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HISTORIC AMERICAN
ROOF TRUSSES
KENNETH ROWER , EDITOR
.
2
INTRODUCTION
.
David C. Fischetti, P.E.
COMPOUND &
RAISED BOTTOM CHORD
TRUSSES
48
Jan Lewandoski
.
6
SCISSOR TRUSSES
Jan Lewandoski
Scissor Truss Analysis
Ed Levin
.
QUEENPOST TRUSSES
17
22
Compound &
Raised Bottom Chord
Truss Analysis
Ed Levin
64
Classical Truss Analysis
Ed Levin
66
THE CLOSE SPACING OF TRUSSES
68
Jan Lewandoski
.
Queenpost Truss Analysis
Ed Levin
33
KINGPOST TRUSSES
36
.
Jan Lewandoski
Jan Lewandoski
.
THE EVOLUTION OF TRUSSES
Jan Lewandoski
Kingpost Truss Analysis
Ed Levin
46
GLOSSARY
BIBLIOGRAPHY
INDEX OF PROPER NAMES
INDEX OF SELECTED TERMS
CONTRIBUTORS
82
89
90
91
92
70
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Ed Levin
Fig. 1-1. Traditional truss forms evolved beginning in antiquity. Many variants are possible on the forms shown here.
zontal bottom chord, the truss with the raised bottom chord is less
stiff and highly stressed. In recognition of the problem, the “kink”
in the bottom chord is shown in early pattern books as reinforced
with iron.
Chapter 5 presents two early examples of trusses with raised bottom chords. The first is Christ Episcopal Church in Shrewsbury,
New Jersey, built in 1769. The second is the 1807 St. John’s
Church in Portsmouth, New Hampshire. Ed Levin’s analysis in
Chapter 5 verifies the deficiencies in the St. John’s truss, which are
apparent from a study of its configuration and an examination of
its condition. Besides its low depth-to-span ratio, there is an undesirable “prying” action in the principal rafter because of the vulnerable distance between its intersections with the raised bottom
chord and the posts (Fig. 1-2 overleaf ).
Scissor trusses and trusses with raised bottom chords, besides
being less stiff in resisting vertical deflection, will also deform horizontally. Support fixity is a critical issue for the designer to consider and model correctly in the computer. A scissor truss or truss
with a raised bottom chord can be designed with both supports
hinged, free to rotate, but unable to move vertically and horizontally, or with only one support hinged and the other acting as a
roller in the horizontal direction. These two cases give wildly different values for member forces under the same loading conditions.
The designer must consider whether the supports will allow the
truss to deform horizontally or whether the supports are perfectly
restrained. Obviously, the configuration of a scissor truss resting on
a stud wall or simple pinned column has very little resistance to the
horizontal movement of the supports resulting from deformation
of the truss. In fact, such a building cross-section is unstable unless
other provisions are made. A cross-section with heavy masonry
buttresses in the sidewalls may be capable of resisting the horizontal deflection of the truss. This resistance will result in a horizontal
thrust applied by the truss to the top of the wall. In actuality, the
typical building of this type acts somewhere between a true roller
support for the truss and a perfectly rigid buttress. In the comput-
Differential shrinkage can easily pull an unbraced truss out of
alignment. It is good practice to brace the bottom chords of heavy
timber trusses, which may move out of plumb simply through the
uneven drying of individual members. Uneven drying can result
from variations in initial moisture content for a number of reasons
in the timber’s history or growth characteristics, such as the distribution of knots. Often such movement can occur by the “hothouse” effect of solar radiation on a truss exposed to a large window. In traditional timber trussed roofs of the early 19th century,
we see diagonal bracing in the plane of the roof to ensure stability
during erection. This bracing eventually was deleted from most
buildings in favor of temporary bracing in an attempt to simplify.
As did the kingpost truss, the queenpost truss evolved very early
in time, with the first builder probably attempting to span a distance greater than previously possible with a kingpost. “Why not
split the kingpost in half, and spread the halves apart?” the builder
may have thought. In Chapter 3, Jan Lewandoski provides insight
into the queenpost truss and its numerous variations, and in
Chapter 5 he considers in greater detail compound trusses formed
by combining two or more simple trusses. Again, the early trusses
were influenced by the various carpenter’s guides of the time.
Certainly, in our English colonies carpenters and builders learned
from 18th-century publications printed in London. The queenpost
truss is so easy to analyze that it is difficult to believe that builders
did not from early times have at least an intuitive understanding of
the forces involved.
The scissor trusses discussed in Chapter 2 and the raised bottom
chord trusses in Chapter 5 are responses to the various architectural requirements of the spaces framed, chiefly domed or vaulted ceilings, and to the architectural styles of the buildings. Trusses with
raised bottom chords and scissor trusses present several challenges
to engineers. The bottom chord configuration places critical tension members out of line with the natural path of tensile forces acting between supports. This misalignment magnifies the member
forces within the truss. Having less depth than one with a hori-

INTRODUCTION
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Peacham, 1806, 48 ft. 2 in.
Ed Levin
Waterbury Center, 1831, 40 ft.
Stowe, 1867, 50 ft.
Rindge, 1797, 52 ft.
Fig. 3-18. In the proportional diagrams below, bending stress is shown in yellow in psi, axial force in blue (compression) and red (tension) in lbs.
Peacham Bending Stress (psi)
Peacham Axial Force (lbs)
Ed Levin
Rindge Bending Stress (psi)
Rindge Axial Force (lbs)
Stowe Bending Stress (psi), common rafters unvalued
Stowe Axial Force (lbs), common rafters unvalued
Waterbury Bending Stress (psi)
Waterbury Axial Force (lbs)

QUEENPOST TRUSSES
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Fig. 4-10. Congregational Church, Windham, Vermont, 1800. Struts and upper chords bear on unjoggled mortises.
The Castleton roof system is framed almost entirely in hemlock.
The pins are ash, 1⅛-in. diameter in the larger members and ⅞-in.
in the smaller. Of interest are the white oak poles woven in between
the common and principal rafters toward the front of the church,
reaching into the steeple perimeter. These were likely some of the
rigging used to build the tall steeple once the roof trusses and roofing were already in place. Also located at the rear of the steeple are
braced and now cut off 10x10 posts that probably served as the
bottom of the derrick for erecting the steeple or perhaps the trusses themselves. The trusses are functioning well, even carrying some
of the steeple load on a pair of sleepers crossing the forward three
trusses. Other than small openings at the kingpost-to-tie joints,
they show no signs of stress.
Fig. 4-11. Kingpost-to-tie joint assembled and exploded, Windham Congregational Church, 1800. Joists are inserted at one end and swung into
place at opposite end via pulley mortises, seen on face of tie beam in truss elevation at top.

KINGPOST TRUSSES
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Ed Levin
Fig. 5-37. Truss diagram of loads and reactions, kingpost truss.
Fig. 5-38. Graphical analysis of truss in Fig. 5-37.
Method of Joints. Here we isolate individual panel points and consider the forces and reactions acting on them. Our example is the
left-hand eaves of the truss at A (Fig. 5-39). Applying the equations
of static equilibrium for planar systems, we resolve forces in horizontal and vertical components, summing these such that  Fx = 0
and  Fy = 0.
The truss diagram tells us
that external loads at joint 1
consist of a downward force of
1000 lbs. and an upward reaction of 4500 lbs., yielding a net
upward force Fy of 3500 lbs.
Of the two chords connecting
at 1, only BJ is capable of exerting downward force to counter
this load. Given the 9:12 roof
pitch, via the rule of similar triangles the vertical component Fig. 5-39. The load at joint 1,
of force in BJ is 9/15 FBJ. method of joints.
Therefore,
that moments acting on the section also cancel out. By convention,
moments imparting clockwise rotation are positive, those spinning
counterclockwise, negative.
Fig. 5-40 shows our example, the right half of the truss as
bounded by points 3, 5 and 6, and the reactions and forces acting
upon it. We start by summing the moments around point 6:
 Fy
 M6 = 0 = Fx23 ¥
Thus Fx23 = 3333.
By similar triangles, Fy23 = Fx23 ¥ 9/12 = 2500 and F23 = -Fx23 ¥ 15/12
or -4167 (the negative sign indicates compression).
We repeat the procedure for the moments around points 5 and
3, giving us, respectively, F26 = 1667 and F16 = 4667.
By symmetry, F23 = F34, F26 = F46 and F16 = F56, so we now lack
only solutions for F12 = F45 and F36.
A quick joint analysis gives us F45:
 Fy5
= 0 = 3500 + Fy45, so Fy45 = -3500.
F45 = Fy45 ¥ 15/9 = -5833.
= 0 = 3500 + 9/15 FBJ , FBJ = -3500 ¥ 15 ∏ 9 = -5833.
Finally, we sum vertical force at either 3 or 6 to get F36 :
Turning to the horizontal load sum at joint 1, apart from JH the
only other local source of Fx is the horizontal force component in
BJ. Having solved for FBJ, we can now deduce the corresponding
force in JH:
 Fx
30 + 2000 ¥ 20 – (4500 – 1000) ¥ 40.
Fy3 = 0 = F36 + Fy23 + Fy34 – 2000 = F36 + 2500 + 2500 – 2000.
F36 = 3000.
= 0 = FJH + 12/15 FBJ = FJH – 4667, so FJH = 4667.
Having found the resultant forces at joint 1, we can then move
on to adjacent panel points and repeat the process until all forces
in the truss are known.
Method of Sections. Here we isolate an individual member or section of the truss as a free body, an isolated portion of a structure in
static equilibrium. Since the whole is in equilibrium, the parts
must be as well, so the system of forces and moments acting on the
free body must also sum to zero.
To analyze the truss, we apply all three equations of planar static
equilibrium. The first two equations ( Fx = 0 and  Fy = 0) mandate that the resultant of a system of forces sums to zero, sufficient
to ensure equilibrium when all forces act through a single point.
But for a system of forces acting on one or more members of a
truss, we must also invoke the third equation (Â Mz = 0) to warrant
Fig. 5-40. Loads on right-hand side of truss, method of sections.

CLASSICAL TRUSS ANALYSIS
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.The Evolution of Roof Trusses
It is impossible for a native speaker to speak incorrectly.
—Benjamin Whorf
We must labour to be beautiful.
—W. B. Yeats
V
ERNACULAR ORIGINS. The truss form emerged
from the timber framing methods of classical antiquity
in the Mediterranean region and only during the last
two centuries became shaped by engineering analysis
and design. Truss construction has always been associated with the
high end of vernacular carpentry; trusses are rarely found in private
homes or barns, but almost always in prestigious public buildings
such as temples or churches, or in bridges. While we have only a
small body of evidence for the exact form of the trussed roofs of
antiquity, we have abundant extant examples of long-span roof systems from the Middle Ages through the Renaissance. The variety
of forms and the inventiveness of their framers seem without end.
Many of these premodern roof frames are fully realized trusses with
a captured kingpost hanging the middle of the tie beam, and the
ends of the rafters restrained within the same tie (Fig. 7-1).
Multiple kingpost and queenpost examples exist in Switzerland
in the work of the self-taught designers and builders Jakob, Johannes
and Hans Ulrich Grubenmann. Their longitudinal roof truss in the
Reformed Church at Grub (1752) and their Bridge on the Linth
Will Beemer
Fig. 7-1. Ste. Catherine’s Church, Honfleur, France, late 15th century.
(1766) represent the culmination of an established central
European tradition of hängewerk—that is, using posts in tension to
suspend tie beams or truss bottom chords (Figs. 7-2, 7-3).
Grubenmann-Sammlung Teufen, Switzerland, used by permission
Fig. 7-2. Extraordinary longitudinal truss, Reformed Church at Grub, Switzerland, 1752.
HISTORIC AMERICAN ROOF TRUSSES
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