Building Design and Construction Handbook - Merritt - Ventech!

10.70 SECTION TEN
Chord splices are accomplished in a similar manner by application of a plate on
each side of the splice.
The ultimate and design load capacity of a plate depends on the steel gage and
the number, size, and design of the steel projections (teeth) that are pressed into
the wood. These characteristics are determined by the manufacturer. Consequently,
metal connector plates from different plate manufacturers are generally not interchangeable
for a specific truss. Most plate manufacturers, in conjunction with their
licensed truss assemblers, provide computerized truss design support for their products.
Thus, designers need show only the required truss configuration and design
loading conditions on the design drawings and the truss supplier will provide all
applicable truss-member designs. Virtually any truss profile can be manufactured
for metal-plate connected wood trusses. More information on this type of truss can
be obtained from the Truss Plate Institute (TPI) and the Wood Truss Council of
America, both located in Madison, Wis.
A variation of the lightweight metal-plate connected wood truss employs much
thicker metal plates in conjunction with heavier wood chord and web members,
such as dimension lumber with a nominal width of 3 or 4 in or glued-laminated
timbers. Use of the heavier components provides much greater load-carrying capacity.
As a result, these trusses may economically be spaced much farther apart;
for example, 8 ft c to c or more, and span much longer distances than the lighter
trusses.
Another form of preengineered, preassembled, lightweight wood truss, commonly
referred to as a composite wood-metal truss, consists of lumber chords and
steel webs. One type uses metal-angle webs with nail plates formed on each end.
At a joint, the teeth of the nail-plate end are pressed into a wood chord. Another
configuration of composite wood-metal truss uses steel tubular web members, which
are joined to the wood chords with patented steel-pin connections. The chords are
typically machine stress-rated lumber or laminated veneer lumber (Art. 10.30.4).
Because of the proprietary nature and uniqueness of the various types of composite
wood-metal trusses, design data should be obtained from the manufacturers.
(See K. F. Faherty and T. G. Williamson, ‘‘Wood Engineering and Construction
Handbook,’’ McGraw-Hill Publishing Company, New York, for additional information.)
10.22.2 Heavy Timber Trusses
Type of heavy timber truss and arrangement of members may be chosen to suit the
shape of structure, the loads, and stresses involved. The configuration most commonly
used are bowstring, flat or parallel chord, pitched, triangular, and scissors
(Fig. 10.17) For most commercial construction, trusses usually are spaced 8 to 24
ft apart.
Joints are critical in the design of a truss. Use of a specific truss type is often
governed by joint considerations. Chords and webs may be single-leaf (or monochord),
double-leaf, or multiple-leaf members. Monochord trusses and trusses with
double-leaf chords and single-leaf web system are common. Web members may be
attached between the chords, or web members may be in the same plane as the
chords and attached with straps or gussets.
Individual truss members may be solid-sawn, glued-laminated, or mechanically
laminated (rarely used in new construction). Steel rods or other steel shapes may
be used as members of timber trusses if they meet design and service requirements.