Lateral Forces Flashcards
How does the horizontal ground movement of an earthquake affect a building?
Initially, the inertia of a building tends to keep it in place. However, the ground acceleration imposes lateral loads on the building and shear at the base as the building begins to move horizontally in response to ground movement. As the ground acceleration changes direction, the building vibrates back and forth. The acceleration of the building is dependent upon its period of vibration which depends on the mass and stiffness of the structure.
The intensity of an earthquake as experienced by a building is affected by:
1 - The building’s distance from the epicenter.
2 - The type of soil below the building.
3 - The magnitude of the earthquake
The horizontal deformation (or displacement) of a structure when subjected to a load is called:
A - The P-Delta Effect
B - The Orthogonal Effect
C - Base Shear
D - Drift
D- Drift
Drift, horizontal deformation, is analogous to floor joist deflection in the vertical direction. Whereas joists are designed to limit bounce, the structural frame of a building must be sufficiently stiff in order to limit the amount of horizontal displacement when subjected to a lateral force. The displacement of one floor level relative to the level above or below is called story drift. it is limited by building codes and is based on a percentage of the story height (h). Building codes also limit the overall drift of a structure which is based on a percentage of the overall building height (H).
Examples of limits on Drift:
Story Drift = 0.013h
Overall Drift = H/500
A major concern for non-structural damage, excessive drift can result in falling fixtures, broken pipes and glass, cracked finishes, and occupant discomfort.
Name TWO methods of minimizing mechanical vibrations in a building?
1 - Isolate mechanical equipment from the structure with springs or neoprene isolator pads. This is a beneficial type of construction discontinuity.
2 - Increase mass by using a heavy concrete base for the equipment. This will dampen motion and lower freqency.
Impact Load
A dynamic, short-term load affecting a structure. Also called a kinetic load, it is caused by travelling elevators, vibrating machinery, and moving vehicles, and forms the part of the total live load. Kinetic loads are calculated by multiplying the magnitude of the static load by an impact factor between 1.2 - 2.0.
In the structural design of steel and timber beams, what cross section property determines the section’s moment capacity?
A - Moment of Inertia (Ix)
B - Area of the Section (A)
C - Depth of the Section (h)
D - Section Modulus (Sx)
D - Section Modulus (Sx)
The Moment Capacity (MR) of a cross section is equal to the Section Modulus (Sx) mulitplied by the Allowable Bending Stress (Fb).
Therefore:
MR=Sx x Fb
Where: (MR) = Moment Capacity (lb-ft. or Kip-ft.)
(Sx) = Section Modulus of the cross section (in3)
(Fb) = Allowable bending stress of the material (psi or ksi)
Period of Vibration:
Short
vs.
Tall buildings
Generally an earthquake has a greater effect on a building with a short period of vibration. Typically these buildings are short in height and composed of a stiff lateral resistive system, such as a shear wall system.
Tall structures such as high-rise office buildings tend to be relatively flexible and have longer periods of vibration.
Short and stiff buildings typically have a period T_<_0.3 seconsd and oscillate rapidly. Medium Height buildings typiclly have a period 0.3<t><1.0 seconds. Tall and flexible buildings typically have a period T</t>>1.0 second and oscilate slowly. Through its flexibility, the building can absorb and dampen the energy exerted by the earthquake.
The amount of dynamic force that is directed to the lateral resistive system depends on the system’s stiffness. The stiffer the system, the more earthquake load it attracts. Shear walls are stiffer than braced frames which are stiffer than Moment-Resiting Frames (MRF is the most ductile).
With regard to a structure, the greater the stiffness, the shorter the period of vibration. Conversly, a decrease in the stiffness (making the building more flexible) will increase the period of vibration.
Ductility is most dependent upon which of the following?
A - A Rigid Frame
B - A long period of vibration
C - Deformation
D - Torsion
C - Deformation
Ductility is the property of a material that enables it to deform, without failure, when subjected to a force.
Steel, for example, is a highly ductile material that can withstand a great amount of deformation without collapse.
Concrete, however, is a brittle, non-ductile material that cannot withstand much deformation and will collapse without warning. Reinforcing concrete with steel rebar increases its ductility if properly designed and detailed.
Which of the following is the renowned engineer who specialized in reinforced concrete shell construction?
A - Auguste Perret
B - Eugene Freyssinet
C - Robert Maillart
D - Pier Luigi Nervi
D - Pier Luigi Nervi
An italian contractor and engineer known for elegant structural solutions in reinforced concrete. He designed the Palazzo dello Sport (1957), Rome. An indoor stadium covered with a thin concrete dome supported by 36 Y-shaped columns. Nervi also pioneered the use of the Lamella system in vault construction.
Auguste Perret - Explored the potential for reinforced concrete in common building types.
Eugene Freyssinet - An engineer who pioneered the use of the space frame and is known for the design of large reinforced concrete airship hangars
Robert Maillart - A designer of elegant reinforced concrete bridges.
Which of the following is a support that receives both vertical and horizontal loading?
A - An abutment
B - A form tie
C - A fascia
D - A backup bar
A - An abutment
An abutment is the foundation of an arch. It provides horizontal and vertical support to an arch depending on the inclination of the arch. The shallower the arch, the larger the horizontal load transfered to the abutment. The steeper the arch, the larger the vertical load transfered.
A form tie is a steel rod with fasteners used to hold concrete formwork together during a pour. A form tie is subjected to horizontal axial tension as a result of the lateral thrust of the wet concrete on the formwork walls.
A fascia is an exposed vertical face of an eave and is subjected to horizontal wind pressure.
A backup bar is a strip of steel used to provide a solid base for begining a structural steel weld.
Identify the systems below.
One- and Two-way slab action
Both are horizontal spanning structural elements. Often monolithic, cast-in-place concrete and used for roof and floor decks.
1 - One-way Slab: Supported by walls or beams on the longer sides of the bay. The spanning structure resists bending in one direction, between the supports. The reinforcing bars in a one-way slab span the short direction. A one-way slab may be solid (for shorter spans) or coffered (one-way pan joist) to reduce the weight of the slab for longer spans.
2 - Two-way Slab: Supported by walls or beams at the perimeter, this system resists bending in two directions. It is most effective when the span is approximately equal in both directions, but is sometimes used for bays that have an aspect ratio (length to width of bay) of up to 2:1
A bundled tube framing system is used in which of the following buildings?
A - First National Bank, Chicago
B - Seagram Building, New York
C - John Hancock Tower, Chicago
D - Sears Tower, Chicago
D - Sears Tower, Chicago
The Sears Tower, built in 1976, has nine tubes bundled together. Essentially a tubular column cantilevered from its foundation, this nine-square grid plan offers additional strength and resistance to lateral wind forces. Each of the nine tubes has lateral resisting ability on its perimeter and gains additional lateral strength when bundled to adjacent tubes.
Name TWO planning methods used to reduce the risk of exposure to seismic activity.
1 - Fault Zone Mapping: Identify and map active fault zones or surface faults. Those faults below the surface can be mapped by tracking small earthquakes. These zones have been defined as 200 yards on either side of the fault line. No construction is permitted within 25 feet of that line and occupancy and construction can be limited within the zone.
2 - Seismic Zone Identification: Regional mapping intended to identify potential risk of earthquake ground failure (see model code seismic zone map of the U.S.). In addition, mapping with respect to stability, flood plains, or landslide-prone areas can help to make informed land-use decisions.
Why are earthquake loads calculated in a similar manner to wind loads?
Both wind loads and earthquake loads are dynamic in nature and occur with varying velocity, intensity and direction.
The most damaging structural effect of these forces is their horizontal action. In the static lateral force procedure, seismic loads are calculated as acting parallel to the earh’s surface. This is similar to the static method of calculating wind loads, which measures constant positive wind force acting on a windward elevation and a constant negative pressure on the leeward elevation.
Seismic forces are dependent upon the mass (weight) the structure, while wind forces depend on the surface area of a windward or leeward direction. Wind loads induce significant bending on a building while seismic loads induce shear at the base of the building.
What is the C factor from the formula
V=(ZIC/Rw)W
C=(1.25S)/T2/3
The numeric coefficient that describes the relationship of fundamental period of vibration of a building (T) to that of the site (S) and the interaction of the two during seismic activity. The longer the period of the building (T) (more flexible structure), the lower (C) the lower shear (V) at the base of the structure. The softer the site (for example, clay is softer than sand), the longer the period of the site (S), the larger the base shear (V). Resonance, and therefore more damage will occur when (S) and (T) are comparable.
(S) is a factor of the depth of the soil above bedrock and varies from 1 to 2 as the depth increases. (T) is a function of the building’s height (hn) and the type of lateral resistaice system used in the building (CT).
T=CT(hn)3/4 where:
CT = 0.035 for steel Moment Resisting Frames (MRF)
CT = 0.030 for reinfoirced concrete moment resisting frames and for Eccentrically Braced Frames (EBF)
CT = 0.020 for other structures.
Neutral Axis
In a beam, compressive stresses and tensile stresses are separated by horizontal plane called the neutral surface. The line at which this plane intersects the beam cross section is called the Neutral Axis (N.A.).
The amount of tension and compression increases with the distance from the neutral axis. Positive bending (simply supported beams) occurs when all the fibers above the neutral axis are in compression and all the fibers below are in tension.
Negative bending (overhangs and cantilevers) occurs when the fibers above th N.A. are in tension and those below are in compression.
The N.A. in a bending member is the location of no stress (neither tension nor compression).
Consider a cantilevered retaining wall 65 feet long and 11 feet high, with no surcharge.
What is the total hotizontal load exerted on the wall?
A - 1.8 kips
B - 118 kips
C - 60 kips
D - 330 kips
B - 118 kips
Model codes allow the design for a wall retaining well-drained soil on the basis of an equivalent fluid pressure of 30 lbs. per cubic foot. The pressure on the retaining wall increases with the depth of the soil retained. Accordingly, the lateral pressure exerted by the soil at a depth of 1’ it is 1’ x 30lbs/ft3 = 30lbs/ft<span>2</span>; at a depth of 2’ it is 2’ x 30lbs/ft3 = 60 lbs/ft2; at 3’ it is 90lbs/ft2, etc.
At 11’ the pressure on the wall is 11’ x 30lbs/ft3 = 330 psf.
Using the formula:
P = (ph2)/2
Where:
p = the equivalent fluid pressure
h = the height of the retained soil,
Then: the total horizontal load is P = (30(11)2)/2 = 1,815 lbs per linear foot of wall. 1,815 x 65 = 117,975 lbs or 118 kips.
Resultant of triangular load distribution P:
P = (330psf)(11’)/2 = 1815 lbs/ft of length
PTotal = 1815plf x 65’ = 118 kips
Diaphragm
A horizontal roof or floor system designed and detailed to distribute lateral forces to vertical resisting frames or walls. This component of a structural system functions by absorbing lateral loads and redistributing them to the vertical gravitational elements in the system and then to the foundation. May be made of various materials including wood, steel and concrete.
Battering in seismic design is a form of which of the following?
A - Retaining
B - Coupling
C - Pounding
D - Damping
C - Pounding
This is when two adjacent structures repeatedly bump into each other due to ground accelleration and shaking. A degree of physical separation is required to keep the two masses from causing damage to each other. The required separation is generally taken as the sum of the drifts of the two structures multiplied by a factor of safety.
1 - Resonance
vs.
2 - Damping
Resonance - The cyclic, rhythmic motion of a body whose magnitude increases with each successive cycle. It occurs when the force acting upon an object is in harmonic rhythm with the displaced object’s period of vibration, thus the vibration is amplified. This may occur when the period of the building is similar to the period of the soil on which it rests. Flexible buildings should be designed for stiff sites and stiff buildings should be designed for flexible sites.
Damping - Any influence that tends to reduce the amplitude (magnitude of displacement from neutral) in succeeding cycles. This may take the form of shock absorbers (springs) or cushioning material at the foundation, or a heavy mass counterbalance on the top of the building. Both of which attempt to isolate the building from lateral forces.
A shear wall is like a vertical cantilever in that it transfers compression, tension and overturning moment to which of the following?
A - Column
B - Floor diaphragm
C - Ceiling
D - Base of wall
D - Base of wall
Shear forces are transferred through the plane of the wall to its base and are carried down to the foundation. Shear walls transfer forces by direct shear resistance in their plane, cantilevered moment resistance to lateral forces perpendicular to their plane, and develop horizontal sliding resistance resulting from friction between the base of the building and the supporting soil.
Hertz
A unit of frequency, measured in cycles per second. One (Hz) is equal to one cycle or one vibration per second
Which of the following building components is particularly prone to wind stresses?
A - Spandrel
B - Caryatid
C - Collector
D - Parapet
D - Parapet
A parapet is a vertical extension of a wall plane that rises above the roof line. It is subject to wind pressure that acts as both a direct (positive) force and a suction (negative) force. Depending on its height and construction it may require additional bracing for stability.
A Spandrel panel is located in the space between a window head and the sill of the window in the story above it. Non-visual spandrel panels are usually placed to conceal floor construction
A Caryatid is an element carved or molded into the form of a draped female figure, that serves the function as a column.
A Collector is a structural member that transfers lateral forces to another member.
Uplift
As a result of wind, it is the net upward force or suction acting on a roof, eave or the entire building itself. Special framing connectors may be required for adidtional anchorage, especially for lightweight members and structures. In general, the use of either wind load design method (projected area method or normal force method) will account for this action. Dead Load is generally an advantage and considered a stabilizing factor in resisting uplift forces.
Additional vertical uplift forces to consider may include seismic action, hydrostatic pressure or frost action in expansive soils.
Positive wind pressure occurs primarily at what areas of a building?
A - The windward face
B - The leeward face
C - The sides
D - The roof
A - The windward face
Also called direct pressure, it affects the building surface that is facing the wind and perpendicular to its path. The direct impact of this air mass generally produces the major portion of the wind force (qs) on a building.
The leeward face and side of the building experience negative pressure (suction). The roof of the building experiences suction if it is flat or shallow (
Identify the areas of this building most prone to an increase in wind speed.
As an air mass encounters a stationary object it must find a way around it. This creates areas of increased positive wind pressure. The increase in pressure results in higher wind velocity at a building’s corners, eaves, ridges, rakes and openings in the windward facade.
While it may not affect structural design, it may produce an undesirable funneling effect in openings or unpleasant outdoor spaces and loud whistling, especially between closely spaced buildings.
What is the resulting action of the following wind effect diagram?
Wind has the general effect of direct positive pressure on the facade facing the wind, and suction or negative pressure on a flat roof, and suction on the leeward side of a building.
Identify this type of wall.
Gravity retaining wall
A retaining wall that uses its own (massive) weight to resist the thrust of retained soil. This gravity retaining wall incorporates a shear key at its base to help resist sliding. Used for moderate to low walls, up to 6’ high.
1 - Parabolic
vs.
2 - Catenary
1 - A Parabolic curve is assumed by a cable carrying a uniformly distributed load projected on the horizontal. The deck of a suspension bridge is an example load distribution on a cable.
2 - A Catenary Curve is assumed by a cable carrying a uniformly distributed load along the length of the cable itself, overhead power lines for example. The load distribution on a cable due to its own weight is a catenary curve load distribution.
Eccentric Diagonal Bracing
vs.
Concentric Diagonal Bracing
1 - Eccentric Diagonal Bracing is diagonal bracing in which one or more of the connections does not intersect a column-beam joint. It results in additional bending and rigid frame behavior in the members.
2 - Concentric Diagonal Bracing is bracing in which the bracing members intersect a column-beam joint and are subjected primarily to axial forces.
A braced frame system of trussing or triangulation used to stabilize pinned post and beam systems against lateral loading. Since these axial elements (including steel rods or cable) are ususally best at resisting tension, two members are often used to eliminate the need for a compression member. The resulting increase in stiffness is beneficial for seismic and wind resistance and is usually incorporated into steel frames. Eccentrically braced frames are more ductile than concentrically braced frames and are thus able to absorb seismic energy through torsion on the frame.
The discontinuity of the shear wall below makes it most susceptible to:
A - Horizontal Shear
B - Torsion
C - Vertical Shear
D - Base Shear
C - Vertical Shear
The middle section of the wall above and below the windows is the link between two solid sections of the shear wall in this single wall plane. These links in coupled shear walls are prone to the effect of vertical shear. The change in direction of the lateral force in addition to the gravitational loads on the wall result in X-shaped diagonal cracking commonly found in concrete and masonry construction after an earthquake.
Which loading combination is NOT considered critical to structural design?
A - Dead Load + Live Load + Seismic Load
B - Dead Load + Live Load
C - Dead Load + Live Load + Wind Load
D - Dead Load + Live Load + Wind Load + Seismic Load
D - Dead Load + Live Load + Wind Load + Seismic Load
Wind and seismic forces are not calculated as forces acting simultaneously on a structure. There is an extremely low probability that the two phenomena will occur simultaneously. In certain regions a snow-load factor may be required in addition to the seismic zone requirements.
A tall pole supports an antenna and is supported by a cable as shown. What is the force in the cable?
7,637 lbs
1 - The total horizontal wind force (P) is 75 lb/ft. x 120’ = 9,000 lbs. applied at mid height (60’)
2 - Sum the moments about the base of the tower (A).
3 - Assuming clockwise moments are positive,
ΣMA = 0 ; (9000 lbs. x 60’) - (H x 100’) = 0
H = (9000 lbs. x 60’)/100; = 5400 lbs.
4 - Since the slope of the cable os 1:1 then: The force in the cable = 5400 x sqrt(2) = 7,637 lbs.
In a three cable scheme, this assumes that the cable is acting alone, where the force (wind) is parallel to the direction of the cable.
Identify the areas of this building most prone to negative wind pressure.
Negative pressure (suction) is an outward force and occurs primarily on the leeward side of the building (the side opposite from the wind direction).
Negative pressure also occurs on the flat roof, corners, eaves, rakes and ridges and all other non-windward sides of the building. Any windward side parapet is subjected to both positive and negative pressure. All other parapets are subject to negative pressure
Bent
A component of a rigid frame system.
A planar framework of beams and columns with moment-resistive connections designed to resist lateral forces in the plane of the frame, in addition to vertical forces.
Wind and its characteristics
It is the movement of air caused by differences in the surface temperature in various parts of the world. This temperature differential is caused by unequal levels of heat gain in the earth’s surface, over oceans and in the air.
Air has mass, density and kinetic energy, and as it encounters a stationary object, such as a building, it exerts a pressure (qs). Therefore, certain requirements for structural design must be employed in order to resist that force.
An increase in velocity translates to an increase in force and thus, in pressure. The primary concern is the maximum sustained velocity and the impact effect of gusts exacerbates the force and accounts for the most critical situation.
Response Spectrum
A graph curve that describes the effect of dynamic forces on a building. It is a ratio of the period of vibration of a structure to the period of vibration of the ground. The graph curve shows that buildings with a short period of vibration (stiffer buildings) will be subject to greater direct force when constructed on bedrock sites and that buildings with a long period of vibration (flexible buildings) will be subject to lesser direct force.
Why is dead load considered a disadvantage in earthquakes?
V = (ZIC/Rw)W
**Dead load or mass is directly proportional to lateral force. **Therefore, the greater the dead load, the greater the impact of the force on the building. The heavier the building (concrete frame, shear walls, etc.) the higher the lateral load it attracts. The lighter the building (steel frame, bracing, etc.) the less lateral load it attracts.
In addition to the stiffness of a building’s lateral resistive system (Rw), the relationship of the building’s period to that of the site (C), the seismic zone in which it is located (Z) and the importance of its occupancy (I), the dead load of the building (W) is the most important factor in determining the base shear exerted on the building.
Gradient Height
The height above the ground at which friction from obstacles on the ground no longer affect wind speeds. These heights have been determined for three differant regional conditions
1 - Open areas: 900’ above the ground.
2 - Suburban areas: 1,200’ above the ground.
3 - Urban areas: 1,500’ above the ground.
The density of buildings and other structures on the ground determine how much friction and resistance to wind pressure will be encountered. The less the density, the less the interference with the path of wind. Open bodies of water offer very little resistance to wind and will have a low gradient height.
Name the FIVE cases of Plan Structural Irregularities, according to model codes.
1 - Torsional Irregularity: A result of asymmentrical plans or a discontinuity in the bracing system. A factor of the variations in story drift in different parts of a building.
2 - Re-Entrant Corners: L T U C O + shaped plans
3 - Diaphragm Discontinuity: Diaphragms with variations in stiffness or having open areas or cutouts such as those for stairs, especially when these are close to the edge or corners of the diaphragm.
4 - Out-of-Plane Offsets: Discontinuities in a lateral force resistance path.
5 - Non-Parallel Systems: Where elements in the structural system are not parallel or symmetrical with respect to the major axis of the system. Curved or angled walls, for example.
Regarding building codes for seismic conditions, what are the characteristics of a properly designed building?
Buildings designed in an earthquake zone should be able to:
1 - Resist minor earthquakes with no structural damage and some damage to non-structural components.
2 - Resist moderate earthquakes with some structural damage.
3 - Resist major earthquakes without collapse but with structural and non-structural damage.
For practical reasons, all buildings cannot be designed to withstand major earthquakes without damage. Therefore there is some compromise in the approach to seismic design.
According to model codes, the general purpose of earthquake design is to guard against major structural failure and loss of life, not limit damage and maintain function.
Name the FOUR basic types of lateral force resisting systems.
1 - Shear Wall, Bearing Wall or Box System: Consists of vertical (walls) and horizontal (roof and floor diaphragms) planar elements.
2 - Braced Frame: Can be Concentric or Eccentric. Incorporates diagonal bracing members, struts, guy wires, etc. to stabilize a rectangular pinned frame.
3 - Moment-Reisting Frame (Rigid Frame): Joints in the frame are fixed, that is, capable of transmitting moment forces between horizontal and vertical elements. Model codes classify different frames based on their stiffness and ductility and the way they are detailed. Some examples include:
SMRF: Special Moment-Resiting Frame IMRF: Intermediate Moment-Resisting Frame OMRF: Ordinary Moment-Resisting Frame
4 - Dual System: Combines the ductility of a Moment-Resisting Frame with the stiffness of a shear wall or braced frame.
The choice of system is based on the loading conditions and seismic response characteristics requried.
What is the I factor from the formula:
V = (ZIC/Rw)W
Importance Factor
The Importance Factor that identifies essential and hazardous facilities (after an earthquake) from structures that are less essential. These buildings are designed to be 25% stronger than other structures ( I = 1.25) in order to remain safe and functional after an earthquake or other natural disaster.
How does the pressure exerted by wind vary with wind speed?
The pressure is proportional to the square of the wind speed.
The wind stagnation pressure (qs) at a standard height of 33’ is given by:
qs = 0.00256 V2
Where qs is the wind stagnation pressure (psf) at a height of 33’. This pressure increases with height.
V is the wind speed (MPH) as it strikes the structure.
For example, if the wind speed (mph) is doubled, the pressure (psf) is four times larger.
A square-plan building withstands the given wind pressure better than a rectangular-plan building with the same height and floor area.
What are the main reasons for this resistance?
1 - Building A has more surface area facing the wind than B
2 - Building A has less shear resistance than building B, due to the shorter length of the shear walls.
3 - The narrower diaphragm in ‘A’ is more prone to in-plane buckling.
Chord Force is the force that is developed horizontally along the edge of the diaphragm between the supports. The diaphragm acts like a simply supported, uniformly loaded beam, and the chords act as the flanges. Determine the chord force in the following example.
First, find the external moment applied on the diaphragm due to the lateral load (w). M = (mL2)/8
The resisting moment developed by the diaphragm is C x d. Torsion will not occur as long as the external moment is equal to the resisting moment. Then, to compute the compression chord force (C), or the tension chord force (T), divide the external moment by the depth of the diaphragm C = M/d
Note that the reaction for each wall is (wL)/2. If the uniform load coefficient (w), is 500 lbs/ft. the length of the diaphragm is 100’ and its width d = 50’, then:
M = (500 x 1002)/8 = 625,000 ft-lbs.
Then the chord force = M/d = 625,000/50 = 12,500 lbs.
Which plan form is least acceptable in seismic zone 4?
Plan C
This plan is most susceptible to earthquake damage for two reasons.
1 - the re-entrant corner will cause independent movement in the two masses of the building.
2 - the asymmetrical footprint exposes the building to the effects of torsion.
Seismic seperation joints will provide a physical separation and allow the wings of the building to move independently.
Plan B has two re-entrant corners but the symmentry of the wings will help to counteract the effects of torsion.
What is the magnitude of hydrostatic pressure at the bottom of a 12’ deep water storage tank?
The magnitude of hydrostatic pressure at a given point is equivalent to the unit weight of the retained liquid (water weighs 62.4 lbs/ft3) multiuplied by the height of the wall containing the liquid.
For example, 62.4 lbs/ft3 x 12’ = 748.8 lbs/ft2 acting in a triangular distribution perpendicular to the storage tank walls at the base of the tank.
Name the FIVE cases of Vertical Structural Irregularities, according to model codes.
1 - Stiffness Irregularity: Also called the soft story, where one story lacks stiffness of the story above or below it.
2 - Weight or Mass Irregularity: Where the weight of one story is more than 150% of the weight of the adjacent story.
3 - Vertical Geometric Irregularity: Where the horizontal dimension of the structural system in any story is more than 130% of that in an adjacent story. Very large setbacks, for example.
4 - In-Plane Discontinuity in Vertical Lateral Force-Resisting-Elements: Where there is an offset of structural elements. An interrupted shear wall, or a staggered system, for example.
5 - Discontinuity in Capacity: Also called a weak story, where one story lacks the strength of the story above or below it.
Name the types of Moment-Resisting Frame Systems.
These frames provide resistance to lateral forces primarily by the flexural (bending) action of its members, and the transfer of moments from the horizontal components of the frame to the vertical components through rigid connections.
1 - SMRF: Special Moment-Resisiting Frame: A steel or concrete frame specially designed to provide ductile behavior.
2 - MMRWF: Masonry Moment-Resisting Wall Frame: A masonry wall designed to resist forces through ductile behavior.
3 - IMRF: Concrete Intermediate Moment-Resisting Frame: Essentially a concrete frame, generally prohibited in seismic zones 3 and 4. This frame is also detailed for ductile behavior.
4 - OMRF: Ordinary Moment-Resisting Frame: Ordinary steel frames are generally acceptable in all seismic zones whereas ordinary concrete frames are prohibited in zones 2A, 2B, 3 and 4. OMRF are not specifically designed for ductility
5 - STMF: Special Truss Moment Frame: A steel frame specially designed to provide ductile behavior.
The greater the ductility of a frame through detailing, the higher the Rw in the base shear equation and the lesser the resulting Shear (V).
Irregular Structures
vs.
Regular Structures
With regard to seismic response, these terms define the two types of building configuration classifications as defined by model codes. They refer to the building’s size, shape and physical characteristics, which are integral to the structure’s response to lateral forces.
Irregular Structures are those characterized as having significant physical discontinuities in configuration or in their structural system. These features include those discontinuities defined as vertical or plan structural irregularities. These structures may require the dynamic method of structural design analysis.
Regular Structures are defined as having none of these characteristics.
How does a Moment-Resisting Frame react to lateral forces?
The structural frame absorbs lateral forces through deformation and strength. The deformation softens the loading on the structure (damping), so there is actually less resistance required of the frame. Though flexible compared to shear walls or trussed frames, Moment-Resisting Frames are also called rigid frames and must be made of a ductile material such as steel (or reinforced concrete in zones 1 and 2). Reinforced concrete frames are acceptable in Zones 3 and 4 when they are detailed for ductile behavior.
A concrete beam in a desert parking structure is subjected to a range of temperatures. In winter, the lowest recorded temperature was 22oF and the beam was exctly 36’-0” long. In summer, the maximum temperature was 114oF. At the extreme temperatures what is the difference in beam length.
A - 2.20”
B - 0.02”
C - 1.02”
D - 0.22”
D - 0.22”
The change in length = (the change in temperature) x ( the original length) x (the coefficient of thermal expansion of the material).
Since the coefficient of thermal expansion of concrete:
a = 0.0000055 in/in/oF
_/_L = _/_T x Lo x (coefficient)
_/_L = ( 114o - 22o ) x ( 36 ft. x 12 in/ft ) x ( 0.0000055 in/in/oF)
_/_L = .22”
Note: that _/_L is in inches and _/_T is in degrees F, while the coefficient of thermal expansion is in inches/inch/oF
Normal Force Method
vs.
Projected Area Method
Used in the application of design wind pressure and for the value and appropriate use of the Sq factor in calculating wind loads, the two methods are:
Normal Force Method: Required for gabled rigid frames and may be used for any building less than 400’ tall. Wind pressures in this method are assumed to act simultaneously normal (perpendicular) to all exterior surfaces.
Projected Area Method: Total wind effect is a combination of an inward (positive, horizontal) pressure acting on the building’s projected area, and a negative, upward pressure acting on the full projected area of the building’s roof. May be used for any structure less than 200’ tall except for gabled rigid frames.
Calculate the Overturning Moment and the Stabilizing Moment for the wall shown.
The Overturning Moment about A is:
OTM = (4k x 12’ )+ (4k x 24’) + (4k x 36’) + (4k x 48’) = 480 k-ft.
The Stabilizing Moment results from the dead weight (W) of the building:
SM = 60k x 10’ = 600 k-ft.
As long as the Stabilizing Moment is greater than the Overturning Moment, the building is safe from overturning. Model codes usually require that the SM be 1.5 times the
OTM.
Why are soil conditions important in seismic design?
Each soil type has a different period of vibration (S) which is a component of the C factor in the base shear calculation. The period is dependent upon the grain size, grain shape, density, compaction and degree of water saturation. The soil resisting the forces that cause overturning must be considered in lateral force design. Soils that have high water content (clays and silts) may be subject to liquefaction.
Characteristics of Finishes
Building finishes such as stucco, sheetrock, plaster or wall tile are relatively stiff, rigid and brittle. Often these vertical finishes are attached to a frame that is more flexible. Initally, these non-structural elements bear the force of a lateral load. This may result in cracking and failure of the finishes. Proper detailing of finish connections can minimize the damage of excessive lateral forces.
Graphic representation of the static methods for calculating wind pressure.
1 - Normal Force Method
2 - Projected Area Method
1 - The pressure on the leeward roof is negative and constant. The pressure on the leeward facade of the building is also negative and constant.
2 - Windward face pressure varies with height.
Identify this type of retaining wall
A cantilevered, heel-type retaining wall.
The most common type of retaining wall. A freestanding wall with no transverse support at its top, such as a floor or roof plane. The wall shown uses the weight of the earth and the upward pressure of the soil at the toe to counteract overturning.
When mapping the intensity of an earthquake, an isoseismal map is sometimes used. Intensity ratings are plotted and areas of similar rating are enclosed by a dark contour line. Intensity ratings usually increase closer to the epicenter; however, soil conditions can cause unusually high intensities.
Name and define TWO methods of measuring the strength of earthquakes.
1 - Richter Scale: Devised by Professor Charles Richter in 1935. It is a measure of the magnitude or energy released at a distance of 100 kilometers from the epicenter. While accounting for the distance from the source, the magnitude is measured logarithmically. Each unit increase on the scale represents a 10-fold increase in amplitude or approximately a 31-fold increase in energy. This method does not account for the duration or frequency; both of which are significant in determining damage.
2 - Modified Mercalli Scale: A measure of intensity, originally developed in 1902 and modified in 1931 to fit construction conditions. It is a scale based on subjective observations of the effects on buildings, ground and people. Somewhat dated, since damage is usually localized in older buildings described in this scale. It has an intensity rating scale of I to XII and is mapped (as shown on opposite side of card) where I is not felt and XII is total destruction. It can be correllated with ground acceleration.
Locate the centroid of the cross section below.
The Y-axis is centroidal since it has an axis of symmetry.
The total area is 120 + 160 + 48 = 328 in2
Sum the statical Moment about the bottom of the member
M = 120(22) + 160(8) + 48(2) = 4016 in3
To locate the centroid (X), divide the statical moment by the sum of the areas of the component peices
X = (4016/328) = 12,244 in
The P-Delta effect
The action of combined stress in a structural member. A result of a combination of gravity load, for example: axial compression on a column, plus lateral loads due to wind or seismic forces. This produces an additional moment when the centroid of the column deflects away from the axial line of action of the compressive force. The resulting axial compression loads in addition to the moment resulting from story drift caused by this effect, are used to evaluate the overall stability of the structural frame.
When the heavy mass of a building rests on a soft story and is then laterally displaced (story drift) due to lateral load, the mass crushes the columns. This is a result of the P-Delta effect.
Identify this retaining wall.
Counterfort retaining wall
A retaining wall with fin walls in tension. The fin walls are placed at regular intervals and are connected to the heel and stem. This type of wall can be used when heights exceed the capability of a cantilevered retaining wall and clear open space is required on the unretained side. Not to be confused with a buttressed retaining wall where the fin walls are in compression.
Identify the beam-to-column connections below.
A - Angle Bracket Connection
B - Strutural Tee
C - Welded Connection
D - Stub Bracket
All the connections shown are rigid, moment-resisting connections.
What term(s) may be used to describe floor A.
Soft Story
A vertical structural irregularity in which the lateral stiffness of one story is less than 70% of the stiffness of the story above or below it.
A Weak Story is one in which the story strength is less than 80% of the story above or below it.
This can occur at any level but is most common at the first story as a result of large openings for vehicles, etc. Parking garages are an example
Discontinuity
Forces flow through a structure from element to element. Discontinuity is the interruption in that normal path of travel. Stacked columns and shear walls for example, provide a direct path for the forces of gravity. If these elements are not in vertical alignment, gravity forces are transferred through other building elements as they travel to the ground. In addition to openings in shear walls for windows or vehicles, this interruption of forces may also occur in horizontal diaphragms in the form of openings for stairs or elevators.
In the following building plan diagram, the structure is least resistant to lateral forces in which direction?
East-West Axis
There is less shear resistance due to the lack of resistant structure on the North and South elevations. the two shear walls on the East and West elevations are very strong in their own plane (N and S loads) and are weak in a direction perpendicular to their plane (E and W loads). In the absence of lateral resistive members in the E-W orientation, the two shear walls will topple like dominos under E-W loads.
How deep does a friction pile need to be to withstand an axial load of 350 kips?
Assume that the pile diameter is 2’-6” and that the friction between the pile and the soil is 1500 psi.
The surface area of the pile is providing the resistance of 1500 psi. This area is called the Skin Friction Area and is equal to (2πr)l or the circumference multiplied by the length.
F = P/A ⇒ A = P/F = (350k x 1000lbs/k)/(1500 psi) = 233.33 ft2
Skin Friction Area = (2πr)l = (2π(2.5/2))xl
So, π x 2.5 x l = 233.33 ft2
l = 29.7 ≈ 30’ deep pile
Stressed Skin Panel
This diaphragm is typically composed of equally spaced panels, continuously attached to supportive members at the top and/or bottom, along the member’s length. These assemblies act as a composite material to resist external loads. Could be a floor, roof or walls.
An ‘L’ shaped plan is found to be the best design solution for a particular site. Name THREE design solutions that will help minimize the problems associated with the re-entrant corner.
The three general solutions to consider are:
1 - A seismic separation joint located at the inside corner will separate the building into two parts and allow for independant movement of each. This joint is also called a seismic joint or an isolation joint.
2 - The corner may be reinforced with additional structure such as a collector element.
3 - The inside corner may be splayed which helps the structure act more like a single unit, without excessive stress concentrations at the re-entrant corner.
Describe the classification of buildings and other structures with regard to Importance factors, according to model codes.
What condition is caused by the area labeled ‘A’
Surcharge
The pressure exerted by sloping earth above a retaining wall. This pressure can also be a result of vehicles or pedestrians above the retained soil. It can be converted into an increase in the fluid pressure.
30 PCF for level ground
45 PCF for 2:1 slope
Name THREE self-stabilizing structures
These are systems that achieve stability from the inherent form of the structure. They are relatively stiff, tend to resist deformation and are often symmetrical, of uniform stiffness and have no discontinuities in plan configuration or elevation. The pyrmaid is an ideal building form to withstand seismic loads.
Shear Wall
A component of a structural system that resists lateral forces by developing shear in its own plane. It transfers the horizontal shear to its base. Common construction materials include: reinforced concrete and masonry, and wood stud walls with plywood sheathing.
Shear walls are often used to support gravitational loads transferred from roofs to floors. In high-rise construction the walls around the core typically serve as shear walls and also receive floor loads.
Identify this type of retaining wall and describe the primary function of the component labled ‘x’
A Buttressed Retaining Wall
The fins strengthen the wall by resisting lateral and compression forces and are often used to resist the lateral outward thrust associated with roof vaults such as those found in Gothic Cathedrals (flying buttresses). They are also used to retain earth. This type of wall has the fins on the downhill face.
It is not to be confused with a counterfort retaining wall.
A building represented by the plan diagram below is subjected to an earthquake force in the direction shown. Which diagram represents the approriate reaction of the building.
Diagram A
The ground movement in the East-West direction causes a twisting action in the structure and the stress concentrates at the inside corner. The difference in the building’s cener of mass (centroid, CM) and the center of rigidity (CR) (lack of shear wall resistance in the North-South wing) results in torsion.
Is it an advantage or disadvantage to have redundancy in a structural frame?
It is an Advantage
Consider the structural frames below:
Frame A has 3 bays in each direction with all connections rigid.
Frame B also has 3 bays in each direction with rigid connections only at the perimeter and pinned connections at the interior.
Frame A has more “reserve” frame action and a higher degree of redundancy, so that if the perimeter connections fail, the interior “backup” connections will continue the intended frame action. Frame B does not have “backup” frame action and will fail totally if the perimeter connections fail, since the remaining connections are pinned.
In response to terrorist activities, structural engineers are adding several degrees of redundancy to the structural system of buildings to prevent progressive collapse which results in loss of life.
Name the structural member that is used to transfer horizontal forces to vertical resisting elements.
A - Grade Beam
B - Drag Strut
C - Dual Brace
D - Bent
B - Drag Strut
Typically, it is an element in a diaphragm, parallel to the lateral force, that collects and transfers the shear force to the structural system or distributes the load within the diaphragm. These members are subject to axial tension or compression.
In a shear wall with punched openings a horizontal load is distributed in proportion to the stiffness of each pier. Assuming the wall is of uniform thickness, the stiffness of the pier is approximately proportional to the cube of the pier dimension.
P1 = ((d1)3/(d13 + d23 + d33))P or Px = (dx3/Σ(d3))P
If P = 20,000 lbs and d1 = 5’-0”, d2 = 10’-0”. d3 = 8’-0” what is the horizontal force in each of the piers?
P1 = ((d1)3/(d13 + d23 + d33))P
53 + 103 + 83 = 125 + 1000 + 512 = 1637 ft3
P1 = (53/1637) x 20,000 = 1,527.18 lbs or 1.5k
P2 = (103/1637) x 20,000 = 12,217.47 lbs. or 12.2k
P3 = (83/1637) x 20,000 = 6,255.35 lbs. or 6.3k
Name these lateral force resistive systems for tall buildings.
A - Bay-Type System: Rigid Frames in plan and in elevation. Lateral Loads are transferred to the foundation at interior and exterior bays.
B - Tube System: Perimeter walls are rigid and rigidly connected to each other at the corners. Lateral loads are transferred to the foundation at the perimeter.
Examples: John Hancock, Chicago (trussed tube); Sears Tower, Chicago (bundled tube); World Trade Center, NYC (tube in tube).
C - Core System: Shear walls are placed (symmetrically, whenever possible) inside the structure. Structure-free plates are cantilevered from the core. This allows for uninterrupted glass on the perimeter. Usually used in conjunction with tube systems.
D - Suspension System: Floors are suspended from the structure and loads are transferred to massive compression piers (similar to a suspension bridge).
Examples: HongKong and Shanghai Bank; Federal Reserve Bank, Minneapolis.
Identify these THREE types of retaining wall failure.
1 - Overturning: Rotation due to settlement at the toe or excessive moment stress.
2 - Sliding: Due to a lack of horizontal friction at its base or lack of soil pressure resistance in front of the toe or shear key.
3 - Settlement: or sinking due to the insufficient bearing capacity of the supporting soil.
To resist failure, retaining walls are usually designed with a safety factor of 1.5, or the ability to withstand 1.5 times their overturning moment.
Period of Vibration
The time elapsed during one complete oscillation or back-and-forth sway of a building. It is dependent upon the weight or mass of the building and its stiffness or rigidity.
The following building is subjected to horizontal seismic or wind forces. H is the resultant of all horizontal forces and y is the location above the foundation. VDL is the total dead load and x is its location.
What is the product of H x y and VDL x x and what is their relationship?
The product of H x y is called the Overturning Moment.
The product of VDL x x is called the Stabilizing Moment.
The stabilzing moment resists the overturning effect of the horizontal load.
Note that only dead load is used.
VDL x x must be greater than H x y. to assure stability. and should be about 1.5 times greater.
Model codes will require that stabilizing moment equals 1.5 x the overturning moment.
P = ph2/2
Formula for Horizontal Pressure on a retaining wall
P = ph2/2
Where:
P = Total pressure
p = the equivalent fluid pressure (p = 30 pcf)
h = the total retained height
The total pressure (P), in lbs. per linear foot of wall length, is considered to act at the centroid of the triangle (h/3). The area of the triangle is the resultant pressure P on the elevation of the retaining wall. The height of the triangle (h) and the base of the triangle (hp) so the area of the triangle is (hp)h/2=ph2/2. Retaining walls must be designed to support forces equal to that of a fluid weighing 30pcf when the soil supported has no surcharge and is as deep as the height of the wall. At a height h, the pressure is h x p (or h x 30).
Computing the moment of inertia for a built-up area often requires moving the axis to which the moment of inertia is referenced.
IX = Moment of Inertia about the X-axis
A = Area of Section
d = The distance from the axis being considered
X1 = New axis
IX1 = Moment of Inertia of the Area about X1
IX1 = Ix + A(d2)
This is called the transfer equation.
How are shear stresses transferred through separate panels of a plywood diaphragm?
The shear forces are transferred between plywood panels through the structural memberes attached to the edge, such as joists, beams and blocking. Additional plywood fasteners such as hurricane clips also help join separate sheets of plywood to create diphragm action.
Name THREE design solutions that will increase the rigidity of a soft story.
1 - Cross brace some of the open bays.
2 - Increse the number and stiffness of the columns.
3 - Use tapered columns to avoid stress concentrations at corners.
The addition of interior shear walls at the ground level will provide increased rigidity and resistance to lateral force. The symmetrical placement of shear walls will cause the center of rigidity to align with the center of mass, thus avoiding possible torsion effects.
Which force diagram of a 3-story structure approximates static lateral force analysis?
Diagram D
This diagram represents the distribution of diaphragm forces using the static lateral force analaysis method. It assumes equal floor heights, mass, distribution and diaphragm strength. Although the inverted force triangle shows the greatest diaphragm reaction force at the top of the building, the shear for each floor is added to the sum from the floor above and maximum shear occurs at the base. Additionally, there is a whiplash effect experienced by the roof diaphragm.
Overturning Moment
OTM = (F1 x h1) + (F2 x h2) + (F3 x h3) + (F4 x h4)
A toppling effect caused by lateral loads, which is a crucial concern particularly in tall, slender buildings.
Overturning moment is the sum of all lateral loads at each floor line multiplied by the the length of each moment arm (the height of the floor above the level where overturning is to be measured). The overturning moment is resisted by the stabilizing moment due to the building’s dead weight.
OTM = Σ(Fx x hx)
Shear Key
A short longitudinal stub of concrete below a footing used to prevent sliding by increasing friction between the footing and the soil.
For larger abutments with greater hydrostatic pressure, H-piles (wide flange) are driven deep into the ground and the protruding portion is embedded in concrete.
