Seismic Forces Flashcards
All of the following statements about shear walls are correct except
a. a shear wall resists lateral forces from wind of earthquake by developing shear in its own plane
b. a shear wall must be made of reinforced concrete or structural steel
c. shear walls may be used with moment resisting frames to form a dual system
d. a shear wall is analagous to a vertical cantilever beam
b. a shear wall must be made of reinforced concrete or structural steel
Base Shear
Calculated total shear force acting at the base of a structure, used in
codes as a static representation of lateral earthquake forces. Also referred to as
“equivalent lateral force.”
Seismic Design Category, per IBC:
A=
B=
C=
D=
E=
F=
A = building in regions with little probability of earthquake
B = ordinary occupancy that could experience shaking
C = structures of ordinary occupancy that experience strong shaking or important
D=Structures that experience medium shaking
E = Ordinary building close to a fault line
F = Important building close to a fault line
To find base shear (units = –1–)
1) kips
Base Shear (V) = Seismic response coefficient (Cs) x effective seismic weight of “ building(W)”
Describe importance factor when calculating base shear.
The importance factor is essentially an extra safety adjustment used to increase the calculated load on a structure based on its occupancy and/or function. Essential facilities (such as hospitals, fire and police stations, etc.) have the highest seismic importance factors (IE= 1.5), while buildings where people congregate (such as schools, auditoriums, etc.) also have relatively high seismic importance factors (IE = 1.25). Other structures have a seismic importance factor of unity (IE = 1.0). Higher importance factors are intended to insure that structural integrity is not compromised and important facilities remain operational during emergencies and natural disasters. Based on typical occupancy classifications for most wood structures, wood buildings are frequently designed using an importance factor of unity (IE = 1.0).
seismic response coefficient
I(importance factor)
R(response modification coefficient)
Sds
T (Actual period of building)
Cs(seismic design coefficeint)
To solve this problem you need to know the formula for base shear and seismic response coefficient.
W=1,000,000 pounds as given
I(importance factor)=1
R(response modification coefficient) = 6 as given
Sd1(Design spectral response at period of 1.0 sec )=0.55g as given
T (Actual period of building) = 0.4 seconds as given
Fundamental Period:
the rate at which an object will move back and forth if they are
given a horizontal push
Period:
the time (in seconds) that is needed to complete one cycle of a seismic wave
Frequency:
the inverse of period, or the number of cycles that will occur in 1 second
measured in Hertz.
Hertz:
a measurement of frequency, 1 Hertz = 1 cycle per second.
Newton’s second law of motion
• Force (F) = Mass (M) x Acceleration (A)”
• Mass is equivalent to the weight of the building at ground level
• Acceleration measured in terms of acceleration due to gravity (1g = 32 ft per second
per second)
Poorly constructed buildings begin to suffer damage at about …
10 percent g (or 0.1g).
A useful measure of strong-motion duration is termed…
… the bracketed duration.
-the time between the first and last peaks of motion that exceeds
a threshold acceleration value of 0.05g
Velocity refers to…
the rate of motion of the seismic waves as they travel through the
earth in inches per second. It’s VERY fast… the P-Wave travels at 7,000 – 18,000
mph, the S-Wave travels at 4,500 – 11,000 mph
Displacement:
the distance that points on the ground are moved from their initial
locations by the seismic waves, measured in inches.
Earthquake shaking is initiated by a fault slippage in the underlying rock. As the shaking propagates to the surface, it may be amplified, depending on …
…the intensity of shaking, the nature of the rock and, above all, the sur- face soil type and depth.
earthquake damage tends to be more severe in areas of
…
…soft ground.
Amplification
A relative increase in ground motion between one type of soil and
another, or an increase in building response as a result of resonance.
A rule of thumb is that the building period equals the number of stories divided by …
10; therefore, period is primarily a function of building height.
When a reinforced concrete structure experiences severe ground shaking, it begins to crack: this has the effect of …
…increasing the structure’s period of vibration: the structure is “softening”.
When a vibrating or swinging object is given further pushes that are also at its natural period, its vibrations increase dramatically in response to even rather small pushes and, in fact, its accelerations may increase as much as four or five times. This phenomenon is called …
…resonance.
buildings suffer the greatest damage from ground motion at a frequency close or equal to their own …
…natural frequency.
The amplification in building vibration is very undesirable. The possibility of it happening can be reduced by trying to ensure that the building period will not co- incide with that of the ground. Thus, on soft (long-period) ground, it would be best to design what type of building?
a short, stiff (short-period) building.
A building with stell moment-resisting frames assigned to SDC D is 120 feet high and has a fundamental period of vibration of 1.29 seconds. If the building height were 240 feet and all other factors remained the same, the period would
A. double
B. decrease by half
C. remain the same
D. increase about 75 percent
D. increase about 75 percent
Tall buildings have longer periods than short buildings, and therefore B and C can be ruled out. But the period does not vary directly with the height, but rather with the 0.8 power of the height for steel moment resisting frames; therefore, even without using a calculator, you can see that choice A is incorrect.
kaplan q 12
Building period =
number
of stories / 10
1 story bldg = 0.1 sec nat. period = 10 hertz
20 story bldg. = 0.5 sec nat. period = .5 hertz
• Other factors (eg: structural system, materials, contents, geometric proportions) also
affect the period, but height is the most important
• Building period may also be changed by earthquake damage.
The braciing of nonstructural elements must be designed to resist seismic forces that are
usually greater than those used for the design og the building
For example, if the seismic force on a building is 15 percent of the dead load of the building, the seismic force on a given nonstructural element might be 30 percent of the wieght of that element.
A hole or notch for a pipe must be provided in a reinforced concrete beam. Which of the diagrams shows the hole or notch that will LEAST affect the beams’ load carrying capacity?
Openings in beams have the least effect on the beam’s load carrying capacity if they are located in areas of low stress. The two main types of stress and bending, or flexural, sress. The shear stress is usually greatest near the supports and least near the midspan. Thus , an opening in an area of low shear stress would be near the center of the span (ABC). FOr a simply supported beam, the beding stesses are greatest neat the middle of the span and least near the supports. Within the beam depth, the bending stresses are greatest near the top and bottom and least at about the mid depth of the beam. Therefor an opening in a n area of low bending stress would be near the supports or at the mid depth of the beam .The only location in an area of both low shear and low bending stress is A.
Nonstructural:
systems and components that are part of a building that don’t like in
the primary load bearing path of the building
Four types of safety hazards presented by nonstructural components are:
- Direct hazard - the possibility of casualties because of broken glass, light fixtures, appendages, etc.,
- Loss of critical function - casualties caused by loss of power to hospital life support systems in bed panels, or functional loss to fire, police or emergency service facilities,
- Release of hazardous materials - casualties caused by release of toxic chemicals, drugs, or radioactive materials and,
- Fire caused by nonstructural damage - damage to gas lines, electrical disruption, etc.
Select corret statement about the frame shown
A it is more ductile than a concentric braced frame
B. It is not permitted to be used to resist seismic loads
C. It may be used to resist seismic loads except in SDC D and higher
D. It is called a special moment resisting fram
A it is more ductile than a concentric braced frame
Frame shown is an eccentric braced frame, in which at least one end of each brace is eccentric to the beam column joint or the opposing brace. The intent is to make the braced frame more suctile and therfore able to absorb a significant amount of energy without buckling the braces.
The load capacity of a strucutral steel column depends on the slenderness ratio Kl/r. In this ratio, K depends on
A. the length of the column
B. the grade of steel
C. the moment of interia and area of the column
D. the end condition of the column
D. the end condition of the column
Slenderness Ratio (SR) = end condition (k) x unbraced length in inches (L) / radius of gyration (r)"
The seismic loads on the building shown in plan are restricted by shear walls 1,2,3, and 4. The diaphragm is rigid, and the center of gravity does not coincide with the center of mass. Which walls will have increased shears caused by horizontal torsional moment for seismic loads in the noThe seismic loads on the building shown in plan are restricted by shear walls 1,2,3, and 4. The diaphragm is rigid, and the center of gravity does not coincide with the center of mass. Which walls will have increased shears caused by horizontal torsional moment for seismic loads in the north south direction?rth south direction?
wall 3 only
For wall 1 the shear caused by the torsional moment tis in the opposite direction from that caused by the seismic load
However for wall 3, the shear caused by torsioinal moment is in the same direction as that caused by the seismic load, resulting in increased shear in wall 3
THe two span continuous beam show supports a uniform load on AB only and is connected to supports A.B.C. Neglecting the weight of the beam, the reaction at C is…
acting downward
beacause the beam is connected to C the reaction pulls downward as the beam tries to deflect upward.
A two story moment resisting frame with hinged bases resists the lateral earthquake loads as shown. Neglecting dead loads, what resists the overturning caused by the lateral laods
Uplift in two columns and compression in two columns
because the bases are hinged not moments can exists there
