Earthquake Resistant Building
designs
• 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
horizontal 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 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
around one particular frequency known as its natural,
or fundamental, frequency. When the building and
ground share the building's natural frequency, they're
said to be in resonance. That's bad. Resonance amplifies
the 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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What do you notice about Seismic Hazards in the US?
Where are they located?
How do they affect us in California?
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.
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
component of the
horizontal structure
• Includes floors and roofs of
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
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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.