1. Project Goals/Objectives:

Understanding the nonlinear behavior of shallow building foundations under large amplitude loading (moment, shear and axial loading) is an important aspect of performance-based earthquake engineering (PBEE). The goals of PEER researchers (at UCI, USC, and UCD) on this topic are to develop and test procedures to account for the foundation nonlinearity in PBEE. So far, it is established that soil yielding beneath foundations can be a very effective energy dissipation mechanism. However, foundation yielding may lead to excessive permanent deformations. The primary goal of the research at 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. For the present study, the typical range of parameters investigated in the experiments is relevant to shear walls for low- to mid-rise buildings. A second goal of the researchers at Davis is to begin to develop a plasticity-based 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 data is being made rapidly available to collaborating PEER researchers at USC and UCI. These researchers will be performing numerical analysis of the test results using OpenSees. After the data is processed, organized and checked by preliminary analysis, the data will be made available over the internet.

2. 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. We are now focusing on investigation of parameters that are considered critical to performance-based design of foundations for shear walls in non-ductile concrete frame buildings. Therefore, this project directly supports the overall theme of performance based earthquake engineering and is relevant to performance-based design of nonductile concrete frame buildings. As the project also involves development of new elements for the OpenSees platform, it also supports that aspect of the PEER mission.

3. Methodology Employed:

Centrifuge model tests are being conducted on the large centrifuge at UC Davis to generate test data at the prototype stress levels. Models of shear wall-foundation systems are being tested 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 UC Davis centrifuge. Each model container contains several shear wall-footing systems. Results from each container are being posted at http://cgm.engr.ucdavis.edu/ for use by collaborators and others.

Researchers at UCI are using the centrifuge data, and other data available in the literature to test the implementation of interface elements in OpenSees. A plasticity-based macro-element constitutive model is also being developed and implemented in OpenSees by UC Davis researchers as an alternative to the sub grade spring approach being developed at UC Irvine.

4. Brief Description of past year’s accomplishments (Year 6) & more detail on expected Year 7 accomplishments:

The behavior of the wall-footing-soil system is analyzed in terms of the resultant of the vertical, horizontal, and moment load acting at the center of the base of the footing and the corresponding displacements (settlement, sliding and rotation). The following figure shows the behavior of footing-soil interface for one test. The moment-rotation behavior is highly nonlinear and indicates a large amount of energy dissipation. Rocking of the footing progressively rounds the foundation soil, and this rounding causes a reduction in contact area between the footing and soil thereby causing a nonlinearity and stiffness reduction on the moment-rotation relationship. Permanent deformations beneath footing continue to accumulate with the number of cycles of loading, though the rate of accumulation of settlement decreases as the footing embeds itself. For large moment to shear ratios, the settlement per cycle a correlation has been established between the amplitude of the cyclic rotation and the factor of safety with respect to vertical load. For low moment to shear ratios, the settlement is also associated with cyclic sliding of the footing.

In year 7 we are extending the parametric study to include footings of different shape and footings in intermediate soil types. We are also planning a new unique model test of a soil-footing-shearwall-moment frame system on the centrifuge. This will provide unique system level data that should enable direct observation of how footing behavior is propagated into demand on the building structure.

The macro-element model development is rapidly progressing now that we have discovered a conceptually simple way to treat the soil-footing interface using a moving contact model. The insight from this work is complementing a parallel development of a bounding surface type model for the macro-element. The macro-element model(s) are being implemented in OpenSees and will be compared to the centrifuge test data and to a parallel modeling effort underway at UC Irvine.

5. 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.

6. Plans for Year 8 if project is expected to be continued:

7. Describe any actual instances where you are aware your results have been used in industry:

8. Expected Milestones & Deliverables: