Hello,
I am currently conducting a modal analysis on a reinforced concrete structure using Abaqus, and I am encountering an issue where all the natural frequencies calculated are negative. The model consists of a concrete slab reinforced with steel bars. Here are some specifics of my model setup:
- Element Types: Concrete is modeled with C3D8 elements, and the steel reinforcement is represented with B31 beam elements.
- Interaction: I have used a constraint with an embedded region for the interaction between the concrete and steel reinforcement.
- Material Properties: The materials are defined with realistic properties: Concrete (Young's Modulus: 51,000 MPa, Poisson's Ratio: 0.19, Density: 2.4E-09 tonne/mm³) and Steel (Young's Modulus: 210,000 MPa, Poisson Ratio: 0.3, Density: 7.85E-09 tonne/mm³).
- Boundary Conditions: The structure is constrained with fixed (Encastre) boundary conditions on two sides.
Despite ensuring the mesh quality and reviewing the boundary conditions, material properties, and element connectivity, the simulation consistently results in negative frequencies. I have checked for common issues such as inappropriate constraints, incorrect material properties, and numerical instabilities but have yet to find a solution.
Has anyone encountered a similar issue or can offer insights into potential causes and solutions for this problem in Abaqus? Any suggestions or guidance would be greatly appreciated.
In the case of a reinforced concrete structures, negative frequencies is caused by the interaction between the concrete and steel reinforcement, which can introduce additional stiffness and affect the natural frequencies of the system. Try add damping to system and re-simulate.
Engr. Tufail, thank you for your response. I added damping to the system and re-simulated, yet encountered the same error. The problem might be with the interaction. When I remove all reinforcement and re-simulate, the system functions properly. I employed an embedded region constraint. However, I am uncertain about the root of the problem.
Engr. Tufail and Nils Wagner Thank you for your assistance. I solved the problem, which originated from using a 2D wire and a beam profile for the reinforced steel. Replacing the beam with trusses resolved the negative frequency issue
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