Performance-based Design of Soil-Foundation Interface in Buildings - 1412005b

Project Title—ID Number	Performance-based Design of Soil-Foundation Interface in Buildings - 1412005b
Start/End Dates	10/1/05 – 11/30/06
Funding Source	PEER-NSF
Project Leader (boldface) and Other Team Members	Bruce L. Kutter (UCD/F), Sivapalan Gajan (UCD/GS)

Project goals and objectives

Soil foundation interaction associated with heavily loaded shear walls during large seismic events may produce highly nonlinear behavior. Geotechnical components of the foundation are known to have a significant effect on the building response to seismic shaking. The nonlinearity of the soil may act as an energy dissipation mechanism, potentially reducing shaking demands exerted on the building. This nonlinearity, however, may result in permanent deformations that also cause damage to the building.

The goals of this research are to develop and test procedures to account for the foundation nonlinearity in performance based earthquake engineering. The primary goal of the research at UC Davis is to produce archived test data at prototype stress levels, regarding the cyclic and permanent deformation behavior of shallow foundations over a typical range of moment to shear ratio, shear to axial load ratio, foundation embedment, and soil type. A second goal of the researchers at Davis is to develop a coupled macro-element "constitutive model" to simulate the cyclic rotation, sliding, and settlement of a shallow foundation subject to combined moment, shear and axial loading.

The specific goals of this project in Year 9 are:

1. To summarize centrifuge test data and the effects of various parameters on the performance of shallow foundations
2. To provide verified and calibrated capability (macro-element constitutive model) in OpenSees to model the nonlinear behavior of shallow footings that are loaded into the nonlinear range.

Role of this project in supporting PEER's mission (vision)

It is now well understood that for many buildings, shallow foundations may suffer large loads that cause yielding or non-linear behavior in the soil beneath the foundation. A better understanding of the foundation non-linearity is needed in order to accurately assess the performance of the supported structures with particular focus on foundations for shear walls in non-ductile concrete frame buildings. This project directly supports the overall theme of performance based earthquake engineering. As the project also involves development of new elements for the OpenSees platform, it also supports that aspect of the PEER mission.

Methodology employed

Centrifuge models of shear wall-foundation systems and frame-shear wall-foundation systems are being tested in the using a variety of foundation dimensions, embedment depths and footing shapes. Some footings are tested only in axial loading, others are being tested under a constant axial load while slow-cyclic lateral load is applied to the wall at different heights above the foundation to provide different moment to shear load ratios as shown in the following figure. In other tests, model buildings are subject to base shaking using the shaking table mounted on the centrifuge. The data is being used to test, calibrate, and develop material models and new elements for OpenSees that will enable numerical simulation of nonlinear Soil-Structure Interaction. Results from each container are being posted at http://cgm.engr.ucdavis.edu for use by collaborators and others. The results are shared with other PEER researchers at UC Irvine, USC, and are available to other researchers.

Brief Description of previous year's achievements, with emphasis on accomplishments during last year (Year 8)

SUMMARIZING CENTRIFUGE TEST DATA

Seven series of centrifuge experiments, including about 60 shear wall-footing models, have been conducted at UC Davis. The eighth test series included nonlinear frame structures supported by shear wall-shallow foundations. The effects of parameters such as vertical static factor of safety, depth of embedment, applied moment to shear ratio, footing geometry and soil type will be systematically analyzed and summarized in the final report. Data reports for completed tests are already in the project website. The data from test series 8 are available to collaborating PEER researchers at USC and UCI.

DEVELOPMENT OF MACRO-ELEMENT MODEL

We have coded a single-element constitutive model (macro-element model) to simulate the foundation-soil interface behavior in C++. Footing and the soil beneath the footing were considered as a single element. By keeping track of the shape of the soil surface beneath the footing, maximum past pressure and settlement experienced by the soil, and the location of the contact area of the footing with soil, the nonlinear cyclic load-displacement behavior of the footing-soil system was modeled. We have used bounding surface plasticity theory to capture the coupling effect of combined vertical-shear-moment (V-H-M) loading. The major model parameters are: friction angle, vertical stiffness and the rebounding ratio of soil. An example measured and predicted moment, rotation, lateral force and sliding displacement for a shear wall footing are presented in the figure below.

Implementation of macro-element model in OpenSees

The macro-element model has been implemented in OpenSees in Year 8. We run the model in OpenSees to simulate the footing-soil interface behavior for a rigid shear wall structure. One student (George Hu) has completed his MS thesis in UC Davis using our model in OpenSees. We have done simulations and compared to the simulations form UCI and USC for a shear wall structure.

SENSITIVITY ANALYSIS

We will run OpenSees parameter studies with the nonlinear shear wall footing element and with a moment frame attached (vary the FS of footing, effective height of building, footing embedment, frame stiffness, fraction of inertial frame load transferred to shear wall, ground motions). We will provide recommendations on how the new footing element in OpenSees can be used in a probabilistic PBEE framework. This will include analysis of propagation of the uncertainties in field measurements of soil properties to uncertainties in the new constitutive model parameters and finally into the demands placed on the building.

Other similar work being conducted within and outside PEER and how this project differs

Work performed at UCI (Tara Hutchinson, PI) focuses on developing numerical tools for modeling this rocking behavior and predicting associated foundation and building settlements, and validating these models against available experimental data. Numerical studies at UCI will be based on a nonlinear Winkler-type framework for modeling the soil response (i.e., using nonlinear springs and dashpots, with gapping elements). Experimental data provided from centrifuge tests conducted at UCD, as well as other available data, will be used for validation of the analytical approach. Initial validation of the numerical models will lead to further parametric studies, which consider the combined dissipation of energy through non-linearity in structural elements (e.g. in shear walls, at beam-column joints) and non-linearity of foundation elements (through yielding of the soil). Parametric studies will consider moment resisting frame (MRF) structures as well as coupled structural systems (MRF's and shear walls combined). The work conducted at USC entails the oversight and integration of work performed at UCD and UCI. This includes sequencing and prioritizing model tests and analysis directions and implementing analysis and experimental data into the framework of a performance based engineering design approach. The work performed by USC will also include interfacing with practicing engineers in the US and Europe involved in implementation of nonlinear SSI into seismic design guidelines or codes.

A new project at UCLA (Jonathan Stewart, PI) is attempting to use the findings and developments at Davis and Irvine and implementing them in the context of performance based engineering.

Describe any instances where you are aware that your results have been used in industry

We have held a workshop in which we have discussed our results with several structural and geotechnical practicing engineers. Thus the results have been used to clarify the issues and mechanisms, which require attention.

Expected milestones & deliverables

• Ph.D. thesis (June 2006 – Sivapalan Gajan)
• Final project summary report to PEER (Sept. 2006)

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