Engineering and Construction

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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.

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The heavy timber structure poses some special problems for the designer because the exposed framing and decking do not provide the concealed cavities that are present in light frame structures and other conventional building systems. Roof  thermal insulation cannot be hidden in spaces between ceiling joists  or roof rafters and, instead, must be placed on top of the roof deck. If the  roof is nearly flat, it can be insulated and roofed in the manner shown for  low-slope roofs in Chapter 16, or, if  the roof is steeply pitched, a nailing surface for the shingles or other roofing must be added on the outside of  the insulation. Electrical wiring for lighting fixtures that are mounted on the underside of the decking must  either run through exposed metal  conduits below the deck, which may  be visually unsatisfactory, or be channeled through the insulation above the deck. Ductwork for heating and  cooling must remain exposed. If  the walls and partitions are made of  masonry or stressed-skin panels, special arrangements are necessary for  installing wiring, plumbing, and me- chanical system components in the  walls as well (Figure 4.19).

Figure 4.19 When floors and roofs are heavy timber framed, lighting, communications wiring, ductwork, fi re sprinklers, and other services are exposed to view.

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A heavy timber frame building with an exterior masonry or concrete bearing  wall is normally braced against wind and seismic forces by the shear resistance of its exterior walls, working  together with the diaphragm action of its roof and floor decks. In areas of high seismic risk, the walls must be  heavily reinforced both vertically and horizontally, and the decks may have to be specially nailed or overlaid with plywood to increase their shear resistance, as well as specially anchored  to the perimeter walls. In buildings with framed exterior walls, diagonal bracing or shear panels must be provided. Seismic upgrades to historic  heavy timber and masonry buildings often require the insertion of new steel-braced frames or reinforced  concrete shear walls in order to meet contemporary lateral force resistance requirements.

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Building codes require that Type IV Heavy Timber buildings have floors and roofs of solid wood construction without concealed cavities. Figure 4.18 shows different types of decking used for these purposes. Minimum permissible thicknesses of decking are given in Figure 4.7. To meet code requirements, floor decking must also be covered with a finish  floor consisting of nominal 1-inch (19-mm)  tongue-and-groove boards laid at right angles or diagonally to the struc- tural decking. In some circumstances,  ½-inch (13-mm) plywood or other composite wood panels are also per- mitted as the finish layer.

Figure 4.18

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Where heavy timber beams join masonry or concrete walls, three problems must be solved: First, the beam  must be protected from decay caused by moisture seeping through the wall.  This is done by leaving a ventilating  airspace of at least ½ inch (13 mm) between the masonry and all sides of the beam except the bottom, unless the beam is chemically treated  to resist decay. The second and third  problems have to be solved together:  The beam must be securely anchored  to the wall so that it cannot pull away under normal service, yet it must be able to rotate freely so that it does not pry the wall apart if it burns through and collapses during a severe fire (Figure 4.12). Traditional and more contemporary methods of accomplishing these dual needs are shown  in Figures 4.11 and 4.13, respectively.

Figure 4.11 Two alternative details for the bearing of a beam on masonry in traditional Mill construction. In each case, the beam end is fi recut to allow it to rotate out of the wall if it burns through (Figure 4.12), but it is also anchored against pulling away from the wall by means of either lag screws or a lug on the iron bearing plate.

Figure 4.12 A timber beam that burns through in a  prolonged fi re is likely to topple its sup- porting masonry wall (left) unless its end  is fi recut (right). In this illustration, the beam is anchored to the wall in each case  by a steel strap anchor.

Figure 4.13 An example of contemporary heavy timber construction, based on steel plate connectors and an insulated cavity wall.

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Where the edges of ß oors and roofs of a heavy timber frame are supported on concrete or masonry, special attention must be given to the potential for differential shrinkage between  the outer walls and the interior wood  column supports. Wood, compared  to masonry or concrete, expands  and contracts more with changes in  moisture content, particularly in the  direction perpendicular to its grain.

These changes occur over a period of  years as large timbers gradually dry  and seasonally with changes in ambient conditions. A heavy timber frame  building is detailed to minimize the  effects of this differential shrinkage  by eliminating cross-grain wood from  the interior lines of support. In traditional Mill construction, cast iron

Figure 4.7 Minimum sizes for solid wood members used in Type IV Heavy Timber construction, as specifi ed in the IBC.

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In addition to the issues of sustainability of wood production and use that were raised in the previous chapter,  there are issues that pertain especially to heavy timber  frame construction:

It is wasteful to saw large, solid timbers from logs:
In most instances, only one or two timbers can be  obtained from a log, and it is often difficult to saw  smaller boards from the leftover slabs.

Glue-laminated timbers and composite timbers utilize  wood fiber much more efficiently than solid timbers.

Recycled timbers from demolished mills, factories,  and barns are often available. Most of these are  from old-growth forests in which trees grew slowly,  producing fine-grained, dense wood. As a result,  many have structural properties that are superior to  those of new-growth timbers. Recycled timbers may be  used as is, resurfaced to give them a new appearance,  or resawn into smaller members. However, they  often contain old metal fasteners. Unless these are  meticulously found and removed, they can damage  saw blades and planer knives, causing expensive mill shutdowns while repairs are made.

Continuous bending action of beams may be created  by splicing beams at points of inß ection rather than  over supports, as shown in Figures 4.15, 4.20 and 4.21.  This reduces maximum bending moments, allowing  timber sizes to be reduced substantially.

Timbers do not lose strength with age, although they  do sag progressively if they are overloaded. When a  heavy timber building is demolished at some time in the future, its timbers can be recycled, even if they  were obtained as recycled material for the building  that is being demolished.

A heavy timber frame enclosed with foam core  sandwich or stressed-skin panels is relatively airtight  and well insulated, with few thermal bridges. Heating  and cooling of the building will consume relatively  little energy.

The glues and finish coatings used with glue-laminated  timbers may give off gases such as formaldehyde that  can cause indoor air quality (IAQ) problems. It is  wise to determine in advance what glues and coatings  are to be used, and to avoid ones that may cause  IAQ problems.

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Large timbers, because of their greater capacity to absorb heat, are  much slower to catch Þ re and burn  than smaller pieces of wood. When  exposed to fire, a heavy timber beam,  though deeply charred by gradual  burning, will continue to support its load long after an unprotected steel  beam exposed to the same conditions has collapsed. If the fire is not  prolonged, a fire-damaged  heavy  timber beam or column can often be  sandblasted afterward to remove the  surface char and continue in service.

For these reasons, building codes  recognize heavy timber  framing that meets certain specific  requirements as having fire-resistive properties.

In the International Building  Code (IBC), for a building to be classified as Type IV Heavy Timber (HT)  construction, its wooden structural  members must meet certain mini- mum size requirements and its exterior walls must be constructed of  noncombustible materials. Minimum  permitted sizes for solid wood timbers  are summarized in Figure 4.7. Glue- laminated members used in this type of construction must meet similar  requirements. Exterior walls may be  constructed of concrete, masonry, or  metal cladding. Historically, this combination of fire-resistive wood framing  and noncombustible exterior was referred to as Mill construction, reflecting  its origins in 19th-century brick ma- sonry mill structures, or as Slow-burning construction (Figures 4.8-4.11).

Figure 4.7 Minimum sizes for solid wood members used in Type IV Heavy Timber construction, as specifi ed in the IBC.

Traditionally, the edges of the timbers  were also chamfered (beveled at 45 degrees) to eliminate the thin edges of  wood that catch fire most easily, but  this is no longer a code requirement.

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Wood beams have been used to span roofs  and floors of buildings since the beginning of  civilization. The fi rst timber-framed buildings  were crude pit houses, lean-tos, teepees, and  basketlike assemblages of bent saplings. In  earliest historic times, roof and fl oor timbers  were combined with masonry loadbearing walls to  build houses and public buildings. In the Middle  Ages, braced wall frames of timber were built for  the first time (Figures 4.1 and 4.2). The British  carpenters who emigrated to North America in  the 17th and 18th centuries brought with them  a fully developed knowledge of how to build  efficiently braced frames, and for two centuries  North Americans lived and worked almost  exclusively in buildings framed with hand-hewn wooden timbers joined by interlocking wood-to- wood connections (Figures 4.3 and 4.4). Nails were rare and expensive, so they were used only  in door and window construction and, sometimes,  for fastening siding boards to the frame.

Until two centuries ago, logs could be converted to boards and timbers only by human muscle power. To make timbers, axemen  skillfully scored and hewed logs to reduce them  to a rectangular profi le. Boards were produced  slowly and laboriously with a long, two-man pit  saw, one man standing in a pit beneath the log,  pulling the saw down, and the other standing  above, pulling it back up. At the beginning of  the 19th century, water-powered sawmills began to take over the work of transforming tree trunks into lumber, squaring timbers and slicing  boards in a fraction of the time that it took to  do the same work by hand.

Most of the great industrial mills of 19th-century  North America, which manufactured textiles,  shoes, machinery, and all the goods of civilization,  consisted of heavy sawn timber fl oors and roofs  supported by masonry exterior walls (Figures 4.5 and 4.6). The house builders and barn raisers of the  early 19th century switched from hand-hewn to sawn timbers as soon as they became available. Many  of these mills, barns, and houses still survive, and  with them survives a rich tradition of heavy timber  building that continues to the present day.

Figure 4.1 Braced wall framing was not developed  until the late Middle Ages and early Renaissance, when it was often exposed  on the face of the building in the style of construction known as halftimbering.  The space between the timbers was fi  lled with brickwork or with wattle and daub, a  crude plaster of sticks and mud, as seen here in Wythenshawe Hall, a 16th-century  house near Manchester, England.

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 4.3 The European tradition of heavy timber framing was brought to North America  by the earliest settlers and was used for houses and barns until well into the  19th century.

Figure 4.4 Traditional timber framing has been  revived in recent years by a number of builders who have learned the old  methods of joinery and updated them with the use of modern power tools and  equipment. (a) Assembling a bent (a plane of columns, beams, rafters, and  braces). (b) The completed bents are laid out on the fl oor, ready for raising. (c)  Raising the bents, using a truck-mounted crane, and installing fl oor framing and  roof purlins (smaller, secondary framing members that span across the primary  beams or rafters). (d) The completed frame. (e) Enclosing the timber-framed  house with sandwich panels consisting of waferboard faces bonded to an  insulating foam core.

Figure 4.5 Most 19th-century industrial buildings in North America were constructed of  heavy timber roofs and fl oors supported at the perimeter on masonry loadbearing  walls, a construction method that came to be known to as Mill construction.  This impressive group of textile mills stretches for a distance of 2 miles (3 km) along the Merrimac River in Manchester, New Hampshire.

Figure 4.6 The generously sized windows in the mills provided plenty of daylight to work  by. Columns were of wood or cast iron. Most New England mills, like this one,  were framed very simply: Their fl  oor decking is carried by beams running  at right angles to the exterior walls, supported at the interior on two lines of  columns. Notice that the fi nish fl  ooring runs perpendicular to the structural  decking. Overhead sprinklers provide additional fi re safety to a construction  method that already has inherent  fire-resistive qualities.

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Wood construction has evolved into two major systems of on-site construction: The hand-hewn frames of centuries past have become the heavy timber frames of today, used both for single-family houses and for larger buildings. And from the heavy timber frame has sprung wood light frame construction, which is the dominant  system for houses, small commercial  structures, and apartment buildings  even as tall as four or five  stories.

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Adhesives are widely employed in the factory production of plywood and panel products, laminated wood, wood structural components, and cabinetwork. Some such adhesives, most notably urea-formaldehyde, can release formaldehyde or other unhealthful gases long after the manu-facturing process is complete and the products have been placed into service in buildings. When used for interior carpentry, such products can be the source of indoor air quality problems.

Where products manufactured with adhesives or binders will be used indoors, those made with a more chemically stable exterior adhesive called phenol-formaldehyde, or with alternative formaldehyde-free adhesives, should be considered. Finishes or veneers that encapsulate composites also reduce emissions from the glues and binders used in their manufacture.

Adhesives are used less on the construction site, where it is more difficult to clamp and hold glued  joints and to maintain controlled environmental conditions until the adhesive has cured. In rough carpen-try work, adhesive is most commonly used for securing subflooring panels to wood framing (Figure 5.27). In this application, mastic sealant is applied using a sealant gun. The joint between the subß oor panel and the joist framing is then clamped by nailing or screwing the panels to the framing, the fasteners serving as the primary structural connection and the adhesive acting primarily to reduce squeaks in the floor. Adhesives of various types are also used in finish carpentry, usually in combination with nails or screws, to improve the strength and stability of joints in finish trim, site-built cabinets, and other architectural woodwork.

Figure 5.27 For a stiffer, quieter fl oor, subflooring  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.

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Dozens of ingenious sheet metal and metal plate devices are manufactured for strengthening common connections in wood framing. The most frequently used is the joist hanger, but all  of the devices shown in Figures 3.45 and 3.46 find extensive use. There are two parallel series of this type of  device, one made of sheet metal for  use in light framing and one made of  thicker metal plate for heavy timber  and laminated wood framing. The  devices for light framing are attached  with nails and the heavier devices  with bolts or lag screws. Where such framing devices will be exposed to  the weather or to lumber treated  with corrosive chemical treatments,  they should be made from corrosion- resistant metal such as heavily galvanized steel or stainless steel. To  prevent galvanic corrosion between  dissimilar metals, fasteners for metal  framing devices should always be  made of the same material as the  framing device: Galvanized framing  devices should be fastened with galvanized fasteners, and stainless steel  devices should be fastened with stain-less steel fasteners.

Figure 3.45 Joist hangers are used to make strong connections in fl oor framing wherever wood joists bear on one another at right angles. They are attached with special short nails driven through the holes punched in the hangers. The heavier steel beam hangers are used primarily in laminated wood construction. Post bases serve the twofold function of preventing water from entering the end of the post and anchoring the post to the foundation. The bolts and lag screws used to connect the wood members to the heavier connectors are omitted from this drawing.

Figure 3.46 The sheet metal connectors shown in this diagram are less commonly used than those in Figure 3.45, but are invaluable in solving special framing problems and in reinforcing frames against wind uplift and earthquake forces.

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Fasteners have always been the weak link in wood construction. The interlocking timber connections of the past, laboriously mortised and pegged, were weak because much of the wood in a joint had to be removed to make the connection. In todays wood connections, which are generally based on metal devices, it is usually impossible to insert enough nails, screws, or bolts in a connection to develop the full strength of the members being joined.

Adhesives and toothed plates are often capable of achieving this strength but are largely limited to factory installation. Fortunately, most connections in wood structures depend primarily ondirect bearing of one member on an-other for their strength, and a varietyof simple fasteners are adequate forthe majority of purposes.

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Hardboard is a thin, dense panel made of highly compressed wood fibers.

It is available in various thicknesses  and surface finishes, and in some  formulations is durable against weather  exposure. Hardboard is produced  in configurations for residential siding and roofing as well as in general-purpose and pegboard panels of standard dimension.

Insulating fi  berboard  sheathing is a low-density panel, usually ½ or ¾ inch (13 or 19 mm) thick, made of  wood or vegetable fibers and binders, and coated with asphalt for water  resistance. It has some thermal insulating value and is used in wood construction chieß  y as nonstructural wall  sheathing, although some panels of  this type are strong enough to also  contribute lateral force resistance to  a building frame. Other  cellulosic fiberboard panels are made from finely  processed recycled paper waste and  are used for wall sheathing, acoustical isolation, carpet underlayment, and  even structural roof decking. Panels  of these types are low in cost, make  productive use of waste or recycled  materials, and help to conserve forest  resources.

Hardwood plywood panels, made  from birch, maple, poplar, or alder veneers, are popular for use in  cabinetry and other interior finish carpentry.

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For structural uses such as sub-flooring  and sheathing, wood panels may be specified either by thickness or by  span  rating. The span rating is determined  by laboratory load testing and is given  on the gradestamp on the back of the  panel, as shown in Figures 3.33 and  3.34. The purpose of the span rating  system is to permit the use of many  different species of woods and types  of panels while achieving the same  structural objectives. Under normal  loading conditions, any panel with  a span rating of 32/16 may be used  as roof sheathing over rafters spaced  32 inches (813 mm) apart or as sub-flooring over joists spaced 16 inches  (406 mm) apart. The long dimension of the sheet must be placed perpendicular to the length of the supporting members. A 32/16 panel may be  plywood, composite, or OSB, may  be composed of any accepted wood  species, and may be any of several thicknesses, so long as it passes the  structural tests for a 32/16 rating.

Figure 3.33 Typical gradestamps for structural wood  panels. Gradestamps are found on the back of each panel. (Courtesy of APA–The Engineered Wood Association)

Figure 3.34 A guide to specifying structural panels. Plywood panels for use in wall paneling, furniture, and other applications where  appearance is important are graded by the visual quality of their face veneers rather than their structural properties.

The designer must also select from  three  exposure durability classifi  cations  for structural wood panels: Exterior,  Exposure 1, and Exposure 2. Panels marked Exterior are suitable for  use as siding or in other applications  permanently exposed to the weather. Exposure 1 panels have fully water- proof glue but do not have veneers of  as high a quality as those of Exterior panels; they are suitable for structural  sheathing and sub-flooring, which will  be protected from the weather once  the building is finished, but must often endure long periods of wetting during  construction. Exposure 2 is suitable  for panels that will be fully protected  from weather and will be subjected  to only a minimum of wetting during  construction. About 95 percent of  structural panel products are classified as Exposure 1.


For plywood panels intended as finish surfaces, the quality of the face  veneers is of obvious concern and  should be specified by the designer  (Figure 3.32). For example, A-B  plywood (with an A grade veneer on  one face and a B grade veneer on the  other) might be specified for cabinet construction, while less expensive  C-D plywood is adequate for exterior  sheathing that will be concealed in  the finished construction. For higher-quality architectural woodwork,  fine  flitch-sliced hardwood face ve- neers may be selected rather than  rotary-sliced softwood veneers and  the matching pattern of the veneers specified.

Figure 3.32 Veneer grades for softwood plywood. Ply- wood with A grade face veneer is costly and limited in availability. It is used only in applications demanding the highest quality appear- ance. Plywood with B face veneer is used for  concrete formwork construction and for less demanding appearance applications. Plywood  with C-plugged face veneer is used for under- layment and combination subfl  ooring/under- layment, where an especially smooth substrate is required to avoid telegraphing irregularities  through the fi nish fl ooring material. Plywood for sheathing and subfl ooring is most com- monly specifi ed as “CDX,” consisting of a C grade face, a D grade face, and constructed  with exterior glue (the panel is installed with the C face oriented toward the weather). C  grade veneer is also the lowest grade permitted in any ply of an Exterior rated panel, that is, a  siding panel intended for permanent exposure to the weather.

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Structural wood panel products fall into three general categories (Figure 3.29): Plywood panels are made up of thin layers of wood  veneer glued together. The grain on the front and  back face veneers runs in the long direction of the sheet, whereas the grain  in one or more interior crossbands  runs perpendicular, in the shorter direction. There is always an odd  number of layers in plywood, which  equalizes the effects of moisture movement, but an interior layer may  be made up of a single veneer or of  two veneers with their grains running  in the same direction. Composite panels have two parallel face veneers bonded to a core of reconstituted wood fibers. Nonveneered panels are of several  different types:

Oriented strand board (OSB) is made of long shreds (strands) of wood  compressed and glued into three to five layers. The strands are oriented  in the same manner in each layer  as the grains of the veneer layers in  plywood. Because of the length and  controlled orientation of the strands,  OSB is generally stronger and stiffer  than the other types of nonveneered panels. Because it can be produced  from small trees and even branches,  OSB is generally more economical than plywood. It is the material  most commonly used for sheathing  and sub flooring of light frame wood buildings. OSB is also sometimes  called waferboard, a name that applies  more precisely to a similar type of  panel made of large flakes of wood  that has been largely replaced by  OSB.

Particleboard is manufactured in different density ranges, and is made up  of smaller wood particles than OSB  or waferboard that are compressed and bonded into panels. It  finds use  in buildings mainly as a base material  for wood veneer and plastic laminate.



It is also used commonly as an under- layment panel to create an especially  smooth substrate for the application  of resilient flooring.

Fiberboard is a very fine-grained  board made of wood fibers  and  synthetic resin binders intended for  interior uses only. The processing of  the raw wood products in fiberboard  manufacturing is more intensive  than that in the manufacture of particleboard, resulting in a panel that  is dimensionally more stable, stiffer, better able to hold fasteners, and  superior in its working and finishing  characteristics. The most commonly  used form of fiberboard  is  medium- density fi  berboard (MDF). It is used in  the production of cabinets, furniture,  moldings, paneling, and many other  manufactured products. Though the  names are similar, care should be taken not to confuse MDF panels with  medium-density overlay (MDO) ply- wood panels, discussed below.

Figure 3.29 Five different wood panel products, from top to bottom: plywood, composite panel, waferboard, OSB, and particle- board.

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Wood in panel form is advantageous for many building applications (Figure 3.28). The panel dimensions are usually 4 by 8 feet (1220 by 2440 mm).

Panels require less labor for installation than individual boards because  fewer pieces must be handled, and  wood panel products are fabricated  in such a way that they minimize  many of the limitations of boards and
dimension lumber: Panels are more nearly equal in strength in their  two principal directions than solid  wood; shrinking, swelling, checking,  and splitting are greatly reduced.

Additionally, panel products make  more efficient use of forest resources  than solid wood products through  less wasteful ways of reducing logs  to building products and through  utilization in some types of panels  of material that would otherwise be  thrown awayÑbranches, undersized trees, and mill wastes. Many wood  panel products are made largely  from recovered or recycled wood  waste, and panels made from rapidly  renewable vegetable fibers are also  available.

Figure 3.28 Plywood is made of veneers selected to give the optimum combination of economy and performance for each application. This sheet of roof sheathing plywood is faced with a D veneer on the underside and a C veneer on the top side. These veneers, though unattractive, perform well structurally and are much less costly than the higher grades incorporated into plywood made for uses where appearance is important.

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Of the major building materials, wood is the only one that is organic in origin. This accounts for much of its uniqueness as a material of construction. Trees grow naturally in the forest, and most of the work of ÒmanufacturingÓ wood is done for us by the solar-powered processes of growth within the tree itself. This makes wood a renewable resource and inexpensive to produce: Trees need only be harvested, cut to size, and dried before they are ready for use in construction. On the other hand, we have only limited control ofthe quality and properties of wood. Unlike steel, concrete, or masonry, we can do little to refine solid wood tosuit our needs. Rather, we must accept its natural strengths and limitations.

Wood is well suited to our building structures. The tree itself is a wooden structure, a tower, erected for the purpose of displaying leaves to the sun.* The tree is sub-ject to the same forces as the buildings we erect: It supports itself against the pull of gravity; it withstands the pressures of wind and the accumulation of snow and ice on its limbs; and it resists the natural stresses of the envi-ronment, such as changes in temperature and moisture, and attack by other organisms. Wood has evolved to per-form these jobs well in the tree. Not surprisingly, it also performs them well in our buildings.

In nature, the greatest forces acting on a tree, such as the pressure of the wind, tend to act in bursts of relatively short duration. In response, wood has evolved to be strongest when resisting forces that act brieß y rather than those that act over longer periods of time. When we build with wood, we recognize this unique property. In the engineering design of a wooden structure, the maximum loads that the structure may carry are increased as the length of time these loads are expected to act decreases.

This increase, called the load duration factor, ranges from 15 to 100 percent over the basic design strength of the material and is applied to wooden structures when considering the effects of wind, snow, earthquake, and impactÑ the same types of loads the tree has evolved to withstand most effectively.

The structural form of the tree is also well suited to its environment. Tree branches are supported at one end onlyÑan arrangement called a  cantilever. The joint sup-porting a cantilevered branch must be strong and stiff, efficiently utilizing the materials structural capabilities to the utmost. Yet despite the stiffness of thejoint, the branch it supports can deß ect relatively large amounts since it is fastened at one end only. Underheavy loads, such as wet snow, the branch can droop and shed its load. Likewise, under strong winds,the tree and its branches can sway and shed much of the force acting upon them.

The trees capacity to bend and sway helps it to survive forces that might otherwise break it. When we build with wood, we cannot directly exploit the efficient structural form of the tree. The large deß  ections characteristic of this form are unsuitable for buildings. They would cause discomfort to building occupants and place undue stress on other building components.

Nor can we exploit the naturally strong joints that join a trees limbs, despite their benefits in stiffness and economy of material. In preparing wood for our uses, we cut the tree into pieces of convenient size and shape, destroying the continuity between its parts. When we reassemble these pieces, we must devise new ways to join them. Despite the many methods developed for fabricating wood connections, joints withstrength and stiffness comparable to the trees are rarely achievable. Rather, we rely on much simpler types of connections that suffice when a beam is supported at both ends.

Wood in a tree performs many functions that wood in a building does not, because wood is involved with all aspects of the trees life processes and growth. In order to meet numerous and specialized tasks, wood has evolved a complex internal structure that is highly directional.

The fact that wood is anisotropic, that is, it has different properties in different directions, affects virtually all of its properties. The direction of the grain in a piece of wood influences its production, shaping, fastening, strength, durability, aging, and beauty. No other major building material, perhaps with the exception of some specialized reinforced concrete systems, has such directional characteristics.

Consider two wood building systems, the log cabin and the heavy timber frame. Log cabin construction is appropriate to the most primitive construction technology. Logs found on or near the site are prepared for use with a minimum of labor and simple tools. Once assembled, the logs provide structural support and act as interior finish, exterior cladding, and insulation. Though a practical solution to providing shelter, log cabins use wood inefficiently and result in a structure  that is dimensionally unstable due to the large stacks of  cross-grain lumber. Where more sophisticated tools and methods are available, heavy timber takes the place of log cabin construction. In this system, wood members are assembled in ways that more closely resemble the tree form. The resulting structure is stronger, requires less material, and is more stable. A simple change in the orientation of the wood members produces a completely different building system, one that is more efficient, durable, and comfortable.

Modern developments in the wood construction materials industry reß ect efforts to overcome the limitations of wood in its natural state and to make the most  efficient use of the material. Wood panels made of ply- wood are larger and more stable than those that can  be produced from solid boards while minimizing the
adverse effects of wood grain and natural defects. Panel  products such as OSB and hardboard make use of wood waste that might otherwise be discarded. Glue-laminated and composite lumber beams can be produced in sizes and of a quality exceeding those available in solid timbers. Chemical treatments can protect wood from fire or decay. Thus, the trend is toward using wood less in its natural state and more as a raw material in sophisticated manufacturing processes to produce more refined and higher-quality construction components. Yet despite such advances, the tree itself remains an inspiration in its grace and strength, as wellas in the lessons  it offers for our understanding of building materials and methods.

*Brayton F. Wilson and Robert R. Archer, ÒTree Design: Some  Biological Solutions to Mechanical Problems,Ó BioScience, Vol. 29, No. 5, p. 293, 1979.

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Wood-plastic composite (WPCs) products are made from wood lumbers and  plastics of various types, mixed with  other ingredients, such as ultraviolet stabilizers, pigments, lubricants,  and biocides, which are then heated  and pressed, extruded, or injection-molded into final form. In comparison to their solid wood counterparts,

WPC materials offer more consistent  material quality, freedom from defects and distortion, and, depending  on their formulation, superior resistance to moisture. They are used  most notably for exterior decking,  as well as for exterior railing systems  and finish trim, both interior and  exterior. Like structural composites,  WPC products make productive use  of rapidly renewable or waste materials. Some WPCs also have high recycled materials content.

WPC decking, most commonly  made from blends of polyethylene  or polypropylene and wood lumber,  is  available in sizes matching conven-tional solid wood decking in lengths  up to 20 feet (6.1 m). It may be fastened with corrosion-resistant nails or  screws, or with concealed hardware  that engages the edges of the boards.

A variety of maintenance-free colors  and textured finishes are available,  some remarkably similar in appearance to genuine hardwoods.

Ingredients and manufacturing processes used in the making of  composite wood trim vary widely, as do  the workability, surface qualities, and  durability of the finished  products.

Blends of plastic and wood similar  to those used for the manufacture  of composite decking may be used.

Alternatively, formulations with a  higher proportion of wood lumberÑ more similar in composition to traditional wood panel products, such as plywood, OSB, and fiberboard,  discussed later in this part may be  used. Products may be preÞ  nished, factory primed, encapsulated within a dense plastic outer shell, or coated with a resin-impregnated paper that  improves the quality of field-applied finishes.

In comparison to solid lumber, nonstructural composite lumber expands and contracts more with changes in temperature, so greater allowance for thermal movement must be made during installation.

And in the case of spanning members such as decking, the lesser stiffness of composites necessitates closer spacing of joists or other members on which the members are supported.

Wood trim made up from shorter lengths of finger-jointed  and  glued solid wood material is also available as an alternative to conventional finish lumber. In comparison to solid pieces, finger-joint stock is more stable and more consistently free of defects. It also makes use of short-length scraps that might otherwise be treated as waste.

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Structural composite lumber, also called engineered lumber, products are substitutes for solid lumber made from wood veneers or wood fiber  strands and glue.  Laminated strand lum-ber (LSL) and  oriented strand lumber (OSL) are made from shredded wood strands, coated with adhesive, pressed into a rectangular cross section, and cured under heat and pressure (the wood strands used in the manufacture of LSL are longer than those used in OSL). LSL and OSL are the least strong and least expensive of the composite lumber products. They are used mainly for rim boards and short-span headers.  Laminated veneer lumber (LVL) is made from thin wood veneer sheets, as wide as the member is deep, that are glued and laminated into thicker members. LVL is similar in appearance to plywood except without crossbands (Figure 3.26).

Figure 3.26 An LVL beam, made of veneers similar to those used in the manufacture of plywood, resting on its side in preparation for installation on a concrete foundation wall. When placed in its fi nal position, the beam will be rotated so that the laminations are oriented vertically in the beam. Also visible in the photograph are a preservative- treated wood sill (left) and an LSL rim board (right), both of which are discussed later

Parallel strand lumber (PSL) is made from long, thin strips of wood veneer glued and pressed in a process similar to that for LSL and OSL, but with the veneer strips arranged more uniformly parallel than the strands in those other products. PSL is the heaviest, strongest, and most expensive of the composite lumber products (Figure 3.27). LVL and PSL are most commonly used for longer-span headers and oor beams.

Figure 3.27 A view of corner framing for a  residential garage. The lower beam is a PSL. A high-strength member such as this  is needed to span the roughly 9-foot (2.7-m)-wide garage opening while  supporting the fl oor load above. There are two solid wood top plates above  the PSL. Above the top plates is an LSL rim board that encloses and steadies  the fl oor framing behind it. This lower-strength member is adequate for  this nonstructural function. Several manufactured wood products discussed  later in this chapter are also in view: Above the rim board, the laminated edge  of the plywood subfl oor is visible, and to the left of the rim board, an I-joist is  exposed where the fl oor framing projects beyond the plane of the wall below.

Studs and posts may also be made of any of the composite lumber types described in the previous paragraph, as well as from structural finger-jointedlumber in which short lengths of solid  lumber scrap are finger-jointed  and glued end-to-end into longer lengths.

Composite studs and posts are well  suited for use in the framing of walls that are especially tall or that have very large openings, or wherever else long, straight, and especially strong, stiff members are needed.

Structural composite lumber products make productive use of wood materials that are rapidly renewable or that might otherwise be treated as waste, and they offer many of the benefits of glue-laminated wood: dimensional stability, structural strength up to three times that of con-ventional solid material, availability of large sizes and long lengths, and consistent quality. Particularly where these products are used in exposed interior applications, however, attention should also be paid to the types of adhesives and binders used in the products and the potential for offgassing of volatile organic compounds or formaldehyde.

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Lumber sizes in the United States are given as nominal dimensions in inches,  such as 1 x 2 (one by two), 2 x 10, and so on. At one time, sawn lumber may have approached these actual dimensions. Today however, subsequent to sawing, seasoning, and surfacing,  true sizes are less. By the time a kilndried 2 10 reaches the lumberyard, its actual dimensions are close to 1½ by 9¼ inches (38 by 235 mm). The relationship between nominal lumber  dimensions (which are always written without inch marks) and actual dimensions (which are written with inch marks) is given in simplified form in Figure 3.22 and in more complete  form in Figure 3.23. Anyone who designs or constructs wooden buildings soon commits the simpler of these  relationships to memory. Because of  changing moisture content and manufacturing tolerances, however, it is  never wise to assume that a piece of  lumber will conform precisely to its  expected dimensions. Wood members vary in size seasonally with changes in  humidity and temperature. In hot, dry  locations such as attics, wood framing  may shrink to dimensions substantially below its original measurements.

Figure 3.22 The relationship between nominal and actual dimensions for the most common sizes of kiln-dried lumber is given in this simplifi ed chart, which is extracted from the complete chart in Figure 3.23.

Figure 3.23 A complete chart of nominal and actual dimensions for both framing lumber and fi  nish lumber.

Members in older buildings may have been manufactured to full nominal  dimensions or to earlier standards of  actual dimensions such as 15/ 8 inches or 1¾ inches (41 or 44 mm) for a  nominal 2-inch member.

Pieces of lumber less than 2 inches in nominal thickness (38 mm  actual thickness) are called  boards.
Pieces ranging from 2 to 4 inches  in nominal thickness (38 to 89 mm  actual thickness) are referred to collectively as  dimension lumber. Pieces  nominally 5 inches (actual size 114  mm) and more in thickness are  termed timbers.  Dimension lumber is usually supplied in 2-foot (610-mm) increments  of length. The most commonly used  lengths are 8, 10, 12, 14, and 16 feet  (2.44, 3.05, 3.66, 4.27, and 4.88 m),  but retailers frequently stock rafter  material in lengths to 24 feet (7.32 m).

Actual lengths are usually a fraction of  an inch longer than nominal lengths.

Lumber in the United States is  priced by the  board foot. Board foot measurement is based on nominal  dimensions, not actual dimensions.

A board foot of lumber is deÞ  ned as  a solid volume 12 square inches in  nominal cross-sectional area and 1  foot long. A 1 x 12 or 2 x 6 10 feet long contains 10 board feet. A 2 x  4 10 feet long contains [(2 x 4)/12] 10 6.67 board feet, and so on. Prices of dimension lumber and timbers  in the United States are usually quoted on the basis of dollars per thou- sand board feet. In other parts of the world, lumber is sold by the cubic meter. Because the board foot is based on lumber nominal dimensions and  metric sizes use actual dimensions,  there is no direct conversion between  board feet and cubic meters.

The architect and engineer specify lumber for a particular construction use by designating its species, grade,
seasoning, surfacing, nominal size, and chemical treatment, if any. When ordering lumber, the contractor must additionally give the required lengths of pieces and the required number of pieces of each length.