Engineering and Construction

ANCHORAGE OF TIMBER BEAMS AND MASONRY WALLS

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.

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 4.11 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.

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 4.12 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.

Wood Shrinkage in Heavy Timber Construction

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.

CONSIDERATIONS OF SUSTAINABILITY IN HEAVY TIMBER CONSTRUCTION

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.

FIRE-RESISTIVE HEAVY TIMBER CONSTRUCTION

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.

HEAVY TIMBER FRAME CONSTRUCTION

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.

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 4.1 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.

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 4.2 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.

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 4.3 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.

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 4.4 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.

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 4.5 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.

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.

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.

TYPES OF WOOD CONSTRUCTION

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.

Adhesives - Use in the construction

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.

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.

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.

Sheet Metal and Metal Plate Framing Devices

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.

Wood Fasteners

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.

Other Panel Products

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.

Specifying Structural Wood Panels

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.

Structural Wood Panel Types

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.

Wood Panel Products

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.

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.

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.

A NATURALLY GROWN BUILDING MATERIAL

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.

Wood-Plastic Composite Decking and Nonstructural Lumber

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.

Structural Composite Lumber

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.

Lumber Dimensions - Contruction

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.

Structural Properties of Wood

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.

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.

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.

Foundation Design and the Building Codes

Because of the public safety considerations that are involved, building codes contain numerous provisions relating to the design and construction of excavations and foundations. The IBC deﬁ nes which soil types are considered satisfactory for bearing the weight of buildings and establishes a set of requirements for subsurface exploration, soil testing, and submission of soil reports to the local building inspector. It goes on to specify the methods of engineering design that may be used for the foundations. It sets forth maximum loadbearing values for soils that may be assumed in the absence of detailed test procedures (Figure 2.5). It establishes minimum dimensions for footings, caissons, piles, and foundation walls and contains lengthy discussions relating to the installation of piles and caissons and the drainage and waterprooﬁ ng of substructures. The IBC also requires engineering design of retaining walls.

In all, the building code attempts to ensure that every building will rest upon secure foundations and a dry substructure.

Figure 2.5

Presumptive surface bearing values of various soil types, from symbols the 2006 IBC. Classes 3, 4, and 5 refer to the soil group in Figure 2.2.

Figure 2.2
The Uniﬁ ed Soil Classiﬁ cation System, from ASTM D 2487. The group symbols are
a universal set of abbreviations for soil types, as seen for example, in Figure 2.8.

Figure 2.8
A typical log from a soil test boring indicating the type of soil in each stratum and
the depth in feet at which it was found. The abbreviations in parentheses refer to the
Uniﬁ ed Soil Classiﬁ cation System, and are explained in Figure 2.2.

Designing Foundations and Different Design Thresholds

It is a good idea to begin the design of the foundations of a building at the same time as architectural design work commences. Subsurface conditions beneath a site can strongly influence fundamental decisions about a building—its location on the site, its size and shape, its weight, and the required degree of flexibility of its construction. On a large building project, at least three designers are involved in these decisions: the architect, who has primary responsibility for the location and form of the building; the structural engineer, who has primary responsibility for its physical integrity; and the foundation engineer, who must decide, on the basis of site exploration and laboratory reports, how best to support it in the earth. More often than not, it is possible for the foundation engineer to design foundations for a building design dictated entirely by the architect. In some cases, however, the cost of the foundations may consume a much larger share of the construction budget than the architect has anticipated, unless certain compromises can be reached on the form and location of the building. It is safer and more productive for the architect to work with the foundation engineer from the outset, seeking al- ternative site locations and building configurations that will result in the fewest foundation problems and the lowest foundation cost.

In designing a foundation, a number of different design thresh-olds need to be kept in mind. If the designer crosses any of these thresholds, foundation costs take a sudden jump. Some of these thresholds are:

• Building below the water table. If the substructure and foundations of a building are above the water table, minimal effort will be required to keep the excavation dry during construction. If the water table is penetrated, even by an inch, expensive steps will have to be taken to dewater the site, strengthen the slope support system, waterproof the foundation, and either strengthen the basement floor slab against hydrostatic uplift pressure or provide for adequate drainage to relieve this pressure. For an extra inch or foot of depth, the expense would probably not be justified; for another story or two of useful building space, it might be.

• Building close to an existing structure. If the excavation can be kept well away from adjacent structures, the foundations of these structures can remain undisturbed and no effort and expense are required to protect them. When digging close to an existing structure, and especially when digging deeper than its foundations, the structure will have to be temporarily braced and may require permanent underpinning with new foundations. Furthermore, an excavation at a distance from an existing structure may not require sheeting, while one immediately adjacent al-most certainly will.

• Increasing the column or wall load from a building beyond what can be supported by a shallow foundation. Shallow foundations are far less expensive than piles or caissons under most conditions. If the building grows too tall, however, a shallow foundation may no longer be able to carry the load, and a threshold must be crossed into the realm of deep foundations. If this has happened for the sake of an extra story or two of height, the designer should consider reducing the height by broadening the building. If individual column loadings are too high for shallow foundations, perhaps they can be reduced by increasing the number of columns in the building and decreasing their spacing.

For buildings at the scale of one-and two-family dwellings, foundation design is usually much simpler than for large buildings because foundation loadings are low. The uncertainties in foundation design can be reduced with reasonable economy by adopting a large factor of safety in calculating the bearing capacity of the soil.

Unless the designer has reason to suspect poor soil conditions, the footings are usually designed using rule-of-thumb allowable soil stresses and standardized footing dimensions. The designer then examines the actual soil when the excavations have been made. If it is not of the quality that was expected, the footings can be hastily redesigned using a revised estimate of soil-bearing capacity before construction continues. If unexpected groundwater is encountered, better drainage may have to be provided around the foundation or the depth of the basement decreased.