iDesign

The primary objective is the development of a simple yet accurate method for performance-based design of structures in seismic areas. In essence, we seek to revolutionize the standard process that every professional structural engineer undertakes to design a structure subject to seismic forces. The reason is that recent earthquakes have shown that buildings reflecting current design approaches may reduce the rate of fatalities, but often result to staggering monetary losses and disruption of functionality. Thus, earthquakes can still financially cripple entire cities, or even countries.

To holistically quantify such effects, the concept of “seismic performance” is employed. This characterizes the behavior of a given structure under seismic loads. Ideally this is based on the use of metrics that are of immediate use to engineers, e.g., story forces and deformations, but also to stakeholders, such as monetary losses, human casualties and time-to-repair (or replace). At a simpler level, one may quantify damage by using simpler engineering metrics such as global ductility or maximum interstory drift. “Performance-based” earthquake engineering is concerned with tackling the dual problems of assessment and design. Assessment is the direct process of estimating the performance of a given (existing) structure. Design is the inverse problem, whereby a (new or rehabilitated) structure, its members and properties are sought to assure a desired performance under a given seismic hazard. As typically befitting such dualities, the direct path of assessment is by far the simpler of the two. Designing a structure to achieve a desired level of performance is an indirect process that is currently solvable only through arduous iterations: It is simply not applicable in practice. To make things worse, the considerable uncertainty inherent in earthquakes (i.e., when, where, how intense) and structures (what has been constructed versus what was designed on paper, what are the material properties, issues of corrosion, aging etc.) make this problem even more difficult.

To offer a practicable path for performance-based design, three important innovations have been introduced. First, a computationally efficient approach is proposed for rapidly establishing the effect of model uncertainties on the seismic performance (Figure 1). This allows the quantification of the consequences of uncertainties and their inclusion in subsequent analyses. Second, a simplified, yet accurate formula is offered for evaluating the seismic performance in terms of the mean annual frequency of damage occurring in the building. Thus, intuition is gained on the structural parameters that influence the seismic behavior of the building, while an analytical estimation of the building performance becomes possible. Finally, the concept of Yield Frequency Spectra (Figure 2) is developed, whereby an engineer can directly determine the required strength and stiffness of the structure given the seismic hazard at the building site and the owner’s requirements on how frequently it sustains low or hign levels of damage. Both analytical approximations and accurate numerical solutions are available, encoded in open-source software that can estimate Yield Frequency Spectra within seconds to provide a reliable design basis for any building and any user requirements.