Earthquakes/Buildings Flashcards
Range of Richter magnitude that would identify a moderate earthquake.
5.0-6.0
Range of Richter magnitude that would identify a strong earthquake
6.0-7.5
Range of Richter magnitude that would identify a great earthquake
Greater than 7.5
What does the Richter magnitude scale measure?
The numerical value represents a measure of energy release on a logarithmic scale.
Epicenter
The projection of the source of the earthquake at the earths surface. (Two-dimensional location).
Hypocenter (or Focus)
The source of the earthquake at a location below the earths surface. (Three-dimensional location).
Focal depth
The distance from the earths surface to the hypocenter/focus. Depth of the earthquake source.
Modified Mercalli Intensity (MMI)
12 increasing measures of earthquake intensity (MMI - MMXII). Intensity of an earthquake is based on the damage and other observed effects on people/buildings.
Define Peak Ground Acceleration (PGA) and common units
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).
Stiffness v Rigidity
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.
Flexibility v Ductility
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.
Pier Rigidity relationship to: height, thickness & depth
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
Ductility Factor
Ratio of a materials strain energy at fracture to its strain energy at yield.
(Sec 5.6)
μ = Ut / Ur
=toughness (rupture) / resilience (yield)
What factors influence the ductility factor?
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
Minimum recommended ductility factor
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)
Why specify a minimum ductility factor?
Obtain ductility margin (between yield and collapse) sufficient enough to ensure survival in a design earthquake
A theoretical analysis of elastic response of a structure will usually (overestimate or underestimate) the stresses resulting from an earthquake and why?
Overestimate
A structure will not behave elastically. Energy will be dissipated through deformation and Inelastic behavior through local yielding, reducing the seismic energy.
Two components of drift
- Shear drift
Sideways deflection due to lateral loads - Chord drift
Sideways deflection due to axial/vertical loads
P-delta Effect
Additional column bending stress caused by eccentric vertical loads. (P*deltax)
How are drift and Pdelta effect related?
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).
Natural (fundamental) period of a building
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)
Redundancy
Distributed excess capacity and multiple load paths within a structure.
Redundant design has a safety factor.
Trend toward or away from redundancy in recent high-rise building design?
Toward.
Increased reliability of structure with redundancy!
Behavior not predictable, need to be able to handle tolerable loss of excess capacity.
Torsional Shear
Occurs when an EQ acts on a structure whose center of mass and center of rigidity in a structure do not align.
Negative Torsional Shear
The force that counteracts the direct shear caused in a lateral element from an earthquake.
How should negative torsional shear be treated?
Negative torsional shear should be disregarded since it reduces shear demand on element.
Rayleigh Method
Method in determining the mode shape of a MDOF system through iterative process.
Critical Damping
The amount of structural damping that causes oscillation to die out and return to the equilibrium position faster than any other amount of damping.
Damping Ratio
Actual damping coefficient / critical damping coefficient
Sec 4.8
Practical range of damping ratios
- 02 (steel frame)
- 15 ( wood frame)
(Sec 4.8)
To what extent does damping affect the natural period of vibration
Damping increases actual period slightly.
Damping is disregarded in calculation building natural/fundamental period.
Response Spectrum
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.
How does the portal method deal with the effects of column strengthening and shortening?
Portal method disregards changes in column length.
Sec 8.2
What is liquefaction? What type of soul does it occur in?
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
