Off-The-Shelf vs. Roll-Your-Own
Fenves says that the codes used for earthquake simulation have come either from the commercial sector or have emerged from a haphazard development process within academia. Commercial applications like Ansys, Abacus, Adina, and the structural engineering program SAP200 have some advantages: they are tested, documented and supported. "For design engineers with a job to do, these are good solutions, but they are not adequate for cutting-edge research because the modeling capability is not enough. Large models are at the core of our research and the commercial packages aren't equipped to handle them. The same is true for the testing of new algorithms—commercial code, being closed source, can't be modified to include them."
Researcher-developed code is more accommodating of experimentation, but in the past, it has run into a different sort of problem: a lack of coordinated development and no code repository. "Structurally, development has too often branched at the root, never to re-converge. With some widely used applications, there could be 200 versions out there, with some unable to interoperate with others. With the open source model, there's a whole process involved. People communicate about who is working on what, and someone takes responsibility for the repository." The OpenSees website has linked some of the basics and culture of open source, creating a primer for interested students. There are links to Eric Raymond's seminal essay "The Cathedral and the Bazaar," the Apache project, GNU, Red Hat Linux, and the Open Source Initiative—the non-profit corporation behind the movement.
When PEER was established in 1997, OpenSees fit right in to the project's overall goal of "performance-based" earthquake engineering. The term defines broader criteria for designing structures to withstand earthquakes. Today, earthquake design is prescriptive—you design structures to meet the building codes, and if they pass, you have done your job. "Building codes are there to save lives, but they don't say anything about whether the building can be used afterward, the damage it might sustain, or the cost of repair," Fenves says. "Nor do the codes say whether there will be loss of life due to non-structural components: ceilings falling, pipes breaking. Performance-based engineering looks into all of this. It means identifying performance goals—not just building codes—then designing a structure to meet those goals. The key to performance-based engineering is the ability to predict what will happen to a given structural design given a certain type of earthquake."
An inevitable byproduct of performance-based engineering is that it puts more "stress" on the simulation software. The creators of commercial packages don't worry about performance-based goals, "because an engineer is not going to use a model that is more sophisticated than is needed to satisfy the code Right now, if a commercial firm actually added sophisticated models, I don't think anyone would use them. The design process needs to change, first." That means not just crunching numbers with existing software, but writing entirely new code. "The fidelity of our models far exceeds what's available commercially, with the ability to replicate experimental data throughout a full range of loads. Whether we are looking at soil models, reinforced concrete models, or non-linear solution methods—our solvers are much more capable than what's available commercially."
When combined, these various software models can lead to surprising results. That was the case for the analysis of a bridge near the California-Oregon border, conducted by a research group at the University of California, San Diego. Fenves thinks it may be the most advanced model of a bridge ever created. When researchers ran the simulation, the results predicted that soil on the hill could slump and liquefy, squeezing the bridge together. "People have talked about that happening, but the model actually showed us that if the earthquake is big enough, with the right site conditions, the soil slides in and the bridge follows. You couldn't model this on a shake table." Nor could you do so in commercial software, because the building codes don't require it.