The nonlinearity of the soil and the interaction between the soil and foundation is shown to cause the building's stiffness and period to change to varying degrees. On the one hand, the nonlinearity of the soil may act as an energy dissipation mechanism, potentially reducing demands exerted on the structural components of the building. This associated nonlinearity, however, may result in permanent deformations (rotation or settlement) that cause damage to the building. The goal of this research is to further the understanding of soil-foundation-structure interaction with regards to seismic response.

The project is conducting centrifuge model tests to study the cyclic and dynamic response of building foundations. The test data are being used by researchers at UC Davis, UC Irvine, and USC to develop design procedures, including elements in OpenSees for prediction of the behavior of the foundations.

Two series of model tests were completed in year five of PEER and on series of model tests was completed in year 6. The model test consisted of models of shear walls for buildings supported on shallow foundations. Many tests involved slow cyclic lateral loading of the shear wall models and obtained data regarding the relationships between load amplitude and the rotation, sliding, and settlement of the footings. Other tests involved dynamic shaking of the base of a sand layer upon which the shear wall models are placed. Parametric studies were done to investigate the effect of factor of safety, building height, footing embedment, soil density and soil type.

A workshop was held on March 5, 2003 at UC Davis to discuss the status of the project status of the project and to obtain input focus for the research. The workshop was valuable in that it enabled excellent interaction between practitioners and researchers in both structural and geotechnical engineering fields. The status report for this project can be found with other data reports at: http://cgm.engr.ucdavis.edu/research/projects/krr.

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 nonlinearity in structural elements (e.g. in shear walls, at beam-column joints) and nonlinearity 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).

Figure 2. Typical results from a slow cyclic lateral load test FS = 6.7,
embedment = 0.0m, load height = 4.9m, footing length = 2.84m
Larger View

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. In addition, input and expertise on foundation design issues will be provided for the Van Nuys building test bed including potential retrofit solutions using shear walls and shallow foundations.

There is now useful related work being conducted in France, Italy, and England. We have developed contacts with these researchers. Our project is different because we are looking at foundation response in earthquakes, experiments are being done on a centrifuge (most other tests were done at 1-g), and we are comparing results from ground shaking to those measured in slow cyclic tests. None of the other known projects are integrating all of these factors in one test program.

One key finding of the research to date is that the moment-rotation behavior observed in slow cyclic tests was similar to that observed in dynamic tests. The permanent settlements, however, were significantly greater than that in slow cyclic tests. Most experimental data available from other researchers has involved slow cyclic loading. This raises a big question that must be solved: How important is the difference in settlement behavior for dynamic and static tests?

We also recommend that the analysis and physical modeling be extended to combine nonlinear soil and nonlinear structure behavior simultaneously. This has not been done before and it could have significant implications on PBEE.

From the recent workshop, we received recommendations to further study SFSI for non-symmetrical systems (where permanent rotations may accumulate), to look at c-phi soils (so far we have looked at undrained clay and dry sand), and to extend the analysis efforts to encompass continuum modeling of SFSI.

Our workshop status report will be converted to a final report that covers research through year 6. It will be prepared together with researchers UCD, UCI, and USC; this will be completed at the end of year 6. We are planning to conduct one more model test series by September 2003.

So far we have completed: Three data reports, one workshop was held, ones status report, one MS thesis have been completed. We will also deliver 3 more data reports, one more MS thesis, and a final report at the end of year 6. The UCI team is using our data to verify a procedure for OpenSees to model shallow foundation SFSI using a nonlinear Winkler Spring approach. We are complementing this by developing a macro element model for SFSI to be implemented in OpenSees.