge E Modulus of Elasticity of soil

      F Force in an energy dissipation unit G Shear modulus

      g Acceleration due to gravity H Average story height

      I Moment of Inertia

      1. Soil spring stiffness, stiffness matrix Kh Spring constants for horizontal cases ku Linear soil spring stiffness

      2. Length of footing

      m Mass of oscillating body N Number of layers

      P Force

      U Total acceleration

      u Independence function Independence function

      U1 Acceleration due to kinematic interaction

      u1 Linear displacement

      U2 Acceleration due to dynamic interaction uf Free translation relative to ground

      motion

      y1 Model linear displacement yg Ground acceleration

      Coefficient of friction Poissions ratio of soil Mass density

      f frequency

      T Fundamental natural period Mode shape

      pk Modal participation factor mk Modal mass

      Qlk Design lateral force

      kx,sur Translation stiffness along x- axis (foundation at surface)

      ky,sur Translation stiffness along y- axis (foundation at surface)

   3. MATHEMATICAL FORMULATION(MANUALLY)

      For the mathematical calculation of base shear of building as shown in fig

      By using response spectrum method as per IS 1893-2002 part- II

      Step-I Calculation of lumped masses to various floor levels roof /floor = mass of infill + mass of column +mass of beam in longitudinal and transverse direction of that floor + mass of slab + imposed load on that floor if permissible

      ** 50% of Imposed load, if imposed load is greater than 3 kN/m2

      Step- II : Frame considering the stiffness of infill

      The frame considered in previous section is again analyed by considering the stiffness of infill walls. The is modelled as equivalent diagonal strut.

      The mass matrix [M] for lumped plane frame model is

      kz,sur Translation stiffness along z- axis (foundation at surface)

      kxx Rocking stiffness along x- axis (foundation at surface)

      kyy Rocking stiffness along y- axis (foundation at surface)

      kzz Rocking stiffness along z- axis

      Stiffness of infill is determined by modeling the infill as an equivalent diagonal strut in which, width of strut ,

      Where,

      elastic modulus of frame material N/m2 elastic modulus of frame material N/m2

      t = thickness of infill wall mm

      h = height of infill wall m l = length of infill wall

      = moment of inertia of beam m4

      For the frame with two bays there are two struts participating in each direction total lateral stiffness of each storey

      Stiffness matix [k]=

      For the above stiffness and mass matrix, eigen values and Eigen vectors are work out as follows

      =0

      the mode shapes corresponding to each natural frequency is determined from the equation

      Column stiffness of storey

      =0

      Step III Mode Shapes

      Eigen vectors (mode shapes):

      Degree of freedom	Stiffness of the foundation at surface
      Translation along x- axis
      Translation along y- axis	Ky,sur=
      Translation along z- axis
      Rocking along x- axis
      Rocking along y- axis
      Torsion about z- axis

      Step IV Determination of modal participation factor The modal participation factor

      Step V Determination of modal mass

      Where

      G= acceleration due to gravity

      Step VI Determination of lateral forces at each floor at each mode

      The design lateral force is given by(Qik) at floor i in mode

      The design horizontal seismic coefficient Ah, for various mode are

      Step VII Determination of storey shear forces in each mode

      Vik=

      Step VIII Determination of storey shear forces due to all modes V1=[( V11)2+( V12)2+( V13)2+( V14)2]0.5

      Step IX Determination of lateral forces at each storey Final base shear at each floor

      F4 at roof floor F3 at third floor = V3- V4

      F2 at second floor = V2- V3 F1 at first floor = V1- V2

      Total Base shear = F1+ F2 + F3 + F4

      1. Calculation of soil parameters

         The stiffness is calculated by as per the guidelines of FEMA 356 by considering the elastic parameters of soil dimensions of foundations the main elastic parameters is requird is shear modulus which is calculated by

         Where E= modulus of elasticity kN/m2

         For the group I soil (soft clay, firm and stiff clay, silty sand and loose sand)

         For the stiffness of soil the following formulae to be used from FEMA356

      2. Formulation of Problems

         Sr. No.	Constant	Values	Remarks
         1	Z	0.1	Structure assumed in Zone II
         2	I	1.5	Importance Factor
         3	R	3	Response Reduction Factor
         4	M-25		Grade of Concrete
         – 5	Fe-500		Grade of Steel

         Table 3.1 : Constants which are considered for calculation

      3. Response Spectrum Analysis

      Figure. 2.4 Response Spectrum of Bhuj Earthquake (2001) in X direction[2]

   4. ANALYSIS BY ANSYS

      Proposed building Models with SSI

      Model 1: building without SSI effect and infill wall.

      Model 2: building with SSI effect and infill wall.

   5. RESULTS & DISCUSSION

      The study of effect of soil structure interaction with infill wall stiffness is more effective on base shear calculation of building Here one type of soil group is to be considered and along X direction is considered for analysis This study will be show decrease in the base shear by considering effect of SSI

   6. FUTURE WORK

      The further study shows that the effect of SSI will be the major parameter affecting the calculation of base shear calculation. The different model will considered with infill wall considerations for the analysis and conclusion will be made. High rise complicated geometrical building for rigorous analysis is going to be considered In present study only one type of soil strata is considered beneath the structure which is rarely possible, so the study has been prolonged for layered soil media. For this study only response spectrum method is considered, time history method can be used

   7. REFERENCES

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