Seismic retrofit & Shear failure in lowest story

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Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. Other retrofit techniques are applicable to areas subject to tropical cyclones, tornadoes, and severe winds from thunderstorms. Methods to reduce hazards within households and also for general disaster preparedness are found in the related article Household seismic safety. It is important to keep in mind that there is no such thing as an earthquake proof structure, although performance can be greatly enhanced through proper initial design or subsequent modifications.

Infill shear trusses — University of California dormitory, Berkeley, California

Pleasing use of detail - note "waist" effect and integrated bench in base. Pedestrians exiting shops are protected from collision with bench by railings behind original footing

Levels of modification

Seismic retrofit is primarily applied to achieve public safety, with various levels of structure and material survivability determined by economic considerations:

Public safety only. The goal is to protect human life, ensuring that the structure will not collapse upon its occupants or passers by, and that the structure can be safely exited. Under severe seismic conditions the structure may be a total economic write-off, requiring tear-down and replacement.

Structure survivability. The goal is that the structure, while remaining safe for exit, may require extensive repair (but not replacement) before it is generally useful or considered safe for occupation. This is typically the lowest level of retrofit applied to bridges.

Primary structure undamaged and the structure is undiminished in utility for its primary application. A high level of retrofit, this ensures that any required repairs are only "cosmetic" - for example, minor cracks in plaster, drywall and stucco. This is the minimum acceptable level of retrofit for hospitals.

Structure unaffected. This level of retrofit is preferred for historic structures of high cultural significance.

[edit] Shear failure in lowest story

In many buildings the ground level is designed for different uses than the upper levels. Low rise residential structures may be built over a parking garage which have large doors on one side. Hotels may have a tall ground floors to allow for a grand entrance or ballrooms. Office buildings may have stores in the ground floor which desire continuous windows for display.

Traditional seismic design assumes that the lower stories of a building are stronger than the upper stories and where this is not the case the structure will not respond to earthquakes in the expected fashion. Using modern design methods, it is possible to take a weak story into account.

Several failures of this type in one large apartment complex caused most of the fatalities in the 1994 Northridge earthquake.

Typically, where this type of problem is found, the weak story is reinforced to make it stronger than the floors above by adding shear walls or moment frames. Moment frames consisting of inverted U bents are useful in preserving lower story garage access, while a lower cost solution may be to use shear walls or trusses in several locations, which partially reduce the usefulness for automobile parking but still allow the space to be used for other storage.