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Seismic Loads

Origins

Sectional view of how we currently understand Earth’s inner structure

First a look at geology…

Plate Structure of Earth’s Crust

Plate Boundary

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Pangaea

ca. 500,000,000 B.C.

Gap

Overlap

Edge of Continental Shelf

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San Andreas Fault From Air

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P (Primary) Waves (Mostly Direct, Push-Pull in nature)

S (Secondary) Waves (Mostly Reflected, Side-to-side in nature)

Seismometers

Seismograph

Hours

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3 Types of Seismic Wave

Actions

Types of Seismic Waves

http://www.analog.com/library/analogDialogue/archives/35-01/earthquake/index.html

� P-waves � S-waves

� Rayleigh-waves � Love-waves

Terraforming:

Effects of Earthquakes on the Landscape

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1964 Earthquake near Anchorage, Alaska

“Sand Boil” (note film canister for scale reference)

Niigata, Japan, 1964 — Kawagishi-Cho Apartment Buildi ngs Collapse

“Liquefaction”• Same type buildup of water pressure in soil that causes sand

boils creates a weakening of the soil and loss of bearing capacity by dispersing soil particles and turning moist soil into mud

http://wapi.isu.edu/envgeo/EG5_earthqks/eg_mod5.htm

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What Produces the Damaging Forces In Structures?

• Ground motion below building…• Results in inertial reaction of building

trying to “stay still” by Newton’s 1st law of motion…

• Which leads to lateral displacement behaving as a lateral structural force

What Produces the Damaging Forces In Structures?

• Dynamic Forces: Newton’s Second Law of Motion: F= M·A

• F=W·A/g or F=W·c (F = Seismic loading force)

• W= Building weight• A = Ground acceleration• g = Gravitational constant (32.2 ft/s2 or 9.8 m/s2)

• c = Seismic base shear coefficient – Says lateral force is a % of building weight

Other Factors Affecting Seismic Loads on Structures

• Magnitude of Ground Acceleration• Building Inertia (directly proportional to mass)• Natural Vibrational Period of Building• Natural Vibrational Period of Soil• Nature of Structural Framing System

Other Factors Affecting Seismic Loads on Structures

• If building infinitely rigid with steady acceleration, then F=W·c would be true

• But real buildings are flexible to some extent, the accelerations are anything but steady, and the founding soils are of complex variations.

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Code-Prescribed Seismic Load Equation

• Allows us to look at the actual dynamic force on the structure as a static load.

• Based on previous equation, F=W·c, with additional modifiers

• Z = Seismic Zone Coefficient (fig. 16-2, table 16-I)• I = Importance Factor (table 16-K)• C = Coefficient for ground acceleration taking into

consideration the interaction between the soil and building vibration period (C=2.75 max)

• W = Building Weight• Rw = Building Frame Coefficient (table 16-N)• “Old Method” based on 1994 Uniform Building Code

Code-Prescribed Seismic Load Equation

WRZICW

V =

1994 UBC Seismic Zone MapCurrent (2003) International Building Code

Map of Peak Seismic Accelerations