TRUSS-FRAMED CONSTRUCTION
The rec recommendatio ndations ns containe ntained d in this manual nual are based upon the collective experience of engineers and builders builders who are famil famil iar wit with h the T russ-Framed System Syst em.. T hese suggesti suggestions ons do not cover cover all all tech niques of truss-frame fr amed d constr constructi uctio on. N or do the they prescribe the only acceptable or preferred standard or practice practi ce.. T he autho author s are sol sol el y responsi sponsible ble for for the t he accuracy of the statements and interpretations con tained in this publication and such interpretations do not necessarily reflect the views of the Government. Notwithstanding the paragraph immediately above, nei nei ther the U .S. Depart Departme ment nt of of Agric Agri cultur ul ture e nor the N A H B Re R esearch search F oundatio undati on, I nc. assume assume any lega legall l i abi abi l ity it y or re r esponsibi sponsibill ity it y for for the t he acc accurac ur acy y or appli appli cabil cabilii ty of of any informati in formatio on, meth metho ods or techni technique ques s in this thi s manual. manual.
F or sale sale by the N A H B Re R esearch search Fo F oundati undatio on, I nc. nc. P .O. B ox 16 1627 27,, Rockvi Rockvill l e, M D 20 2085 850 0
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 N AH B Re R esearch search Foundation, Foundation, Inc I nc.. in cooperation with F orest Produc P roducts ts L abo aborato rat ory, F or est Servi Service ce,, U. U . S. Depart Departme ment nt of of Agric Agri cultur ul ture e
ACKNOWLEDGEMENTS
The autho authors, H ila il a W. Anderso Anderson, n, P.E . of the NAH B Rese Research arch F oundati undatio on, Inc I nc., ., and and Gunard Gunard E. E . H ans, ans, R.A., R .A., of of the U.S. U .S. Depart Departme ment nt of of Agric Agri cultur ul ture e, F orest Service, Service, Fo F orest P roducts roducts L abor aboratory, gr gratefull ateful ly acknowledg acknowledge e the the techni technical cal assistance provided by the following key contributors: Hugh D. Angleton, NAHB/RF J ohn E . M eeks, P.E ., Cons Consulting ulting Eng E ngine inee er 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 NAHB Research Research Foundati F oundatio on, the Fo Forest est Se S ervice vice,, and other inter i nteres es ted organizati organizat ions who have ser served ved as reviewer reviewers of of thi th is manuscript.
I llustrations ll ustrations by L aurenc aurence e W. Mil M il ler
2
PREFACE
The Trus Tr uss s-Framed System (TFS) described in this manual is an innovative wood-framed const constrr uction ucti on meth method od devel devel oped oped by the th e F orest P roduc roducts ts L abo aboratory for r esidenti sidential al buildings buil dings.. The unitized unitized-frame system provides for rapid and storm-resistant construction. nstr uction. T his hi s 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. I nformation on spec speci fic fi c matter matter s regardi regarding ng T F S construction nstr uction can can be obtai obtaine ned d from Dr. Dr . E r win L . Schaffe Schaffer, r, P .E., .E ., State and P ri vate F orestr restry, y, F or est Produc P roducts ts Labo L aborat rato or y, P.O. P.O . B ox 513 5130, 0, M adison, WI 53 5370 705. 5. 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 M anage anagement ment Br anch anch 6505 Belcrest Road Room 524, Federal Building Hyattsville, Maryland 20782
3
CONTENTS Page No. 6
INTRODUCTION Builders' Evaluations
6
Code Acceptances
7
Use of This Manual
7 10
DESIGN Basic Design
10
Design Methodology
10
Single-family Home Design Examples
14
Multi -famil y and Commercial Bui ldings
20
Non-rectangular B uil dings
20
Size Considerations
20
Variations F rom Basic Truss-Frame
21
Partial Truss-F rames
21
Split Truss-Frames
21
Stacked Truss-Frames
21
I ntegration With Conventional F raming
23 26
DETAILS
26
Basic Framing
27
The B uilding Envelope Permanent Bracing
27
Racking Panels
27
Roof Sheathing
29
Floor Sheathing
29
4
Interior Partitions
29
Roof Overhangs and Soffits
30
Cantilevered and Raised Trusses
30 31
Discontinuities and Openings I nterrupted Members
31
Door and Window Framing
31
Emergency Egress
34
Stairwell Framing
35
Fireplace Framing
36 36
I ntegration of Subsystems Foundations
36
Elevated Foundations
37
Anchoring Truss-Frames
37
Firestops and Draftstops
38 39
Thermal Design Mechanical Equipment I nstallations
39 42
CONSTRUCTION
42
Fabrication
42
Transportation Handling and Storing
43
Erecting Truss-F rames
44
Placing
44
Aligning
45
Bracing
47 48
SUMMARY
5
strated the adaptability of the system to varying design and construction requirements.
INTRODUCTION The Tr uss-Framed System (TFS) is a new lightframe wood construction concept that inte grates customary construction components roof trusses, floor trusses, and wall studs--into unitized frames. I t offers a new alternative in prefabrication and field assembly methods without basic departures from established building practices. I t represents an engineered building system adaptable to a wide variety of design requirements and construction proce dures, as illustrated by this manual.
Builders' Evaluations
-
The TF S concept was developed by the Forest Products Laboratory, Forest Service, USDA in Madison, Wisconsin, i n response to the need for an economical, high-quality, and disaster-re sistant framing system. A public patent, No. 4,005,556 has been issued to Roger L . Tuomi on this system and it is available to anyone who wishes to make use of it. TFS evolved from field observations that framing failures commonly occurred at connections between floor, walls, and roof. I t became apparent that increased continuity from the foundation to the roof would lead to greater structural integrity without increased material require ments. I n the TFS, continuity between in dividual framing members is developed by connectors, such as metal truss plates or plywood gusset plates, capable of transmitting bending moment, shear, and axial forces. Advantages offered by the TFS include savings in both construction materials and time. The system establishes consistent 24-inch spacing between frames and prevents a possible mix of 16- and 24-inch spacing in floor, walls and roof. Elimination of floor beams, interior columns, and headers leads to further lumber savings. Factory assembly of frames all ows maximum utilization of short lengths and re duces waste or loss at the construction site. Rapid field assembly of prefabricated frames reduces open time and leads to earlier com pletion. Truss floor construction allows easy installation of utilities, and the floor cavity can be used as a heating or cooling air supply or return plenum.
Many builders and designers have expressed interest in TFS construction. Reactions of the first builders who gained field experience with the system have varied with differences be tween their previous construction practices and successes in solving the initial truss-frame supply problem. •
David Skinner of EC-ON -ERGY Cor poration in Tampa, Florida, reports significant cost savings over previous construction practices for his singlefamily detached and attached houses. He considers time, material, and sup ply cost savings. EC-ON -ERGY's construction team of four workmen can erect four locked-in houses in two working days.
•
Bill Pilgrim of Douglasville Building Components, I nc., in Douglasville, Georgia, reports that they save about $2,000 on a 1,200-square-foot house. The company has put up four homes per day using a crew of 10. These cost savings are comparable to the $2,300 difference in rough framing bids of the initial TFS demonstration house near Madison, Wisconsin.*
•
R. R. Patterson, Construction Man ager for the Daniel Shelter Systems Division of the Fortis Corporation in K ing, North Carolina, estimates, after building a prototype truss-framed house, that if all areas of possible material savings were employed to their best advantage the material for a truss-framed house would cost $349 more than their conventional model. They also recorded an additional $520 in labor cost but added that "most of this additional cost is attributable to
*R.L . Tuomi, G.E. Hans, and D.J . Stith. 1978. Fabri cation, Transportation, and Erection of the Prototype Truss-Framed House, Forest Products L aboratory, Forest Service, U SDA, P.O. Box 5130, Madison, WI 53705.
Extensive experience building TFS houses in the U.S. and in foreign countries has demon 6
the framing crew's unfamiliarity with the truss-frame and the problem of job-site conditions". The Fortis Cor poration's study also notes that pro jects designed around only three truss-frame configurations to assure large production runs could yield fif teen or more different house designs. Skinner initially fabricated his own frames, but now buys them from various truss manufacturers. Pilgrim fabricates frames in his own truss plant. Patterson's analysis is based on purchased frames.
ment has been reviewing requests from its field offices on an individual -case basis. I t is suggested that up-to-date information on code acceptances be verified with the Forest Pro ducts L aboratory. Use Of This Manual The manual was prepared to familiarize builders with the TF S construction concept. I t is organized in three sections covering design, detailing, and construction aspects. The ex amples of design solutions and construction procedures are not intended to serve as spec ific guidelines, but rather as illustrations of the variety of options available for working with TF S.
Savings associated with truss-framed con struction will depend upon the degree to which builders can adapt the TF S to their designs and construction procedures. Al The design section outlines structural analysis ternate sources of supply may have to be procedures and variations from basic trussexplored, most notably in the area of frame configurations. The detail section il lustrates adaptations of common carpentry windows, to avoid custom fabrication. Fac tors to consider in planning for cost- details to TFS requirements. The construction effective use of TFS include: section shows some of the handling methods used by different builders, and calls attention • L ocating a component designer and to special field considerations. Although fabricator. Whereas ini tial trusswritten for the builder, the manual should also frame manufacturing cost may be be valuable to others interested in TFS appli higher because of a fabricator's un cations. familiarity with the system and setup charges, such costs may be reduced This is not intended to be an all-inclusive for larger production runs. residential design manual. I t covers only those design and construction details that directly • Planning an efficient truss transpor interact with truss-framed construction. I ts tation and handling system. L arger format is predicated on the assumption that frames may require special truck- effective design and construction details com loading, transporting, and handling mon to conventional construction will also be procedures. applied. •
Training erection crews to achieve optimal labor times and attain the desired quality of workmanship.
Code Acceptances Recommended TFS design and construction procedures are based on established building code requirements. TFS is a fully engineered system and technical backup data attesting to the validity of the analysis and design pro cedures are available. Where specific approval has been needed, builders have gained accep tance by local code officials. The U.S. Department of Housing and Urban Develop 7
8
DESIGN
T h e t r u ss- f r a m ed b u i l d i n g sy st em a l l o w s f u l l co or d i n a t i o n o f en g i n e er i n g an d ar chitectur al design obj ectiv es. Th e basic design pr ocedu r e is d e sc r i b e d i n t h i s sec t i o n i n t er m s o f en g i n eer i n g d e si g n m et h o d o l o g y , a r c h i t e ct u r a l d e si g n ex a m p l e s, a n d d e si g n v a r i a t i o n s .
BASIC DESIGN A truss-frame consists of a roof truss and a floor truss joined by exterior wall studs. The wide variety of possible roof and floor truss designs and combinations is illustrated by figure 1. End walls may be truss-framed with field-assembled stud infill, prefabricated in conventional construction, or built on site. One builder of truss-framed houses has char acterized the system in this way, " The TrussFramed System uses smart engineering instead of excess material to give me superior con struction at lower cost ". Many TF S designs will use 2x4 and 2x3 mem bers, but special spans or spacing situations may call for larger members. I n high-velocity wind areas it may be necessary to use 2x6 studs in truss-frames because available grades of 2x4 stud materials may be overstressed at the typical two-foot truss-frame spacing. Sim ilarly, heavy snow loading, seismic loading, or flood plain location may require heavier mem bers and/or framing connections. The TF S can be used in advanced energy-conserving struc tures such as double-wall, box stud and enve lope designs. DESIGNMETHODOLOGY The Truss-Framed System consists of engi neered building components capable of pro viding superior structural integrity. Frames for each application must be specifically de signed. Structural design should follow re cognized engineering methodology such as given by the Truss Plate I nstitute's TP I -78 and PCT -80 design specifications.* Appropriate design loadings must be selected to suit the specific location and building usage. L oading *Truss Plate I nstitute, I nc. 1978. TPI -78. Design Specifications for Metal Plate Con nected Wood Trusses, Recommended Design Practice, and 1980. PCT-80. Design speci fications for Metal Plate Connected Parallel Chord Wood Trusses. 100 West Church Street, F rederick, M D 21701.
conditions are the greater of either code requirements or actual design load. The Truss Plate I nstitute design methods are commonly used by truss fabricators. For the Truss-Framed System, design of the roof truss and floor truss portions of the truss -frame should follow these methods and the wall stud portion be designed with conventional en gineering procedures (Figure 2). The design solution provides data on required lumber sizes and grades. I t also identifies metal truss plate requirements at joints, whi ch may vary be tween the plates available from different manufacturers. As a result, the truss speci fications prepared for any given type of plate are not applicable to assembly with other plates. An example of structural design output using the TPI methods for a one-story truss frame with typical residential loading is shown in Figure 4. The example shows lumber sizes and grades that can be used for a truss frame of the given configuration on a 26 -foot span. Sizes are not shown because they vary with truss details and plate properties. As the TFS offers considerable design flexibility, no single configuration or truss depth is specifically recommended. Member sizes and required lumber grades may change with any departure from thi s example. These factors must all be considered by the engineer in the structural design. I n the development of the TF S, member stresses and deflections were predicted by a sophisticated modeling technique known as the Purdue Plane Structures Analyzer (PPSA)†.
†F orest P roducts L aboratory, Purdue Plane Structures Analyzer, A computerized Wood Engineering System, USDA Forest Service Res. Paper F PL 168, 1972. Forest Products L aboratory, Box 5130, Madison, Wis. 53705. NOTE: The computer program is being revised to include recent changes in design recom mendations. 10
Queen
Howe
H i p
Scissor
Mono
Fink
Dual
M a n sa r d T a i l ed
Pratt Pratt
Warren
Cantilevered Second Floor
Spliced Over Center Support
Cantilevered First Floor
Off-center Spliced In-line Joist
Concrete Slab-on-grade
Fi g u r e 1 . So m e o p t i o n a l t r u ss- f r a m e co n f i g u r a t i o n s. T h e u p p e r en d o f ea c h st u d i s a m em b e r o f i t s r o o f t r u s s. T h e l o w e r e n d of ea c h st u d extends to t he l ower edge of all w ood fl oor t r usses or joists.
11
L aboratory tests of full size truss frames show good agreement with deflections predicted by this computer method, which analyzes the complete truss frame as a unit without sepa rating into components (Figure 3). PPSA, however, requires predetermination of member properties and other decisions on structural performance modeling; it may therefore, be of only limited interest to most designers and of limited application to typical design tasks. As the TPI design methods disregard structural continuity between the roof and floor truss
portions, questions have arisen regarding the effect of neglecting this continuity. To ad dress this concern, the truss frame shown in Figure 4 was analyzed by the PPSA method using two different assumptions: (a) separate floor and roof trusses, and (b) complete frame. Calculated stresses varied by less than 5% between the TP I method and the two PPSA analyses. L arger studs (such as 2x6) can have a greater effect on stress distribution in trusses, but these considerations must be re solved by design engineers on the basis of builders' specifications.
C o n v e n t i o n a l en g i n eer i n g analysis PCT-80 design
Fi g u r e 2 . T r u s s- f r a m e b r e a k d o w n f o r T PI a n a l y si s.
PPSA an al yzes th e f u l l t r u ss-f r a m e a s a unit
Fi g u r e 3 . T r u s s- f r a m e a n a l y si s b y PPSA m et h o d .
12
ROOF TRUSS CHORDS
SI ZE
LUMBER
DESCRI PTI ON
1- 2 2- 4 4- 5 6- 7 7- 1 ALL WEBS
2X4 2X4 2X4 2X4 2X4 2X3
NO. NO. NO. NO. NO. NO.
D. FI R- LARCH DENSE D. F. - LARCH DENSE D. F. - LARCH D. FI R- LARCH MC15 D. F. - LARCH D. FI R- LARCH
2 2 2 2 2 3
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%
STUDS Si 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 1- 2 2- 9 9- 10 10- 1 WEBS 9- 11
SI ZE 2X4 2X4 2x4 2X4 2X3
LUMBER NO. SEL. NO. SEL.
2 STR. 2 STR.
NO. 2
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 Figure 4.
DESCRI PTI ON
DESI GN CRI TERI A
D. FI R- LARCH MCl 5 D. F. - LARCH D. FI R- LARCH MC15 D. F. - LARCH
TOP CH. LL = 40 PSF DL = 10 PSF BOT CH. LL = 0 PSF DL = 5 PSF TOTAL LOAD = 55 PSF SPACI NG = 24 I N. O. C. I NPUT DEFL. L/ 360 I NCREASE I N DESI GN VALUES = 0%
D. FI R- LARCH D. FI R- LARCH
Typical loading and lumber data for one truss-frame. 13
L aboratory tests of full size truss frames show good agreement with deflections predicted by this computer method, which analyzes the complete truss frame as a unit without sepa rating into components (Figure 3). PPSA, however, requires predetermination of member properties and other decisions on structural performance modeling; it may therefore, be of only limited interest to most designers and of limited application to typical design tasks. As the TP I design methods disregard structural continuity between the roof and floor truss
portions, questions have arisen regarding the effect of neglecting this continuity. To ad dress this concern, the truss frame shown in Figure 4 was analyzed by the PPSA method using two different assumptions: (a) separate floor and roof trusses, and (b) complete frame. Calculated stresses varied by less than 5% between the TPI method and the two PPSA analyses. L arger studs (such as 2x6) can have a greater effect on stress distribution in trusses, but these considerations must be re solved by design engineers on the basis of builders' specifications.
TPI-78 design
c on v e n t i o n a l en g i n e er i n g analysis PCT-80 desig n
F i g u r e 2 .
T r u s s- f r a m e b r ea k d o w n f o r T P I a n a l y s i s
PPSA an al yz es th e a m e as a unit
f r f u l l t r u s s-
Fi g u r e 3 . T r u s s- f r a m e a n a l y si s b y PPSA m et h o d .
12
ROOF TRUSS CHORDS
SI ZE
LUMBER
DESCRI PTI ON
1- 2 2- 4 4- 5 6- 7 7- 1 ALL WEBS
2X4 2X4 2X4 2X4 2X4 2X3
NO. NO. NO. NO. NO. NO.
D. FI R- LARCH DENSE D. F. - LARCH DENSE D. F. - LARCH D. FI R- LARCH MC15 D. F. - LARCH D. FI R- LARCH
2 2 2 2 2 3
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%
STUDS Si 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
DESCRI PTI ON
DESI GN CRI TERI A
1- 2 2- 9 9- 10 10- 1
2X4 2X4 2X4 2X4
NO. 2 SEL. STR. NO. 2 SEL. STR.
D. FI R- LARCH MCl 5 D. F. - LARCH D. FI R- LARCH MC15 D. F. - LARCH
TOP CH.
WEBS 9- 11
2X3
NO. 2
D. FI R- LARCH
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
Figure 4.
D. FI R- LARCH
LL = 40 PSF DL = 10 PSF BOT CH. LL = 0 PSF DL = 5 PSF TOTAL LOAD = 55 PSF SPACI NG = 24 I N. O. C. I NPUT DEFL. L/ 360 I NCREASE I N DESI GN VALUES = 0%
Typical loading and lumber data for one truss-frame. 13
Single-FamilyH ome Design Examples To demonstrate the feasibility of TF S appli cations in various localities, preliminary structural designs for three example houses were analyzed.* These houses were located in different regions and design conditions were determined by local code requirements. Per formance requirements were easily resolved using lumber species and grades likely to be available to local truss fabricators. Details of
these preliminary designs are not included because the responsibility lies with each de signer for (a) dimensioning the trusses, (b) selecting proper lumber species, grades and sizes, and (c) specifying the connectors to carry the design loads required by local code. *I ll ustrated in U.S. F orest Products L abora tory. Truss Framed Systems. Forest Products L aboratory, Forest Service, USDA, P .O. Box 5130, Madison, WI 53705.
Astoria (Ranch). The Astori a house (Figure 5) was adapted to the market and local codes of the city of Astoria, Oregon. The ori ginal TFS demonstration house in Arlington, Wisconsin is similar to this design. The ranch house can be built with full truss -frames over basement or with partial truss-frames on a concrete slab. The garage with its ridge running in the direction shown is most easily framed by conventional methods, but may be constructed with partial truss -frames by turning the ridge through 90 degrees.
Figure
5a.
Truss-frame
14
for
Astoria
model.
Fi g u r e 5 b .
A s t o r i a D e si g n
15
Bowling Green (Split-foyer Bilevel). This bilevel or raised ranch model (Figure 6) was designed for Bowling Green, K entucky. I ts eight-foot-wide split-foyer entry is framed with three interrupted truss-frames, designed with truncated floor trusses. The lower level end wall garage door opening does not require a structural header if the end wall above is constructed as a truss-frame with conventional stud fill-in.
Fi g u r e 6 a .
T r u s s- f r a m e f o r B ow l i n g G r e en m o d el .
16
Fi g u r e 6 b .
Bo w l i n g G r e en D e si g n .
17
New Bedford (Two Story). This traditional N ew England model (Figure 7) was designed to meet the code of New Bedford, Massachusetts. The two floor trusses are combined into a single truss frame. Second floor studs and roof form a partial truss frame. New Bedford loading requirements indicated 2x6 studs for the first story. Both first and second stories were designed with 2x6 studs to meet first story loading conditions and to provide for R -19 batt insulation in the walls of both stories.
Figure 7a. Truss-frame for New Bedford model.
18
Figure 7b. New Bedford Design.
19
Multi-Familyand Commercial Buildings The Truss-Framed System is also readily adap table to multi -family residential and light commercial buildings. I t offers clear spans that permit flexible floor plans. The floor trusses have adequate depth to accommodate wiring, plumbing, ducting or heating/cooling plenums. The structural integrity of the TFS is especially valuable to light commercial structures.
Fi g u r e
I f an interior load bearing partition is not desired beneath the roof intersection, a beam or an engineered girder truss can be used to support the roof trusses in the area of cut away studs. Framing of the roof surface at the point of intersection can be completed by conventional construction methods.
A " polynesian " r o o f (EC O N ERGYCor p.)
design.
- -
Non-Rectangular Buildings The truss-framed system is highly versatile in conforming to non-rectangular designs. Com posite "L ", " T ", "U" and "H "building shapes are configured as readily as with conventional trusses. Non-rectangular truss-framed resi dences are shown in Figures 8 and 9.
9.
Size Considerations The TF S concept is not subject to any pre scri bed size li mitati ons. On the other hand, individual truss configurations, manufacturing facilities, or transportation routes may effec tively limit the maximum size of truss-frames. Transportation clearances may restrict the design of larger truss-frames while smaller houses may not require any further con sideration. For example, a truss-frame with the following characteristics is under 14 feet in height: 26-foot span 3 in 12 roof pitch 12-inch heel joint clearance for insulation 7'6" finished ceiling height (a conventional rough opening height is 8'1-1/8"). This truss can be loaded flat for transportation on 14-foot maximum-width roads. Where size limitations are encountered, options include:
Fi g u r e 8 .
A n L - sh a p e d t r u s s- f r a m ed d esi g n . - (EC O N - ERGY Cor p.)
20
•
Using alternate truss designs to im prove structural efficiency.
•
Using upgraded lumber species and grades to achieve increased spans.
•
I ncreasing lumber dimensions to im prove structural characteristics.
•
•
Decreasing ceiling heights to improve road clearances, to improve energy conservation, and to permit use of lower -grade studs.
•
Using reduced roof slopes to avoid road clearance problems.
•
Fabricating truss-frames as sub-as semblies when they exceed the di mensions of manufacturing facilities and then assembling them either at the factory or on site.
•
Using lower -profile hip truss-frames. I f a traditional peaked roof appear ance is desired, triangular roof -peak elements can be added on site (F igure 1).
VARIATI ONS FROM BASIC TR USSFRAME Some of the suggested design options require variations from the basic one-story truss-frame configuration. Such adaptations include com binations of full and partial truss-frames and use of conventional framing techniques along with truss-framed components.
Conventional lumber or manufactured floor joists or off -center spliced joists* may be mated with partial truss frames where clearspan floor construction is not desired.
Split Truss-Frames
Tr uss-frames can also be factory-built in sec tions and assembled on site with truss fabri cator assistance, (Figure 10). These split truss-frames may simplify transporting and handling tasks for wide-span or high -slope truss-frames. On-site assembly normally re quires a portable hydraulic press unit to embed the toothed connector plates in position. Such presses can sometimes be site-furnished on loan from the truss manufacturer. When properly designed, nailed metal plates or gluenailed plywood plates are also structurally acceptable for on-site assembly. The field assembly crew must be trained in assembly procedures to avoid incorrect loca tion of truss plates or handling stresses that could endanger the integrity of plate connec tions. Aligning fixtures should be used to assure precise alignment of elements being assembled.
Partial Truss-Frames
The TF S lends itself to production of separate truss-frame elements and assembly of such elements in the field. Partial truss-frames can be manufactured with one or more elements missing (Figure 10). This may be done for several reasons: •
Floor trusses may be omitted for building on concrete floor slabs or conventionally built decks.
•
Any element may be site installed to facilitate handling and transportation. As previously noted, site assembly must be supervised or stipulated by the truss manufacturer.
•
Multistory truss-framed buildings may be framed using partial truss frames in the upper stories (Figure 10). This method is described under Stacked Truss-frames.
Stacked Truss Frames
Multistory truss frames are restricted in size by transportation and handling limitations. They are, therefore, manufactured in sections and stacked duri ng the erection process. I t is quite possible to site-assemble multistory truss-frames. However, practical considera tions favor an assembly technique similar to conventional multi story platform framing. This starts with a full truss-frame composed of a ground-floor truss, studs, and a second-floor truss. After erection of the truss-frames and sheathing of the floors, the upper story is framed with partial truss-frames consisting of wall studs and roof trusses as shown in the multistory illustrati on of F igure 10. Stacking *See NAH B. 1981. Off -Center Spliced Floor J oists. Research Report No. 4. NAH B Pub lication Sales, 15th & M. Streets N.W., Washington, D C 20005.
21
Si t e -A ss em b l e d
U p p er Pa r t i a l
Multistory
Pa r t i a l O n Sl a b
L o w er P a r t i a l Si t e -A sse m b l e d
Figure
10.
Partial
22
truss-frames.
multistory truss-frames simplifies transporta tion, facilitates erection and provides the desired structural integrity. Some precautions should be observed: the stacked truss-frame studs must be aligned vertically and the partial truss-frame must be securely connected through the sole plate to underlying structures by nailing plates, straps, or structural sheath ing or siding.
•
Entry A, a conventional stoop.
•
Entry B, a deck that provides an architectural feature as well as a raised entry.
•
Entry C, a platform entry.
•
Entry D, a shed extension of the truss-framed roof to provide a ground level doorway and interior steps. This can also serve as an air lock entry.
•
Entry E, an entry in a non- TF S sec tion of the house, in this case the garage structure. I t has a ground level doorway and interior steps.
•
Entry F, a split foyer.
Integration With Conventional Framing
Some irregular framing conditions are more easily resolved by conventional construction in the field rather than by prefabrication. One such condition is the intersection of roofs in non-rectangular buildings, as mentioned pre viously. Another conditi on is illustrated by entry-floor -level adaptations. The basic truss-frame often incorporates a 20 inch-high floor truss. This raises the floor level about 10 inches higher than would con ventional lumber floor joists. This elevation is sometimes reduced by excavating soil under the crawl space and dropping truss -frame support onto a ledge lower than the foundation top. Cast-in -place concrete ledges or header concrete blocks may be used for this purpose. Clearance between soil and untreated lumber must meet local code requirements for pro tection of the wood members. F igures 11(a) and 11(b) show some methods of stepping up to floor level, using exterior or interior steps: A.
B.
Conventional
Deck
entry.
Figu r e 11 (a).
23
stoop.
En t r y
V a r i a t i on s.
Entries D and E treat the floor elevation as an advantage by featuring "raised" living rooms.
D. Shed extension entry.
E.
C.
Garage-level
entry.
Platform.
F.
Split
Figu r e 11(b). Ent r y Var iat ions.
24
foyer.
DETAILS
This section covers construction details to be considered in the design of truss-framed buildings. Organized in three subsections it discusses basic framing, discontinuities and openings, and integration of subsystems. BASIC FRAMING
Although the TFS construction method is con siderably different from conventional construc tion, the completed building frame is very similar to a stick -built structure.
2x4 spacer , fi r e stop a n d d r y w a l l b a ck u p
St e el o r anchor plate
Pl y w o o d
2x2 dryw all and t r i m b a ck u p
2 x 6 sp a c e r , floor edge support and fire stop
Alternate end clip detail
Fi g u r e 1 2 .
Ba si c b u i l d i n g sh e l l c on s t r u c t i o n .
26
Permanent Bracing
The Building Envelope
The shell of the TF S building is similar to The structural designer must designate loca conventional in-line construction for 24-inch tion, size, and attachments for all permanent spacing of framing members. The most not bracing required in the truss-framed struc able departure from conventional framing as ture.* Truss-frames are plane structural com shown by F igures 12 and 13, is the absence of ponents, and their design analyses assume that every truss member will remain in its assigned top and bottom plates, which have been re position under load. Permanent bracing must placed by multipurpose spacer blocks. A number of options are available for the type provide adequate support to hold every truss member in its design position, and to resist and use of spacer blocks. They may be applied with cli p angles or nailed in place (Figure 12). lateral forces due to wind or seismic loads. The spacer block at the floor line may be built Racking Panels up as shown; or two separate spacers may be used – one for drywall backup and firestop in the wall and the other as floor edge support Truss-framed walls require code-approved bra cing as do conventional stud walls, with the between trusses. following further qualification: Bl o ck s or c li p s f o r d r y w a l l su p p o r t
CAUTION: LET -I N DIAGONAL BRACIN G OR LE T -IN METAL BRACING WIL L COM PROMISE THE STRUCTURAL I NTEGRITY OF TRUSS-FRAM ES. EDGES OF TRUSSFRAME STUDS SHOULD NEITHER BE NOTCHE D NOR KE RF CUT F OR BRACES OR M ECHANICAL I NSTALL ATIONS.
G a b l e en d w a l l
Fireblocking
The essential point is that it would be un reasonable to design in the superior structural integrity of truss-framed construction, only to degrade it by weakening the studs. I f shear panels are used as bracing, local codeapproved materials, locations, and nail patterns are to be followed. The most effecti ve shear panel is plywood, but other structural sheath ing panels, board sheathings, or other materials may be code-acceptable as shown in Figures 14 and 15. Flat metal X-bracing, shown in Figure 16, is also permitted by many codes but will not provide the same rigidity and strength as Su b f l o o r s u p p o r t l e d g er wil l most structural sheathings. I f foam boardtype insulation is used for wall sheathing, the T h i s t w o -st u d c or n e r i s i n s u l a t ed f o r e n er g y racking panels frequently use a shear panel of sa v i n g . M u l t i p u r p o se b l o c k s sh o w n i n F i g u r e plywood under a layer of board insulation. 1 2 m a y a l so b e u s ed a s su b f l o o r s u p p o r t s a n d f i r est o p s o r a p l y w o od o r g y p s u m b o a r d d r ea t st o p m a y b e a p p l i e d t o st u d s b el o w t h e su b - *See Truss Plate I nstitute. I nc. 1976. Bracing f l o o r l e d g er si m i l a r t o Fi g u r e 3 3 . Wood Trusses: Commentary and Recommen Figure
13.
End
wall
detail.
dations. BWT -76. 100 W. Church Street, F re derick, MD 21710.
27
Fi g u r e
14.
Si n g l e w a l l si d i n g c a n b e n a i l e d d i r e c t l y t o s t u d s and sill.
Ed g e b l o c k i n g
Fi g u r e
15.
Fi g u r e
I f sh e a t h i n g p a n e l s a r e n o t f u l l w a l l h ei g h t , backup edge blocking is required at the joint by som e codes.
28
16.
Fl a t m e t a l X - b r a c i n g i s accept abl e un der som e codes but structural sheathing provides stronger and stiffer racking panels.
Roof Sheathing Any code-approved roof sheathing is adaptable to the TFS. Nailing patterns should be carefully followed as with any other roof structural system. C-D INT plywood is re commended over 24-inch o.c. roof trusses. The left-hand number of the I dentification I ndex in the grade-trademark must be equal to, or higher than, the truss-frame o.c. spacing, for example 24/0, 32/16, etc. Plywood is assumed to be applied continuously across two or more spans and applied face grain across trussframes. I nteri or grade plywood with exterior glue should be specified for best durability. Exterior grade plywood should be used for the underside of the roof deck exposed to the weather and for closed soffits. Diagonal board sheathing, straight board sheathing, spaced boards, or other materials are also acceptable under many codes.
outside walls are shown in Figures 12 and 13, and an installation is shown in Figure 17. Interior Partition I nterior partitions for the TFS are identical to those installed in conventional wood framed houses with one exception. Because there are no top or sole plates in TFS exterior walls, the tie-in to the exterior wall may vary. One cost-effective method is illustrated in Figure 18.
Floor Sheathing Truss-framing can utilize any floor sheathing that is code-approved for the truss-frame spacing. The American Plywood Association's Glued Floor System* is widely used for either single-floor or two-layer-floor constructions. Details of sheathing edge support along the
2 x 4 m i d sp a n s u p p o r t
Pa r t i t i o n e n d st u d
D r y w a l l c l l ip s o r w o o d b a c k u p c l ea t s
Figure
17.
Floor sheathing installation. (Fo r e st Pr o d u c t s L a b o r a t o r y ) Figure
*See APA Glued Floor System. U405, Ameri can Plywood Association, 1119 A Street, Ta coma, WA 98401. 29
18.
Tying
in
interior
partitions.
Roof Overhangs and Soffits Roof overhangs used as passive solar design elements, weather protection features, or aes thetic expressions are as readily incorporated into the TFS as into conventional roof designs. Customary roof overhang and soffit details are fully adaptable to TFS construction. Figure 19 shows three popular soffi t designs. Occa sionally, ledgers or nailer blocks may be required to support the soffit returns. Gable overhangs can be framed with conventional ladder panels. Raised truss with i n t e g r a l so f f i t r e t u r n .
Cantilevered and Raised Trusses A truss is cantilevered when both top and bottom chords extend beyond the point of truss support. Roof trusses are frequently canti levered to "raise" the upper chord so an adequate thickness of insulation can be in stalled above the wall top plate. They are also cantilevered for aesthetic effect in designs such as mansard roofs. Floor trusses are cantilevered to provide overhanging floors as shown in Figure 20. E xamples of raised roof trusses are shown in Figure 1.
R a i se d t r u s s w i t h l o o k o u t s o f f i t r et u r n .
Tr uss-frames can be designed for cantilevering. L ike conventi onal trusses, cantilevered trussframes require careful structural design to minimize structural weaknesses and dimen sional changes associated with variations in service conditions. Most raised or "energy" truss designs require that an additional web member be brought down to the bottom chord. This web carries compressive forces from the top chord, and it is imperative that the designer determine if it will require lateral bracing. Sloping soffit
Figure
19.
Truss-frame
soffit
treatments.
30
I f cantilevered trusses are shipped as parti al truss-frames with studs to be fastened on site, the bearing locations should be conspicuously tagged to avoid any possibility of reversal or displacement of bearing points.
Interrupted Members The basic TF S concept of identical trussframes standing in succession to form the sturdy framework of a structure seldom occurs in real life. I nterruptions in the symmetry of the structure are often necessary for doors, windows, stairways and fireplaces. These interruptions can be accommodated by the U p p e r f l o o r p a r t i a l t r u ss-f r a m e truss-framed system quite as readil y as by conventional stick-built systems.
Bo t t o m p l a t e a n d f l o o r sh e a t h i n g a s in conventional platform framing.
Cantilevered
floor
truss
L ow e r f l o or f u l l t r u ss-f r a m e
Wall openings may be conventionally rough framed and roof framing at such discontinui ties can be filled out with conventional trusses. Some builders prefer to erect the building shell of identical truss frames and then cut studs for oversized wall openings. I n either case, the rough framing of openings follows the same code-accepted structural details as in conven tional stud construction. I f floor or roof trusses are modified, the discontinuity must be structurally designed. Examples include provisions for stairwells and large through-the-roof chimneys. I t should be noted that truss-frames having truncated trusses will probably require temporary bracing to facilitate handling and erecting.
Door and Window Framing The general rules for layout of openings in truss-framed houses are similar to those in conventional 24-inch o.c. framing: Fi g u r e 2 0 .
C a n t i l e v er e d t r u sses i n two-story construction.
•
L ocate wide door and wi ndow openings in the gable end walls, as in Figure 21, if possible. Gable ends are non-loadbearing and require only non structural framing as shown in Fig ure 22.
•
Maintain window horizontal dimen sions in truss-framed walls at be tween-the-studs (nominal 22 ½ -inch) width, as shown in F igure 23, where feasible. The 22½ -inch windows are often installed in a side-by-side series to form a picture window. Windows may also be surface-mounted outside the studs, as shown in F igure 24.
DISCONTINUITIES AND OPENINGS The typical 24-inch spacing of truss-frames cannot accommodate all needed or desired design options. Wider spacing is needed for stairs and doorways and, sometimes, for win dows. L arger wall openings can be more easil y accommodated in end walls, and stairways may be positioned outside the truss-framed portion of the structure. Where discontinuities in truss-frames are necessary, their effects on structural integrity can be minimized by pro per design.
31
•
•
Locate wider -than-22½ - inc h windows and doors next t o a stud wherever practical. Rough -framing details for these wider windows and doors in truss - framed houses can be identical to details in conventional wood fram ing such as those shown in Figures 26 and 27. Rough framing can be ei th er Any in pr efabricated or site- built. terrupted truss - frame members should be tied in t o the rough framing with metal framing anchors, as shown in Figure 26 , to avoid building in a " weak link "; if th e members ar e tied in with structural - grade sheathing (such as plywood or fiberboard struc tural or nail- base) or siding, the an chors ar e not necessary.
w i d e w i n d o w s , d o o r s o r s l i d i n g g l a ss d oo r s c a n b e i n st a a l l e d i n g a b l e en d w a l l s w i t h o u t c u t t i n g t r u ss-f r a m es a n d w i t h o u t st r u c t u r a l headers. Fi g u r e 2 2 . Fr a m i n g f o r o p e n i n g s i n e n d w a l l s.
Install a sidelight adjacent to an insulated hinged door in lieu of a sliding glass door, for structural and energy efficiency.
I t should be noted that if 22 ½ - inch windows are installed between the studs of truss frames, load - bearing lintels or headers are not requir ed. The head and sill of th e rough window opening can each be formed by a single flat 2x4 identical to firestop and spacer blocks, as shown in Figure 25.
- Fi g u r e 2 1 . Wi n d o w s i n n o n b ea r i n g g a b l e end w all (EC O N ERGYCo r p .)
- -
Fi g u r e 2 3 .
32
Between-the-studs window i n s t a l l a t i o n .
Figure 24 .
A su r f a c e- m oun ted, between - t h e - stud s em er gency egress window. (Wausau Metals Cor por ation.)
33
The insulated plywood b o x h e a d er i n a n en e r g y conserving design.
Figure
25.
Preparations for doors and windows in side wal ls (Forest Products Laboratory)
Figur e 27.
Figur e 26.
Conventional t r u s s- f r a m e d
header in wall.
a
Insulated plyw ood box header for oversized w i n d o w s or d o o r s.
Emergency Egress Truss-Framed System "Engineered 24" (for merly "MOD 24") or other 24-inch-o.c. layouts must conform with required emergency egress provisions. Any between-the-studs window designated as an emergency egress (most fre quently a bedroom window) must be selected
34
and specified to meet applicable code require ments. Factors to consider in emergency window egress include:
•
Determining rooms that require win dow egress under applicable codes.
•
Designating any specific egress win dows.
•
Selecting between-the-studs windows meeting your code's clear -opening re quirements. The sliding hinges of some casement window models reduce the clear opening width in open posi tion, clear -throw hinges may be re quired to maintain the required egress width. Single-hung or double-hung windows may have adequate width but insufficient height of clear opening to meet code requirements.
•
Using surface-mounted egress win dows, as shown in F igure 24.
•
Cutting studs and installi ng wider than-stud-space windows if necessary. Second floor framing truss-frames
with
Stairwell Framing Stairways may be oriented either parallel or perpendicular to the direction of the floor joists. Parallel orientation, as shown in Figure 28, requires fewer interruptions in the trussframe layout. I n layouts requiring wider floor openings, it may be more practical to support the floor truss header on posts. Where such supports are not desired, the clear -span floor trusses along the opening (or trimmer trusses) can be de signed for the increased loading. Another common alternative is the use of double trusses along the opening; in TFS, one such member would be a full truss frame, and the other a separate floor truss. I n either case, the structural adequacy of such trusses is assured by engineering analysis for the required open ing. WARNING: TRUSS-FRAMEMEMBERS MU ST NOT BE SITE -CUT IN ANY MAN N ER EX CE PT AS SPECIFIED B Y TH E DESIGNER. I n conventional construction, stairway openings are framed with trimmer joists. I n trussedfloor construction, such trimmer joists can be used but do not perform as structural mem bers.
partial
Non-structural double trimmer joist
wall
plate
Lower level frames
Truss-frame designed for stairway opening
truss-
Fir esstop s
Fi g u r e
28.
R ou g h
o p en i n g
f or
st a i r w a y .
35
Fireplace Framing Prefabricated fireplaces can be installed with out major structural interruptions. A zero clearance fireplace fully projected into the room, as shown i n Figure 29, or in a corner location will require no truss -frame cutting. A flush or chase location requires cutting one or more studs. F actory -built triple-wall deco rative chimney packages permit ceiling and roof penetrations between truss -frames, and a chase installation can avoid such penetrations altogether. Masonry fireplaces should be installed in end walls. F ireplace designs that require truss interruptions should be avoided, but if they are used, the truss frames must be engineered and manufactured for the specific application.
INTEGRATION OF SUBSYSTEMS All considerations in integration of subsystems in conventional construction remain equally applicable to truss-frame construction. Such considerations include anchoring of the wood frame to foundations, fire safety, thermal performance, and mechanical equipment in stallations.
Foundations The Truss-F ramed System is highly adaptable to different foundation types. Any substruc ture that allows effective anchoring of sill plates or other secure tie-downs for the trussframe may be used. The structural integrity of the foundation and its anchoring system must equal that of the truss-frames to avoid building a weak link into an otherwise effi ciently engineered structure.
A z e r o -c l ea r a n c e f i r ep l a c e p r o j e ct e d i n t o t h e r o o m d o es n o t r eq u i r e c u t t i n g t r u s s- f r a m es. C h i m n e y s c a n p e n et r a t e t h e w a l l a s sh o w n , o r t h e c ei l i n g a n d r o o 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 2 9 .
F oundation systems that can be used with the TF S include: •
Concrete or masonry walls
•
All -weather wood foundations
•
Post, pile, pole, pier, or concrete frame structures
•
Installation fireplace.
of
pr efabr icated
CAUTION: IT IS ESSENTI AL THAT EVERY STEP OF FOUNDATION CONSTRUCTION FROM LAYOUT TO FINAL SILL INSTAL LATION BE PRECISELY MEASURED TO ENSURE THAT THE FOUNDATION IS SQUARELY AND ACCURATELY BUIL T.
Concrete slab-on-grade.
36
su p p o r t b ea m
T i e - downanchors
Figur e 30.
Tr uss - f r a m e a ssem b l y o n concrete fr am e. (Forest Pr oducts Laboratory)
Foun dat ion pale Figur e 31.
Elevated Foundations The TF S offers unusual advantages in house construction on concrete frame or wood pole foundations in hurricane zones and sloping sites. The TFS allows the assembly of all rough framing in one simple step. Access from grade level to the floor line on such sites may be difficult. A truss-framed house on a concrete subframe is shown in F igure 30. The subframe, also could have been built in wood pole construction as in Figure 31. Such construction requires consideration of a few further factors. Tr uss-frames must be anchored to their sup porting beams with designer-specified con nectors. Truss-frames are commonly assem bled with hot dipped galvanized truss plates. For damp salt -air environments, metal con nectors may require added protection from corrosion. E xposed truss plates in ocean-front areas can be coated with epoxy resin to further improve long-term corrosion resist ance. Extreme exposures combining damp conditions with ammoniacal copper arsenate (ACA) or chromated copper arsenate (CCA) treated wood may call for more costly stain less steel plates.
Sp l i t gr id
r i n g o r sp i k e conn ector
Tr uss - f r a m es o n p o l e foundation.
Anchoring Truss-Frames The method of anchoring a structure to its foundation is a potential weak link in any system, including the TFS. I n most TFS structures, a sill plate can be anchored to the foundation, then each truss-frame securely anchored to the sill plate. Sills are conven tionally tied down by threaded anchor bolts embedded in the foundation wall. Straps may also be placed in concrete for securing sills. Such fasteners should extend at least 7 inches into cast concrete and 12 to 18 inches into masonry block walls or bond beams. Other sill -to-foundation anchors include caulking an chors, expansion anchors, and powder -actuated studs, although some of these may not comply with applicable codes. Anchorage of trussframes to a concrete beam without a sill plate is shown in Figure 32. I n the case of the All-Weather Wood F oun dation, the foundation's cap plate takes the place of a sill plate. The cap plate must be well anchored to the wall below, usually by plywood sheathing. Perhaps the most effective method of tying a
37
truss-frame to a wood foundation or sill plate is by a structurally acceptable sheathing or siding such as plywood securely nailed to the sill and the truss-frame. Metal strapping or metal framing anchors can also provide codeacceptable tie downs.
escape route for inhabitants and to permit safer access for firefighters. The National Forest Products Association (NFoPA) devel oped recommended fire-blocki ng practices to be considered in updating the model building codes*. NFoPA recommendations may be used in the absence of more detailed applicable code requirements.
Toe-nail ing does not furnish adequate trussframe anchoring to the sill plate unless a supplementary anchor such as sheathing or metal ties is applied. I t would be deplorable to tie down integrated truss-frames with a poor anchoring system.
Figu r e 32 .
Firestopping and draftstopping limit the spread of fire by preventing the movement of flame, hot gases and smoke to other areas of the building:
•
Fir estops limit movement through re latively small concealed passages such as under stairs and inside wall s. Firestopping material may consist of at least 2" nominal lumber, two thick nesses of 1" nominal lumber with broken lap joints, or 3/4" plywood, or other approved materials.
•
Draftstops limit movement through large concealed passages such as open-web floor trusses. Draftstopping material may consist of at least 1/2" gypsumboard, 3/8" plywood, or other approved materials usually applied pa rallel to the main framing members.
Firestopping is required at both ceiling and
Tr uss-fr am es secur ed to a floor levels in concealed spaces of stud walls. concrete frame foundation with I n the TF S it is usuall y provided by the upper str ap an chor s. (For est Pr odu cts and lower spacer blocks, as shown in Figure 13. Laboratory)
Draftstopping is recommended in concealed floor or ceiling cavities parallel to the floor trusses, as shown in Figure 33. At least one draftstop is recommended for each floor or ceiling truss cavity. I n a large house, the isolated cavities should not exceed areas al lowed by the applicable code or N FoPA re commendations.
Partial truss-frames applied to slabs-on-grade or to conventional wooden decks, as in upper story construction, also need adequate anchor age. Again, this is usually provided by codeaccepted sheathing or siding properly nailed to both sills and studs. An acceptable alternate is the use of metal straps or framing anchors.
In multi-family dwellings, the NFoPA recom mends that, unless approved sprinklers are installed, draftstops should be provided in floor
Firestops and Draftstops Building codes vary in their requirements for firestopping and draftstopping in concealed spaces within a building. Model codes and local codes are being updated to consider new constr uction techni ques. Both modes of fire blocking are intended to limit the spread of a fire through structure cavities to protect an
*I mproved F ire Safety: Design of Firestopping and Draftstopping for Concealed Spaces. Na tional F orest Products Association. 1619 Mas sachusetts Avenue N.W., Washington, DC 20036. 38
and ceiling cavities in line with party walls. Also, in attics, mansards, overhangs and other concealed roof spaces, draftstops should be provided if the party wall does not extend to the roof sheathing. Draftstop
I nsulati on, vapor retarder and caulking appli cation recommendations for TFS are similar to those for conventional wood frame construc tion. The only notable - and a favorable difference is in the application of under floor insulation. I n truss-framed construction, floor insulation can be applied in the plane of the floor truss bottom chord. Because side walls of the truss cavity can also be insulated, the truss cavity's mechanical installations are en closed within the building's thermal envelope. This arrangement reduces energy losses from heating/air conditioning ducts and hot water systems. I t also reduces the possibility of frozen water pipes. The TF S has fewer thermal bridges that short circuit heat flow than does conventional con struction. Such thermal bridging members are represented by second top plates, let -in braces and band joists, which are not needed in truss framed structures.
Mechanical Equipment Installations
Figure
33.
Draftstop installation floor truss cavity.
in
a
Thermal Design The Truss-Framed System accomodates all popular energy design features including:
The Truss-F ramed System provides underfl oor chases for convenient mechanical installations. Electrical, plumbing, and HVAC components should not be installed in exterior walls unless absolutely necessary. I nstallations usually can be planned so as to require no cutting of structural members. Fixture locations can be selected to provide clearance between struc tural members and ducts or fittings such as closet flanges. Time-consuming joist dril ling is not required in the TF S because floor framing is of open web construction. Plumbing -supply pipes in exterior walls are more vulnerable to freezing and, are likely to create thermal bridges. HVAC ducts in exterior walls are less subject to heating (or cooling) losses to the outside and also interrupt the insulation en velope.
•
Mineral wool or cellulose insulations, either blanket or loose fill.
•
Foam insulation board sheathing.
•
I nfiltration barriers.
•
2x6 wall studs.
•
Double-wall, box-stud and envelope house designs.
•
Floor trusses can be designed with duct races, as shown in Figure 34.
Raised or "energy" trusses for full ceiling insulation.
•
Open-web floor trusses can accom modate small ducts without designedin races. Trusses are closely spaced
•
•
Mechanical services can be distributed in sev eral ways:
Passive and active solar systems. 39
and, if the duct size is a close fit to the web openings, it will be difficult to string lengths of ducting into the cavity. Temporary openings can sometimes be provided through the end walls to simplify the task.
CAUTION: TRUSS-FRAME MEMBERS, INCLUDING STUDS, MUST NOT BE CUT OR NOTCHED WITHOUT SPECIFIC EN GINEERING INSTRUCTIONS FROM THE TRUSS MAN UF ACT URE R. ONL Y TH E STUDS MAY BE DRIL LE D THROUGH THE CENTER OF A NOMINAL 4-INCH OR WIDER FACE WITH UP TO 1-INCH DIA METERHOLES.
O p en i n g t o a c co m m o d a t e a i r d u c t
Air
duct
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 floor t r uss cavity.
•
The floor -truss cavity can provide an ideal HVAC underfloor plenum.* I t can be used as a supply plenum combined with conventional return air ducting or as split supply/return air plenums.
•
Conventional dropped-soffit duct ra ces or plenums can be used.
*See The Plen-Wood System - Underfloor Heating/Cooling Method. Southern F orest Pro ducts Association, Box 52468, N ew Orleans, L A 70152.
40
CONSTRUCTION
A significant key to the potential benefits offered by the truss -framed system is effective coordination of the fabrication, tr ansportation, and erection stages. The advantages can be further enhanced by cooperative effort between the truss fabricator and the builder. This section discusses the major considerations in fabricating and erecting truss -frames.
FABRICATION Truss-frames should be fabri cated by a truss manufacturing plant. Criteria and procedures should follow Truss Plate Institute or similar design and fabrication specifications including quality-control requirements. I f the plant's fabricating equipment cannot process full height or full -size truss-frames, they can be manufactured in sections and assembled after ward, either in the plant or on site. F or self help or isolated construction projects,* pro perly designed truss-frames with nail -glued plywood gusset plates can be used. Fabri cators should not make substitutions for specified connectors without approval by the design engineer. Even heavier gauge plates cannot be routinely substituted, as they may have lower gripping values because of different features.
Figure 36. Handling truss-frames in fabricatioin plant. (Forest Products Laboratory)
Typical truss-framed fabrication practices are shown in Figures 35 and 36.
TRANSPORTATION Possible constraints on truss -framed configu ration and size imposed by transportation factors have already been discussed from the designer's standpoint. Special considerations of concern to the fabricator may include:
Figu r e 35.
Setting up tr uss-fr am e assem bly. (For est Pr oducts Labora tor y)
•
L oading truss-frames on trucks at an angle to improve road clearances.
•
Obtaining greater road clearances for secondary road routings.
•
Shipping truss-frames knocked-down to partial truss frames for final as sembly on site. (I n these instances the field connection must be speci fically designed and the field assem bly supervised or stipulated by the truss manufacturer.)
Trucking options for maintaining road clear ances are shown in F igure 37.
*M idwest Plan Service. 1981. Designs for Glued Trusses. I owa State University, Ames, I owa 50011 42
HANDLING AND STORING Truss-frames can be awkward to handle and vulnerable to damage if handled incorrectly as indicated by Figure 38. They should be protected fr om excessive lateral deformation, which can lead to damaged joints or members. Manual handling requires a crew of at least five to cover intermediate lifting positions and to avoid undue distortion of the truss plane. Truss-frames should be unloaded and stored only on relatively flat areas free of obstruc tions to avoid distortion of joints, as shown in Figure 39. With sufficient care truss-frames can be handled and stored either lying hori zontally or standing in the vertical position.
For hori zontal storage, stacked truss-frames should be placed on enough supports to protect them from unsupported long spans and ground moisture. For vertical storage, they should be adequately blocked or braced to prevent top pling. I n either case, they should be covered for protection from the elements and ade quately ventilated to prevent moisture build up.
Fi g u r e 3 8 .
Be n d i n g o f t r u s s- f r a m es i n h a n d l i n g . ( E C O N ERGYCor p . )
- -
A.
L o a d ed
verti cally
B. L o a d e d a t a n a n g l e to reduce height and width. C. Fi g u r e 3 7 .
Loaded
flat Fi g u r e 3 9 .
T r u c k i n g op t i o n s f o r 1 4 a n d 1 5 f o o t t r u s s-f r a m es.
-
-
T r u s s- f r a m e st o r a g e i n horizontal position. (EC O N ERGYCor p.)
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43
ERECTING TRUSS FRAMES Speed of erection is a major benefit of TF S construction. Erecting the unitized frames completes the assembly of floor, wall, and roof framing in a single operation. An entire building can be erected almost as fast as conventional roof trusses can be set. One experienced TFS builder reported that he erected the frames for a typical house in 90 minutes and had it under lock and key that same day. Another reported that his experi enced production crew of four erected two houses in one day. The erection task includes three operations: placing,aligning and bracing.
Placing I n the placing operation, the truss-frame is lifted from the stock pile and located in its approximate final position. This can be done in several ways: •
Carrying and tilting-up into position. This usually requires a crew of five to avoid undue distortion of the truss-frames.
•
Mechanical erection with a light crane, as shown in Figures 40 and 41. A crew of three plus a crane operator is common ly used.
•
Mechanical setting with a fork lift, if the ground level is accessible to the fork lift truck. L ow foundations or wide openings such as garage door openings can provide such accessibility, as shown in F igure 42.
Figur e
40.
Setting t r uss-fr am es by cran e. (Forest Products Laboratory)
Lift
at
During loading, unloading and placing opera tions. control and safety can be improved by using a long-handled quick-disconnecting clamp such as the one shown in Figure 43.
Sp r ea d e r b a r needed
Figur e 41.
as
Recom m ended liftin g pr actice for residential truss f r a m e s .
-
44
Tag line
panel
points.
Fi gu r e 42.
Setti ng t r uss-fr am es w i th a fork lift. (Douglasville Bui l di n g Com ponent s, Inc.) Figur e 43.
Aligning
b e
The task of aligning truss-frames is simple but it is essential that each step be executed carefully. The first end wall is erected and exactly squared and anchored to the founda tion. I t is plumbed in the center and at each end, and braced to assure it is precisely vertical. The plumbing operation requires a heavy-weight carpenter’s plumb bob; a spir it level is not sufficiently accurate. Temporary braces can be adjusted and shimmed to hold the end wall ri gidly vertical. Adjustable framing braces can simplify the aligning task. The first truss-frame is also squared and plumbed. NOTE: THE FI RST END WALL AND I TS ADJ ACENT TRUSS-FRAME MUST BE PRECISEL Y SQUARED AND PLUM BED TO SERVE AS A GUI DE FOR THE REM AINI NG TRUSS-FRAMES. Subsequently erected truss-frames are spaced and squared by means of precut spacer blocks installed near the top and bottom of each stud as shown in Figure 12. The lower spacer blocks in F igure 12 are cut shorter than actual truss-frame spacing to allow for the thickness of truss plates and/or anchor plates. Matching
45
ling
t r u s s-f r a m es.
edges and corner of wall sheathing panels is helpful in aligning truss-framestuds, but every truss-frame should be checked for alignment with a carpenter’s spirit level. E very fourth truss-frame erected should be plumbed at the center and at both walls to assure that accurate alignment is being maintained, as shown in Figure 44. Flat metal shims may be applied to correct spacing, or over - or under length spacer blocks can be cut. I f non standard spacer blocks are precut, they should be prominently marked to identify their dif ferent lengths. Reusable spacing fixtures as shown in Figures 45 and 46 can assist in aligning floor trusses while the floor sheathing is applied, however they should not be used as substitute for accurate location marks along each sill plate. Small inaccuracies in location can be either compensating or cumulative. I n the absence of direct layout marks, cumulative errors could creep in.
Figure 45.
Fi g u r e 4 4 .
A l i g n i n g a n d p l u m b i n g t r u s s- fr am es. (For est Pr odu cts Laboratory)
Figure
46
46.
Aligning floor trusses for application of sheathing. (Forest Products Laboratory)
Bracing Temporary bracing serves two purposes: •
to secure truss-frames in their design positions until permanent bracing is applied.
•
to prevent a potential erection dis aster of plane structures - - domino coll apse. This possibility of progres sive collapse must be prevented until permanent bracing is secured. Failure to do so could have embarrassing, even fatal results.
Temporary bracing starts with bracing the first end wall . An example of temporary bracing is shown in Figure 47. Any brace that could undergo compression must be constrained from bowing laterally. L ateral buckling would re duce or destroy the compressive strength of such members.
Figure
47.
Lateral bracing lateral buckling braces.
prevents of ground
47
CAUTI ON: TE MPORARY BRACING MU ST BE CAPABLE OF RESTRAINING MOVE MEN T I N ANY DI RECTION, AGAINST ANY WIND LOAD, WORKMAN AND EQUIP MENT LOAD, OR ACCIDENTAL IMPACT UNTIL THE PERMANENT BRACING IS IN PLACE. PARTICUL AR CARE MUST BE EXERCISED IN L OOSENING TEMP ORARY BRACING IN ORDER TO ADJ UST, RE PLACE, OR IN STALL PERMANE NT BRAC ING. Permanent wall sheathing should be installed as soon as the first end wall and first trussframe are aligned, as shown in F igure 49. By the time an end wall and two succeeding trussframes are wall -sheathed and anchored on both sides, the sheathed frames are self -supporting. The wall sheathing should be applied to sub sequent frames as they are erected. I t may be advisable to provide temporary bracing be tween truss-frames before application of shea thing, as shown in F igure 48.
