Engineering and Construction

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Floor framing (Figure 5.19) is laid out in such a way that the ends of un-cut 4-foot by 8-foot (1.2-m by 2.4-m) subflooring panels will fall directly over joists; otherwise, many panels will have to be cut, wasting both materials and time. Standard joist spacings are 16 or 24 inches o.c. (406 or 610 mm o.c.; o.c. stands for on  center,  meaning that the spacing is measured from center to center of the joists). Though less common, a joist spacing of 19.2 inches (486 mm) can also be used. Any of these spacings automatically provides a joist to support each end of every 8-foot (2.4-m) panel (Figure 5.23).

Figure 5.19 Step Three in erecting a typical platform frame building: the ground- floor platform. Compare this drawing with the framing plan in Figure 5.17. Notice that the direction of the joists must be changed to construct the cantilevered bay on the end of the building. A cantilevered bay on a long side of the building could be framed by merely extending the  floor joists over the foundation.

Figure 5.23 Installing floor joists. Blocking will be inserted between the joists over the two interior beams to prevent overturning of the joists.

Wood composite I-joists or manufactured trusses are frequently used in place of solid wood joists. Their advantages include the ability to span greater distances, straightness  and uniformity, and reduced drying  shrinkage of the floor  platform. These joist types are most frequently  spaced at 19.2 or 24 inches o.c. (488  or 610 mm o.c.) I-joists, with their thin webs, often require reinforcing where concentrated loads occur,  such as at joist ends, at cantilevers, or where interior loadbearing partitions bear on the joists (Figures 5.44 and 5.45).

Figure 5.44 Alternative details to those shown in Figure 5.43 for a fl oor framed with I-joists rather than solid lumber. For clarity, the web of the I-joist is rendered light gray and blocking with a darker shade. The web stiffener shown in A is cut slightly shorter than the height of the web of the joist, and is installed so that a small space remains between the top of the blocking and the underside of the top fl ange. This prevents the blocking from prying the fl anges apart if the I-joist itself shrinks or is compressed slightly (see also Figure 5.45). The I-joist blocking panel in B-1 is exactly analogous to the solid blocking shown in Figure 5.45 B. A squash block is shown in B-2. This is a short section of 2x (38-mm) framing, installed vertically like a very short stud on either side of the I-joist. Squash blocks are used under points of concentrated load, such as under loadbearing posts or studs on either side of a large opening in a wall above. They are cut slightly longer than the full height of the joist to ensure that the loads are transmitted through the blocks and not the joist

Figure 5.45 Blocking where an I-joist frames into a glue-laminated wood beam, as illustrated in Figure 5.44A. This blocking acts as a web stiffener and also prevents the I-joist from rotating within the metal hanger, which is not tall enough to restrain the upper fl ange of the joist. The need for blocking depends on the size of the I-joist, the magnitude of the loads, and the type of hanger used.

After floor framing is complete, subflooring is fastened in place. To reduce squeaking in the finished floor and to increase floor  stiffness, adhesive may be used in conjunction with mechanical fasteners (Figures 5.26 and 5.27). Deformed shank nails or screws, either of which hold subflooring more securely than common nails, also contribute to the prevention of squeaks. Plywood and OSB panels must be laid with  the grain of their face layers perpendicular to the direction of the joists because these panels are considerably stiffer in this orientation (that is, a standard 4-foot by 8-foot (1.2-m by 2.4-m) panel should be laid with its  longer dimension perpendicular to the joists). Sheathing and subflooring panels are normally manufactured  1 /8  inch (3 mm) short in each face dimension so that they may be spaced slightly apart at all their edges  to prevent floor buckling during construction from the expansion of storm-wetted panels. Bridging, consisting of solid blocking or wood or metal crossbracing inserted between joists at  midspan or at intervals not exceeding  8 feet (2.4 m), is a traditional feature of floor framing (Figures 5.19, 5.28). (The term blocking refers, in general, to short lengths of lumber inserted between framing members for any  of a variety of purposes, such as bracing, reinforcing, or providing solid backing where fasteners are required for attachment of finish materials or equipment.) Its function is to hold the joists straight and to help them  share concentrated loads. The International Residential Code requires bridging only for joists deeper than 2 x 12 feet (38 x 286 mm). In better-quality construction, however, bridging is frequently used in all floors, regardless of depth, to improve stiffness and reduce vibration. Solid blocking is also required wherever floor joists span continuously across supporting beams to resist overturning of the joists (Figure 5.18B, D).

Where the ends of joists butt into supporting headers, as around stair openings and at changes of joist di- rection for projecting bays, end nails and toe nails cannot transfer the full  load between members, and sheet  metal joist hangers must be used. Each hanger provides a secure pocket  for the end of the joist and punched holes into which a number of special short nails are driven to make a safe connection.

Figure 5.26 Applying OSB subfl ooring. Note that the longer dimension of the panel runs perpendicular to the joist framing.

Figure 5.27 For a stiffer, quieter fl oor, subfl  ooring should be glued to the joists. The adhesive is a thick mastic that is squeezed from a sealant gun on the tops of the joists just before the subfl  ooring is laid in place.

Figure 5.28 Bridging between joists may be solid blocks of joist lumber, wood crossbridging, or, as seen here, steel crossbridging. During manufacture, the thick steel strip is folded across its width into a V- shape to increase its stiffness. Steel crossbridging with one end toothed requires only one nail per piece and no cutting to size, so it is the fastest to install.

Figure 5.18 Ground-fl oor framing details, keyed to the lettered circles in Figure 5.19. A compressible or resilient sill sealer strip should be installed between the sill and the top of the foundation to reduce air leakage but is not shown on these diagrams. Using accepted architectural drafting conventions, continuous pieces of lumber are drawn with an X inside and intermittent blocking with a single diagonal. The metal termite shield in detail C is used in areas where the risk of termite infestation is high. It prevents subterranean termites from traveling undetected from cracks in the concrete into the wood framing above.

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The foundation sill plate, usually made of preservative-treated wood for added resistance to insects and moisture, is bolted to the foundation as a base for the wood framing (Figure 5.18,  A, B, and C). A single sill, as shown in the details here, is all that is required by code, but in better-quality  work, the sill may be doubled or made from thicker stock for greater stiffness. A thicker sill plate may also  be required in areas of high seismic risk, to provide a stronger connection between the wood framing and  the foundation anchor bolts. Because the top of a foundation wall is usually somewhat uneven, the sill may  also be shimmed up at low spots with wood shingle wedges or plastic shims to provide a more level, even base  for subsequent framing. A  sill sealer, made of compressible or resilient  material, should be inserted between  the sill and the foundation to reduce air infiltration through the gap. Some  sill sealer materials also discourage  moisture wicking up from the foundation into the wood framing (Figure 5.22). Conventional anchor bolts  are sufficient to hold most buildings on their foundations, but tall frames or those subject to high winds  or strong earthquakes may require more elaborate attachments (Figures 5.39-5.41).

Figure 5.22 Carpenters apply a preservative-treated wood sill to a sitecast concrete foundation. A strip of compressible glass fiber sill sealer has been placed on the top of the concrete wall, and the sill has been drilled to fit over the projecting anchor bolts. Before each section of sill is bolted tightly, it is leveled as necessary with wood shingle or plastic shims between the concrete and the wood. As the anchor bolts are tightened, the sill sealer squeezes down to a negligible thickness. A length of bolted-down sill is visible at the upper right. The basement windows were clamped into reusable steel form inserts and placed in the formwork before the concrete was cast. After the concrete was cast and the formwork was stripped, the steel inserts were removed, leaving a neatly formed concrete frame around each window. A pocket in the top of the wall for a steel beam can be seen at the upper left.

Figure 5.39 Strap tie hold-downs are made of galvanized steel straps with hooked or deformed ends cast into the concrete foundation wall (top). After the wall is framed, the exposed length of strap may be nailed directly to framing or, as seen here, nailed through the sheathing into the studs or posts behind (bottom). The number, size, and spacing of nails used to fasten the strap depend on the magnitude of the loads that must be resisted and the capacity of the wood member to hold nails without splitting. In the top image, anchor bolts cast into the top of the foun- dation wall that will be used to secure the sill plate in place are also visible.

Figure 5.40 Hold-downs made from threaded rod and steel plate anchors can resist much greater forces than strap tie hold-downs. They may be used at each fl oor level to securely tie the full height of the building frame to its foundation. For the type shown here, the nuts at the ends of the threaded rods may require retightening after the fi rst heating season to compensate for wood shrinkage, which can mean that access holes must be provided through the interior wall surfaces. Other models rely on spring-loaded tapered shims or other mechanical compensation so as to be self-adjusting.

Figure 5.41 A heavy-duty seismic hold-down similar to that illustrated in Figure 5.40. The anchor rod with one end cast into the concrete foundation wall protrudes through the preservative-treated wood sill where its threaded end is bolted to the anchor. The anchor in turn is bolted to a 4 4 (89 89 mm) post with fi ve bolts of substantial diameter. A conventional foundation anchor bolt with an oversized square washer is also partially visible to the right of the hold-down. Also note the thicker-than-normal, 3-inch nominal (64-mm) sill plate, a common feature in wood light framing designed for high seismic forces.

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The erection of a typical platform  frame (referred to, imprecisely, as  rough carpentry in the architectÕs specifications) can best be understood by following this post s sequential  isometric diagrams, beginning with  Figure 5.19. Notice the basic simplici- ty of the building process: A platform  is built on top of the foundation.  Walls are assembled horizontally on the platform and tilted up into place.

Another platform or a roof is built on top of the walls. Most of the work is accomplished without the use of ladders or scaffolding, and temporary  bracing is needed only to support the walls until the next level of framing is  installed and sheathed.

The details of a platform frame  are not left to chance. While there  are countless regional and personal  variations in framing details and techniques, the sizes, spacings, and  connections of the members in a  platform frame are standardized  and closely regulated by building  codes, even down to the size and  number of nails for each connection (Figure 5.21) and the thicknesses and nailing patterns of the sheathing panels.

Figure 5.19 Step Three in erecting a typical platform frame building: the ground- fl oor platform. Compare this drawing with the framing plan in Figure 5.17. Notice that the direction of the joists must be changed to construct the cantilevered bay on the end of the building. A cantilevered bay on a long side of the building could be framed by merely extending the floor joists over the foundation.

Figure 5.20 Alternative ways of constructing an interior line of support for ground-fl oor joists. Space is left over the tops of the beams in B and D to allow for drying shrinkage in the joists.

Figure 5.21 Platform framing members are fastened according to this nailing schedule, which framing carpenters know by memory and which  is incorporated into most building codes and construction standards.

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An experienced carpenter can frame a simple building from the most minimal drawings, but the framing for a  larger or custom-designed structure may need to be planned as carefully as for a steel- or concrete-framed building (Figure 5.16). The architect or engineer determines an effcient layout and the appropriate sizes for joists and rafters, and communicates this information to the carpenters by means of  framing plans (Figures 5.17 and 5.51). For most purposes, member sizes can be determined using standardized structural tables that are part of residential building codes, or, for more complex framing or special conditions, custom engineering may be required. Larger-scale  section details, similar to those seen throughout this chapter, are prepared for major connections in the building system. The architectural floor plans serve to indicate the locations and dimensions of walls, partitions, and openings, and the exterior elevations show the outside faces of the building, with vertical dimensions or elevations indicated as required. For most buildings,  building sections are also drawn that cut completely through the building, showing the dimensional relation- ships of the various floor levels and roof planes and the slopes of the roof surfaces.  Interior elevations are often prepared for kitchens, bathrooms, and other rooms with elaborate interior features.

Figure 5.16 A floor plan and building section are two important components of the construction drawings for a simple house with wood light framing. The ground floor is a concrete slab on grade.

Figure 5.17 A framing plan for the ground-floor platform of the building

Figure 5.51

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Foundations for light framing, originally made of stone or brick, are  now made in most cases of sitecast  concrete or concrete block masonry  (Figures 5.5-5.11). These materials  are highly conductive of heat and  usually must be insulated to meet  code requirements for energy con- servation (Figures 5.8, 5.9, 5.12, and  5.13). Where concrete and masonry  construction methods are not practical, such as in extremely cold regions, foundations may be constructed entirely of preservative-treated wood  (Figures 5.14 and 5.15). Such  permanent wood foundations can be con- structed in any weather by the same crew of carpenters that will frame the building; they are readily insulated in the same manner as the frame  of the house they support; and they easily accommodate the installation of electrical wiring, plumbing, and interior finish materials.

Figure 5.5 Step One in the construction of a simple platform frame building: establishing the position, shape, and size of the building on the site. After the corners of the foundation have been staked, batter boards are erected safely beyond the limits of the excavation area. String lines crossing over the corner stakes are stretched from the batter boards, and their positions on the boards are preserved with notches or nails in the boards. Later, after excavation, the strings are stretched again, and the foundation corners can be relocated at the bottom of the excavation. This drawing begins a series of isometric drawings that will follow the erection of a wood light frame building step by step throughout the course of this chapter.

Figure 5.6 Typical details for a sitecast concrete foundation and basement for a platform frame building. Details A, B, and C are keyed to the circled portions of the drawing in Figure 5.7.

Figure 5.7 Step Two in the construction of a typical platform frame building: excavation and foundations. The letters A, B, and C indicate portions of the foundation that are detailed in Figure 5.6.

Figure 5.8 Most building codes require thermal insulation of the foundation. These are three different ways of adding insulation to a  cast-in-place concrete or concrete masonry basement and one way of insulating a crawlspace foundation. The crawlspace may  alternatively be insulated on the outside with panels of plastic foam. The interior batt insulation shown in B is commonly used but  raises questions about how to avoid possible problems arising from moisture accumulating between the insulation and the wall.

Figure 5.9 Some typical concrete slab-on-grade details with thermal insulation. In areas where termites are common, there must be a metal fl ashing (termite shield) similar to that shown in Figure 5.18 C or other interruption that passes through the foam insulation above ground level to prevent termites from tunneling undetected through the insulation to reach the wood structure. This applies to the foam-insulated basement details in Figure 5.8 as well.

Insulating  concrete formwork (ICF) foundation  systems using permanent insulating  forms are easy to construct, eliminate the need for formwork removal, and provide integral insulation  (Figure 14.12).  Proprietary  precast  concrete foundation systems relying on factory-fabricated reinforced  concrete panels are rapidly erected  on site and can be used to construct  basement structures of consistent  strength and quality. They may be  manufactured with insulation integral to the precast panel or designed  to readily accept insulation added in  the field.

All basements also need to be carefully dampproofed and drained  to avoid flooding with groundwater
and to prevent the buildup of water pressure in the surrounding soil  that could cause the walls to cave in  (Figures 5.6, 5.7, and 5.11).

Figure 5.10 Erecting formwork for a sitecast concrete foundation wall. The footing has already been cast and its formwork removed. It is visible in front of the worker at the left.

Figure 5.11 Masons construct a foundation of concrete masonry. The fi rst coat of parging, which is portland cement plaster applied to help dampproof the foundation, has already been applied to the outside of the wall, and the drainage layer of crushed stone has been backfi lled in place. The projecting pilaster in the center of the wall will support a beam under the center of the main fl oor. After a second coat of parging, the outside of the foundation will be coated with an asphaltic dampproofi ng compound. (Reprinted with permission of the Portland Cement Association from Design and Control of Concrete Mixtures, 12th edition; Photos: Portland Cement Association, Skokie, IL)

Figure 5.12 A concrete masonry foundation with polystyrene foam insulation applied to its outer surface.

Figure 5.13 Following completion of the exterior siding, a worker staples a glass fi  ber reinforcing mesh to the foam insulation on the exposed portions of the basement wall. Next, he will trowel onto the mesh two thin coats of a cementitious, stucco- like material that will form a durable fi nish coating over the foam.

Figure 5.14 Erecting a permanent wood foundation. One worker applies a bead of sealant to the edge of a panel of preservative- treated wood components as another prepares to push the next panel into position against the sealant. The panels rest on a horizontal preservative-treated plank, which, in turn, rests on a drainage layer of crushed stone. Wood founda- tions can be constructed in any weather and can be insulated in the same way as the superstructure of the building.

Figure 5.15 The difference in color makes it clear where the preservative-treated wood foundation leaves off and the untreated superstructure of the building begins.

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A wood light frame building can be designed to mini- mize waste in several ways. It can be dimensioned to utilize full sheets and lengths of wood products. Most small  buildings can be framed with studs 24 inches (610 mm) o.c.  rather than 16 inches (406 mm). A stud can be eliminated  at each corner by using small, inexpensive metal clips to support the interior wall finish materials. If joists and rafters are aligned directly over studs, the top plate can be a  single member rather than a double one. Floor joists can  be spliced at points of inflection rather than over girders;  this reduces bending moments and allows use of smaller  joists. Roof trusses often use less wood than conventional  rafters and ceiling joists.
Laminated strand lumber and rim joists, wood I-joists,  laminated veneer lumber beams and headers, glue-laminated girders, parallel strand lumber girders, and OSB sheathing are all materials that utilize trees more efficiently  than solid lumber. Finger-jointed studs made up of short  lengths of scrap lumber glued together may replace solid  full-length wood studs.

Framing carpenters can waste less lumber by saving  cutoffs and reusing them rather than throwing them  automatically on the scrap heap. In some localities,  scrap lumber can be recycled by shredding it for use  in OSB production. The burning of construction scrap should be discouraged because of the air pollution it  generates.

Although the thermal efficiency of wood light frame  construction is inherently high, it can be improved  substantially by various means, as shown in Figures 7.17-7.21. Wood framing is much less conductive of heat than  light-gauge steel framing. Steel framing of exterior walls is  not a satisfactory substitute for wood framing unless the heat  flow path through the steel framing members can be broken  with a substantial thickness of insulating foam.

Figure 7.17 Insulation levels in walls of light frame buildings can be increased from the R13 to R-15 (RSI-90 to RSI-104) of a 2 X 4 stud wall to R19 (RSI-132) or more by using either 2 X 6 framing and thicker batt insulation (left) or 2 X 4 framing with plastic foam sheathing in combination with batt insulation (right). The foam sheathing insulates the wood framing members as well as the cavities between them but can complicate the process of installing some types of siding.

Figure 7.18 Window and door headers in 2 X 6 framing require special detailing. Two alternative header details are shown here in section view: (a) The header members are installed fl  ush with the interior and exterior surfaces of the studs, with an insulated space between. This detail is thermally effi cient but may not provide suffi cient nailing for interior fi  nish materials around the window. (b) A 3 X 2 spacer provides full nailing around the opening.

Figure 7.19 Two alternative corner post details for 2 6 stud walls, shown in plan view: (a) Each wall frame ends with a full 2 X 6 stud, and a 2 X 4 nailer is added to accept fasteners from the interior wall fi nish. (b) For maximum thermal effi ciency, one wall frame ends with a 2 X 4 stud fl ush with the interior surface, which eliminates any thermal bridging through studs.

Figure 7.20 With the two framing methods shown here, walls can be insulated to any desired level of thermal resistance.

Figure 7.21 A raised-heel roof truss provides plenty of space for attic insulation at the eave.

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Although complex in its details, the platform frame is simple in concept  (Figure 5.3). A floor  platform  is  built. Loadbearing walls are erected  upon it. A second-floor platform is  built upon these walls and a second  set of walls upon this platform. The  attic and roof are then built upon  the second set of walls. There are,  of course, many variations: A concrete slab that lies directly on the ground is sometimes substituted for  the ground-floor platform; a building may be one or three stories tall  instead of two; and several types of  roofs are frequently built that do not  incorporate attics. The essentials,  however, remain: A floor  platform  is completed at each level, and the  walls bear upon the platform rather  than directly upon the walls of the  story below.

Figure 5.3 The concept of platform framing, shown in cross section, reading from left to right: A foundation wall is constructed. A ground-fl oor platform is framed and sheathed. Ground-fl oor wall frames are assembled horizontally on the platform, then tilted into their fi nal positions. A second-fl  oor platform is constructed on top of the ground-floor walls, and the process of wall construction is repeated. The attic floor and roof are added.

The advantages of the platform  frame over the balloon frame are several: It uses short, easily handled lengths of lumber for the wall framing. Its vertical hollow spaces are  automatically Þ restopped by the  platform framing at each floor.  Its  platforms are convenient working  surfaces for the carpenters who build  the frame. The major disadvantage  of the platform frame is that each  platform constitutes a thick layer of
wood whose grain runs horizontally.  This leads inevitably to a relatively  large amount of vertical shrinkage  in the frame as excess moisture dries  from the wood, which can cause dis- tress in the exterior and interior finish surfaces.

A conventional platform frame is made entirely of nominal 2-inch  members, which are actually 1½  inches (38 mm) in thickness. These are ordered and delivered cut to  the nearest 2-foot (600-mm) length,  then measured and sawn to exact  length on the building site. All  connections are made with nails,  using either face nailing, end nailing, or toe nailing as  required by the characteristics of  each joint. Nails are driven either by  hammer or nail gun. In either case,  the connection is quickly made be- cause the nails are installed without  drilling holes or otherwise preparing the joint.

Each plane of structure in a  platform frame is made by aligning a number of pieces of framing  lumber parallel to one another at specified intervals, nailing these to crosspieces at either end to main- tain their spacing and flatness, and then covering the plane of framing with  sheathing, a facing layer of  boards or panels that join and stabilize the pieces into a single struc- tural unit, ready for the application of finish materials inside and out (Figure 5.4). In a floor structure, the parallel pieces are the floor  joists,  and the crosspieces at the ends of the joists are called headers, rim joists,  or  band joists. The sheathing on a floor is known as the  subfl  oor. In a wall structure, the parallel pieces  are the  studs, the crosspiece at the  bottom of the wall is the sole plate or  bottom plate, and the crosspiece at the  top (which is doubled for strength if  the wall bears a load from above) is  called the top plate. In a sloping roof,  the rafters are headed off by the top  plates at the lower edge of the roof  and by the ridge board at the peak.

Figure 5.4 The basic components of platform frame construction. (a) Walls are framed with repetitive vertical studs that are connected at the top and bottom by horizontal plates. (b) Floors are framed with repetitive joists that are connected at their ends by headers. (c) Roofs are framed with rafters. Every surface is sheathed with wood panels of either plywood (as shown) or, more commonly, less expensive OSB.

Openings are required in all  these planes of structure: for windows  and doors in the walls; for stairs and  chimneys in the floors; and for chimneys, skylights, and dormers in the  roofs. In each case, these are made  by heading off the opening: Open- ings in  floors are framed with headers and trimmers, which
usually must be doubled to support the higher loads placed on them by  the presence of the opening. In walls,  sills head off the bottoms of window  and door openings, while trimmer  studs on the sides provide support to loadbearing headers across the tops.

Sheathing, a layer of wood panels nailed over the outside face of  the framing, is a key component of  platform framing. The end nails that  connect the plates to the studs have  little holding power against uplift  of the roof by wind, but the sheath- ing connects the frame into a single  strong unit from foundation to roof.  The rectilinear geometry of the parallel framing members has no useful  resistance to wracking by lateral forces  such as wind, but rigid sheathing  panels brace the building effectively  against these forces. Sheathing  also furnishes a surface to which  shingles, boards, and ß  ooring  are  nailed for finish surfaces. In build- ings constructed without sheathing,  or with sheathing materials that are  too weak to brace the frame, such  as insulating plastic foam, diagonal  bracing must be applied to the wall  framing to impart lateral stability.

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Wood light frame construction is the most versatile of all building systems. There is scarcely a shape it can-
not be used to construct, from a plain rectilinear box to cylindrical towers to complex foldings of sloping
roofs with dormers of every description. During the century and a half since it first came into use, wood
light framing has served to construct buildings in styles ranging from reinterpretations of nearly all the
historical fashions to uncompromising expressions of every contemporary architectural philosophy. It
has assimilated without difficulty during this same period a bewildering and unforeseen succession of
technical improvements in building: central heating, air conditioning, gas lighting, electricity, thermal in-
sulation, indoor plumbing, prefabricated components, and electronic communications cabling.

Light frame buildings are easily and swiftly constructed with a minimal investment in tools.  Many observers of the building industry have criticized the supposed inefficiency of light frame construction, which is carried out largely by hand methods on the building site, yet it has successfully  fought off competition from industrialized building systems of every sort, partly by incorporating their  best features, to remain the least expensive form of durable construction. It is the common currency  of small residential and commercial buildings in North America today.

Wood light frame construction has its deficiencies: If ignited, it burns rapidly; if exposed to  dampness, it decays. It expands and contracts by significant amounts in response to changes in  humidity, sometimes causing chronic difficulties with cracking plaster, sticking doors, and buckling floors. The framing itself is so unattractive to the eye that it is seldom left exposed in a building. These  problems can be controlled, however, by clever design and careful workmanship, and there is no arguing with success: Frames made by the monotonous repetition of wooden joists, studs, and rafters  are likely to remain the number one system of building in North America for a long time to come.

Figure 5.1 Carpenters apply plywood roof sheathing to  a platform-framed apartment building. The ground floor is a concrete slab on grade.  The edge of the wooden platform of the upper floor is clearly visible between the  stud walls of the ground and upper floors. Most of the diagonal bracing is temporary,  but permanent let-in diagonal braces occur between the two openings at the lower left  and immediately above in the rear building. The openings have been framed incorrectly,  without supporting studs for the headers.

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Heavy timber cannot span as far or with such delicacy as steel, and it cannot mimic the structural continuity or smooth shell forms of concrete, yet many people respond more positively to the idea of a timber building than they do to one of steel or concrete. To some degree, this response may stem from the color, grain figure, and warmer feel of wood. In part, it may also come from the pleasant associations that people have with the sturdy, satisfying houses our ancestors erected from handhewn timbers only a few generations
ago. Most people today live in dwellings where none of the framing is exposed. Exposed ceiling beams in
one's house or apartment are considered an amenity, and shopping or restaurant dining in a converted mill  building of heavy timber and masonry construction is generally thought  to be a pleasant experience. There  is something in all of us that derives satisfaction from seeing wood beams  at work.

In economic reality, heavy timber must compete successfully on the basis of price with other construction materials, and as our forests have diminished in quality and shipping costs have risen, timber is no longer an automatic choice for building a mill or any other type of structure. For many  buildings, however, heavy timber is an economical alternative to steel and concrete, particularly in situations  where the appearance and feel of its large wooden members will be highly  valued by those who use it, in regions  close to commercial forests, or where code provisions or fire insurance premiums create a financial incentive.

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The table in Figure 1.2 shows the range of height and area limits for buildings of different occupancies built of Heavy Timber (Type IV) construction and of Light Frame  (Type V) construction (applicable when heavy timber framing  and light frame construction are mixed). Notice that when consid- ering any particular occupancy,  the allowable heights and areas for Heavy Timber construction are  comparable to those for 1-Hour-Protected Noncombustible (Type IIA) construction (steel, concrete,  or masonry) and are greater than  those for protected and unprotected Type V construction or unprotected steel (Type IIB). These limits  are an explicit recognition of the inherent fire-resistive qualities of heavy timber framing.

Figure 1.2 Height and area limitations of build- ings of various types of construction, as defi ned in the 2006 IBC. (Portions of this publication reproduce tables from the 2006 International Building Code

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Estimate the nominal depth of wood roof decking at 1 /45 of its span. Estimate the depth of wood floor decking at  1 /35 of its span. Standard nominal depths of wood decking  are 2, 3, 4, 6, and 8 inches (actual size 38, 64, 89, 140, and  184 mm).

Estimate the depth of solid wood beams at 1 /15 of their span and the depth of glue-laminated beams at  1/ 20 of their span. Add a nominal 6 inches (150 mm) to these depths for  girders. The width of a solid wood beam or girder is usually  1 /4 to  1 /2 of its depth. The width of a glue-laminated beam  typically ranges from  1 /7 to 1 /4 of its depth.

Estimate the depth of timber triangular roof trusses at  1 /5 to  1 /2 of their span and the depth of bowstring trusses at  1 /2 to  2/ 3 of their span.

To estimate the size of a wood column, add up the total  roof and ß oor area supported by the column. A nominal 6- inch (actual size 140 mm) column can support up to about  400 square feet (37 m2) of area, an 8-inch (actual size 184  mm) column 1000 square feet (93 m2), a 10-inch (actual  size 235 mm) column 1500 square feet (140 m2), a 12-inch (actual size 286 mm) column 2500 square feet (230 m2),  and a 14-inch (actual size 337 mm) column 3500 square  feet (325 m2). Wood columns are usually square or nearly  square in proportion.

For actual sizes of solid timbers in conventional units,  see Figures 3.22 and 3.23. Standard sizes of glue-laminated  timbers are given in Chapter 3. For a building that must  qualify as Type IV Heavy Timber construction under the  IBC, minimum timber sizes are given in Figure 4.7.

Figure 3.23 A complete chart of nominal and actual dimension for both framing lumber and finish lumber

These approximations are valid only for purposes  of preliminary building layout, and must not be used  to select final member sizes. They apply to the normal  range of building occupancies such as residential, office, commercial, and institutional buildings. For manufacturing and storage buildings, use somewhat larger  members.

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Long curved timbers for making heavy timber vaults and heavy timber domes are  easily fabricated in laminated wood  and are widely used in athletic arenas,  auditoriums, suburban retail stores,  warehouses, and factories (Figures  4.29-4.31). Arched structures, like  rigid frames, exert lateral thrusts that  must be countered by tie rods or suit- ably designed foundations.

Figure 4.29 Semicircular laminated wood arches support the timber roof of the Back Bay Station in Boston. Notice the use of horizontal steel tie rods to resist the thrust of the arches at  the base.

Figure 4.30 A typical hinged foundation connection for an arch or dome, made of welded steel plates. The hinge pin allows for rotation between the arch and the foundations, which avoids many kinds of forces that might otherwise be placed on the structure by wood shrinkage and foundation movement.

Figure 4.31 This laminated wood dome spans 530 feet (161.5 m) to cover a 25,000-seat stadium and convention center in Tacoma, Washington.

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The cruck (Figure 4.2), cut from a  bent tree, was a form of  rigid frame or  portal frame. Today's rigid frames are glue-laminated to shape and find wide use in longer-span buildings.  Standard configurations are readily available (Figures 4.22-4.24), or the  designer may order a custom shape.  Rigid frames exert a horizontal  thrust, so they must be tied together  at the base with steel tension rods, also  called  tie rods. In laminated wood construction, rigid frames are often called arches, acknowledging that the  two structural forms act in very nearly  the same manner.

Figure 4.2 European timber house forms generally followed a progression of development from crude pit dwellings, made of earth and tree trunks, to cruck frames, to braced frames. The crucks (curved timbers), hewn by hand from appropriately shaped trees, were precursors to the laminated wood arches and rigid frames that are widely used today.

Figure 4.22 Three-hinged arches of glue-laminated wood carry laminated wood roof purlins. The short crosspieces of wood between the purlins are temporary ladders for workers.

Figure 4.23 Typical details for three-hinged arches of laminated wood. The tie rod is later covered by the fl oor slab.

Figure 4.24 The shear plates in the crown connection of the arch, shown here in a larger-scale detail, are recessed into grooves in the wood and serve to spread any force from the steel rod across a much wider surface area of wood to avoid crushing and splitting.

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With large, old-growth trees no longer readily available, very large timber  beams are usually built up of laminations or strands rather than sawn. Such beams are stronger and more  dimensionally stable than sawn wood  beams and can be made in the exact  size and shape desired (Figures 4.20  and 4.21). Hybrid or FRP reinforced  glulam beams, can be useful when especially lon- ger span are needed or heavy loads  must be supported.

Figure 4.20 Installing tongue-and-groove roof decking over laminated beams and  girders.

Figure 4.21 This roof uses glue-laminated beams to support proprietary long-span trusses made of wood with steel tube diagonals.