Hello everyone,

Details of Model

I have modeled a simple 2D, 1Bay, and 1Story portal frame in OpenSees using elastic beam-column elements from the OpenSees library.

Soil is modeled using quad elements by giving PressureIndependent Multi Yield material. The nodes at the base of the soil deposit were restrained against vertical translation and constrained to have the same displacement in the horizontal direction. The nodes placed at the same depth on the two opposite lateral boundaries were tied together. Lysmer Kuhlemeyer's dashpot is modeled on the bottom left of the soil domain. Dimensions of soil domain are 60m width and 16m thick, while out of plane thickness quad elements are considered as 6m.

The interaction is modeled using giving equal DOF to the Soil nodes and structure nodes.

Problem

The time history analysis of the above structure leads to the following observations

1.The first mode time period obtained after SSI modeling is very high (>20000sec). Whereas the Second and third mode time periods are fine.

2. Most Importantly, As I increase the size of the element in the X direction the response of the structure is also increasing at a higher rate. (Drift, roof acceleration, and roof displacement).

[3. As I increase the size of the element in the X direction, the output acceleration magnitude is increasing at a higher rate for the 1D wave propagation analysis too.

It seems there is a problem with modeling of soil]

Can anyone help me to solve the following issue..?

Thanks in advance