Chapter 6- Charleson Earthquake

Question 1

35. Measurement of an earthquake’s magnitude is made on the Richter scale (of which there are several variances).

What is the expected number of occurrences per annum of earthquakes with magnitudes 7 and 8,

and how much more energy is there in a magnitude 8 relative to a smaller one of magnitude 6

Answer 1

1. The earthquakes magnitude is determined using the logarithm of the amplitude of the largest seismic wave calibrated to a scale by a seismograph.
2. Magnitude of 8 is 100 times bigger than 6 as the magnitude increases by a factor of 10 each time
3. Each whole number increase represents 32 times more energy released from the earthquake
4. Only 1 8 occurs in the world each year
   However 7 is a weekly occurrence, but happen in sparsely populated areas.

Question 2

36. One of the two strongest U.S. earthquakes occurred in December 1811 and was centred in New Madrid, Missouri. What material was used to construct the common buildings?

What structural property inherent in the form of their construction made them well suited to resist the shaking motion from earthquake forces

, and how did this property help to prevent the common buildings from catastrophic collapse

Answer 2

1. log cabins were the most common (therefore wood) due to their flexiable joints to withstand the shaking.
2. A wooden house with all its joint properly braced and pinned together will sway and stretch behaviouring in an elastic manner
3. this is called ductile. The structure did not fail in a catospgoric manner
4. Modern steel and reinforced concrete are made to be ductile like the wooden houses

Question 3

37. Why did masonry buildings perform poorly in the Charlestown earthquake of 1886?
    What type of force tries to slide an upper level of bricks over a lower one, and is therefore the most destructive

Answer 3

1. masonry buildeing performed badly in the earthquake due to be made from bricks and concrete blocks joined with a cement mortar. These materials have high compression strength but little tensile strength.
2. A shearing force tried to slide the lower bricks over the upper bricks. The inertia force tries to counter act this force therefore causing a crack to form in the brick wall.
3. Since the masonary is weak in tensile strength the walls crack in both diagonals way therefore totally disjointed and eventually crumbles

Question 4

38. Why did the masonry walls supported on cast-iron columns and beams survive almost intact

and what approach used today has this concept given us to design buildings to resist earthquakes better?

How can this approach be applied in modern buildings, while limiting the lateral movement that causes irreversible damage, such as occurred in 1971 to the Olive View Hospital

Answer 4

1. masonary walls supported by cast iron columns and beams survived due to the iron frame acting as a horizontal shock absorber and damping the intensity of the seismic force before it reached the wall
2. Base isolation is the process known as today where buildings are supported in reinforced rubber pads that allow the earth to move under the buildings. However limits the lateral movement of the buildings

Question 5

39. What is liquefaction

Answer 5

Liquefaction is when the earth totally fails in an earthquake. Occurs in sandy soil that is completely saturated with water that suddenly becomes liquid when subjected to shaking.

Question 6

How do earthquakes show that the soil under a structure greatly influences how the structure responds to the shaking

Answer 6

soil type under a structure influences its behaviour.
The conditions modify the incoming seismic wave by amplifying frequencies, ground motion and the extension of the duration of shaking. The soil can accelerate the force which buildings needs too resit more.
Softer soil intensity the shock of the earthquake which results in more damage.