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TRUSS-FRAMED CONSTRUCTION

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The recommendations contained in this manual arebased upon the collective experience of engineers andbuilders who are familiar with the Truss-FramedSystem. These suggestions do not cover all techniques of truss-framed construction. Nor do they

prescribe the only acceptable or preferred standard orpractice. The authors are solely responsible for theaccuracy of the statements and interpretations contained in this publication and such interpretations donot necessarily reflect the views of the Government.

Notwithstanding the paragraph immediately above,neither the U.S. Department of Agriculture nor theNAHB Research Foundation, Inc. assume any legalliability or responsibility for the accuracy or applicability of any information, methods or techniques inthis manual.

For sale by the NAHB Research Foundation, Inc.P.O. Box 1627, Rockville, MD 20850

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TRUSS-FRAMED CONSTRUCTION

A M A N U A L O F B A S I C P R A C T I C E

Prepared by NAHB Research Foundation, Inc.

in cooperation with

Forest Products Laboratory, Forest Service, U. S. Department of Agriculture

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ACKNOWLEDGEMENTS

The authors, Hila W. Anderson, P.E. of the NAHB ResearchFoundation, Inc., and Gunard E. Hans, R.A., of the U.S.Department of Agriculture, Forest Service, Forest ProductsLaboratory, gratefully acknowledge the technical assistanceprovided by the following key contributors:

Hugh D. Angleton, NAHB/RF

J ohn E . Meeks, P.E., Consulting Engineer

Russell C. Moody, P.E., FPL

Robert C. Stroh, Ph.D., NAHB/RF

Roger L. Tuomi, P.E., USFS

Appreciation is also extended to others in the NAHBResearch Foundation, the Forest Service, and other interested organizations who have served as reviewers of thismanuscript.

I llustrations by Laurence W. Mil ler

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PREFACE

The Truss-Framed System (TFS) described in this manual is an innovative wood-framed construction method developed by the Forest Products Laboratory for residential buildings.

The unitized-frame system provides for rapid and storm-resistant construction. This manual covers the basic details of design, fabrication and erection of the TFS, with sufficient detail to allow the designer, builder, and code official to evaluate and utilize the system. Information on specific matters regarding TFS construction can be obtained from Dr. Erwin L. Schaffer, P.E., State and Private Forestry, Forest Products Laboratory, P.O. Box 5130, Madison, WI 53705. Application forms for a USDA nonexclusive license to use this patented technology are available from: U.S. Department of Agriculture, S&E Administrative Services Division Chief, Program Agreements and Patents

Management Branch6505 Belcrest RoadRoom 524, Federal BuildingHyattsville, Maryland 20782

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CONTENTS

INTRODUCTION Builders' Evaluations Code Acceptances Use of This Manual

DESIGN Basic Design Design Methodology

Single-family Home Design Examples Multi

-family and Commercial Buildings

Non-rectangular Buildings Size Considerations

Variations From Basic Truss-Frame Partial Truss-Frames Split Truss-Frames Stacked Truss-Frames Integration With Conventional Framing

DETAILS Basic Framing

The Building Envelope Permanent Bracing Racking Panels Roof Sheathing Floor Sheathing

Page No.

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Interior Partitions

Roof Overhangs and Soffits

Cantilevered and Raised Trusses

Discontinuities and Openings

Interrupted Members

Door and Window Framing

Emergency Egress

Stairwell Framing

Fireplace Framing

Integration of Subsystems

Foundations

Elevated Foundations

Anchoring Truss-Frames

Firestops and Draftstops

Thermal Design

Mechanical Equipment Installations

CONSTRUCTION

Fabrication

Transportation

Handling and Storing

Erecting Truss-Frames

Placing

Aligning

Bracing

SUMMARY

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INTRODUCTION

The Truss-Framed System (TFS) is a new light-frame wood construction concept that integrates customary construction components-roof trusses, floor trusses, and wall studs--intounitized frames. It offers a new alternative inprefabrication and field assembly methods

without basic departures from establishedbuilding practices. It represents an engineeredbuilding system adaptable to a wide variety ofdesign requirements and construction procedures, as illustrated by this manual.

The TFS concept was developed by the ForestProducts Laboratory, Forest Service, USDA inMadison, Wisconsin, in response to the need foran economical, high-quality, and disaster-resistant framing system. A public patent, No.4,005,556 has been issued to Roger L. Tuomion this system and it is available to anyonewho wishes to make use of it. TFS evolvedfrom field observations that framing failurescommonly occurred at connections betweenfloor, walls, and roof. I t became apparent thatincreased continuity from the foundation tothe roof would lead to greater structuralintegrity without increased material requirements. In the TFS, continuity between individual framing members is developed byconnectors, such as metal truss plates orplywood gusset plates, capable of transmittingbending moment, shear, and axial forces.

Advantages offered by the TFS include savingsin both construction materials and time. Thesystem establishes consistent 24-inch spacingbetween frames and prevents a possible mix of16- and 24-inch spacing in floor, walls androof. Elimination of floor beams, interiorcolumns, and headers leads to further lumbersavings. Factory assembly of frames allowsmaximum utilization of short lengths and reduces waste or loss at the construction site.Rapid field assembly of prefabricated framesreduces open time and leads to earlier completion. Truss floor construction allows easyinstallation of utilities, and the floor cavitycan be used as a heating or cooling air supplyor return plenum.

Extensive experience building TFS houses inthe U.S. and in foreign countries has demon

strated the adaptability of the system tvarying design and construction requirements

Builders' Evaluations

Many builders and designers have expresseinterest in TFS construction. Reactions of thfirst builders who gained field experience wit

the system have varied with differences between their previous construction practices ansuccesses in solving the initial truss-framsupply problem.

David Skinner of EC-ON-ERGY Coporation in Tampa, Florida, reportsignificant cost savings over previouconstruction practices for his singlefamily detached and attached housesHe considers time, material, and supply cost savings. EC-ON-ERGYconstruction team of four workmecan erect four locked-in houses in twworking days.

Bill Pilgrim of Douglasville BuildinComponents, Inc., in DouglasvilleGeorgia, reports that they save abou$2,000 on a 1,200-square-foot houseThe company has put up four homeper day using a crew of 10. Thescost savings are comparable to th$2,300 difference in rough framinbids of the initial TFS demonstratio

house near Madison, Wisconsin.*

R. R. Patterson, Construction Manager for the Daniel Shelter SystemDivision of the Fortis Corporation iK ing, North Carolina, estimates, aftebuilding a prototype truss-framehouse, that if all areas of possibmaterial savings were employed ttheir best advantage the material foa truss-framed house would cost $34more than their conventional mode

They also recorded an additional $52in labor cost but added that "most othis additional cost is attributable t

*R.L . Tuomi, G.E. Hans, and D.J . Stith. 1978Fabrication, Transportation, and Erection othe Prototype Truss-Framed House, ForesProducts Laboratory, Forest Service, USDAP.O. Box 5130, Madison, WI 53705.

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the framing crew's unfamiliarity withthe truss-frame and the problem of

job-site conditions". The Fortis Corporation's study also notes that pro

jects designed around only threetruss-frame configurations to assurelarge production runs could yield fifteen or more different house designs.

Skinner initially fabricated his own frames,but now buys them from various trussmanufacturers. Pilgrim fabricates framesin his own truss plant. Patterson's analysisis based on purchased frames.

Savings associated with truss-framed construction will depend upon the degree towhich builders can adapt the TFS to theirdesigns and construction procedures. Alternate sources of supply may have to beexplored, most notably in the area of

windows, to avoid custom fabrication. Factors to consider in planning for cost-effective use of TFS include:

Locating a component designer andfabricator. Whereas initial truss-frame manufacturing cost may behigher because of a fabricator's unfamiliarity with the system and setupcharges, such costs may be reducedfor larger production runs.

Planning an efficient truss transportation and handling system. Largerframes may require special truck-loading, transporting, and handlingprocedures.

Training erection crews to achieveoptimal labor times and attain thedesired quality of workmanship.

Code Acceptances

Recommended TFS design and construction

procedures are based on established buildingcode requirements. TFS is a fully engineeredsystem and technical backup data attesting tothe validity of the analysis and design procedures are available. Where specific approvalhas been needed, builders have gained acceptance by local code officials. The U.S.Department of Housing and Urban Develop

ment has been reviewing requests from itsfield offices on an individual-case basis. I t issuggested that up-to-date information on codeacceptances be verified with the Forest Products Laboratory.

Use Of This Manual

The manual was prepared to familiarizebuilders with the TFS construction concept. Itis organized in three sections covering design,detailing, and construction aspects. The examples of design solutions and constructionprocedures are not intended to serve as specific guidelines, but rather as illustrations ofthe variety of options available for workingwith TFS.

The design section outlines structural analysisprocedures and variations from basic truss-frame configurations. The detail section il

lustrates adaptations of common carpentrydetails to TFS requirements. The constructionsection shows some of the handling methodsused by different builders, and calls attentionto special field considerations. Althoughwritten for the builder, the manual should alsobe valuable to others interested in TFS applications.

This is not intended to be an all-inclusiveresidential design manual. It covers only thosedesign and construction details that directly

interact with truss-framed construction. Itsformat is predicated on the assumption that

effective design and construction details common to conventional construction will also beapplied.

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DESIGN

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The tru ss-f r am ed bui ld ing system al lows fu l l coor d inat ion of engineer ing

an d ar chitectur al design obj ectiv es. Th e basic design pr ocedu r e is

described in this section in term s of engin eer ing design m ethodology,

ar chitectur al design exam ples, an d design v ar iat ions.

BASIC DESIGN

A truss-frame consists of a roof truss and afloor truss joined by exterior wall studs. Thewide variety of possible roof and floor trussdesigns and combinations is illustrated byfigure 1. End walls may be truss-framed withfield-assembled stud infill, prefabricated inconventional construction, or built on site.One builder of truss-framed houses has characterized the system in this way, "The Truss-Framed System uses smart engineering insteadof excess material to give me superior construction at lower cost".

Many TFS designs will use 2x4 and 2x3 mem

bers, but special spans or spacing situationsmay call for larger members. In high-velocitywind areas it may be necessary to use 2x6studs in truss-frames because available gradesof 2x4 stud materials may be overstressed atthe typical two-foot truss-frame spacing. Similarly, heavy snow loading, seismic loading, orflood plain location may require heavier members and/or framing connections. The TFS canbe used in advanced energy-conserving structures such as double-wall, box stud and envelope designs.

DESIGNMETHODOLOGY

The Truss-Framed System consists of engineered building components capable of providing superior structural integrity. Framesfor each application must be specifically designed. Structural design should follow recognized engineering methodology such asgiven by the Truss Plate Institute's TPI-78 andPCT-80 design specifications.* Appropriatedesign loadings must be selected to suit thespecific location and building usage. Loading

*Truss Plate Institute, Inc. 1978. TPI-78.Design Specifications for Metal Plate Connected Wood Trusses, Recommended DesignPractice, and 1980. PCT-80. Design specifications for Metal Plate Connected ParallelChord Wood Trusses. 100 West Church Street,Frederick, MD 21701.

conditions are the greater of either coderequirements or actual design load.

The Truss Plate I nstitute design methods arecommonly used by truss fabricators. For the

Truss-Framed System, design of the roof trussand floor truss portions of the truss-frameshould follow these methods and the wall studportion be designed with conventional engineering procedures (Figure 2). The designsolution provides data on required lumber sizesand grades. I t also identifies metal truss platerequirements at joints, which may vary between the plates available from differentmanufacturers. As a result, the truss specifications prepared for any given type of plate

are not applicable to assembly with otherplates.

An example of structural design output usingthe TPI methods for a one-story truss framewith typical residential loading is shown inFigure 4. The example shows lumber sizes andgrades that can be used for a truss frame ofthe given configuration on a 26-foot span.Sizes are not shown because they vary withtruss details and plate properties. As the TFSoffers considerable design flexibility, no singleconfiguration or truss depth is specifically

recommended. Member sizes and requiredlumber grades may change with any departurefrom this example. These factors must all beconsidered by the engineer in the structuraldesign.

In the development of the TFS, memberstresses and deflections were predicted by asophisticated modeling technique known as thePurdue Plane Structures Analyzer (PPSA)

Forest Products Laboratory, Purdue PlaneStructures Analyzer, A computerized WoodEngineering System, USDA Forest ServiceRes. Paper FPL 168, 1972. Forest ProductsLaboratory, Box 5130, Madison, Wis. 53705NOTE: The computer program is being revisedto include recent changes in design recommendations.

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Queen

H ip

Fink

Pratt

Warren

Canti levered

Second Floor

Howe

Mono

Spliced OverCenter Support

Canti levered

First Floor

Scissor

Dua l

Man sar d Tai led

Pratt

Off-center Spliced

In-line Joist

Concrete

Slab-on-grade

Figur e 1. Some optional tru ss-f r am e conf igur at ions. The upper end of

each stud is a m em ber of its r oof tr uss. Th e lower end of each stud

extends to t he l ower edge of all w ood fl oor t r usses or joists.

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Laboratory tests of full size truss frames showgood agreement with deflections predicted bythis computer method, which analyzes thecomplete truss frame as a unit without separating into components (Figure 3). PPSA,however, requires predetermination of memberproperties and other decisions on structuralperformance modeling; it may therefore, be of

only limited interest to most designers and oflimited application to typical design tasks.

As the TPI design methods disregard structuralcontinuity between the roof and floor truss

portions, questions have arisen regarding theeffect of neglecting this continuity. To address this concern, the truss frame shown inFigure 4 was analyzed by the PPSA methodusing two different assumptions: (a) separatefloor and roof trusses, and (b) complete frame.Calculated stresses varied by less than 5%between the TPI method and the two PPSA

analyses. Larger studs (such as 2x6) can havea greater effect on stress distribution intrusses, but these considerations must be resolved by design engineers on the basis ofbuilders' specifications.

Conventional engin eer ing

analysis

PCT-80 design

Figur e 2. Tr uss-f r am e breakdown for TPI analysis.

PPSA an al yzes th e

fu l l t r uss-f r am e as

a un i t

Figur e 3. T r uss-fr am e ana lysis by PPSA m eth od.

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ROOF TRUSS CHORDS SI ZE

1- 2 2X4 2- 4 2X4 4- 5 2X4 6- 7 2X4 7- 1 2X4

ALL WEBS 2X3

Si ze 2X4

CHORDS SI ZE 1- 2 2X4 2- 9 2X4 9- 10 2x4

10- 1 2X4 WEBS 2X3 9- 11 WEBS 2X3

LUMBER DESCRI PTI ON NO. 2 D. FI R- LARCH NO. 2 DENSE D. F. - LARCH NO. 2 DENSE D. F. - LARCH NO. 2 D. FI R- LARCH NO. 2 MC15 D. F. - LARCH NO. 3 D. FI R- LARCH

STUDS

DESI GN CRI TERI A TOP CH. LL = 30 PSF

DL = 10 PSF BOT CH. LL = 0 PSF

DL = 10 PSF TOTAL LOAD = 50 PSF SPACI NG = 24 I N. O. C. I NPUT DEFL. L/ 240 I NCREASE I N DESI GN VALUES = 15%

Lumber Descr i pt i on Desi gn Loadi ng NO. 2 D. FI R- LARCH 25 PSF ( BENDI NG)

ROOF- CEI LI NG DEAD + LI VE LOADS ( AXI AL) ASSUME FI XED ENDS

FLOORTRUSS LUMBER NO. 2 SEL. STR. NO. 2

SEL. STR. NO. 2

NO. 3

DESCRI PTI ON DESI GN CRI TERI A D. FI R- LARCH TOP CH. LL = 40 PSF MCl 5 D. F. - LARCH DL = 10 PSF D. FI R- LARCH BOT CH. LL = 0 PSF MC15 D. F. - LARCH DL = 5 PSF

TOTAL LOAD = 55 PSF SPACI NG = 24 I N. O. C.

D. FI R- LARCH

I NPUT DEFL. L/ 360

I NCREASE I N DESI GN D. FI R- LARCH

VALUES = 0% 2- 16 3- 16 3- 15 4- 15 4- 14 5- 14

6- 13 7- 13 7- 12 8- 12 8- 11

Figure 4. Typical loading and lumber data for one truss-frame.

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Laboratory tests of full size truss frames showgood agreement with deflections predicted bythis computer method, which analyzes thecomplete truss frame as a unit without separating into components (Figure 3). PPSA,however, requires predetermination of memberproperties and other decisions on structuralperformance modeling; it may therefore, be of

only limited interest to most designers and oflimited application to typical design tasks.

As the TPI design methods disregard structuralcontinuity between the roof and floor truss

portions, questions have arisen regarding theffect of neglecting this continuity. To adress this concern, the truss frame shown Figure 4 was analyzed by the PPSA methousing two different assumptions: (a) separatfloor and roof trusses, and (b) complete frameCalculated stresses varied by less than 5between the TPI method and the two PPS

analyses. Larger studs (such as 2x6) can hava greater effect on stress distribution trusses, but these considerations must be rsolved by design engineers on the basis builders' specifications.

TPI-78 design

convention al engi neer ing

analysis

PCT-80 desig n

Figure2. Truss-f r am e br eakdown f or TPI analys is

PPSA an al yz es th e

f u l l t russ-f r ame as

a un i t

Figu r e 3. Tr uss-fr am e analy sis by PPSA m eth od.

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ROOF TRUSS CHORDS SI ZE LUMBER DESCRI PTI ON

1- 2 2X4 NO. 2 D. FI R- LARCH

2- 4 2X4 NO. 2 DENSE D. F. - LARCH

4- 5 2X4 NO. 2 DENSE D. F. - LARCH

6- 7 2X4 NO. 2 D. FI R- LARCH7- 1 2X4 NO. 2 MC15 D. F. - LARCH

ALL WEBS 2X3 NO. 3 D. FI R- LARCH

STUDS

DESI GN CRI TERI A TOP CH. LL = 30 PSF

DL = 10 PSF BOT CH. LL = 0 PSF

DL = 10 PSF TOTAL LOAD = 50 PSF SPACI NG = 24 I N. O. C. I NPUT DEFL. L/ 240 I NCREASE I N DESI GN VALUES = 15%

S i z e Lumber Descr i pt i on Desi gn Loadi ng 2X4 NO. 2 D. FI R- LARCH 25 PSF ( BENDI NG)

ROOF- CEI LI NG DEAD + LI VE LOADS ( AXI AL) ASSUME FI XED ENDS

FLOOR TRUSS CHORDS SI ZE LUMBER

1- 2 2X4 NO. 22- 9 2X4 SEL. STR.

9- 10 2X4 NO. 2

10- 1 2X4 SEL. STR.

WEBS 2X3 NO. 2

9- 11

WEBS 2X3 NO. 3 2- 16 3- 16 3- 15 4- 15 4- 14 5- 14

6- 13 7- 13 7- 12 8- 12 8- 11

DESCRI PTI ON DESI GN CRI TERI A D. FI R- LARCH TOP CH. LL = 40 PSF MCl 5 D. F. - LARCH DL = 10 PSF D. FI R- LARCH BOT CH. LL = 0 PSF MC15 D. F. - LARCH DL = 5 PSF

TOTAL LOAD = 55 PSF SPACI NG = 24 I N. O. C.

D. FI R- LARCH I NPUT DEFL. L/ 360 I NCREASE I N DESI GN VALUES = 0%

D. FI R- LARCH

Figure 4. Typical loading and lumber data for one truss-frame.

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Single-FamilyHome Design Examples

To demonstrate the feasibility of TFS applications in various localities, preliminarystructural designs for three example houseswere analyzed.* These houses were located indifferent regions and design conditions weredetermined by local code requirements. Per

formance requirements were easily resolvedusing lumber species and grades likely to beavailable to local truss fabricators. Details of

these preliminary designs are not includedbecause the responsibility lies with each designer for (a) dimensioning the trusses, (b)selecting proper lumber species, grades andsizes, and (c) specifying the connectors tocarry the design loads required by local code.

*I llustrated in U.S. Forest Products Labora

tory. Truss Framed Systems. Forest ProductsLaboratory, Forest Service, USDA, P.O. Box5130, Madison, WI 53705.

Astoria (Ranch). The Astoria house (Figure 5) was adapted to themarket and local codes of the city of Astoria, Oregon. The original TFSdemonstration house in Arlington, Wisconsin is similar to this design.

The ranch house can be built with full truss-frames over basement or withpartial truss-frames on a concrete slab. The garage with its ridge runningin the direction shown is most easily framed by conventional methods, butmay be constructed with partial truss-frames by turning the ridge through90 degrees.

Figure 5a. Truss-frame for Astoria model.

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Figur e 5b. Astoria Design

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Bowling Green (Split-

foyer Bilevel). This bilevel or raised ranch model(Figure 6) was designed for Bowling Green, Kentucky. I ts eight-foot-widesplit-foyer entry is framed with three interrupted truss-frames, designedwith truncated floor trusses. The lower level end wall garage dooropening does not require a structural header if the end wall above isconstructed as a truss-frame with conventional stud fill-in.

Figur e 6a. Tr uss-f r am e for Bowl in g Green m odel .

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Figu r e 6b. Bowlin g Green Design .

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New Bedford (Two Story). This traditional New England model (Figure7) was designed to meet the code of New Bedford, Massachusetts. Thetwo floor trusses are combined into a single truss frame. Second floorstuds and roof form a partial truss frame. New Bedford loadingrequirements indicated 2x6 studs for the first story. Both first andsecond stories were designed with 2x6 studs to meet first story loadingconditions and to provide for R-19 batt insulation in the walls of bothstories.

Figure 7a. Truss-frame for New Bedford model.

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Figure 7b. New Bedford Design.

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Multi-Familyand Commercial Buildings

The Truss-Framed System is also readily adaptable to multi-family residential and lightcommercial buildings. It offers clear spansthat permit flexible floor plans. The floortrusses have adequate depth to accommodatewiring, plumbing, ducting or heating/coolingplenums. The structural integrity of the TFSis especially valuable to light commercialstructures.

Non-Rectangular Buildings

The truss-framed system is highly versatile inconforming to non-rectangular designs. Composite "L", "T", "U" and "H"building shapes areconfigured as readily as with conventionaltrusses. Non-rectangular truss-framed residences are shown in Figures 8 and 9.

I f an interior load bearing partition is notdesired beneath the roof intersection, a beamor an engineered girder truss can be used tosupport the roof trusses in the area of cutaway studs. Framing of the roof surface atthe point of intersection can be completed byconventional construction methods.

Figur e 8. An L-sha ped tr uss-f r am ed design.

(EC-ON-ERGY Cor p.)

Figu r e 9. A "polynesian"roof design

(EC-ON-ERGYCor p.)

Size Considerations

The TFS concept is not subject to any pscribed size limitations. On the other hanindividual truss configurations, manufacturifacilities, or transportation routes may effe

tively limit the maximum size of truss-fram

Transportation clearances may restrict tdesign of larger truss-frames while smalhouses may not require any further cosideration. For example, a truss-frame wthe following characteristics is under 14 fein height:

26-foot span

3 in 12 roof pitch

12-inch heel joint clearance forinsulation

7'6" finished ceiling height (aconventional rough opening heightis 8'1-1/8").

This truss can be loaded flat for transportation 14-foot maximum-width roads.

Where size limitations are encountered, optioinclude:

Using alternate truss designs to iprove structural efficiency.

Using upgraded lumber species agrades to achieve increased spans.

Increasing lumber dimensions to improve structural characteristics.

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Decreasing ceiling heights to improveroad clearances, to improve energyconservation, and to permit use oflower-grade studs.

Using reduced roof slopes to avoidroad clearance problems.

Fabricating truss-frames as sub-assemblies when they exceed the dimensions of manufacturing facilitiesand then assembling them either atthe factory or on site.

Using lower-profile hip truss-frames.I f a traditional peaked roof appearance is desired, triangular roof-peakelements can be added on site (F igure1).

VARIATIONS FROM BASIC TRUSSFRAME

Some of the suggested design options requirevariations from the basic one-story truss-frameconfiguration. Such adaptations include combinations of full and partial truss-frames anduse of conventional framing techniques alongwith truss-framed components.

Partial Truss-Frames

The TF S lends itself to production of separatetruss-frame elements and assembly of such

elements in the field. Partial truss-frames canbe manufactured with one or more elementsmissing (Figure 10). This may be done forseveral reasons:

Floor trusses may be omitted forbuilding on concrete floor slabs orconventionally built decks.

Any element may be site installed tofacilitate handling and transportation.As previously noted, site assemblymust be supervised or stipulated bythe truss manufacturer.

Multistory truss-framed buildings maybe framed using partial truss framesin the upper stories (Figure 10). Thismethod is described under Stacked

Truss-frames.

Conventional lumber or manufacturedfloor joists or off-center spliced

joists* may be mated with partialtruss frames where clearspan floorconstruction is not desired.

Split Truss-Frames

Truss-frames can also be factory-built in sections and assembled on site with truss fabricator assistance, (Figure 10). These splittruss-frames may simplify transporting andhandling tasks for wide-span or high-slopetruss-frames. On-site assembly normally requires a portable hydraulic press unit to embedthe toothed connector plates in position. Suchpresses can sometimes be site-furnished onloan from the truss manufacturer. Whenproperly designed, nailed metal plates or glue-nailed plywood plates are also structurally

acceptable for on-site assembly.

The field assembly crew must be trained inassembly procedures to avoid incorrect location of truss plates or handling stresses thatcould endanger the integrity of plate connections. Aligning fixtures should be used toassure precise alignment of elements beingassembled.

Stacked Truss Frames

Multistory truss frames are restricted in sizeby transportation and handling limitations.

They are, therefore, manufactured in sectionsand stacked during the erection process. I t isquite possible to site-assemble multistorytruss-frames. However, practical considerations favor an assembly technique similar toconventional multistory platform framing.

This starts with a full truss-frame composed ofa ground-floor truss, studs, and a second-floortruss. After erection of the truss-frames andsheathing of the floors, the upper story isframed with partial truss-frames consisting ofwall studs and roof trusses as shown in themultistory illustration of F igure 10. Stacking

*See NAHB. 1981. Off-Center Spliced FloorJ oists. Research Report No. 4. NAH B Publication Sales, 15th & M. Streets N.W.,Washington, D C 20005.

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Upper Pa r t i a l

M u l t i s t o r y

Pa r t i a l On Slab

L ower Pa r t i a l

Si t e -A ssem bl ed

Si t e -A ssem bl ed

Figure 10. Part ia l truss-frames.

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multistory truss-frames simplifies transportation, facilitates erection and provides thedesired structural integrity. Some precautionsshould be observed: the stacked truss-framestuds must be aligned vertically and the partialtruss-frame must be securely connectedthrough the sole plate to underlying structuresby nailing plates, straps, or structural sheathing or siding.

Integration With Conventional Framing

Some irregular framing conditions are moreeasily resolved by conventional construction inthe field rather than by prefabrication. Onesuch condition is the intersection of roofs innon-rectangular buildings, as mentioned previously. Another condition is il lustrated byentry-floor-level adaptations.

The basic truss-frame often incorporates a 20

inch-high floor truss. This raises the floorlevel about 10 inches higher than would conventional lumber floor joists. This elevation issometimes reduced by excavating soil underthe crawl space and dropping truss-framesupport onto a ledge lower than the foundationtop. Cast-in-place concrete ledges or headerconcrete blocks may be used for this purpose.Clearance between soil and untreated lumbermust meet local code requirements for protection of the wood members. F igures 11(a)and 11(b) show some methods of stepping up tofloor level, using exterior or interior steps:

A .

Entry A, a conventional stoop.

Entry B, a deck that provides anarchitectural feature as well as araised entry.

Entry C, a platform entry.

Entry D, a shed extension of thetruss-framed roof to provide a groundlevel doorway and interior steps. Thiscan also serve as an air lock entry.

Entry E, an entry in a non-TFS section of the house, in this case thegarage structure. I t has a groundlevel doorway and interior steps.

Entry F, a split foyer.

Conventional stoop.

B. Deck entry.

F igu r e 11 (a ) . En t r y Var ia t ions.

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Entries D and E treat the floor elevation as anadvantage by featuring "raised" living rooms.

D. Shed extension en

C. Platform.

E. Garage-level entry.

F. Split foyer.

Figu r e 11(b). Ent r y Var iat ions.

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DETAILS

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This section covers construction details to be considered in the design oftruss-framed buildings. Organized in three subsections it discusses basicframing, discontinuities and openings, and integration of subsystems.

BASIC FRAMING

Although the TFS construction method is considerably different from conventional construction, the completed building frame is very

similar to a stick-built structure.

2x4 spacer , fi r e stop

and dr ywa l l backup

Steel o r Plyw ood

an c h o r p l a t e

2 x 2 d r y w a l l a n d

t r i m b a ck u p

2x6 space r ,

f l o o r e d g e

s u p p o r t

a n d f i r e

stop

A l t e r n a t e e n d c l i p d e t a i l

Fi g u r e 1 2 . Basi c bu i l d i n g she l l c ons t r u c t i o n .

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The Building Envelope

The shell of the TFS building is similar toconventional in-line construction for 24-inchspacing of framing members. The most notable departure from conventional framing asshown by Figures 12 and 13, is the absence oftop and bottom plates, which have been re

placed by multipurpose spacer blocks. Anumber of options are available for the typeand use of spacer blocks. They may be appliedwith clip angles or nailed in place (Figure 12).

The spacer block at the floor line may be builtup as shown; or two separate spacers may beused one for drywall backup and firestop inthe wall and the other as floor edge supportbetween trusses.

Bl o ck s or c li p s f o r d r yw a l l su p p o r t

Gab l e end w a l l

F i r e b l o c k i n g

Sub f l o o r suppo r t l e dger

Th i s tw o -st u d cor ne r i s i n su l a t ed f o r ener g ysav i n g . Mu l t i p u r po se b l o c k s show n i n F i gu r e

12 m ay a l so be used as sub f l o o r suppo r t s and

f i r est o p s o r a p l yw ood o r g y p s um boa r d d r ea t

st op m ay be app l i e d t o st u d s bel ow t he sub -

f l o o r l e d g er sim i l a r t o Fi g u r e 3 3 .

F i g u re 13 . End wa l l de t a i l .

Permanent Bracing

The structural designer must designate location, size, and attachments for all permanentbracing required in the truss-framed structure.* Truss-frames are plane structural components, and their design analyses assume thatevery truss member will remain in its assigned

position under load. Permanent bracing mustprovide adequate support to hold every trussmember in its design position, and to resistlateral forces due to wind or seismic loads.

Racking Panels

Truss-framed walls require code-approved bracing as do conventional stud walls, with thefollowing further qualification:

CAUTION: LET-IN DIAGONAL BRACINGOR LET-IN METAL BRACING WILL COMPROMISE THE STRUCTURAL INTEGRITYOF TRUSS-FRAMES. EDGES OF TRUSS-FRAME STUDS SHOULD NEITHER BENOTCHED NOR KERF CUT FOR BRACESOR MECHANICAL I NSTALLATIONS.

The essential point is that it would be unreasonable to design in the superior structuraintegrity of truss-framed construction, only to

degrade it by weakening the studs.

If shear panels are used as bracing, local codeapproved materials, locations, and nail patternsare to be followed. The most effective shearpanel is plywood, but other structural sheathing panels, board sheathings, or other materialsmay be code-acceptable as shown in Figures 14and 15. Flat metal X-bracing, shown in Figure16, is also permitted by many codes but wilnot provide the same rigidity and strength aswil l most structural sheathings. I f foam boardtype insulation is used for wall sheathing, the

racking panels frequently use a shear panel ofplywood under a layer of board insulation.

*See Truss Plate Institute. Inc. 1976. BracingWood Trusses: Commentary and Recommendations. BWT-76. 100 W. Church Street, F rederick, MD 21710.

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Figu r e 14. Sing le w all sidin g can benai led direc t ly t o studs

and s i l l .

Figu r e 15. If sheath ing panels ar eno t f u l l wa l l heigh t ,

backup edge blocking is

required at the jo in t by

some codes.

Figu r e 16. Flat m etal X-brac ing isaccept abl e un der som e codes

but struc tura l sheath ing

prov ides stronger and

st i f fer rack ing panels .

28

Edge b lock i n g

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Roof Sheathing

Any code-approved roof sheathing is adaptableto the TFS. Nailing patterns should becarefully followed as with any other roofstructural system. C-D INT plywood is recommended over 24-inch o.c. roof trusses. Theleft-hand number of the Identification I ndex in

the grade-trademark must be equal to, orhigher than, the truss-frame o.c. spacing, forexample 24/0, 32/16, etc. Plywood is assumedto be applied continuously across two or morespans and applied face grain across truss-frames. Interior grade plywood with exteriorglue should be specified for best durability.Exterior grade plywood should be used for theunderside of the roof deck exposed to theweather and for closed soffits. Diagonal boardsheathing, straight board sheathing, spacedboards, or other materials are also acceptable

under many codes.Floor Sheathing

Truss-framing can utilize any floor sheathingthat is code-approved for the truss-framespacing. The American Plywood Association'sGlued Floor System* is widely used for eithersingle-floor or two-layer-floor constructions.Details of sheathing edge support along the

F i g u r e 1 7 . F loo r shea th i n g i n s t a l l a t i o n .(Fo re st Pr oduc t s Labo r a t o r y )

*See APA Glued Floor System. U405, American Plywood Association, 1119 A Street, Tacoma, WA 98401.

outside walls are shown in Figures 12 and 13and an installation is shown in Figure 17.

Interior Partition

Interior partitions for the TFS are identical tthose installed in conventional wood framehouses with one exception. Because there ar

no top or sole plates in TFS exterior walls, thtie-in to the exterior wall may vary. Oncost-effective method is illustrated in Figur18.

2x4 m idspan support

Pa r t i t i o n end st u

Dr ywa l l c l l ip s o r wood backup c lea ts

F i g u r e 1 8 . T y i n g i n i n t e r i o r p a r t i t i o n s

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Roof Overhangs and Soffits

Roof overhangs used as passive solar designelements, weather protection features, or aesthetic expressions are as readily incorporatedinto the TFS as into conventional roof designs.Customary roof overhang and soffit details arefully adaptable to TFS construction. Figure 19shows three popular soffit designs. Occasionally, ledgers or nailer blocks may berequired to support the soffit returns. Gableoverhangs can be framed with conventionalladder panels.

Raised t russ wi th

i n t eg r a l so f f i t r et u r n .

Cantilevered and Raised Trusses

A truss is cantilevered when both top abottom chords extend beyond the point of trusupport. Roof trusses are frequently canlevered to "raise" the upper chord so adequate thickness of insulation can be stalled above the wall top plate. They are acantilevered for aesthetic effect in desigsuch as mansard roofs. Floor trusses acantilevered to provide overhanging floors shown in Figure 20. Examples of raised ro

trusses are shown in Figure 1.

Truss-frames can be designed for cantileverinLike conventional trusses, cantilevered trusframes require careful structural design minimize structural weaknesses and dimesional changes associated with variations service conditions.

Most raised or "energy" truss designs requthat an additional web member be brougdown to the bottom chord. This web carricompressive forces from the top chord, and

is imperative that the designer determine if will require lateral bracing.

I f cantilevered trusses are shipped as partiatruss-frames with studs to be fastened on sitethe bearing locations should be conspicuousltagged to avoid any possibility of reversal o

Ra i sed t r uss w i t h l ookou t

s o f f i t r et u r n .

S l op i ng

s o f f i t

displacement of bearing points.F i gu r e 19 . T r uss - f r ame so f f i t t r ea tmen t s .

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Up p e r f l o o r p a r t i a l t r u ss-f r am e

Bottom plat e and f loor sheath ing as

in conven t iona l p la t fo rm f raming .

Can t i l e v e r e d f l o o r t r u s s

L owe r f l o or f u l l t r u ss-f r am e

Figu r e 20 . Can t i l e ver ed t russes in

two - s t o r y cons t r u c t i o n .

DISCONTINUITIES AND OPENINGS

The typical 24-inch spacing of truss-framescannot accommodate all needed or desireddesign options. Wider spacing is needed for

stairs and doorways and, sometimes, for windows. Larger wall openings can be more easilyaccommodated in end walls, and stairways maybe positioned outside the truss-framed portionof the structure. Where discontinuities intruss-frames are necessary, their effects onstructural integrity can be minimized by proper design.

Interrupted Members

The basic TFS concept of identical trusframes standing in succession to form tsturdy framework of a structure seldom occuin real life. Interruptions in the symmetry the structure are often necessary for doo

windows, stairways and fireplaces. Theinterruptions can be accommodated by ttruss-framed system quite as readily as conventional stick-built systems.

Wall openings may be conventionally rouframed and roof framing at such discontinties can be filled out with conventional trusseSome builders prefer to erect the building shof identical truss frames and then cut studs foversized wall openings. In either case, trough framing of openings follows the sam

code-

accepted structural details as in convetional stud construction.

I f floor or roof trusses are modified, tdiscontinuity must be structurally designeExamples include provisions for stairwells alarge through-the-roof chimneys. I t should noted that truss-frames having truncattrusses will probably require temporary bracto facilitate handling and erecting.

Door and Window Framing

The general rules for layout of openings truss-framed houses are similar to those conventional 24-inch o.c. framing:

Locate wide door and window opings in the gable end walls, as Figure 21, if possible. Gable ends anon-loadbearing and require only nostructural framing as shown in Fure 22.

Maintain window horizontal dime

sions in truss-framed walls at

tween-the-studs (nominal 22 -inwidth, as shown in F igure 23, whfeasible. The 22 -inch windows often installed in a side-by-side serto form a picture window. Windomay also be surface-mounted outsthe studs, as shown in F igure 24.

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Locate wider-than-22-inch windowsand doors next to a stud whereverpractical. Rough-framing details forthese wider windows and doors intruss-framed houses can be identicalto details in conventional wood framing such as those shown in Figures 26and 27. Rough framing can be ei ther

prefabricated or site-

built. Any interrupted truss-frame members should

be tied in to the rough framing withmetal framing anchors, as shown inFigure 26, to avoid building in a"weak link"; if the members ar e tiedin with structural-grade sheathing(such as plywood or fiberboard structural or nail-base) or siding, the anchors ar e not necessary.

Install a sidelight adjacent to aninsulated hinged door in lieu of asliding glass door, for structural andenergy efficiency.

I t should be noted that if 22 -inch windowsare installed between the studs of truss-frames, load-bearing lintels or headers are notrequired. The head and sill of the roughwindow opening can each be formedby a singleflat 2x4 identical to firestop and spacer blocks,as shown in Figure 25.

Figur e 21. Wind ows in non-bear ing gableend wall (EC-ON-ERGYCor p .)

w ide win dows, doors or sl id ing g lass d

can be instaal led in ga ble end w al ls wcut t ing tr uss-f r am es and w i thout str uheaders.

Figur e 22. Fr am ing f or openings in end

Figur e 23. Between-the-studswindow i n s t a l l a t i o n .

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Figure 24. A sur face-m oun ted, between-the-stud s em er gency

egress window. (Wausau Metals Cor por at ion.)

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box header in an en

conserving design.

The insulated plywoo

Figure 25. Preparat ions for doors and

windows in side wal ls

(Forest Products Laboratory)

Figur e 27. Insulated plyw ood box

header for oversized

win dows or doors.

Emergency EgressTruss-Framed System "Engineered 24" (formerly "MOD 24") or other 24-inch-o.c. layoutmust conform with required emergency egresprovisions. Any between-the-studs window

Figur e 26. Convent ional header in adesignated as an emergency egress (most fre

t russ-f ramed wal l .quently a bedroom window) must be selected

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and specified to meet applicable code requirements. Factors to consider in emergencywindow egress include:

Determining rooms that require window egress under applicable codes.

Designating any specific egress win

dows.

Selecting between-the-studs windowsmeeting your code's clear-opening requirements. The sliding hinges ofsome casement window models reducethe clear opening width in open position, clear-throw hinges may be required to maintain the required egresswidth. Single-hung or double-hungwindows may have adequate width butinsufficient height of clear opening to

meet code requirements.

Using surface-mounted egress windows, as shown in F igure 24.

Cutting studs and installing widerthan-stud-space windows if necessary.

Stairwell Framing

Stairways may be oriented either parallel orperpendicular to the direction of the floorjoists. Parallel orientation, as shown in Figure28, requires fewer interruptions in the truss-frame layout.

In layouts requiring wider floor openings, itmay be more practical to support the floortruss header on posts. Where such supports arenot desired, the clear-span floor trusses alongthe opening (or trimmer trusses) can be designed for the increased loading. Anothercommon alternative is the use of doubletrusses along the opening; in TFS, one suchmember would be a full truss frame, and theother a separate floor truss. I n either case, thestructural adequacy of such trusses is assuredby engineering analysis for the required open

ing.

WARNING: TRUSS-FRAMEMEMBERSMUST NOT BE SITE-CUT IN ANY MANNER EXCEPT AS SPECIFIED BY THEDESIGNER.

In conventional construction, stairway openingsare framed with trimmer joists. In trussed-floor construction, such trimmer joists can beused but do not perform as structural members.

Figur e 28. Rough opening f or stai r way .

35

Second f loor f raming wi th par t ia l

truss-frames

wal l p la te

Lower level truss-

f rames

Fir esstop s

Non-st ructural double

t r immer jo i s t

Truss-frame desfor stai rway op

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Fireplace Framing

Prefabricated fireplaces can be installed without major structural interruptions. A zero-clearance fireplace fully projected into theroom, as shown in Figure 29, or in a cornerlocation will require no truss-frame cutting. Aflush or chase location requires cutting one or

more studs. Factory-built triple-wall decorative chimney packages permit ceiling and

roof penetrations between truss-frames, and achase installation can avoid such penetrationsaltogether.

Masonry fireplaces should be installed in endwalls. Fireplace designs that require trussinterruptions should be avoided, but if they areused, the truss frames must be engineered andmanufactured for the specific application.

INTEGRATION OF SUBSYSTEMS

All considerations in integration of subsystemsin conventional construction remain equallyapplicable to truss-frame construction. Suchconsiderations include anchoring of the woodframe to foundations, fire safety, thermalperformance, and mechanical equipment installations.

Foundations

The Truss-Framed System is highly adaptable

to different foundation types. Any substruc

ture that allows effective anchoring of sillplates or other secure tie-downs for the truss-frame may be used. The structural integrityof the foundation and its anchoring systemmust equal that of the truss-frames to avoidbuilding a weak link into an otherwise efficiently engineered structure.

Foundation systems that can be used with theTFS include:

Concrete or masonry walls

All-weather wood foundations

Post, pile, pole, pier, or concreteframe structures

Concrete slab-on-grade.

A ze r o -c l ea r ance f i r ep l a ce p r o j e ct ed i n t o

t h e room does no t r equ i r e cu t t i n g t r u s s-

f r am es. Ch im neys can penet r a t e t he w a l l

a s show n , o r t he cei l i n g and r oo f a s i n

m o r e t r a d i t i o n a l d esi g n .

Fi g u r e 29 . I n s t a l l a t i o n o f p r e f ab r i c a t e

f i r e p l a ce .

CAUTION: IT IS ESSENTI AL THAT EVERYSTEP OF FOUNDATION CONSTRUCTIONFROM LAYOUT TO FINAL SILL INSTALLATION BE PRECISELY MEASURED TOENSURE THAT THE FOUNDATION ISSQUARELY AND ACCURATELY BUILT.

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T ie-downanchors

Figur e 30. Tr uss-

f r am e assem bly on Spl i t r ing or sp

suppor t beam

concrete fr am e. (Forest Pr oducts Foun dat ion pale gr id conn ectorLaboratory )

Elevated Foundations

The TFS offers unusual advantages in houseconstruction on concrete frame or wood polefoundations in hurricane zones and slopingsites. The TFS allows the assembly of allrough framing in one simple step. Access fromgrade level to the floor line on such sites may

be difficult. A truss-framed house on aconcrete subframe is shown in Figure 30. The

subframe, also could have been built in woodpole construction as in Figure 31. Suchconstruction requires consideration of a fewfurther factors.

Truss-frames must be anchored to their supporting beams with designer-specified connectors. Truss-frames are commonly assembled with hot dipped galvanized truss plates.For damp salt-air environments, metal connectors may require added protection from

corrosion. Exposed truss plates in ocean-frontareas can be coated with epoxy resin tofurther improve long-term corrosion resistance. Extreme exposures combining dampconditions with ammoniacal copper arsenate(ACA) or chromated copper arsenate (CCA)treated wood may call for more costly stainless steel plates.

Figur e 31. Tr uss-f r am es on pole

foundat ion .

Anchoring Truss-Frames

The method of anchoring a structure to itsfoundation is a potential weak link in anysystem, including the TFS. In most TFSstructures, a sill plate can be anchored to thefoundation, then each truss-frame securely

anchored to the sill plate. Sills are conven

tionally tied down by threaded anchor boltsembedded in the foundation wall. Straps mayalso be placed in concrete for securing sills.Such fasteners should extend at least 7 inchesinto cast concrete and 12 to 18 inches intomasonry block walls or bond beams. Othersill-to-foundation anchors include caulking anchors, expansion anchors, and powder-actuatedstuds, although some of these may not complywith applicable codes. Anchorage of truss-frames to a concrete beam without a sill plateis shown in Figure 32.

In the case of the All-Weather Wood Foundation, the foundation's cap plate takes theplace of a sill plate. The cap plate must bewell anchored to the wall below, usually byplywood sheathing.

Perhaps the most effective method of tying a

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truss-frame to a wood foundation or sill plateis by a structurally acceptable sheathing orsiding such as plywood securely nailed to thesill and the truss-frame. Metal strapping ormetal framing anchors can also provide code-acceptable tie downs.

Toe-nail ing does not furnish adequate truss-

frame anchoring to the sill plate unless asupplementary anchor such as sheathing ormetal ties is applied. It would be deplorable totie down integrated truss-frames with a pooranchoring system.

Figu r e 32 . Tr uss-fr am es secur ed to a

concrete f rame foundat ion wi thstr ap an chor s. (For est Pr odu cts

Laboratory )

Partial truss-frames applied to slabs-on-gradeor to conventional wooden decks, as in upperstory construction, also need adequate anchorage. Again, this is usually provided by code-accepted sheathing or siding properly nailed toboth sills and studs. An acceptable alternateis the use of metal straps or framing anchors.

Firestops and Draftstops

Building codes vary in their requirements forfirestopping and draftstopping in concealedspaces within a building. Model codes andlocal codes are being updated to consider newconstruction techniques. Both modes of fireblocking are intended to limit the spread of afire through structure cavities to protect an

escape route for inhabitants and to permsafer access for firefighters. The NationaForest Products Association (NFoPA) deveoped recommended fire-blocking practices tbe considered in updating the model buildincodes*. NFoPA recommendations may be usein the absence of more detailed applicablcode requirements.

Firestopping and draftstopping limit the spreaof fire by preventing the movement of flamehot gases and smoke to other areas of thbuilding:

Firestops limit movement through relatively small concealed passages sucas under stairs and inside walls. Firestopping material may consist of aleast 2" nominal lumber, two thicknesses of 1" nominal lumber wit

broken lap joints, or 3/4" plywood, oother approved materials.

Draftstops limit movement througlarge concealed passages such aopen-web floor trusses. Draftstoppinmaterial may consist of at least 1/2gypsumboard, 3/8" plywood, or otheapproved materials usually applied parallel to the main framing members

Firestopping is required at both ceiling anfloor levels in concealed spaces of stud wall

In the TFS it is usually provided by the uppeand lower spacer blocks, as shown in Figure 1

Draftstopping is recommended in concealefloor or ceiling cavities parallel to the flootrusses, as shown in Figure 33. At least ondraftstop is recommended for each floor oceiling truss cavity. In a large house, thisolated cavities should not exceed areas alowed by the applicable code or NFoPA rcommendations.

In multi-

family dwellings, the NFoPA recommends that, unless approved sprinklers ainstalled, draftstops should be provided in floo

*Improved Fire Safety: Design of Firestoppinand Draftstopping for Concealed Spaces. National Forest Products Association. 1619 Masachusetts Avenue N.W., Washington, D20036.

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and ceiling cavities in line with party walls.Also, in attics, mansards, overhangs and otherconcealed roof spaces, draftstops should beprovided if the party wall does not extend tothe roof sheathing.

Draf ts top

Figure 33. Draftstop instal lat ion in a

floor truss cavity.

Thermal Design

The Truss-Framed System accomodates allpopular energy design features including:

Mineral wool or cellulose insulations,either blanket or loose fill.

Foam insulation board sheathing.

Infiltration barriers.

2x6 wall studs.

Double-wall, box-stud and envelopehouse designs.

Raised or "energy" trusses for fullceiling insulation.

Passive and active solar systems.

Insulation, vapor retarder and caulking application recommendations for TFS are similar tothose for conventional wood frame construction. The only notable - and a favorabledifference is in the application of under floorinsulation. In truss-framed construction, floorinsulation can be applied in the plane of thefloor truss bottom chord. Because side walls

of the truss cavity can also be insulated, thetruss cavity's mechanical installations are enclosed within the building's thermal envelope

This arrangement reduces energy losses fromheating/air conditioning ducts and hot watersystems. It also reduces the possibility ofrozen water pipes.

The TFS has fewer thermal bridges that shortcircuit heat flow than does conventional construction. Such thermal bridging members arerepresented by second top plates, let-in brace

and band joists, which are not needed in trussframed structures.

Mechanical Equipment Installations

The Truss-Framed System provides underfloochases for convenient mechanical installationsElectrical, plumbing, and HVAC componentsshould not be installed in exterior walls unlesabsolutely necessary. Installations usually canbe planned so as to require no cutting ostructural members. Fixture locations can bselected to provide clearance between struc

tural members and ducts or fittings such acloset flanges. Time-consuming joist dril ling inot required in the TFS because floor framinis of open web construction. Plumbing-supppipes in exterior walls are more vulnerable tfreezing and, are likely to create thermabridges. HVAC ducts in exterior walls are lessubject to heating (or cooling) losses to theoutside and also interrupt the insulation envelope.

Mechanical services can be distributed in sev

eral ways:

Floor trusses can be designed withduct races, as shown in Figure 34.

Open-web floor trusses can accommodate small ducts without designedin races. Trusses are closely spaced

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and, if the duct size is a close fit tothe web openings, it will be difficultto string lengths of ducting into thecavity. Temporary openings cansometimes be provided through theend walls to simplify the task.

Opening to accomm odate air du ct

A i r duc t

Fi g u r e 3 4 . A i r d u c t i n st a l l a t i o n i n a f l o o r t r u s s c a v i t y .

The floor-truss cavity can provide anideal HVAC underfloor plenum.* Itcan be used as a supply plenumcombined with conventional return air

ducting or as split supply/return airplenums.

Conventional dropped-soffit duct races or plenums can be used.

*See The Plen-Wood System - UnderfloorHeating/Cooling Method. Southern Forest Products Association, Box 52468, New Orleans, LA70152.

CAUTION: TRUSS-FRAME MEMBERSINCLUDING STUDS, MUST NOT BE CUOR NOTCHED WITHOUT SPECIFIC ENGINEERING INSTRUCTIONS FROM TH

TRUSS MANUFACTURER. ONL Y THSTUDS MAY BE DRILLED THROUGH THCENTER OF A NOMINAL 4-INCH OWIDER FACE WITH UP TO 1-INCH DIAMETERHOLES.

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CONSTRUCTION

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A significant key to the potential benefits offered by the truss-framedsystem is effective coordination of the fabrication, transportation, anderection stages. The advantages can be further enhanced by cooperativeeffort between the truss fabricator and the builder. This section discussesthe major considerations in fabricating and erecting truss-frames.

FABRICATION

Truss-frames should be fabricated by a truss

manufacturing plant. Criteria and proceduresshould follow Truss Plate Institute or similardesign and fabrication specifications includingquality-control requirements. I f the plant'sfabricating equipment cannot process full-height or full-size truss-frames, they can bemanufactured in sections and assembled afterward, either in the plant or on site. For self-help or isolated construction projects,* properly designed truss-frames with nail-gluedplywood gusset plates can be used.

Fabricators should not make substitutions for

specified connectors without approval by thedesign engineer. Even heavier gauge platescannot be routinely substituted, as they mayhave lower gripping values because of differentfeatures.

Typical truss-framed fabrication practices areshown in Figures 35 and 36.

Figu r e 35. Sett ing up tr uss-fr am e assem bly.(For est Pr oducts Labora tor y)

*Midwest Plan Service. 1981. Designs forGlued Trusses. Iowa State University, Ames,Iowa 50011

Figure 36. Handling truss-frames infabricatioin plant. (ForesProducts Laboratory)

TRANSPORTATION

Possible constraints on truss-framed confiration and size imposed by transportatfactors have already been discussed from designer's standpoint. Special consideratiof concern to the fabricator may include:

Loading truss-frames on trucks at angle to improve road clearances.

Obtaining greater road clearances secondary road routings.

Shipping truss-frames knocked-doto partial truss frames for final sembly on site. (In these instan

the field connection must be spefically designed and the field assebly supervised or stipulated by ttruss manufacturer.)

Trucking options for maintaining road cleances are shown in Figure 37.

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Aligning Figur e 43.be

Figu r e 42. Sett ing t r uss-fr am es w ith afork l i f t . (Douglasvi l le

Bui ldin g Component s, Inc.)

The task of aligning truss-frames is simple butit is essential that each step be executedcarefully. The first end wall is erected andexactly squared and anchored to the foundation. I t is plumbed in the center and at eachend, and braced to assure it is preciselyvertical. The plumbing operation requires aheavy-weight carpenters plumb bob; a spiritlevel is not sufficiently accurate. Temporary

braces can be adjusted and shimmed to holdthe end wall rigidly vertical. Adjustableframing braces can simplify the aligning task.The first truss-frame is also squared andplumbed.

NOTE: THE FIRST END WALL AND ITSADJ ACENT TRUSS-FRAME MUST BEPRECISEL Y SQUARED AND PLUMBED TOSERVE AS A GUIDE FOR THE REMAININGTRUSS-FRAMES.

Subsequently erected truss-frames are spacedand squared by means of precut spacer blocksinstalled near the top and bottom of each studas shown in Figure 12. The lower spacerblocks in F igure 12 are cut shorter than actualtruss-frame spacing to allow for the thicknessof truss plates and/or anchor plates. Matching

l ing tr uss-fr am es.

edges and corner of wall sheathing panels ishelpful in aligning truss-frame studs, but everytruss-frame should be checked for alignmentwith a carpenters spirit level. Every fourthtruss-frame erected should be plumbed at thecenter and at both walls to assure thataccurate alignment is being maintained, asshown in Figure 44. Flat metal shims may beapplied to correct spacing, or over- or underlength spacer blocks can be cut. I f nonstandard spacer blocks are precut, they shouldbe prominently marked to identify their different lengths.

Reusable spacing fixtures as shown in Figure

45 and 46 can assist in aligning floor trussewhile the floor sheathing is applied, howevethey should not be used as substitute foaccurate location marks along each sill plateSmall inaccuracies in location can be eithecompensating or cumulative. In the absence odirect layout marks, cumulative errors couldcreep in.

45

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Figure 45.

Figur e 44. A l ign ing and plumbing t russ-

fr am es. (For est Pr odu cts

Laboratory )

46

Figure 46. Al igning f loor trusses for

appl icat ion of sheathing.

(Forest Products Laboratory)

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Bracing

Temporary bracing serves two purposes:

to secure truss-frames in their designpositions until permanent bracing isapplied.

to prevent a potential erection dis

aster of plane structures - - dominocollapse. This possibility of progressive collapse must be prevented untilpermanent bracing is secured. Failureto do so could have embarrassing,even fatal results.

Temporary bracing starts with bracing the firstend wall. An example of temporary bracing isshown in Figure 47. Any brace that couldundergo compression must be constrained from

bowing laterally. Lateral buckling would reduce or destroy the compressive strength ofsuch members.

Figure 47. Latera l brac ing preventsla tera l buck l ing of ground

braces.

47

CAUTION: TEMPORARY BRACING MUSTBE CAPABLE OF RESTRAINING MOVEMENT IN ANY DIRECTION, AGAINST ANYWIND LOAD, WORKMAN AND EQUIPMENT LOAD, OR ACCIDENTAL IMPACTUNTIL THE PERMANENT BRACING IS INPLACE. PARTICULAR CARE MUST BE

EXERCISED IN LOOSENING TEMPORARYBRACING IN ORDER TO ADJ UST, RE

PLACE, OR INSTALL PERMANENT BRACING.

Permanent wall sheathing should be installedas soon as the first end wall and first truss

frame are aligned, as shown in Figure 49. Bythe time an end wall and two succeeding trussframes are wall-sheathed and anchored on bothsides, the sheathed frames are self-supportingThe wall sheathing should be applied to subsequent frames as they are erected. I t may be

advisable to provide temporary bracing between truss-frames before application of sheathing, as shown in Figure 48.

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igure 48. Temportary diagonal brace securestruss-frames unti l sheathing is

applied (EC-ON-ERGY Corp.)

Wall sheathing or single-layer siding must be:

Code approved material to developadequate racking strength.

Anchored to truss-frames and sillplates using code-specified nail spacing.

Permanent bracing, including wall sheathing orsiding, roof sheathing, floor sheathing and

lateral bracing, must be installed as specifiedby the structural designer. See the permanentbracing discussion in the DETAI LS section.

Figure 49. appl icat ion of wall sheathingfor bracing. (Forest Products

Laboratory .)

SUMMARY

From the standpoint of the completed assembly, there is little difference betweentruss-framed and conventionally-built light-frame wood houses. The TFS uses the same

components, such as roof or floor trusses, andis designed to the same code requirements asis conventional construction. The truss-framedhouse benefits from better structural performance attained with less material than a

comparable stick-built house assembled witheven the best workmanship. The feature mostappreciated by the builder and the home buyer,however, may be the acceleration of theconstruction process permitted by this method.As compared to panelized construction, the

TFS offers a new option in prefabrication forbuilders who want to improve their productionefficiency in the rough framing stage whilemaintaining full control over the finishingstages