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Nominal and Dressed Sizes of Lumber
Grain direction in a sheet of plywood

Engineering Aid 3 - Beginning Structural engineering guide book
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Figure 6-1.-Examp1e of a laminated lumber. a board 1 ft long by 1 ft wide by 1 in. thick. Since the length of lumber is usually measured in feet, the width in inches, and the thickness in inches, the formula for the  quantity  of  lumber  in  board  feet  becomes  the following: Example: Calculate the board measure of a 14-ft length of a 2 by 4. Applying the formula, you get Lumber less than 1 in. thick is presumed to be 1 in. thick for board measure purposes. Board measure is calculated on the basis of the nominal, not the dressed, dimensions of lumber. The symbol for board feet is bm, and the symbol for a unit of 1,000 is M. If 10,000 board feet of lumber were needed, for example, the quantity would  be  10Mbm. LAMINATED LUMBER Laminated  lumber  is  commonly  used  when increased wood load-carrying capacity and rigidity are required.  Usually  made  of  several  pieces  of  1 1/2-in. -thick lumber, called laminations, the pieces are nailed,  bolted,  or  glued  together  with  the  grain  of  all pieces running parallel (fig. 6-1). When extra length is needed, the pieces are spliced with the splices staggered so that no two adjacent laminations are spliced at the same point. Built-up beams and girders are examples of  laminated  lumber. Laminations may be used independently or with other materials in the construction of a structural unit. Trusses  can  be  made  with  laminations  for  the  chords and  sawed  lumber  for  the  web  members  (fig.  6-2). Special  beams  (fig.  6-3)  may  be  constructed  with laminations for the flanges and sawed lumber for the webs. Probably the greatest use of laminations is in the fabrication of large beams and arches. Beams with spans larger than 100 ft and depths of 8 1/2 ft have been constructed  with  2-in.  boards.  Laminations  this  large are factory-produced. They are glued together under pressure.  Most  laminations  are  spliced  using  scarf joints (fig. 6-4), and the entire piece is dressed to ensure uniform  thickness  and  width. PLYWOOD Plywood is a panel product made from thin sheets of wood called veneers. An odd number of veneers, such as three, five, or seven, is generally used so the grains on the face and back of the panel run in the same direction. Cross-lamination (fig. 6-5) distributes the grain strength in both directions, creating a panel that resists  splitting  and,  pound  for  pound,  one  of  the strongest  building  materials  available. Figure 6-2.-Truss using laminated and sawed lumber. 6-8







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