13 th Worl Conference on Earthquake Engineering Vancouver, B.C., Canaa August 1-6, 24 Paper No. 3233 COMPUTER SIMULATION ON DYNAMIC SOIL-STRUCTURE INTERACTION SYSTEM Peizhen Li 1, Xilin Lu 2, Bo Chen 3 an Yueqing Chen 4 SUMMARY Three-imensional finite element analysis in time omain on ynamic soil-structure interaction of a practical engineering is carrie out in this paper. General-purpose finite element program ANSYS is use in the analysis. Commonly use equivalent linearity moel is chosen as constitutive relation of soil. Viscous bounary of soil is implemente in ANSYS program. The influences of parameters are iscusse, such as soil property an the rigiity of structure, excitation, on ynamic characteristics, seismic response an interaction effect of SSI system. In orer to analyze the effect of the liquefaction of san on the seismic response of the SSI system, the effective stress metho of consiering the soil as equivalent linear material in every time interval is introuce in this paper. The above proceure is realize in ANSYS program by using the ANSYS Parametric Design Language (APDL). The earthquake response of san-pile-high-rise builing is analyze, an the analysis results show the liquefaction of san has large effect on the seismic response of structure in san-pile-structure interaction system. Key Wors: Computer simulation, Soil-structure interaction, ANSYS program, Liquefaction, Seismic response. INTRODUCTION Over the last 4 years, the ynamic Soil-Structure Interaction (SSI) has attracte an intensive interest among researchers an engineers in the fiels of structural ynamics, wave mechanics an soil ynamics over the worl. The methos of their investigations inclue experimental stuy an analysis research. The analysis methos are generally classifie into two kins, analytical metho an numerical simulation methos. Due to the unerevelopment of computer technology, analytical metho was popular in the 197 s. However, the analytical metho can only be use to solve simple problems. Along with the rapi 1 Lectuer, State Key Lab. for Disaster Reuction in Civil Eng., Tongji Univ., Shanghai, 292, China. 2 Professor, State Key Lab. for Disaster Reuction in Civil Eng., Tongji Univ., Shanghai, 292, China. Email: lxlst@mail.tongji.eu.cn 3 Engineer, Office of Guangzhou Construction Science & Technology Committee, Guangzhou, 513, China. 4 Associate Professor, Institute of Civil Eng., Wuhan Univ., Wuhan, 4372, China.
progress in the art of computer science, now numerical simulation methos are wiely use to the stuy on SSI. Numerical simulation methos are roughly sorte into three kins, such as substructure metho (Chopra [1]), finite element metho an hybri metho (Toki [2]). Substructure metho is commonly applie to linear analysis ue to the use of the superposition principle. Finite element metho is applicable for most complex cases. Hybri metho can be consiere as the integrating of analytical metho an numerical metho, or the integrating of ifferent numerical methos in ifferent fiels, which can take full avantage of these methos. Three-imensional finite element analysis on a practical engineering consiering SSI is carrie out in this paper. In the computer simulation on SSI system, the nonlinear behavior of layere soil is simulate with the commonly use equivalent linearity moel, an viscous bounary is aopte as bounary of soil. A computational metho of investigation on practical engineering consiering SSI by general-purpose finite element program ANSYS is explore in this paper, which is of great avantage to the popularization of SSI stuy an promote the stuy outcomes to guie practical engineering. Furthermore, in orer to analyze the effect of the liquefaction of san on the seismic response of the pile an the superstructure, the effective stress metho of consiering the soil as equivalent linear material in every time interval is introuce in this paper. The above proceure is realize in ANSYS program by using the APDL, an earthquake response of san-pile-high-rise builing is analyze. The calculation results show that the liquefaction of san has large effect on the seismic response of structure in san-pilestructure interaction system. THREE-DIMENSIONAL FINITE ELEMENT ANALYSIS ON SSI SYSTEM Combining general-purpose finite element program ANSYS, three-imensional finite element analysis on SSI system has been carrie out. The rationality of the moeling metho has been verifie in previous stuy by comparison analysis between the calculation an the shaking table moel tests (Lu [3]). Brief Description of a Practical Engineering A cast-in-place frame structure supporte on pile-raft founation is stuie in this paper. The layout of column gri is shown in Fig.1. The frame structure has 12 stories abovegroun an one story unergroun. The height of unergroun floor is 2.8m, while the height of groun floor is 4.5m an the height of other floors is 3.6m. The thickness of cast-in-place floorslab is 12mm, the imensions of column, bounary beam, an walkway beam are 6 6mm, 25 6mm, an 25 4mm, respectively. The raft thickness of pile-raft founation is.8m, the imension of pile is 45 45mm, an the length of pile is 39m with.7m entering the bearing stratum. The layout of pile-raft founation is shown in Fig.2. The eforme bar of grae II (The yiel strength f y is 34MPa) is use as main reinforcement, an the concrete grae is C3 (The compressive strength f c is about 21MPa). 21 6 6 141 141 6 6 6 6 6 6 6 42 Fig.1 Layout of Column Gri 42 Fig.2 Layout of Pile-Raft Founation
The istribution of soil near the First Shimen Roa of Shanghai is use (DGJ8-11-1999 [4]). Accoring to the classification of soil category efine in Shanghai local founation esign coe of DGJ8-11-1999, the layers of soil from top to bottom are 1fill, 3very soft gray silty clay, 4very soft gray clay, 5-1 gray clay, 5-2 gray silty clay, 5-3 terreverte clay, 7strawyellow-gray silty san. The shear wave velocity an mass ensity of soil are shown in Table 1. Dynamic Constitutive Moel of Soil In this paper, equivalent linearization moel of soil is aopte to simulate the material nonlinearity of soil. Base on the relationships of G -γ an D-γ, a set of G, D anγ is obtaine harmoniously by iteration. G -γ enotes the relationship between ynamic shear moulus G an ynamic shear strain γ, while D-γ enotes the relationship between amping ratio D an ynamic shear strainγ. The soil s skeleton curve of Davienkov moel is aopte in this paper, an the relationship of G /G max - γ is shown as Equation 1. G = 1 H ( γ ) (1) G max A 2B r H = ( γ / γ ) ( γ ) B (2) 1 + ( γ 2 / γ r ) 2 G = ρv (3) max s γ (.1σ ) 1 3 γ γ γ = (4) G max is the maximum ynamic shear moulus of soil, ρis the mass ensity of soil, an V s is the shear wave velocity of soil. γ r is a shear strain for reference. σ is the average effective confining pressure of soil, an its unit is kpa. The values of parameter A, B an γ γ are shown in Table 2. The hysteresis loop of soil D/D max -γ is expresse as following empirical formula. D G β = (1 ) (5) Dmax G max D max is the maximum amping ratio of soil. βis the shape factor of curve D /D max -γ, an 1. is chosen as β for soft soil of Shanghai area. The value of D max is reference to Table 2. No. Bottom Depth (m) Table 1 Soil Property Mass Density (kg/m 3 ) Shear Wave Velocity (m/s) 1 3.5 18 9 3 8. 174 14 4 17.6 17 14 5-1 26.5 177 16 5-2 35.2 181 18 5-3 41.3 199 21 7 >41.3 196 >21 Table 2 Parameter of Davienkov Moel for Shanghai Soil γ γ Soil Type A B D max (1-3 ) clay 1.62.42.3.6 silt 1.12.44.25.8 san soil 1.1.48.25 1. meium coarse san 1.1.48.25 1.2
In ANSYS program, there is a kin of parametric esign language name APDL, which is a scripting language. Users can use it to automate common tasks or even buil moels in terms of parameters. The equivalent linearity moel is realize in ANSYS program by using the APDL, an the calculation of material nonlinearity is realize automatically. Viscous Bounary of Soil The use of finite element metho for SSI stuy ictates that the infinite meium is truncate along certain bounaries (calle artificial bounaries) an thus is reuce to a finite region (calle near fiel). In orer to have meaningful results, the artificial bounaries, which actually o not exist, can transmit waves from near to far fiel without reflections, or at least the wave reflections back into near fiel can be ignore. The artificial bounary conitions may be also interprete as the constitutive equations for the interaction forces between near an far fiels; thus their performance in SSI analysis epens on how correctly they escribe these forces. The artificial bounary conitions can be classifie as viscous bounary, superposition bounary, paraxial bounary, extrapolation bounary an so on (Lysmer [5]) (Lysmer [6]) (White [7]). The viscous bounary is the most commonly use bounary conitions in practice as it has a simple form suitable for finite element formulation an nonlinear analysis. The viscous bounary is aopte in this paper. Viscous bounary is equivalent to setting a series of ampers on artificial bounary to absorb wave energy. The amping coefficients of ampers have no relation to frequency. In this paper, viscous bounary is implemente by spring-amper element in ANSYS program. By using symmetry principle, the meshing of above practical engineering consiering SSI is shown in Fig.3. Earthquake wave is inputte along transverse irection of the structure. Tenfol transverse size of structure is chosen as soil size, an viscous bounary is put on transverse bounary of soil. The viscous bounary is not rawn in Fig.3 in orer to see the meshing clearly. z x y Fig.3 Meshing of SSI System El Centro earthquake recor, whose peak values of acceleration is ajuste to.1g, is inputte from the bottom of soil along X axis. In orer to analyze the effect of the viscous bounary of soil, the computational analysis is carrie out uner following three conitions in this paper. 1 Thirtyfol transverse size of structure is chosen as soil size, an free bounary is put on transverse bounary of soil. 2 Tenfol transverse size of structure is chosen as soil size, an viscous bounary is put on transverse bounary of soil. 3 Tenfol transverse size of structure is chosen as soil size, an free bounary is put on transverse bounary of soil.
The plane center of groun floor is chosen as origin of coorinates. The comparison of isplacement response between the above-mentione conition 2 an conition 1 is shown in Fig.4. Abovementione conition 1 is consiere as infinite fiel in half space approximately. Point A13 in Fig.4 is the central point on the top of structure, an its elevation is 44.1m. Point CX1 is on the surface of soil intersection with structure, an its coorinate is (7.5,, -.6). Figure 4 shows that the isplacement timehistory curves of corresponing points are approximately coincient. This conclusion can also be rawn from the comparison between other corresponing points in the soil-pile-structure system..4.8 Displacement (m).2. -.2 A13_conition 1) A13_conition 2) Displacement (m).4. -.4 CX1_conition 1) CX1_conition 2) -.4 8 16 24 32 4 Time (s) Fig.4 Comparison of Displacement Response -.8 8 16 24 32 4 Time (s) Table 3 shows the comparison of isplacement peak value of points on soil surface along vibration irection between the above-mentione three conitions. It shows that there is small ifference between results uner conition 2 an conition 1, while the ifference between results uner conition 3 an conition 1 is much larger. Therefore the size of computational moel an computer resource can be reuce greatly by using viscous bounary, an the computational accuracy is still assure. Table 3. Displacement Peak Value of Points on Soil Surface along Vibration Direction Coorinate X (m) Conition 1 Conition 2 Conition 3 Peak Value of Displacement (m) Peak Value of Displacement (m) Error to 1) (%) Peak Value of Displacement (m) Error to 1) (%) 7.5.6229.6154-1.2179.7664 23.4647 1.575.5964.5951 -.22941.7455 24.9932 14.1.5877.5893.27211.7415 26.1542 17.625.5819.5889 1.2171.7439 27.84853 21.15.5784.5886 1.76267.7471 29.16283 24.675.5757.59 2.4784.7529 3.7771 28.2.5735.5921 3.2378.76 32.5813 31.725.5721.5949 3.97624.7683 34.28527 35.25.574.5982 4.8676.7777 36.33333 Input Excitation El Centro recor an Shanghai artificial wave is aopte as input excitation. Frequency content of these two seismic waves is shown in Fig.5 an Fig.6.
Fig.5 Frequency Content of El Centro Fig.6 Frequency Content of Shanghai artificial SSI Stuy on Different Soil Properties Suppose the equivalent ynamic shear moulus of above-mentione soil of Shanghai area after iteration is G. Three-imensional finite element calculation on SSI with ifferent soil properties, such as.2g,.5g, 1G, 2G, 3G an 5G, is carrie out..2g,.5g, 1G, 2G, 3G an 5G enote that the ynamic shear moulus of soil are.2,.5, 1, 2, 3 an 5 times of G, respectively. El Centro earthquake recor, whose peak values of acceleration is ajuste to.1g, is inputte from the bottom of soil along X axis. Natural frequency Table 4 shows the natural frequency of SSI system of ifferent soil properties. It is clear that natural frequency increases along with the increase of the ynamic shear moulus of soil, an furthermore, the increment of higher orer is larger than that of lower orer. The natural frequency of SSI system is lower than that of structure supporte on rigi groun, that is to say, the natural frequency of the structure system ecreases an perio increases uner consieration of SSI. No. Table 4 Natural Frequency of SSI System with Different Soil Properties Frequency (Hz) A:.2G B: 1G C: 5G D: Rigi Groun Error to B (%) Frequency (Hz) Frequency (Hz) Error to B (%) Frequency (Hz) Error to B (%) 1.22-52.3.423.471 11.3.57 19.7 2.29-38.3.471 1.17 115.9 1.58 235.4 3.321-51.2.658 1.458 121.7 2.845 332.5 4.364-49.4.719 1.525 112.1 4.224 487.6 5.47-54.1.888 1.67 81. 5.783 551.3 Seismic response of structure Fig.7 presents the acceleration peak value, isplacement peak value, interstory shear an overturning moment of the structure. It is obvious that the seismic response of structure is very complicate along with the change of ynamic shear moulus of soil. By analysis of natural frequency of SSI system, it is foun that participation of the first moe shape is most notable along the vibration irection when ynamic shear moulus is.2g, while participation of the secon moe shape enhances graually along with the increase of ynamic shear moulus of soil.
5 5 4 4 3 2 1.2G.5G 1G 2G 3G 5G 3 2 1.2G.5G 1G 2G 3G 5G -1..5.1.15.2.25 Acceleration Peak Value (g) -1..5.1.15.2.25.3 Displacement Peak Value (m) 5 4 3 2 1.2G.5G 1G 2G 3G 5G 5 4 3 2 1.2G.5G 1G 2G 3G 5G -1 3 6 9 12 15 18 Interstory Shear Amplitue ( Á1 3 kn) -1 1 2 3 4 Overturning Moment Amplitue ( Á1 3 kn.m) Fig.7 Seismic Response of Structure (Soil of Different Properties) Effect of SSI on isplacement peak value of structure supporte by ifferent soil Table 5 inicates the effect of SSI on isplacement peak value of the structure supporte by soil with ifferent properties. When SSI is not taken into account, the groun shock, which is inputte from structure bottom, is the acceleration time-history of surface point, aequately far away from the structure. Issues rawn from Table 5 are as follows. 1) Displacement peak value of the structure uner consieration of SSI is commonly larger than that of the structure supporte by rigi groun. 2) SSI has notable effect on isplacement peak value of the structure at bottom part, while has less effect on isplacement peak value of the structure at top part. 3) The effect of SSI on isplacement peak value of the structure becomes larger along with the ecrease of the shear moulus of soil. Effect of SSI in Shanghai soft soil area In Fig.8, the effect of SSI in Shanghai soft soil area on acceleration peak value, interstory rift, interstory shear an overturning moment of the structure are presente. From Fig.5, the acceleration peak value, interstory shear an overturning moment of the structure uner consieration of SSI are smaller than those uner conition of rigi groun, respectively. The maximum value of reuction is 1.4%, 8.2% an 7.7%, respectively. The interstory rift of the structure uner consieration of SSI is larger than that uner conition of rigi groun near the top of the structure, while less than that near the bottom of the structure.
The maximum change is 36.6%. The maximum of acceleration peak value is at the top of the structure, while maximums of interstory shear an overturning moment are at the bottom of structure. The maximum of interstory rift is between groun floor an the secon floor above the groun floor, because the stiffness of unergroun floor is much larger than that of the groun floor. Table 5 Displacement Peak Value of Structure along Vibration Direction (Different Soil Property) Soil A:.2G B: 1 G C: 5 G Property I (m) II (m) III (%) I (m) II (m) III (%) I (m) II (m) III (%) -2.8.53 --.59 --.28 --.54.2 2379.63.9 622.39.4 559 4.5.59.11 443.78.43 83.2.37.21 7.9 8.1.63.18 254.94.7 34.4.41.34 2.7 15.3.73.32 131.133.123 8.5.62.55 12.5 22.5.83.44 88.8.181.17 6.8.82.79 3.6 29.7.92.54 69.7.222.29 6.4.13.13.16 36.9.99.62 61.5.253.237 6.9.12.122-1.57 44.1.16.66 59.8.274.253 8.2.131.133-1.63 5 5 4 4 3 2 1 SSI Rigi Groun 3 2 1 SSI Rigi Groun.5.1.15.2.25.3 Acceleration Peak Value (g).1.2.3.4 Interstory Drift Amplitue (m) 5 5 4 SSI Rigi Groun 4 SSI Rigi Groun 3 3 2 1 2 1-1 4 8 12 16 2 Interstory Shear Amplitue ( Á1 3 kn) -1 1 2 3 4 5 Overturning Moment Amplitue ( Á1 3 kn.m) Fig.8 Seismic Response of Structure (Comparison between SSI an Rigi Groun)
Effect of SSI on Displacement Peak Value of Structure with Different Rigiity Table 6 shows the effect of SSI on isplacement peak value of the structure with ifferent rigiity. The ifferent rigiity of the structure is realize by aopting ifferent grae of concrete. Issues rawn from Table 6 are as follows. 1) SSI has a notable effect on isplacement peak value of the structure at bottom part, while has less effect on isplacement peak value of the structure at top part. 2) The effect of SSI on isplacement peak value of the structure is larger along with the increase of the structure rigiity. Table 6 Displacement Peak Value of Structure along Vibration Direction (Different Concrete Grae) Grae of A: C2 B: C3 C: C4 Concrete I (m) II (m) III (%) I (m) II (m) III (%) I (m) II (m) III (%) -2.8.56 --.59 --.58 --.59.1 482.63.9 623.62.7 731 4.5.72.5 43.5.78.43 83.2.77.37 19 8.1.88.83 6.67.94.7 34.44.91.6 51.4 15.3.131.145-9.64.133.123 8.53.123.15 16.4 22.5.185.21-8.15.181.17 6.81.165.146 12.9 29.7.231.247-6.51.222.29 6.4.21.179 11.8 36.9.266.279-4.79.253.237 6.94.228.23 12.1 44.1.289.298-3.1.274.253 8.25.246.217 13.3 Note: I -- Displacement peak value of structure uner consieration of SSI; II -- Displacement peak value of structure without consieration of SSI; III -- Relative error between I an II, namely III=(I-II)/II*1%. Seismic Response of Structure uner Different Excitation Fig.9 isplays the acceleration peak value, isplacement peak value, interstory shear an overturning moment of the structure uner excitation of El Centro wave an Shanghai artificial wave. The acceleration peak values of El Centro wave an Shanghai artificial wave are both ajuste to.1g. It shows that the seismic response of structure uner the excitation of Shanghai artificial wave is obviously bigger than that uner the excitation of El Centro wave. The main reason is that the low frequency of Shanghai artificial wave is very abunant, an the frequency of SSI system is very low. TWO-DIMENSIONAL EFFECTIVE STRESS ANALYSIS ON SSI SYSTEM Constitutive Moel of Soil In this paper, Drucker-Prager moel is aopte as static constitutive moel of san. An the soil s skeleton curve of Davienkov moel is applie as ynamic constitutive moel of soil. As shown in Fig. 1, the ynamic shear moulus an shear intensity of saturate soil ecline uner excitation of cycle loa. The ecline may be cause by increasing of pore water pressure an can be escribe by ecreasing
the G max an τ max. Supposing the ecrease maximum shear moulus an shear intensity as G mt anτ mt, then: 5 5 4 4 3 2 1 El Centro Shw2 3 2 1 El Centro Shw2-1..1.2.3.4.5.6 Acceleration Peak Value (g) -1..2.4.6.8 1. Displacement Peak Value (m) 5 5 4 El Centro Shw2 4 El Centro Shw2 3 3 2 1 2 1-1 1 2 3 4 Interstory Shear Amplitue ( Á1 3 kn) -1 2 4 6 8 1 12 Overturning Moment Amplitue ( Á1 3 kn.m) Fig.9 Seismic Response of Structure (Comparison between El Centro an Shanghai Wave) τ G max Gmt Initial hysteresis loop Following hysteresis loop Following skeleton curve γ Initial skeleton curve Fig. 1 Stress-strain curve uner excitation of cyclic loa
G mt ( 1 * ) 2 ( ( v ) ) = G max 1 u (6) τ = τ max 1 (7) mt u * * u is the pore water pressure ratio. v is a constant an the value of v is usually taken as 3.5~5.. The hysteresis loop of soil D/D max -γ is expresse as following empirical formula. D D max (1 G ) β = (8) D max is the max amping ratio of soil. βis the shape factor of curve D /D max -γ, an 1. is chosen as β for soft soil of Shanghai area. The value of D max is reference to Table 2. G mt Pore Water Pressure Moe of San Soil The increment moe of pore water pressure of Shanghai san soil coul be expresse as follows: u * 2 ( N N ) 1 θ = u / σ = (1 mα s ) 2 π arcsin f (9) u (1 mα ) N * s 1 2θ u = = ( ) (1) σ 1 θ πθn 1 ( N N f ) N f u is pore water pressure inuce by seismic vibration within the time of σ is initial average effective stress. m is a constant from test, an the value is usually taken between 1. an 1.2. α s is the level of static stress, an can be figure out by equation 12. N T. N is equivalent vibration times in every time interval, an can be figure out by equation 15. N is total vibration times, an N = N. θ is a constant, an See eems that.7 can be chosen as the value for most soil. N f is the vibration times when liquefaction occur. An it can be calculate from equation 11. b an = τ (11) f σ τ is peak value of cycle shear stress. a an b are constant from test. Assuming the estroye area is the maximum cycle shear area uner plain strain state, initial static shear ratio α s an ynamic shear stress ratioα can be rawn from following equations (Chen, [8]). α 2 2 s = 2τ xy ( σ x + σ y + 2σ c ) 4τ xy (12)
α τ = = σ ( σ x + σ y 2τ + 2σ ) c 2 xy, 4τ 2 xy + σ x σ y (13) σ x, σ y an τ xy is static normal effective stress an static shear stress on horizontal area, respectively. σ = c ctgϕ c, c an ϕ is cohesive strength an internal friction angle, respectively. The value of c is zero for pure san soil. τ xy, is equivalent cycle peak value of seismic shear stress on horizontal area. N is equivalent vibration times in every time interval, an can be figure out as follows. First, the uration time T an the effective vibration time energy of the seismic wave in time interval of time T is calculate: N eq is looke up in Table 7. Then, the ratio between Ti an the corresponing energy in the whole uration SA( T ) = i t i t i 1 2 a ( t) t T 2 a ( t) t (14) N = N SA T ) (15) eq ( i Dynamic analysis metho with effective stress by step by step iteration in every time interval. The analysis metho is shown in etail as follows. (1) The static effective stress σ x, σ y an τ xy are worke out through static analysis. (2) The whole perio of the seismic wave is ivie into some equal intervals. The initial ynamic shear moulus an the initial amping ratio are etermine for each of element. An analysis using the above-mentione ynamic constitutive moel of san soil is carrie out by step by step iteration metho in the first time * interval. (3) N in this time interval an the accumulative value N are figure out. (4) u in this * time interval an the accumulative value u are figure out using equation 9 for each of element. (5) G mt an D mt consiering the effect of the pore water pressure are figure out for each of element. An the calculate value is use as the initial value for the next time interval. (6) Using the restart function of ANSYS program, the ynamic shear moulus an amping ratio are change into the value worke out in the step (5). An the calculation of next time interval is carrie out without exiting the program. In this way, the continuity of result can be ensure. The above step (2)~(6) shoul be repeate for each of time interval until the seismic wave finishes. The magnitue of time interval has an effect on the calculation result. Reasonable magnitue of time interval relates with the site conition an the property of inputte seismic wave. Generally, the pore water pressure shoul have at least one whole cycle in each time interval.
DYNAMIC ANALYSIS OF SAND-PILE-TALL BUILDING SYSTEM CONSIDERING SAND LIQUEFACTION Brief Description of a Practical Engineering A cast-in-place frame structure supporte on pile-raft founation is stuie in this paper. The layout of column gri is shown in Fig.1. The frame structure has 16 stories abovegroun an one story unergroun. The height of unergroun floor is 2m, while the height of groun floor is 4m an the height of other floors is 2.8m. The imensions of column, bounary beam, an walkway beam are 6 6mm, 25 6mm, an 25 4mm, respectively. The raft thickness of pile-raft founation is 1.m, the imension of pile is 45 45mm, an the length of pile is 43m. The layout of pile-raft founation is shown in Fig.2. The groun soil is uniform saturate san an is 7 meters in thickness, which is on the top of the berock. The constitutive moel of soil, the equation of amping an some other parameters in the calculation can refer to corresponing item mentione above. The soil an pile are simulate by twoimensional plain strain element. The column an beam are simulate by two-imensional beam element. Simple truncation bounary, which is 6 meters far from the structure, is use as the lateral bounary. El Centro recor is applie as inputte excitation. An the peak value is ajuste to.3g. One secon is taken as the time interval. Table 7 The value of N eq an T Earthquake N Magnitue eq (time) T (secon) 5.5~6 5 8 6.5 8 14 7 12 2 Pore pressure ratio 1..8.6.4.2 A B C A_-4.5m D_-27.5m B_-14m E_-33m C_-22.5m F_-48m D E 7.5 2 4. F 8 3 6 9 18 27 36 Time (s) Fig.11 The increment curve of pore pressure ratio The increment curve on pore water pressure ratio of san soil is shown in Fig. 11. Liquefaction occurre at the thir secon in point 4.5m eep, an this phenomenon occurre at the fourth secon in point 14~22.5m eep, while no liquefaction occurre in point 3~7m eep. Especially in san at the top of pile, the pore water pressure ratio is smaller than.1. The comparison of acceleration response between the effective stress metho an general stress metho is shown in Fig. 12. An the comparison of corresponing Fourier spectra is shown in Fig. 13. In the figure, soil inner is the point in soil among piles, which is 25m eep from surface of soil. An soil surface is on the surface of soil, which is 3m far away from the structure. It shows that the liquefaction an softening of groun soil coul magnify low frequency wave an filter high frequency wave. Wave frequency higher than 5Hz are almost filtere. Having taken the affect of the pore water pressure into account in the effective stress metho, the whole system becomes more flexible comparing with that in the general stress metho. The effect of wave filter on high frequency become more eviently an the
peak value of Fourier spectrum curve move to lower frequency. The peak value of acceleration response of the soil inner an soil surface, which is calculate from the effective stress metho, is 1% smaller than that from the general stress metho. The peak value of acceleration response of the groun floor an the top floor is 1% an 17% smaller than that from the general stress metho, respectively. Because liquefaction occurs in the soil near the surface at the thir secon an the seismic wave can not be transferre any more, the acceleration response on the surface of the groun soil becomes very small after the thir secon. The acceleration response of the super structure iminishes graually after liquefaction occurs in most of soil. The acceleration response of the soil inner point keeps large because liquefaction oes not occur in this point. 1.2.6 Top floor_effective stress metho 1.2.6 Top floor_general stress metho.. -.6 -.6-1.2 9 18 27 36-1.2 9 18 27 36.8.4 Groun floor_effective stress metho.8.4 Groun floor_general stress metho.. Acceleration (g) -.4 -.8 9 18 27 36.6.3. -.3 Soil inner_effective stress metho -.6 9 18 27 36 Acceleration (g) -.4 -.8 9 18 27 36.6.3. -.3 Soil inner_general stress metho -.6 9 18 27 36.6.3 Soil surface_effective stress metho.6.3 Soil surface_effective stress metho.. -.3 -.3 -.6 9 18 27 36 Time (s) -.6 9 18 27 36 Time (s) Fig.12 The comparison of acceleration response between the effective stress an general stress metho.48.36 Soil inner_effective stress metho.8.6 Soil inner_general stress metho.24.4 Acceleration (g/hz).12.. 2.5 5. 7.5 1..2.15 Soil surface_effective stress metho Acceleration (g/hz).2.. 2.5 5. 7.5 1..8.6 Soil surface_general stress metho.1.4.5.2.. 2.5 5. 7.5 1... 2.5 5. 7.5 1. Frequency (Hz) Frequency (Hz) Fig.13 The comparison of acceleration Fourier spectra between the effective stress an general stress metho 14
CONCLUSIONS In this paper, combining general-purpose finite element program ANSYS, research on practical engineering consiering SSI through both the general stress metho an the effective stress metho have been carrie out, which is of great avantage to the popularization of SSI stuy an to the stuy outcomes to guie practical engineering. Issues rawn from the stuy are as follows. 1) Natural frequency of SSI system increases along with the increase of ynamic shear moulus of soil. 2) Seismic response of structure uner consieration of SSI is very complicate along with the change of ynamic shear moulus of soil. 3) SSI has notable effect on isplacement peak value of structure at bottom part, while has less effect on isplacement peak value of structure at top part. 4) The effect of SSI on isplacement peak value of structure becomes larger along with the ecrease of shear moulus of soil. 5) The effect of SSI on isplacement peak value of structure becomes larger along with increase of structure rigiity. 6) The liquefaction of san has large effect on the seismic response of structure in san-pile-structure interaction system. ACKNOWLEDGEMENT This project is carrie out uner the sponsorship of the key project (No.525821) an the youth project (No.53818) of National Natural Science Founation of China. REFERENCES 1. Chopra AK, Perumalswami PR. Dam-founation interaction uring earthquakes. Proceeings of 4th Worl Conference Earthquake Engeering, Santiago, Chile.1969. 2. Toki K, Sato T. Seismic response analysis of surface layer with irregular bonaries. Proceeings of 6th Worl Conference On Earthquake Engeering, New Dehli, Inia. 1977. 3. Lu XL, Li PZ, Chen B, an Chen YQ. Numerical Analysis of Dynamic Soil-Box Founation- Structure Interaction System, Journal of Asian Architecture an Builing Engineering, 22; 1(2): 9-16 4. Founation Design Coe (DGJ8-11-1999). Shanghai, China, 1999. 5. Lysmer J, Kulemeyer RL. Finite Dynamic Moel for Infinite Meia. J. Eng. Mech. Div., ASCE 1969; Vol.95: 759-877. 6. Lysmer J, Wass G. Shear Waves in Plane Infinite Structures. J. Eng. Mech. Div., ASCE 1972; Vol.98: 85-15. 7. White W, Valliappan S, Lee IK. Unifie Bounary for Finite Dynamic Moels. J. Eng. Mech. Div., ASCE 1977; Vol.13: 949-964. 8. Chen GX, Xie JF, Zhang KX. Effect of Founation Soil Liquefaction on Earthquake Response of Pile-supporte High-rise Builing System. Earthquake Engineering an Engineering Vibration 1995; 15(4): 93-13. 15