LESSON 3: HISTORY OF THE DEVELOPMENT OF MITIGATION STRATEGIES FOR THE EFFECTS OF SEISMIC HAZARDS

What’s in
We just got through the groundbreaking introductory of earthquakes. You now have an idea regarding the main structural
effects of earthquake and its types. This time, we will shake our way to the history of the development of mitigation of earthquakes.
Let’s go back in time, reminisce, remember, relapse, or what you may call it, and identify the events that transpired for the
earthquake engineers to come up with the current earthquake mitigation systems.
What I need to know
At the end of this topic, you will be knowledgeable about how modern earthquake mitigation were developed. You will also
realize how important it is to know the past and learn from the experiences compiled by our predecessors.
What’s new
Activity 3.1. Identify the chronological order of the events related to the development of mitigation strategies for effects of
seismic hazards. Put the letter in the box in its correct chronological order.
a. The development of the response spectrum theory marked a major step forward.
b. Development of construction practices to mitigate the effects of shaking.
c. This stage has implicitly been included in building codes since the 1970s and deals with performance-based design
d. Focused on characterizing the effect of shaking using lateral forces.
e. Concept of structural dynamics were incorporated into design practice.

What is it
HISTORY OF THE DEVELOPMENT OF MITIGATION STRATEGIES FOR THE EFFECTS OF SEISMIC HAZARDS
Until the middle of the last century, humans did not fully understand the process that causes earthquakes. To make up for
this, they developed construction practices to reduce the structural damages caused by ground shaking. Studies of pre-historic
buildings showed that there are two main strategies that were followed:
increase the strength or change the stiffness of the lateral force-resisting
systems. These practices were often applied to important buildings, while
ordinary structures remained vulnerable.

INCAN ENGINEERING
The Incas are renowned for their advanced building techniques,
particularly in Machu Picchu, where they used interlocking stones without
mortar (ashlar masonry). This method increased friction between the stones,
allowing them to shift slightly during seismic activity without collapsing,
effectively providing a form of passive energy dissipation and enhancing the
structures' earthquake resistance.

Source: https://incatrailmachupicchu.org/important-temples-machu-picchu/
LESSON 3: HISTORY OF THE DEVELOPMENT OF MITIGATION STRATEGIES FOR THE EFFECTS OF SEISMIC HAZARDS

Inca construction followed prescriptive specifications based on historical experience with successful designs, a practice
still used in modern engineering. In contrast, engineered seismic-resistant design, which relies on scientific principles and rational
methods, developed more recently. The understanding of seismic effects on structures began to take shape in Europe after major
earthquakes in the 18th century, particularly the 1755 Lisbon earthquake, which led to new building regulations. Subsequent
studies, including those on the Calabria earthquake in Italy, laid the foundation for future research and the development of
seismology.

LATERAL FORCES IN SEISMIC DESIGN

Seismic-resistant design evolved in response to major earthquakes worldwide. After the 1881 Nobi earthquake, Japan
introduced the concept of lateral force calculations based on Newton’s second law. Similarly, following the 1908 Messina-Reggio
earthquake, Italy developed early seismic design guidelines, laying the foundation for the equivalent static procedure used today.
In the U.S., the 1906 San Francisco earthquake led to indirect seismic considerations in building codes, while the 1933 Long
Beach earthquake resulted in stricter regulations like the Field Act (for public schools) and the Riley Act (for most buildings in
California). By the 1940s, U.S. building codes (1943 City of Los Angeles building code and the 1947 San Francisco building code)
began incorporating building mass, height, and soil conditions into seismic force calculations.

While local building codes have existed for centuries, including the Hammurabi Code from ancient Babylon (which
enforced good construction through penalties rather than specifications), regional model codes were first introduced with the
Uniform Building Code (UBC) in 1927. The 1930 edition of the UBC included seismic provisions and recommended their adoption
in earthquake-prone cities. These provisions were based on two key observations: (1) most damage in earthquakes was caused
by lateral shaking, and (2) buildings on soft soil suffered more structural damage—making the UBC likely the first to
recognize soil effects in seismic design. The code required buildings to withstand a lateral force equal to 10% of the dead plus
design live loads, reducible to 3% for structures on firm soil. Later UBC editions introduced seismic risk maps for the U.S.,
identifying regions at risk for large-magnitude earthquakes and improving earthquake-resistant construction guidelines. These
advancements shaped modern seismic design standards worldwide.

RESPONSE SPECTRUM THEORY

In 1948, a joint committee in California proposed model lateral-force provisions for building codes, marking a major
advancement in seismic design. A key innovation was the introduction of the earthquake-response spectrum concept, originally
developed by Maurice A. Biot in 1943 and later refined by George W. Housner as a design tool. This concept considers both
ground motion and the building’s dynamic properties, particularly its period, making building period an explicit factor in seismic
force calculations. The development of the response spectrum was made possible by strong-motion recordings from newly
deployed accelerographs.

In 1952, a second committee, now involving the Structural Engineers Association of California (SEAOC), created a
comprehensive guide called the Recommended Lateral Force Requirements and Commentary (known as the SEAOC Blue Book),
first published in 1959. This guide explicitly related lateral forces to building periods using the design response spectrum, shaping
modern seismic-resistant building codes.

STRUCTURAL DYNAMICS

In the 1960s, structural dynamic concepts were further developed and incorporated into seismic codes, especially after
the 1971 San Fernando earthquake. The inclusion of response spectrum theory in the design of hospitals and other critical facilities
marked a significant step in applying structural dynamics to seismic design. These concepts, however, are computationally
intensive, and their application advanced alongside the development of computers. Despite significant progress in mitigating
earthquake-induced ground shaking, structures designed with modern seismic principles have still experienced much larger forces
during major earthquakes than anticipated by design codes, sometimes leading to substantial damage or even collapse.

This has led researchers to conclude that while buildings can survive intense shaking without collapsing, they can still suffer
damage if the lateral force-resisting systems are tough enough to absorb seismic energy and provide a continuous path for forces
to transfer to the ground. Additionally, engineers now recognize that the static analysis principles used for gravity loads don’t fully
account for the dynamic forces structures experience during earthquakes. Modern seismic design focuses on ensuring that
buildings can deform without overstressing from load reversals and have adequate ductility, strong connections, and resilient
structures to meet specific performance goals:

 Resist minor earthquakes without damage.

 Resist moderate earthquakes without structural damage, though with some nonstructural damage.
 Resist major earthquakes without collapse, though with both structural and nonstructural damage.
LESSON 3: HISTORY OF THE DEVELOPMENT OF MITIGATION STRATEGIES FOR THE EFFECTS OF SEISMIC HAZARDS

This approach aims to enhance a building’s ability to withstand earthquakes of varying magnitudes while minimizing risks to
human life.

EVOLUTION OF PERFORMANCE-BASED SEISMIC DESIGN

Since the 1970s, seismic performance criteria have remained largely unchanged, focusing on ensuring that buildings can
resist different levels of earthquake intensity with varying degrees of damage. However, a more flexible approach has recently
emerged, allowing real estate owners to determine the desired level of structural performance and safety based on their priorities—
whether prioritizing occupant protection or capital investment. This new approach, called performance-based design (PBD),
provides more customized seismic resilience options rather than a one-size-fits-all standard. PBD is expected to be included in
future standards, such as the 2010 ANSI/ASCE "Minimum Design Loads for Buildings and Other Structures", to guide engineers in
designing buildings that meet specific performance goals based on the owner's needs.

What’s more

Activity 3.2. Identify the chronological order of the events related to the development of mitigation strategies for effects of
seismic hazards. Put the event in the box in its correct chronological order. Other instructions will be presented face-to-face for a
more exciting set-up.

a. Response Spectrum Development

b. Incorporation of Structural Dynamics
c. Incan Engineering (Ashlar masonry)
d. Uses the approach called performance-based design.
e. The Uniform Building Code (UBC) was introduced.
f. Humans did not understand the phenomenon of the ground shaking.
g. Lisbon and Calabria Earthquake
h. Nobi Earthquake
i. San Fernando Earthquake
j. SEAOC Blue Book
k. Uniform Building Code was introduced
l. Stricter regulations are implemented (Riley Act and Field Act)
m. Uses the approach called Performance-Based Design (PBD)

17th-18th 1880s-
40s-50s
Century 1930s

70s-
60s-70s
present