EARTHQUAKE LOADS
CETS461 Earthquake Engineering
EARTHQUAKE LOADS
OUTLINE
• OVERVIEW
OF
NSCP
SECTION
203
COMBINATONS OF LOADS
• OVERVIEW OF NSCP SECTION 204 DEAD
LOADS
• OVERVIEW
OF
NSCP
SECTION
208
EARTHQUAKE LOADS
CETS461 Earthquake Engineering
EARTHQUAKE LOADS
• Earthquake loads are the forces and
deformations induced in a structure due to
ground shaking during an earthquake.
• These loads are dynamic and vary based on
factors like the building’s mass, height,
structural configuration, site conditions, and
proximity to the earthquake source.
• Designing for earthquake loads ensures that
buildings can withstand seismic events with
minimal damage, protecting both the structure
and its occupants.
CETS461 Earthquake Engineering
SEISMIC DESIGN PARAMETERS
NSCP 2015 Section
1.
Occupancy Category
103/Table 103 -1
2.
Importance Factor (I)
208.4.2/Table 208-1
3.
Soil Profile Type
208.4.3/208.4.3.1/Table 2082
4.
Seismic Zone
208.4.4.1/Figure 208-1
5.
Seismic Zone Factor (Z)
208.4.4.1/Table 208-3
6.
Seismic Source Type
208.4.4.2/Table 208 - 4
7.
Distance from the nearest Active PHIVOLCS FAULTFINDER APP
Fault / Closest Distance to Known
Seismic Source
8.
Near Source Factor (𝑵𝒂 )
208.4.4.3/Table 208-5
9.
Near Source Factor (𝑵𝒗 )
208.4.4.3/Table 208-6
CETS461 Earthquake Engineering
SEISMIC DESIGN PARAMETERS
NSCP 2015 Section
10. Seismic Coefficient (𝑪𝒂 )
208.4.4.4/Table 208-7
11. Seismic Coefficient (𝑪𝒗 )
208.4.4.4/Table 208-8
12. Basic Seismic Force Resisting 208.7.2/ Table 208-11A,B,C &
System
D
13. Response Modification Factor, 208.7.2/Table 208 -11A,B,C &
R
D
14. 𝑪𝒕 & 𝒉𝒏
208.5.2.2
15. Structure Period (T)
208.5.2.2
16. Seismic Dead Load (W)
17. Design Base Shear (V)
208.5.2.1
18. 𝑭𝒕
208.5.2.3
CETS461 Earthquake Engineering
OCCUPANCY CATEGORY
• Occupancy categories are classifications used
in building codes to define the function and
usage of a building, reflecting the level of risk
associated with its occupancy in the event of
structural failure or seismic activity.
• These categories influence the structural
design requirements, particularly in seismic
design, because buildings with certain
functions (like hospitals or schools) require
higher safety standards due to their critical
nature or high occupancy.
CETS461 Earthquake Engineering
OCCUPANCY CATEGORY
• In the National Structural Code of the
Philippines (NSCP) and similar codes
internationally, buildings are grouped into
occupancy categories that directly impact the
importance factor (I) used in seismic, wind, and
other load calculations.
CETS461 Earthquake Engineering
OCCUPANCY CATEGORY
CETS461 Earthquake Engineering
IMPORTANCE FACTOR
• The Importance Factor (I) is a multiplier applied
in structural design calculations to enhance the
resilience of a building based on its occupancy
type, usage, and the level of risk it poses to
human life or societal function.
• It accounts for the consequences of structural
failure and ensures that critical facilities or highoccupancy buildings have greater structural
safety.
CETS461 Earthquake Engineering
IMPORTANCE FACTOR
• In seismic, wind, and other load calculations,
the importance factor directly adjusts the design
loads applied to the structure, requiring more
robust construction for higher-risk facilities.
CETS461 Earthquake Engineering
IMPORTANCE FACTOR
CETS461 Earthquake Engineering
SOIL PROFILE TYPE
• The Soil Profile Type is a classification used in
structural and seismic design to describe the
soil conditions at a building site.
• Different soil types affect how seismic waves
travel and influence the level of ground shaking
that a structure experiences during an
earthquake.
• Soil profile types are crucial seismic design
parameter, as soils with certain characteristics
can amplify ground motion, increasing the
seismic forces that a building must resist.
CETS461 Earthquake Engineering
SOIL PROFILE TYPE
CETS461 Earthquake Engineering
SEISMIC ZONE
• A Seismic Zone is a geographic area classified
based on the likelihood and intensity of seismic
activity.
• Seismic zoning helps engineers and builders
understand the earthquake risk of an area,
enabling them to design structures accordingly
to withstand expected ground shaking.
CETS461 Earthquake Engineering
SEISMIC ZONE
• In structural design, seismic zones determine
the seismic design coefficients used in
calculating earthquake loads on buildings.
• This approach ensures that buildings in higherrisk areas are designed with stronger seismic
resistance than those in lower-risk areas.
CETS461 Earthquake Engineering
SEISMIC ZONE
• In building codes like the National Structural
Code of the Philippines (NSCP), regions are
divided into seismic zones, each with an
associated seismic zone factor or zone
coefficient.
• This coefficient adjusts the seismic forces used
in design calculations, reflecting the probability
and severity of earthquakes in that area.
• While different codes may have specific ways of
defining these zones, the core purpose is to
assess and mitigate earthquake risk.
CETS461 Earthquake Engineering
SEISMIC ZONE
CETS461 Earthquake Engineering
SEISMIC ZONE FACTOR
• The Seismic Zone Factor (often denoted as Z) is
a numerical value that represents the expected
level of seismic activity in a given geographic
region.
• This factor is used in structural design to adjust
the intensity of seismic loads based on the
earthquake risk specific to that area.
• It’s a core part of calculating the base shear—
the total lateral force that a structure must
withstand during an earthquake.
CETS461 Earthquake Engineering
SEISMIC ZONE FACTOR
• The seismic zone factor scales the design forces
based on the likelihood and strength of
earthquakes in the region.
• Areas prone to stronger, more frequent
earthquakes have a higher seismic zone factor,
leading to a more robust design to ensure
structures can withstand the anticipated
seismic activity.
CETS461 Earthquake Engineering
SEISMIC SOURCE TYPE
• A Seismic Source Type refers to the
classification of earthquake-generating sources
based on their location, tectonic characteristics,
and potential to cause ground shaking.
• Seismic sources can vary in the type of faults
they originate from, the distance to structures,
and the type of tectonic activity involved.
• These
distinctions
are
essential
for
understanding earthquake risk and accurately
designing structures to resist seismic forces,
especially in high-risk regions
CETS461 Earthquake Engineering
SEISMIC SOURCE TYPE
CETS461 Earthquake Engineering
DISTANCE FROM THE NEAREST ACTIVE FAULT
CETS461 Earthquake Engineering
DISTANCE FROM THE NEAREST ACTIVE FAULT
• The distance from the nearest active fault is a
crucial factor in seismic design and hazard
assessment for structures.
• It refers to the horizontal distance between a
given location (e.g., a construction site or a
building) and the closest fault that is known to
be actively moving or capable of generating an
earthquake.
• This distance impacts the level of seismic forces
that a building may experience.
CETS461 Earthquake Engineering
DISTANCE FROM THE NEAREST ACTIVE FAULT
• Structures closer to active faults are more likely
to experience higher ground accelerations and
intensities during an earthquake.
• The PHIVOLCS FaultFinder is an application
capable to do proximity searches to active
faults.
• It may be used to determine the location of
active faults in an area and to measure the
shortest distance between an active fault and a
user’s current location, which is determined by
the gadget’s tracking device.
CETS461 Earthquake Engineering
DISTANCE FROM THE NEAREST ACTIVE FAULT
• It may also be used to measure the shortest
distance between an active fault and a specific
site, which is identified by a user.
CETS461 Earthquake Engineering
NEAR – SOURCE FACTOR
• The Near-Source Factor (or Near-Fault Factor) is
a parameter used in seismic design to account
for the increased seismic forces experienced by
structures located close to an active fault.
• When buildings are near the source of an
earthquake, they can be subject to amplified
ground motions due to the proximity to the
rupture zone, which can result in higher ground
accelerations and velocities.
CETS461 Earthquake Engineering
NEAR – SOURCE FACTOR
• Many seismic codes provide tables or formulas to
determine the appropriate Near-Source Factor,
often denoted as Na (for acceleration) and Nv (for
velocity).
• These values are typically multiplied by the design
base shear or seismic forces calculated for the
structure, leading to increased design loads for
buildings near the fault.
• The values of Na and Nv increase as the distance to
the fault decreases, ensuring that structures are
designed to resist the intense demands of nearby
ground motion.
CETS461 Earthquake Engineering
NEAR – SOURCE FACTOR
• Na: This factor specifically adjusts the shortperiod acceleration response (typically affecting
low-rise, stiffer buildings).
• Nv: This factor adjusts the 1-second period
response, which is significant for taller or more
flexible buildings.
CETS461 Earthquake Engineering
SEISMIC COEFFICIENTS
• Seismic coefficients are numerical factors used
in structural design to represent the level of
ground motion or seismic demand a structure
may experience during an earthquake.
• They are essential in the analysis and design of
structures to resist earthquake forces, ensuring
safety and structural performance.
CETS461 Earthquake Engineering
SEISMIC COEFFICIENTS
Seismic coefficients account for:
Intensity of Ground Shaking:
• They reflect the expected earthquake forces
based on the structure's location and soil
conditions.
Dynamic Behavior of Structures:
• Different structures respond differently to
seismic forces depending on their height,
stiffness, and mass distribution.
CETS461 Earthquake Engineering
SEISMIC COEFFICIENTS
Seismic Risk:
• They align design requirements with the local
seismic hazard level to protect life and property.
Ca: Short-Period Seismic Coefficient
• Represents the ground motion effects for
structures with short natural periods (e.g., lowrise buildings or stiff structures).
• Affected by site-specific seismic hazard levels
and soil type.
CETS461 Earthquake Engineering
SEISMIC COEFFICIENTS
Cv: Long-Period Seismic Coefficient
• Represents the ground motion effects for
structures with long natural periods (e.g., tall,
flexible buildings).
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
• Most buildings can be categorized using one of
several common structure types.
• Over the years, engineers have observed that
each structure type has unique earthquake
vulnerabilities.
• For this reason, building codes classify
structures by their structural system type and
designate specific design requirements for each
system.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
• Structural systems are categorized based on
the material of construction (e.g., concrete,
masonry, steel, or wood), the way in which
lateral (i.e., horizontal) forces from earthquake
shaking are resisted by the structure (e.g., by
walls or frames), and by the relative quality of
earthquake-resistant design and detailing
provided.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
• There are five broad categories of structural
systems used in areas with earthquake hazard:
• Bearing wall systems,
• Building frame systems,
• Moment frame systems,
• Dual systems, and
• Cantilever column systems.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
• Building codes categorize quality and extent of
earthquake-resistant detailing using the terms
special, intermediate, and ordinary.
• Systems that employ extensive measures to
provide for superior earthquake resistance are
termed special systems, while systems that do
not have such extensive design features are
called ordinary systems.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
• The building codes also include design rules for
structural systems intended to provide seismic
resistance that is superior to that of ordinary
systems but not as good as special systems;
these systems are called intermediate
systems.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
1. BEARING WALL SYSTEMS
• Bearing wall buildings are the oldest form of
building construction.
• In bearing wall systems, structural walls located
throughout the structure provide the primary
vertical support for the weight of the building as
well as the resistance to lateral forces produced
by wind and earthquakes.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
1. BEARING WALL SYSTEMS
• Bearing wall buildings are commonly used for
residential construction, warehouses, and lowrise commercial buildings of concrete, masonry,
and wood construction.
• Walls in bearing wall buildings can be of
reinforced brick, concrete, or stone masonry;
reinforced concrete; cold-formed steel; or wood
construction.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
2. BUILDING FRAME SYSTEMS
• Building frames are a common structural system
for buildings constructed of structural steel and
reinforced concrete.
• In building frame structures, the weight of the
building is typically carried by vertical elements
called columns and horizontal elements called
beams.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
2. BUILDING FRAME SYSTEMS
• Lateral resistance is provided either by
diagonal steel members called braces that
extend between the beams and columns to
provide horizontal rigidity or by concrete,
masonry, or timber shear walls that provide
lateral resistance but do not carry the weight of
the structure.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
3. MOMENT FRAMES
• Moment-resisting frames, also called moment
frames, are commonly used for both structural
steel and reinforced concrete construction.
• In this form of construction, the horizontal
beams and vertical columns provide both
support for the weight of the structure and the
strength and stiffness needed to resist lateral
forces.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
3. MOMENT FRAMES
• Stiffness and strength are achieved using rigid
connections between the beams and columns that
prevent these elements from rotating relative to one
other.
• Although more expensive to construct than bearing
wall and braced frame structural systems, moment
frame systems are popular because they do not
require braced frames or structural walls, therefore
permitting large open spaces and facades with
many unobstructed window openings.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
3. MOMENT FRAMES
• Just as with steel structures, the building codes
permit construction of special concrete
moment frames without height restrictions.
• Ordinary and intermediate moment frames,
either of steel or concrete construction, are
limited in height in regions of higher seismicity.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
4. DUAL SYSTEMS
• Dual systems, an economical alternative to
moment frames, are commonly used for tall
buildings.
• Dual system structures feature a combination of
moment frames and concrete, masonry, or steel
walls, or steel braced frames.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
4. DUAL SYSTEMS
• The moment frames provide vertical support for
the weight of the structure and provide a portion
of the earthquake resistance of the structure
while most of the earthquake resistance is
provided either by concrete, masonry, or steel
walls, or by steel braced frames.
• Some dual systems are also called frame-shear
wall interactive systems.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
4. DUAL SYSTEMS
• Because these structures are inherently
redundant, as they have multiple systems to
resist earthquake forces, they are permitted to
be designed for reduced seismic forces relative
to bearing wall and building frame systems.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
5. CANTILEVER COLUMN SYSTEMS
• Cantilever column systems are sometimes used
for light single-story structures, such as carports
or in the top story of multi-story structures.
• In these structures, the columns cantilever
upward from their bases, where they are
restrained from rotation.
• The columns provide both vertical support of the
weight of the building and lateral resistance to
earthquake forces.
CETS461 Earthquake Engineering
BASIC SEISMIC FORCE RESISTING SYSTEM
5. CANTILEVER COLUMN SYSTEMS
• Structures using this system have performed
poorly in past earthquakes and severe
restrictions are placed on its use in zones of high
seismic activity.
• The building codes also require that design of
these systems consider relatively large seismic
forces to minimize the potential for damage.
CETS461 Earthquake Engineering
RESPONSE MODIFICATION FACTOR
• The numerical coefficient representing the
inherent over-strength and global ductility
capacity of lateral-force-resisting systems is the
Response Modification Factor (R).
• The R factor, as outlined in Section 208 of the
National Structural Code of the Philippines
(NSCP 2015), accounts for the capacity of a
structural system to dissipate energy and
accommodate inelastic deformation during
seismic events.
CETS461 Earthquake Engineering
RESPONSE MODIFICATION FACTOR
• It reflects the system's ability to resist forces
beyond the elastic range while maintaining
structural integrity.
• Higher R values are assigned to systems with
greater ductility, which dissipate more energy
during seismic events.
• The R factor reduces the seismic base shear
forces calculated from elastic analysis, making
designs more practical and cost-efficient.
CETS461 Earthquake Engineering
RESPONSE MODIFICATION FACTOR
• Higher R values lead to lower base shear
forces in design, reflecting the structure’s ability
to sustain larger deformations and absorb
seismic energy.
• Systems with low R values, such as cantilevered
columns, must be designed to resist nearelastic forces due to their limited ductility.
CETS461 Earthquake Engineering
STRUCTURE PERIOD
• The structure period refers to the fundamental
period of vibration (T) of a structure,
representing the time it takes for the building to
complete one cycle of free vibration in a specific
direction when subjected to dynamic loads,
such as an earthquake.
• It is a natural property of the structure
determined by its mass and stiffness.
• The period affects how the structure interacts
with ground motion.
CETS461 Earthquake Engineering
STRUCTURE PERIOD
• Structures with short periods (stiffer structures)
resonate with high-frequency ground motions,
while structures with long periods (more flexible
structures) resonate with low-frequency ground
motions.
• Taller structures tend to have longer periods due
to increased flexibility.
• Stiffer structures have shorter periods.
CETS461 Earthquake Engineering
STRUCTURE PERIOD
𝑪𝒕 = Coefficient based
on
the
structural
system
𝒉𝒏 = Height of the
building
(meters)
above the base
CETS461 Earthquake Engineering
LATERAL FORCE PROCEDURE
• In seismic design, there are various methods
used to estimate the lateral forces acting on a
structure due to an earthquake.
• These methods are based on the level of
complexity and the type of building being
designed.
CETS461 Earthquake Engineering
LATERAL FORCE PROCEDURE
1. SIMPLIFIED STATIC PROCEDURE
• The Simplified Static Procedure is used for
regular, low-rise buildings where seismic loads
are estimated using simplified static formulas.
• This method does not account for detailed
dynamic behavior but provides a quick
estimation of seismic forces.
• It is typically used for buildings with a height of 3
stories or fewer, or those with a simple layout
and low seismic response.
CETS461 Earthquake Engineering
LATERAL FORCE PROCEDURE
2. STATIC PROCEDURE
• The Static Procedure is the most commonly
used method for seismic design of buildings.
• It calculates the seismic base shear and
distributes it as lateral forces along the
building’s height.
CETS461 Earthquake Engineering
LATERAL FORCE PROCEDURE
2. STATIC PROCEDURE
• This procedure assumes that the building's
lateral
force-resisting
system
behaves
elastically, with no consideration for the
building's dynamic response during the
earthquake.
• This method is typically used for buildings that
are not extremely tall or complex.
CETS461 Earthquake Engineering
LATERAL FORCE PROCEDURE
3. DYNAMIC PROCEDURE
• The Dynamic Procedure accounts for the
building's response to seismic forces through a
more detailed analysis of its dynamic behavior,
including the effects of vibration and resonance.
• This method is more complex and involves the
use of response spectra, modal analysis, or
time-history analysis to determine the seismic
forces and responses at different points in the
structure.
CETS461 Earthquake Engineering
LATERAL FORCE PROCEDURE
3. DYNAMIC PROCEDURE
• It is typically used for tall buildings or structures
with irregular shapes or complex behavior
during an earthquake.
CETS461 Earthquake Engineering
LATERAL FORCE PROCEDURE
CETS461 Earthquake Engineering
LATERAL FORCE PROCEDURE
CETS461 Earthquake Engineering
VERTICAL DISTRIBUTION OF FORCE
CETS461 Earthquake Engineering
BASE SHEAR
• Base shear is the total horizontal seismic force that
acts at the base of a structure due to ground motion
during an earthquake.
• It represents the sum of all lateral forces that the
structure experiences and must resist to prevent
collapse or significant damage.
• Base shear is an estimate of the maximum
expected lateral force on the base of the structure
due to seismic activity.
• Base shear is a fundamental concept in seismic
design and is calculated to determine the lateral
load distribution on a structure.
CETS461 Earthquake Engineering
SEISMIC DEAD LOAD
CETS461 Earthquake Engineering
SEISMIC DEAD LOAD
• The total dead load includes both the structure’s
self-weight and any superimposed dead loads.
• Superimposed dead loads refer to any
additional permanent or stationary loads that
are applied to a structure after its initial
construction.
• These loads are typically above the structure's
own weight (its self-weight, which is the basic
dead load) and are considered as part of the
overall permanent loads on the building.
CETS461 Earthquake Engineering
SEISMIC DEAD LOAD
• Superimposed dead loads are often
associated with:
• Finished Flooring Materials ( Tiles,
carpets,etc.)
• Partitions
• Ceiling Finishes
• Roof Coverings
• Fixed equipment (HVAC, Lighting fixtures,
etc.)
• Any other permanently installed components
or features.