Abaqus Earthquake Analysis -

Mastering Abaqus Earthquake Analysis: A Comprehensive Guide to Seismic Simulation

6. Practical Example Outline (Explicit Time-History)

** JOB: Building on soft soil – M7.0 earthquake
*HEADING
*PREPRINT, MODEL=YES
*PART, NAME=SOIL
... nodes and elements ...
*ELSET, ELSET=INF (for infinite elements)
*SOLID SECTION, ELSET=SOIL, MATERIAL=SOILMAT
*PART, NAME=BUILDING
... beams/columns using B31 elements ...
*MATERIAL, NAME=SOILMAT
*ELASTIC
 50e6, 0.3   (E, nu for soft clay)
*DAMPING, BETA=0.01   (Rayleigh beta only)
*STEP, NAME=Geostress
*GEOSTATIC
*DLOAD
 SOIL, GRAV, 9.81
*STEP, NAME=Earthquake, NLGEOM=YES
*DYNAMIC, EXPLICIT
 , 20.0   (20 sec duration)
*BOUNDARY, TYPE=VELOCITY
 BASE, 1, 1, v_x(t)   (velocity history)
*CONTACT
*END STEP

7. Conclusion

Abaqus provides powerful tools for earthquake analysis, but success requires correct boundary treatments, appropriate damping models, and solver selection. The Explicit solver with infinite elements and Rayleigh damping (ALPHA=0) is a robust starting point for non-linear SSI problems. Engineers must always verify energy balance and mesh resolution to avoid spurious reflections. For critical infrastructure, validation against shaking table tests or benchmark problems (e.g., NEEShub) is essential.

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3. Damage Visualization

• For CDP concrete models, output the variables DAMAGE-T (Tension) and DAMAGE-C (Compression). Plotting these contours allows you to visualize where cracks have formed (value of 1.0 implies full damage).

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4. Step-by-Step Implementation in Abaqus/Standard (Implicit)

Step 1: Build the FE model

• Use solid (C3D8R) or beam (B31) elements. For soil, use continuum elements with pore pressure (CPE8RP) if undrained response is important.

Step 2: Perform eigenvalue extraction

*STEP, NAME=Eigen, PERTURBATION
*FREQUENCY, EIGENSOLVER=LANCZOS, NORMALIZATION=MASS
20
*END STEP

Step 3: Apply gravity load (static step)

*STEP, NAME=Gravity, NLGEOM=YES
*STATIC
0.01, 1.0
*DLOAD
ALL_ELEMS, GRAV, 9.81, 0., -1., 0.
*END STEP

Step 4: Seismic time history step

*STEP, NAME=Earthquake, NLGEOM=YES, INC=10000
*DYNAMIC, HHT-ALPHA=-0.05
0.01, 30.0, 1e-7, 0.01
*BOUNDARY, TYPE=ACCELERATION, LOAD CASE=1
BASE_NODE, 1, 1, 9.81
*AMPLITUDE, NAME=ACC_X, INPUT=eq_x.txt
*DAMPING, ALPHA=0.12, BETA=0.002
*END STEP

5. Advanced: Nonlinear Time History in Abaqus/Explicit

Abaqus/Explicit is the tool of choice for:

• Collapse simulation.
• Pounding between adjacent buildings.
• Seismic response of base-isolated structures (nonlinear isolator elements).
• Rocking foundations.

Transition from Implicit:

• Replace Static, General with Dynamic, Explicit using Smooth Step Amplitude to slowly apply gravity over 0.1–0.5 seconds to avoid shock loading.
• Use Mass Scaling carefully to increase computational stability without adding excessive mass. Target a stable time increment (( \Delta t \leq L_e / c_d )).

Adding Earthquake in Explicit:

• Apply a Boundary Condition with prescribed acceleration to the base nodes.
• Use Tabular Amplitude referencing your acceleration record.
• Output at intervals of 0.01–0.02 seconds for manageable file sizes, but use an increment of ~1e-5 to 1e-4 seconds for stability.

Energy Balance Monitoring: Always request ALLIE (internal energy), ALLSE (strain energy), ALLKE (kinetic energy), and ALLVD (viscous dissipation). The sum should remain constant. abaqus earthquake analysis

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