Earthquake Resistant Building designs

• Buildings are designed to withstand vertical forces. Buildings are designed to withstand vertical forces. If earthquakes only moved the ground vertically, buildings might suffer little damage because all structures are designed to withstand vertical forces associated with gravity

• But the rolling waves of an earthquake exert extreme But the rolling waves of an earthquake exert extreme horizontal forces on standing structureshorizontal forces on standing structures.

• These forces cause lateral accelerations, which scientists measure as G-forces. – A magnitude-6.7-quake, for example, can produce an

acceleration of 1 G and a peak velocity of 40 inches (102 centimeters) per second. Such a sudden movement to the side (almost as if someone violently shoved you) creates enormous stresses for a building's structural elements, including beams, columns, walls and floors, as well as the connectors that hold these elements together. If those stresses are large enough, the building can collapse or suffer crippling damage.

The Finer Points of Frequency The Finer Points of Frequency When seismic waves reach the Earth's surface, they cause the ground, and anything sitting on it, to vibrate at certain frequencies. During an earthquake, a building will tend to vibrate a building will tend to vibrate around one particular frequency known as its naturalaround one particular frequency known as its natural, or fundamentalfundamental, , frequencyfrequency. When the building and ground share the building's natural frequency, they're said to be in resonance. That's bad. ResonanceResonance amplifies amplifies the effects of an earthquakethe effects of an earthquake, causing buildings to suffer more damage. In September 1985, a temblor in Mexico City created waves with a frequency perfectly aligned to the natural frequency of a 20-story building. As a result, more buildings of this height were damaged than taller or shorter structures. In some cases, a damaged 20-story building stood right next to an undamaged building of different height

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• Before a major construction project begins, engineers must first evaluate the seismic activity of the building site. In the U.S., they have access to a resource to aid in this process -- National Seismic Hazard Maps prepared by the U.S. Geological Survey (USGS)

• In high-hazard areas, engineers and architects must adhere to more rigorous standards when designing buildings, bridges and highways to make sure these structures withstand earthquake shaking.

• Once engineers determine the seismic risks of a site, they must propose an appropriate building design. In general, they avoid irregular or asymmetrical designs at all costs.

• Seismic engineers prefer to keep buildings symmetrical so that forces are distributed equally throughout the structure.

What do you notice about Seismic Hazards in the US?Where are they located?How do they affect us in California?

Diaphragms• Even symmetrical buildings

must be able to withstand significant lateral forces.

• Engineers counteract these forces in both the horizontal and vertical structural systems of a building.

• Diaphragms are a key Diaphragms are a key component of the component of the horizontal structurehorizontal structure

• Includes floors and roofs of Includes floors and roofs of levels.levels.

Cross Bracing and Shear Walls• Cross-bracing, which

uses two diagonal members in an X-shape, is a popular way to build wall trusses.

• Instead of braced frames or in addition to them, engineers may use shear walls -- vertical walls that stiffen the structural frame of a building and help resist rocking forces.

Moment Resisting Frames• Shear walls do, however,

limit the flexibility of the building design. To overcome this downfall, some designers opt for moment-resisting frames.

• In these structures, the columns and beams are allowed to bend, but the joints or connectors between them are rigid. As a result, the whole frame moves in response to a lateral force and yet provides an edifice that's less obstructed internally than shear-wall structures.

Base Isolation• Another solution -- known as base isolation

-- involves floating a building above its foundation on a system of bearings, springs or padded cylinders.

Active Mass Damping• In increasingly more

earthquake-resistant buildings, designers are installing damping systems. Active mass damping, for example, relies on a heavy mass mounted to the top of a building and connected to viscous dampers that act like shock absorbers.

The Future

• Interestingly, much of the future of seismic engineering involves looking back, not forward. That's because retrofitting old buildings with improved designs and materials is just as important as constructing new buildings from scratch.