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Seismic Design > Earthquakes/Buildings > Flashcards

Earthquakes/Buildings Flashcards

1
Q

Range of Richter magnitude that would identify a moderate earthquake.

A

5.0-6.0

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2
Q

Range of Richter magnitude that would identify a strong earthquake

A

6.0-7.5

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3
Q

Range of Richter magnitude that would identify a great earthquake

A

Greater than 7.5

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4
Q

What does the Richter magnitude scale measure?

A

The numerical value represents a measure of energy release on a logarithmic scale.

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5
Q

Epicenter

A

The projection of the source of the earthquake at the earths surface. (Two-dimensional location).

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6
Q

Hypocenter (or Focus)

A

The source of the earthquake at a location below the earths surface. (Three-dimensional location).

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7
Q

Focal depth

A

The distance from the earths surface to the hypocenter/focus. Depth of the earthquake source.

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8
Q

Modified Mercalli Intensity (MMI)

A

12 increasing measures of earthquake intensity (MMI - MMXII). Intensity of an earthquake is based on the damage and other observed effects on people/buildings.

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9
Q

Define Peak Ground Acceleration (PGA) and common units

A

Corresponds to infinity rigid soil and a period = zero. Maximum amplitude of ground acceleration measured in “g”‘s

(Ft/s2/32.2100 = g, m/s2/9.8100 = g).

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10
Q

Stiffness v Rigidity

A

Stiffness (k):
K = F / x
The force that will deflect a structure elastically a unit amount in a given direction. Can be calculated for individual LFRS.

Rigidity: R
A normalized stiffness. Only used when forces are being distributed among several members. Ratio.

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11
Q

Flexibility v Ductility

A

Flexibility: (1/k)
The reciprocal of stiffness. The deflection obtained when a unit force is applied. Elastic deformation.

Ductility:
ability of a material to distort and yield without fracture or collapse. Inelastic deformation.

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12
Q

Pier Rigidity relationship to: height, thickness & depth

A

x = Fh^3 / 12EI
K = F/x
K = 12EI / h^3
I=td^3/12

Height: h^3
Thickness: t (linearly related - first power)
Depth: 1/d^3

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13
Q

Ductility Factor

A

Ratio of a materials strain energy at fracture to its strain energy at yield.

(Sec 5.6)

μ = Ut / Ur
=toughness (rupture) / resilience (yield)

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14
Q

What factors influence the ductility factor?

A 
Temperature
(I.e. steel - higher temp, more ductile)
Previous stress/strain history
(I.e. steel - more brittle if it’s been worked in previous cycles/events)
Type of construction 
Structural system
Quality 
Detailing
Redundancy
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15
Q

Minimum recommended ductility factor

A

No less than 2.2 to 2.5 for modern structures

4-6 concrete frames
6-8.5 steel frames

(Sec 5.5 , 5.6)

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16
Q

Why specify a minimum ductility factor?

A

Obtain ductility margin (between yield and collapse) sufficient enough to ensure survival in a design earthquake

17
Q

A theoretical analysis of elastic response of a structure will usually (overestimate or underestimate) the stresses resulting from an earthquake and why?

A

Overestimate

A structure will not behave elastically. Energy will be dissipated through deformation and Inelastic behavior through local yielding, reducing the seismic energy.

18
Q

Two components of drift

A
  1. Shear drift
    Sideways deflection due to lateral loads
  2. Chord drift
    Sideways deflection due to axial/vertical loads
19
Q

P-delta Effect

A

Additional column bending stress caused by eccentric vertical loads. (P*deltax)

20
Q

How are drift and Pdelta effect related?

A

When a structure drifts, it’s vertical loads become eccentric. The eccentric loading increases the column stress, and the stress increase is called the p-delta effect.

If drift is large enough, it causes pdelta effects. (Check theta factor).

21
Q

Natural (fundamental) period of a building

A

The time is takes the building to complete one full swing in its primary mode of oscillation (units in seconds).

(Sec. 3.8 & 4.6)

22
Q

Redundancy

A

Distributed excess capacity and multiple load paths within a structure.

Redundant design has a safety factor.

23
Q

Trend toward or away from redundancy in recent high-rise building design?

A

Toward.

Increased reliability of structure with redundancy!

Behavior not predictable, need to be able to handle tolerable loss of excess capacity.

24
Q

Torsional Shear

A

Occurs when an EQ acts on a structure whose center of mass and center of rigidity in a structure do not align.

25
Q

Negative Torsional Shear

A

The force that counteracts the direct shear caused in a lateral element from an earthquake.

26
Q

How should negative torsional shear be treated?

A

Negative torsional shear should be disregarded since it reduces shear demand on element.

27
Q

Rayleigh Method

A

Method in determining the mode shape of a MDOF system through iterative process.

28
Q

Critical Damping

A

The amount of structural damping that causes oscillation to die out and return to the equilibrium position faster than any other amount of damping.

29
Q

Damping Ratio

A

Actual damping coefficient / critical damping coefficient

Sec 4.8

30
Q

Practical range of damping ratios

A
  1. 02 (steel frame)
  2. 15 ( wood frame)

(Sec 4.8)

31
Q

To what extent does damping affect the natural period of vibration

A

Damping increases actual period slightly.

Damping is disregarded in calculation building natural/fundamental period.

32
Q

Response Spectrum

A

Graph of the maximum response (acceleration, velocity, displacement) to a specified excitation of a SDOF system plotted as a function of the SDOF systems natural period.

33
Q

How does the portal method deal with the effects of column strengthening and shortening?

A

Portal method disregards changes in column length.

Sec 8.2

34
Q

What is liquefaction? What type of soul does it occur in?

A

Sudden drop in shear strength and bearing capacity in soils. Soil turns to liquid, allowing structure on the soil to sink/settle.

Earthquake - cycles of reversed shear
Pore water pressure increases
Effective stress / shear strength decreases

Occurs in saturated, cohesion-less soil
Aka SAND

Seismic Design (4 decks)

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