Engineering and Construction

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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.