Lateral Forces Flashcards

Question

Identify the areas of this building most prone to an increase in wind speed.

Answer 1

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.

Answer 2

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.

Answer 3

**Gravity retaining wall** ## Footnote 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.

Answer 4

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.

Answer 5

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.

Answer 6

**C - Vertical Shear** ## Footnote 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.

Answer 7

**D - Dead Load + Live Load + Wind Load + Seismic Load** ## Footnote 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.

Answer 8

**7,637 lbs** ## Footnote 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, ΣM_A = 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.

Answer 9

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

Answer 10

**A component of a rigid frame system.** ## Footnote 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.

Answer 11

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 (q_s). 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.

Answer 12

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.

Answer 13

V = (ZIC/R_w)W ## Footnote **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 (R_w), 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.

Answer 14

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.

Answer 15

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.

Answer 16

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.

Answer 17

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.

Answer 18

**Importance Factor** ## Footnote 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.

Answer 19

**The pressure is proportional to the square of the wind speed**. The wind stagnation pressure (q_s) at a standard height of 33' is given by: q_s = 0.00256 V² Where q_s 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.

Answer 20

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.

Answer 21

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.**

Answer 22

**Plan C** ## Footnote 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.

Answer 23

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.

Answer 24

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.

Answer 25

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 R_w in the base shear equation and the lesser the resulting Shear (V).

Answer 26

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.

Answer 27

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.

Answer 28

**D - 0.22"** ## Footnote 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 = ( 114^o - 22^o ) 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

Answer 29

Used in the application of design wind pressure and for the value and appropriate use of the S_q 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.

Answer 30

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.

Answer 31

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.

Answer 32

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.

Answer 33

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.

Answer 34

**A cantilevered, heel-type retaining wall.** ## Footnote 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.

Answer 35

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.

Answer 36

D - **R_w = 8, T = 1.5** _Base Sear_ V = (ZIC/R_w)W The greater the structural system coefficient, R_w, the more flexible and ductile the lateral system is. For concrete shear walls R_w = 4.5 For concrete frame with shear wall R_w = 5.5 For concrete Special Moment Resisting Frames (SMRF) R_w = 8.5 The greater the period of the building (T), the more flexible the lateral system is, the less the 'C' factior is (soil period/building period).

Answer 37

They are walls supported by their own weight in addition to vertical and sometimes lateral loads. The Three classifications are: 1 - Vertical Load-Bearing Walls support the weight of the walls above as well as floor and roof loads in addition to their own weight. 2 - Shear walls are structural walls that resist lateral and vertical loads in the plane of the shear wall in addition to their own weight. 3 - Retaining walls are structural walls that resist the horizontal pressure of the soil in addition to their own weight and any surcharge.

Answer 38

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

Answer 39

Two methods of Quantifying wind velocity ## Footnote 1 - **Fastest-Mile Wind Speed** is measured using an anemometer. It is the highest sustained average of a one-mile long air sample, based on the time required for it to pass over a given point. This information can be obtained from wind velocity maps prepared by the National Oceanographic and Atmoshperic Administration. 2 - **Basic Wind Speed** is used in design calculations. This is the highest fastest-mile wind speed for a particular region. It is measured at a point ten meters (33') above grade due to wind friction at the ground level and is associated with an annual probability of 0.02.

Answer 40

A** Flexible Diaphragm** deforms more than twice the story drift. It can be constructed of either wood or steel. It distributes loads based on tributary area allocated to each diaphragm. A **Rigid Diaphragm** (rigid plane) distributes forces in proportion to the stiffness of the load resisting elements. It can be constructed of either steel, reinforced concrete or composite concrete and steel decking. Shear studs (Nelson studs) are often used to attach the composite diaphragm to a steel frame.

Answer 41

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.

Answer 42

If the lateral resistive system in a building is not symmetrical, the result of wind or seismic forces may be a twisting effect (**Torsion**). This is due to the inability of the system to resist forces from any direction equally. The torsional effect occurs when the centroid of the lateral force resistive system (center of rigidity) in the horizontal diaphragm does not coincide with the center of stiffness (center of mass) of the vertical resistive system.

Answer 43

**Counterfort retaining wall** ## Footnote 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.

Answer 44

**A - Considered an advantage** ## Footnote It is generally considered an advantage in wind design because it aids in resisting uplift, overturning moment and sliding. It also tends to reduce the occurance of vibration and flutter due to wind forces. In seismic conditions weight is considered a disadvatage as it results in larger base shear

Answer 45

Retaining walls must support hydrostatic forces equal to a fluid weighing 30 lbs. per cubic foot when the earth supported is level and is as deep as the wall is high. The equivalent fluid pressure of a soil depends on its moisture content. The wetter the soil (clay, silts) the closer the equivalent fluid pressure is to that of water (62.4 lbs. per cubic foot).

Answer 46

It is considered a special type of Live Load. Because portions of a building are subject to accumulation, they must be designed to resist the increased load. This potentially unbalanced accumulation, particularly on roofs, in valleys and at parapets, must be considered. However, these design loads may be reduced where roofs have sufficient pitch. In heavy snow areas, local building codes should be consulted.

Answer 47

During a seismic event, these dynamic forces occur in all directions. Since buildings are routinely designed for gravity loads (Vertical Forces), these forces are calculated, for design purposes, as acting horizontally parallel to the ground surface. Similar to the effect of wind loads in that both are horizontal and dynamic. Whereas seismic loads are applied as shear at the base where the building comes in contact with the ground, wind loads are applied at a higher location on the building and thus result in severe bending. Horizontal loads are typilcally the most damaging to a structure.

Answer 48

1 - Reinforced Concrete = 144-150 PCF 2 - Fresh Water = 62.4 PCF 3 - Snow = 8 PCF 4 - Dry Clay Soil = 63 PCF 5 - Sandy Gravel, dry or wet = 119 PCF 6 - Cast Aluminum = 165 PCF 7 - Steel = 490 PCF 8 - Asphalt Shingles = 1.7 - 2.8 PSF 9 - 1/2" Gypsum wall board = 2 PSF 10 - 2x4 Wood stud wall with GWB on both sides = 8 PSF (or 35-40 PCF assuming avg. weight of wood)

Answer 49

A - Angle Bracket Connection B - Strutural Tee C - Welded Connection D - Stub Bracket All the connections shown are rigid, moment-resisting connections.

Answer 50

This instrument measures ground or building acceleration. It provides data on the accleration and earth movement during earthquakes. In seismic zones 3 & 4 buildings over six stories tall and 60,000 s.f., as well as every building over ten stories are typically required to contain three of these instruments. They are located near the top, at mid-hieght, and at the bottom level of the building. Data provided by these instruments is used for research and in structural design.

Answer 51

Soft Story ## Footnote 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

Answer 52

Z = The Seismic Zone Factor ## Footnote The seismic zone factor is taken from the seismic zone map. It is used to adjust the base shear simulated at the base of a building based on the seismic history and risk of the region. It identifies the risk of earthquake for a particular region. The risk factor ranges in value from .075 in Zone 1 (the least risk) to .40 in Zone 4 (the greatest risk). These factors are then applied to the base shear equation to augment the amount of shear simulated at the base of the structure.

Answer 53

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.

Answer 54

**Epicenter** is a point on the earth's surface, directly above the origin or focus of an earthquake. **Hypocenter** is called the focus. It is the point of maximum stress or slippage, or the location of a fault break, miles below the surface of the earth, where an earthquake originates. A fault is a break or fracture in the rearth's crust along where there is slippage and a displacement of one side of the break relative to the other. This slippage can result in an earthquake.

Answer 55

**East-West Axis** ## Footnote 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.

Answer 56

T = C_T(h_n)^3/4; Type of strucrure (C_T) = 0.035; height of building (h_n) = 135' T = 0.035(135)^3/4 = 1.386 seconds T^2/3 = 1.24 seconds W = 9000 kips C = (1.25 x S)/T^2/3 = (1.25 x 1.5)/1.24 = 1.51 Base Shear = V = (ZIC/R_w)W V = ((0.15 x 1 x 1.51)/6)9000 = 339.75 kips

Answer 57

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**

Answer 58

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.

Answer 59

All buildings move or deform as a result of lateral forces. These loads are transferred through the structure to other building components. 1 - Door and window frames should be detailed or installed to allow for movement of the building structure without transferring the load to the door leaf or window glass 2 - Suspended elements such as ceilings, light fixtures, and HVAC equipment are prone to failure if they are not properly secured to resist the effects of horizontal movement (drift). 3 - Like the suspended elements, plumbing pipes and sprinkler systems are susceptible to swinging action. Not only should piping be designed to allow for thermal expansion and contraction, but it should also be installed with flexible connections or in some way be isolated from the structure to allow movement.

Answer 60

The three general solutions to consider are: ## Footnote 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.

Answer 61

**Surcharge** ## Footnote 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

Answer 62

A factor of the total dead load (MASS) of a building including some permanent live loads (storage, fixed equipment, furnishings, etc.) This is a factor of the mass of a building that will be accelerated during an earthquake.

Answer 63

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.

Answer 64

D - 1 and 2 ## Footnote Though basic wind speed is measured at 33' above the ground, wind speed increases as the height above the ground increases. In addition, some forces caused by wind, such as that of a wind flowing across a flat roof or a leeward face produce negative pressure, also called suction.

Answer 65

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.

Answer 66

The equivalent static pressure or force (P) to be applied normal (perpendicular) to the exterior surfaces of a building as determined from the following formula: P = C_eC_qq_sI where: P = design wind pressure, in psf C_e = combined height, exposure and gust factor coefficient. C_q = pressure coefficient for the structure or a portion of the structure (using _Normal Force Method_ or _Projected Area Method_) q_s = Wind stagnation pressure measured at a height of 33' in psf. I = Importance Factor Each of these values can be found in model codes. For most buildings I = 1.0, however for buildings considered essential for public health and safety, like hospitals or those with over 300 or more occupants, the importance factor is 1.15. This pressure may be positive (inward) or negative (outward or suction) on the building surface in question.

Answer 67

Moment that also involves twisting or rotation in a plane perpendicular to the axis of a member or structure. This phenomenon occurs in a building when the center of rigidity (centroid) of the lateral resistive system does not coincide with the centroid of the gravitational system (centroid of building mass). This can happen as a result of an asymmetrical wind silhouette or an asymmetrical layout of shear walls for example.

Answer 68

**A Buttressed Retaining Wall** ## Footnote 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.

Answer 69

**Diagram A** ## Footnote 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.

Answer 70

**Base Shear Formula** The formula for calculating _base shear_ in a given direction. V = Total lateral force or shear at the base (kips) Once this variable is known, it is used to determine the loads imposed on the various structural members. Z = Seismic Zone factor I = Importnace factor C = Numerical coefficient of the building's period of vibration. (T), in seconds R_w = Numerical coefficient for the lateral resistance of the structure. W = Total dead load to be moved by the seismic event.

Answer 71

The Formula for calculating the total lateral force or base shear in a given direction. ## Footnote V = Total Lateral Force or shear at the base (in kips). Once this variable is known, it is used to determine the loads imposed on the various structural members. Z = Seismic Zone Factor I = Importance Factor C = Numerical Coefficient of the building's period of vibration (T) (in seconds) R_w = Numerical Coefficient for the lateral resistance of the structure. W = Total Dead Load to be moved by the seismic event.

Answer 72

Force equation The formula for Newton's second law of motion. Force = Mass x Acceleration. A dynamic force is defined by this formula. The larger the mass (M) of the building (its dead load), the larger the resulting force (F). The larger the acceleration (a) of the ground due to shifting tectonic plates, the larger the resulting force (F).

Answer 73

**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.

Answer 74

The rate of change of velocity of a moving body. ## Footnote Velocity changes constantly during an earthquake and is measured as a percentage of the rate of change of velocity of gravity (32 ft./second²) . Ground acceleration has greater potential to cause structural damage than does displacement (drift). Acceleration is often responsible for damage to the structure whereas drift is mostly responsible for non-structural damage.

Answer 75

P = C_eC_qq_sI ## Footnote C_e = The surrounding surface environment and its degree of friction (exposure), the height of the building and the gust factor. These variables are combined into one coefficient. C_q = This factor describes the configuration of the structure (flat roof, pitched), the face of the building (windward, leeward, etc.) and the components of the building that are subjected to wind loads. q_s = Wind Speed. The wind stagnation coefficient. q_s depends directly on the square of the wind speed ( q_s = 0.00256v² ) I = The Importance Factor of the structure Essential facilities: I = 1.15 Non-essential facilities: I = 1.0

Answer 76

_Steel Moment-Resisting Frame (Rigid Frame)_ **Advanatges**: Small dead load, short construction time, flexiblity in space planning, simple unobstructed rectangular spaces, permits construction in bad weather. **Disadvantages**: Largest Drift, greater fabrication time. _Braced Frame_ **Advantages**: Stiffer than rigid frames, most efficient use of materials, pin connections are cheaper than rigid frame. **Disadvantages**: Drift occurs by elongation or shortening of the diagonals. Buckling in slender compression members may occur. _Shear Wall_ **Advantages**: Most rigid, least drift. **Disadvantages**: Heaviest, will receive larger seismic loads.

Answer 77

**B - Drag Strut** ## Footnote 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.

Answer 78

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

Answer 79

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.

Answer 80

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.

Answer 81

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.

Answer 82

Using the formula for horizontal force for parts and attachments. F_p = ZIC_pW_p F_p = 0.15 x 1.0 x 0.75 x 2500 **F_p = 281 lbs.**

Answer 83

^{Without specific information about building configuration, the following general comments apply.} _Poor_ These systems do not have continuity at the connections and between components and fall easily under seismic loads 4 - Unreinforced masonry system. 6 - Precast concrete system _Medium_ Both systems need to be detailed for ductility. 1 - Wood Frame: Absorb energy well. 2 - Reinforced Concrete walls with openings: acceptable as long as the openings are not too large. _Good_ Steel is an excellent material for absorbing seismic loads and it responds by flexing. Reinforced concrete is a material, but performs well when detailed for ductility. 3 - Steel frame with rigid infill. 5 - Reinforced concrete Ductile Moment-Resisting Frame.

Answer 84

The product of **H** x **y** is called the **Overturning Moment**. The product of **V_DL** 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. **V_DL** 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.

Answer 85

**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).

Answer 86

A dogbone connection (also called a reduced beam section) is a method used to strengthen a beam-to-column connection in a Moment-Resisting Frame. One method of strengthening beam-to-column connections is to add stiffener plates to the column or angles to the beam flanges. In a **dogbone** connection, however, the top and bottom flanges of the beam are selectively reduced to purposely weaken the beam, thus making the connection itself stronger. While giving up some beam strength, this is a very econimical method of introducing additional ductility to a Moment-Reisiting Frame in seismic regions. Failure of the beam is safer than failure of the connection.

Answer 87

**A - Moment-Resisting Frame** ## Footnote This type of structural system is economical, provides unobstructed bays and sufficient lateral resistance for buildings under 30 stories. Drift on this lateral resistive system is more appreciable than braced frames and shear wall systems.

Answer 88

I_X = Moment of Inertia about the X-axis A = Area of Section d = The distance from the axis being considered X₁ = New axis I_X1 = Moment of Inertia of the Area about X1 I_X1 = I_x + A(d²) This is called the transfer equation.

Answer 89

**B - Thermal Stress** ## Footnote This internal stress, caused by temperature changes in the material, results in dimensional changes in the material. Structural members expand/elongate with an increase in temperature and contract/shorten with a decrease in temperature.

Answer 90

Characterized by motion that varies with time, such as vibrations, as opposed to the effects of a simple static force. Dynamic loads are non-static in nature and include those caused by wind, earthquakes, hydrostatic pressure, vibrating machinery, traveling vehicles, and moving people. Dynamic load sources cause a dramatic temporary increase in load.

Answer 91

It is the measure of the property of Inertia. This is the property of an object that causes it to resist changes in its state of motion. A more common term for dealing with this expression is weight, which is defined as the force produced by the acceleration of gravity. It is the equivalent of the (W) factor in the formula for base shear. Buildings are in a condition of static equilibrium in the absence of a dynamic force such as wind and earthquakes. As soon as an earthquake strikes, this equilibrium ceases. As the ground shakes its acceleration causes the mass of the building to be displaced. The inertia needed to move the building depends on the building mass or its dead weight (W).

Answer 92

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.

Answer 93

These joints are located where there is a change in structural system, major building material, height, or where differntial movement from either moisture or temperature is expected. The width of the expansion joint is calculated as the sum of the anticipated movements of each component piece being joined.

Answer 94

An analysis of seismic loads using a computer program that utlizes actual or simulated earthquake data obtained from accelerographs implanted in existing buildings. This method is much more detailed than the static method, which treats seismic loads as equal to constant lateral load acting at the base level of the structure. This method is required by building codes in seismic zones 3 & 4 and for every building over ten stories, or having a slenderness ratio \> 5.

Answer 95

This is a category of structures defined in model codes as having special requirements for building construction. These structures include atria, covered mall buildings, pedestrian walkways, aviation control towers, etc. This category also includes special provisions for other occupancy groups.

Answer 96

Defined in model codes as a Group H occupancy. These facilities include those that involve the manufacturing, production or storage of materials that pose a physical, or health hazard in excess of defined units.

Answer 97

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.

Answer 98

**Diagram D** ## Footnote 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.

Answer 99

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(F_x x h_x)

Answer 100

Typically, it is desirable to tie an entire building structure together so it moves as a single unit when subjected to a lateral force. However, due to discontinuities in building mass, period, etc., different parts of a building tend to move independently and in proportion to the rigidity of the lateral reisting system. This type of connection allows these building parts some freedom of movement without resulting damage.

Answer 101

1 - **Rigid Diaphragm**: Cast-in-place reinforced concrete slabs. Precast Tees connected together to resist Shear Steel decking with concrete topping. Plywood/OSB with wood framing, nails and steel connectors. 2 - **Flexible Diaphragm**: Steel deck (wihtout concrete topping; depends on guage, depth of deck, and the pitch of the corrugations). Requires vertical shear supports at both supported sides and in both directions.

Answer 102

Earthquakes are dynamic and vary in intensity, length and acceleration (period). A building's dynamics can be altered by construction practices as well as their uses. Seismic design, therefore, focuses on engineering that will prevent loss of life and injuries. Seismically designed structures should survive minor earthquakes with minmal damage.

Answer 103

The seperation of a building's foundation from direct, rigid contact with the ground. Base isolators are typically made of special rubber or rubber and steel laminates. They are intended to absorb the seismic acceleration of the ground. Employing this technique allows the earth and building to move independently during an earthquake. It increases the building's period of vibration which reduces the impact of lateral forces. Base isolators in a building are similar to shock absorbers in a car. They are used when the contents of the structure, such as computer equipment, health care equipment, etc. are susceptible to damage due to excessive movement.

Answer 104

This factor accounts for the type of lateral resistive system when determinging base shear for a building. Stiff structural systems such as shear walls have a low value, while a flexible or ductile Moment-Resisting Frame structure has a higher value.

Answer 105

Determine the basic scheme. Determine the loads. Determine load propagation. This involves following a load through the elements of a structure. Design of elements. Design for interaction between the elements. Design documentation.

Answer 106

**B - A Rooftop water storage tank for the building** ## Footnote Water storage tanks are heavy when full (water weighs 62.4 PCF). The weight of the component is the single most important factor in determining the amount of acceleration the component will undergo. The fact that the water storage tank is on the roof causes it to act like a pendulum when subjected to seismic force. This amplifies the seismic effects on the building.

Answer 107

The modulus of elasticity is used to measure this characterisitc of a mterial and the moment of inertia is used to measure this characteristic of a cross section. The product of E x I is called the stiffness (of a certain section that is made of a certain material). A flexible structure is one that lacks stiffness. It is the ability of a material to resist deformation, as opposed to the strength of a material which is its ability to resist bending forces without failure.

Answer 108

A closed, flat building surface will allow the wind to pass over it in a fluid motion. However, openings or recesses for doors and windows tend to capture the wind and can result in a significant increase in wind force. According to model codes, all doors and windows are considered to be openings unless they are specially detailed to resist the additional loads. Also, individual building components, such as parapets, that extend beyond the main body of the building are often subject to increased wind pressure and may be required by code to withstand the additional force.

Answer 109

**A - Fine Sand** ## Footnote The amount of horizontal pressure for granular soils depends on the soil's _angle of repose_. In soil with a steep angle of repose, more of the pressure from weight of the soil is vertical, and thus exerts less horizontal pressure on the retaining wall. In soil with a shallow angle of repose, the pressure from the weight of the soil is more horizontal, and thus exerts more horiz. pressure on the retaining wall. A - Fine Sane: Angle of Repose = 35^o (most horiz. pressure) B - Peebles: Angle of repose = 45^o (most. vert. pressure) C - Coarse Sand: Anbg;le of repose = 40^o D - Clay: Clay is a cohesive soil which in which particles stick together, and thus the angle of repose does not apply.

Answer 110

A **simply supported beam** is supported at its ends only and is statically determinate. It has a larger moment and therefore deflects more than a continuous beam with the same clear span and loading. A **continuous beam** has three or more supports and is statically indeterminate. It is subject to both positive and negative moments along the the length of the beam. The negative moment reduces the amount of positive moment and thus a continuous beam is subjected to less bending moment and will deflect less than simply supported beam with the same span and load.

Answer 111

_Structural Components_ 1 - Planar Loading: Roof and floor diaphragms, shear walls. 2 - Linear Loading: Joists, beams, girders 3 - Point Loading: Columns 4 - Mixed loading: Foundations _Non-Structural Components_ 1 - Architectural, Exterior: Cladding, parapets, elevator penthouse, canopies, chimneys, and stacks. 2 - Architectural, Interior: Partition walls, suspended ceilings, access floors, egress and ingress routes, non-structural stair enclosures. 3 - Building Systems and Equipment: HVAC Equipment, plumbing distribution network systems, electrical/lighting components, communication systems, conveying systems including elevator/counterweight/guardrail 4 - Building Contents: Furniture, office equipment, shelving, etc.

Answer 112

**Formula for horizontal force for parts and attachments.** Seismic Codes require that parts of structures and attachments, such as precast concrete panels, must be designed for lateral loads. For each such attachment a horizontal force must be considered. F_p = ZIC_pW_p F_p = Horiz. Force for part Z = Seismic Zone Factor I = Importance Factor C_p = Horizontal force factor. Coefficient from table in the code and is either 0.75 or 2.0 W_p = Weight of the part being checked

Answer 113

1 - Due to relative lack of stiffness compared to braced frames or shear wall systems, rigid (moment-resisting) frames experience the most drift. Additional, or larger sized columns may be required to reduce drift. 2 - Deformation causes complex dynamic behavior of the structure and its members. 3 - Stronger joint connections may add to construction cost, construction time, and may increase resonance problems.

Answer 114

The ** Center of Mass (CM)** of a structure is the centroid of all of its structural and non-structural components. The **Center of Rigidity (CR)** of a structure is the centroid of those mebers that participate in the lateral force-resisting system. Unless the CM and CR of a structure coincide, the structuire will experience undesirable torsion (twist in plan) that may cause it to collapse. The placement of the lateral force-resisting members on the perimeter of a structure will give it more stability. The symmetrical placement of these elements will reduce any possible torsional effects. The distance between the CM and the CR is called the eccentricity. The smaller the eccentricity, the smaller the damage due to torsion.

Answer 115

Ratio used as practical or code-specified limits for a structural member's unbraced length-to-thickness (h/t) for colums and span-to-depth (L/d) for beams.

Answer 116

1 - Type fo Soil: Angular, dense and well-graded sand and gravel will develop substantial friction, whereas clay (slippery) will not. The presence of moisture will reduce friction. 2 - Contact Surface: Smooth objects have great potential for slippage. 3 - Pressure Distribution: An increase in surface pressure distribution will incerase friction and reduce slippage.

Answer 117

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.

Answer 118

Nearly every other building system must be designed to resist seismic forces. For example, building cladding, mechanical and electrical systems including ducts, lighting fixtures, and hung ceilings. In addition, plumbing systems, (sprinklers and stand pipes) as well as circulation systems (elevators) warrant special consideration.

Answer 119

A component of a structural system, such as a tie or drag strut, that is used to absorb lateral forces and transfer them from one part of a structure to vertical members in the system. These members are often part of the vertical load resistive system and act as beams.

Answer 120

Only for the prevention of base sliding (base shear) or overturning due to wind or seismic forces is a heavy building (dead load) a benefit. Structural damage to a heavy building may actually be exaggerated when subjected to seismic force. The greater the building's mass, the greater the force acting upon it. A light frame or flexible structure may better withstand some effects of a lateral forcce but may not be able to resist base shear.