Codes & Collapse

Past and Current Seismic Building Codes

The basic life-safety objective in current seismic codes is to prevent the collapse of the building, although the building need not be repairable. In this context, we define collapse as vertical or lateral instability.  Lateral instability occurs when a small increase in lateral earthquake load leads to an uncontrolled increase in sidesway.  Vertical instability occurs with the loss of gravity load-carrying ability of the structural system, as from axial failure of a column from overload or shear failure of a column under seismic drifts.

Collapse probabilities for new, code-conforming normal buildings should be less than 1% for a design basis earthquake (DBE) and 10% chance for maximum considered earthquake (MCE) — see Table C.1.3.1b of the commentary to Chapter 1 in ASCE 7-10.  Requirements for seismic design and detailing have evolved greatly over the past 50 years, as have computational design tools, construction techniques and structural materials.  Older buildings with seismically deficient structural systems in our large urban centers may be prone to collapse in large earthquakes.  Past studies have found that probabilities for collapse of older structures with known deficiencies (e.g., per ASCE 41-13) are much higher than for new, code-conforming construction.

Our building codes examine each building separately, without consideration of interaction with neighboring structures. Current codes require seismic separation of adjacent buildings to prevent pounding as buildings sway in earthquakes, but do not consider collisions from collapse. Older codes either did not require separation of adjacent buildings, or required a seismic gap that is now considered inadequate.  Without adequate seismic separation, buildings will pound each other when the lateral displacements combine to exceed the gap, and pounding has been found to be a significant cause of collapse in past earthquakes.

For closely-spaced tall buildings in the urban core, neglecting potential interactions from collapse can result in unacceptable risks to all of the affected buildings. New, code-conforming buildings may face unexpected high risks if surrounded by older, more vulnerable towers. 

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