ACS SASSI Version 3 is a state-of-the-art highly specialized finite element computer code for performing 3D linear and non-linear soil-structure interaction (SSI) analysis for shallow, embedded, deeply embedded and buried structures under coherent and incoherent earthquake ground motions. The ACS SASSI software is an extremely user friendly, modern engineering software under MS Windows with a unique suite of SSI engineering capabilities. ACS SASSI is equipped with two translators for converting inputs of structural finite element models from ANSYS (CBD file) (ANSYS is a trademark of ANSYS Inc.) or university SASSI2000 (fixed format input files) to ACS SASSI, and also from ACS SASSI to ANSYS (APDL input file format).

ACS SASSI uses an automatic management of all data resources, files, directories, and interconnections between different software modules. ACS SASSI can be run interactively for a single SSI model or batch for single and multiple SSI models.

The latest ACS SASSI Version 3 SSI capabilities incorporate many advanced algorithms and specialized features. In comparison with the standard SASSI methodology, the ACS SASSI incorporates many additional SSI capabilities and specialized features, in addition to its much faster computational speed.

Generation of three-component input acceleration time histories compatible with a given design ground response spectrum with or without time-varying correlation between the components. The user has also the option to generate acceleration histories using the complex Fourier phasing of selected acceleration records (called “seed records” in the new ASCE 4-16 standard). The software provides baseline correction and computes PSD and peak ground accelerations, velocities and displacements to be used by the analyst to check the US NRC SRP3.7.1 requirements for the simulated ground accelerations.

ACS SASSI Version 3 includes state-of-the-art modeling including both isotropic (radial) and anisotropic (directional) incoherency models. Both stochastic and deterministic incoherent SSI approaches could be employed for simple stick models with rigid base mats. These incoherent SSI approaches were validated by EPRI (Short et. al., 2007) for stick models with rigid base mats, and accepted by US NRC (ISG-01, May 2008) for application to the new NPP seismic analysis. ACS SASSI includes six incoherent SSI approaches, namely, two simplified deterministic approaches that are the AS and SRSS approaches benchmarked by EPRI (Short et al., 2007), three other alternate deterministic approaches, and a rigorous stochastic simulation approach that is called “Simulation Mean” approach included in the 2006-2007 EPRI validation studies.

There are seven plane-wave incoherency models that can be used: the Luco-Wong model, 1986 (theoretical, not validated model), five Abrahamson models (empirical, isotropic or anisotropic, based on the statistical dense array records) and user-defined incoherency models. The Abrahamson models include the coherency models published in 1993, 2005 (all sites, surface foundations), 2006 (all sites, embedded foundations), 2007a (rock sites, all foundations), 2007b (soil sites, surface foundations) and user-defined coherency models. The recent ACS SASSI versions include directional or anisotropic Abrahamson coherency models in addition to the isotropic or radial Abrahamson coherency models included in earlier releases and also used in the 2007 EPRI studies.

The user-defined coherence functions are useful for particular sites for which more detailed seismological information is available, or for sensitivity studies including the effect of soil  layering inclination or nonuniformity in horizontal plane. They are based on specific-site response data, eventually coming from the 2D nonlinear site response analysis using the equivalent-linear iterative approach for selected soil profile slices. The slice directions should in principle correspond to the two principal orthogonal directions of the soil layer slopes, namely for the maximum and minimum soil layer slopes. Thus, the user-defined coherence functions are usually different in the two selected orthogonal horizontal directions. For the incoherent SSI analysis using refined FE models with elastic foundations, we highly recommend the use of the stochastic simulation approach that includes no intrusion in the SSI system dynamics. The AS and SRSS deterministic approaches are simplified incoherent SSI approaches that should be applied only to rigid base mat stick models, as validated by the 2007 EPRI studies (Short et al., 2007). The use of AS or SRSS to elastic foundation FE models might not be necessarily appropriate since could produce considerably biased results, mostly overly conservative, but sometime unconservative, especially for the vertical direction for which the foundation base mats are much more flexible, as stated also in the new ASCE 4-16 standard.

The SRSS approach is more difficult to apply since it has no clear convergence criteria for the required number of the incoherent spatial modes to be considered for the incoherent SSI analysis.

For flexible foundations, the number of required incoherent spatial modes required to reconstruct the free-field coherency matrix could be very large, in order of several tens or even hundreds of modes on a case-by-case basis. This makes SRSS impractical and even dangerous for FE elastic foundation problems.

Nonlinear hysteretic soil behavior capability is included in the ACS SASSI main software for  seismic SSI analysis using the Seed-Idriss iterative equivalent linear procedure for both the global (due to wave propagation in free-field) and the local soil nonlinearity (due to SSI effects). The nonlinear soil site response is included in SOIL module based on SHAKE methodology. For 2D and 3D SSI models, the local soil nonlinear behavior can be included using near-field soil PLANE or SOLID elements (defined in the HOUSE model input). The ACS SASSI code uses a fast SSI reanalysis (or restart) solutions for the equivalent-linear soil SSI iterations that takes advantage of the already computed soil impedance matrix available from the SSI initiation run. This feature reduces the run time per SSI iteration by a factor of 2 to 5 times depending of the foundation embedment size. For nonlinear soil SSI analyses performed in batch mode, the simultaneous X-Y-Z input effects can be considered at each SSI iteration using the COMB_XYZ_STRAIN auxiliary program.

The iterative equivalent-linearization can be applied for modeling the cracking and post-cracking reinforced concrete wall behavior using shell elements, or for modeling the rubber-bearing hysteretic materials for the seismic base-isolation using nonlinear spring elements. The nonlinear springs can be also applied for modeling the local pile-soil interface nonlinear effects due to pile slipping in the vertical direction that can be an important aspect during intense seismic motions.

The nonuniform soil motion, or multiple seismic input motion option, includes the capability to consider variable amplitude seismic input motions. The nonuniform motion input is applicable to continuous foundations assuming that the free-field motion complex amplitude varies in the horizontal plane after specific frequency dependent spectral patterns. These patterns are described by the user using complex amplification factors (or relative transfer functions) computed with respect to the reference amplitude motion. The nonuniform motion assumption could be combined with motion incoherency and wave passage to create more realistic seismic environments.

Seven interpolation schemes for the complex responses are implemented for complex responses. The newer interpolation scheme that uses bi-cubic splines is recommended for complex FE models under incoherent seismic inputs (for which the number of SSI frequencies should be usually larger than 200). For such cases, when number of frequencies is sufficiently large, the bi-cubic spline interpolation provides most accurate results for incoherent analysis since it does not create any spurious peaks or valleys. The bi-cubic spline interpolation should be applied only if the number of SSI frequencies is sufficiently large, so that spectral peaks are not clipped by the smooth spline interpolation. Different interpolation techniques could perform differently on a case-by-case basis, especially for highly complex FE models with coupled responses, especially for incoherent motions. The various interpolation options that are available in the code provide the structural analyst a set of powerful tools for identifying and avoiding the occurrences of spurious spectral peaks in the computed transfer functions of structural motions and stresses. The first six options were implemented in the original SASSI 1982 scheme that uses a non-overlapping moving window, the university SASSI2000 scheme that uses a weighted average moving window, and four new interpolation schemes including two non-overlapping window schemes with different shifts and two average overlapping moving window schemes with different numbers of sliding windows. To check the interpolation accuracy, convenient comparative plots of the computed Transfer Functions (TF) versus the interpolated Transfer Functions can be easily obtained using the GUI graphics.

The new Fast Flexible Volume (FFV) method provides accurate and numerically efficient SSI analysis solutions for deeply embedded structures (DES) such as small modular reactors (SMRs). The FFV method, in addition to the interaction nodes defined at the outer surface of excavation volume, includes interaction nodes defined by internal node layers within excavation volume. Using the INTGEN command, the user can automatically generate the interaction nodes for the FFV method or other methods, as FV and FI-FSIN (SM) or FI-EVBN (MSM). The FFV method speeds the SSI analysis of deeply embedded structures by up to tens of times faster than the traditional, reference FV method.

Automatic selection of additional SSI calculation frequencies that are required to improve the accuracy of the interpolated TF that is applicable to both the node acceleration TFs (ATF) and the element stress TFs (STF). This feature that can be implemented using UI commands and macros, is an important practical capability, especially for larger size FE model applications, because it saves a lot of labor effort and ensures high quality of the SSI analysis.

Visualization of complex TF variation patterns within the entire structural model for selected SSI calculation frequencies. The complex TF patterns are visualized on the structure using colored vector plot animations including all three-directional components (red for X, green for Y and blue for Z). The TF amplitude is given by vector length, and the TF phase is given by vector orientation. This capability is extremely useful for checking the correctness of the FE modeling and understanding the structural dynamic behavior.

Computation and visualization of the amplitude TF or spectral accelerations for a selected damping value at a given SSI calculation frequency for the entire SSI model using either structural deformed shape or bubble plots. The deformed shape plots are animated structural plots with a controlled movie frame speed, so that they can be also viewed as static plots. For selected resonant frequencies, the spectral amplitudes or the ZPA values can be plotted as a deformed shape plot.

Computation and visualization of structural acceleration and relative displacement time histories can be performed using structural deformed shape plots. The deformed shape plots can be static structural plots for selected times, or maximum values, or structural animations of the SSI response variation in time during the earthquake action.

Computation and contour plotting of the average nodal seismic stresses (for all six components in global coordinates) in the entire structure, or for selected parts of the structure based on the element center stresses for the SHELL and SOLID elements. Both maximum and time-varying values of nodal stresses are computed and available for plotting. The approximation is based on the assumption that element center and node stresses are equal (no shape function extrapolation is included). For sufficiently refined finite element models this approximation is reasonable. Contour stress plotting can be either static maximum values or animated time-varying values at selected time frames (automatic frame selection is included). Maximum element center stresses values are also available in a convenient text file format.

Computation and contour plotting of seismic soil pressure on foundation walls using near-field SOLID elements. The nodal pressure is computed based on averaging of adjacent element center pressures. Both maximum and time-varying values of nodal seismic pressures are computed and available for plotting. The analyst can also automatically combine the seismic soil pressures with the static soil bearing pressures and then, plot the resultant soil pressure of foundation walls and mat. Contour seismic soil pressure plotting can be either static contour plots of maximum values or animated contour plots of time-varying values at selected time frames (an automatic frame selection capability is included).

Carry-out post-SSI analysis calculations for superposition of the co-directional SSI effects in terms of acceleration, displacement of stress time-histories and in-structure response spectra using UI commands and macros. For time histories both the algebraic summation available. For in-structure response spectra (ISRS) post-processing

• the weighted linear combination and
• the square-root of sum of square (SRSS) for the superposition of the X, Y and Z co-directional effects are implemented.

The analyst can also compute the envelope, broaden and average ISRS from multiple spectral curves. Post-processing calculations can be also used for computing the maximum structural stresses, forces and moments, and/or the maximum seismic soil pressure on walls and mat with or without including the soil static bearing pressure component.

The most recent upgrades, starting with IKTR9, includes a fast-postprocessing option for SSI response time histories using compressed binary databases for compuations and visualizations, and including a new thick shell element to model thick walls and floors in nuclear structures. New visualization of the shell element forces and moments per element is included.

The new thick shell element is accurate for both thick and thin shell elements based on Mindlin-Reissner theory. The latest upgrade IKTR10 also includes a coordinate transformation capability for the shell elements and a principal strain computation capability for the upper and lower surfaces of shell elements, applicable to linear type shell elements. Details on the application of the new coordinate transformation are provided in the User’s Manual for SHELL and TSHELL elements.

The IKTR10 upgrade includes new aux program called Post_BinDB for post-processing SSI response time-histories stored in the binary databases for acceleration, displacement and element stress/force SSI time-history responses. It should be noted that this Post_BinDB program partially duplicates and has less capabilities than the ACS SASSI UI commands for performing post-processing of the SSI response binary databases. It might be still an attractive alternate for some users that prefer running a DOS program instead of using the UI commands.

The ACS SASSI NQA Version 3 has been tested, verified, documented and released under the Ghiocel Predictive Technologies Nuclear Quality Assurance Program which is in compliance with the requirements of 10 CFR50 Appendix B, 10 CFR21, ASME NQA-1 2008 and Addenda Subpart  2.7 2009. The ACS SASSI NQA version comes with a complete set of software documentations that were developed under the quality assurance requirements of the GP Technologies NQA-1 Level Program. The ACS SASSI NQA version documentation includes the user and verification manuals and the V&V computer files for a large set of various seismic V&V problems, including shallow, embedded and buried foundations, rigid and flexible foundations, piles, subjected to various different seismic environments, different surface and body seismic waves, motion incoherency and directional wave passage along an arbitrary horizontal direction, multiple support excitations for isolated foundations, linear or nonlinear SSI analysis.

The current ACS SASSI NQA Version 3 IKTR10 includes a set of 57 SSI verification and validation (V&V) problems, many of these including several subproblems. In these V&V problems, the computed SSI results using ACS SASSI are compared against benchmark results based on published analytical solutions or computed using other validated with computer programs, including SHAKE91, SASSI2000 and ANSYS. Each SSI verification problem tests a different capability of the ACS SASSI NQA code. The total number of the V&V computer input files and output files for all the SSI verification problems of the ACS SASSI NQA version is several thousands of files that require about few hundreds of MB of the hard drive space.

The latest upgrade of the ACS SASSI Version 3 (Installation Kit Revision 10, IKTR10) includes two installer kit options for the same software:

1. IKTR10 for SSI models with up to 100,000 nodes and
2. IKTR10_650K for SSI models with up to 650,000 nodes or 2,500,000 degrees of freedom.

The ACS SASSI Version 3 IKTR9_650K was tested for 3D SSI models with up to 625,000 nodes and up to 35,000 interaction nodes on 128 GB to 512 GB RAM workstations running under MS Windows 8 and 10. SSI model size limitation to 650,000 nodes will be relaxed from time to time in future, as the MS Windows workstation RAM and HDD resources continue increase.

The minimum hardware requirement is a 8-16 GB RAM workstations under the MS Windows 7, 8 or 10 OS that include a video card capable of supporting the OpenGL 4.0 libraries or higher for the 3D plotting and animations. For large size embedded SSI models with many interaction nodes, such as 15,000-20,000 interaction nodes, or even more, we suggest have the use of 192 GB to 512 GB RAM MS Windows 10 workstations.

It should be noted that the latest upgrades of the ACS SASSI Version 3 can run simultaneously up to tens or hundreds of dynamic load cases for a small runtime increase. Thus, for flexible foundation impedance problems that requires a large number of unit amplitude harmonic loads, ACS SASSI can run up to 500 separate load cases in a single run, depending on the size of the problem and the available RAM. Also, for seismic incoherent SSI analysis problems, ACS SASSI can run up to 50 stochastic simulations including X, Y and Z components for each in a single SSI analysis run, depending on the size of the problem and the available RAM. These new capabilities of running many load cases in a single run makes the latest upgrade of ACS SASSI Version 3 to be at tens of time faster than the 2015 ACS SASSI Version 3 IKTR4 for running multiple load cases or incoherent SSI simulations.

For seismic SSI coherent SSI analyses, ACS SASSI does not need any SSI restart analysis for the three-directional seismic input components, since it can solve the X, Y and Z input cases in the internal memory without the need to save the large restart files for each frequency. Restart files can be still useful for “New Structure” and “New Seismic Environment” restart analyses, such as nonlinear SSI analysis runs, or incoherent SSI analysis simulations.

The last upgrades of ACS SASSI Version 3, after IKTR4, include a totally new ACS SASSI. User Interface (UI) that is much more capable than the previous version UI, including the MAIN, PREP and SUBMODELER modules available in the ACS SASSI Version 3 IKTR4. The new UI includes all the PREP and SUBMODELER commands of the previous versions, and also many new commands and powerful graphical capabilities for plotting SSI models and results. The new UI commands offer a much larger number of SSI modeling options than before.

The ACS SASSI UI also ensures the automatic administration of all resources, files, directories, and the interfaces between software modules. Comparing with previous releases in 2015, the ACS SASSI Version 3 upgrades in 2016 have a much more powerful User Interface (UI) that includes many new commands, parametric language macros, much faster post-processing for calculations, plotting and animations.

In addition to the ACS SASSI main software SSI capabilities for performing deterministic linearized SSI analysis, there are number of companion, separate software modules that include additional advanced SSI capabilities. These advanced SSI capabilities are provided in the Option A-AA (ACS SASSI-ANSYS integration that provides a refined integrated two-step approach in accordance with new ASCE 4-16 standard Section 4.6, Option PRO (probabilistic site response and probabilistic SSI analyses per new ASCE 4-16 standard recommendations in Sections 2 and 5.5 and in compliance with USNRC RG1.208, and Option NON (nonlinear structure to simulate the reinforced concrete cracking per new ASCE 4-16 standard Section C.3.3.2 and USNRC SRP 3.7.2, and the post-cracking reinforced concrete behavior per ASCE 43-05 standard and new ASCE 43-16 draft, and USNRC SRP 3.7.2), or simulate the seismic base-isolators hysteretic behavior per new ASCE 4 standard Section 4.7 and 12.4). These advanced SSI analysis capability options are briefly described in the next paragraphs.

The ACS SASSI-ANSYS integration capability included in Options A-AA covers an area of great interest for practical applications. This capability provides an advanced two-step SSI approach that can include more refined FEA structural models in the second step, including some local nonlinear material and/or nonlinear geometric effects in the structure or at foundation interface with the soil. There are two ACS SASSI-ANSYS interfacing options:

Option A is an ACS SASSI-ANSYS interfacing capability is based on an integrated twostep SSI approach, the 1st step is the overall SSI or SSSI analysis using the ACS SASSI SSI model and the 2nd step is the structural stress analysis using a refined ANSYS model with the input boundary conditions defined based on the SSI responses from ACS SASSI. Option A includes a fast automatic export of the seismic SSI boundary conditions for performing a detailed nonlinear or linear ANSYS stress SSI analysis using either quasi-static or dynamic models, or computing soil pressure on the foundation baseslabs and walls including soil separation effects.

The 2nd step can have two distinct functionalities:

1. perform structural stress analysis using refined ANSYS FEA structural models with detailed meshes, eventually including enhanced element types, non-linear material and plasticity effects, contact and gap elements, and
2. compute seismic soil pressure on the basement walls and slabs including the soil material plasticity, foundation soil separation and sliding using refined ANSYS soil models.

Option A assumes that any nonlinearity included in the ANSYS model does not affect the SSI soil motion of the foundation-soil interface.

Option AA in an Advanced ACS SASSI-ANSYS integration capability that permits to run SSI  analysis using directly state-of-the-art ANSYS FE structural models. The Option AA ANSYS models could include advanced ANSYS FE types, pipes, shells including shear flexibility, coupled nodes, constraint equations, MPC elements, and even fluid elements (FLUID80) and the superelements (MATRIX50). The Option AA uses directly the ANSYS FE model dynamic matrices with no need for the ANSYS FE model conversion into an ACS SASSI FE model. Only the model topology from the ANSYS FE model is transferred to ACS SASSI, but no material or constants, or other parameters. For computing structural stresses, the Option A should be used to transfer the SSI response motions at all time steps or selected critical steps as boundary conditions for ANSYS superstructure model.

Option NON or nonlinear structure SSI analysis uses a fast hybrid time-complex frequency approach based on a piece-wise equivalent-linearization for the nonlinear structure SSI analysis (Ghiocel et. Al, 2017). The ACS SASSI nonlinear SSI analysis is extremely fast, at least hundreds of times faster than the time-domain nonlinear SSI approaches based on numerical integration.

Option NON is applicable to the reinforced concrete structures for simulating the concrete cracking and post-cracking behavior in the low-rise shearwalls for the design-level and/or beyondthe- design-level seismic inputs. The Option NON was validated for the low-rise reinforced concrete shearwall buildings that fail primarily due to the in-plane shear deformation. Based on the time-domain hysteretic behavior, the elastic modulus and damping in each concrete wall are modified iteratively based on the local stress and deformation levels. No out-plane nonlinear concrete behavior is considered. However, Option NON can also consider the nonlinear concrete behavior due to the in-plane bending deformation effects. In the same nonlinear structure FE model, the analyst can include wall panels (parts of walls) that fail primarily either due to the shear deformation or the bending deformation, respectively. This has an useful practicality.

Option NON can be also used to include the effects of the hysteretic behavior of the seismic base isolators using nonlinear spring elements. The nonlinear springs also might be used to simulate nonlinear pile-soil interface behavior, or even to identify the potential of the structure sliding on the ground including SSI effects. Option NON is validated for nonlinear translational springs in the three X, Y and Z global directions. Nonlinear rotational springs are not included at this time.

Option PRO for probabilistic site response analysis (PSRA) and SSI analysis (PSSIA) using efficient LHS simulations. Option PRO is consistent with probabilistic site response and SSI procedures in the new ASCE 4-16 standard (Sections 2 and 5) and the USNRC guidance for computing the FIRS for the new site-specific licensing applications (Ghiocel, 2017).

The Option PRO probabilistic modeling includes:

1. Response spectra shape model for the seismic motion input,
2. Soil shear wave velocity Vs and hysteretic damping D profiles, defined for each soil layer for low shear strain values,
3. Soil shear modulus G and hysteretic damping D, as random functions of the soil shear strain values for each soil layer, and
4. Equivalent linear values for the effective structural stiffness and damping for each group of elements depending on the stress levels in different parts of the structure.

The combined use of Options PRO and NON offers highly efficient robust and accurate SSI solutions for the design-basis analysis and/or the seismic PRA/PSA fragility calculations. Using combined Option PRO-Option NON, the structural engineers will have the required SSI analysis tools to compute much more realistically the structural fragilities. Thus, the nonlinear reinforced concrete post-cracking behavior structures on ground could be efficiently evaluated in a business practical manner and not neglected, or crudely represented, as quite often was done in the past due to the lack of efficient nonlinear SSI analysis tools.