For thousands years the use of
wood trusses has been an important architectural and structural engineering
practice. The concept has been considerably improved over the last 70 years to
enable wood roof trusses and wood floor trusses to become common place
construction components.
With hundreds of variations of
a few standard designs, wood trusses have become some of the most versatile
construction elements in use today. Wood floor trusses, wood roof trusses, open
web wood trusses, wood bridge trusses, and a host of other applications are in
use in many current construction applications. While a solid wood beam is
typically limited to spans of less than 25 feet and is usually not cost
effective as an extended span structural element, they are still specified for
certain high visibility architectural design applications. But by incorporating
the chord and strut design, smaller dimension lumber can utilized which not
only drives down costs but greatly increases the spanning and load bearing
capabilities. Spans as long as 70 feet are possible with 2x6 lumber and loads
exceeding 60 pounds per square feet can be achieved with properly engineered
wood trusses.
Whether prefabricated or built
on site, the design of wood trusses used for structural purposes must performed
properly. Engineered wood trusses are typically built according to custom
specifications but “off the shelf" trusses are available for standard applications.
The most common design is the
triangular chord and strut configuration. This design evolved from Roman era
engineering through the timber railroad bridges of the early 18th century.
Following World War II it was realized that incorporating a flat metal
connector plate along with stress testing and inspected lumber components
yielded a much stronger, reliable, and cost effective truss.
The schematic of a
typical triangular chord and strut wood roof truss design is shown in Figure 1.
The top chords typically bear loads directly, and the resulting tension and
compression load distribution through the struts to the bottom chord accounts
for the greater overall load bearing capabilities of the wood truss.
Each intersection will
typically have a steel reinforcing plate with stamped barbs pressed into an
overlapping position on both sides of the truss which serves to hold the
members together. Many modifications of the same basic configuration are
possible including, but not limited to, hip, scissors, vault, dual pitch,
Polynesian, bow string, dual flat, cantilevered mansard, and almost any
architectural design imaginable.
Wood floor trusses follow the
same triangular load distributing design but have a long rectangular profile,
where the top and bottom chords are parallel to each other. Struts can be
spaced to accommodate utility chases, such as air conditioning and heating duct
runs for example.
Knowing the expected live and
dead loads, the bearing points, overhangs, and other structural factors will
allow a manufacturer to calculate the required actual configuration. This
process has evolved to the point of using automated manufacturing equipment to
fabricate wood trusses to specifications, greatly decreasing labor costs. And by specifying low deflection ratings,
such as L/480 or smaller, structural vibrations can be reduced or eliminated as
well yielding a very solid response to live load application.