A: How Buildings Resist Earthquakes
forces can act in all directions. Unlike gravity loads that are
transferred in a downward direction, earthquake loads start at the
supporting soil and are transmitted to the building. The horizontal and
vertical earthquake forces travel in different load paths and may result
in tension, shear compression, bending or torsion
forces. Buildings experience horizontal distortion when subjected
to earthquake motion. When these distortions get large, the damage can be
catastrophic. Therefore, most buildings are designed with
lateral-force-resisting systems (or seismic systems), to resist the
effects of earthquake forces. In many cases seismic systems make a
building stiffer against horizontal forces, and thus minimize the amount
of relative lateral movement and consequently the damage. Seismic systems
are usually designed to resist only forces that result from horizontal
ground motion, as distinct from vertical ground motion.
The combined action of seismic systems
along the width and length of a building can typically resist earthquake
motion from any direction. Seismic systems differ from building to
building because the type of system is controlled to some extent by the
basic layout and structural elements of the building. Basically, seismic
systems consist of axial-, shear- and bending-resistant elements.
In wood-frame, stud-wall buildings, plywood
siding is typically used to prevent excessive lateral deflection in the
plane of the wall. Without the extra strength provided by the plywood,
walls would distort excessively or “rack,” resulting in broken windows
and stuck doors. In older wood frame houses, this resistance to lateral
loads is provided by either wood or steel diagonal bracing.
The earthquake-resisting systems in modern
steel buildings take many forms. In moment-resisting steel frames, the
connections between the beams and the columns are designed to resist the
rotation of the column relative to the beam. Thus, the beam and the column
work together and resist lateral movement and lateral displacement by
bending. Steel frames sometimes include diagonal bracing configurations,
such as single diagonal braces, cross-bracing and “K-bracing.” In
braced frames, horizontal loads are resisted through tension and
compression forces in the braces with resulting changed forces in the
beams and columns. Steel buildings are sometimes constructed with
moment-resistant frames in one direction and braced frames in the other.
In concrete structures, shear walls are
sometimes used to provide lateral resistance in the plane of the wall, in
addition to moment-resisting frames. Ideally, these shear walls are
continuous reinforced-concrete walls extending from the foundation to the
roof of the building. They can be exterior walls or interior walls. They
are interconnected with the rest of the concrete frame, and thus resist
the horizontal motion of one floor relative to another. Shear walls can
also be constructed of reinforced masonry, using bricks or concrete
a building performs in an earthquake depends upon a few key building
characteristics described below:
Structural Building Elements
structural elements of a building that comprise the 'skeleton'
supporting the rest of the building, includes the foundation, load-bearing
walls, beams, columns, floor system and roof system, as well as the
connections between these elements. To carry its own weight ("dead
load"), live loads, and wind and earthquake forces the
building elements and connections are subjected to tension, compression,
shear, bending, and torsion. Buildings are primarily
designed to resist vertical forces from gravity. The roof and floor
systems carry these vertical forces to the supporting beams. The beams
carry the forces to the columns and bearing walls, which then carry the
forces down to the foundation and the supporting soil. This process of
carrying forces from the roof down to the soil is known as a load path.
The failure of any building element or connection along the load path can
lead to building damage or collapse.
Building Materials and Systems Performance Characteristics
Under normal conditions, a building experiences elastic deformations,
deforming as force is applied and returning to its original shape when
removed. However, extreme earthquake forces may generate inelastic
deformations in which the element does not return to its original
shape after the force is removed. Ductility is the property of certain
elements that have inelastic deformation before failing. Building
elements constructed with ductile materials have a "reserve
capacity" to resist earthquake overloads. Therefore, buildings
constructed of ductile elements, such as steel and adequately
reinforced concrete, tend to withstand earthquakes much better than
those constructed of brittle materials such as unreinforced masonry.
and Stiffness. Strength is the property of an element to
resist force. Stiffness is the property of an element to resist
displacement. When two elements of different stiffnesses are forced to
deflect the same amount, the stiffer element will carry more of the
total force because it takes more force to deflect it. When stiff
concrete and masonry elements are combined with more flexible steel or
wood elements, the concrete and masonry take more of the total force.
Resistant Components. Four basic components provide seismic
resistance against lateral forces (Figure 3):
frames consist of beams, columns, and stiff diagonal braces that
perform like shear walls, but use less material.
resistant frames (generally of steel or reinforced concrete)
consist of beams connected to one or more columns to carry
multi-dimensional earthquake forces.
Diaphragms are floor and roof deck systems that carry forces
across the building to shear walls, braced frames, and/or columns.
walls are large structural walls placed in a building to carry
forces from the roof and floor systems to the supporting foundation,
and into the soils.
walls are interior walls and partitions that are not necessarily
continuous to the foundations, but which are attached securely to
two floor diaphragms (the top side of a floor diaphragm to the
underside of the floor above) and that are stiff and strong enough
to resist the independent movement of the two connected diaphragms.
Strong building connections allow forces and displacements to be
transferred between vertical and horizontal building elements. In
addition, strong connections increase the overall structural building
strength and stiffness by allowing all of the building elements to act
together as a unit. Inadequate connections represent a weak link in
the load path of the building and are a common cause of earthquake
building damage and collapse.
When a tuning fork strikes a surface, it vibrates back and forth at a
certain rate - this rate is known as its fundamental period. All
objects, including buildings, have their own unique fundamental period
of vibration. Ground shaking from an earthquake will cause vibrations
in a building. If the ground shaking matches the fundamental period of
the building, the building will resonate with the earthquake, causing
the building vibrations to greatly increase. This can lead to
extensive building damage. "Damping" diminishes this
resonance by pulling the energy out of the system as heat - in the way
that a shock absorber in a car dampens a car's vibrations from bumps
in the road. Damping is imparted to a building by the cracking
and inelastic movement of its structural elements, and it can also be
deliberately added by installing shock absorber-like devices into the
building's structure. In the first case, "controlled"
damage at the onset of shaking can reduce the likelihood of
catastrophic damage as the shaking intensifies, and in the second
case, the damping devices work like vehicle shock absorbers to reduce
the response of the structure to a level below that at which
post-elastic behavior (and thus damage) will occur.
Distribution. Buildings that are wide at their base and have
most of their weight distributed to their lowest floors generally fare
better in earthquakes than tall, top- heavy buildings which act like
an inverted pendulum. Inverted pendulum buildings usually experience
greater displacements than those shorter and heavier near the base.
Configuration. Square or rectangular buildings with floor
plans with symmetrically place lateral force resisting elements tend
to perform better in earthquakes than buildings composed of irregular
shapes or 'those with large foyers or lobbies that create a soft story
condition. Buildings with irregular shapes cannot distribute lateral
forces evenly, resulting in torsional response that can increase
damage at key points in the building.
/ Soil Characteristics. The underlying geology of the
site can also have a significant effect on the amplitude of the ground
motion there. Soft, loose soils tend to amplify the ground motion and
in many cases a resonance effect can make it last longer. In such
circumstances, building damage can be accentuated. In the San
Francisco Earthquake of 1906, damage was greater in the areas where
buildings were constructed on loose, natural and manmade fill and less
at the tops of the rocky hills. Even more dramatic was the 1985 Mexico
City earthquake. This earthquake occurred 250 miles from the city, but
very soft soils beneath the city amplified the ground shaking enough
to cause weak mid-rise buildings to collapse (see Figure F-5).
Resonance (see below) of the building frequency with the amplified
ground shaking frequency played a significant role.
with rock close to or at the surface will be less likely to amplify
motion, and with such sites, generally, the farther from the source of
an earthquake, the less severe the motion. The type of motion felt also
changes with distance from the earthquake. Close to the source the
motion tends to be violent rapid shaking, whereas farther away the
motion is normally more of a swaying nature. Buildings will respond
differently to the rapid shaking than to the swaying motion.. Buildings
can be severely damaged when the soils that support the building
foundations shift, sink, slide, or liquefy. Optimally, structures should
not be located in areas with poor site conditions.
Resonance was a major
problem in the 1985 Mexico City earthquake, in which the total
collapse of many mid-rise buildings (Figure F-5) caused many
fatalities. Tall buildings at large distances from the earthquake
source have a small, but finite, probability of being subjected to
ground motions containing frequencies that can cause resonance. Where
taller, more flexible, buildings are susceptible to distant
earthquakes (swaying motion) shorter and stiffer buildings are more
susceptible to nearby earthquakes (rapid shaking).
It is very beneficial for a rehabilitated lateral-force-resisting
system to have an appropriate level of redundancy, so that any
localized failure of a few elements of the system will not result in
local collapse or instability. This should be considered when
developing rehabilitation designs.