DIAPHRAGMS AND SHEAR WALLS

Please download to get full document.

View again

of 36
13 views
PDF
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Document Description
DIAPHRAGMS AND SHEAR WALLS
Document Share
Document Tags
Document Transcript
  D IAPHRAGMSAND  S HEAR  W ALLS THE ENGINEEREDWOOD ASSOCIATION  APA DESIGN/CONSTRUCTION GUIDE   Wood is the right choice for a host of construction applications. It is theearth’s natural, energy efficient and renewable building material. Engineered wood is a better use of wood. It uses less wood to make more wood products. That’s why using APA trademarked I-joists, glued laminatedtimbers, laminated veneer lumber, plywood and oriented strand board is constructive ... for the environment, for innovative design, and for strong, durable buildings.  A few facts about wood. ■ We’re not running out of trees. One-third of the United States land base– 731million acres – iscovered 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.Mostof that harvestable acreage also is open to other uses, such ascamping, hiking, and hunting. Forests fully cover one-half of Canada’s 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. ■ We’re growing more wood every day.  American landowners plant more than twobillion trees every year. In addition, millions of trees seednaturally. The forest products industry, which comprises about 15percentof forestland ownership, is responsible for 41percent of replanted forestacreage. That works out to more than one billion trees a year, or aboutthreemillion trees planted every day. This high rate of replanting accounts for the fact thateach year, 27percent more timber is grown than is harvested. Canada’s replanting recordshows a fourfold increase in the number of trees planted between 1975 and 1990. ■  Manufacturing wood is energy efficient.  Wood products made up 47percent of allindustrial raw materials manufactured in theUnited States, yet consumed only 4 percentof the energy needed to manufacture allindustrial raw materials, according toa1987study. ■ Constructive news for a healthy planet. For every ton of wood grown, a young forest produces 1.07tons of oxygen and absorbs 1.47tons of carbondioxide. Wood. It’s the constructive choice for the environment. NOTICE: The recommendations inthis guide apply only to panels that bear the APAtrademark. Only panelsbearing the APA trademark are subject to the Association’s quality auditing program.  RA T E D  S H EA T H I N G E X P O S U R E  1  S I Z E D  F O R  S PA C I N G  3 2/ 1 6   1 5/ 3 2  I N C H  0 0 0  P S  1 - 9 5  C - D  P R P - 1 0 8  THE EN GINEERED W O OD  A S S O CI A TI ON  AP A    ©   2   0   0   1    A   P   A –   T   H   E   E   N   G   I   N   E   E   R   E   D   W   O   O   D   A   S   S   O   C   I   A   T   I   O   N   •   A   L   L   R   I   G   H   T   S   R   E   S   E   R   V   E   D .  •   A   N   Y   C   O   P   Y   I   N   G ,   M   O   D   I   F   I   C   A   T   I   O   N ,   D   I   S   T   R   I   B   U   T   I   O   N   O   R   O   T   H   E   R   U   S   E   O   F   T   H   I   S   P   U   B   L   I   C   A   T   I   O   N   O   T   H   E   R   T   H   A   N   A   S   E   X   P   R   E   S   S   L   Y   A   U   T   H   O   R   I   Z   E   D   B   Y   A   P   A   I   S   P   R   O   H   I   B   I   T   E   D   B   Y   T   H   E   U .   S .   C   O   P   Y   R   I   G   H   T   L   A   W   S . Be Constructive  WOOD Percent ofPercent of MaterialProductionEnergy Use  Wood474Steel2348 Aluminum28  CONTENTS Introduction . . . . . . . . . . . . . . .3Diaphragms and Shear Walls Defined . . . . . . . . .4 Advantages of Diaphragm Design . . . . . . . . . .6Rigid and FlexibleDiaphragms . . . . . . . . . . . . . . .9Design Example 1 . . . . . . . . . .10Design Example 2 . . . . . . . . . .18Design Example 3 . . . . . . . . . .24 Appendix A . . . . . . . . . . . . . . .29 Appendix B . . . . . . . . . . . . . . .30 Appendix C . . . . . . . . . . . . . . .33Diaphragm/Shear Wall Design References . . . . . . . . . .34 About APA . . . . . . . . . . . . . . . .35 hen designing a building for lateral loads such as those generated by wind or earth-quakes, a design engineer may have  several alternatives. Lateral loads may betransferred to the foundation via braced framesor rigid frames, diagonal rods or “x” bracing,including let-in bracing in the case of wood frame construction, or other methods. Where structural panels are used for the roof, floors, or  walls in a building, lateral loads can be accom-modated through the use of these ordinary  vertical load bearing elements. This type of construction is easily adaptable to conventionallight frame construction typically used in residences, apartment buildings and offices. The same concept is equally adaptable to larger warehouses and similar industrial or commercial buildings.Buildings can be designed to resist the horizontalloads introduced by the most violent wind or earthquake through the application of a principlecalled “diaphragm design.”This guide from APA – The Engineered Wood Association defines diaphragms and shear wallsand gives examples of how they can be incorpo-rated into building design. W  2001 APA - The Engineered Wood Association  4  with ordinary good construction prac-tice, any sheathed element in a building adds considerable strength to the struc-ture. Thus, if the walls and roofs aresheathed with panels and are adequately tied together, and to the foundation,many of the requirements of adiaphragm structure are met. This factexplains the durability of panel-sheathed buildings in hurricane and earthquakeconditions even when they have not been engineered as diaphragms. For fulldiaphragm design, it is necessary to alsoanalyze chord stresses, connections, andtie-downs.Panel diaphragms have been used exten-sively for roofs, walls, floors and parti-tions, for both new construction andrehabilitation of older buildings. A diaphragm acts in a manner analogous to a deep beam or girder, where the panels act as a “web,” resist-ing shear, while the diaphragm edgemembers perform the function of “flanges,” resisting bending stresses.These edge members are commonly called chords in diaphragm design, and may be joists, ledgers, trusses,  bond beams, studs, etc. A shear wall is simply a cantilevereddiaphragm to which load is applied atthe top of the wall, and is transmittedout along the bottom of the wall. Thiscreates a potential for overturning whichmust be accounted for, and any over-turning force is typically resisted by hold-downs, or tension ties, at each end of the shear element.Due to the great depth of mostdiaphragms in the direction parallel toapplication of load, and to their meansof assembly, their behavior differsslightly from that of the usual, relatively shallow, beam. Shear stresses have beenproven essentially uniform across thedepth of the diaphragm, rather thanshowing significant parabolic distribu-tion as in the web of a beam. Similarly,chords in a diaphragm carry all “flange”stresses acting in a simple tension andcompression, rather than sharing thesestresses significantly with the web. As inany beam, consideration must be givento bearing stiffeners, continuity of websand chords, and to web buckling,  which is normally resisted by the framing members.Diaphragms vary considerably in load-carrying capacity, depending on whether they are “blocked” or “unblocked.” Blocking consists of lightweight nailers, usually 2x4s, framed between the joists or other primary structural supports for the specific purpose of connecting the edges of thepanels. (See Figure 2.) Systems which DIAPHRAGMS  AND SHEAR  WALLS DEFINED  A diaphragm is a flat structural unitacting like a deep, thin beam. The term“diaphragm” is usually applied to roofsand floors. A shear wall, however, is a vertical, cantilevered diaphragm. A diaphragm structure results when aseries of such diaphragms are properly tied together to form a structural unit.(See Figure 1.) When diaphragms andshear walls are used in the lateral designof a building, the structural system istermed a “box system.” Shear wallsprovide reactions for the roof and floor diaphragms, and transmit the forces into the foundation. An accurate method for engineering diaphragms has evolved from extensivetesting, and will allow the engineer tosupply his client with a building resis-tant to hurricanes or earthquakes at very little extra cost.The structural design of buildings using diaphragms is a relatively simple,straightforward process if the engineer keeps in mind the over-all concept of structural diaphragm behavior. Actually,  2001 APA - The Engineered Wood Association  provide support framing at all paneledges, such as panelized roofs, are alsoconsidered blocked. The reason for  blocking in diaphragms is to allow con-nection of panels at all edges for better shear transfer. Another form of blocking for purposes of shear transfer is with acommon piece of sheet metal stapled toadjacent panels to provide shear transfer  between panels (see APA TechnicalNote: Stapled Sheet Metal Blocking for  APA Panel Diaphragms, Form N370).Unblocked diaphragm loads are con-trolled by buckling of unsupportedpanel edges, with the result that suchunits reach a maximum load above which increased nailing will not increasecapacity. For the same nail spacing,design loads on a blocked diaphragmare from 1-1/2 to 2times design loadsof its unblocked counterpart. In addi-tion, the maximum loads for which a blocked diaphragm can be designed aremany times greater than those for diaphragms without blocking.The three major parts of a diaphragmare the web, the chords, and the con-nections. Since the individual pieces of the web must be connected to form aunit; since the chord members in allprobability are not single pieces; since web and chords must be held so thatthey act together; and since the loadsmust have a path to other elements or to the foundation, connections arecritical to good diaphragm action. Their choice actually becomes a major part of the design procedure. 5 Full depth bridging(acts as blocking)Blocking (may also bepositioned flatwise) TC L b ww h  Wind load, F(lb per sq ft)Side wall carries loadto roof diaphragm at top,and to foundation at bottomRoof (horizontal diaphragm)carries load to end wallsEnd wall (vertical diaphragm or shear wall)carries load to foundationv (lb per lin ft of diaphragm width) =w (lb per lin ft of wall) = FT (lb) = C = vh vvv h2wL2b FIGURE 1 DISTRIBUTION OF LATERAL LOADS ON BUILDING FIGURE 2 BLOCKING  2001 APA - The Engineered Wood Association
Similar documents
View more...
Search Related
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks