The effects and consequences of high-rise collapses from earthquake in the urban core may be analyzed in a logical sequence, addressing a number of tractable questions:

- Given
a specific earthquake scenario and building inventory,
**what are the probabilities of collapse (and non-collapse) for each building**, without consideration of interaction with any surrounding structures? The probability of collapse varies by building structural system and age, height and configuration, as well as critical structural deficiencies, but models are already well developed, as a part of Performance-Based Earthquake Engineering (PBEE). These tools generally are able to predict the onset of lateral instability or the failure of gravity load-carrying members, indicating incipient collapse. - How
do towers topple in earthquakes? More precisely stated: for any given tall
building under a given shaking level,
**what are the modes of collapse that may occur, what are their respective probabilities, and how do we define the potential impact zone for each mode?**Collapse may be limited to a single story, with or without gross overturning or impact to adjacent structures. Collapse with sidesway may not occur at the base of the building. Collapse can occur from failure of gravity load-carrying elements from axial load or shear induced by interstory drifts. Or collapse can involve full sidesway, where the impact area extends over a length proportional to the height of the building. Tall buildings have collapsed in pure side-sway in earthquakes in Kobe, Japan (1995), in Taiwan (Chi-Chi Earthquake, 1999), in Mexico City (1985) and in Concepcion, Chile (2011) – See**“Cases."**New NDP techniques such as**element removal**[Mosalem et al.] can be used to analyze buildings from the inception of instability through the process of progressive collapse. **What happens when one building collapses and strikes another building?**A collapsing building will be a total loss, but what about impacted building(s)? The damage to an impacted building will depend upon the relative heights and weights of the two buildings, whether the impact is "head-on" or a "glancing blow," the separation distance between the two buildings (affecting the kinetic energy of the toppling tower at impact), and specific features of each building’s structural and cladding systems.

**Failures at Base: ** Foundation failures leading to tower collapse have not been observed in past earthquakes in the United States, but they have elsewhere. Foundations may not be viewed as a typical “weak link” in earthquakes, so foundations may not be subject to the same level of scrutiny as the above-ground portions of a typical tower. Nevertheless, there are a number of situations under which existing foundation systems may need evaluation, and where deficiencies in the foundation system or soil-foundation-structure interaction can increase the probability of a tower failure under earthquake loads and/or influence the mode and direction of toppling:

• non-seismic foundations

• inadequate pile foundations

• shallow foundations

• sloping site

• failure of an adjacent foundation

In developing a model of urban seismic risk, general urban system models and detailed building-specific models should consider the foundation systems and their influence on tower failure probability, mode and direction.