Engineering and Construction

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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.