Part 3 — Types of Beams You Are Likely to Encounter If a surgeon doesn't possess a foundation of human anatomy and physiology knowledge, would you let that surgeon cut you open and probe around inside your body? Would you let a fire officer send you inside a burning structure if that...
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- Boards — Less than two inches deep and at least two inches wide. Boards are graded for appearance rather than strength. Boards are used for siding, subflooring and interior finish work.
- Dimension lumber — Generally, two to four inches deep and at least two inches wide. Used extensively for general construction, dimension lumber is graded for strength rather than appearance. Dimension lumber includes rafters, joists and light-framing elements such as wall studs. "Light framing" lumber is two to four inches deep and two to four inches wide.
- Structural lumber — At least five inches deep and two inches wide and graded for strength and structural application. Beams and stringers must be two inches wider than the depth. Posts and timbers are at least five inches deep by five inches wide. (Width cannot be more than two inches greater than the depth.)
- Timbers — Five inches or more in the smallest dimension.
- Heavy timbers — Columns must be no less than eight by eight inches when supporting floor loads and not less than six inches in the smallest dimension when supporting a roof or ceiling. Beams and girders must not be less than (nominal) six inches wide and 10 inches deep. The smallest nominal dimension for sawn-wood timber-truss components that will support a floor load cannot be less than eight inches.
Lumber is measured in "board feet," with one board foot equal to the volume of a piece of wood with a nominal dimension of 12 inches square and one inch deep. (A 2,100-square-foot home would require roughly 13,125 board feet of lumber.)
The term "nominal" refers to the dimensions of a piece of lumber before drying and surface planning; the term "dressed" refers to the actual dimensions of lumber after drying and surface planning. Dressed sizes are always shown with inch marks (") and nominal sizes are always shown without inch marks. Sawn lumber is generally available in lengths from six to 24 feet.
2. Glue Laminated (Glulam) Lumber
In 1933, the first glue laminated beam was used in the United States at the Peshtigo School in Madison, WI. When you hear "Glulam," you can be certain it is a substantial beam. Rather than a composite of thin wood veneers or tiny chunks of wood, Glulams are a layered composite of full-dimensional lumber and high-strength glue. It is not difficult to find a contemporary Type III commercial building featuring a panelized roof supported by huge Glulam girders. (Example: a concrete tilt-up warehouse with a panelized wood roof supported by steel columns.)
A typical Glulam girder is basically a stack of glued together two-by-four-inch dimensional lumber and is very strong, thus the complete name: glue laminated lumber beam. When nature and human engineering collaborate, incredible strength and reliability result. Pound for pound, an engineered Glulam girder will always be stronger than the most perfect, defect-free, sawn heavy timber that nature could grow. A unique feature of a Glulam beam is that it can be cambered; cambering is the upward pre-bending of a beam into a frown-shape so that the beam will flatten when loaded, rather than bending downward into a smile-shape when loaded. For additional resistance to tension, a Glulam hybrid is available that incorporates laminated veneer lumber (LVL) along the bottom of the beam.
Although glued together and thus classified "EWB" (engineered wood beam), Glulam girders are not vulnerable to failure when the glue heats. The fire performance of a Glulam girder can be expected to be at least that of a perfect heavy-timber girder of the same dimension. Be more concerned with the fire performance of unprotected steel columns that are often used to support Glulam girders and, as always, pay attention to the weakest part of any structural system: the connections.
Glulam beams can be factory curved and are often used as ground-to-ground arches in plank-and-beam structures. It is often desirable to leave finished Glulams exposed rather than hidden above ceilings. Glulams are also used for contemporary timber truss components.
As strong as they are, Glulam beams can and do fail. The primary cause of Glulam failure is improper design and overloading. Overloading can cause shear failure at the supported ends and mid-span tensile failure along the bottom of the Glulam. Manufacturing defects can also make Glulams vulnerable to overload failure. The good news is that contemporary building codes make overload failure of glue laminated lumber beam unlikely.
During pre-planning visits, look for Glulam beams that have been reinforced with post-tensioning rods along the beam bottom. These rods should be considered a red-flag indicator of a Glulam in distress. The steel tension rod will fail when heated to around 800 degrees Fahrenheit.
3. Laminated-Strand Lumber (LSL)
A laminated-strand lumber beam is an engineered, composite, factory-made beam. LSLs are used routinely as headers above windows and doors within wood frame walls. LSL is manufactured using small strands (up to 12 inches long) of hardwoods that normally would not be considered suitable for structural applications. The face-side appearance of LSL is similar to that of oriented strand board (OSB).