INTRODUCTION TOLATERAL DESIGN

THE ENGINEEREDWOOD ASSOCIATION

APA

Wood is the right choice for a host of construction applications. It is theearths natural, energy efficient and renewable building material.

Engineered wood is a better use of wood. The miracle in todays woodproducts is that they make more efficient use of the wood fiber resource

to make stronger plywood, oriented strand board, I-joists, glued laminated timbers, andlaminated veneer lumber. Thats good for the environment, and good for designers seekingstrong, efficient, and striking building design.

A few facts about wood. Were not running out of trees. One-third of the United States land base 731 million acres is covered by forests. About two-thirds of that 731million acres is suitable for repeated planting and harvesting of timber. Butonly about half of the land suitable for growing timber is open to logging.Most of that harvestable acreage also is open to other uses, such ascamping, hiking, and hunting. Forests fully cover one-half of Canadas land mass. Of thisforestland, nearly half is considered productive, or capable of producing timber on asustained yield basis. Canada has the highest per capita accumulation of protected naturalareas in the world areas including national and provincial parks.

Were growing more wood every day. American landowners plant more than two billion trees every year. In addition, millions of trees seednaturally. The forest products industry, which comprises about 15 percentof forestland ownership, is responsible for 41 percent of replanted forestacreage. That works out to more than one billion trees a year, or about

three million trees planted every day. This high rate of replanting accounts for the fact thateach year, 27 percent more timber is grown than is harvested. Canadas replanting recordshows a fourfold increase in the number of trees planted between 1975 and 1990.

Manufacturing wood is energy efficient.Wood products made up 47 percent of allindustrial raw materials manufactured in theUnited States, yet consumed only 4 percentof the energy needed to manufacture allindustrial raw materials, according toa 1987 study.

Good news for a healthy planet. For every ton of wood grown, a young forest produces 1.07 tons of oxygen and absorbs 1.47 tons ofcarbon dioxide.

Wood, the miracle material for the environment, for design, and for strong, lasting construction.

NOTICE:The recommendations inthis guide apply only topanels that bear the APAtrademark. Only panelsbearing the APA trademarkare subject to theAssociations qualityauditing program.

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WOODThe Miracle Material

Percent of Percent ofMaterial Production Energy Use

Wood 47 4

Steel 23 48

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CONTENTS

Vertical vs. Lateral Loads a Primer . . . . .4Load Path . . . . . . . . . . . . . . . . .4Lateral Design Terminology . . . . .5Differences Between Wind and Seismic Loads . . . . . . . . . . .6What Holds a Building Together? . . . . . . . . . . .7Limitations of Toenails . . . . . . . . .8

Shear Walls and Diaphragms .8Engineered versus PrescriptiveRequirements . . . . . . . . . . . . . . .9Bracing versus Shear Wall . . . . . .9What is Wall Bracing? . . . . . . .10Comparative Tests of Wall Sheathing Products Under Cyclic Loading . . . . . . . .10

Diaphragms . . . . . . . . . . . . .11Using Wood Structural Panels to Design Diaphragms . . . . . . .11

How is a diaphragm designed for a sloped roof? .11What are blocked andunblocked diaphragms? . . . .11Can a continuous ridge vent be used in conjunction with a roof diaphragm? . . . .12

Using the Diaphragm Table . . .12

Shear Walls . . . . . . . . . . . . .14Using Wood Structural Panels in Shear Walls . . . . . . . .14

How does a shear wall differ from a diaphragm? . . .14What is a shear wall segment? . . . . . . . . . . .14Can plywood siding be used as a shear wall? . . . . . .14What is base shear? . . . . . . .15What is overturning? . . . . . .15

Using the Shear Wall Tables . . .15

Becoming a Lateral Design Pro: Tips to Make You a Master of the Tables . . . .17

Other Considerations . . . . . .17Drag Struts . . . . . . . . . . . . . . .17Diaphragm and Shear Wall Chords . . . . . . . . . .18Perforated Shear Walls (Type II Shear Walls) . . . . . . . . .18Lateral Design The Next Step .18

Additional Information . . . . .19About APA The Engineered Wood Association . . . . . . . . . . .19

his publication describes how wood framedbuildings can be designed to withstandlateral loads such as those caused by highwinds (hurricanes) or seismic forces (earth-

quakes). When building or designing in areaswith seismic and high wind loads, it is essentialto understand how lateral loads affect structures.Also important is knowledge of how constructiondetailing and connections affect the ultimateperformance of the building.

The elements of a wood-framed building thatenable it to withstand earthquake and hurricaneforces are its shear walls and diaphragms. Wood-framed construction systems rely completely onthe correct and adequate design and installationof these and all other components of the struc-ture, including framing, wood structural panelsheathing and connections.

Wood-frame construction makes it easy for building professionals to design and constructsafe, strong buildings that meet code require-ments while fulfilling lateral design criteria.Woods inherent characteristics make it an idealmaterial for shear wall and diaphragm design.Its light weight results in less force due to inertiaduring an earthquake, and less force means lessdamage. Woods flexibility allows the building tobetter absorb and resist the forces found in bothearthquakes and high winds. Finally, wood struc-tural panels provide a cost effective means ofmeeting design requirements for shear walls and diaphragms.

If the information you need to use APA trade-marked wood structural panels in shear wall ordiaphragm applications is not found in this pub-lication, or if you need assistance, contact APA atthe number listed on the back cover of this guide.

T

VERTICAL VS.

LATERAL LOADS

A PRIMER

Any building, regardless of size or location, must be designed to safelyresist the structural loads anticipatedduring its lifetime. These loads can bedivided into two categories verticalloads and lateral loads.

Vertical loads are loads acting in the up and down direction. These loadsare the obvious ones; the weight of thebuilding itself (dead load), the weight ofeverything in the building (live load) andvariable loads such as those from snow.Because these loads are easy to under-stand, typical construction practice hasevolved into an efficient system that doesa good job of accommodating them.

The challenge lies with the otherloads, the lateral loads. Lateral loads are those that act in a directionparallel to the ground. The two majorcontributors to lateral load are highwinds, such as those from a hurricane,and seismic (earthquake) forces. Neithertype of load is intuitive and designingfor these loads is typically not fullycovered at the university level, or wellunderstood outside of a few limitedacademic and engineering circles.

To further complicate matters, becausewind and seismic forces can be felt onthe building from any direction, thestructure must be designed to withstandlateral loads in two directions at rightangles to each other. As a result, threeseparate load designs must be calculated

into the foundation. From the founda-tion, the load is then transferred into theground. Therefore, the load path forvertical loads is: from the roof or floor tothe supporting walls, through the floorsbelow, to the foundation and finally tothe ground. Vertical load transfer is sim-plified because of the way elements arestacked on top of each other in con-ventional framing. No such advantageexists for lateral load design.

The lateral load path is less intuitivebut the rules remain the same. The

for every building: one vertical loaddesign and two lateral load designs (onefor each direction). Then, the loadcapacity of all major building elementsand every connection between eachelement must be calculated to makesure each has the capacity to resist allthree loads and transfer lateral andvertical forces between them.

Correct lateral design is essential. Abuilding that has not been specificallydesigned and built to resist lateral loadswill likely collapse when subjected tothese forces. This was proven by themassive destruction seen as a result ofHurricane Andrew and the Northridge,California Earthquake. This publicationis intended to help building designersavoid such disasters through properlateral design and detailing.

Load PathOne of the most basic yet importantconsiderations facing building designersis that each individual building elementmust be designed to carry expectedloads. In addition, these loads must betransferred through the rest of the struc-ture into the foundation. Finally, everyconnection between the elements of thestructure must be designed so that theapplied loads can be transferred from oneelement to the next until they reach theground. This transfer of forces throughthe structure is called the load path.

Understanding a vertical load path isquite straightforward. Vertical loads actupwards or downwards on the horizontalelements of the structure the roof andfloors. The roof and floors transfer theseloads to the load-bearing walls below and

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FIGURE 1

VERTICAL LOAD PATH

FIGURE 2

LATERAL LOAD PATH

major elements of a wood-framed build-ing that enable it to withstand lateralforces are shear walls and diaphragms.These elements must be designed toresist the lateral loads applied, andconnections between elements must bestrong enough to transfer the loadsbetween elements.

While the load path for wind and earthquake forces varies slightly at thebeginning, it is basically the same for alllateral loads. The lateral load is eithertransferred into the roof element (whenwind pushes against the walls perpen-dicular to the wind), or it originatesdirectly in the roof element (as wouldhappen during an earthquake). The load is transferred through the roofdiaphragm and from the diaphragm byfasteners or framing anchors into thetop of the walls that run parallel to thedirection of the load. This force is thentransferred down through the shearwalls (and, in the case of a seismicevent, added to the force generatedwithin the parallel wall), then into andthrough the floor below via fasteners orframing anchors. At this point, the forcefrom the roof and walls above is addedto the force in the floor diaphragmbelow and all are transferred, again byfasteners or framing anchors, to the topof the walls parallel to the load below.This process is repeated through succes-sive floors until the loads are transferredinto the foundation and from there intothe ground.

Therefore, the general lateral load pathis from the roof or floor into the wallsparallel to the force, to the foundationand into the ground. Remember that thisload path is then repeated with the forceacting at 90 degrees to the first one.

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Lateral Design TerminologyLoad Path the path taken by a force acting on a building. Loads are transferred

by the elements in the building and by the connections between those elements into

the foundation.

Diaphragm a flat structural unit acting like a deep, thin beam. The term is usually

applied to roofs and floors designed to withstand lateral loads. Diaphragms are com-

monly created by installing wood structural panels over roof or floor supports.

Shear Wall a vertical, cantilevered diaphragm that is constructed to resist lateral

shear loads by fastening wood structural panels over wall framing.

Chord the edge members of a diaphragm, typically the joists, ledgers, truss elements,

double top plates, etc.

Box-type Structure when diaphragms and shear walls are used in the lateral design

of a building, the structural system is called a box system.

Wall Bracing a building element that resists lateral loads under low load situations;

the configuration and connections are prescribed by the building codes for light framed

wood structures.

Blocked Diaphragm a diaphragm in which all panel edges occur over and are nailed

to common framing; the additional nailing provides a greater number of fasteners avail-

able to transfer shear from one panel to another, thus increasing the overall shear capac-

ity and rigidity or stiffness of the diaphragm.

Unblocked Diaphragm a diaphragm in which only the 4-foot-wide panel ends occur

over and are nailed to common framing; this is the most common type of diaphragm

used in standard residential construction.

Shear Wall Segment a portion of the shear wall that runs from the diaphragm above

to the diaphragm/foundation below; also known as full-height segments, shear wall

segments occur between openings (such as windows or doors) in the shear wall.

Base Shear the reaction at the base of a wall or structure due to an applied lateral

load, a sliding force.

Overturning what happens when a lateral force acts on a wall or structure and the

wall is restrained from sliding; a tip-over or overturning force results.

Drag Strut a structural building component that distributes the diaphragm shear

from one shear-resisting element to another; typically served by the double top plate.

Perforated Shear Wall a shear wall containing door and window openings that is

treated as a single shear wall with a slightly lower capacity than that of a similar

full-height shear wall segment.

6Designs based on both lateral load paths must be completed and the elements and inter-element connectionsdesigned to resist all applicable verticaland lateral loads. (See Figure 3.)

Differences Between Wind and Seismic LoadsAt first glance, hurricanes and earth-quakes seem to have little in common.But from a building design point of view,they affect structures in a similar manner.

Seismic forces are generated by theshaking of the building during an earth-quake. The shaking motion causes thestructures mass to be accelerated backand forth, and up and down. This accel-eration causes a force to develop withinthe structure in locations where thestructures mass is the largest. This is atthe roof, floor and wall lines parallel to

SEISM

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FIGURE 4

SEISMIC FORCES ACTING ON MASS

WIND

LOAD

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FIGURE 5

WIND FORCES ACTING ON AREA

the earthquake forces. Thus, the designloads calculated are applied at thesepoints in the resulting building design.

Wind forces, on the other hand, actagainst the sides of the building, like the

sail on a boat. The forces act over theentire buildings cross section. A largepercentage of the load is transferred upinto the roof/floor and the rest downinto the foundation.

FIGURE 3

LATERAL LOAD ANALYSIS MUST BE CONDUCTED ALONG BOTH AXES OF STRUCTURE

SHEA

R WAL

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SHEA

R WAL

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SHEAR WALL

DIA-PHRAGM

SHEAR WALL(BEHIND)

FORC

E

and then design:

Design ConsiderationsEarthquakes and hurricanes both havevertical and horizontal force compo-nents. Therefore, the structure must bedesigned for the component actingalong both horizontal axes, as well as forthe vertical component. From a designperspective, once the forces along allthree axes of the structure have beendetermined, the actual building designproceeds identically to accommodateboth wind and seismic loads.

The actual determination of the designloads is covered in all of the majormodel building codes. Seismic loads arecomputed by considering the:

Structural type and location of the building Importance factor of the building(1)

Soil type under the foundation Building geometry Actual mass of the structure

Wind loads are also computed afterconsidering a number of factors, including the:

Orientation of the building with respect to the wind Building size and shape Design wind speed Cover provided by surrounding structures and other terrain features Roof slope Porosity of the building envelope Importance factor of the building(1)

What Holds a Building Together?The whole idea of engineering a structure is to insure that the buildingprovides an acceptable level of structuralintegrity so that life-safety is assuredduring an earthquake or hurricane andstructural damage is minimized.

allow the forces acting on one elementto flow into the adjacent element, pro-gressing through the structure until theforces reach the ground.

This flow of forces completes the loadpath and provides the property protec-tion and life safety for which the build-ing was designed. The overall strength ofa building is a function of the strength ofthe individual building elements thewalls, roof, floor and foundation andhow those key components are tiedtogether. Properly designed, detailedand executed connections are key.

How is this accomplished? What holdsthe building together? Connectionsare key to the structural integrity of any building.

Light-framed wood structures areknown as box-type structures. A simpleanalogy of this type of structure is thecommon corrugated cardboard box(hence the name box-type structure).

The corrugations in the boxs sides areoriented vertically and consequently havea high vertical load carrying capacity. Thisis similar to the orientation of studs in alight-framed wood wall. Each of the wallelements in the box (and in the wood-framed structure sheathed with woodstructural panels) is rigid in terms ofracking resistance. In other words, it isextremely hard to force a rectangular sideof the cardboard box out of square.

The corrugations in the top and bottomof the box run side to side, thus distrib-uting vertical loads applied to the top orbottom of the box to the sides. In theroof and floors of box-type structures,vertical loads are similarly transferredthrough the floor and roof trusses orjoists into the walls of the structure.Like the sides of the box and walls of awood framed building, roof and floorelements are also extremely difficult toforce out of square.

With all of these very admirable physical properties, why is a cardboardbox so flimsy until it is assembled? It isonly when all of the edges are tapedtogether that a box gains its true poten-tial strength and ability to withstandlateral and vertical loads. The tape iswhat makes the box work.

Similarly, in a box-type structure, it is theinter-element connections that tapethe sides together. These connections

7

All edges of each element must be attached to all edges of adjacent elements to achieve structural rigidity.

FIGURE 6

PRINCIPLES OF A BOX-TYPE STRUCTURE

(1) Structures that must be available toprovide emergency services after a cata-strophe (hospitals, law enforcement, firefighting facilities and schools) are designedwith a greater factor of safety to insuretheir survivability. This is accomplished by assigning such structures a largerimportance factor during design.

Limitations of ToenailsWhen connecting diaphragms or shear walls together, avoid using toenails they are notthe best connection choice available.

Every structure in a high seismic or hurricane region is subject to loads in all threeorthogonal directions, that is, vertical loads (both down and up), as well as loads alongboth horizontal axes of the structure (loads parallel and perpendicular to the plane ofthe wall). This means that at every joint between elements, roof to end wall, for example,there will be forces trying to pull these elements apart in all three directions. Connectionsare key to the structural integrity of the building.

In a roof diaphragm-to-shear wall connection, there is a vertical load trying to lift theroof away from the wall (wind uplift or vertical seismic component). There also exists acomponent of both high wind and seismic forces that tries to slide the roof in a directionparallel to the axis of the wall. In addition, a force is trying to push the wall into or awayfrom the roof diaphragm in a direction perpendicular to the wall. Because these forcesare coming from a variety of directions, toenails used by themselves are often inadequatefor the following reasons: Toenails are good at resisting loads only in certain directions, such as loads perpendicular to the plane of the nail, or sideways in relation to the nail. Due to the cyclic nature of earthquake and hurricane loads, direction of the loadcycles back and forth. Cyclic loads in the other directions put the nails in withdrawalhalf of the time. Nails should never be used in withdrawal.

It is usually necessary to supplement connection details when toenails are used to trans-fer lateral loads, and often desirable to completely eliminate their use by using a properlydesigned framing anchor. Framing anchor catalogs give the capacities of their productsin all 3 directions as appropriate, for just this reason.

SHEAR WALLS

AND DIAPHRAGMS

Wind and seismic forces are resisted byshear walls that are composed of woodstructural sheathing fastened to woodframing, properly connected to thefoundation below and the roof above.Similarly, the installation of wood struc-tural panels over a roof or floor supportscreates a diaphragm, a flat structuralunit that acts like a deep, thin beam that resists lateral forces.

Shear walls and diaphragms are buildingelements necessary for proper lateraldesign. Aside from the fact that a shearwall is vertical and the diaphragm ishorizontal (or nearly horizontal), theyare essentially the same kind of struc-tural element. A diaphragm is designedas a simply supported beam while theshear wall is designed like a vertical,cantilevered diaphragm.

A diaphragm acts in a manner similar to a deep I-beam or girder, where the panels act as a web resisting shear,while the diaphragm edge membersperform the function of flanges, resist-ing bending stresses. These edge mem-bers are commonly called chords indiaphragm design, and may be joists,ledgers, double top plates, etc.

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RESISTANCE

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RESISTANCE

TOENAILS CANNOT RESIST FORCES IN ALL DIRECTIONS

Engineered versus Prescriptive RequirementsCurrent model building codes allow thedesigner to use either of two methods todesign light frame wood structures.Both are appropriate for detached oneand two family dwellings as well asmany other wood structures. These twomethods are by the use of engineeringdesign or prescriptive requirements.

Chapter 16 of the current editions of all three major model building codesand the International Building Code isthe chapter that provides the informa-tion required to engineer a structure the design-based requirements. Thischapter provides all of the vertical andhorizontal design loads (gravity, snow,wind, seismic, impact, construction, liveand dead loads, etc.) that must be con-sidered when doing an engineeringdesign of any type of structure covered by the building codes.

This chapter provides no guidance onhow to actually build the structure. Itjust provides the loads on the structure.This data must be interpreted by adesigner or engineer whose responsibil-ity it is to provide all of the detailsnecessary to resist the applied loadsand build the structure.

The wood chapter of each of these building codes (Chapter 23) containsprescriptive requirements for the designof wood structures. Prescriptive require-ments provide a cookbook method forthe design of wood structures withincertain limitations. They tell the designer

Tighter-than-conventional nailing of the sheathing/siding Thicker-than-conventionalsheathing/siding Different framing grades, species and sizes Limitations on the placement of theshear wall segments (e.g., shear walls onupper floors must be placed directlyover shear walls below) Special fastening requirements at thetop of the shear wall elements to insureload transfer from the roof/floordiaphragm into the shear wall

When the building is designed usingprescriptive requirements, lateral forcesare resisted by wall bracing instead ofby shear walls. This wall bracing mustbe placed at prescribed locationsthroughout the structure: e.g., Eachend and not more than 25 feet on cen-ter (taken from Table 2308.9.3(1) ofthe 2000 International Building Code).While these bracing panels serve thesame function as the engineered shearwall provide resistance to the lateralforces acting on the structure theyhave few elements in common with theshear walls described above. Their lackof detailing severely limits the strengthand stiffness of the wall bracing whencompared with an engineered shearwall. For this reason, wall bracing isrelegated to low-load situations.

what size and grade lumber to use fordifferent applications, what anchor boltspacing to use, what size joists to use forvarious floor spans, the fastening sched-ule for all applications, etc. Becauseprescriptive requirements ignore specificfactors such as the actual geometry of thestructure, actual loads seen by the struc-ture and their location, this designmethod is limited for use in locationswith low wind and minimal risk of seis-mic activity.

Bracing versus Shear WallsThe design method chosen, engineeredor prescriptive, will determine whethershear walls or wall bracing are used toprovide lateral bracing and resistance in the structure.

In the process of designing a box-typestructure, the engineer/designer will findthat he or she must provide a number ofvertical wall elements designed to resistthe horizontal forces acting on the build-ing (earthquake, wind or both). Theelements used in a box-type structure toresist lateral loads are shear walls. Theseshear walls also act as interior and exte-rior walls, load bearing and non-loadbearing walls required to meet the archi-tectural goals of the building as well asthe requirements of other design loads.

While shear walls may look similar toother walls, they often contain a numberof important differences, including:

Additional base shear anchor bolts inthe bottom plate (the size and numbermay be different from the prescriptiverequirements of the code) Hold-down anchors at each end of the shear wall

9

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FIGURE 7

SHEAR RESISTING ELEMENTSPRESCRIPTIVE CORNER BRACING/WALL BRACING

WoodStructural

Panels

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Relative Strength

Gypsum FoamSheathingwith Let-inBracing

FIGURE 8

COMPARATIVE TESTS OF WALL SHEATHING PRODUCTS UNDER CYCLIC LOADING

In recent tests by APA The Engineered Wood Association, walls fully sheathed with plywood or OSB were three times stronger than walls constructed with gypsum wall sheathing or let-in bracing.

The comparative wall sheathing tests, part of an ongoing research program by APA, were designed to simulate the dynamic effects of seismic events. The tests involved 8 x 8 ft walls constructed according to the bracing provisions and nail schedules of the major model building codes.

What is Wall Bracing?The codes give various options. Mostcommonly used: one unit of wallbracing could be 4 lineal feet of studssheathed with wood structural panels,8 lineal feet of studs sheathed with gyp-sum board on one side, 8 lineal feet ofwall containing a let-in brace at45 degrees (limited to single story orsecond story of a two story building), etc.

ENGINEERED SHEAR WALLS

Specific stud species Base shear anchor bolts Hold-down

FIGURE 7

SHEAR RESISTING ELEMENTS PRESCRIPTIVE CORNER BRACING/WALL BRACING

ENGINEERED SHEAR WALLS

Let-in bracingWall board (8' reqd.)

Wood Structural Panel (4' reqd.)

APA wood structural panels of specific grade and thickness

Specific nail size andspacing requirements

Specific stud species Base shear anchor bolts Hold-down anchors

11

DIAPHRAGMS

Using Wood Structural Panelsto Design Diaphragms

A discussion of diaphragm design invariably raises a number of commonlyasked questions:

How is a Diaphragm Designed for a Sloped Roof?Lateral load design examples are almost always modeled with flat roofdiaphragms. This is done because it iseasier to visualize the load path througha simple box than through a compli-cated architectural exercise in multiple,intersecting roof lines and unusualbuilding shapes. Regardless of the shapeof the structure or pitch of the roof,buildings can be broken down intorectangular segments and each segmentanalyzed as if it had a flat roof.

Once the load on the roof diaphragm is determined, the design of the roofdiaphragm is conducted on the plan view(or vertical projection) of the diaphragm.After the diaphragm material, thickness,fastener type and spacing has been sizedand selected for the plan view, it mustbe used over the entire roof diaphragm,regardless of the actual angle of the roof.

What are Blocked and Unblocked Diaphragms?Diaphragms vary considerably in load-carrying capacity, depending onwhether they are blocked orunblocked. A blocked diaphragm isone in which all four panel edges occurover and are nailed to common framing.This additional nailing provides a greaternumber of fasteners available to transfershear from one panel to another, thusincreasing the overall shear capacity andrigidity or stiffness of the diaphragm.

Blocked Diaphragm

Unblocked Diaphragm

FIGURE 9

BLOCKED AND UNBLOCKED DIAPHRAGMS

In an unblocked diaphragm, only the4-foot-wide panel ends occur over andare nailed to common framing. This isthe most common type of diaphragmused in standard residential construc-tion. Loads are generally low enoughthat the added expense of blockingunsupported edges is not required.

As there are fewer nails available totransfer shear from one panel to thenext, unblocked diaphragms have lowerallowable capacities and are less stiffthan blocked diaphragms.

Can a Continuous Ridge Vent be used in Conjunction with a Roof Diaphragm?If the roof is designed as an unblockeddiaphragm, continuous ridge vents can be used. This is because with anunblocked diaphragm, there is norequirement for a connection to bemade between unsupported edges ofadjacent panels.

With a blocked diaphragm, the use of acontinuous ridge vent is slightly morecomplicated because all edges of alladjacent panels must occur over and beattached to common framing. This is astrue for the ridge as it is for panels occur-ring in a common plane. In many cases,the connection at the ridge can bedetailed, however, to accommodate atleast partial ridge venting. This can bedone by placing blocking in alternatingspaces between the rafters or joists. Atthe location where there is blocking, the

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Blocked

Edge nail spacing

Unblocked

Half of edgenail spacing

(Not to scale)

FIGURE 10

ACCOMMODATING RIDGE VENTS WITH BLOCKED DIAPHRAGMS

required nail spacing shall be halved, sothat twice as many nails are present tomake this connection in every otherspace. This can be done when the normal blocked diaphragm nail spacing is either 4 or 6 inches on center. If thenailing into the blocking after doubling is at 2-1/2 inches on center or less, thesame additional framing-width require-ments given in the diaphragm and shearwall tables shall apply. Fortunately, thediaphragm nailing requirements in build-ings of a size where pitched roofs aresuitable are generally small enough to beaccommodated by this method.

Using the Diaphragm Table

Example One:Given: Residential roof diaphragm,trussed roof, unblocked diaphragmrequired, diaphragm capacity requiredequals 180 plf, load orientationunknown.

Solution: Using Table 1, refer to theUnblocked Diaphragm area of thetable. As orientation is unknown, usethe All other configurations column,since these values will be conservative.Check APA RATED SHEATHINGrows first because Structural I may notbe readily available in all areas. Similarly,check only rows with 2-inch minimumnominal framing width, as the roof framing is made up of trusses. From thetable, see that 8d nails with 15/32-inchsheathing over 2x framing yields a capacity of 180 plf with the standard 6 and 12 nail spacing. As 180 plf isequal to the required 180 plf capacity,this selection is OK for use.

Example Two:Given: Commercial roof diaphragm,trussed roof, diaphragm capacityrequired equals 350 plf with a Case I orientation.

Solution: Using Table 1, refer to theUnblocked Diaphragm area of thetable first. You will find that no solutionis possible. Next, check the BlockedDiaphragm area of the table. CheckAPA RATED SHEATHING rowsfirst. Similarly, check only rows with2-inch minimum nominal framingwidth as the framing is made up oftrusses. From the table, see that 8d nailswith 15/32-inch Sheathing over 2xframing yields a capacity of 360 plf.Nails must be placed at 4 inches oncenter at all diaphragm boundaries andat 6 inches on center at all other paneledges. As 360 plf is greater than 350 plf,this selection is OK for use.

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TABLE 1

DIAPHRAGMS: RECOMMENDED SHEAR (POUNDS PER FOOT) FOR HORIZONTAL APA PANEL DIAPHRAGMS WITH FRAMING OFDOUGLAS-FIR, LARCH OR SOUTHERN PINE(a) FOR WIND OR SEISMIC LOADING

Blocked Diaphragms Unblocked Diaphragms

Nail Spacing (in.) at Nails Spaced 6" max. atdiaphragm boundaries Supported Edges(b)

(all cases), at continuous panel edges parallel to load (Cases 3 & 4),

and at all panel edges (Cases 5 & 6)(b)

Minimum 6 4 2-1/2(c) 2(c) Case 1 (NoMinimum Minimum Nominal unblocked

Nail Nominal Width of Nail Spacing (in.) at edges or All otherPenetration Panel Framing other panel edges continuous configurations

Common in Framing Thickness Member (Cases 1, 2, 3 & 4) joints parallel (Cases 2, 3,Panel Grade Nail Size (inches) (inch) (inches) to load) 4, 5 & 6)

6 6 4 3

6d 1-1/4 5/16 2 185 250 375 420 165 1253 210 280 420 475 185 140

8d 1-3/8 3/8 2 270 360 530 600 240 1803 300 400 600 675 265 200

10d(d) 1-1/2 15/32 2 320 425 640 730 285 2153 360 480 720 820 320 240

5/16 2 170 225 335 380 150 1103 190 250 380 430 170 1256d(e) 1-1/4

3/8 2 185 250 375 420 165 1253 210 280 420 475 185 140

3/8 2 240 320 480 545 215 1603 270 360 540 610 240 180

8d 1-3/8 7/16 2 255 340 505 575 230 1703 285 380 570 645 255 190

15/32 2 270 360 530 600 240 1803 300 400 600 675 265 200

15/32 2 290 385 575 655 255 1903 325 430 650 735 290 21510d(d) 1-1/2

19/32 2 320 425 640 730 285 2153 360 480 720 820 320 240

Case 5Case 2 Case 4Load LoadLoadLoad

Continuous panel jointsContinuous panel joints

Load

Diaphragm boundary

FramingCase 6

Continuous panel joints

Case 3 Blocking,if used

Blocking,if used

Blocking,if used

Case 1Load Framing Framing

(a) For framing of other species: (1) Find specific gravity for species of lumberin the AFPA National Design Specification. (2) Find shear value from tableabove for nail size for actual grade. (3) Multiply value by the following adjust-ment factor: Specific Gravity Adjustment Factor = [1 (0.5 SG)], where SG= specific gravity of the framing. This adjustment shall not be greater than 1.

(b) Space nails maximum 12 in. o.c. along intermediate framing members(6 in. o.c. when supports are spaced 48 in. o.c.).

(c) Framing at adjoining panel edges shall be 3-in. nominal or wider, andnails shall be staggered where nails are spaced 2 inches o.c. or 2-1/2 inches o.c.

(d) Framing at adjoining panel edges shall be 3-in. nominal or wider, andnails shall be staggered where 10d nails having penetration into framing ofmore than 1-5/8 inches are spaced 3 inches o.c.

(e) 8d is recommended minimum for roofs due to negative pressures of high winds.

Notes: Design for diaphragm stresses depends on direction of continuouspanel joints with reference to load, not on direction of long dimension ofsheet. Continuous framing may be in either direction for blocked diaphragms.

APA RATEDSHEATHING,APA RATEDSTURD-I-FLOOR and other APA gradesexcept SpeciesGroup 5

APASTRUCTURAL Igrades

SHEAR WALLS

Using Wood Structural Panels in Shear WallsFollowing are questions that are commonly raised about shear walls:

How Does a Shear Wall Differ from a Diaphragm?They are essentially the same kind ofstructural element both comprised ofstructural lumber components used tocarry the vertical loads, splice the woodstructural panels together and holdthem in place. The shear load is carriedthrough the wood structural panelsheathing to adjacent sheathing panelsthrough the lumber framing. In essence,the shear wall table is really just a special case diaphragm table. In fact,the diaphragm table is sometimes usedto design shear walls.

14

FIGURE 11

SHEAR WALL SEGMENT

h

w w w w

Local building codes typically stipulate a minimum w of h/3.5

A Shear Wall A Diaphragm

Is vertical Is horizontal (or nearly so)

Is designed Is designed like a as a simplycantilevered supportedbeam beam

Table has only Table has both blocked values, blocked and because a shear unblockedwall is always diaphragm blocked* values

*A code requirement.

What is a Shear Wall Segment?A shear wall segment in a conventionalshear wall is the portion of the shearwall that runs from the diaphragmabove (such as a roof) to the diaphragm(floor) or foundation below. Also knownas full-height segments, true shear wallsegments occur between openings (suchas windows or doors) in the shear wall.In conventional shear wall design, onlythe full-height segments may be used totransfer the shear from the diaphragmabove to and through the floor below.

Design practice and building codes putfurther restrictions on the geometry ofshear wall segments to insure that mini-mum height-to-width ratios are main-tained to hold building deformations toa level that can be tolerated.

Can Plywood Siding be used as a Shear Wall?Yes, all three of the major model buildingcodes and the International BuildingCode recognize the use of APA 303Plywood Siding (such as T1-11) for useas shear walls. Sidings have the samecapacity as Sheathing grade wood struc-tural panels of the same thickness whensimilarly attached. The thickness of theplywood siding panel at the point ofnailing is used to determine its capacity.

There are a number of non-plywoodsiding panels known as APA RatedSiding that have also been recognizedfor such uses. The actual design infor-mation for such products is proprietaryand available from the manufacturer.

What is Base Shear?Base shear is the force acting at the baseof the structure in a direction parallel tothe axis of the shear wall. The baseshear connection keeps the buildingfrom sliding off its foundation. As dis-cussed on page 5, the purpose of ashear wall is to transfer the lateral loadacting on the wall from above downthrough the floor below and on into thefoundation. The wood structural panelsheathing on the wall provides therigidity and strength to accomplish thistask. When transferring shear betweenthe second- and first-floor shear walls,the wood structural panel sheathing, ifconstructed so that all panel edgesoccur over and are attached to commonframing, will provide this shear transferconnection. At the foundation wherethe wood structural panel shear wallsheathing from above is fastened to theplate below, the base shear transfer isaccomplished by bolting the plate downto the foundation. These anchor boltsare also known as base shear anchors.

Using the Shear Wall Tables

Example One:Given: Commercial building shear wallrequiring 5/8-inch gypsum sheathing onoutside of building for one-hour fireseparation. Required shear wall capacityequals 437 plf.

Solution:Using Table 2, check the Panelsapplied over 1/2" or 5/8" gypsumsheathing area of table. Check APARATED SHEATHING rows firstbecause Structural I may not be readilyavailable in all areas. From the table, seethat 10d nails with a 3 and 12 nailspacing and any thickness of APA RatedSheathing will provide a capacity of490 plf provided that the framing atadjoining panel edges is 3-inch nominalor wider. As 490 plf is greater than437 plf, this selection is OK for use.

What is Overturning?Once the shear wall is bolted soundly to the foundation with the base shearanchors (and thus prevented from slid-ing), the force acting on the shear wallat the roof level and second floor level acts in such a way as to overturn theshear wall. The taller the structure, thegreater the force. Overturning is pre-vented by anchoring the double studs,3x_, or 4x_ members at each end ofeach shear wall segment down to thefoundation with a tension tie. These tiesare called hold-downs. The tensionties in shear walls located in floors abovethe ground floor must be continuousthrough the lower floor walls and mustbe ultimately tied into the foundation.

The base shear anchors, previouslydiscussed, are placed to prevent slidingonly, and are not intended to holddown the shear wall against overturning.If used for this purpose, the bottomplate will stay in place and the sheathingon the shear wall may unzip from thebottom plate as the wall overturns.

15

FIGURE 12

BASE SHEAR

FIGURE 13

OVERTURNING

16

Load Framing

Shear wall boundary

Blocking

Foundation resistance

Framing

Typical Layout for Shear Walls

TABLE 2

SHEAR WALLS: RECOMMENDED SHEAR (POUNDS PER FOOT) FOR APA PANEL SHEAR WALLS WITH FRAMING OF DOUGLAS-FIR, LARCH, OR SOUTHERN PINE(a) FOR WIND OR SEISMIC LOADING(b)

Panels Applied OverPanels Applied Direct to Framing 1/2" or 5/8" Gypsum Sheathing

Minimum MinimumNominal Nail Nail Size Nail Spacing at Nail Size Nail Spacing at

Panel Penetration (common or Panel Edges (in.) (common or Panel Edges (in.)Panel Grade Thickness in Framing galvanized galvanized

(in.) (in.) box) 6 4 3 2(e) box) 6 4 3 2(e)

5/16 1-1/4 6d 200 300 390 510 8d 200 300 390 510

APA 3/8 230(d) 360(d) 460(d) 610(d)

STRUCTURAL I 7/16 1-3/8 8d 255(d) 395(d) 505(d) 670(d) 10d(f) 280 430 550 730grades 15/32 280 430 550 730

15/32 1-1/2 10d(f) 340 510 665 870

APA RATED

5/16 or 1/4(c) 180 270 350 450 180 270 350 450

SHEATHING; APA3/8

1-1/4 6d200 300 390 510

8d200 300 390 510

RATED SIDING(g)3/8 220(d) 320(d) 410(d) 530(d)

7/16 1-3/8 8d 240(d) 350(d) 450(d) 585(d) 10d(f) 260 380 490 640and other APA15/32 260 380 490 640grades except

15/32 310 460 600 770 species Group 5

19/321-1/2 10d(f)

340 510 665 870

APA RATED Nail Size Nail Size

SIDING 303(g) (galvanized (galvanized

and other APA casing) casing)

grades except 5/16(c) 1-1/4 6d 140 210 275 360 8d 140 210 275 360species Group 5 3/8 1-3/8 8d 160 240 310 410 10d(f) 160 240 310 410

(a) For framing of other species: (1) Find specific gravity for species of lumberin the AFPA National Design Specification. (2) For common or galvanized boxnails, find shear value from table above for nail size for actual grade. (3) Multi-ply value by the following adjustment factor: Specific Gravity Adjustment Factor= [1 (0.5 SG)], where SG = specific gravity of the framing. This adjustmentshall not be greater than 1.

(b) All panel edges backed with 2-inch nominal or wider framing. Install pan-els either horizontally or vertically. Space nails maximum 6 inches o.c. alongintermediate framing members for 3/8-inch and 7/16-inch panels installedon studs spaced 24 inches o.c. For other conditions and panel thicknesses,space nails maximum 12 inches o.c. on intermediate supports.

(c) 3/8-inch or APA RATED SIDING 16 oc is minimum recommended whenapplied direct to framing as exterior siding.

(d) Shears may be increased to values shown for 15/32-inch sheathing withsame nailing provided (1) studs are spaced a maximum of 16 inches o.c., or(2) if panels are applied with long dimension across studs.

(e) Framing at adjoining panel edges shall be 3-inch nominal or wider, andnails shall be staggered where nails are spaced 2 inches o.c.

(f) Framing at adjoining panel edges shall be 3-inch nominal or wider, andnails shall be staggered where 10d nails having penetration into framing ofmore than 1-5/8 inches are spaced 3 inches o.c.

(g) Values apply to all-veneer plywood APA RATED SIDING panels only. OtherAPA RATED SIDING panels may also qualify on a proprietary basis. APARATED SIDING 16 oc plywood may be 11/32 inch, 3/8 inch or thicker.Thickness at point of nailing on panel edges governs shear values.

Becoming a Lateral Design Pro: Tips to Make You a Master of the Tables

Using the diaphragm and shear wall tables is relatively simple. After calculating your

required capacity, simply find the recommended design capacities. However, there are tips

that can be used in both tables that will give you greater flexibility, permit substitutions

and allow more economical designs.

If a Structural I panel is selected from the tables, note that the same value can be

achieved with a non-Structural I panel simply by increasing the thickness 1/8 of an inch.

Because Structural I panels may not be readily available in all areas and are often higher

in cost than non-Structural I sheathing, this alternative provides design flexibility. The

thicker panel will also provide a better nail base and may improve the capacity of the

panel for normal loads, and may also be more inexpensive.

If the capacity of a given panel thickness with a 6 and 12 nail spacing (6" on center at

supported panel edges and 12" on center over interior supports) isnt enough, consider a

tighter nail spacing (4" or 3" on center) before going with a thicker panel.

When designing a diaphragm, only block the portion of the diaphragm that needs

blocking those areas where the diaphragm shear exceeds the unblocked diaphragm

capacity given for the panel type, thickness, fastener type and spacing. This saves

time and labor.

If a shear wall or diaphragm design load gets high enough to require the use of nail

spacings of 2-1/2 inches or less, consider using thicker panels because 3 x framing is

typically required at adjoining panel edges for these tight nail spacings. In these cases,

thicker panels with a wider nail spacing may be a more cost-effective solution.

In the shear wall table, the thickness of the panel at the point of nailing determines the

panel thickness. Thus, if a 19/32-inch-thick siding panel is to be used as a shear wall and

the long panel edges are nailed so that the nail does not go through the shiplapped or

grooved area of the panel edge, then the full 19/32-inch thickness of the panel may be used

to determine the shear wall capacity. This usually means starting the nails a little farther in

from the edge of the panel and slanting them about 10 degrees toward the stud.

OTHER

CONSIDERATIONS

Drag StrutsAs described on page 4, the load path for a box-type structure goes fromthe diaphragm into the shear wallsrunning parallel to the direction of theload. Another way to say this is that thediaphragm loads the shear walls thatsupport it. Because the diaphragm actslike a long, deep beam, it loads each ofthe supporting shear walls evenly alongthe length of the walls. The problem lieswith the fact that seldom is each shearwall solid throughout its full length.Typically, a wall contains windows and doors.

The traditional model used to analyzeshear walls only recognizes wall seg-ments that run full height as shear wallsegments. This means that at locationswith windows or doors, a structuralelement is needed to distribute theuniform diaphragm shear over the top of the opening into the full height segments adjacent to it. This element is called a drag strut.

17

T1-11 nailing

Double top plate acts like a drag strutin these locations (over openings).

FIGURE 14

TOP PLATE DRAG STRUTS

Note:Nailing of both panel edges alongshiplap joints is recommended. Thedouble nailing is required when wallsegment must meet wall bracing orengineered shear wall requirements.

Diaphragm and Shear Wall ChordsBlocked diaphragms are thought to act like long deep beams. This modelassumes that shear forces are accommo-dated by the wood structural panel webof the beam and that moment forcesare carried by the tension or compres-sion forces in the flanges or chords ofthe beam. These chord forces arenormally assumed to be carried by thedouble top plate of the supportingperimeter walls. Given the magnitude ofthe forces involved in most light framedwood construction projects, the doubletop plate has sufficient capacity to resistthe tensile and compressive forces,assuming adequate detailing at thesplice locations. The real problem lies in wall lines that make a continuousdiaphragm chord impossible.

Because shear walls are little more thanblocked, cantilevered diaphragms, theytoo develop chord forces and requirechords. The chords in a shear wall arethe double studs that are required ateach end of each shear wall. Just as thechords need to be continuous in adiaphragm, the chords in a shear wallalso need to maintain their continuity.This is accomplished by the tension ties(hold-downs) that are required at eachend of each shear wall and between thechords of stacked shear walls.

Perforated Shear Walls (Type II Shear Walls)A considerable amount of research is being done by organizations on thesubject of perforated shear walls (alsoknown as Type II shear walls). Thisprocedure allows a wall with door andwindow openings to be treated as asingle shear wall with a slightly lowercapacity. The advantage of this methodis that the entire perforated wall onlyrequires two hold-downs, one at eachend. The traditional methodology wouldhave required that the wall be brokeninto segments, and each segmentanchored with a pair of hold-downs. Ashold-downs can be costly and difficultto install properly, contractors are gener-ally well disposed to eliminating anchorsat the cost of thicker sheathing andadditional nailing. The perforated shearwall method has recently been includedin the International Building Code andhas been in the Standard Building Codefor some years.

Lateral Design The Next StepOnce familiar with the concepts pre-sented in this publication, the designercan work through the detailed designexamples found in APA Design/Construction Guide: Diaphragms andShear Walls, Form L350. This publica-tion can be obtained through your localAPA field representative, from APA head-quarters in Tacoma, Washington, orfrom the Associations web site atwww.apawood.org.

Fortunately, in residential construction,the double top plates existing in moststud walls will serve as a drag strut. Itmay be necessary to detail the doubletop plate such that no splices occur incritical zones. Or it may be necessary tospecify the use of a tension strap at buttjoints to transfer these forces.

The maximum force seen by drag strutsis generally equal to the diaphragmdesign shear in the direction of theshear wall multiplied by the distancebetween the shear wall segments.

Drag struts are also used to tie togetherdifferent parts of an irregularly shapedbuilding:

To simplify design, irregularly shapedbuildings (such as L or T shaped) aretypically divided into simple rectangles.When the structure is reassembledafter the individual designs have beencompleted, drag struts are used to pro-vide the necessary continuity betweenthese individual segments to insure thatthe building will act as a whole.

18

Drag struts(double topplate interiorwalls, bottomchord of roof truss, floorjoists, etc.)

FIGURE 15

DRAG STRUTS USED TO TIE AN L SHAPED BUILDING TOGETHER

19

ADDITIONAL INFORMATION

About APA The Engineered Wood Association APA The Engineered Wood Association is a nonprofit trade associationwhose member mills produce a majority of the structural wood panelproducts manufactured in North America.

The Associations trademark appears only on products manufactured bymember mills and is the manufacturers assurance that the product con-forms to the standard shown on the trademark. That standard may be anAPA performance standard, the Voluntary Product Standard PS 1-95 forConstruction and Industrial Plywood, or Voluntary Product StandardPS 2-92, Performance Standards for Wood-Based Structural-Use Panels.Panel quality of all APA trademarked products is subject to verificationthrough an APA audit.

APAs services go far beyond quality testing and inspection. Research andpromotion programs play important roles in developing and improvingplywood and other panel construction systems, and in helping users andspecifiers better understand and apply panel products.

For additional information on wood construction systems, contact APA The Engineered Wood Association, P.O. Box 11700, Tacoma,Washington 98411-0700.

Other publications from APA:

Design/Construction Guide: Diaphragms and Shear Walls, Form L350 Design Concepts: Building in High Wind and Seismic Zones, Form W650 Engineered Wood Construction Guide, Form E30 APA Publications Index, Form B300

MORE INFORMATION ONLINE

Visit APAs web site at www.apawood.org for more information on engineered wood products, wood design and construction, and technical issues and answers.

Online publication ordering is also available through the web site.

INTRODUCTION TO LATERAL DESIGNWe have field representatives in many

major U.S. cities and in Canada who can helpanswer questions involving APA trademarked

products. For additional assistance in specifyingengineered wood products, contact us:

APA THE ENGINEERED WOOD ASSOCIATION

HEADQUARTERS 7011 So. 19th St. P.O. Box 11700Tacoma, Washington 98411-0700

(253) 565-6600 Fax: (253) 565-7265

PRODUCT SUPPORT HELP DESK(253) 620-7400

E-mail Address: help@apawood.org

The product use recommendations in this publica-tion are based on APA The Engineered WoodAssociations continuing programs of laboratorytesting, product research, and comprehensive fieldexperience. However, because the Association hasno control over quality of workmanship or the con-ditions under which engineered wood products areused, it cannot accept responsibility for productperformance or designs as actually constructed.Because engineered wood product performancerequirements vary geographically, consult yourlocal architect, engineer or design professional toassure compliance with code, construction, andperformance requirements.

Form No. X305C/Revised April 2003/0200

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copyright: 2003 APA - The Engineered Wood Association