APA Engineered Wodd Handbooks Episode 9 potx

DESIGNING FOR LATERAL LOADS 7.3T C w w h Wind load, F lb per sq ft Side wall carries load to roof diaphragm at top, and to foundation at bottom Roof horizontal diaphragm carries load to

CHAPTER 7 DESIGNING FOR LATERAL

as these topics are addressed in complete detail by the applicable model buildingcodes

Average story drift: is the average of the deformations of all lines of shear walls

oriented parallel to the applied load The deformation shall be calculated as aportion in each line of shear walls at the top of the shear wall (see Eq [7.1])

Base shear: the total reaction at the base of a wall parallel to the axis of the

wall or structure due to an applied lateral load; a ‘‘sliding’’ force

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

fastened to common framing; the additional fastening provides a load path totransfer shear at all panel edges, thus increasing the overall shear capacity andrigidity (stiffness) of the diaphragm

Boundary element: diaphragm and shear wall boundary members to which

sheathing transfers forces Boundary elements include chords and drag struts atdiaphragm and shear wall perimeters, interior openings, discontinuities, and re-entrant corners

7.2 CHAPTER SEVEN

Box-type structure: when diaphragms and shear walls are used as the lateral

force resisting system of a building, the structural system is called a box system

Chord: the edge members of a diaphragm or a shear wall, typically the joists,

ledgers, truss elements, double top plates, end posts, etc., that resist axial forces.Chords are oriented perpendicular to the applied lateral load

Collector: a structural building component that distributes the diaphragm shear

from one building element to another; typically served by the double top plate.Collectors are oriented parallel to the applied lateral load Also called drag struts

Diaphragm: a flat, or nearly flat, structural unit acting like a deep, thin beam.

The term is usually applied to roofs and floors designed to withstand lateralloads Diaphragms are commonly created by installing structural-use panels overroof or floor supports

Diaphragm, blocked: a diaphragm in which adjacent sheathing edges are

fas-tened to a common member for transferring shear Examples to achieve blockedstatus are panels being fastened to common framing members, sheet metal fas-tened to adjacent panels typically with staples, or staples driven through thetongue-and-groove for 11⁄8in wood structural panels

Diaphragm, flexible: a diaphragm is flexible for the purpose of distribution of

story shear when the lateral deformation of the diaphragm under the appliedlateral load (see Eq [7.2]) is greater than or equal to two times the average storydrift For analysis purposes, it can be assumed that a flexible diaphragm distrib-utes story shear by tributary area into lines of shear walls oriented parallel tothe applied lateral load Traditionally, wood diaphragms are considered flexible

Diaphragm, unblocked: a diaphragm in which only panel edges in one direction

(i.e., the 4 ft wide panel ends) occur over and are fastened to common framing;the typical diaphragm for standard residential construction

Drag strut: see collector.

Lateral load: horizontal forces that result from wind or seismic forces Wind

forces act on the side of the building and on sloped roofs Seismic forces resultfrom ground accelerations causing inertial forces to act on the structural mass

Lateral stiffness: the slope of the load-displacement for a lateral force-resisting

system

Load path: the path taken by forces acting on a building Loads are transferred

by the elements in the building and by the connections between those elementsinto the foundation

Overturning: occurs when a lateral force acts on a wall or structure and the wall

is restrained from sliding; a ‘‘tip-over’’ or overturning force results

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

shear loads by fastening structural-use panels over wood wall framing Thisstructural system transfers lateral forces from the top of the wall to the bottom

of the wall, and eventually transfers the lateral loads to the foundation

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 segment.Shear wall segments occur between building wall discontinuities such as doors,windows or corners in the shear wall

Structural-use panel: a structural panel product composed primarily of wood

and meeting the requirements of USDOC PS-1, USDOC PS-2, or equivalent

DESIGNING FOR LATERAL LOADS 7.3

T

C

w w

h

Wind load, F

(lb per sq ft)

Side wall carries load

to roof diaphragm at top,

and to foundation at bottom

Roof (horizontal diaphragm)

carries load to end walls

End wall (vertical diaphragm or shear wall)

carries load to foundation

v (lb per lin ft of diaphragm width) =

w (lb per lin ft of wall) = F

wL 2b

FIGURE 7.1 Distribution of lateral loads on buildings.

proprietary standard recognized by the code authority Structural-use panels clude all-veneer plywood, composite panels containing a combination of veneerand wood-based material, and matformed panels such as oriented strand boardand waferboard

in-Subdiaphragm: a portion of a larger wood diaphragm designed to transfer local

forces to primary diaphragm collectors

Tie-down (hold-down): a device used to resist uplift of the chords of shear walls Wall bracing: a building element that resists lateral loads under low load situ-

ations; the configuration and connections are prescribed by the building codesfor light-framed wood structures

7.3 SHEAR WALLS

Shear walls and diaphragms are designed to transfer in-plane forces When thesetwo assemblies are used to resist lateral design forces of buildings, the structuralsystem is sometimes referred to as a box system The shear walls provide reactionsfor the roof and floor diaphragms, and transmit the forces into the foundation (seeFig 7.1)

The structural design of buildings using diaphragms is a relatively simple,straightforward process if the designer keeps in mind the overall concept of struc-tural diaphragm behavior Actually, with ordinary good construction practice, anysheathed element in a building adds considerable strength to the structure Thus, ifthe walls and roofs are sheathed with panels and are adequately tied together andtied to the foundation, many of the requirements of a diaphragm structure are met.This fact explains the good performance of structural-use panel sheathed buildings

• These assemblies act as deep beams.

• In-plane shear resistance is provided by the structural-use panel-to-framing nections

con-• Axial tension and compression resistance is provided by the chord members ogous to an I-beam flange)

(anal-• Nailed assemblies, as shown in Tables 7.1, 7.2, and 7.7, exhibit ductile, absorbing behavior

energy-While shear resistance of these assemblies can be computed by principles ofengineering mechanics1it is recommended that designers use Tables 7.1, 7.2, and7.7 for typical design purposes In addition to eliminating labor-intensive calcula-tions, these tables limit configurations to those that have proven to exhibit theaforementioned ductile behavior by demonstration via structural testing and years

of successful in-use performance

7.3.1 Shear-Wall Testing

Buildings are subjected to a variety of loads during their lifetime All loads resisted

by the structure must be transferred into the foundation Gravitational loads (rooflive load, snow load, dead load) react vertically on the structure and are typicallytransferred to the foundation through load-bearing walls Wind and earthquakesapply lateral load to a structure Lateral loads are transferred to the foundationthrough lateral force-resisting systems For light-frame construction, the gravita-tional forces are typically resisted by nominal dimension lumber in the form ofwall studs, and the lateral loads are commonly resisted by wood structural panelsheathed shear walls

APA—The Engineered Wood Association has conducted research on the ior of shear walls for almost 50 years The first technical report was published in

behav-1953 Two years later the Uniform Building Code (UBC) recognized shear-walldesign values based on the APA tests This recognition allowed shear walls to beused as lateral force-resisting systems in buildings designed per the UBC.The intent of the following sections are to review the methods in which currentshear-wall values are derived, discuss some of the questions raised about shear-wallperformance based on monotonic and reversed cyclic load testing, and briefly dis-cuss research efforts that are currently being conducted by APA

Static Test Methods

ASTM E72 The current version of ASTM E722covers standard tests of wall,floor and roof elements Although ASTM E72 is often thought of as a racking test,the actual standard test method is much more broad-based The racking test portion

of ASTM E72 has the longest history of any of the standard test methods discussed

in this chapter Much of the data that have traditionally been used to support wall design values have been developed from tests following ASTM E72 This

shear-DESIGNING FOR LATERAL LOADS 7.5

Load

Stop Hold down rod

Typically, several incremental monotonic (unidirectional) loads are applied andremoved before the wall is taken to failure These loads allow the measurement ofpermanent wall set to be recorded at different load levels The magnitude of theload cycles specified by this test standard is not related to the sheathing being tested.For example, low-strength and high-strength shear walls follow the same load cycle.The test method is partially intended to eliminate confounding factors of the testthat may affect the overall results The test method is intended primarily to evaluatepanel shear resistance, including the sheathing attachments to framing, not the be-havior of the structural assembly or system

7.6 CHAPTER SEVEN

8 ft (2.4 m)

8 ft (2.4 m)

Hold-down connector

FIGURE 7.3 Schematic diagram of shear-wall tests following the methods outlined by ASTM E564 5

The following observations can be made The vast majority of shear wall testsconducted over the past 50 years follow ASTM E72 One of the problems withfollowing this test method is that it specifies load increments, regardless of the wallsheathing material ASTM E72 was originally intended for evaluating nominal wallbracing for residential applications that exhibited relatively low shear capacities.The load increments for shear walls that are designed for high loads may representonly a fraction of the design load

Although ASTM E72 allows other loading patterns to be used, no suggestion isgiven on how the loads should be chosen APA test methodology, covered in PRP-

108,3deviates from ASTM E72 on this point, in that the loads are determined as

a function of the design load This ensures that the wall will be loaded to designload at least three times The results from ASTM E72 tests are often used to verifythe allowable design values published in the U.S model building codes or the 2000International Building Code (IBC).4A load factor (strength limit state / design shearload) for a test series typically averages about 3.0

ASTM E564 Some designers and agencies have been critical of the ASTM

E72 test method due to the use of the artificial down mechanism (steel down rods); therefore, an alternative ASTM standard was developed, ASTM E564.5The ASTM E564 test method is designed to evaluate the performance of shear wallassemblies instead of focusing only on the behavior of the sheathing Figure 7.3illustrates a schematic diagram of an ASTM E564 assembly test

hold-For ASTM E564, hold-down connectors are required to resist overturning forcesinstead of hold-down rods as used in ASTM E72 Unlike ASTM E72, ASTM E564provides an option to apply vertical loads to simulate gravitational forces Another

DESIGNING FOR LATERAL LOADS 7.7

difference in the two test methods is that ASTM E564 does not utilize a stop atthe sole plate; lateral slippage is prevented only by the sole plate bolts

ASTM E564 test procedures also subject the shear walls to several monotonicload and unload cycles In contrast to ASTM E72, the magnitude of the loads isbased on the expected maximum shear capacity (strength limit state) of the walls.Although, some may view the ASTM E72 and E564 load tests as being cyclic,since the walls are loaded and unloaded several times, the methods do not modeltrue behavior of earthquakes because: (1) the loads are only applied for a smallnumber of cycles, (2) the load is not fully reversing (it only loads the wall mono-tonically in one direction) and (3) the monotonic tests are typically performedslowly, thus modeling static behavior Such loading is considered applicable to windloading conditions Conversely, earthquake loads are typically fully reversing cyclicloads that continue for numerous cycles and cause dynamic loads on the structures

Quasi-static Cyclic Test Methods

Structural Engineers Association of Southern California (SEAOSC ) Test Method Code officials, engineers, and researchers have questioned how well mon-

otonic laboratory tests of shear walls relate to the behavior of full-size shear wallssubjected to reversed cyclic loads After the Northridge, California, earthquake (Jan-uary 17, 1994), the City of Los Angeles adopted several building code changes thateffectively reduced (by 25%) the allowable design loads for shear walls based onmonotonic tests, until cyclic (reversed) load shear wall tests were conducted toconfirm or modify previously recognized values Since very few data were availablefrom matched monotonically and cyclically tested shear walls, the reductions werebased on experience of structural engineers in Southern California and were some-what arbitrary The intention of the interim reductions was to provide conservativedesign values until more informed design recommendations could be formed One

of the hurdles of cyclic testing was that there was no universally recognized protocolfor cyclic load testing of shear walls In 1994, the Structural Engineers Association

of Southern California (SEAOSC) took on the task of developing a test standardfor cyclic (reversed) loading of shear walls The proposed standard was finalized

in 1996,6with minor revisions in 1997

The shear-wall test specimen used in the SEAOSC test method is similar to thewall used for ASTM E564 monotonic tests (Fig 7.3) The overturning forces thatare generated in the wall are resisted by the use of hold-down connectors Loadapplied to the top of the wall is a fully reversing, sinusoidal load with decay cycles,

as illustrated by Fig 7.4

The magnitude of the load cycles is based on the first major event (FME) Bydefinition, the FME is the ‘‘first significant limit state to occur.’’ In lay terms, FMErepresents the point in which permanent damage to the wall begins to occur Thispoint can also be thought of as the proportional limit or yield limit state of thewall The load cycle that is used by the SEAOSC test method is based on a proposedsequential phased displacement (SPD) procedure.7 The proposed procedure wasdeveloped by a joint U.S and Japan Technical Coordinating Committee on MasonryResearch and is commonly referred to as either the SPD or the TCCMAR proce-dure The SPD procedure is designed to provide fully reversing displacements withprogressively increasing displacements

After FME occurs (after nine cycles in Fig 7.4), a degradation cycle is included

in the test cycle This degradation cycle represents 100%, 75%, 50%, and finally25% of the maximum displacement increment The degradation cycle is included

to analyze the effect of systems that develop slack The area of the hysteresis loops

FIGURE 7.4 Load cycle following the SPD procedure 6

represent the energy dissipated by the shear wall Observing the load vs ment hysteresis loops enables a slack system to be identified by the load essentiallyapproaching zero during the degradation cycles A slack system will lose the ability

displace-to dissipate energy at lower displacements Slack systems may occur for somebolted connections (localized wood crushing causing loose joints) or some types ofbrittle systems; however, wood structural panel shear walls with mechanical fasten-ers, such as nails, generally do not behave as a slack system

The next distinct portion of the load cycle is the stabilization load cycles Eachincremental displacement has three stabilization cycles The intent of this cycle isfor the strength and stiffness degradation to stabilize before the next incrementalcycle The stabilization cycle can be thought of as ‘‘at a given displacement, thewall incurs as much damage as it can’’; then the next load cycle increment isinitiated

Although the original SPD proposed procedure was based on quasi-static ing, the SEAOSC procedure specifies that the tests will be conducted at a loadingrate from 0.2–1.0 Hz The specified load rate is at a lower (or at least on the lowend) frequency than what is typically observed in an earthquake There are severalreasons to test at a lower frequency First, it is believed that lower frequency testswill limit the inertial effects that might occur during the load tests The secondreason is that these types of tests are performed with the aid of an effective force-testing system An effective force testing system simulates the dynamic loads ofearthquakes by applying an effective force to the wall This effective force is similar

load-to forces generated by the mass of the structure and the ground acceleration Theeffective force testing system is used in lieu of shake table testing The lowerfrequencies are more attainable by typical effective force-testing systems

In 1997, the International Conference of Building Officials Evaluation Service,Inc (ICBO ES) adopted the SEAOSC test protocol by means of an acceptancecriteria8 for evaluation of prefabricated sheathed shear-wall elements Althoughthe acceptance criteria do not currently apply to site-built wood structural panel

DESIGNING FOR LATERAL LOADS 7.9

sheathed shear walls, it is believed that the recognition of the acceptance criteriasets a precedent for the evaluation for all light-frame lateral force resisting elements.Also in 1997, the SEAOSC test method was presented to ASTM CommitteeE06 for recognition as a consensus test method The SEAOSC test method followsthe guidelines of ASTM format to expedite approval During the time this chapterwas being written, the test method was balloted several times It is expected thatthe standard will be balloted at the main committee level soon

In 1995, APA—The Engineered Wood Association installed a cyclic loadingsystem that has the capability to test shear walls following the SEAOSC procedure.One of APA’s objectives is to gain knowledge about shear-wall performance in thecyclic domain An extensive multiyear test program has been initiated to answermany questions that have not been addressed in the cyclic test domain A few ofthe tests programs are:

• Investigate the behavior of narrow shear walls subjected to cyclic loading anddevelop design recommendations for their design and construction

• Analyze the behavior of walls that contain two dissimilar sheathing materials: forexample, wood structural panel on one side, gypsum wallboard on the other side

• Analyze the effect of sheathing orientation (vertical or horizontal)

• Analyze the effect of blocked and unblocked sheathing

• Study alternative sheathing fastener systems

The test programs are designed to study factors that have raised questions aboutthe behavior of cyclically loaded shear walls Due to the importance of resolvingseveral of the initial issues, APA funded a series of tests at the University ofCalifornia-Irvine (UCI) This test program consisted of eight different shear wallstested with fully reversed cyclic tests in accordance with the SEAOSC procedure(note that the test method was not finalized when these test were conducted, butthe tests were conducted following a draft of the test method) However, no match-ing monotonic tests (either ASTM E72 or ASTM E564) were conducted on theseshear walls The preliminary findings9of these load tests indicate that the maximumshear loads tended to be lower than monotonic tested walls following ASTM E72test procedures Although this finding could be significant, the comparison is notbased on matched specimens For example, fastener fatigue failure, as related tocyclic load protocol, has a significant effect on the final results but is not a failuremode for monotonic testing

Most of the UCI tests were conducted following the SPD test procedure Onetest was conducted following a modified SPD procedure This modified procedureeliminated the degradation cycles that occur after FME Figure 7.5 is a represen-tation of the load cycle followed for one of the eight tests

Matched walls were tested following the SPD procedure (with degradation cyclesand stabilization cycles) and the modified SPD procedure (stabilization cycles only).The two tests indicated that the degradation cycles did not significantly affect theresults of wood structural panel shear-wall tests The degradation cycles may not

be necessary for future shear-wall tests, although other tests in progress with woodstructural panels and other sheathing materials over steel framing indicate that deg-radation cycles may provide useful information

One of the significant observations of the APA tests conducted at UCI was thatsheathing nails around the perimeters exhibited fatigue The majority of the loadcycles in the SPD procedure exceed the FME displacement (yield limit state) Sincewall damage is dependent on total number of cycles and the number of cycles and

FIGURE 7.5 Simplified SPD load cycle as reported by Rose 9

the amount of displacements that exceed FME, it follows that the behavior of thewall will be dependent on the load history

Since nail fatigue was not reported in observations of damaged shear walls afterthe Northridge, California, earthquake or other recent seismic events, Ficcadenti et

al.10 conducted cyclically loaded shear-wall tests following a modification of theSEAOSC procedure Steel hold-down rods, similar to ASTM E72, were utilized.The hold-down rods were required on both ends of the test wall to resist overturningloads in both directions The SPD load cycle was also modified by removing thestabilization cycles, but included the degradation cycles of the SPD procedure.Figure 7.6 illustrates the modification of the load cycle

In these tests, failures due to nails pulling through the structural panel wereobserved Smaller-sized nails and thinner panels were used in these tests Some ofthe differences may be attributed to load history, and some may be attributed to thefastening conditions

These walls also achieved maximum shear loads in excess of 3.0 times designload The maximum shear capacity of the walls was achieved about 2 in (50 mm)displacement Conversely, several of the APA tests reached the maximum shearloads at around the 1 in (25 mm) displacement cycles Figure 7.7 is a typicalhysteresis loop observed in one of the eight APA wall tests conducted at UCI Notethat the cycled shear capacity occurred at about 1 in of displacement

The maximum shear capacity of shear walls tested following the SEAOSC testprocedure will clearly be affected by the fatigue failures of the nails It appearsunrealistic to assume that cyclically tested walls will achieve load factors that are

as high as matching monotonically tested counterparts due to the difference infailure modes

CUREe Protocol and Others As previously discussed, the SPD loading

pro-tocol, in many researchers’ opinions, is a very severe test cycle that causes a failuremode (nail fatigue) that is not typical of seismically loaded shear walls Researchersrealize the need and importance of cycle testing and its importance in aiding ad-vanced shear wall modeling tools However, it is also important to match real shear-

DESIGNING FOR LATERAL LOADS 7.11

FIGURE 7.6 Modified SPD load cycle used by Ficcadenti et al 10

FIGURE 7.7 Load versus displacement response of the APA.

wall performance with lab-tested walls Therefore, there has been a significantamount of research on the effects of load cycles The authors’ experience is mostlywith North American researchers, but some work has been initiated on the inter-national front and will eventually be finalized in an ISO standard

In North America alone, the authors are aware of at least three different cyclictest regimes other than the ISO test cycle and the SPD loading protocol ForintekCanada developed a test cycle that leads to more realistic failures (reported by Lam

7.12 CHAPTER SEVEN

et al.11 Forintek Research Reports available through their website and numerousconference proceedings).12He et al.13modified the Forintek procedure because theauthors’ opinion was that the Forintek load cycle was also too severe Finally, one

of the elements of the CUREe-Caltech Woodframe Project developed yet anothercyclic test cycle This load cycle and others was studied under another element ofthe Woodframe project (unpublished data, but will be available from the CUREewebsite).14 Based on other researchers’ conclusions and preliminary data from theCUREe-Caltech Woodframe Project, the SPD cycle appears to provide the lowestultimate load capacity and lowest deflection at ultimate load

Dynamic Test Methods

Pseudo-dynamic Characterizing the performance of shear-wall assemblies

though quasi-static cyclic testing provides a vast amount of data that can be veryuseful Particularly, the data can be used to develop and refine computer-basedmodels for more advanced design analysis of wood-framed buildings However, animportant concept should not be forgotten about the quasi-static testing Cyclictesting only simulates the affects of an undefined seismic event in an undefinedstructure, which, in the authors’ experience, is a difficult concept for many layper-sons to understand A very common question that laypersons ask when they visitthe APA Laboratory and witness a cyclic test is ‘‘What magnitude earthquake didthe cyclic test simulate?’’ The common answer to this question is that it depends

on soil conditions, mass of the structure, plan of structure, and ground motion.Typically quasi-static cyclic testing, particularly the SPD cycle, subjects walls tomany more displacement repetitions that would be expected from a seismic event.Pseudo-dynamic testing is intended to simulate the effects of an assumed groundmotion on an assumed structure Karacabeyli and Ceccotti15summarize the results

of their pseudo-dynamic tests Based on previous either quasi-static cyclic or otonic shear-wall testing, a nonlinear time history analysis can be conducted on anassumed building Based on the time-history analysis, the deflection of an individualshear wall can be modeled Once this deflection history is developed from thenonlinear time history analysis, a test shear wall can be constructed and tested withthe deflection history under laboratory conditions Furthermore, this testing can beconducted at a much slower testing rate, hence the name ‘‘pseudo-dynamic testing.’’The advantage of this test method is that it actually simulates dynamic perform-ance of shear walls, unlike the quasi-static tests Another advantage is that the testequipment required to conduct pseudo-dynamic tests is much more common thanwhat would be required to conduct a true dynamic test The disadvantage of thismethod is that many assumptions are required to conduct this type of test, such asbuilding geometry and ground motion Thus, to obtain useful results for designinferences, a range of ground motions would need to be conducted on walls from

mon-a rmon-ange of building types

Pseudo-dynamic test methods can also be very effective in estimating the loads

on a particular shear wall subjected to a particular event Of course, detailed datamust be known about this shear wall, so it is unlikely this method could be useful

to simulate real seismic events More likely, this method would be useful for ulating the effects of a laboratory type event, such as a shake table tests One ofthe CUREe-Caltech Woodframe project elements will conduct pseudo-dynamictests on a wall assembly similar to a wall in a full-sized structure that was subjected

sim-to shake table testing

Shake Table Testing As one can imagine, shake table testing has not been a

terribly common occurrence for timber framed construction in North America Thereason for this lack of testing is several-fold: (1) the study of seismic design of

DESIGNING FOR LATERAL LOADS 7.13

wood-framed structures has traditionally not been viewed as academically rigorous

by some universities, (2) historically, wood-framed structures have performed well

in seismic events, which leads to (3) lack of funding for this type of research

To the authors’ knowledge, Dolan16was one of the pioneers in dynamic testing

of wood framed elements Dolan’s tests were conducted on single wall elements,tested on a relatively small shake table at the University of British Columbia Eventhough shake table testing had been conducted on single elements, shake tabletesting for full-sized structures has been limited in North America An interestingpoint is that after the 1995 Kobe, Japan, earthquake, several full-sized structureswere tested using the Kobe ground motion on very large shake tables in Japan.Although the results of these tests are interesting in a general sense, the constructiontechniques are somewhat different for North America and Japan

To the authors’ knowledge, the first full-sized wood-framed structures shake tabletest in North America was conducted in 2000 as part of the CUREe-Caltech Wood-framed project The test structure was intended to be representative of a two-storyresidential structure The findings of this testing are still being studied, but someimportant initial conclusions are that nonstructural elements such as gypsum wall-board and stucco provide significantly more stiffness than assumed There is nodoubt that the design methodology for wood-frame structures will be updated based

on this more recent shake table testing Finally, another element of the Caltech Woodframe project is shake table testing of a multifamily dwelling withtuck-under parking commonly referred to as soft story construction The historicalperformance of these types of structures has not been particularly good, given thetorsional issue caused by the asymmetric soft story associated with the parkinglevel This testing will likely lead to retrofit recommendations for these types ofbuildings

over-by hold-downs, or tension ties, at each end of the shear-wall segments

Table 7.1 and 7.2 present the tabulated values for structural-use panel sheathedwood frame shear walls for wind loading and seismic loading, respectively Thetable applicable to wind loading lists design values that are 40% higher than theshear-wall tables intended for seismic loading This 40% increase is to account forthe better understanding and refinement of wind loading design Furthermore, shearwalls subjected to extreme wind loads have had a proven track record with excellentperformance Some model building codes have adopted the 40% increase Thus,the designer should confirm that the increase is applicable under the local code

Design Tables. Table 7.1 provides recommended shear (lb / ft) for structural-usepanel shear walls with framing of Douglas fir, larch, or southern pine for seismicloading Table 7.2 provides similar shear values for wind loading

Other Design Considerations

Estimated Deflection The deflection (⌬) of a blocked shear wall uniformlynailed throughout may be estimated by use of the following formula:

3

TABLE 7.1 Recommended Shear (lb / ft) for Structural-Use Panel Shear Walls with Framing of

Douglas Fir, Larch, or Southern Pineafor Seismic Loadingb

Panel grade

Minimum nominal panel thickness (in.)

Minimum nail penetration

in framing (in.)

Panels applied directly to framing Nail size

(common or galvanized box)

Nail spacing at panel edges (in.)

Panels applied over 1 ⁄ 2 in or 5 ⁄ 8 in gypsum sheathing Nail size

(common or galvanized box)

Nail spacing at panel edges (in.)

except species 5

group 5

Specification; (2) (a) for common or galvanized box nails, find shear value from table above for nail size for actual

in o.c For other conditions and panel thickness, space nails maximum 12 in o.c on intermediate supports Fasteners

framing as exterior siding.

maximum of 16 in o.c or (2) if panels are applied with strength axis across supports.

spaced 2 in o.c Check local code for variations of these requirements.

requirements.

on panel edges governs shear values.

Shear wall boundary

Blocking

Foundation resistance

TABLE 7.2 Recommended Shear (lb / ft) for Structural-Use Panel Shear Walls with Framing of

Douglas Fir, Larch, or Southern Pine for Wind Loading Onlyb

Panel grade

Minimum nominal panel thickness (in.)

Minimum nail penetration

in framing (in.)

Panels applied directly to framing Nail size

(common or galvanized box)

Nail spacing at panel edges (in.)

Panels applied over 1 ⁄ 2 in or 5 ⁄ 8 in gypsum sheathing Nail size

(common or galvanized box)

Nail spacing at panel edges (in.)

except species 5

group 5

Specification; (2) (a) for common or galvanized box nails, find shear value from table above for nail size for actual

in o.c For other conditions and panel thickness, space nails maximum 12 in o.c on intermediate supports Fasteners

maximum of 16 in o.c or (2) if panels are applied with strength axis across supports.

spaced 2 in o.c Check local code for variations of these requirements.

requirements.

on panel edges governs shear values.

Notes: The recommended shears presented in Table 7.2 are based on the historically used design stresses for shear

walls, which are listed in Table 7.1 Some building codes have allowed a 40% increase due to the better understanding

of wind loading The designer should confirm that the 40% increase is applicable under the local code.

Shear wall boundary

Blocking

Foundation resistance

7.18 CHAPTER SEVEN

TABLE 7.4 Approximate Deflection (in.) Due to

Anchorage Detail d afor Use in Shear-Wall Deflection

Hold-down type Approximate rangea of d a(in.)

aThese numbers are intended to represent an approximate range A better estimate can be achieved by consulting the

TABLE 7.3 Fastener Slip Equations e n(in.) for Use in Calculating Diaphragm and Wall Deflection Due to Nail Slip (Structural I)

Shear-Fastener

Minimum penetration (in.)

For maximum loads up to (lb)

Approximate slip, e n(in.)a,b

Green / dry Dry / dry

a Fabricated green / tested dry (seasoned); fabricated dry / tested dry V n⫽ fastener load (lb / nail).

bValues based on Structural I plywood fastened to lumber with a specific gravity of 0.50 or greater Increase slip by 20% when plywood is not Structural I.

where⌬ ⫽calculated deflection (in.)

v⫽maximum shear due to design loads at the top of the wall (lb / ft)

G v t v⫽rigidity through the thickness lb / ft (see Table 2.22) This value should

be adjusted by the appropriate C gfactor listed in Table 2.22A

e n⫽nail deformation (in.) (see Table 7.3)

d a⫽deflection due to anchorage details (rotation and slip at tie-down bolts).See Table 7.4 for range of hold-down deflections

Overturning Moment Overturning moments result from shear walls being

loaded by horizontal forces The overturning moments are resisted by force couples.The tension couple is typically achieved by a hold-down Figure 7.8 and the equa-tion below present a method for calculating overturning forces

DESIGNING FOR LATERAL LOADS 7.19

V

h L

• Wall requires 5 ⁄ 8 in gypsum sheathing for one-hour fire rating

• Required shear-wall capacity 670 lb / ft (9,780 N / m).

Find:

Panel thickness, nail size and nailing schedule

solution Using Table 7.2, check the ‘‘Panels applied over 1 ⁄ 2 in or 5 ⁄ 8 in gypsum sheathing’’ area of table Check ‘‘Rated Sheathing ’’ rows first since Structural I may not be readily available in all areas From the table, note that 10d nails with 3 in nail spacing at panel edges and 12 in nail spacing at intermediate framing for a sheath- ing thickness of 3 ⁄ 8 , 7 ⁄ 16 , or 15 ⁄ 32 in will provide a capacity of 685 lb / ft provided that the framing at adjoining panel edges is 3 in nominal or wider Because 685 lb / ft (10,000 N / m) is greater than 670 lb / ft (9780 N / m), this selection is OK for use.

Example 7.2 Simple Shear-Wall Design for Seismic Loading

Given:

• Residential building

• Seismic loading

• Typical wall sheathing thickness of 7 ⁄ 16 in.

• Typical nail size of 8d common

• Wall stud spacing of 24 in o.c.

• Required shear-wall capacity is 435 lb / ft (6350 N / m).

Find:

Required nail spacing

solution Using Table 7.1, check the ‘‘Panels applied direct to framing’’ area of table Check ‘‘Rated Sheathing ’’ rows first because Structural I may not be readily avail- able in all areas From the table, note that 7 ⁄ 16 in structural-use panels with 8d nails

spaced at 3 in at the panel edges and 6 in at intermediate framing (see footnote b)

will provide a capacity of 450 lb / ft Because 450 lb / ft (6570 N / m) is greater than 435

lb / ft (6350 N / m), this selection is OK for use.

7.20 CHAPTER SEVEN

8' 6'-8"

2'-4"

24'-0"

3'-0" 2'-4" 8'-0" 2'-4" 3'-0" 3'-0" V

V = 3000 lb/ft, v = 300 lb, H = 2400 lb/ft

H H H

H H

H H

H

FIGURE 7.9 Building elevation for segmented shear-wall example.

Example 7.3 Shear-Wall Design (Traditional Segmented)

Design shear walls for the wall shown in Figure 7.9 The shear load on the wall from the diaphragm is 3 kips (13,344 N) The controlling load is assumed to be from wind pressures.

solution The total length of full-height segments is 10 ft (3.05 m) Note that 3.5:1 is the minimum shear wall aspect ratio for wind loading or for seismic loads in design categories A–C (per the 2000 IBC) 4 or seismic zones 1–3 (per the 1997 UBC) For seismic design category D–F (per the 2000 IBC) or seismic zone 4 (per the 1997 UBC) the maximum shear-wall aspect ratio is 2:1 Thus, the wall segments shown in this example are too narrow to meet the 2:1 aspect ratio criteria, but do meet the 3.5:1 criteria.

1 The unit shear is:

capacity, vallowof:

vallow ⫽ 365(0.92) ⫽ 335 lb/ft (4890 N/m) ⱖ 300 lb/ft (4380 N/m) Note the increase to 15 ⁄ 32 in (11.9 mm) panels is taken assuming studs will be placed 16 in (406 mm) o.c and the panel will be oriented with the 8 ft (2.44 m) direction vertical.

3 The hold-downs must be located at the ends of each full-height segment as shown

in Figure 7.9 and designed to resist, H:

H ⫽ vh ⫽ 300(8) ⫽ 2400 lb (10,700 N)

DESIGNING FOR LATERAL LOADS 7.21

Example 7.4 Shear-Wall Deflection

Calculate the deflection of the shear wall in Example 7.3.

solution The total shear-wall deflection will be considered to be a function of the deflection of the full-height segments For this example a weighted average based on wall rigidities will be used to calculate the total shear-wall deflection Wall rigidities will be assumed to be relative to wall length, assuming consistent framing and nailing patterns Another approach to this problem would be to iterate using the load-deflection equation to converge on the displacement caused by the total load, assuming the wall segments all deflect an equal amount.

Shear-wall deflection analysis usually involves engineering judgment In this ample the walls are narrow, with an approximate aspect ratio of 3.5:1 The accuracy

ex-of the shear wall deflection equation at aspect ratios greater than 2:1 is questionable However, in the absence of formal guidelines for narrow shear-wall deflection, the code equation will be used Other aspects that would reasonably influence the accuracy of wall deflection calculations (by stiffening the wall) are the presence of sheathing above and below openings, and wall finish materials (such as siding, stucco, and gypsum).

No formal guidelines currently exist for these aspects either, but there are several large research projects investigating shear wall behavior, and in the near future guidelines will likely be developed In the meantime, an estimate will be made as follows The deflection of the 2.33 ft (0.71 m) wall segment is calculated with the following equation (using English units):

E⫽ 1,200,000 psi, for s-p-f studs—stud grade

A⫽ 10.5 in 2 , for two 2 ⫻ 4 vertical end studs

G v t v ⫽ Gt ⫽ 27000(3.1) lb/in., for7 ⁄ 16 OSB (from Chapter 2)

b⫽ 2.33 ft, wall width

Load per nail, vnail:

vnail⫽ v/(12/S) ⫽ 300/(12/6) ⫽ 150 lb/nail (where S ⫽ nail spacing, in.) Nail slip, e n(equation from Table 7.3 using English units):

⌬ ⫽ 3(0.23)0.725 ⫹ 0.3(0.632) ⫽ 0.69 in (17.53 mm)

Shear-wall deflection is important for checking drift limitations and determining whether the diaphragm should be considered rigid or flexible.

a R⫽ relative rigidity based on wall length.

bShear distributed to wall segments in proportion to wall length.

7.3.3 Advanced Shear-Wall Topics

Principles of Mechanics. The 2000 International Building Code4allows ples of mechanics to be used to calculate the resistance of shear walls based onfastener strength and sheathing shear resistance per Section 2305.1.1 Tissell pro-vides guidance in the principles of mechanics calculations, published in APA Re-search Report 154.17 Tissell’s approach is based on single-fastener design valuesadjusted by various empirically determined factors that account for such things asnail spacing, panel grade, framing size, stud spacing, and the potential for panelbuckling APA originally derived Table 7.1 based on this type of approach Throughthe years, the load-duration factor, diaphragm factor, and nail design values havechanged, and therefore following Tissell’s approach will not provide perfect agree-ment with Table 7.1 However, in most cases the agreement is reasonable Whenthe individual fasters design values changed, APA chose not to change the shear-walls tables accordingly because, through testing, the design values have been dem-onstrated to be appropriate

princi-The principles of mechanics approach can be a very useful tool, especially when,for some reason or another, the shear wall is beyond the scope of Table 7.1 or 7.2

It should be noted that Tissell’s approach does not consider crushing of the top andbottom plate, which, based on recent testing experience, may result in further em-pirical adjustments to account for this Crushing of the top and bottom plate can

be an issue for highly loaded shear walls; it does not appear to be a significantissue for walls within the scope of Table 7.1 or 7.2 APA recommends that of Table7.1 or Table 7.2 be used in most cases over the principles of mechanics approach;however, sometimes a tabulated solution is not an option

Shear Transfer Around Openings ( pier method ). Traditional shear walls onlyconsider full-height segments of wood structural panels as being effective in re-sisting shear forces Therefore, any realistic wall line that contains openings willconsist of one or more segments of shear walls Each full-height segment must also

be detailed to resist overturning forces, which often requires tie-downs at each end

of the full-height segments

Another approach is to treat the entire wall as one unit, hence reducing thenumber of tie-downs This method requires the shear forces to be transferred aroundthe openings using the sheathing above and below openings, and some time mayrequire additional metal strapping to supplement the strength of the sheathing The

1999 SEAOC Bluebook18credits Ed Diekmann, S.E., with developing this approachcirca 1982 The approach is similar to the ‘‘pier method’’ in masonry design, and

DESIGNING FOR LATERAL LOADS 7.23

TABLE 7.6 Shear Capacity Adjustment Factor for Use with the Perforated Shear-Wall Method

Wall height

Maximum opening heighta

8 ft wall 2 ft, 8 in 4 ft, 0 in 5 ft, 4 in 6 ft, 4 in 8 ft, 0 in.

10 ft wall 3 ft, 4 in 5 ft, 0 in 6 ft, 8 in 8 ft, 4 in 10 ft, 0 in Percent full-height

appears in the 2000 International Building Code4 in Section 2305.3.7.1 under the

title Force Transfer Around Openings.

Two main advantages of this method are that it reduces the number of tie-downsrequires for a wall line and allows the designer to use narrower piers since theaspect ratio limitations in the building code apply to the pier, as opposed to thefull-height segments The disadvantage of this method is that it can require a largeamount of detailed calculations, especially if the wall becomes very complicated

It also requires special field detailing, extra blocking, extra nailing, and / or tional straps to achieve the field detailing

addi-Three good references for additional information are Ed Diekmann’s shear-wallchapter in the Faherty and Williamson Handbook,19 Duquette’s Timber Solutions Manual,20 and the SEAOC Seismic Design Manual, volume 2.21

Perforated Shear-Wall Approach. Intuitively, one could argue that there are twoways to design walls with openings One method is to reinforce the openings,discussed in the previous subsection; the other option is to place a penalty on theshear wall in some form or fashion The concept of providing an empirically de-veloped penalty is the basis for the perforated shear-wall approach Based on asignificant amount of monotonic and cyclic testing in both the United States andJapan, an empirically developed penalty was developed that accounts for maximumwall opening size and percent wall sheathing (Table 7.6) The shear capacity ad-justment factor is applied as a reduction factor to Tables 7.1 and 7.2 Technically,

any wall with openings is classified as a perforated shear wall Perforated shear walls can have several meanings, but in general the term implies the empirically

developed approach

7.24 CHAPTER SEVEN

The main advantage of the perforated shear-wall approach is that it reduces thenumber of tie-downs, is simple to apply, and can lead to very minor penalties forsmall openings The disadvantage of this system is that it can lead to very largepenalties for large opening sizes, which essentially forces the designer to use thepier method or the traditional segmented approach or redefine the perforated wall

to eliminate large openings (see Example 7.5 and 7.6 for this comparison) Anotherdisadvantage is that currently there is no theoretical derivation of the empiricaladjustment factors Some engineers have difficulty accepting a so-called black boxapproach without an engineering mechanics backup It is also the authors’ experi-ence that many engineers are comfortable using the method for wind design butargue that it is not appropriate for seismic design As previously noted, a reasonableamount of cyclic testing has been conducted on this methodology, and the empiricaladjustment factors have always proven to be conservative with respect to the ulti-mate strength of the tested walls

This method was first introduced in the building codes in 1995 The StandardBuilding Code was the first to accept the methodology The most current buildingcode reference for this method is the 2000 International Building Code4in Section

2305.3.7.2 under the title No Force Transfer Around Openings In addition, the

2000 NEHRP Provisions22provide the most current thinking on the subject, which

is slightly more up to date than the 2000 IBC The 2000 NEHRP Commentary21also provides a detailed design example for the perforated shear-wall method

Example 7.5 Shear Wall Designed with Openings (Using the Perforated Shear-Wall Approach)

This example will show an empirical shear-wall design method that accounts for the effect of openings using the perforated shear wall method.

The same wall section as shown in Example 7.3 will be redesigned as a perforated shear wall to highlight the differences between the two methods The entire wall, not just full-height segments, will be considered as the shear wall and the openings will

be accounted for with a shear capacity adjustment factor (SCAF) Hold-downs are always required at the ends of shear walls With this design method hold-downs are required only at the ends of the wall since the entire wall is treated as one shear wall with openings.

Design the shear walls for the wall shown in Figure 7.10 The shear load on the

wall from the diaphragm, V, is 3 kips (13.34 kN) from wind pressures The length of the perforated shear wall is defined by hold-down location, H, as shown in Fig 7.10.

solution

1 The unit shear in the wall is 300 lb / ft (4380 N / m) (see Example 7.3)

2 Design variables:

The specific gravity adjustment factor (SGAF) is 0.92 (from Example 7.3).

The allowable shear capacities can be increased by 40% for wind loads, per section 2306.4.1 4 Alternatively, Table 7.2 can be used which implements the 40% increase The length of full-height segments is 10 ft (3.05 m) See note from Example 7.3 regarding the narrow full-height segments shown here Two items are needed for finding the shear capacity adjustment factor (SCAF):

a percent full height sheathed

b maximum opening height

The percent full-height sheathed is the length of the full-height segments divided

by the total length of wall ⫽ 10/24 ⫽ 0.42% The maximum opening height is 6

ft, 8 in From Table 7.6 or Section 2305.3.7 of the 2000 IBC the SCAF is 0.53 (conservative using 40% without interpolation).