Dmn:- AI.rlq

Queen-rod Roof Truss.

The Evolution of

THE TRUSS
AE 685
University of Florida
Department of Architecture
Historic Preservation Option
David E. Ferro
May 18, 1976

INTRODUCTION

SLIDE 1

The purpose of this study is to trace the development of

truss structures from ancient times to the dawn of modern

construction--the beginning of this century. Although this

is primarily a study of the truss forms as they have been

utilized in roof construction, many of the most important

of these structural forms have been the result of the de-

velopment of bridges, especially the railroad bridges of

the nineteenth century, and these forms will also be traced.

Instead of attempting to divide the innumerable truss forms

developed prior to 1900 into classes according to their

characteristic features, considering how one system is mixed

with another and then with another, a few forms have been

selected which will directly or indirectly include most of

the varieties which had been developed.

DEFINITION

SLIDE 2

SLIDE 3

A truss is basically a structural framework of wood, metal,

or both (a composite truss), generally composed of straight

members which are fastened to form a rigid, undeformable,

geometric unit. Trusses function as long-span systems which

distribute loads to supports through a linear arrangement of

various sized members placed in a single plane. In an ef-

ficient truss system these members, each shorter than the

total span, theoretically are subjected only to axial tensile

and compressive forces. Such a system has no horizontal reac-

tion component at the wall structure upon which it rests--

no inward or outward thrust which the wall must be structured

to resist (see Fig. 1).

The conventional truss is designed with the assumption that

all joints are pinned, or free to rotate, and derives its

rigidity, and often its form, through the geometry of tri-

angulation. No rigid framework can be made which does not

consist of a a repitition of the triangle. Any framework of

more than three members can therefore alter its shape unless

it is divided into triangles by diagonal braces.

The basic truss frame triangle may also be used to produce

other structural forms including: rigid frames, arch trusses,

and two-way flat span systems (see Fig. 2).

D

RooF
Elements
Braces. Diagonal subsidiary timbers
inserted to strengthen the framing of
a roof. They can be straight or arched
(the arched brace is a refined version
of the cnUCK), and connect either a
tie-beam with the wall belov, or a
collar-beam with the rafters below.
Collar-beam. A tie-beam applied
higher up the slope of the roof.
Haniiierbeamn. A horizontal bracket
roof, usually projecting at the wall
plate level, to carry arched braces and
struts, and supported by braces. Ham-
merbeams lessen the span and thus
allow shorter timbers. They also help
to reduce lateral pressure.
King-post. The middle upright post
in a roof TRUSS cotIecting the tie-
beam or collar-beam with the RIDGE.
Principals. The main rafters of a
roof, usually corresponding to the
main bay divisions of the space below.
Purlin. A horizontal timber laid
parallel with the wall plate and the
ridge beam some way up the slope of
the roof, resting on the principal
rafters and forming an intermediate
support for the common rafters.
Queen-posts. A pair of upright posts
placed symmetrically on a tie-beam
(or collar-beam), connecting it with
'!h rafters above.
Rifier or com1m1on rafter. A roof
timber sloping up from the wall plate
to the RIDGE.
Strut. A timber, either upright,
connecting the tie-beam with the
rafter above it, or sloping, connecting
a King- or Queen-post to the rafter.
Tic-beamn. The horizontal transverse
beam in a roof, connecting the feet of
the rafters, usually at the height of the.
wall plate, to counteract the thrust.
IWallplate. A timber laid longitudin-
ally on the top of a wall to receive the
ends of the rafters.
Wind-braces. Short, usually arched,
braces connecting the purlins with the
principal rafter and the wall plate, and
fixed fat against the rafters. Wind-
braces strengthen the roof area by in-
creasing resistance to wind pressure.
They are often made to look decora-
live by foiling and cusping

Key:
Ridge. 9. Tie-beam
2. Coin on rafter o1. Wall plate
3. 'Principal rafter IIt. Collar-beam
4. King-post 12. Arched brace
5. Queen-post I3..Hammcrbeam.
6. I'urlin 14. Ilrace
7. Strut S1. Wall post
8. Sole plate 16. Corbel
Roof: Elecents
zs31pl&

(a) KING POST

VERTICAL (

(c) ENGLISH (or HOWE)

DIAGONAL-7 36-..5-3

(b) WARREN

(d) HOWE

(e)PRATT (f)PRATT

(g) FINK (h)BOWSTRING

Fig. 3-46. Common types of roof trusses.

FiP,. ri. General view of the Exhibition building under construction during November, 1850.
24, p .4,6

37, P 194

EARLY TRUSS DEVELOPMENT

;LIDE 4

LIDE 5

The history of building is to a great extent a record of the

efforts of man, the builder, to roof larger and larger floor

spaces, unimpeded by supports.

The truss is one of the four devices that builders have

developed for holding up the roofs or upper stories of struc-

tures--the other being the corbel, post-and-lintel and arch-

and-vault (see Fig. 3). These devices were developed by vari-

ous civilizations according to the materials the ancients had

at their disposal: brick and stone for the corbel and the arch-

and-vault; stone and wood for the post-and-lintel; and wood

for the truss. Greece and China had stone, clay and wood

to chose from and long adhered to post-and-lintel construc-

tion. Mesopotamia had clay but no stone or wood to speak of

and developed the corbel and arch-vault extensively. It was

in wood-rich Europe that the true truss was developed (de

Camp, p. 35).

The multiplicity of gabled roofs of all styles and periods

can be classified in three basic types (see Fig. 4);

I. The bearer-beam type roof is one designed

to be propped in the center with its

rafters acting as beams. Its main disad-

FOUR TYPICAL BUILDING METHODS. (Drawn by the author.) A. POST-AND-
LINTEL. a. ARCH AND VAULT. c. CORBEL OR CANTILEVER. D. TRUSS.

p. 176

TYPE I
Prmct.3al J/reJs
Bending .

I

I-L Dayran.
in ce/re /No
/hrst

1-3
Ac
SCr

I-5
e.e

o/V
ed

!)/e/nedld/e

Or~ory

_tA

rl ~--- by ms

I

F4. /orie t/,e

I p'na3
2o

y/and Curtr I
arer fat .y,&

Yort Cu/dl aA7

Ca/hdA l

1r- Arh-iraced
tol/bou cda/r. Jr.
A/fer/ A^anron//
h'orwich

TYPE 11

IM
Prnci! rl /rei
Co ,n;rjj/on

hr-l Dltrated

k, Jor.s nd
Co//ar. Afon, t
ley/. Jbfoe.
I

CharCh.

I
,-1 v '.

111-4.

111- 5 ThnI S .fars o k
m/cr.ned,-.'e orJ

e#/,Tnm r, .Af'2,

IT-6 SW/ -Col/tn.
rnO loned i ene ri*

SRooF CONSTRUCTION. THE THREE M[AIN CLASSES OF ROOFS WITH INTERMEDIATE TYPES.

TYPE lE
ihe /rue 7aifs.J'resisS

Znl/er/ned/a'e I

11 A

d 4e 6&-"
,I, cj

A-f-,d a" 111-2 fIwruwLrS'
A/or~vean arzde.1 oof0for/dye
urc4h. Co/ rrcefr J/ '.

vantage is that its spanning capability

depends upon the dimensions of single

timbers available.

II. The coupled rafter type roof is not propped

in the center but exerts an outward thrust

which must be resisted by the mass of the

walls or masonry abutments.

III. The true truss type roof is one in which

the outward (horizontal) thrust component

is counteracted by a pull upon or tie-beam.

Although it is the most scientific of the

three classes of roof structures its tie-

beam restricts interior headroom and di-

minishes the apparent height of an interior.

LIDE 6 The invention of the truss is attributed by some authorities

(Americana, Vol. 27, p. 180) to the Hellenistic builders of

the third century B.C. The theoretical basis for its inven-

tion, they hypothesize, may have been the discovery by Greek

geometers that the triangle is the only rigid figure.

LIDE 7 Most authorities agree that the Greeks were not the inventors

of the true truss. Instead, they relied upon an elaborate

system of posts and lintels to produce their low-gabled

roofs. The great wooden roof of the arsenal at the Piraeus,

the port of Athens (see Fig. 5) constructed between 340 and

330 B.C. by the architect Philos is an example of this sys-

tem. A full contemporary specification for this roof has

survived and is recorded by Singer (vol. II, p. 402).

) SECTION OF THE ARSENAL OF THE PIRAEUS AT ATHENS ILLUSTRATING THE ROOF
CLAUSES OF PHILO'S SPECIFICATION. (From Choisy's Etudes Epigraphiqutes.)

3, .205

fig.1

*TYPICAL-ClCMNE3S CONSTrRDvCTON.
*RLUAlwNr~iomrYIaHGT3AO.PA-StilH 15issU 1103-

52 ,vol. ?p403

From an engineering point of view, this Greek system, essen-

tially a bearer-beam system, was a comparatively unprogres-

sive and static approach to the roofing problem. Some

Greek structures remained unroofed due to their wide span

and lack of roofing technology (Atkinson, p. 200). The tem-

ple at Miletus is an example of such a structure.

Chinese and Japanese builders were never limited to one or

two materials as, for example, the Mesopotamians were limited

to brick. At an early date they developed a composite sys-

tem of architecture utilizing stone, wood, tile and brick.

Despite their skillful use of wood, they did not discover

the truss until relatively late in the course of their ar-

chitectural development; instead (see Fig. 6) they relied

upon an extremely complex system of posts, lintels and brack-

ets to support their roof structures (de Camp, p. 290).

Roman Truss Technology

Probably the first definite example of the truss ap-

pears on a bas relief on the Column of Trajan in Rome (see

Fig. 7). The relief shows a bridge built by Apollodoros

of Damascus in 104 A.D. for the Emperor Trajan (de Camp,

p. 221). Spanning the Danube, the bridge, about two-thirds

of a mile in length (EW.A., p. 449), rested on twenty stone

piers, 170 feet apart. The bridge's superstructure con-

sisted of a series of wooden arches which supported a wooden

deck. If the sculptor who carved the relief was accurate,

SLIDE 8

1DE 9

LIDE 10

13, Plate XIV Trajan's bridge over the Danube, with the emperor sacrificing in the foreground.
A relief on Trajan's Column in Rome

O Z A\ A\ A\ A\

Trajan'a bridge over the Dnubeo.

F'

0i;

0%,

SLIDE 11

LIED 12

LIDE 13

Apollodoros used diagonal bracing in the wooden part of

the bridge.

The Roman architect and theorist Marcus Vitruvius Pollio

(46-30 B.C.) recommends two roof types (see Fig. 8), in his

ten-book treatise on architecture "De Architectura."

In the first scheme (see Fig. 9), for small spans, purlins

span from wall to wall and act as loaded beams, a system

identical to that employed by the Greeks in the Piraeus ar-

senal.

In the second scheme "if the roof is of a larger span, a

ridge piece columne) is laid on top of the post columnn)

. . and a tie-beam (transtrum) and struts (capreoli) will

be necessary" (Atkinson, p. 209). Note that in this con-

figuration the columne" (ridge beam) carries no load at

all; the "capreoli" (rafters) are in compassion and transmit

the loads which are in turn resolved into a pull on the

"transtrum" (tie-beam) and a simple vertical load on the

wall structures.

Vitruvius' mention of the tie-beam marks a major development

in structural history--the development of the true truss.

Without this true truss, the large spans of the Roman Basili-

cas such as the basilica Julia, the basilica of Trajan, and

Constantine's nave of St. Peters (see Fig. 10) (spanning

9RoRMa ROOF CONSTRUCTION ACCORDING TO VITRUVIUS. (After Choisy.)
Roo CON TRU TIO -- .ITRUVIUS, _--- p ^ g

BASILICA OF VITRUVIUS AT FANO.
(After H. L. Warren.) p. 209

Jt P ud /_ tfitde the lr//
'210

SPinfh/eon 7brtico
,p. 2-----

2 p. 535

eighty feet) could not have been covered by timber construc-

tion.

Vitruvius built a wooden-roofed basilica at Fano around

46 B.C. (see Fig. 11). In his description of its roofing

system (De Architectura, Book V, Chapter 1), he noted the

"agreeable effect" of the roof structure, both inside and

out, and the economy and simplicity of his design (Atkinson,

p. 211).

Early Christian Truss Technology

At the time of Constantine, the vaulted masonry construction

that had been "one of the glories of imperial Rome," came to

an end. The old capital abandoned vaulting and, even after

1000 A.D., no other type of church than the wooden-roofed

basilica was constructed.

The trusses of old St. Peter's (333 A.D.) and of St. Paul's

outside the walls (see Fig. 12) are examples of Roman timber

construction of the late empire. In these examples, the

"king-post" and "queen-posts" are recognized to be in tension,

as evidenced by their pinning beneath the tie-beam. These

vertical members are treated as hangers or straps as can

be seen in the section of the St. Peter's truss. The prin-

cipal rafters and tie-beams are doubled in each truss, and

the tie-beams grip the king-post between them. The queen-

post members of the St. Paul's truss (see Fig. 13) were

SLIDE 14

SLIDE 15

doubled with the other members, including the king-post,

functioning as in the St. Peter's truss (Atkinson, p, 211).

In the roofs of the basilica of Ulpia and of the portico of

Hadrian's Pantheon (120 A.D.) (see Fig. 14), bronze truss

members were used, suggesting the recognition of tensile

stress, as well as an attempt to achieve fire resistance

(Atkinson, p. 211).

Both king-post and queen-post trusses, separately or in

combination, were common in Early Christian architecture.

The early basilican church was a structure of great strength

and simplicity and had a wide-spread influence in the history

of structure. Evidence of the far-reaching effects of the

Early Christian forms can be seen in the truss system of

the Great Mosque of Damascus, constructed in 707 A.D.

Another outstanding example of the Early Christian wooden-

roofed basilica is S. Apollinare in Classe, constructed in

Ravenna between 530 and 549 A.D.

SLIDE 16

SLIDE 17

MEDIEVAL TRUSS DEVELOPMENT

3LIDE 18

3LIDE 19

,LIDE 20

Early Romanesque Truss Technology

In Romanesque central Italy, king-post and queen-post

trusses continued to achieve spans greater than the later

Gothic vaults would achieve. Internally the open timber

trusses were often carved and painted in reds, white and

black (Atkinson, p. 211). An excellent example of this

treatment is found in the church of St. Miniats, constructed

near Florence between 1140 and 1180. Here white horizontal

tie-beams contrast strikingly with the transverse wall ar-

ches.

The fifth and early sixth century churches of the north had

been merely imitations, more or less, of the early Roman

churches of Rome itself (Hamlin, A.T.A., p. 217). Three

centuries of incessant warfare and brawling, and gold-grabbing

chieftains, had left the greater part of Roman construction

technology behind, including the truss and an understanding

of its form.

At the beginning of the Middle Ages, as the Roman estate

system rapidly developed into the feudal system, a gradual

substitution of wood for stone in smaller buildings reflected

the disintegration and eclipse of the old Roman masonry.skills

In their place developed new skills in wood construction.

SLIDE 21

SLIDE 22

SLIDE 23

SLIDE 24

Perhaps it is significant that the earliest surviving manu-

script of Vitruvius in the British Museum dates from the

early ninth century--shortly after the reign of Charle-

magne as king of the Franks (Hamlin, A.T.A., p. 221).

Advantages of the new form of construction were many . .

wood was cheap, easily obtained, the timber building could

be rapidly constructed, and if it was destroyed--it was no

great matter (Hamlin, A.T.A., p. 217).

The result of the mad, anarchic fighting of the time, and

the development of skill in wood construction was a new

architecture which was to dominate building in northern

Europe through the Middle Ages. Before 1000 A.D., construc-

tion was of rough timber with little refinement (see Fig.

15), but after 1000 A.D. (see Fig. 16) the new technical

skills of the builder began to be reflected in the increas-

ing scale and technical sophistication of building. The

timber-framed structure became a carefully interlocking

rigid frame suitable for monumental structures (see Fig. 17)

(Acland, p. 48).

As has been noted, knowledge of the advanced concept of

the true truss of the earlier basilican church of Rome

seems not to have reached the northern Romanesque builder.

In both Romanesque England and France, the early tie-beam

(see Fig. 18) was of great thickness and designed to support

BLIDE 25

9, p. 107 f\ Early notched frame. From
SThe Family House in England.
51,p.21

O asilical, with galleries over the aisles, Sta Agnes e, Rome, after 625.

In the hammer-beam truss, arched braces stepped
out from corbelled brackets to extend the span:
Pilgrims Hall, Winchester, 1325-6. (after Wood)
'i p. 59

mple, added about 15oo, the carpenters used
nplex hidden mortises and splayed wedges for
Curved braces. (after Hewett)

The post, rafter, and tie beam intersections were
mortised and pegged together. (after Horn and
Betl)

About 1230 carpenters used alternating braced
frame and arched and braced crucks to ensure
rigidity in the Tithe Barn of the Abbey of Beaulieu
at Great Coxwell, Berkshire. (after Horn and Born)

the roof at its center rather than to tie in the feet of

the principal rafters. In reality these roof structures

were of the bearer beam type (Atkinson, p. 214). The tra-

dition of a heavy bearer-type tie-beam persisted in England

for several centuries.

The twelfth century roofs of Norwegian churches, such as Bor-

gund (see Fig. 19), Gol, and Stedje reflect a highly developed

wood structure derived from ship building (Atkinson, p. 184).

A strong influence from these forms can be seen in the later

work of the English builder.

In general, trusses have been most highly developed in coun-

tries that have more timber construction. In such countries,

structural members tend to be more slender and are used in

great numbers. Thus the development of truss forms in these

areas during the Middle Ages was marked by inefficiencies

produced by the complication and clutter of extra members

that add nothing to their strength. These inefficiencies,

also evident in the bridge structures of the period (see

Fig. 20), are the result of an inadequate understanding of

the nature of stresses in truss structures and the total

lack of methodology for the analysis of these stresses (de

Camp, p. 327).

Two representative examples of the state of Romanesque truss

technology are the church of Maulbronn in Southern Germany

and the church of St. Germain des Pres in Paris. At Maul-

SLIDE 26

SLIDE 27

SLIDE 28

REIMS. SECTIONS OF NAVE. (From T. H. King.)
S .3.... ., p.

Scale oF Feet
5 0 10 20 30 40 50 60 70 80 90 100
The stone piers of Westminster bridge and the wooden superstructure originally proposed. The
masonry arches actually built are shown by dotted lines.

) evCIo et of -U. C-rvcK Pre.22

-J)CVC1Or"1e'1t Of- tkhc Cruck Fr?,YnCe

AA

ODE 29

MLIDE 30

3LIDE 31

bronn a truss form similar to the queen-post form of the

Romans is employed (see Fig. 21), although its tie-beam also

acts as a bearer-beam for the two queen-posts, which are in

compression. At St. Germain a system of seven radiating

struts, intersected by a straining beam, bear on the tie-

beam (E.W.A., p. 450).

Because of their bearer-beam structural apporach, the great

Norman churches of the tenth and eleventh centuries were

limited in width by the length of the huge timber tie-beams

by which they were spanned. The nave of Winchester Cathe-

dral, constructed in 1090, was 45 feet in width and was

spanned by timbers measuring 12 inches by 20 inches (Atkin-

son, p. 214).

English Cruck Development

The traditional structural system of the early English

builder was the cruck (see Fig. 22), a form which paralleled

all other English development even into the fifteenth cen-

tury. The cruck or arch brace in its earliest form con-

sisted of two tree trunks rammed into the ground and lashed

together at the top to form a crude isosceles triangle (see

Fig. 23) (Atkinson, p. 219). Later the cruck was constructed

of naturally curved trees, split in half and used as oppos-

ing rafters. The cruck in this configuration supported

the roof ridge and distributed the roof structure's thrust

;LIDE 32

SThe long curved bents or blades used in arched
braced timber roofs were derived from the frequent
use of 'crucks' or curved braces in barns.

l,p.56
The 'cruck' had long been used in primitive barns
and shelters throughout Europe to replace straight
rafters, as in this Ukrainian peasant hut.

EsNLISH CRUCK TRADITION CO.M-
BINING WITH THE BEARER TRADITION.
(After Innocent.)

FLAN or cCJ H6y -Ec..D0O
The Black Death of 1.749 was the indirect cause of a revolution
in domestic building at all levels. Almno., overnight the beavy
mortality rate caused an acute shortage of labour, thereby
doubling its market value. The farmer, left with no one to work
his land, changed from arable fearing to pasturage for sheep
and unwittingly laid the foundation of his own future wealth
and the general prosperity of the country. Within a short time
the wool trade was booming and sheep farmers, weavers, and
merchants became rich, many of them becoming free men for
the first time in their lives. Wealth spread rapidly in the
fifteenth century and there was a great social upheaval in
England, followed by many changes. The new middle classes
emerged with a great desire to better themselves and to build
houses which would bear comparison with that of the lord of
the manor, so by the first decades of the sixteenth century
there were innumerable owners of houses between the wealthy
nobleman and the lowly cottager.
7he Timber-frame House in England

111-1 3, P.220

@ ho,p. 15
RooPr ovlt NAVE ANU AISLES 01 NO TlI VWALaiAIL CIUU CII.

1,p. 56
@ When the cruck was adapted to arch braced roof
construction it gave a rigid connection to the wall.

SLIDE 33

SLIDE 34

far down the wall surface or carried it right to the ground<

as at the church at Luntly Court (see Fig. 24).

As the cruck developed further a configuration resembling

a truss, with a collar or cross-beam was introduced for in-

creased rigidity (see Fig. 25). The cross beam was extended.

by the thirteenth century, to a point above the cruck's bas

and was connected to the base by posts (Shelter, p. 22).

Although the cruck gradually gave way to more sophisticated

building forms, its influence in the form of curved braces

is evident in many of the great variety of English roof

forms to follow. A strong cruck influence can be seen in

the roofs of the Morton Church, at Lincolnshire (see Fig.

26), the church of St. Mary at Pulman, Norfolk, and the church

of North Walsham (see Fig. 27).

Gothic England

During the Gothic period there was a pronounced difference

in the French and English approach to the roofing of struc-

tures, due basically to the nature of the materials avail-

able to builders in the two countries. The primary material

for roof construction was the same species of Oak in both

countries. The difference is that the English Oak, grown

on park-like land, have short trunks and large curved limbs

while the French Oak, often deliberately planted in tight

groves, have long straight trunks (Atkinson, p. 39). Thus

SLIDE 35

SLIDE 36

the French had long, straight, thin, strong members at hand

while the English were forced to utilize massive curbed

branches.

Due to the arch-brace or cruck tradition of early English

architecture, the tie-beam was not seen to be a necessity

in roof frame construction (see Fig. 28) and was seldom

utilized except as a bearing member. It is not certain whe-

ther the early English builder was ignorant of the advantages

of the tie-beam or if he ignored the tie-beam purposely be-

cause of its interference or inconvenience as an obstruc-

tion of interior space.

In all of the English Gothic "truss" forms, thrust is partly

resolved by collars, and partly carried down and distributed

on the surface of the wall (Atkinson, p. 184). Therefore,

it must be noted that virtually none of these forms may be

classified as true trusses.

The English timber roof follows no distinct evolutionary

development, and all of the forms, with the exception of

the hammer beam, which developed in the late fourteenth cen-

tury were used indiscriminately throughout the following

centuries (Nellist, p. 129).

The only uniform change in English roof form during the

Gothic period was in the gradual flattening of the external

pitch of the roof. Twelfth, thirteenth and fourteenth

century English churches were low and dark. In the fifteenth

century, with the passion for light, clerestory windows

were introduced, and the pitch of the roof was flattened

to ensure that the roof's outward thrust was not too much for

the light clerestory walls (Nellist, p. 219).

In England, far more than in continental European countries,

most churches and halls were covered with open timber roofs

instead of vaulting. The chief types of these roofs were:

the tie-beam, the trussed after, the hammer-beam and the

collar-braced roofs.

Oak was the primary material employed in the construction of

these roofs although elm, beech, and sweet chestnut were

also occasionally used (Singer, Vol. III, p. 262). Except

for the rafters, the members of such roof structures were

often elaborately moulded (see Fig. 29).

The tie-beam (see Fig. 30) was used throughout the Gothic

period, but most commonly in the early centuries. In this

type of roof, the ridge-beam is supported by a king-post

which bears on the tie-beam at its weakest point, its center.

To prevent the tendency of the tie-beam to sag, the

was often slightly cambered upwards or two queen-posts re-

placed the central king-post (Nellis, p. 129). Examples of

the tie-beam form include Swardestone, Norfolk (see Fig. 31),

and Woodensborough in Kent. During the fifteenth century,

SLIDE 37

SLIDE 38

CoUa.r at B.

25 M =

^^^s^~

Scale of DJetals

Section

ROOF OVER NAVE
KNAPTON CHURCH, NORFOLK.

q 0 1.

30'. 6

Enncipal and. Common Rafters.

Flowe3: aE Dh

Rjdtie.

(D, PI. 5r

One of t!Ie Angeil
upou ends- of hammer Beams.

1 9 6 3 0

1 i'

at C.

40,p. 13

3 The tie-beam

3,p. 219

ENGLISH BEARER-BEAM ROOF, SWARDESTONE
CHURCH, NORFOLK. (After Brandon.)

Roup: EARLY ENOIIS11; SOLAR OFr HOUSE AT CIIARNICY, BIII.:uKxEIRK v. 1270; *WITII (CAlIwnRED
CRosS BEAMS 8UPP'oRLTING P'oSTS AND A I'UIRLIN P'LAT" BIY DMICT RESISTANCE TO P'RESSUUI(R
1:3'rr CHNIURY. 56,vol. 3, p.359

SLIDE 39

SLIDE 40

SLIDE 41

SLIDE 42

when the pitch was exceedingly flat, the space between the

tie-beam and principal rafters was so small- that it became

a pierced carved panel (see Fig. 32).

The trussed rafter roof (see Fig. 33) consisted of pairs

of rafters unconnected except at the ridge and at the wall

plate where a trussed base was constructed. The inherent

weakness of this form was offset to some extend by the addi-

tion of a collar-beam and other interior bracing which re-

sulted in a polygonal or waggon-shaped roof frame. The nave

at Wimbotssham, Norfolk, is an example of such a roof (see

Fig. 34).

The arch-braced roof, derived from the earlier cruck tradi-

tion, consists of large curved brace members (see Fig. 35)

which take the place of the tie-beam. These braces run

from the roof plate to a high collar beam or they can be

supported lower on the wall by a corbel structure.

The hammer-beam roof "truss" (see Fig. 36), introduced in

the late fourteenth century, is easily the most ingenious

form of English Gothic roof construction. This form,

possibly originating in France (Nellist, p. 130), incorpora-

ted the experience gained from all previous English Gothic

roof forms to produce a structure particularly well suited

for wide spans. The principal of the hammer beam structure

involves the building up of a series of cantilevered brack-

ets which are able to carry the roof load and distribute

a61 P 1575
St. ary' Church Ules r 2c, p.137

Occasionally struts
/continued to form X
. & the collar omitted

Large curved members \
take the place of ti.e
beams. These can run
from sole-piece to collar
or they can run down
wall face to rest on corbel
40,p. i32

( The trussed rafter

Note similarity
of construction
to trussed /
rafter /

The arch brace

Hammer beam

A The hammer-beam
B The double hammer-beam
C Section through Westminster Hall

40, 132

Rafters at
18"-21' ana

Triangular
truss at
base of

Wall plate

/'i* '

s-- -.-, ... .-....I _

-,- I
S_-...
= ---L- _' -, 7.- =t "'-- -'

- - ir.-' -'- 7 :'.:- ;,. -:.,. --

-:--E _:_::- ::::-:-' ": -: -: ::-: - '' -:.

ipn a Lof Hoof "I PU ill
in dluar of Willh

ROOF OVER NAVE I ini m'
ScI:aiUiilh of alIt,.rs .1 x .I
6T MARY'S CHURCH, WIMBOTSHAM, NORFOLK. I u i,' apaIarl frill c. t ltio (In 1r I tr:.
G ,,pl 5

ROOF OVER NAVE,
STARSTON CHURCH, NORFOLK.

Flower at A.

SCALE OF E 0 _I 3 4. 6 8 9 ]0 FEET

Common RaTter.

"^-*-J **>

Scazle byo D~tails.
II q 9 7 9

Flower at A.

Piirin.

Wall Fiece.

a,pl. 2
Prncipal Rafter.

~ ~1-----11---1- .I- --

ROOF OVER NAVE

OF WYMONDHAM CHURCH,
NORFOLK.

c,, pi. 20

SLIDE 43
*

the thrust down the wall below the plate line. The struc-

ture's equilibrium depends largely on the rigidity of its

joints (see Fig. 37).

English carpenters were highly skilled in jointing heavy

timbers, but, inspite of the best jointing, the structure

exerts some amount of thrust against the walls on which it

rests (Atkinson, p. 220). With age, the amount of outward

thrust increases as joints loosen and deteriorate, neces-

sitating the installation of metal tie rods at the level of

the wall's plate line.

Two of the finest examples of this type roof structure are

the Great Halls of Westminster and Hampton Court. West-

minster, constructed in 1397 (see Fig. 38), is one of the

earliest hammer-beam trusses and also is longest, spanning

almost 70 feet. The hammer-beam structure of Hampton Court

(see Fig. 39), erected between 1531 and 1535, spans 40 feet.

It is essentially medieval in its construction, with Renais-

sance ornamentation and a truncated form reminiscent of the

French mansard (see Fig. 40) (Singer, Vol. III, p. 265).

The hammer-beam truss "sums up the English medieval genius

for construction, and reveals both the confusion of prin-

ciples and the mastery of practice characteristic of the

race" (Atkinson, p. 221).

STDE 44

SLIDE 45

1o01 s o1 i5 :aoTeet l 'o r 2 ) 4-
Jca/e fr TusI

W\LSTMINSTElRl 11ALtL. MliMlilE S OIr THI ARCh AND IIAMMElil-ULtAM 1OOF.
(After Rceport by Sir Frank Baines, l.M. Office of WIorks.)

S V/////// V///////A
Hammer-beam roof of the Great Hall, Hampton Court Palace, c I53r-5. Typica.
Renaissance features are the double slope and the ornamentation of the spandrels (AA), the pendants (BB),
and the corbels (cc). 52 ,vol. 3, p. 24-

Junclon beA~&en 7Tussed
\ iur//Ji andC'Go-'sBejmn

3,p.222

Romanesque France

The Romanesque French development of a thin tie-beam and

relatively efficient light truss came about due to the

straight, slender Oak timbers available to French carpen-

ters.

The Romanesque roofs of the south of France did not, in

most cases, differ from the common Roman king-post system.

The truss's tie-beam and king-post were stop-chamfered and

and the loads transmitted by the purlins to the principal

rafters were counteracted by upright struts, bearing on

the tie-beam (see Fig. 41). Later Romanesque builders re-

located these struts (see Fig. 42), springing them from the

foot of the king-post, proving their understanding of the

stress relationships in a true truss (Gwilt, p. 633).

In the north of Romanesque France, difficulty was encountered

in roofing over the masonry vaults of major ecclesiastical

structures. The vaults of these structures were actually

masonry "ceilings," constructed below the huge timber trusses

which supported the roof (see Fig. 43) (Shelter, p. 153).

If a tie-beam truss system was employed, main walls had to

be carried up as high as the main vault rib; this was ex-

pensive and impractical where the high walls were relatively

thin. An alternative was to modify the truss structure to

eliminate the tie-beam. A scissors-type truss (see Fig. 44)

was develoepd for this purpose, although it soon proved to

SLIDE 46

SLIDE 47

[From Violet-le-Due.
E.RLY FRENCH RooFS. 3, P 2/5

GOTHIC PERIOD 441

A B C

^D E

JACK LEG
STRUT

GH
RomantSii and Gothic timber roofs: (A) early Romanesque (Italian): rafters resting on
extrados of vault; (a) Romanesque (French): tie-beam introduced to relieve vault; (c) Romanesque (French):
collar-beam used instead of tie-beam; (D) Gothic (French): diagonal braces used instead of tie-beam; (E) early
Gothic roof of Peterborough cathedral with wooden ceiling; (F) Gothic (English): trussed rafter roof with
collar-beam; (G) Gothic (English): curved-brace roof with collar-beam; (H) late Gothic (English): so-called
tie-beam roof. Note low pitch; (i) late Gothic (English): hammer-beam roof.

52, vol.2,p.44/

20,p. /19

be unsatisfactory. The principal rafters of the scissors

structure would in time draw away from the tenons of the

scissor braces, destroying any effective wall-connecting

tie, and allowing the masonry walls to spread (Gwilt, p.

634).

Gothic France

The failure of the scissors truss pointed up the need for

a more efficient tie-beam truss structure. The result was

adoption of the high pitched roof (see Fig. 45) which be-

came characteristic of the French Gothic period. This roof

required neither great width of footing nor the heavy mem-

bers of trusses with lower pitches.

From the thirteenth century the roofs of new French churches

were constructed before the vault in order to stiffen the

whole building while the vault was constructed beneath it

(see Fig. 46) (Atkinson, p. 183).

This time was one of experimentation and searching for the

ultimate system of light trusses capable of spanning large

spans. At Auxerre the traditional purlins were discarded

with the resulting truss racking being checked by the con-

struction of trussed partitions between trusses (see Fig.

47). The partitions were mortise-and-tenoned into the king-

post and queen-posts were added to the framework to restrain

the tie-beam's sag under its new load (Gwilt, p. 634).

SLIDE 48

SLIDE 49

GZ ,vo/. l,p.675

A.IENS TRANSVIeHSE SECTION OF THE NAVE

[From Viollet-l-Duc.
AUXERRE. FRENCH WOOD BARREL CEILING WITH
TIE-BEAM SHOWING. j, P. 2/

AT AUXElE.
2rp'f.634

7-THe VAU-TS OF TH5 cQ cTHic L-r TWCPfo ..warpe 04 T+) ^WTH ei-& HLt-4E TIMPeIE- T IMIc.4ES SF4NEeV
cavp TtE VAlu-LW TC' Surro-r RFoof. We cw7 VAiT.-r.

3,p.2/7

c- oe n.
7f'o W,
c 7- AD X CC'A.7

c(: 1

-3, p. 40

I "N

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211' IA-~

/
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The roof above the vault of St. Ouen at Rouen, dating from

the fourteenth century, is another example of the French

Gothic builder's complete understanding of the true-truss.

The tie-beam is chamfered and the three posts are clamped

over it. The tie-beam and posts are all recognized to be in

tension and the side posts (see Fig. 48) are actually cut

LIDE 50 away in the center for lightness (Atkinson, p. 183).

LIDE 51 The original truss system over Reims, illustrated by Viollet-

le-Duc (see Fig. 49), consists of equally light timbers,

scarcely more than 10 inches in section at any point (At-

kinson, p. 217). The vertical posts of this structure

are often double timbers which act, below a certain point,

as tie-beam hangers.

LIDE 52 In French secular building, the tie-beam is employed as a

tension member in the wooden barrel or waggon roof (see Fig.

50) in which the beam and hanging post are exposed. This

form evidently came about as the result of a desire to

utilize the space above the level of the wall plate. A

similar roof structure is described by Viollet-le-Duc as

LIDE 53 a cradle roof, common in France from the twelfth to the

sixteenth century. An example of this type of roof-framing

system is that of the Prefecture, formerly the Episcopal

Palace at Auxerre (see Fig. 51), which spans 30 feet (Gwilt,

p. 634).

X Pl -nS,

T
;N N '.

B *\

K,
V''N

35,p. 26

5Les elements d'une Jerme de charpente
adaptee a gauche a une couverture de
chaume, d droite a une couverture en miles
plates :
I Faitage. 11 Jamb
2 Perche. 12 Coya
3 PoinCon. 13 Sabli

4 Lien.
5 Chevron.
6 Arbaletrier.
7 Entrait retrousse.
8 Panne.
9 Echantignole.
Latte.

14 Bloch
15 Soliv
16 Somn
17 Dalla
18 Remp
19 Parqi

e de force.
u.
ere.
ret.
e du plafond.
nier servant d'entrait.
'ges des combles.
lissage de terre.
iet.

3,2p. 2/

.y f oof Frivninw tZ/ ils
frtvm Enrly pnestiApc/ hitMcture ofjconnectart

NRI' v' nt, /IA., a,/i.a,
fq6. 66.

/ ,.i/ S., Pdt at e '//nc
!T uart. J-',.

- -----..- _.._... 7' ft- --- .------- ----------.----

Fio, ,. (7.
riq 67-

SLIDE 54

SLIDE 55

The highest development of medieval carptentry and joinery

in eastern Europe seems to have occurred at the end of the

fifteenth and the beginning of the sixteenth centuries. As

a rule, all roof trusses developed before the 'Renaissance

were joined together either with dovetail joints (see Fig.

52) (at joints in tension) or mortise-and-tenon joints (see

Fig. 53), which were secured with oak pegs or trenails

(Daujas, Vol. II, p. 523). A rare exception to this rule

is found at Eltham Palace Hall, constructed in 1479, where

iron "dogs" were used to strengthen truss joints (Singer,

Vol. II, p. 442).

Fifteenth-Century Italy

In Italy during the fifteenth century, refinement of

the traditional Roman truss forms continued. At the basili-

ca of S. Paulo fuori la mura, double trusses spanning almost

79 feet, were constructed (see Fig. 54). In this example

the principal rafters abutted on short king-posts. A piece

of timber was suspended between the short king-posts and

sustained by a strong wooden key passing through it and the

king-posts. This timber hanger suspended the two tie-

beams by means of another peg passing through it below them.

The tie-beams, in two lengths, were scarfed and held together

by three iron straps (Gqilt, p. 632).

RENAISSANCE TRUSS DEVELOPMENT

Renaissance Italy

During the Renaissance, an Italian architect, Andrea Pal-

ladio (1518-1580) made the most significant advances in

truss technology since Vitruvius. In his "Quattro libri

del' architettura," Palladio illustrates several simple but

well designed wooden true trusses which he developed for

use in roof and bridge structures (Timoshenko, p. 182).

He applied four different truss systems to bridge struc-

tures--the first-known application of the truss principle

to this type of construction (see Fig. 55) (Steinman, p.

76).

Palladio was the first architect to construct structures

of any great span. One of these was the bridge (see Fig.

56) he constructed between Trent and Bassano over the Bren-

ta, having a span of almost 100 feet (Kirby, p. 144). An-

other of his bridges achieved a span of 108 feet over the

Cismore (see Fig. 57). His Cismore timber bridge was an

arched truss structure, perhaps the earliest to employ

"framed voussoirs," a form which was later further developed

and used extensively in iron as well as timber bridges

(Gwilt, p. 648).

SLIDE 56

S10 "S 19 20 30 Feet

Roof of St Paul's Church, Covent Garden. (Above) 1636; (below) 1796.

56p.1/82.

Palladjo's truss bridge.
Palladio's truss bridgO.

Pilladio's bridge near Trent.

Design for the composite roof-trusses of the Sheldonian Theatre, O.ford, by Wren. (Above)
Elevation of a single truss; (below) cross-bracing betweren the trusses.

In the Netherlands, Simon Stevin. (1548-1620), a contemporary

of the Italian architect, discovered the concept of the

triangle of forces, a discovery which finally gave birth

to the scientific analysis of actual loads on members of

simple truss structures (de Camp, p. 369). Stevin also in-

vented the decimal system.

Renaissance England

In the English Renaissance, except for a few low-pitched

roofs introduced from Italy by Inigo Jones, which might

have been designed by Vitruvius, there was no great change

in English roof structure design before Wren.

;LIDE 57 An example of an early Renaissance English truss is that

designed by Jones for the roof of St. Paul's Covent Garden,

constructed between 1631 and 1638 (see Fig. 58). In compari-

son with French forms of the time, this truss's members

were extremely massive. Upright and diagonal timbers, ill

disposed and indeterminate in their function, were halved

and lapped at their intersections, considerably weakening

them.

Jones' truss seems to be a clumsy assembly of props and

struts when compared with the truss designed by Hardwich

for the reconstruction of the building in 1796 (see Fig.

59). Hardwick's truss consisted of two principal rafters

and a horizontal straining bar which met at queen-posts.

From this structure the tie-beam was suspended, no longer

a beam but a tie-in to restrain the feet of the truss. An

idea of the difference in efficiency between the two trusses

is provided by Peter Nicholson's comparison of the timber

content of the two roofs. Jones' roof contained 298 cubic

feet of material while Hardwick's new roof contained 198

cubic feet of material (Nicholson, Vol. II, p. 381).

Sir Christopher Wren (1632-1723) may well have been the

first English architect to make a clear, functional approach

to the design of a timber roof truss.

LIDE 58 Wren's roof for the Sheldonian Theater at Oxford, designed

in 1663, marks an innovation regarded by most authorities

as one of the structural wonders of its age. The building's

68-foot width was too great to be spanned by any normal form

LIDE 59 of timber construction. In solving this problem, Wren

developed an ingenious design (see Fig. 60) composed of tim-

bers of reasonable length, dovetailed and mortise-and-tenoned

into each other, and bound together with heavy iron bolts,

straps, and plates (Singer, Vol. III, p. 267). Recalling

the mansard-type roof, with its double slope, the truss was

stiffened by a system of wooden diagonal cross-braces.

Although he had not yet visited France, Wren may have been

influenced in his design for the trusses of his Sheldonian

Theatre by Philibert de l'Orme's book on built-up, long-span

timber trusses. De l'Orme is noted for his sixteenth-century

development of a method of construcing relatively long-span

wooden arched structures from small members. In these struc-

tures, short planks were laid side by side with the flat

faces vertical and ends cut to butt along radial lines.

Joists between planks in adjacent layers were staggered and

the planks were held together by oak pegs. This method, ex-

cept that iron nails have replaced oak pegs (trenails), is

used in the construction of temporary centering for arches

(see Fig. 61) (Singer, Vol. V, p. 469).

SLIDE 60

EIGHTEENTH-CENTURY TRUSS DEVELOPMENT

The eighteenth-century architect made much freer use of

the smith's work. Timber has always been liable to shear

LIDE 61 at the ends and at fastenings of members in tension. The

pegs and shoulderings of king-posts and the long projections

of the butts of tie-beams of ancient trusses reflect these

LIDE 62 weaknesses (see Fig. 62). It was only logical that iron

was first used as straps or stirrups for joining timber

members and for roof members in tension (see Fig. 63). The

LIDE 63 tie-beam came to be suspended from the foot of the king-post

or queen-post by a forged strap secured by a gib and cotter

(see Fig. 64). With improved saws and bolts (first developed

in the fifteenth century), a scarfed joint in a long tie-

beam became practical.

Two outstanding examples of eighteenth-century British

development are the truss designed by James Stuart about

1785 for the Greenwich Hospital (see Fig. 65), and that

constructed by Edward Gray in 1793 for the Drury-lane thea-

tre (see Fig. 66).

All of the joints of the Greenwich Hospital truss, which

spanned 51 feet, were well-secured by iron straps. An

iron king-post, 2 inches square, served to support the tie-

/ / /'J

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[From .o't:r.n S"stlmn d'Arrs.
@ a A FRENCH TIMBER TRUSS FOR WIDE SPANS PROJECTED BY COLONEL EMY IN 1828.

/s-
"1-

SiI
4. ~'
,4 t *

E 64 beam at its center (Gwilt, p. 630). The span of the Drury-

lane theatre roof truss is over 80 feet (Gwilt, p. 631).

It was a form of extreme simplicity and compositional logic.

By dividing the span into three parts, the roof was kept

low and usable space was created within the trusses' depth.

All tension and high-sheer connections were reinforced by

iron straps. Here is an example of the use of the basic

king-post and queen-post truss forms combined to create an

efficient long-span structure. Such basic truss forms were

similarly combined and recombined in late nineteenth-century

rion industrial structures to create an infinite variety of

long-span roof systems.

Iron members would probably have been used in roof struc-

tures even earlier except that oak tends to accelerate the

deterioration of early iron making a satisfactory longlasting

connection almost impossible (Atkinson, p. 226).

Exchanges, market places, riding schools and other structures

of the late eighteenth century required longer uninterrupted

roof spans than could be achieved by the truss technology

of the time. One of the most ingenious of the timber engin-

eering solutions to the long-span problem was introduced by

A. R. Emy, in France. Emy made use of the principles origin-

ally developed by L'Orme to achieve spans up to 125 feet.

He published the results of his work in "Nouveau Systeme d'

28

Arcs" in 1828. In the same year he projected a 235-foot

arched truss, for a riding school in Moscow (see Fig, 66a).

The structure was never constructed as designed (Atkinson,

p. 225.

EARLY AMERICAN TRUSS DEVELOPMENT

Early colonial architecture derived its structural base, for

the most part, from medieval origins. The seventeenth-century

colonist used timber framing for every kind of building that

required any elaborate construction. Religious, residen-

tial and utilitarian structures resembled their counterparts

of the late medieval period in England, the Netherlands,

Germany and the Scandinavian countries.

JIDE 65 The medieval English cruck form (see Fig. 67) appeared early

in Jamestown and the Massachusetts Bay colonies. After 1630,

houses of framed construction began to appear, and with the

advent of the power-driven saw mill in 1633 (Condit, p. 12)

and improved hand tools, the wooden frame of posts, beams and

rafters soon took the form it was to retain until the early

nineteenth century.

The materials of the New England frame structure, as in

Europe, was oak, hand-hewn in early times and later power-

JIDE 66 sawn into square or rectangular sections. Early joints were

generally of the straight-butt type and later of the mortise-

and-tenon type, with wooden pins being used exclusively

(see Fig. 68) (Condit, p. 12).

( Two types of cruck house
The aisles in the larger hall-type of
building were used as stable space
for the animals, leaving the central
space for the family

joints for timber framing. Plate C, A Treatise
on (C(larpenwu, Francis PIrice, London, 1733. Builders'
handbooks such as this were commonly used by colonial
carpenters and builders.

K

Roof framing in the form of the truss was rare in colonial

SLIDE 67 building. The interior construction of the Old.Ship Meeting

House, constructed at Hingham, Massachusetts, in 1681 (see

Fig. 69), illustrates the beginning of truss framing in

SLIDE 68 America. In this structure, the roof is carried by a true

truss system "with an elaboration which betrays the empiri-

cal 'rule of thumb'" (Condit, p. 16) structural approach

which prevailed until the 1840s. Spanning 20 feet, the sys-

SLIDE 69 tem resembles the inverted framework of a ship's hull (see

Fig. 70), as did much of north and west Europe medieval truss

framing. Othe examples of this type structure are Richard

Munday's Trinity Church at New Port, Rhode Island, construc-

ted between 1725 and 1726, and Peter Harrison's'King's

Chapel at Boston, constructed between 1749 and 1754 (Condit,

p. 16).

SLIDE 70 In the eighteenth-century frame houses of Tidewater, Vir-

ginia, a modified queen-post truss (see Fig. 71) developed,

composed of two principal rafters, a collar beam, and two

queen-posts. The rafters had tenons that dropped into the

tie-beam mortises and were then pegged. The rafters were

connected at the ridge with an open mortise-and-tenon joint,

secured with one wood peg. Two horizontal purlins were

tenoned into each principal rafter to support lightweight

common rafters and provide the requisite bracing against

DECK

PRINCIPAL
RAFTER

Old Ship Church, Hingham, Massachusetts, 1681. Cross section of the nave showing the
roof truss.

As the single-masted Hansa cog a shell of wood
which was a timber vault in reverse evolved
into the three-masted carrack of 15oo, the internal
structure had to be braced and trussed to take the
racking stress of wave action. (after Landstr6m)
p. 46

' CLoJj J.t i 10toA

JTY L L L HovI I BLAAIrOLp A/t II i A Hlov/it CV I iotL

104. DIAGRAMMATIC SECTION SHOWING CHARACTERISTIC COLONIAL *
HOUSE FRAMING. Left: TYLER HOUSE, BRANFORD, CONN. Right:
ACADIAN HOUSE, GUILFORD, CONN. (Kelly: Early Domestic Architecture -
of Connecucit.) 27,p. 516

X L

-f -

Joints for timber framing. Plate C, A Treatise
.[t on Carpentry, Francis Price, London, 1733. Builders
'' 'handbooks such as this were commonly used by colonial
carpenters and builders.

. Detail of queen-post truss at b.
. Vertical section through queen-post.
cDetail of queen-post truss at head; purlin and wrought-iron straps are
S omitted for the sake of clearness.
17, vol. 23, p.696.

i~r

Fio. 312. SrMPLE ROOF FRAN- S

(2, vol. I,p. 700

racking. Diagonal bracing was sometimes used between trusses

of larger buildings (Peterson, p. 66).

Timber framing belonged largely to the world of strict util-

ity during the end of the eighteenth century. The main stream

of braced frame residential development ended with the develop-

ment of balloon framing in 1833 (see Fig. 72). In commercial

and industrial construction, mills, warehouses, railroad

stations and other commercial structures were developed into

the repetitive post-and-beam systems of the nineteenth cen-

tury.

In the early nineteenth century, wood was still cheaper than

easier to handle than other structural materials. It was

DE 71 during these years that large-scale timber framing began in

America, in the textile mills of New England. The Slater

Mill, the first cotton texture mill in the United States,

was constructed in 1793 at Pawtucket, Rhode Island. This

three-story, non-trussed, gable-roofed structure remained the

standard for timber mill construction for sixty years (see

Fig. 73) (Condit, p. 19).

Other examples of this standard mill type include the Lip-

pitt Mill, constructed at West Warwick in 1810, and the mill

designed by J. C. Bucklin at Woonsocket in 1820 (Condit,

p. 19).

(a) (c)

(b) (d)

FIGe). 313. WO TR S Lo
FiG. 313. WOOD TRUSSES FOR LOXG SPA~s

52,vol5, p.4a7,
O A 'balloon frame' for a house.

Iron members, used in the construction of British mills since

the early 1800s, did not appear in American mills until 1850.

ILIDE 72 In 1853, the Georgia Mill at Smithfield, Rhode Island, was

redesigned, introducing triangular trusses to carry its

gabled roof. A new standard of mill construction was de-

veloping, making use of composite king-post roof trusses,

consisting of wood rafters and wrought-iron tension members

(see Fig. 74) (Condit, p. 19).

In the introduction of iron rods as auxiliary tension members

in truss framing, they were first simply added to the under or

tension surface of wood members. Later the iron rods replaced

wood tension members entirely. A particular fine example of

LIDE 73 the composite truss of the mid-nineteenth century survives in

the Inbound Freight House of the Illinois Central Railroad,

originally constructed in Chicago in 1855 (see Fig. 75) (Con-

dit, p. 22).

Inbound Freight House, Illinois Central Railroad, Chicago, Illinis, 1855, 1872. Cross section
showing the roof truss.
O, p./Co

Roof at Cardiff Station.

WOODEN BRIDGE TRUSSES

Early European Bridges

During the eighteenth and nineteenth centuries advances in

bridge design contributed immeasurably to the advancement of

truss structure technology.

In Switzerland, three significant bridges were designed by

Hans Ulrich Grubermann and his brother Johannes. Hans' two-

MIDE 74 span bridge over the Rhine at Schaffausen, constructed in

1755 (see Fig. 76), consisted o' struts radiating from the

masonry abutments and joined together by horizontal and

vertical members. Although the structure achieved spans

of 171 and 193 feet, its trusses were too ponderous and its

parts were too dependent on one another. Johannes' bridge

at Reichenan was similar to the Schaffausen structure except

that it had a single span of 240 feet (Steinmeyer, p. 112).

The finest work of the Grubermann brothers was their bridge

at Wettingen over the Limmat near Zurich, constructed in

1758 (see Fig. 77). This bridge differed considerably from

the other two structures. Seven oak beams, from 12 to 14

feet in length, were built up and fastened to form an arch

by means of a series of metal straps, fixed by bolts and keys

(Steinman, p. 113). This was perhaps the first timber bridge

structure to use the true arch,

52,vo/.3,p. 425

Grubenmann's bridge over the Lim-
mat (half of one span).

QUEEN POST ir/ oa _pa__
^ ------ ^ ----- ^ ^^y --^ _e-^ -----

'- "' -U PPER CHORD .___
SLOqWE2 CHORD i
ee nposS 53.p.9
k~

Fq. 1. art. 25V. Fq. IO.( art 263./ .
Fig/^ 100. a;rt. 2.^,P. Figf. 106'! art 26:3.

PLAT XIVIIn.

fig. 101.

Fig 9W. art. 264.

Fqg. 103. art. 260.

F' 104. art. e.1.

'

Flq. 110. arl. 265.

ft. 105. art.f .6'2.

t.nororid v ,Tamer. DaSim
GO, p. /1

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F,'ATE A77.

GO, pI. l6

F q.-''.. .llyt .

tq.P,. J'ridar ,,\'rr tl/r lrmn ,i. Art. 266.

L,n.ln I),hi:r~nl II 1 7:111,: N!,lr N~8lsrr./,,al. I I(~'rl

All three of the Grubermann bridges combined the arch and

truss principles, as did most early long-span bridges due

to the designer's uncertainty as to the strength of the

truss (Steinman, p. 113).

Early American Bridges

In the American colonies the construction of wood bridges

reached its highest development in the half-century after

the revolution.

LIDE 75 The first truss forms employed by American bridge builders

were the king-post and queen-post trusses (see Fig. 78).

The Pine Brook bridge in Waitsfield, Vermont, is an early

Bridge of the king-post type (Sloane, p. 97).

An interesting indication of the state of eighteenth-century

American bridge technology is a comparison between the design

jIDE 76 for a timber arched truss structure (see Fig. 79) from a

1797 American handbook on building, and an arch truss bridge

designed by Andrea Palladio in 1549 (see Fig. 80). They

are obviously virtually identical!

The first long-span American timber trusses functioned essen-

tially as a stiffening or bracing structure for an arched

rib. As mentioned earlier, there was undoubtedly a lack

of early complete faith in the truss as a self-sufficient

structural form.

tbZvir~ie kwz-" /a/ ANDREATRALLADIO1I549

ZlIrawaqft '"ColIsSUS 4 io0i

I.

MPONROD)S

rk iG NFANNY KIEBL$
53,,0.95

The Permanent Bridge, Schuylkill River, Philadelphia. Three arch-
truss spans. Over-all length: 530 feet. Timothy Palmer, builder. Com-
pleted: 1805. Replaced: 1850.

Palmer Truss Bridges

Timothy Palmer of Newburyport, Massachusetts was the first

of the great American bridge builders. A self-taught ar-

chitect, he began building bridges in 1780. In 1792 he com-

pleted the Essex-Merrimac Bridge near Newburyport, utilizing

two Palladian arched trusses, with spans of 113 and 160 feet.

LIDE 77 The most famous fo Palmer's bridges was the so-called "Per-

manent Bridge," constructed over the Schuylkill River at

Philadelphia in 1806 (see Fig. 81). In this bridge, actually

three contiguous spans, the horizontal thrusts of adjacent

arches were utilized to balance'one another. The truss was

a single continuous structure whose top chord followed the

curve of the arched deck. Posts were set along the radial

lines of the ribs and each truss panel contained a single

diagonal (Kirby, p. 142).

LIDE 78 A later bridge, using the Palladian arched truss, was de-

signed by Louis Wernwag at Fairmont on the Schuylkill River

(see Fig. 82). Built in 1812 and named the "Colossus," this

bridge achieved a clear span of 340 feet (Sloan. p. 95).

The main structural elements of the Colossus' superstructure

were five parallel, laminated arched ribs, and a double

diagonal truss. The truss was apparently stressed only under

eccentric loading.

In another bridge, constructed over the Delaware River

at New Hope, Pennsylvania between 1813 and 1814, Wernwag

employed wrought-iron diagonals in the truss panels. This

was perhaps the first notable use of iron in an American

bridge (Condit, p. 87). In 1830 he designed the first timber

railraod bridge in this country, although nothing is known

about the structure of this bridge (Condit, p. 88).

Burr Truss Bridges

3LIDE 79 Theodore Burr began building bridges shortly after 1800,

utilizing a combination structure of arch and truss forms

which he patented in 1817 (see 'Fig. 83). The main member of

Burr's bridge was a flat truss with parallel top and bottom

chords and either single or double diagonals. The total

load was distributed between this truss and a distinct arch

rib. Burr's basic approach in this design seems to have

been to increase the weight and strength of the structural

forms fo the Palmer bridge, revealing his "mature grasp of

their structural character" (Condit, p. 82).

Burr's first bridge was constructed across the Hudson River

at Waterford, New York between 1803 and 1804. Its four spans

varied form 154 to 180 feet in length. Materials included

hand-hewn white and red pine, white oak pins, and fasteners

of wrought iron.

rD/0,LW P.63
1,PI Y .~A~ ihuI IJ iAIC 6wCAffjCOo~Cgbw 4a~p

27 Bridge over the Delaware River, Trenton, New Jersey, 1804-6. Theodore Burr, builder.

Bridge over the Hudson River, Waterford,
New York, 1803-4. Theodore Burr, builder.

Other notable designs by Burr included the Delaware River

bridge at Trenton, New Jersey and the McCall's Ferry bridge

over the Susquehanna River near Lancaster, Pennsylvania.

The Delaware River bridge, constructed between 1804 and

1806, consisted of five spans with a maximum span of 203

feet. In this structure he integrated the arch and truss

systems by connecting the lower chord of the truss to the

arch by means of a series of braces which were spiked to

the lower chord of the truss and joined to the arch by iron

bands. The McCall's Ferry bridge, constructed between 1814

and 1815 was Burr's longest span,. achieving a single clear

span of 360 feet (Condit, p. 85').

E 80 One of many contemporary examples of the Burr truss was

constructed in 1941 across the Cocagna River at Cocagna,

New Brunswick, and consists of four 144-foot spans (see Fig.

84) (Plowden, p. 48).

Thomas Pope, a New York architect and landscape gardener,

published his "Treatise on Bridge Architecture" in 1811.

His work marked the first systematic and analytic work on

bridge construction in America (Condit, p. 86).

Town Truss Bridges

The first bridge designer to free the truss from its unneces-

sary dependence on the arch was Ithiel Town. A student of

Asher Benjamin and later a partner in the prominent archi

tectural firm of Town and Davis, Town patented his lattice

SLIDE 81 truss in 1820 (see Fig. 85). Its chief feature was its

closely spaced array of intersecting diagonal members which

formed the web of the truss. Without posts, it formed a con-

tinuous structure for short or long spans.

Although the Town truss was an indeterminate structure, it

was a true truss, existing only verticle reaction components

SLIDE 82 at its supports. It was widely adapted to railroad use

and became "the bridge builder's equivalent to the balloon

frame" (Condit, p. 90), made of simple, common sizes of lum-

ber, and requiring few bolts and metal rods. Due to its

simplicity, any carpenter could erect such a truss in a short

period of time and at a minimum of expense. The structure's

main weakness was its tendency to warp in long spans, es-

pecially as it aged.

SLIDE 83 In 1831 Town proposed the adoption of his truss to cast and

wrought iron. The first metal trusses in England, constructed

in 1845 (Timonshenko, p. 186), were of the Town type, al-

though it was not until 1859 that the iron form appeared in

American (Condit, p. 90).

LIDE 84 By 1840 railroads were placing the greatest demands on the

bridge builder and an extensive reconsideration of the exist-

ing inadequate structural forms became necessary. Since wood

was still the dominant bridge material, the railroad's major

need in truss construction was a structure which would in-

/7/fl. c/*avJprsv

TOWN LATTICE TRUSS.

i--4i

The lattice truss patented by Ithiel Town, 1820.

1oP. 9/

r

Design for a Single track Railroad thidge. constructed. of wro'uht iron .-_. I.attice Priciple.

Le tqth bethr,'e picrs 67 /fet.

o. :6 .

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-iLI

111111~1Ii

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I I 11 1 1 / / jiLLL /

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trt11'ot ot //ttt" .I. H.

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ARCHITECTURAL IRON WORKS,_ NEW-YORK.

Setiorn. at C.

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(>47,pl 67
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volve a relatively small number of members, sufficiently

massive to withstand the increasing vertical and.lateral

impacts of locomotives. The ideal structure had to be eas-

ily assembled, determinate (eliminating redundant members

and reducing the number of points of maximum wear), and

reasonably inexpensive.

Long Truss Bridges

ILIDE 85 The forerunner of the famous Howe and Pratt trusses was

Stephen Long's "assisted truss," patented in 1830 (see Fig.

86). Long's original truss consisted of a row of rectangular

panels, each containing two diagonals. This structure was

"assisted" by the addition of a king-post truss over the

two center panels. In 1836 and 1839 he was granted two

subsequent patents for forms in which he eliminated the

redundant king-post truss, creating an "improved brace

bridge" (Condit, p. 93). The precise form and proportioning

of Long's 1939 truss suggest that he had knowledge of some

accurate method of truss stress analysis.

Howe Truss Bridges

LIDE 86 Of all the timber trusses it was the Howe truss (see Fig.

87) that came the closest to the ideal described above.

Consequently, it enjoyed the longest and most vigorous life

of any nineteenth-century truss.

SO, 92

LoNx's BRIDGE.

DETAILS OF SPLICED CORD.
SThe "assisted truss" patented by Stephen H. Long, 1830.

SThe truss patented by William Howe, July 10 and August 3, 1840.

L /0, Op. 94
,1 ... ... J., ... .. .

Sjb

/, p. 95

@ The Howe truss as modified by Amasa Stone, 1841.

I i I I I I

Scalt of ftct.
rf o u

William Howe, the uncle of inventor Elias Howe, completed

his first railroad bridge in 1838 at Warren, Massachusetts.

It has been claimed that the idea for this truss form was

suggested to Howe by the old roof truss of an old church in

Brookfield, Massachusetts (Kirby, p. 146). This first Howe

truss was rectangular in profile with single diagonals ex-

tending across two panels. All members were wood except

the vertical pieces which were wrought iron (indicating their

design as tension members).

In 1840 Howe was granted two patents on a modification in

which he introduced a larger number of diagonals, each inter-

secting others at three points (Condit, p. 94).

Howe was granted a final patent in 1846 for a variation on

his standard truss in which a timber arch was introduced

along the side of the truss--a form very similar to the Burr

arch/truss configuration. This type of truss was employed

in the roof system of a train shed constructed at Boston

in 1842 by the Boston and Worchester Railroad (Condit, p.

96).

SLIDE 87 The truss form, commonly called the Howe truss, which domi-

nated railroad timber bridge construction from the 1840s

through the end of the century, was actually a simplifica-

tion of Howe's 1840 truss (see Fig. 88). First built by

Amasa Stone and D. L. Harris, this truss was similar to

Long's improved brace truss. Its rectangular profile was

divided into a series of panels, each with two diagonals.

The vertical end .posts, top and bottom chords, and diagonals

were heavy timbers, while the other vertical members (see

Fig. 89) were either wrought iron rods or wood. All connec-

tions throughout the truss were bolted (Condit, p. 96).

A structurally determinate Howe truss was finally developed

around 1851 (see Fig. 90) (Condit, p. 96) when the number of

diagonals in each panel was reduced to,one, with all diagonals

sloping in the same direction in either half of the truss.

The contemporary steel Howe railroad bridge is similar in

form to this timber structure.

SLIDE 88

BILL OF MATERIAL. 10 IDraces, 10 x 12" 8 4 Blocks, No.. holes 11" 150 Bolta, 1" dlia. 39"
4 pcs. 10x12" .8' 2 8x12" 1' 8 Upset Rods, 1" di. 13' 11" 24 24"
12 32 0 8 x 8" 13' 4 S 1' 8" 2 30"
8 8 8 x iO' 10' 8 13a' 8" 30 Wansher, i" hol. 2 x ("
8 2" 4 Blocks, No. 1, holes l1" 8 2" 13' 0" 34 Steel -beams, 15"-150 44'
4 i' o.4 N., 8 2}" 18' 10" 6 "
12 8 x 0" 12 No. 3, 31" 8 21 0""1 ORDEAM sspnDERs.
24 Fplice Clamps. 1 No.2, "" 1lStraps,6x}x34"holes3|"di. 12 iRod, 1i"tim. '4"
ou I'Pckinug iBlcks. 8 2!" 16 " 2}" 24 Washers, 6"Square.
3 21" 12 1""
S .... ---------

SCALE OF FEET
1 I 1 3 a 6 0

PLAN OF 80 FT. HOWE TRUSS.

SCALE OF FEET

4/, /. //

Plate XI.

(c) 1o0, p./32
(C)
Later modifications of the Howe truss designed to make it a determinate structure. a. Stand-
ard triangular roof truss. b. Cambered roof truss. c. Standard trapezoidal bridge truss.

( l The truss patented by August Canfield, 1833.

0O,p.-o4

r\

THE IRON TRUSS BRIDGE

The four decades beginning with 1840 may be regarded as the

period of development of the modern truss bridge.

The same demands that led to the substitution of cast and

wrought iron for wood in buildings lay behind its use in

bridges, strength, durability and fire resistance, Since

wood had physical properties comparable (on a lower scale)

to those of iron, the bridge makers continued utilizing the

techniques and forms of timber construction with little change.

The primary difference was the saving of material made possible

by the use of thin rods for members subjected only to tension.

Later, as iron became more widely accepted, special truss

forms were evolved, suited to the most economical use of the

new material (Finch, p. 73).

,IDE 89 The first patent for an all-iron bridge was granted to August

Canfield of Patterson, New Jersey in 1833 (see Fig. 91). It

was a hybrid structure consisting of a flat suspension rod

(under tension) from which a truss was hung (Condit, p. 104).

The first iron railroad bridge in America was constructed by

Richard Osborne at Manayunk, Pennsylvania in 1845. The

bridge, spanning 34 feet, consisted of three Howe trusses of

the standard double-diagonal form. The two diagonal members,

considered to be compression members, were cast-iron, while

the top and bottom chords and vertical members were wrought-

iron (Condit, p. 106). The structure's wrought-iron top

chord, indicating its design for tension resistance, illus-

trates the designer's confusion as to the types of stresses

to which the various.truss members were subjected.

In spite of inadequate knowledge of the physical properties

of iron and of the distribution of stresses in an iron or

timber truss, there was a growing enthusiasm among railroad

builders for making greater practical use of the material.

In 1846 Frederick Harbach patented a variation of the Howe

truss based on a different estimate of the distribution of

tensile and compressive forces. The top chord and diagonals

of this truss were cast-iron, all except the counter-diagonals

which were in the form of hollow cylinders. The structure's

bottom chord and vertical members were wrought-iron.

The failure of two Harbach bridges on the Erie Railroad,

in 1849 and 1850, caused Erie officials to halt their pro-

gram of iron construction and to replace all then-existing

iron structures by wood (Condit, p. 107).

JIDE 90 Amasa Stone's Howe deck truss over a stream at Ashtabula,

Ohio, constructed in 1865, was one of the pioneer efforts in

the substitution of wrought-iron for cast-iron. The 154-foot

SLiDE 91

span was a bold design but in 1876 the top chord of one truss

sheared, due to a material defect in the member, and a train

went down with the structure, claiming 92 lives (Condit, p.

108).

These and other such unfortunate failures underscore the

inadequacies which existed in iron technology and structural

analysis up to the mid-nineteenth century.

Pratt Truss Bridges

Perhaps the first scientifically designed truss was created

by Thomas Pratt of Boston, America's most thoroughly educated

bridge builder during the early railroad age. Pratt's first

truss, probably designed in 1842, was a modification of the

Stone-Howe form (see Fig. 92), rectangular in profile. The

structure's posts and chords (top and bottom) were timber,

while its double-diagonals were wrought iron. Unlike Howe,

Pratt treated posts as compression members and diagonals as

tension members, thereby shortening the compression members

and reducing lateral buckling (Condit, p. 111).

In 1844, Pratt and his father Caleb were granted a joint

patent on a truss which could be constructed with either a

parallel or polygonal top chord (see Fig. 93). Proposed for

either composite or all-iron construction, its posts and top

chord were in comperssion, while its bottom chord, diagonals

(screw rods) and end posts were in tension. This design,

SLIDE 92

IO,p.//O

@ The first truss designed by Thomas Pratt, 1842.

The trusses patented by Thomas and Caleb Pratt, 1844.

0, p. ///
Simplifications of the Pratt truss developed in the latter part of the nineteenth century. Top:
Deck truss. Bottom: Through truss.

I

superior to the Howe truss because of its more functional

distribution of tensile and compressive stresses in its vari-

ous members, was simplified in the latter part of the nine-

teenth century to a form which came to be widely used for

iron and steel bridges (see Fig. 94) (Kirby- p. 159). The

LIDE 93 Penn Central Railroad bridge, constructed over a back channel

of the Allegheny River at Herrs Island, Pittsburgh in 1908,

is an example of this modified configuration (see Fig. 95).

Pratt constructed a thirteen-span iron railroad bridge at

Portage, New York in 1875. Still in use today, the entire

superstructure was erected in ohly 82 days (Condit, p. 112).

Whipple Truss Bridges

During the years after 1840 Squire Whipple, Wendell Bollman,

and Albert Fink designed many railroad bridges with trusses

of the types which bear their names. All of these forms are

obsolete now.

The most widely known American bridge engineer was Squire

LIDE 94 Whipple. Whipple's first patent was granted in 1841 for the

bowstring truss (see Fig. 96). The structure's distinguish-

ing feature was its polygonal top chord which was apparently

designed to function as an arch as well as a truss member.

Panels were generally braced by two diagonals of wrought iron,

while the top and bottom chords were of cast iron (Condit,

p. 113).