Engineering and 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.

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.