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Continuing
Education
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Use
the following learning objectives to focus your study while reading
this month’s ARCHITECTURAL RECORD / AIA Continuing Education article.
Learning
Objective:
After
reading this article, you will be able to:
1.
Describe the characteristics of timber trusses.
2.
Explain the differences between solid timber, glulam, parallel
strand lumber, and steel hybrid trusses.
3.
Explain how trusses operate differently than beams.
4. Describe how timber trusses
effect the aesthetics of a space.
5.
Understand timber truss connections and how they control the design
of the truss.
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Trusses made of heavy timber or hybrids of timber and steel
can be remarkably efficient assemblies for spans ranging from
25 to more than 200 feet. In houses, restaurants, churches, museums,
and commercial structures, timber trusses offer advantages over
other types of trusses or conventional framing. As illustrated
by the many aging covered bridges standing today, timbers can
withstand weathering if sheltered from precipitation or treated
with preservatives. Heavy timbers can have a fire resistance of
up to an hour-better than steel trusses, which lose their strength
in high temperatures. Timber trusses may be part of a timber-framed
building, or they may be married to conventional framing. Either
way, they visually convey warmth, solidity, and durability.
The timber components of a truss may come from logs of virtually
any species, though Douglas fir is often favored. Unseasoned wood
will shrink and twist as it dries, pulling the attached parts
of the frame along for the ride. "Even with seasoned wood, the
structure must be detailed to handle movement-by building in tolerances
and even slip joints," says Ben Brungraber, an engineer with Benson
Woodworking Company, truss fabricators, erectors, and timber framers
in Alstead Center, N.H. "A relaxed attitude in the client is also
helpful. If they ask for solid wood, they should appreciate its
propensities," including some twisting and checking, he says.
Glulams, commonly made from select two-by boards of Southern
yellow pine or Douglas fir, also qualify as timber. For their
size, glulams are stronger than solid lumber because of the arrangement
of the wood: The best-quality laminations are reserved for the
top and bottom of a member, where most of the stress is applied.
Parallel strand lumber (PSL), which consists of slender veneer
strips that, pressed together, form a sort of loaf of wood, is
also used. Like glulams, PSL is less wasteful and destructive
to the environment than solid timber since the low-grade raw materials
yield a homogeneous end product. Both types of engineered lumber
are stronger and more dimensionally stable than sawn timbers of
equivalent size and, as a result, are less likely to twist or
check. They are also more expensive, and their availability varies
by region.
Recycled timber of virtually any species often comes from dismantled
industrial buildings. These timbers start out less expensive than
solid or engineered varieties, "but by the time they are denailed,
debugged, planed, patched, cleaned, and cut to size, they wind
up costing a lot more," Brungraber says. Recycled wood, however,
is dimensionally stable and has a priceless patina. "It also doesn't
require cutting down a new tree and adds 'karma' to the space,"
he adds.
Though timber trusses tend to appear solid and heavy, they can
take on a more delicate look when steel cables or rods are substituted
for tension members. The tensile strength of steel is much greater
than that of timber. Steel components can simplify the connections
in a truss and reduce its actual and visual heft. While adding
steel creates a pleasing mix of materials, it adversely affects
the fire resistance of the truss. Also, because steel expands
and contracts with temperature changes, architects must look carefully
at the connections to wood members. This is especially important
when a hybrid truss is part of a building's exterior.
Truss technology
A truss is a framework of linear elements, triangulated for
stability. The strength of this assembly is a function of its
geometry, connections, and members. Architects know that a triangle
will hold its shape under load. The superstructure's appearance
should be a function of both the framework-the trusses plus their
lateral bracing-and the degree to which it is exposed to view.
Once the members are sized to accommodate the various live and
dead loads, the aesthetics are up to the architect.
To understand how a truss behaves, one must comprehend the stresses
it undergoes. The structural behavior of each member in a timber
truss differs from that of a solid-timber beam or girder. A downward
load along the length of a beam will cause it to bend. But such
a load on a truss generates tension or compression that is shared
by each of its members in concert. For that reason, timber trusses
are structurally much more efficient than timber beams.
Even if a specific truss member acts as a column or collar tie,
the overall truss still functions as a unit. So a truss can be
lighter than a beam for a given span and load. In addition, the
rate at which member weight rises with increasing span is generally
lower with a truss than with a beam. For short spans and light
loads, however, the expenses of engineering and fabricating timber
trusses make them too costly to compete with beams.
Building codes set minimum
timber sizes that qualify as heavy-timber construction to achieve
fire resitance.
Truss fasteners and connectors are usually steel-galvanized
if intended for exterior applications. Holes, grooves, and recesses,
ranging from 1/2 inch to one inch or more in diameter, are required
for some types of fasteners. These voids reduce the structural
capacity of the member. Unfortunately, the fasteners with the
greatest structural capacity-bolts, split rings, and shear connectors-require
the largest holes. As a result, such connections often control
the structural design of the truss and necessitate the use of
larger timbers. For example: Four 1/2-inch holes across the width
of a four-by-six will reduce its net width by two inches, thereby
limiting its carrying capacity to that of a four-by-four. Fasteners
that require few or no holes are recommended wherever the objective
is to minimize member sizes. Building codes, however, set minimum
timber sizes to achieve a reasonable degree of fire resistance.
To qualify as heavy-timber construction, these members can't be
smaller, even if they would meet structural requirements.
Determining aesthetics
The shapes of roof trusses are a function of the shape of the
roof and the character of the room below it. The bottom chord
of a truss becomes, in effect, the ceiling of a room. A flat-bottom
chord is static; an angled one is dynamic. The arrangement, size,
and quantity of web members also affect the look. Other considerations-whether
the members are curved or straight, painted or stained, or treated
with preservatives or fire retardants-also determine the style
of the truss. Chamfering softens the appearance of the members
and enhances their fire resistance. (Rounded corners are less
flammable.) Pressure-treated wood is more difficult to finish,
because paints and stains don't adhere well to saturated wood.
Connectors for trusses with long spans are usually custom made
because the heavy loads they carry need greater connection capacity
than off-the-shelf products can afford. Whether the connections
are concealed or exposed drives the design of the truss. Visible
joinery-in the form of big steel plates and bolts-is desirable
in some settings. "We have a project in which a simple K-shaped
connection would work, but the architect wants a big, bold connector,"
says Paul Swanson, heavy timber specialist for TrusJoist Macmillan.
"Concealing the connection can mean larger members-a larger portion
of the member is chewed up for counterbores."
Truss members and connectors must be able to accommodate loads
not only of different magnitudes, but also from different directions.
A truss member that is in compression under a heavy snow load
might be in tension under an uplifting wind load. The structural
engineer must determine which load combination induces the highest
forces in each member. The structural engineer's findings, in
conjunction with analysis of the connections, will establish the
minimum structural size of each member.
Designing and erecting a timber truss takes longer and often
requires more engineering than conventional framing. "But in the
end, you have a strong, durable building," Brungraber says. "And
the timbers themselves can be reused when the building has outlived
its purpose."
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Case
Study
Timber
has a long and distinguished history in framing the
roofs of religious buildings, such as the structure between
the inner and outer domes of St. Paul's Cathedral in London.
In the 19th century, American carpenters began applying
Gothic motifs to exposed-wood framing, giving rise to the
Carpenter Gothic style.
At
St. Mary's Church in Richmond, Va., Heimsath Architects
of Austin, Tex., reinterpreted that style, used in the existing
church, for a sanctuary addition. The trusses in the new
sanctuary, which are made of Southern pine glulams, are
wider and taller than those in the main church. Still, the
new room is clearly reminiscent of the old.
The
sloping bottom chords of the simple scissor trusses, which
span a distance of almost 44 feet and are spaced about 8
feet on center, draw the eye upward. This geometry is structurally
appropriate: The roof is so steep (with a pitch of 19 in
12) and the trusses so deep that the slope enables the trusses
to be lightweight, despite their considerable span. They
were, however, too large to ship, so they had to be assembled
on-site. Temporary bracing was required between the trusses
until the top chords were stabilized by the roof decking.
Each
column is laterally braced by a stepped shear wall that
is expressed as a buttress on the outside of the building.
Although the sizes of the timbers and the thickness of the
deck are great enough to classify this building as heavy-timber
construction, the 7,000-square-foot structure is small enough
to permit conventional unsprinklered wood-frame construction.
The
trusses provide the framework for the lighting scheme: Custom
Gothic-style pendants hang from them, spotlights in the
chancel are mounted on them, and wiring is concealed along
their top edges. Custom steel plates are bolted through
the timbers at each connection. Tongue-and-groove sawn-timber
decking spans the distance between trusses. Decorative brackets
ease the visual transition from the tops of the columns
to the trusses, and the bottoms of the kingposts are pointed.
The result is a quiet and inspirational interior, perfectly
suited to spiritual contemplation.
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Case
Study
At
Sharon's California House II in Manhattan Beach, Calif.,
designed by Chicago-based Holabird & Root, the top chords
of the trusses are arched three-by-eight-inch mahogany timbers.
Although this spanning device is not exactly a truss, it
behaves like one. The timber arches bear fully on steel
plates perpendicular to their cut ends.
While
the overall span of the trusses is 17 feet, the tie is only
about two-thirds that length. The untied portion at each
end of the frame consists of a sculptural steel-plate connector,
bolted between the pair of steel channels that compose each
column. The trusses are spaced three feet, four inches on
center.
The
curved top chord is made up of two pieces, joined by a steel-plate
midspan connector. A welded assembly of steel plates below
it is triangulated to resist the compression induced by
the shallow vee of the bottom chord. The lower ends of this
assembly are welded to a short, slender length of pipe,
which provides an ideal bearing surface for the bottom chord.
The
architects selected a 1/2-inch steel rod for the bottom
chord, which provides strength without making the trusses
appear clunky. The ends of rod are screwed into clevises-horseshoe-shaped
iron pieces that allow the length of each bottom chord to
be fine tuned.
To
keep the arches from shifting, each pair is bolted through
a steel plate projecting from the adjacent connector. The
bolts and their associated nuts and washers stand out from
the timber arches to which they are fastened.
The
timbers are finished with a clear sealer, while the steel
connectors are painted. The hollow boxlike design of these
connectors transforms the joints into dramatic and unanticipated
voids. Daylight streaming in through the clerestories between
the trusses accentuates their rhythm and form.
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Case
Study
It
is difficult to comprehend the enormity of the trusses
in the 237,000-square-foot New South Wales Royal Agriculture
Society Exhibition Hall, outside Sydney, Australia. Each
spans a distance of 220 feet. The timbers, made of radiata
pine glulams finished with penetrating oil preservative,
are 32 inches deep, and range from 7 inches to 10 inches
in width. The trusses are arranged in pairs, each sharing
a bottom chord, and are spaced at 120 feet on center. Each
pair defines the boundaries of the six sections that make
up the exhibition halls that will house sporting events,
including volleyball and gymnastics, during the Summer Olympics
in 2000.
The
project engineers, Ove Arup & Partners, compared the costs
of all-steel trusses to timber hybrids and concluded that,
while the latter were slightly more expensive, they offered
environmental advantages. The wood for the glulams is new-growth
timber from a pine plantation in New Zealand. The efficiency
of the glulams, which consist of finger-jointed two-by material,
also required less wood than conventional trusses. Even
so, the scale of the project was such that, during peak
production, six laminating plants were involved, taking
material from four different mills.
The
behavior of these trusses is more complex than that of conventional
vertical trusses. They act, in unison with the purlins,
columns, and other components of the structural grid, as
part of the vaulted roof design, carrying different loads
and offsetting different forces than those normally encountered
in truss construction. The truss pairs are rotated 45 degrees
about their longitudinal axes, putting them at 90-degree
angles to one another. This configuration enables them to
help support the weight of the structure while resisting
both horizontal and vertical forces.
Sydney
architects Ancher Mortlock Woolley incorporated steel rods,
ranging from 1 1/2 inches to 2 1/2 inches in diameter, for
the web members. Triangulated in both directions, the rods
handle stress reversals from tension to compression forces.
Turnbuckles enable careful adjustment of the rods. A continuous
16-inch-diameter steel rod works with the bottom chord to
increase overall truss stiffness. Custom steel-plate connectors
were used at the joints.
The
bottom chords of some of the trusses support rails for operable
acoustic doors. These doors allow for subdivision of the
hall, thus accommodating different sporting and agricultural
society events.
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Continuing Education