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.
Prestressing of concrete is done by what two processes
1) pretensioning: high strength steel is tensioned before the concrete is cast. After the concrete hardens, the prestress wires are cut and the prestress force is applied to the concrete through bond. Therefore no end anchorages are required. Most prestressed members are pretensioned and again NO END ANCHORAGES ARE REQUIRED!
2) postensioning: steel tendons are stressed after the concrete is cast on the sire, by jacking against the anchorages at the ends of the member.
Prestressing results in more efficient use of material, making smaller sections possible.
Overturning:
what happens when a lateral force acts on a wall or building and the wall
restrained from sliding
Base Shear:
the reaction at the base of a wall/structure due to an applied lateral load
Measurements of seismic forces include all of the following EXCEPT
A. acceleration
B. Displacement
C. Momentum
D. Velocity
C. Momentum
MOMENT ARM =
the perpendicular distance from an axis to the line of action of a force
At a certain earthquake load, F, the overturning moment caused by the force of an earthquake equals the resisting moment. What is the value of F?
0.5W
The force created by an earthquake acts through the center of the mass of a structure, and the dead load of a building also acts through the building’s center of mass. To resist overturning, the weight of the structure, W, multiplied by the moment arm (or half the width), x must be equal to or greater than the moment caused by the force, F, acting through the center of the mass
Wx = or > Fh
F(20 ft) = W(10 ft)
=0.5W
The allowable shear in a plywood diaphragm depends on all of the following EXCEPT
A. plywood grade and thickness
B. direction of framing
C. nail size
D. width of framing
B. direction of framing
The allowable shear in a plywood diaphragm depends on the plywood grade and thickness, the nail size and spacing, and the width of framing members
The allowable shear in a plywood diaphragm depends on
the plywood grade and thickness, the nail size and spacing, and the width of framing members
Shear Wall:
vertical, cantilevered diaphragm that is constructed to resist lateral shear
loads
Chord:
the edge members of a diaphragm (e.g.: joists, ledgers, truss elements, double
top plates)
Box-Type Structure:
term used to when diaphragms and shear walls are used in the
lateral design of a building
Wall Bracing:
building element that resists lateral loads under low load situations
Diaphragm:
Diaphragm: flat structural unit acting like a deep, thin beam. Typically applies to roofs/
floors designed to withstand lateral loads
Blocked Diaphragm:
in light frame construction, all sheathing edges not occurring on
a framing member are supported on and fastened to blocking…more nailing provides a
greater number of fasteners able to transfer shear from one panel to another
Diaphragm Boundary:
in light frame construction, a location where shear is
transferred into or out of the diaphragm sheathing, either to a boundary element or to
another force resisting element
Diaphragm chord:
a diaphragm boundary element perpendicular to the applied load
that is assumed to take axial stresses due to the diaphragm moment
Diaphragm flexible:
a diaphragm is flexible for the purpose of distribution of story
shear and torsional moment
Rigid Diaphragm:
a diaphragm is rigid for the purpose of distribution of story shear
and torsional moment when the lateral deformation of the diaphragm is ≤2x the average story drift
Compared to steel frame building, a reinforced concrete building has
greater seismic load and creater weight
seismic load is proportional to a building weight
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.
A special moment resisting frame has an R value of 8. A concrete shear wall has an R value of 5. If all other factors are the same, the special moment resisting frame should be designed to resist a seismic force that
A. is 50 percent greater than that for the shear wall
B. is 37.5 percent smaller than that for the shear wall
C. is the same as that for the shear wall
D. may be greater or smaller than that for the shear wall, depending on the SDC
B. is 37.5 percent smaller than that for the shear wall
Which of the following types of stress are important in building desig. Check all that apply.
A. Shear
B. Tension
C. Compression
D. Strain
shear
tension
compression
Strain is not a type of stress; it is the deformation or change of size of a body caused by external loads
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)"
True or false. Wood frame buildings do not pose a significant life safety hazard during an earthquake
True
True or false. cripple walls below a first floor level must be braced to resist shear forces
In general, will a more flexible longer period design be expected to experience proportionately LESSER or MORE accelerations than a stiffer building.
LESSER
Three basic characteristics of buildings help resist and dissipate the effects of
seismically induced motion:
damping, ductility, and straight/stiffness
Response spectrum:
shows the accelerations that may be expected at varying
periods
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 natural or fundamental period of a vibration of a building is primarily a function of the buildings…
…height.
Determine Seismic Importance Factor
Per Occupancy Category IBC 1604.5
I or II = 1.0
III = 1.25
IV = 1.5
Base isolation
- A method whereby a building superstructure is detached from its foundation in order to change the characteristics of earthquake forces transmitted to the building.
- is becoming common for rehabilitation of historic structures
-Base isolation, in which the superstructure of the building is partially isolated from
ground by motion and use of bearings
-Base isolation is based on shifting the building period towards the long period of the spectrum where the response is reduced
The IBC allows two different methods for determining seismic forces. What are they and when are they used?
The two methods are dynamic lateral force procedures and static method
The dynamic analysis is always acceptable for dsign.
The static lateral force procedure is allowed only under certain conditions of regularity, occupancy, and height.
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 flexible diaphragm usch as plywood acts as a simple beam between the vertical resisting elements, while a rigid diaphragm, such as concrete,
distributes horizontal forces to teh vertical resisting elements in proportion to their relative rigidities.
Drag Struts …
…distribute the uniform diaphragm shear over the top of an opening for a
window or door in the the full height shear wall adjacent to them.
Torsional forces only occur in which type of diapragm?
rigid diaphragms
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
Is a three hinged arch staticcally determinate or indeterminate?
statically determinate
WHen a member undergoes stress, its length changes. If the member does not return to its original length when the stress is removed, the action is called
ineleastic
If no strain remains when the stress is removed, it is called elastic action. However, if some strain remains the action is termed inelastic. Steel is elastic up to the elastic limit. Concrete and wood are nearly elastic at low stress, but inelastic at higher stresses
•Three important preservation principles should be kept in mind when
undertaking seismic retrofit projects:
- Historic materials should be preserved and retained to the greatest extent possible and not replaced wholesale in the process of seismic strengthening
- New seismic retrofit systems, whether hidden or exposed, should respect the character and integrity of the historic building and be visually compatible with it in design
- Seismic work should be “reversible” to the greatest extent possible to allow removal for future use of improved systems and traditional repair of remaining historic materials.
Historic Preservation efforts include upgrades to building structure to protect the
building from seismic and wind forces
•Historic buildings are especially vulnerable to seismic/wind forces as they have
not been designed and constructed to absorb swaying ground motions…can
have major structural damage, or outright collapse
•More and more communities are beginning to adopt stringent requirements for
seismic retrofit of existing buildings.
•Although historic and other older buildings can be retrofitted to survive
earthquakes, many retrofit practices damage or destroy the very features that
make such buildings significant.
•Life-safety issues are foremost and there are various approaches which can
save historic buildings both from the devastation caused by earthquakes and
from the damage inflicted by well-intentioned but insensitive retrofit procedures.
The materials that cannot function as a diapragm include
light guage corrugated metal
straight tongue and groove sheathing
The location deep within the earth where the siesmic movement of rock begins is called the
hypocenter or focus
An epicenter is the projection of a hypocenter onto the surface of the earth.
What type of building is best suited for the flat plate concrete deck?
On the Richter Scale, compared to a magnitude 5 earthquake, and earthquake of magnitude 6 releases about …
