Unexpected high initial stiffness ASDConcrete3D

[Sebastian Torres]

Dear STKO staff,

I am currently working on using STKO Opensees to model reinforced partially grouted concrete block masonry walls subjected to cyclic loading. The experimental results I am referring to are from the paper titled "In-Plane Cyclic Performance of Confined Partially Grouted Masonry Walls with Joint and Vertical Reinforcement" by Hidalgo-Leiva, Diego A., Andrés Picado-Arguedas, and Natalia Sánchez-Vargas (Engineering Structures, 245, 15 October 2021, 112881, https://doi.org/10.1016/j.engstruct.2021.112881).

The general geometry and experimental setup can be seen in the attached figures:

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In my modeling approach, I am using layered shell elements with plate fiber reinforcement. The reinforcements are represented as smeared rebar layers in both the horizontal and vertical directions, with equivalent thickness. The confinement frame concrete and concrete block masonry (assumed to be homogenized) are modeled using the ASDConcrete3D material model. Additionally, I have information about the experimental masonry compression strength, which is fm = 16.4 MPa. The typical elasticity for this masonry material is approximately 900 times E = 14760 MPa.

However, I am facing a consistent problem regardless of the stress-strain relation defined for the masonry. The stiffness of the initial cycles in my simulations is unexpectedly high, even when I vary the mesh size, as shown in the following figure:

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To investigate further, I attempted removing the wall reinforcements to determine if the steel layer approach was causing the issue:

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I have also previously tried modeling the columns as beam elements, but this did not resolve the stiffness problem.

Based on my observations, I suspect that the issue may be related to the interaction between the confinement frame and the masonry. Is it possible that introducing some form of interaction between these elements could help resolve the problem?

Considering the information provided, I would greatly appreciate any suggestions or ideas you may have to address this issue. Do you believe that the current approach could adequately represent the behavior of this complex material?

I have also attached the model that represents the simplest version, utilizing the 1P ASDConcrete3D material for both the masonry and concrete. It's worth noting that this simplified model is suitable for addressing the specific issue I am seeking assistance with - the initial stiffness problem.

M01.zip

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Thank you for your attention and assistance.