earthquake forces in buildings

8/13/2019 Earthquake Forces in Buildings

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Briefing Paper 1, Part A

ATC/SEAOC Joint Venture Training Curriculum

B u i l t t o R e s i s t E a r t h q u a k e sT h e P a t h t o Q u a l i t y S e i s m i c D e s i g n a n d C o n s t r u c t i o n

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There are more than 160 knownactive faults located in California.

This Briefing Paper 1, Building Safety and

Earthquakes, consists of four parts describing

earthquakes and their effects on buildings.

Part A provides an overview of how earthquakes

occur and the ground shaking motion they

produce. It also explains why different indi-

vidual buildings respond differently to the same

ground motion. Parts B to D build on that

information to explain how earthquake motion

creates forces acting on a building, to describe

the structural systems used

to resist earthquakes, and

to define the “load paths”

of earthquake forces

within buildings.

Severely damaging earthquakes have repeatedly

demonstrated the importance of improving the

quality of both earthquake design and construc-

tion. The objective of Briefing Paper 1 (Parts A

to D) is to inform the stakeholders and partici-pants in the design and construction process,

including building inspectors and owners, about

the basic principles of earthquake-resistant

building design.

Most earthquakes are caused by rock movement

along rupturing faults located in the earth’s

crust. On a global scale, the earth’s crust is

divided into separatesections known as plates,

as shown in Figure 1.

Major faults are typically

located at plate bound-

aries. In California, many lesser faults occur

near the boundary of the Pacific and the North

American plates, which, in California, is

defined by the San Andreas fault. However,

° ° ° ° ° ° °

°

°

°

°

°

Figure 1. Global plates and plate boundaries.

60

40

0

60

40

180 120 60 0 60 120 180





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other parts of California also contain faults. In

fact, there are more than 160 known active

faults located in this state. New faults continue

to be discovered, usually when an unexpected

earthquake occurs. Essentially, earthquakes can

affect any location within California, potentially

causing significant damage and loss of life.

Faults move or “slip” when shear stresses deep

underground exceed the ability of the com-

pressed faulted rock to resist those stresses.

Fault slip can move the

nearest ground surface

vertically, laterally, or in

some combination. When

this slip occurs suddenly, it

causes seismic shock waves

to travel through the ground,similar to the effect seen

when tossing a pebble onto the surface of still

water. These seismic waves cause the ground

shaking that is felt during an earthquake.

Ground motion contains a mix of seismic waves

having two primary characteristics as shown in

Figure 2. One is the wave amplitude, which is a

measure of the size of the wave. The other is its

period, which is a measurement of the time

interval between the arrival of successive peaks

or valleys, known as one cycle. This concept of a time measurement can also be expressed as

frequency = 1/period, the number of cycles

occurring per second.

Everything in the path of a seismic wave will be

shaken. However, the amount of ground motion

at any given location depends on three primary

factors. One factor is the distance between the

site and the source location of the earthquake,

known as the focus or hypocenter, which in

California may range from 2 to 15 miles under-ground. The shallower the focus, the stronger

the waves will be when they reach the surface.

As a general rule, the intensity (severity) of

ground shaking diminishes with increasing

distance from the source.

Buildings located less than

15 kilometers (9.3 miles)

from certain types of faults

are required by the 1997

Uniform Building Code

(UBC) to be designed to

withstand the stronger

shaking expected in these near-source zones.

Maps produced by the California Division of

Mines and Geology and available from the

International Conference of Building Officials

(ICBO) indicate where these faults are located.

The second factor is the total energy released

from the earthquake, measured by its magni-

tude. Because the magnitude scale is logarith-

mic, a magnitude 7.0 earthquake releases 31.5

times more energy than does a magnitude 6.0earthquake. The ground shaking intensity at a

given location is greater for the magnitude 7.0

earthquake, but not 31.5 times greater. Instead,

the larger energy release produces shaking that

A magnitude 7.0 earthquakereleases 31.5 times moreenergy than does a magnitude6.0 earthquake.

Figure 2. Cyclic wave of constant amplitude and period.

Amplitude

Time

Period(one cycle)





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is felt over larger distances because the ruptured

fault length is greater. Also, the shaking from a

larger-magnitude earthquake often lasts longer,

because more time is needed for the longer

rupture to release the greater energy.

The last of the three primary factors is the

nature of the soil or rock at the site. Generally,

sites with deep soft soils or loosely compacted

fill will be more strongly shaken than sites with

stiff soils, soft rock, or hard rock. For example,

during the 1989 Loma Prieta earthquake, the

shaking experienced in the San FranciscoMarina District, which is underlain by mud

nearly 100 feet thick, was from three to four

times stronger than the shaking measured only a

few blocks away on bedrock, near the Golden

Gate Bridge. The building codes for new

construction (e.g., the 1997 UBC) and the

NEHRP Guidelines for the Seismic Rehabilita-

tion of Buildings (FEMA-273 report) use

adjustment factors to account for the stronger

shaking at soft soil sites and fill sites.

To summarize: the intensity of ground motionat a specific site, caused by a specific earth-

quake, depends primarily on three factors: the

distance between the source (also known as

focus or hypocenter) and the site, the magnitude

of the earthquake (amount of energy released),

and the type of soil or rock at the site. These

factors are illustrated in Figure 3, which also

shows the location of the epicenter (point on

ground surface directly above the hypocenter).

More complex factors, such as the type of

faulting action, the direction of propagation of

the fault rupture, and the frequency range of the

waves, can increase or decrease the severity

(intensity) of the local shaking. Consequently,

actual ground motion cannot be precisely

predicted. However, based on the recordedmotions of past earthquakes obtained from

instruments located both inside and outside

buildings, it is possible to estimate the probable

maximum ground motion given the values for

the three factors. These estimates form the basis

for seismic design requirements contained in

modern building codes.

Different individual buildings shaken by the

same earthquake respond differently. The

effects of earthquake ground shaking depend on

the specific response characteristics of the type

of structural system used. One important

Figure 3. Common terms and factors affecting shaking intensity at a given site.

Distance from

epicenter

Fault

Focus or hypocenter

Earthquakemagnitude

Soi l atthe site

D i s

t a n c

e f r o

m f o c u s





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building characteristic is the fundamental period

of vibration of the building (measured in

seconds). The fundamental period of a building

depends in a complex way on the stiffness of

the structural system, its mass, and its total

height. Seismic waves with periods similar to

that of the building will cause resonance, and

amplify the intensity of earthquake forces thebuilding must resist.

Structural systems using concrete or masonry

shear walls are stiff and result in buildings with

short periods, whereas more flexible moment-

frame systems have longer periods. In general,

a large portion of the earthquake energy is

contained in short-period waves. Therefore,

short-period buildings with stiff structural

systems are designed for larger forces than long-

period, flexible, buildings. This concept is also

applicable to the amount of force individual

structural seismic elements and their compo-

nents must resist. Stiff elements must be made

stronger because they will attempt to resist

larger earthquake forces than flexible elements

in the same structural system.

Shape or configuration is another important

characteristic that affects building response.

Earthquake shaking of a simple rectangular

building results in a fairly uniform distribution

of the forces throughout the building. In a morecomplex T- or L-shaped building, forces

concentrate at the inside corners created by

those shapes. Similar problems arise when a

building has floor or roof levels of adjacent

portions offset vertically (split levels), or when

Figure 4. Examples of buildings with irregular configurations.

Split Levels Cruciform Plan L-Shaped PlanTall, Soft, or Weak

First Story

the first story is taller or “softer” than the other

stories. Irregularly shaped buildings, shown in

Figure 4, are subject to special design rules

because otherwise they can suffer greater

damage than regularly shaped buildings.

ATC, 1997, NEHRP Guidelines for the Seismic

Rehabilitation of Buildings, prepared by the

Applied Technology Council for the Building

Seismic Safety Council, published by the

Federal Emergency Management Agency,

FEMA 273 Report, Washington, DC.

ICBO, 1997, Uniform Building Code, Interna-

tional Conference of Building Officials, Whit-

tier, California.

Briefing papers in this series are concise, easy-to-read summary

overviews of important issues and topics that facilitate the

improvement of earthquake-resistant building design and construc-

tion quality.

This briefing paper was prepared by the ATC/SEAOC Joint Venture,

a partnership of the Applied Technology Council (ATC) and the

Structural Engineers Association of California (SEAOC). Funding

for the series was provided by the California Seismic Safety

Commission, Proposition 122 Retrofit Practices Improvement

Program.

Copies of Briefing Papers can be downloaded from ATC’s World

Wide Web site (www.atcouncil.org), or are available from:

ATC/SEAOC Joint Venture

c/o Applied Technology Council

555 Twin Dolphin Drive, Suite 550

Redwood City, California 94065



Briefing Paper 1, Part B


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This Briefing Paper 1, Building Safety and Earthquakes, consists of four parts describingearthquakes and their effects on buildings.Part A describes the causes of earthquakes andresulting ground motions. This Part B describeshow earthquake groundmotions create variousforces acting on a buildingand explains how thoseforces result in building drift.Parts C and D discussstructural systems that resistearthquakes and the “loadpath” of earthquake forceswithin buildings.

Designing buildings to resist earthquakes re-quires that ground motions be translated intoforces acting upon a building. Earthquakeforces are called lateral forces because theirpredominant effect is to apply horizontal loads toa building. Although earthquake waves doimpart a vertical component of force to buildings,

the weight of the building normally providessufficient resistance. Therefore, vertical earth-quake forces are usually only accounted for inspecial cases.

The general method for determining the totallateral earthquake force to be applied to abuilding is based on a simple equation, F = ma.It relates the force (F ) to the mass (m) of thebuilding and to the horizontal acceleration (a),imparted from the ground shaking. The buildingmass consists of the sum of the weights of all itsstructural and nonstructural components. The

acceleration is expressed as a fraction of theacceleration due to gravity, commonly called“g.”

We all experience the force associated with1.0 g of constant vertical acceleration everyday

due to the earth’s gravity. The presence of this

force, and hence the gravitational acceleration, isnoticeable when we perform work that requireslifting, such as climbing stairs or moving objectsfrom a lower to a higher level. The physicalexertion involved in these activities is in responseto the force that must be overcome, which we

can call the resisting force.The resisting force is equalto the mass of the objecttimes the acceleration due togravity. Because gravity isconstant, one must over-come a greater resistingforce to move heavierobjects. This concept of

objects resisting movement, actually in anydirection, derives from the object’s “inertia.”

We all experience inertia in response to thehorizontal acceleration that occurs while travel-ing in a vehicle. When quickly accelerating fromrest, the inertia of a person’s body resists movingforward, and there is pressure from the seatback. During braking, the resisting force can befelt as the pressure from a seat belt. Figure 1

Earthquake forces are called

lateral forces because theirpredominant effect is to applyhorizontal loads to a building.

1. Acceleration of truck or ground

1. Deceleration (Braking)

2. Inertial force

2. Inertial force on person or building

Figure 1. Inertial forces are a reaction to

acceleration.





2

illustrates this familiar experience and relates itto the effect of building inertia during an earth-quake.

Regularly shaped buildings and many irregularbuildings have their structural elements oriented

in two perpendicular directions. These directionsare known as the primary or-thogonal axes. When designingfor earthquakes, lateral forces areassumed to be split into theirorthogonal components, and to actparallel to each of these direc-tions. This assumption is madebecause there is no specific orientation of theseismic waves that will pass through a buildingsite. Furthermore, once a building begins toshake, the internal forces that are generated willbe transmitted in components aligned along the

primary orthogonal axes.

A force acting along one axis of a buildingcauses resisting forces in walls or frames parallelto that direction. These are called in-planeforces, because the direction of force is parallelto the plane of the wall or frame. Forces thatare perpendicular to this direction are known asout-of-plane forces. Earthquakes produce bothin-plane and out-of-plane forces, as shown inFigure 2.

For example, in-plane forces acting on a ply-wood-sheathed wood-frame wall are resisted by

the nails attaching the sheathing to the top plate,studs, and sill, and by the anchor bolts attachingthe wall sill plate to the foundation. The out-of-plane forces acting on this same wall are resistedby nailed connections between the top plate of the wall and the adjacent floor or roof framingand at the bottom by anchor bolts in the sill plate.Most building seismic elements are not designed

to resist forces from both these directionssimultaneously. However, theforces in each direction must beseparately accounted for, andstructural elements must beprovided with adequate capacityto resist forces acting in both in-plane and out-of-plane directions.

The resisting force or inertia depends on themass of an object, and the design of buildingsmust take this into account. Just as columns atthe lower-story levels of a building must supportthe combined weight of the levels of the struc-ture above them, lateral resisting forces alsoaccumulate at lower levels. At the roof level,the inertia is based on the weight of the roof andthe weight of one-half of the story height of thewalls immediately below the roof. At the floorimmediately below the roof, the inertia is basedon the tributary weight of the floor and the wallshalfway above and halfway below that floor.Figure 3 illustrates how the inertia at each levelis calculated. The total resisting force increases

at progressively lower levels, culminating at the

Earthquakes produceboth in-plane and

out-of-plane forces.

Figure 2. In-plane and out-of-plane forces on a shear wall.

In-Plane Forces Parallel to Wall Out-of -Plane ForcesPerpendicular to Wall





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foundation level. The resisting force at thefoundation level is based on the sum of thecontributing forces from each level and is knownas the base shear.

In the design of a multi-story building, a portionof the total base shear is applied, as a horizontalforce, at each floor level and at the roof. Thedesign lateral force applied at each level is basedon both the tributary mass at that level and theheight of that level above the base of the build-

ing. The result is a seismic force distributionover the height of a building that is larger at thetop than at the bottom and is described as atriangular shape. This distribution is a simplifiedapproximation that generally matches the actualdistribution of forces during earthquakes. Thedistribution is derived from calculations usingrecordings from strong motion instrumentsinstalled at various levels within buildings. Whendesigning tall structures or very flexible struc-tures with long natural periods, an extra lateralload is added at the roof level to account foradditional stresses, from the “whiplash” effectthat occurs at the roof level of a building.

A horizontal force applied to an object tends topush it sideways. If it is unrestrained at its base,it slides in the direction of the applied force.With buildings, sliding is counteracted by thefrictional sliding resistance between the bottomof the foundation and the soil and by the lateralbearing resistance of the soil against the verticalfaces of the foundation and piles. Lateral forcesacting above the foundation push the superstruc-ture sideways until the resistance of the structure

reaches an equilibrium with that force. Theamount of horizontal displacement that occurs iscalled drift. Drift causes stress in structuralseismic elements and nonstructural elementsbecause it forces them into deformed shapes.Maximum drift usually occurs at the top of abuilding, but each story level is subjected to acertain amount of story drift as shown inFigure 4.

Maximum drift limits and individual story driftlimits are specified in building codes to controlthe horizontal displacement a building experi-

ences during an earthquake. Because drift andassociated accelerations increase toward the topof a building, the 1997 UBC requires roof-mounted equipment to resist forces four timeslarger than equipment located on the ground

Assumed directionof forces

14'

7’

16'

This 7-ftportion(14’/2)

contributesto inertia atroof level

8’

This 15-ftportion(14’/2+16’/2)contributesto inertia atfloor level

This 7-foot portioncontributes toinertia at roof level

This 15-foot portioncontributes toinertia at floor level

Figure 3. Calculating tributary forces at roof and second-floor levels.





4

Briefing papers in this series are concise, easy-to-read

summary overviews of important issues and topics that

facilitate the improvement of earthquake-resistant building

design and construction quality.

This briefing paper was prepared by the ATC/SEAOC Joint

Venture, a partnership of the Applied Technology Council

(ATC) and the Structural Engineers Association of California

(SEAOC). Funding for the series was provided by the California

Seismic Safety Commission, Proposition 122 Retrofit Practices

Improvement Program.

Copies of Briefing Papers can be downloaded from ATC’s

World Wide Web site (www.atcouncil.org), or are available

from:





Buildings must haveadequate separationto avoid the damaging

effects of poundingduring earthquakes.

floor. The FEMA 273 N EHRP Guidelines for the Seismic Rehabilitation of Buildings havesimilar requirements. Building drift is also animportant consideration when determining howclosely two buildings can be spaced. Buildingsmust have adequate separation to avoid thedamaging effects of pounding during earth-quakes.

Drift considerations are particularly importantfor columns and for connections of heavyprecast cladding components whose failurecould lead to injuries or loss of life. All struc-tural seismic elements and their connectionsmust be designed to accommodate the expecteddrift, regardless of their role in resisting lateralforces. The collapse of a newly constructedparking garage at Cal State Northridge duringthe 1994 Northridge earthquake can be partlyattributed to insufficient capacity inits interior concrete columns toaccommodate the story drift.

Adherence to drift limits can alsoreduce economic losses, especiallywith respect to nonstructural compo-nents.

Some older buildings with drift-sensitive brittle finishes have been retrofittedusing a technique known as seismic isolation.Seismic isolation typically uses viscous bearingsor sliding friction bearings to support and isolatethe building from horizontal earthquake groundmotion similar to the way a car’s suspension

isolates it from vertical bumps in the road.Seismic isolation can reduce both seismic forcesand drift-induced damage.

ATC, 1997, NEHRP Guidelines for the Seismic

Rehabilitation of Buildings, prepared by the

Applied Technology Council for the Building

Seismic Safety Council, published by the Federal

Emergency Management Agency, FEMA 273

Report, Washington, DC.


tional Conference of Building Officials, Whittier,

California.

Inertia Force

Inertia Force

NormalPosition

Story Drift Total Drift

GroundAcceleration

Floor

Figure 4. Drift in a building subjected to lateral earthquake forces.



Briefing Paper 1, Part C


1


This Briefing Paper 1, Building Safety and Earthquakes, consists of four parts describingearthquakes and their effects on buildings. PartsA and B describe the causes of earthquakes andresulting ground motions and explain howearthquake motions create various forces actingon a building. This Part C describes the types of structural systems and lateral-force-resistingelements used in buildings and how they can beused in combinations. Part D discusses the“load path” of earthquake forces within build-ings.

The Uniform Building Code (UBC) earth-quake provisions define three basic types of building structural systems: bearing wall sys-tems, building frame systems, and moment-resisting frame systems.

Bearing wall systems consist of vertical load-carrying walls located along exterior wall linesand at interior locations as necessary. Many of these bearing walls are also used to resist lateralforces and are then called shear walls. Bearingwall systems do not contain complete vertical-load-carrying space frames but may use somecolumns to support floor and roof vertical loads.This type of system is very common and in-cludes wood-frame buildings, concrete tilt-upbuildings and masonry wall buildings.

Building frame systems use a complete three-dimensional space frame to support verticalloads, but use either shear walls or bracedframes to resist lateral forces. Examples of these include buildings with steel frames orconcrete frames along the perimeter and at

intervals throughout the interior supportingvertical loads from floors and roof. Buildingframe systems typically use steel braced framesor concrete or masonry shear walls to resist

lateral forces. A building frame system withshear walls is shown in Figure 1(a).

Moment-resisting frame systems can be steel,concrete, or masonry construction. They providea complete space frame throughout the buildingto carry vertical loads, and they use some of those same frame elements to resist lateralforces. Shear walls (and braced frames) are notused in this system, as shown in Figure 1(b).

Occasionally buildings are defined as dual

systems when they have a complete spaceframe that supports vertical loads and combinemoment-resisting frames with either shear wallsor braced frames to resist lateral loads.

Lateral-force-resisting elements must be pro-vided in every structure to brace it against windand seismic forces. The three principal types of resisting elements are shear walls, bracedframes, and moment-resisting frames. Shearwalls can be made of sheathed wood-frame

walls, reinforced masonry, or reinforced con-crete. Steel braced frames are often used incombination with concrete shear walls ormasonry shear walls. Braced frames areessentially vertical, cantilevered trusses and may

Figure 1. Building frame systems.

Building Frame Systemwith Shear Walls

(a)

Moment ResistingFrame System

(b)



Briefing Paper 1, Part C

ATC/SEAOC Joint Venture Training Curriculum2

be either concentric or eccentric in configuration.Concentric frames have diagonal braces locatedso that the lateral forces act along the directionof their longitudinal axis. Eccentric bracedframes use both axial loading of braces andbending of sections of horizontal beams to resistthe forces. Figure 2 shows typical braced frameconfigurations.

Moment-resisting frames can be constructed of steel, concrete, or masonry. Moment framesconsist of beams and columns in which bendingof these members provides the resistance tolateral forces. There are two primary types of moment frames, ordinary and special. Specialmoment-resisting frames are detailed to ensureductile behavior of the beam-to-column joints andare normally used in zones of higher seismicity.

Because of damage observed following the 1994Northridge earthquake, steel moment-resistingframes have been under intensive study and

testing. The goal is to determine the causes of the damage and to recommend changes in steelmoment-resisting frame design and constructionto ensure ductile behavior of the joints. Addi-tional information on that subject is availablefrom the SAC Joint Venture. Contact theApplied Technology Council (see box).

The selection of the type of lateral-force-resisting elements to use in a building is oftenbased on economics. A single type of resistingelement is commonly used in most building types,

such as in houses where wood-framed shearwalls are used, or in concrete tilt-up buildingswhere concrete shear walls are used. However,other types of buildings may need to use combi-nations of more than one type of seismicelement.

The building code allows combinations to beused but they are also subject to very specific

structural design rules. For example, if concreteshear walls that are also bearing walls arecombined with braced frame elements along oneaxis and ordinary moment-resisting frames areused along the other axis, the braced frameelements need to be designed using slightlylarger forces than if they were the only type of resisting element used along that axis. On theother axis, the moment-frame elements alsoneed to be designed for forces larger than if they were the only type of resisting element inthe building. These adjustments in design forcesare required to account for the differences instrength, stiffness, and ductility among the threetypes of resisting elements when used in combi-nation.


tional Conference of Building Officials, Whittier,

California.












World Wide Web site (www.atcouncil.org), or are available

from:





Figure 2. Types of braced frame elements.

X-Brace Chevron Brace

K-Brace Eccentric Brace



Briefing Paper 1, Part D


1


This Briefing Paper 1, Building Safety and Earthquakes, consists of four parts describingearthquakes and their effects on buildings.Parts A and B describe the causes of earth-quakes and resulting ground motions and explainhow earthquake motions create various forcesacting on a building. Part C describes typicalstructural systems and lateral-force-resistingelements used in buildings. This Part D definesthe seismic load path elements, describes theirfunctions and the necessary interconnectionsbetween them to resist earthquake forces. Also

included in this part of Briefing Paper 1 areresources for additional reading on buildingsafety and earthquakes.

Within every building, there are multiple ele-ments that are used to transmit and resist lateralforces. These transmitting and resisting ele-ments define the building’s lateral-load path.This path extends from the uppermost roof orparapet, through each element and connection,to the foundation. Load-path elements vary in

scale from massive multi-story moment-resistingframes to individual nails connecting woodmembers. An appreciation of the criticalimportance of a complete load path is essentialfor everyone involved in the design, construction,and inspection of buildings that must resistearthquakes.

There are two orientations of primary elementsin the load path: those that are vertical, such asshear walls, braced frames, and moment frames,and those that are essentially horizontal, such asthe roof, floors, and foundation. The roof and

floor elements are known as diaphragms.Diaphragms serve primarily as force-transmit-ting or force-distributing elements that takehorizontal forces from the stories at and abovetheir level and deliver them to walls or frames inthe story immediately below. Diaphragms are

classified as either flexible or rigid, and the

method of distributing earthquake forces fromthe diaphragm to the resisting elements dependson that classification. Wood-framed diaphragmscan be considered either flexible or rigid andconcrete diaphragms are considered rigid.

Shear walls and frames are primarily lateral-force-resisting elements but can also performforce-transmitting functions. For example andwhile not necessarily desirable, an upper-storyinterior shear wall may not continue to the baseof the building and therefore must transmit itsforces to a floor diaphragm. Also, at the base of

a frame or a shear wall, forces are transmittedinto a foundation element. The primary struc-tural elements that participate in the earthquakeload path are shown in Figure 1.

Foundations form the final link in the load path bycollecting the base shear and transmitting it tothe ground. Foundations resist lateral forcesthrough a combination of frictional resistancealong their lower surface and lateral bearingagainst the depth of soil in which they are embed-ded. Foundations must also support additionalvertical loads caused by the overturning forces

from shear walls and frame columns.

FloorDiaphragm

BracedFrame Bay

Foundations

Shear

Wall

Roof Diaphragm

Figure 1. Primary structural load path

elements.





2

Within the primary load-path elements, there areindividual secondary elements needed to resistspecific forces or to provide specific pathwaysalong which lateral forces are transmitted.Particular attention must be given to transmittingforces between horizontal seismic elements(diaphragms) and vertical seismic elements. Two

important secondary elements are chords andcollectors. A chord is a structural member alongthe boundary of a diaphragm that resists tensionand compression forces. A collector is a struc-tural member that transmits diaphragm forces intoshear walls or frames. Figure 2 depicts theoverall function of chords and collectors.

In the case of floors and roofs, the perimeteredges or boundaries are critical locations be-cause they form the interface between thediaphragms and the perimeter walls. Theperimeter is typically the location for vertical

seismic elements, although many buildings alsohave shear walls or frames at interior locations.An interior line of resistance also creates adiaphragm boundary. Boundary elements indiaphragms usually serve as both chords andcollectors, depending on the axis along whichlateral loads are considered to be applied.

As shown in Figure 2, the forces acting perpen-dicular to the boundary elements tend to bend thediaphragm, and the chord member must resist theassociated tension and compression. Similar to auniformly loaded beam, a diaphragm experiences

the greatest bending stress and largest deflectionat or near the center of its span between verticalresisting seismic elements. The chord on the sideof the diaphragm along which the forces arebeing applied is in compression, and the chord onthe opposite side is in tension. These tension andcompression forces reverse when the earth-quake forces reverse. Therefore, each chordmust be designed for both tension and compres-sion.

Walls that structurally support diaphragm edgesmust also resist out-of-plane forces caused by

diaphragm bending. In wood-frame walls, thedouble top plates usually act as chords for thediaphragm at that level. In concrete and masonrywalls, reinforcing steel is placed at the diaphragmlevel to resist the out-of-plane bending in the wall.

Collectors are needed when an individual shearwall or frame in the story immediately below thediaphragm is not continuous along the diaphragmboundary (See Figure 3). This is a very commonsituation because shear walls are often inter-rupted by openings for windows and doors, andbecause resisting frames are normally located inonly a few of the frame bays along a diaphragm

boundary. A path must be provided to collect thelateral forces from portions of a diaphragmlocated between vertical resisting seismicelements and to deliver those forces to eachindividual shear wall or frame. The collectormember provides that path. Collectors arecommonly called drag struts or ties. Collectorsare also needed when an interior shear wall orframe is provided (see Figure 3). In this case,the collector is placed in the diaphragm, alignedwith the wall or frame, and extends to thediaphragm edges beyond each end of the wall orframe. Collectors can occur in wood-framed

walls using headers and top plates to transfer theforces and can occur in spandrel beams, of concrete or masonry construction, that link sections of shear walls together.

The following statements contained in the 1997UBC clearly require that a complete load path beprovided throughout a building to resist lateralforces.

“All parts of a structure shall be intercon-nected and connections shall be capable of transmitting the seismic force induced bythe parts being connected.”

“Any system or method of constructionshall be based on a rational analysis... Such

Figure 2. Function of diaphragm chords and

collectors.

Boundary Chord in Compression

Boundary CollectorTransferring Shear

Lateral Force

Boundary Chord in Tension





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analysis shall result in a system that pro-

vides a complete load path capable of transferring all loads and forces from theirpoint of origin to the load-resisting ele-ments.”

To fulfill these requirements, connections mustbe provided between every element in the loadpath. When a building is shaken by an earth-quake, every connection in the lateral-force loadpath is tested. If one or moreconnections fail because theywere not properly designed orconstructed, those remaining in

parallel paths receive additionalforce, which may cause them tobecome overstressed and to fail.If this progression of individualconnection failures continues, itcan result in the failure of acomplete resisting seismicelement and, potentially, theentire lateral-force-resisting system. Conse-quently, connections are essential for providingadequate resistance to earthquakes and must begiven special attention by both designers andinspectors.

Connections are details of construction thatperform the work of force transfer between theindividual primary and secondary structuralelements discussed above. They include a vast

array of materials, products, and methods of

construction. For example, forces are resisted inwood-framed diaphragms by the action of nailsor other fasteners used to attach structuralsheathing to the joists, trusses, beams, ledgers,and blocking that make up the diaphragmframing. Nails, bolts, and prefabricated metalconnectors are used for diaphragm chord andcollector splices of wood members. In steelconstruction, metal deck diaphragms use welds

to resist diaphragm forces and chordand collector beams are connected bybolts, welds, or a combination of both.In concrete construction, diaphragmreinforcing steel resists forces in thediaphragm and chord tension stresses,and reinforcing dowels are generallyused to transfer forces from thediaphragm boundaries to concretewalls or frames.

Connection capacity is determined byperforming a detailed analysis of the individualforces the connection must transfer. Thecapacity actually provided, however, is highlydependent on the implementation of the specificdetails of its construction. Therefore, two

specimens of the same connection can havesignificantly different capacities, even when thedifferences in construction are imperceptible.Some common examples can illustrate this point.The capacity of a 3/16” fillet weld is 25 percent

When a building is

shaken by anearthquake, every

connection in the

lateral-force load

path is tested.

Figure 3. Use of collector element at interior shear wall.

InteriorShearWall

Lateral Force

Interior

ShearWall

DiaphragmBoundaryCollector

Isometric View Plan View



Briefing Paper 1, Part D4

less than that of an equal length of 1/4” filletweld; the capacity of an 18-gauge strap to resisttensile forces is 36 percent less than for a 14-gauge strap of equal width; the shear capacity of a 1/2”-diameter foundation bolt in a 2×4 woodsill is 33 percent less than that of a 5/8”-diam-eter bolt; and the shear capacity in wood of an8d box nail is 22 percent less than that of an 8d

common nail.Construction tolerances play an equal role indetermining the actual capacities of connections.Parameters such as minimum edge and enddistances, required embedment or penetrationdepths, round versus slotted holes for bolts,spacing of reinforcing ties in concrete, andmisalignment of parts causing eccentric loads, canall significantly reduce connection capacity.Careful design and detailing on the drawings andthorough inspection of every connection in theload path is necessary to avoid creating weak links that lead to excessive earthquake damage.

ICBO, 1997, Uniform Building Code, Interna-tional Conference of Building Officials, Whittier,California.

Algermissen, S.T., 1983, An Introduction tothe Seismicity of the United States, EarthquakeEngineering Research Institute, Oakland,California.

ATC, 1997, NEHRP Guidelines for the Seismic Rehabilitation of Buildings, prepared by theApplied Technology Council for the BuildingSeismic Safety Council, published by the FederalEmergency Management Agency, FEMA 273Report, Washington, D.C.

Dames & Moore Inc., 1988, How Earthquakes Affect Buildings - A Video, Final Cut Video,Dames & Moore, Inc., Walnut Creek, Califor-nia.

FEMA, 1998, Seismic Retrofit Training for

Inspectors and Contractors, FEMA Emer-gency Management Institute, Emmitsburg,Maryland.

SAC, 1995, Interim Guidelines: Evaluation, Repair, Modification, and Design of Welded

Steel Moment-Frame Structures, prepared bythe SAC Joint Venture (Report SAC-95-02),published by the Federal Emergency Manage-ment Agency, FEMA 267 Report, Washington,D.C. (Other SAC publications are availablethrough the Applied Technology Council; seebox.)

Stratta, J.L., 1987, Manual of Seismic Design,

Prentice Hall, Englewood Cliffs, New Jersey.

Yanev, P., 1974, Peace of Mind in EarthquakeCountry, Chronicle Books, San Francisco,California.












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