study of soil-structure interaction effects on seismic

Proceedings of 9th IOE Graduate ConferencePeer Reviewed

ISSN: 2350-8914 (Online), 2350-8906 (Print)Year: 2021 Month: March Volume: 9

Study of Soil-Structure Interaction Effects on Seismic Analysis

Sabin Maharjan a, Kamal Bahadur Thapa b

a, b Department of Civil Engineering, Pulchowk Campus, IOE, TU, NepalCorresponding Email: a [email protected], b [email protected]

AbstractIn the past for the design of building, the structure and soil were analyzed in full isolation. But in a dynamicloading scenario, the interaction between structure and soil becomes of great importance. In general designpractice, buildings are modeled as fixed at their base. This assumption is only valid when structure is foundedon solid rock or stiff soil. But in general the soil is flexible and the soil supporting the structure gets displacedand offers some resistance to motion of structure and vice versa. So it becomes important to understand theSSI effect and its proper modeling. In the present work, G+3, G+5 and G+7 storied traditional RCC buildingsregular in plan and elevation are considered. The study is done for different soil condition with different shearwave velocities. This research focuses on the effect of Soil structure interaction under earthquake loads.Direct method is applied to model soil and structure for analysis. The seismic analysis was performed by usingEquivalent Static Method from NBC 105:2020. The results in form of lateral displacement, inter story drift,base shear, and time period are compared for models with and without soil interaction. The results obtainedfrom the study shows increase in time period of the system, and also increase in lateral displacement andstorey drift of the buildings, while magnitude of shear force decreases, which is due to introduction of soilflexibility in analysis. The analysis for the study is performed using SAP2000.

KeywordsSoil Structure Interaction(SSI), Direct Method, NBC 105:2020

1. Introduction

Soil-Structure Interaction refers to the response of astructure due to influence of soil and the response ofsoil due to the influence of motion of existingstructure. SSI evaluates the collective response of thestructure, the foundation and the soil underlying andsurrounding the foundation to a specified groundmotion. When a structure is built anywhere on thesurface, the free field ground motion is obstructedresulting different types of interaction between soiland structure. The energy transfer mechanism fromsoils to buildings is critical for earthquake resistancedesign of structures [1]. Although soil structureinteraction has its origins trace back to the late 19century, and gradually progressed and developed inthe subsequent decades, only due to recentdevelopment of dominant computers and simulationtools such as finite elements, this field has seen moredevelopments as reviewed by [2, 3]

In perivious reseacrh work author [4] presented thatthe soil structure interaction can have a benificial

effect on structure although the research [5] presentedboth benificial and detrimental aspect of SSI. Sincethe presence of soil increases the flexibility ofstrutural system, time period and damping of structurealso increases which results in decrease of base shearbut at the same time increases the displacement of thestructural system. The consequences of this can beinstability of the whole structure. The lack ofinclusion of SSI in deisgn codes is presented inprevious work by [6, 7] and the paper provides asimplified procedure by which SSI effects can beincluded during analysis of structure. The result from[8] concluded that soil structure interaction played asignificant role to increase the seismic base shear oflow-rise building frames, and the effect was stronglyinfluenced by the frequency content of the earthquakeground motion. The research paper [9] studied theSSI on multi story RC frame building founded in softsoil (flexible base) and compared it with fixed base byusing dynamic seismic analysis. Author in researchpaper [10] critically reviewed the available and wellknown modeling techniques for dynamic SSI analysisand discussed the challenges and issues with SSI

Pages: 294 – 301

Proceedings of 9th IOE Graduate Conference

modelling techniques.

In this current work, the study for SSI effects has beenconducted on buildings with regular base dimensionbut with varying height. Different soil type fromvarious location of Kathmandu valley has beenincorporated in this study. Direct method[7, 9, 11, 12] has been applied to model soil andstructure for analysis. Separate SSI systems has beenmodeled for varying soil parameter and varyingbuilding height. The seismic analysis has beenperformed by using Equivalent Static Method fromcodal provision as suggested by NBC 105:2020. Theconsequence of soil structure interaction under lateralloads has been studied. The study has been carried outto identify the changes in lateral displacement, interstorey drift, fundamental time period and base shearby incorporating the effects of SSI. The results fromSSI models and fixed base model were compared todraw a logical conclusion.

2. Idealization of the System

2.1 Structural Idealization

For the study, two-dimensional regular 4, 6 and 8storey RC frame building with 3 bays has beenmodeled. The frames have been modeled using twonodded frame elements. 3m storey height and 4mlength of each bay has been selected for modeling.The dimensioning of members of buildings has beendone according to preliminary analysis. Thedimension of beam was taken as 230 x 350 mm. Thedetail dimensions for column used for modeling ispresented in Table 1. M20 grade concrete for beamand column with Fe500 grade of rebar has beenconsidered. Live and dead loads were considered asper IS 875: Part II.

Figure 1: 2D SAP2000 model for fixed supported 4,6, and 8 storied buildings

Table 1: Description of Buildings

Number ofStorey

TotalHeight(m)

ColumnDimension(mm x mm)

4 12 300 x 3006 18 350 x 3508 24 400 x 400

2.2 Soil Properties

Soil properties for five different locations aroundKathmandu valley has been adopted from paper [13].The required values of shear wave velocity (Vs),compressional wave velocity (Vp), poissons ratio, unitweight modulus of elasticity (E) and shear modulus(G) were adpoted from paper [13]. These parameterswere for shallow depth of 10m. Since shear wavevelocity averaged over top 30m (Vs30) of soil is usedfor seismic analysis so it has been modified for 30m.The modified values are presented in Table 2

Table 2: Soil Properties

SN Site Vs30 PoissonsRatio

Unit wt(kN/ m3)

1 Koteswor 65.44 0.442 12.282 Horizon,

Dhapasi117.89 0.235 14.22

3 Balaju 126.03 0.202 14.464 Balkhu 160.63 0.129 15.375 Sankhu 182.9 0.22 15.87

2.3 Soil Structue Modelling

In this study, direct approach has been used formodeling. The direct analysis evaluates SSI bymodeling a limited soil domain along with thefoundation system, superstructure, transmittingboundaries along the perimeter of the soil domain,and interface elements between the foundation systemand soil. Therefore, the direct solution considers thecomplete soil-structure system and solves thisproblem in one step. This method has been adoptedfrom papers [7, 9, 11, 12]

Plane Strain elements has been used to model soilhalf space. Plane strain elements are quadrilateralelements with two degrees of freedom at each node.For modelling of infinite soil surrounding structure,the soil depth D has been considered as 3B, where Bis the width of building in short direction and distance

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Figure 2: Finite element modeling of soil structure system [12]

from structure to boundary has been considered as 4Bas suggested by [9]. The total width of half space hasbeen considered as 10B for this study. The meshingof soil half space has been carried out as suggestedby [14]. The boundary conditions of half space soilfor the analytical model is fixed boundary at the baseof the soil model. Transmitting boundaries has beenapplied on the vertical boundaries of soil half spaceas suggested by [14]. The infinite soil media has beenreplaced by viscous dampers at the vertical boundaries.Link properties with damper type support were used inSAP2000 to imitate the viscous dampers as suugestedby [14]. The damping coefficients for horizontal andvertical boundaries are given by

Ch =−ρ ∗V p∗A

Cv =−ρ ∗V s∗A

Here, Ch and Cv are the horizontal and verticaldamping coefficients at the model boundaries, ρ is thematerial density, V p and V s are the P-wave andS-wave velocities and A is the Effective nodal area forthe node connected to the damper which is calculatedas the total area of all elements around the node atboundary.

The interface between the foundation and soil hasbeen represented by normal (kn) and shear (ks)

springs between two planes contacting each other andhas been modeled using a linear spring system assuggested by [12]. The relative interface movement iscontrolled by interface stiffness values in the normaland tangential directions. Normal and shear springstiffness values for interface elements of thesoil-structure model is given by

kn = ks = 10∗ K +4G/3∆zmin

Here K and G are the bulk and shear modulus of theneighbouring zone and ∆zmin is the smallest width ofan adjoining zone in the normal direction.

2D SAP model for direct method is shown in Figure 3.

2.4 Methodology

Seismic analysis of the soil structure system has beencarried out by equivalent static method as instructedby NBC 105:2020. Static analysis has been performedby considering the building structure as stationary andthe loads acting on the structure as constant and nottime dependent. For performing static analysis spectralshape factor is selected as 2.25, seismic zone factoris selected as 0.35, Ductility factor of 4, over strengthfactor of 1.5 and importance factor of 1 is selected.Soil type D is considered for analyisis. SAP2000 hasbeen used for analysis of system.

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Figure 3: 2D SAP2000 model for direct method of analysis

3. Results and Discussion

For different soil condition static analysis of buildingswith and without SSI effects of G+3, G+5, G+7storied buildings is performed in SAP2000. Bareframe models are used for analysis of structure. Theresults are presented below .

3.1 Results on Storey Dispalement

Storey displacement is the total displacement of anystorey with respect to base or ground. Soil-structureinteraction mostly for MRF buildings constructed oncomparatively soft soils could considerably intensifythe lateral displacements. The lateral displacementprofile are presented in Figure 4, 5 and 6. As observedfrom the results, it is seen that the displacementincreases with introduction of SSI effects on model.The relative displacement on foundations located onsoft soil is more and the value of lateral displacementdecreases with increasing soil rigidity.

Figure 4: Displacement comparison of G+3Buildings for different sites and fixed support

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3.2 Results on Storey Drift

The story drift is examined for RCC building of 4,6 and 8 storey buildings. The story drift ratio overthe building’s height for different location and soilconditions are presented in the graphical form. Figures7,8 and 9 show the story drift distribution over building

height compared to that response value of fixed basedmodel for 4, 6 and 8 story model. As seen from theresults, the story drift ratio increases over the buildingheight as the supporting soil changes. This increasetrend is more significant in the lower stories. The inter-storey drift in bottom stories for soft soil are largecompared to fixed base model. This is due to the factthat the base shear at bottom decreases significantlywhich leads to larger drift.

Figure 7: Drift comparison of G+3 Buildings fordifferent sites and fixed support


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3.3 Results on Fundamental Time Period

In modelled 4 storey, 6 storey and 12 storey buildings,the computed periods from empirical expressions aresignificantly shorter than those computed fromstructural models especially for building structureswith soft soil.

Figure 10: Time Period comparison of G+3Buildings for different sites and fixed support

It is observed that the time period for soft soil atkoteshwor area is highest among the models whereastime period is minimum for fixed base model. Also,the study reveals that time period is increased

considering the flexibility of the soil. This meansfundamental time period of the building consideringfixed base condition is shorter than the flexible basecondition. It is because, considering soil-structureinteraction makes a structure more flexible comparedto the corresponding rigidly supported structure.



3.4 Results on Base Shear

The difference in base shear for fixed base conditionand considering SSI for different site and differentstorey buildings is shown as graphical representationin Figure 13,14 and 15. It is observed that base sheardecreases considering the flexibility of soil. It is dueto the increase in effective damping ratio and natural

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time period of the structure. Base shear is maximumfor fixed base condition, minimum for Koteshwor site.Minimum base shear in Koteshwor area consideringSSI is about 64% of base shear in fixed base conditionfor 4 storey building and the minimum base shearin Koteshwor area considering SSI is about 59% ofbase shear in fixed base condition for 6 and 8 storeybuilding.

Figure 13: Base Shear comparison of G+3 Buildingsfor different sites and fixed support

Figure 14: Base Shear comparison of G+5 Buildingsfor different sites and fixed support

Figure 15: BaseShear comparison of G+7 Buildingsfor different sites and fixed support

4. Conclusion

The present study is an attempt on understanding theseismic response and behavior of structure when soilflexibility is introduced in the structural system. Withthe use of varying soil and structural parameters aneffort has been made to shed light on the effect of soilstructure interaction on seismic performance ofbuildings. The study reveals that time period isincreased considering the flexibility of the soil. Thismeans fundamental time period of the buildingconsidering fixed base condition is shorter than theflexible base condition. It is because, consideringsoil-structure interaction makes a structure moreflexible compared to the corresponding rigidlysupported structure. This could lead to resonance ofstructure. As observed from the results, it is seen thatthe displacement increases with introduction of SSIeffects on model. The displacement on foundationslocated on soft soil is more and the value of lateraldisplacement decreases with increasing soil rigidity.As seen from the results, the story drift ratio increasesover the building height as the supporting soilchanges. This increase trend is more significant in thelower stories. The interstorey drift in first, secondstorey for soft soil are large compared to fixed basemodel. This could be very serious concern when softstorey structure are constructed over soft soil. It isobserved that base shear decreases considering theflexibility of soil. It is due to the increase in effectivedamping ratio and natural time period of the structure.

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