Ask a CTBUH Expert: Ron Klemencic
Why are Tall Buildings Often Considered Safer
Than Low-Rise Buildings During Earthquakes?
There are physical, code-prescribed, and practical reasons why tall buildings tend to be safer than
low-rise buildings, as outlined in greater detail below:
Physical
When the ground
shakes, although
Ron Klemencic, Chairman seemingly random,
& CEO, Magnusson
there is a fundamental
Klemencic Associates
frequency of motion
that dominates each and every building site. If
a site is largely composed of rock, the
fundamental frequency of shaking is relatively
short, whereas if the site is largely comprised
of soft soils, the fundamental frequency can
be relatively long. A range of fundamental
ground shaking for any given site might be
between 0.5 seconds (rock) to 2.5 seconds
(soft soil).
Buildings also possess a fundamental dynamic
response frequency. In rough terms, the
fundamental period of sway for a building is
equal to 0.1 seconds times the number of
stories. For example, a 50-story tower might
have a fundamental period of vibration of 5.0
seconds, whereas a 25-story tower might
have a fundamental period of 2.5 seconds.
Damage experienced by buildings is
amplified when the site and the building
respond in resonance. Consider the childhood
pursuit of making a swing go higher and
higher. This is accomplished by pumping
one’s legs at just the right interval. Too quick,
and the swing won’t go. Too slow, and the
swing won’t go. Just right, and you take off for
the sky. This analogy summarizes how
58 | Ask a CTBUH Expert
buildings respond to ground shaking. When
the fundamental dynamic response of a
building is in resonance with the ground,
greater damage tends to occur. So, if the
ground shakes at somewhere between 0.5 and
2.5 seconds, buildings in the 5 – 25 story range
are at greatest risk. Tall buildings fall well
outside of this range of resonance, and are
therefore inherently safer.
Code-Prescribed
In many jurisdictions around the world, tall
buildings are subject to more strict building
code regulations, given their importance and
the greater risk associated with large building
populations. Higher demand levels for wind
and seismic loading are prescribed and/or
stricter performance objectives are required.
For instance, in the United States, buildings
with a calculated population of more than
5,000 people are classified as Occupancy
Category III, which requires that these buildings
consider wind forces and seismic forces equal
to 1.15 and 1.25 times (respectively) the
code-prescribed demands. In addition, many
tall buildings in high seismic zones are
designed using a performance-based
methodology, which more explicitly considers
how a specific building design responds to the
site on which it is located. Finally, many
jurisdictions require detailed design reviews be
performed by independent peer reviewers, also
citing the importance of these structures.
Practical
Given the significance of tall buildings, the
level of experience and sophistication of all
those involved in bringing the building to life
tends to be at the top of the industry. Owners,
developers, architects, engineers and builders
generally executing these projects have
seasoned protocols, mature quality assurance
programs and highly experienced staff.
Extensive site investigations are generally
carried out, including site-specific
characterizations of seismic hazards and wind
demands. Great care and skill is exercised in
the development of the architectural and
engineering systems designs, generally based
on state-of-the-art practices and technologies.
Detailed construction programs, logistics and
protocols are planned and executed. Rigorous
testing and inspection protocols are
developed and carried out. The end result
tends to be a building of much higher quality,
leading to more reliable safety. 
About the Author
Ron Klemencic is a past Chairman of the CTBUH
and is co-chair of the CTBUH Seismic Design
Working Group. Ron is Chairman and CEO of
Magnusson Klemencic Associates, a structural
and civil engineering firm headquartered in
Seattle, Washington, USA.
CTBUH Journal | 2014 Issue III