NONLINEAR

Using SSI substructuring methods, the ACS SASSI software solution in complex frequency is restricted to linearized SSI analysis solutions. However, nonlinear SSI analysis including soil and structure hysteretic behavior can be performed using an iterative equivalent linearization algorithm.

For nonlinear 3D solid or 2D plane elements, typically used to model soil local nonlinear behavior, only the HOUSE and STRESS modules should be run differently while ANALYS should be run using the restart for “New Structure”. The local nonlinear soil hysteretic behavior modeled by 3D solid or 2D plane elements is handled automatically by running iterative SSI iterations by looping the HOUSE-ANALYS(restart)-STRESS modules. For the 2D soil layer models, PLANE elements should be used, while for the 3D soil models (typically in the adjacent zone to foundation), SOLID elements should be used. It should be noted that the user can select of using either the effective soil

  1. shear strain component, or
  2. maximum shear strain in 2D soil elements, or
  3. the octahedral shear strain in 3D soil elements

For the 1D soil models that implies horizontal soil layers, the SOIL module can be used for modeling the soil hysteretic behavior using either the Seed-Idriss iterative equivalent-linear model based on the SHAKE methodology, or a true nonlinear soil model based on the DEEPSOIL theory.

The Option NON includes the two modeling choices:

  1. nonlinear RC wall panel elements (macroshell elements) and
  2. nonlinear spring elements with translational dofs.

For the nonlinear structural behavior in 3D SSI analyses, the Option NON (NONLINEAR module) is required. The Option NON can address the reinforced concrete cracking and post-cracking behavior for low-rise shearwalls, nonlinear rubber base-isolators behavior, nonlinear pile-soil interface behavior, and even limited foundation sliding effects.

The NONLINEAR result files include nonlinear structure displacements, accelerations, forces in elements, ductility ratios with respect to the initial element stiffness (cracking for concrete hysteretic models), and inelastic absorption reduction factors for the forces in the nonlinear elements. These inelastic absorption factors are defined by the ratio between the initial elastic forces and the effective nonlinear forces. The inelastic absorption factors depend on the shear strain level. For concrete shearwall buildings these computed inelastic factors correspond to the inelastic absorption reduction factors defined in the ASCE 43-05 standard for different state limits that are used to evaluate the structure wall functional or ultimate capacities.

The nonlinear structure SSI or SSSI analysis also provide better predictions of the inter-building displacements for establishing the appropriate gap sizes. The motion incoherency is an additional important influential factor that can reduce substantially the gaps between the neighboringbuildings during the earthquake motion.

For the low-rise reinforced concrete shearwall buildings, the following hysteretic models are currently implemented:

  • Cheng-Mertz Shear (CMS) model,
  • Cheng-Mertz Bending (CMB) model (not included in this version) and
  • Takeda (TAK) model.

Comparison of these hysteretic models can be found in the User’s Manual.