CVEEN 7330 Modeling Exercise 2c

Transcription

1 CVEEN 7330 Modeling Exercise 2c Table of Contents Table of Contents... 1 Objectives:... 2 FLAC Input:... 2 DEEPSOIL INPUTS:... 5 Required Outputs from FLAC:... 6 Required Output from DEEPSOIL:... 6 Additional Calculations and Discussion:... 6 FLAC Helps... 7 Seismosignal Helps... 9 FLAC Solution: DEEPSOIL Solution: Required Calculations and Discussion: FLAC Source Code DEEPSOIL Input File Printout

2 Objectives: This modeling exercise compares the nonlinear dynamic results for a 1D homogeneous soil column form FLAC with hysteretic damping to those of DEEPSOIL using modified hyperbolic model. FLAC is a 2D nonlinear dynamic code. DEEPSOIL is a 1D nonlinear code. Thus, we will develop a 1D FLAC model and compare the results with DEEPSOIL output for a simple ground response analysis using a homogenous sand profile. FLAC Input: To develop the FLAC 1D model, we will use an example file and modify it. The example project file is found in: C:\Program Files\Itasca\flac500\Options\3-Dynamic\ op_03_12.prj Geometry: 20 m high model by 1 m width. Boundary Conditions: Fix all elements in the y direction Model Type: 2

3 Elastic with hysteretic damping Hysteretic damping model (use default model (2 parameter model) with following properties (default ) (see pink lines in Figures 3.27 and 3.28 below, which represent the default model parameters of and 0.823) Elastic Material Properties: density = 2000 kg/m 3 shear modulus = 0.97e8 N/m 2 = 97 MPa bulk modulus = 2e8 N/m 2 = 200 Mpa Input Acceleration Time History: Taft Record (unscaled) (obtained from course website) 3

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5 DEEPSOIL INPUTS: Type of Analysis Nonlinear Total Stress Metric Pressure-Dependent Hyperbolic Model: Masing Criteria Input Properties by Modulus Geometry: 20 m deep soil model with 20 1-m thick sand layers. Material Properties: density = kn/m 3 (2000 kg/m 3 ) shear modulus = 0.97e8 N/m 2 = 97 Mpa = kpa Shear modulus reduction curves from table below User-Defined 11 points Use the points that corresponds to the pink lines in Figure 3.27 and Figure 3.28 (see below) Shear strain (%) G/Gmax Damping (%) Bedrock Properties Rigid 5

6 Input Acceleration Time History: Taft Record (unscaled from course website) Required Outputs from FLAC: 1. Acceleration time history at base of model (node i = 1) 2. Acceleration time history at middle of model (node i=11) 3. Acceleration time history at top of model (node i=21) 4. Shear stress versus shear strain time history for middle of model (between nodes j=11 and j=10) 5. Surface acceleration response spectrum (5 percent damped) as a function of period. (Use Seismosignal and the results from no. 3 above). 6. Printout of source code for FLAC model. Required Output from DEEPSOIL: 1. Acceleration time history at base of model (layer 20) 2. Acceleration time history at middle of model (layer 10) 3. Acceleration time history at top of model (layer 1) 4. Shear stress/effective vertical stress versus shear strain time history for middle of model (in layer 10) 5. Surface acceleration response spectrum (5 percent damped) as a function of period. (Use Seismosignal and the results from no. 3 above). 6. Printout of DEEPSOIL input profile (*.dp) file Additional Calculations and Discussion: 1. Calculate the fundamental period of the 20 m thick soil column using Eq. (7.16) in Kramer. 2. Compare the maximum shear strain that develops in the middle of the layers for the FLAC and DEEPSOIL results. Discuss how well the maximum shear strains compare for both layers for both models. 3. Make a composite plot of the surface response spectra for both FLAC and DEEPSOIL. Discuss how well the response spectra compare. 6

7 FLAC Helps 1. Since you are using the dynamic option of FLAC, you must configure the dynamic extension with the following command: conf dyn ext 5 2. You need to generate a 1D soil column that is 1 m wide by 20 m high: grid Use will be using an elastic model with hysteretic damping in FLAC to compare with DEEPSOIL. The elastic model with hysteretic damping is involved with: model elastic ini dy_damp hyst default (The default hysteretic damping model in FLAC produces shear modulus and damping curves that are given Figures 3.27 and 3.28 (see pink lines). 4. You must fix the nodes in the y direction to not allow vertical movement. We only want horizontal movement so that an SH wave can propagate. fix y 5. To read in the Taft acceleration time history and apply it to the base of the model, your FLAC code should have his read 100 taft_flac.acc apply xacc 9.81 his 100 j = 1 apply yvel 0.0 j = 1 The taft_flac.acc file must be present in the same directory as the flac model. It has already been formatted to be read into flac and is found on the course website. (Note that the 9.81 multiplier in the second line is applied to the Taft record to convert the record from acceleration (g) to acceleration (m/s). You must use units that are consistent with the FLAC analysis (m, s, N, Pa, etc.) (Note also that the command apply yvel 0.0 j = 1 prevents rocking of the model along the grid point j = 1.) 6. Shear strain is not directly calculated by FLAC, so you have to create code to do so. The following subroutine needs to be in your code: 7

8 def strain1 deltay = 1; one m vertical spacing between nodes strain1 = (xdisp(1,11) - xdisp(1,10))/deltay end 7. Time history plots of acceleration, shear stress and shear strain can be created using the following commands: his 1 dytime his 2 sxy i 1 j 10 his 3 strain1 his 4 xacc i 1 j 1 his 5 xacc i 1 j 11 his 6 xacc i 1 j We want the FLAC output to be at an even timestep for plotting in Seismosignal. This can be done by the following command: set dydt = This sets the time increment to eight ten- thousandths of a second. This small timestep is also required for numerical stability. 9. We are now ready to add the command to start solving. Most of the strong motion ends after about 74 seconds, so we will solve for only the first 74 seconds of the acceleration time history. This is done with the following command: solve dytime After solving, we want to output the surface acceleration time history from FLAC so that we can create a response spectrum in Seismosignal: set hisfile surface.his his write 6 vs 1 save model.sav This creates a file called surface.his for the output and stores the results of history 6 (acceleration at surface) versus history 1 (dynamic time). The last line in the code saves the FLAC model and all output as a file called model.sav 8

9 Seismosignal Helps Note that the time increment for acceleration time history in surface.his is 8 x10-3 seconds. When you read surface.his into Seismosignal, use should use the following parameters (see below). Remember also that acceleration time history from surface.his is in units of m/s 2 and not g. To convert this FLAC file to units of g, you can use the scaling factor. In the above screen shot the scaling factor has been set to , which is equal to 1 / Once you have successfully read the time history into Seismosignal, you should create a 5 percent damped, elastic, pseudo-acceleration response spectrum in Seismosignal for the time history for comparison with DEEPSOIL. 9