Building Design and Construction Handbook - Merritt - Ventech!

FIGURE 10.17 Types of wood trusses.
WOOD CONSTRUCTION 10.71
Bowstring Trusses. Spans of 100 to 200 ft are common, with single top and
bottom chords of glued-laminated timber, webs of solid-sawn timber, or glulam,
and steel heel plates, chord plates, and web-to-chord connections. This system is
lightweight for the loads that it can carry. It can be shop or field assembled. Attention
to the design of the top chord, bottom chord, and heel connections is of
prime importance, since they are the major stress-carrying components. Since the
top chord is nearly the shape of an ideal arch, stresses in chords are almost uniform
throughout a bowstring truss and web stresses are low under uniformly distributed
loads.
Parallel-Chord Trusses. Spans of 100 ft or more are possible with chords and
webs of solid-sawn lumber or glulam timbers. Parallel-chord trusses are commonly
used in floor systems and long-span commercial roofs. Advantages for this type of
truss include ease of installation, relatively long spans, flexibility for installation of
ductwork, plumbing, and electrical wiring, and overall economics.
Triangular and Scissor Trusses. Spans of 100 ft or more are possible with chords
and webs of solid-sawn lumber or glulam timbers. These trusses are used extensively
in residential roof construction and long-span applications in commercial
roofs. Advantages are the same as those described above for parallel-chord trusses.
Truss Joints. Connectors used in truss joints include bolts, split rings, timber
rivets, shear plates, lag screws, and metal connector plates, as described in Art.
10.22.1. The type of connector used depends on the type and size of wood truss
components, loads to be transferred, fabrication capabilities, and aesthetics. Lag
screws, bolts, timber rivets, and shear plates are suitable for field fabrication,
whereas metal connector plates are almost always installed in a truss assembly
plant.
Framing between Trusses. Longitudinal sway bracing perpendicular to the plane
of the truss is usually provided by solid-sawn X bracing. Lateral wind bracing may
be provided by end walls or intermediate walls, or both. The roof system and
horizontal bracing should be capable of transferring the wind load to the walls.
Knee braces between trusses and columns are often used to provide resistance to
lateral loads.