Engineering and Construction

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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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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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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.

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The strength of a piece of wood depends chießy on its species, its grade, and the direction in which the load acts with respect to its grain.

For example, in both tension and compression, wood is typically several times stronger parallel to grain  than perpendicular to grain. With its usual assortment of defects, it is  stronger in compression than in tension. Allowable strengths (structural  stresses that include factors of safety) vary tremendously with species and  grade. For example, allowable com- pressive strength parallel to grain  for commercially available grades  and species of framing lumber varies  from 325 to 1700 psi (2.24 to 11.71 MPa), a difference of more than five times. Figure 3.21 compares the average structural properties of framing lumber to those of some other  common structural materialsÑbrick  masonry, steel, and concrete. Of the  four materials, only wood and steel  have useful tensile strength. Defect- free wood is comparable to steel on  a strength-per-unit weight basis, but  with the ordinary run of defects, an  average piece of lumber is somewhat  inferior to steel by this yardstick.

Figure 3.21 Comparative physical properties of four common structural materials: wood (shaded row), brick masonry, steel, and concrete. Wood has signifi cant strength in both tension and compression. The ranges of values of wood strength and stiffness refl ect differences among species and grades of lumber.

When designing a wooden struc- ture, the architect or engineer determines the maximum stresses that are  likely to occur in each of the structural  members and selects an appropriate  species and grade of lumber for each.

In a given locale, a limited number of  species and grades are usually avail- able in retail lumberyards, and it is  from these that the selection is made.

It is common practice to use a stronger  but more expensive species (Douglas  for or Southern pine, for example) for  highly stressed major members and  to use a weaker, less expensive species  (such as Eastern hemlock) or species  group for the remainder of the structure.  Within each species, the designer se- lects grades based on published tables of allowable stresses. The higher the  structural grade, the higher the allowable stress. But the lower the structural  grade, the less costly is the lumber.

There are many factors other  than species and grade that inß  uence  the useful strength of wood. These include the length of time the wood will  be subjected to its maximum load, the  temperature and moisture conditions  under which it serves, and the size  and shape of the piece. Certain fire retardant treatments also reduce the  strength of wood slightly. All these factors are taken into account when engineering a building structure of wood.

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It is generally recognized that single-family wood frame structures that include shear walls in their construction are very resistant to collapse from earthquake shaking. This is due to several factors, such as their flexibility, strength, and light dead loads, which produce low earthquake-induced inertia loads. These factors make the wood-frame construction much better at resisting shear forces and hence more resistant to collapse.

There are exceptions to the general rule that wood frame structures are resistant to collapse. For example, in the 1995 Kobe earthquake, the vast majority of deaths were due to the collapse of one- and two-story residential and commercial wood-frame structures. More than 200,000 houses, about 10 percent of all houses in the Hyogo prefecture, were damaged, including over 80,000 collapsed houses, 70,000 severely damaged, and 7000 consumed by fire. The collapse of the houses has been attributed to several factors, such as (EQE Summary Report, 1995):

• Age-related deterioration, such as wood rot, that weakened structural members.
• Post and beam construction that often included open first floor areas (i.e., a soft first floor), with few interior partitions that were able to resist lateral earthquake loads.
• Weak connections between the walls and the foundation.
• Inadequate foundations that often consisted of stones or concrete blocks.
• Poor soil conditions consisting of thick deposits of soft or liquefiable soil that settled during the earthquake. Because of the inadequate foundations, the wood-frame structures were unable to accommodate the settlement.
• Inertia loads from heavy roofs that exceeded the lateral earthquake load-resisting capacity of the supporting walls. The heavy roofs were created by using thick mud or heavy tile and were used to resist the winds from typhoons. However, when the heavy roofs collapsed during the earthquake, they crushed the underlying structure.

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Figure 11.19 shows a picture of a temporary retaining wall that consists of steel I-beams, wood lagging, and tieback anchors. The temporary retaining wall is being used to support the excavation during the construction of the foundation. This type of temporary retaining wall consists of steel I-beams that are either driven into place or installed in pre-drilled holes with the bottom portion cemented into place. During the excavation of the interior area, wood lagging is installed between the steel I-beams and the earth pressure exerted on the wood lagging is transferred to the steel I-beam flanges. In order to reduce bending moments in the steel I-beams and to reduce lateral deflections, tieback anchors are often installed as the excavation proceeds.

In some cases, the steel I-beams will be braced with struts and the earth pressure distributions would be similar to those as shown in Fig. 11.18. For cantilevered steel I-beam and wood lagging retaining walls, there will be sufficient movement of the top of the wall to develop the active earth pressure (i.e., Eq. 11.1).