Numerical Simulation of Structure Reaction in Different Soil.pdf

8/9/2019 Numerical Simulation of Structure Reaction in Different Soil.pdf

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Numerical Simulation of the Upper

Structure Reaction in Different

Properties of Foundation Soil and

Underground Water Levels During an

Earthquake

Ming-xun HouSchool of Civil Engineering and Transportation, South China University of

Technology, Guangzhou, Guangdong, 510641, China

e-mail: [email protected]

Ying-guang FangSchool of Civil Engineering and Transportation, South China University of



Ren-guo Gu * School of Civil Engineering and Transportation, South China University of



Li-min WuGuangzhou City Construction College, Guangzhou, Guangdong, 510641,

China


ABSTRACTIn this paper, use the PLAXIS finite element program, through the calculation of a 6-storey

frame structure in different geological conditions, diff erent foundation stiffness and

different groundwater conditions, to analyze the effects on structure seismic response ofthree factors mentioned above which cannot be fully considered in the current seismic

design seismic, whereby some useful suggestions benefit to conceptual design and seismic

calculation has been obtained.

KEYWORDS: finite element; PLAXIS; foundation; structure

INTRODUCTIONThe issues regarding combined action of superstructure and foundation is hot subject in

geotechnical engineering filed. For a long time, due to the limit of calculation methods,

superstructure and foundation are usually separated for calculation in actual engineering design.

In the current seismic design, the seismic influence coefficient of the building structure is

determined according to site (foundation) category, design earthquake grouping, seismic

intensity, structure natural frequency and damping ratio, which can determine the seismic force

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structure suffered, and then make the corresponding cross-sectional anti-seismic checking

calculation and anti-seismic deformation calculation, to determine safety performance of anti-

seismic capacity.

In the seismic design, soil is to be considered by the site classification and design responsespectrum. But in the actual process of earthquake action, earthquake action is generallyconsidered as periodic excitation source on overlaying bedrock; seismic force effects on

foundation and superstructure after passing through upper soil layer, and the properties of

foundation soil have important impact on characteristics of structure seismic response. [1-4]

Under dynamic load effect, soil body and structure as a weighted whole, deformation and

motion controls each other. Regardless the properties of soil or structure, such as the

distribution, deformation modulus and damping of soil, the rigidity, shape and size of structure

etc., will affect the dynamic characteristics of soil and structure system[5-9].

Therefore, the research achievement of dynamic soil-structure interaction has a very

important practical significance for guiding actual engineering design[10-13], improving structural

design methods, making calculation more reasonable and economic, guaranteeing safetyeconomic efficiency of structure and reducing project cost. In order to measure the factors that

cannot be fully considered in current specification, now we apply PLAXIS geotechnical finite

element program, conduct the numerical simulation for a 6-storey building.

FINITE ELEMENT MODEL ESTABLISHMENT

Geometric model

The building is constituted with foundation and overground six stories. 8m of width and30m of length, height above ground of 6 × 3m = 18m, foundation depth of 3m. is The sum of

dead load and live load of each storey is 5kN/m2. The length of building is much larger than the

width, only consider the structural response of the width direction. Therefore, use plane strainmodel, 15-node elements for the simulation. The foundation includes 20m thick soil layer, the

damp of building is simulated with Rayleigh damping [2].

Boundary conditions

As the impact on the soil body away from structure of load can be negligible, and therefore

virtual boundary may be introduced relatively distant from structure, and set artificial damping

boundary conditions on the virtual boundary [3], to make energy is consumed on the boundary

and do not reflect into the soil which have been intercepted.

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Material properties

Table 1: Building materials' properties Parameter Subsoil Unit

Material model MC

Drainage condition TYPE Drained

UNSAT 16.5 kN·m-3

SAT 18.5 kN·m-3

Osmotic coefficient k x 0.003 m·day-1

Osmotic coefficient k y 0.001 m·day-1

Poisson's ratio ν 0.25

Young modulus( static state) 6.00E+04 kN·m-2

Cohesion c 15 kN·m-2

Internal friction angle φ 25 o

Table 2: Foundation materials' properties

Parameter Plank Pillar Unit

Material model Elastic Elastic

Normal Stiffness EA 8.00E+06 7.00E+05 kN/m

Flexural rigidity EI 8.00E+04 6.00E+03 kN·m2

/m

Specific weight ω 5 2 kN/m/m

Poisson's ratio ν 0 0

Rayleigh damping 0.01 0.01

Calculation

Calculation is processing in two steps: the first step is "plastic analysis" in the construction

process of the building; the second step is "dynamic analysis" of earthquake simulation, using

real the 1989 seismic acceleration data recorded by USGS, to read-in by SMC (Strong MotionCD-ROM) format [2].



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Figure 1: The finite element model

CHANGES OF FOUNDATION SOIL PROPERTIES

According to the classification of soil in Table 4.1.3 of Code for Seismic Design of Building

GB50011-2010, select five kinds of soil calculated as foundation [4]. Its physical and mechanical

properties [5] show in the following table.

Table 3: Different soil materials' properties

parameter Rock Hard soil Mediumhard soil

Mediumsoft soil

Soft soil Unit

Density 22 19 17 15 12 kN/m3

Young modulus 2.00E+06 8.00E+05 3.00E+05 6.00E+04 8.00E+03 kN/m2

Poisson's ratio 0.21 0.24 0.26 0.29 0.30

Rayleigh

damping0.01 0.01 0.01 0.01 0.01

Shear wave

velocity840 550 280 180 50 m/s

Under this circumstance that the form of superstructure and the inputted seismic

acceleration data is unchanged, changing the properties of soil, calculate the seismic response in

a variety of foundation soil. Perform the comparison with the seismic response data at structure

apex.



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Figure 2: The displacement response curve of Different soil materials' properties

Through the above calculation, it shows that: (1) in the process of the foundation soil

changes from rock to medium hard soil, the seismic response of structure gradually increases,

but increased significantly smaller.

（2）In the process of the foundation soil changes from medium hard soil to medium soft

soil, seismic response of structure is significantly increased, its maximum displacement

increases about 30%. According to the classification of field in Table 4.1.6 of Code for Seismic

Design of Building GB50011-2010, for the medium hard soil and medium soft fields with cover

layer thickness of 5-50m all belong to class Ⅱ field. Therefore, the seismic response of same

structure with same eigenperiod in the same area (same design earthquake grouping), should be

approximately the same, while this is inconsistent with the above calculation results.

（3）Through analysis of first four kinds of soil, it can be seen that with the decrease of

foundation soil strength, seismic response of structure is increasing; but when foundation soil

strength continues to decrease and turns to soft soil, the displacement of structure decreases on

the contrary, and its response spectrum shape has been changed greatly.

Reason analysis: In the soil-structure dynamic system, due to wave scattering of thestructure interface, dynamic response of the free field has changed. The presence of soil usually

amplifies the input wave of bedrock, and relative softness of soil reduces the frequency of soil -

structure system. Therefore the conditions of foundation are different, the properties of caused

ground motion (cycles, acceleration amplitude, etc.) are different, and thus the earthquake

effects on building are different, and the extent of generated damage to building is not the same.



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CONCLUSIONS

(1) Along with the strength of the foundation soil becomes smaller; seismic response of

structure progressively increases, under normal circumstances it is larger on soft soil than on

bedrock, but not always. For the soft soil whose strength is relatively weak, because of itssuperior long period, it might occur " shock insulation" phenomenon under severe earthquake.

(2) In the current seismic design calculations, only consider the difference of factors such

as different eigenperiod caused due to different foundation conditions, for foundation itself is

unstable under earthquake, occurring seismic subsidence, liquefaction, crack, slip, etc.

phenomena, could not be effectively considered in the calculation, but this kind of effect is

particularly severe in soft foundation. Therefore, in the seismic design of soft foundation, on the

basis of the calculation, necessary measures shall be taken to prevent foundation instability and

structure overturning.

CHANGES OF GROUNDWATER LEVELOverburden thickness in the model is 20m, process calculation when selecting groundwaterof -12m,-6m,-3m, 0m respectively.

Figure 3: The displacement response curve in different underground water level

Result analysis: when groundwater level is below -6m, no significant effect on the seismic

response of structure; underground water changes within 4-6m below ground level, has greater

effect.



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CONCLUSIONS

The underground water level is shallower; generally earthquake damage is more serious.

This is mainly due to two reasons, first the shallow underground saturated soil is easy to cause

instability because of the generated super hydrostatic pressure under dynamic stress. On theother hand, the existence of groundwater changes the dynamic characteristics of soil, making

the periodicity and acceleration amplitude of ground motion changed. Overall, the underground

water level depth has great impact on soft clay, yarn thickness such this kind of fine particles

soil, and has minor impact on coarse grained soil. Underground water changes within 4-6m

below ground level, has great effect; groundwater level is below -6m, has minor effect.

ACKNOWLEDGEMENTS

This work was financially supported by State Key Laboratory of Subtropical Building

Science (Grant 2012ZA04), Natural Science Foundation of China (NO. 2009ZX07423-004) and

Fundamental Research Funds for the Central Universities (Grant 2014ZZ0011).

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