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In wood frame design many times strong woods are used such as Douglas Fir, Larch, Southern Pipe and Oak. However, due to the decrease in supply of large diameter, old-grown trees, there has been in increase in the use of engineered wood materials, which is made from laminated strand lumber (LSL). LSL processes young, more abundant, small diameter trees such as aspen and poplar and processes them into small wood stands up to 12 inches longs. This wood is then coated with adhesive and compresses into large billets that are sawn into lumber dimensions such as panels, boards and framing materials. This wood can be used instead of traditional 2 x 4 and 2 x 6 dimensional lumber and installed with the same process, tools and fasteners. Below is a link to many common types of woods.

Types of Wood

Since the wood frame design was broken down into the foundation, studs, sheathing, joists and rafters, the different materials and their normal sizes used is divided in a similar manner below.

Foundation

As earlier described, there are three main foundation types, the basement, slab-on grade and crawlspace. For these three types, there are 5 main construction systems used: permanent wood foundation, cast-in-place concrete, concrete or masonry blocks, insulting concrete forms and pre-cast concrete.

  • Permanent wood foundation (PWF) construction is similar to wood-framed exterior wall construction. However, since PWF walls are used in below-grade applications, all lumber and plywood is pressure-preservative treated for decay and termite resistance. Because wood foundations are significantly lighter than concrete or block walls, they may be placed on gravel beds, which eliminate the need to cast and cure a concrete footing. Walls are normally framed with 2x8 treated studs on 16" centers, or as specified by the designers. The exterior foundation surfaces are clad with " or 5/8" treated plywood, and polyethylene film is applied before backfilling.

 

  • Cast-in-place concrete construction has rigid foam board insulation normally placed between removable forms with non-conductive ties holding it in place. Concrete is poured on either side of the foam and steel rebar is also normally used to add strength to the wall. After the concrete has cured, the forms can be removed and used again up to 3,000 times without much maintenance.

     

  • Concrete or masonry blocks are used for a foundation by filling the block cores with insulation. High-pressure foam works better than most block-filling methods, such as poured-in insulation like polystyrene beads and vermiculite. Foam inserts are also available for the block cores and are installed as the blocks are mortared into place. Materials such as polystyrene or wood chips can be added to the concrete mix to increase the thermal resistance.

 

  • Insulting concrete forms (ICFs) are used as both the foundation structure and insulation. These forms are made of rigid foam insulation and are either pre-formed interlocking blocks or separate panels connected with plastic ties. These forms stay in place as a permanent part of the wall assembly and provide both a continuous insulation and backing for drywall on the inside.

 

  • Pre-cast concrete foundation walls or panels are normally preinsulated and also have rigid foam board. The panels normally come in lengths of up to 16 feet and in standard heights of 4, 8 and 10 feet. Additional insulation can be added inside the wall cavity to achieve a high R-value.  

 

Studs

Normally strong woods such as Douglas Fir, Larch, Southern Pin and Oak are used, but engineered wood materials can be used as well. Normally for studs supporting a roof only either 2x4 @ 24"o.c. or 2x6 @ 24"o.c are used. For studs supporting one floor and roof, normally 2x4 @ 16"o.c or 2x6 @ 24"o.c. When studs are used to support two floors and a roof, 2x6@16"o.c. are used. If studs are used to to support three floors and roof the normal size used is 2x8@ 16"o.c.

Sheathing

Sheathing is a rough covering over the framing of a building, either roof or wall, which is not exposed when a finish material is applied. Normally plywood or oriented strandboard (OSB) panels are used by nailing them to the studs with the long directions of the panels being oriented perpendicular to the joist. Plywood panels are made by bonding veneers together under heat and pressure. Normally the grain is at right angles to each other and symmetrical about the center ply. A veneer is a thin sheet of wood rotary cut, sliced or sawn from a log or flitch. Oriented strandboard (OSB) is a non-veneered wood panel product made by bonding three or five layers of long, thin wood strands under heat and pressure using a waterproof adhesive. The surface strands are aligned parallel to the long axis of the panel which makes the panel stronger along its length.   

 

Joists

Normally strong woods such as Douglas Fir, Larch, Southern Pin and Oak are used, but Engineered wood materials can be used as well. Normal woods can also be used as long if there is a smaller span is used joists. Therefore, the type of wood affects the span of the joists. Below is a chart of the different wood floor joists with residential floor loads. If larger loads are needed, the joist size should be increased by one size. 

The Architect's Studio Companion

Rafters

Normally strong woods such as Douglas Fir, Larch, Southern Pin and Oak are used, but engineered wood materials can be used as well. Normal woods can also be used as long if there is a smaller span of used joists. As a result, the type of wood affects the spacing of the rafters. This difference between normal and strong wood can be seen in the chart below which is for wood rafters with a pitch of at least 3:12 and no finish ceiling attached to the underside of the rafters. Spans should be decreased by 1 to 2 feet for rafters with lower slopes and the rafter size should be increased by one size for unusually heavy loads.  

The Architect's Studio Companion

Connections

While there are many ways to join wood members, the use of nails, screws, glue and gussets are the most practical for wood frame design. These methods are very simple and economical, with nails, screw and glue being normal things most people have used in their life. A gusset is a flat wood, plywood, metal plate or similar type member used to provide a connection at intersection of wood member. The gussets are fastened by nails, screws or adhesives. Many other methods to connect wood members are not practical. For example, a connection such as a mortise and tenon, could be used, but do to the amount of connections needed in a wood frame design, it would be too time consuming and unpractical. In this connection, a member is cut to insert into a hole in another member and the connection is often held in place by a peg. 

A Gusset Connection
http://www.pre-engineering.com/resources/tenconn/tconnection.htm

Beam to Column Connection
Architectural Graphic Standards

Beam to Girder Connection
Architectural Graphic Standards

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This site was last updated 04/18/05