Lecture 8 - Reducing Seismic Risk Flashcards
Why do we say that “Earthquakes don’t kill people, buildings do”?
Building materials are important. Masonry buildings, made out of blocks such as bricks, are especially weak when subject to strong horizontal shearing, as can occur in large earthquakes.
e.g. Bam 2003 Mw 6.6 - most structures in the city collapsed as they were made of adobe brick.
Why are wood buildings much more likely to withstand earthquakes than masonry ones?
Wood is flexible, lightweight and elastic.
e.g. In the 1906 San Francisco earthquake wood-frame houses were damaged, but did not collapse. They fared better than brick buildings.
What material is the best choice when building large structures to be earthquake resistant?
Steel, as it is strong and flexible. It is expensive so is usually only used for larger buildings such as the Levi’s Stadium.
Why is it important the buildings be properly anchored to their foundations?
Small residential buildings are prone to sliding off their foundations.
What is a common failure mode in buildings during earthquakes?
“Soft first story” collapse.
What are some ways of reducing the likelihood of a building collapsing or sliding off its foundations?
Reinforcing first storeys with braces, brackets, shear walls, bolts and steel frames.
What types of building may need to undergo expensive seismic retrofits?
Historical masonry buildings in North America are of particular concern.
What is resonance?
A natural frequency of vibration.
What does the resonance of a building depend on?
size weight distribution shape materials foundations substrate (whether hard bedrock or loose sediment)
What frequency do buildings tend to resonate at?
~10 Hz divided by the number of storeys
What does seismic wave frequency depend on?
earthquake magnitude
mechanism
distance
local geology
A building will be vibrated more by seismic waves of its resonant frequency.
Why are more people being killed by earthquakes?
growth of megacities
population growth
urbanisation
What type of oscillations do tall buildings respond to?
low-frequency oscillations
What type of oscillations do medium height buildings respond to?
medium-frequency oscillations
What type of oscillations do small buildings respond to?
high-frequency oscillations