Seismic Loads Introduction
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Transcript of Seismic Loads Introduction
1
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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P (Primary) Waves (Mostly Direct, Push-Pull in nature)
S (Secondary) Waves (Mostly Reflected, Side-to-side in nature)
Seismometers
Seismograph
Hours
5
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