Structural Modeling for Lateral Stiffness in

Historic Truss Bridges

Inclusion of stiffening elements, such Introduction use permits ready public access to his-
as decks, into structural-analysis toric structures and provides incentives
Historic truss bridges from the late-nine-
for the bridges’ continued maintenance.
models can aid engineers in teenth and early-twentieth centuries are
Unfortunately, the engineer involved
preserving historic bridges. vanishing rapidly; it is estimated that
with a historic-bridge preservation pro-
half of the nation’s truss bridges that
ject often finds that the bridge has insuf-
existed just 20 years ago have since been
ficient lateral strength to satisfy modern
removed.1 At this rate of attrition, the
requirements.2 This perceived deficiency
engineering legacy of the truss bridge
is due to two related circumstances: the
may soon be relegated to the history
present-day requirement for wind load is
books.
significantly higher than that used for the
One avenue for preservation is reha-
original design a century ago, and the
bilitating such bridges for pedestrian use.
use of traditional structural analysis can
While the principles espoused in this
lead to an incorrect conclusion that the
paper are equally applicable to bridges
wind load will result in structure over-
still intended for vehicular traffic, reha-
stress, causing the bridge to fail. The
bilitation of former highway bridges to
issue of wind pressure required in mod-
pedestrian use is the focus of this study.
ern design is not addressed here. Rather
Conversion of truss bridges to pedestrian
than engaging in often-futile argument
with code officials over the allowance for
wind pressure mandated for design, a
methodology for addressing the second
circumstance by utilizing modern struc-
tural-analysis tools is presented.

Goals
Research on this topic at the University
of Colorado at Denver has focused on
the stiffening effect of decks in historic
truss bridges. There is strong evidence
that decks stiffen a bridge both vertically
and laterally, although traditional analy-
sis methods, limited to structural skele-
tons only, typically ignore this influence.
Accounting for the stiffening effect of a
bridge deck is analogous to including
such effects from floors, interior parti-
tions, and roofs in buildings. While the
overall purpose of this more-comprehen-
sive approach is to aid in preservation
efforts for historic iron- and steel-truss
bridges, the specific goal of this project is
to demonstrate a new methodology to
Fig. 1. Fruita Bridge over the Colorado River, near Fruita, Colorado. This Parker truss, built in 1907, has
three 155-foot (47-meter) spans, each with eight bays. Its deck consists of timber deck planks spiked
account for lateral strength provided by
to timber stringers, which bear on steel floor beams. The deck is discontinuous, with gaps between nontraditional (but real) load paths.
the deck planks. All images by the authors. Although the focus of this study is sur-

33
34 A P T B U L L E T I N : J O U R N A L O F P R E S E RVAT I O N T E C H N O L O G Y / 3 8 : 1 , 2 0 0 7

Table 1. Summary of Wind-Pressure Recommendations for Bridges from the Late-Nineteenth and Early-Twentieth Centuries
Source Span
60 ft. (20m) 100 ft. (30m) 200 ft. (60m) 1,000 ft. (300m) 1,500 ft. (450m)

C. Shaler Smith, “Wind Pressure Upon 30 psf (1.44 kPa) 30 psf (1.44 kPa) 30 psf (1.44 kPa) 30 psf (1.44 kPa) _
Bridges,” Engineering News (Oct. 1, 1881):
395.

J. A. L. Waddell, The Designing of _ 40 psf (1.92 kPa) 35 psf (1.68 kPa) 30 psf (1.44 kPa) 30 psf (1.44 kPa)
Ordinary Iron Highway Bridges
(New York: John Wiley & Sons, 1884), 6.

J. A. L. Waddell, De Pontibus 40 psf (1.92 kPa) 40 psf (1.92 kPa) _ _ 25 psf (1.20 kPa)
(New York: John Wiley & Sons, 1898),
224 and Plate VIII.

Theodore Cooper, “What Wind Pressure 50 psf (2.40 kPa) 30 psf (1.44 kPa) 30 psf (1.44 kPa) _ _
Should be Assumed in the Design of
Long Bridge Spans?” Engineering News
(Jan. 5, 1905): 15-16.

J. A. L. Waddell, Bridge Engineering 35 psf (1.68 kPa) 35 psf (1.68 kPa) 35 psf (1.68 kPa) 25 psf (1.20 kPa) _
(New York: John Wiley & Sons, 1916),
149-154.

viving metal-truss bridges, the principles Modeling and Analysis Background the literature.7 This paper is offered to
hold true for timber trusses as well. help fill a void in the literature by de-
Traditional structural analyses of truss
bridges are based on a skeleton-frame scribing a method the authors used to
Rehabilitation for Preservation analysis, the classic textbook method, evaluate five different metal-truss bridges.
Rehabilitation of truss bridges for pedes- which has been used since Squire Whip-
ple published the method of joints in Loads
trian use is a practical and popular way
to preserve these historic structures. 1847.5 The “computer” in the nine- Superimposed dead load and superim-
However, the American Association of teenth-century design office was the posed live load are still computed manu-
State Highway and Transportation individual who performed the calcula- ally in the same way that was used by
Officials’ (AASHTO) Guide Specifica- tions, using the classic methods of joints the nineteenth-century designer. Self-
tions for the Design of Pedestrian and sections or perhaps graphical meth- weight may be computed manually or
Bridges mandates a relatively stringent ods that simplified some of the arith- determined by software. The AASHTO
wind-load design criteria.3 Structural metic, to determine bridge member Guide Specifications for the Design of
engineers attempting to rehabilitate forces. Today’s practitioner using one of Pedestrian Bridges prescribes the live-
historic bridges from highway to pedes- the many readily available computer load and wind-load values. Current live
trian use often discover that the old programs is really utilizing matrix alge- loads may vary from 65 psf to 85 psf
structures lack the strength to resist the bra. The computer is now a machine, (3.11 kPa to 4.07 kPa), depending on the
AASHTO wind-load criteria. This ap- but it does the same job the human area of the walkway.
parent inadequacy can lead either to a “computer” once did — it completes the In the absence of standards, nine-
draconian structural retrofit, which is calculations. While the techniques of teenth-century bridge engineers based the
both expensive and detrimental to the analysis have changed from hand calcu- determination of wind loads on their
historic character to be preserved in the lations to computer analysis, the basis own reasoning (Table 1). The recommen-
first place, or to condemnation of the for analytical models has remained dations of C. Shaler Smith, J. A. L.
bridge. Although traditional structural basically unchanged. Waddell, and Theodore Cooper appear
analysis may deem many historic bridges There are many instances of the re- to be based on their own conclusions,
inadequate, in case after case observa- sults of gravity-load tests demonstrating formed from years of experience in
tions reveal no physical evidence to that the vertical stiffness is actually bridge engineering. While the thinking of
suggest that wind has caused damage or greater than that calculated by skeleton all bridge engineers probably evolved
distress, even after a century of expo- analyses.6 While there are examples over this period, that of Waddell is well
sure.4 At this age, bridges have weath- (such as the Cornish-Windsor Covered documented in his three books, which
ered many severe windstorms. Thus, the Bridge, which spans the Connecticut provide insight into the reasoning behind
evidence suggests these structures have River between Vermont and New Hamp- bridge-design criteria in his day.8 Note
better resistance to wind pressure than shire) of engineers having included the that for spans in the range of 100 to 200
what is revealed by traditional analysis deck in the lateral analysis, little infor- feet (30 to 60 m), typical for many sur-
alone. mation on this method can be found in viving truss bridges today, the design
 STRUCTURAL MODELING FOR HISTORIC TRUSS BRIDGES 35

Fig. 2. Blue River Bridge over the Blue River, near Silverthorne and Dillon, Fig. 3. Prowers Bridge over the Arkansas River, near Lamar, Colorado. The
Colorado. This steel Pratt truss has five bays and an 80-foot (24-meter) span. span selected for study is a nine-bay camelback Pratt through truss of 160-
It is believed to have been built in about 1895 as Two-Mile Bridge near foot (49-meter) span, built in 1909. It has steel floor beams with steel
Breckenridge, Colorado, and moved to this site at a later, but unknown, date. stringers, covered by a corrugated-metal deck with asphalt pavement.

wind pressure was on the order of 30 to County (Figs. 1 through 5). The five • deck construction (an important
40 psf (1.44 to 1.92 kPa). This is signifi- bridges had a number of features in aspect of this study)
cantly lower than today’s AASHTO common:
Guide Specifications for the Design of • pin-connected trusses Methodology Overview
Pedestrian Bridges, which mandates 75 • through trusses
psf (3.59 kPa), about double the original In this study, all five bridges were ana-
design value. It should be noted that • Pratt trusses or Pratt-truss derivatives lyzed by the traditional skeleton ap-
typical allowable stresses for iron and • late-nineteenth- or early-twentieth- proach using 3-D structural-analysis
steel are presently at higher levels than century construction software typical of the software tools
when the bridges were designed.9 commonly utilized by practicing engi-
• metal, either wrought iron or steel
neers. All analysis was performed with
• abandonment (except one) either RISA-3D or RAM Advanse, soft-
The Five Bridges Studied ware which includes both frame ele-
• geographic region
Five historic truss bridges located in Other features varied among the bridges: ments and plate/shell elements and is
Colorado were studied: Fruita Bridge in readily available to practicing engi-
• spans
Mesa County, Blue River Bridge in Sum- neers.10 Other software with similar
• railings capabilities is also available.
mit County, Prowers Bridge in Bent
County, Rifle Bridge in Garfield County, • varying degrees of deterioration and As-built dimensions and section prop-
and San Miguel Bridge in Montrose damage erties were used, making this a study of
real-world bridges, not a theoretical ex-

Fig. 4. Rifle Bridge over the Colorado River at Rifle, Colorado. With a 240- Fig. 5. San Miguel Bridge over San Miguel River in western Montrose
foot (73-meter) span, this steel Pennsylvania truss, built in 1909, comprises County, Colorado. This wrought-iron bridge with a 142-foot (43-meter) span
the longer of two different spans at that location. The deck is similar to that was built in 1886. In 1964 its timber deck and stringers were replaced with
at Prowers: steel floor beams with steel stringers, covered by a corru- steel stringers with semicircular segments of corrugated-metal pipe that
gated-metal deck with asphalt pavement. bear on the bottom flanges of the stringers, topped with gravel roadbed.
36 A P T B U L L E T I N : J O U R N A L O F P R E S E RVAT I O N T E C H N O L O G Y / 3 8 : 1 , 2 0 0 7

Fig. 6. A typical 3-D skeleton model, illustrating the traditional skeleton Fig. 7. A typical 3-D deck model. The deck is modeled as plate elements,
based on the steel members only. Frame elements were used for all added to the skeleton model. The stringers were added to the skeleton
members. The boundary conditions — pinned at one end and rollers, model as frame elements, and the deck was added using plate elements.
restrained from translation in the lateral direction, at the other end — are This model is of Prowers Bridge, which has the plate elements intercon-
indicated. This model is of San Miguel Bridge. nected at all their nodes, or corners.

ercise. For all five bridges studied, skele- It is important to note that all five of Various connections were found in
ton-frame models were used, based on: the bridges had deck elements layered the five bridges, including connections
• AASHTO wind load determined from one on top of the other: the deck is that were:
a pressure of 75 psf (3.59 kPa). above the stringers, and the stringers are • puddle welded, such as Prowers and

• pin boundary conditions (that is, re- above the floor beams. It would be in- Rifle bridges’ corrugated deck to steel
strained from translation in all direc- correct to model all of these elements in stringers.
tions) for both bearings at one end a single plane, as that would overstate • bolted, such as Blue River Bridge’s
and roller boundary conditions (simi- the stiffness of the deck system. Because deck planks to stringers.
lar to “pin” except permitted to move frame elements and plate elements lie in
different horizontal planes, a modeling • riveted, such as Prowers Bridge’s
in the bridge longitudinal direction) stringers to floor beams.
for both bearings at the other end. contrivance in the form of offset ele-
ments — specifically, frame members • spiked, such as Fruita Bridge’s deck
• pin connections used for internal inserted between the centerlines of the planks to timber stringers.
member-to-member connections. floor beams and stringers and again • friction, such as Fruita Bridge’s timber
• a 3-D skeleton analysis (although between the centerlines of the stringers stringers to steel floor beams and
some engineers still use 2-D analysis and the deck elements — was used to San Miguel Bridge’s gravel roadbed
of the vertical trusses and of the top connect these members (Fig. 10). There against corrugated metal pipe seg-
and bottom horizontal trusses and are no real offset members in the bridges; ments.
combine the results). The skeleton these members in the model serve to For all of these cases, pin joints were
model includes structural members connect the different frame elements that modeled to improve the accuracy of the
but ignores other features, such as the represent the floor beams and the string- analysis because it was believed that
deck or the railings (Fig. 6). ers to one another, as well as to connect member rotation could occur at these
After the initial standard analyses the frame elements that represent the connections. Although the physical con-
were completed, the skeleton models stringers to the plate elements that repre- structions may be complex, the models
were modified to include the stiffening sent the decks. As such, the offset mem- for this second analysis are relatively
effect of their respective decks and then bers are modeled as weightless and stiffer simple.
analyzed again. Frame elements that rep- than the elements that accurately repre- Finally, a third analysis was com-
resented stringers and plate elements that sent bridge members. These artificial pleted after the models were modified to
represented the deck were added to the members have fixed ends at their connec- treat the decks as structural diaphragms,
skeleton models (Fig. 7). Pinned joints tion to the “real” frame and plate ele- that is with in-plane rigidity, by mathe-
also approximated the stringer-to-floor- ments; thus the deformations at the ends matically locking the plate joints in the
beam connection. The frame elements of the connected elements will be the plane of the deck from deformation. This
that represent the stringers were offset same for strain compatibility. The offset change further reduces the axial forces
from floor beams to represent the stack- members also have rotational releases in the bottom-chord eye-bars. The dia-
ing of actual stringers on the floor beams where the “real” stringers interface with phragm model is presented as a potential
(Fig. 8). The plate elements were offset in the floor beams and where the “real” upper bound for lateral stiffness of the
a similar manner, again to represent the deck interfaces with the stringers, be- deck.
stacking of actual deck elements on the cause member rotation can occur at These techniques are described in
stringers (Fig. 9).11 these locations. greater detail in the report on a research
 STRUCTURAL MODELING FOR HISTORIC TRUSS BRIDGES 37

Fig. 8. Typical offset members and release locations. The rotational release Fig. 9. Computer-generated rendering of the timber deck planks on timber
point is located at the intersection of the bottom of the stringer and the top stringers on steel floor beams, including offset elements. There is a
of the floor beam. This is a representation of Blue River Bridge. rotational release at the location of the deck/stringer interface. This model
is for the Fruita Bridge deck, so gaps are present between the rows of
plate elements.

project completed by the Department of The moment of inertia was not input elasticity was input directly in the deck
Civil Engineering at the University of directly because the stiffening effect of model, as was plank thickness. Again,
Colorado at Denver, as well as in other the plank’s geometry is accounted for in there was no need to input the moment
sources cited here.12 the finite-element analysis by the soft- of inertia, as the software’s finite-element
ware. The diaphragm model was then analysis accounts for stiffness due to geo-
Timber Decks analyzed as a potential upper bound for metry. The behavior of interconnected
stiffness, although it is considered unre- deck elements is quite different from the
Two of the bridges had timber decks, alistically stiff in the lateral direction deck at Fruita Bridge, which had gaps
Fruita Bridge and Blue River Bridge. because of its in-plane rigidity, which between the individual deck planks. The
However, configurations of the decks actual wood decks with gaps between deck of Blue River Bridge was modeled
were different. Each 155-foot (47-meter) the planks clearly do not possess. with a grid of interconnected plate ele-
span of Fruita Bridge has eight bays, The other timber-deck bridge, located ments, which were connected to the sup-
with steel floor beams and timber string- over the Blue River near Silverthorne and porting stringers with rigid offset frame
ers covered by a timber deck (Fig. 1). Dillon, Colorado, is believed to have elements (Fig. 10). One might expect this
Steel eye-bars serve as bottom chords been built in about 1895 as Two-Mile virtually solid deck to behave more
and principal diagonals, and steel rods Bridge, near Breckenridge, Colorado, closely to a rigid diaphragm than the
provide counterbracing and cross-brac- and moved to the Blue River site at an deck at Fruita Bridge. This theory was
ing in the plane of the top and bottom unknown date (Fig. 2). This steel Pratt confirmed by the analyses.
chords. The bridge served highway traf- truss has five bays and a span of 80 feet
fic from 1907, when it was constructed, (24 meters), with a timber deck consist- Corrugated-Metal Decks
until a replacement bridge was built ing of longitudinal “running boards” on
about one-half mile downstream in transverse planks on steel stringers. The Three of the bridges had had their origi-
1970. The bridge has been abandoned steel stringers bear on and are mechani- nal timber decks replaced with corru-
since then. The City of Fruita would like cally attached to the steel floor beams. gated metal decks. Prowers and Rifle
to reopen the bridge for pedestrian and The bridge has steel eye-bar bottom bridges have corrugated-steel bridge
bicycle use as part of a bikeway leading chords and diagonals and steel-rod cross- decks topped with asphalt pavement.
to nearby tourist attractions but has been bracing at the center bay. The railing is a San Miguel Bridge has an unusual con-
stymied by the expense of rehabilitation. steel lattice with double-angle top and figuration of semicircular segments of
The pin-connected, skeleton-frame bottom rails. corrugated-metal pipe topped with
model of Fruita Bridge was analyzed While it has transverse deck planks gravel roadbed. These three decks were
under AASHTO loads. For the deck similar to Fruita Bridge, longitudinal all much heavier than the original timber
model, individual deck planks, with gaps running boards have been added on top decks.
between the planks, were approximated of the deck planks. The orthogonal Prowers Bridge over the Arkansas
(Fig. 9). The actual deck planks are crisscrossing of running boards and deck River, near Lamar, Colorado, consists of
spiked to the timber stringers, so the planks creates a much more continuous six spans of various constructions (Fig.
model approximated the spiked connec- deck than that at Fruita Bridge. The two 3). The span selected for study was a
tion as pinned. The modulus of elasticity layers of mutually orthothropic timbers, 160-foot (49-meter) camelback Pratt
for the wood deck planks was input well spiked together, approximate a through truss that was built in 1909 and
directly, as was thickness of the planks. single solid deck. The wood’s modulus of abandoned in 1994, chosen because it
38 A P T B U L L E T I N : J O U R N A L O F P R E S E RVAT I O N T E C H N O L O G Y / 3 8 : 1 , 2 0 0 7

Fig. 10. Computer-generated rendering of timber deck on steel stringers on Fig. 11. Windward bottom-chord force for timber decks and as-built decks.
steel floor beams. The deck has been modeled using plate elements For all cases, the windward bottom chord is in compression for the timber
interconnected at all their nodes (corners) and frame elements for the deck cases and would require significant cost to remedy. Fruita Bridge has
stringers and floor beams. The offsets are weightless “dummy” frame the same values for both cases because the existing (as-built) deck is the
elements of high stiffness, used for connectivity only. This model is of the same configuration as its original timber deck. The other bridges have
Blue River Bridge deck. higher forces in the windward bottom chord, because the higher as-built
deck weights increase tension in the bottom chords.

was the longest span. It has steel eye-bar not nearly so in the transverse direc- roadbed on semicircular segments of cor-
bottom chords and diagonals with steel- tion.13 Because of software limitations, rugated-metal pipe supported on steel
rod counterbracing. The railing is a steel the deck was modeled with a grid of stringers, was installed in 1964. Gravel
lattice with single-angle top and bottom elastic plate elements of constant stiff- roadbase, identical to that used on road,
rails. Rifle Bridge, over the Colorado ness in all directions, as if the deck were was placed over the pipe segments. The
River at Rifle, consists of two spans (Fig. an isotropic solid (Fig. 10).14 Values for thickness of the gravel roadbase varied
4). The 240-foot (73-meter) Pennsylva- the modulus of elasticity were input for from about 4 inches (above the apex of
nia truss was selected for study because plate and shell elements and apply in all the pipe segments) to about 16 inches
it was the longer span. It has steel floor directions; thus, this simple approach to (above the interface of the corrugated
beams with steel stringers, steel eye-bar a complex problem was adopted. De- metal pipe segment and the bottom
bottom chords and diagonals, and steel- spite this limitation, it was felt that a flange of the stringers). This construction
rod counterbracing. The railing is a steel methodology utilizing readily available results in a very heavy deck. At approxi-
lattice with double-angle top and bottom software tools would be more beneficial mately 74 psf (3.54 kPa), the San Miguel
rails. It has been abandoned since the to preservation efforts than the use of deck had the highest of all the deck dead
late 1960s, when a replacement bridge more expensive software with greater loads studied. This deck was modeled
was constructed. analytical precision. However, different using RISA 3D, and interconnected plate
For both Prowers and Rifle bridges, stiffnesses were studied in the course of elements were used to represent the
the corrugated-metal-and-pavement deck analysis, and the stiffness with the best gravel roadbed. One change from the
construction was modeled with intercon- fit to field-acquired test data was adopt- previous example is that the offset ele-
nected elastic plate elements similar to ed.15 Prowers Bridge was modeled using ments were modeled so that the deck
Blue River Bridge. This is a simplifica- RISA 3D, and Rifle Bridge was modeled elements were in the same plane as the
tion: the corrugated metal is much stiffer using RAM Advanse. stringer top flanges. This change in the
in the direction of the flutes and more San Miguel Bridge had been con- model is relatively minor, although the
flexible transverse to the direction of structed originally with a timber deck on physical deck construction was quite
flutes. Further complicating the modeling timber stringers with five spans as Fifth different. As with the Prowers and Rifle
is the fact that high flexural stresses in Street Bridge over the Colorado River at models, this decision was made for
the bridge’s lateral direction, induced by Grand Junction in 1886. When that modeling simplicity.
wind pressure, would tend to occur near bridge was replaced in the 1930s, one
mid-span, where the deck was most flex- of the spans was relocated to the San Comparisons
ible laterally, but high shear stresses Miguel River site (Fig. 5). This wrought-
would tend to occur near the span ends iron bridge with a 142-foot (43-meter) Historically, these bridges were built
where the deck was considerably stiffer span was subject to heavy live loads with timber decks. Fruita Bridge still has
laterally. The lateral flexibility of the cor- from ore-carrying trucks in an active a timber deck, albeit a replacement, in its
rugations leads to a greater sensitivity to mining region of the Colorado Plateau. original configuration. Longitudinal
stress in the longitudinal direction, but The current deck, consisting of gravel running boards were added on top of the
transverse deck timbers of Blue River
 STRUCTURAL MODELING FOR HISTORIC TRUSS BRIDGES 39

Fig. 12. Force versus deck dead load for skeleton, deck, and diaphragm Fig. 13. Windward bottom-chord force versus deck dead load for deck and
models. A linear regression curve for each data set from five different diaphragm models. Where there are only small differences between the
bridges is shown. For a given deck dead load, the skeleton models show deck and diaphragm values, the deck is about as laterally stiff as theoreti-
the least bottom-chord force, which represents compression, if negative. cally possible, e.g., Blue River Bridge. Large differences indicate that the
The deck models show higher bottom-chord forces, considered to be more deck is not nearly as stiff as theoretically possible, such as at Fruita Bridge.
realistic. Finally, the diaphragm models show a theoretical, albeit unrealis-
tic, upper bound for bottom-chord force.

Bridge. The other bridges have replace- sion forces in the windward members. more pronounced in the lighter, timber-
ment decks of other configurations, all Net compression can occur in the wind- deck models.
heavier than the original timber deck. ward bottom chords if the wind-induced Figure 12 summarizes the findings for
In the models discussed here, the compression exceeds the self-weight- the case of axial force in the midspan
alternative load path of the deck as a induced tension. For eye-bar members bottom chord on the windward side. The
lateral stiffening feature has been intro- intended for tension only, the net com- results from the skeleton, deck, and dia-
duced. It is concluded that the combina- pression calculated by the skeleton phragm models for all five bridges are
tion of skeleton and deck reveals the method is often sufficient to result in the examined. Two correlations can be seen:
stiffness of the bridge in the lateral direc- buckling of the member. (Note that the first, a higher deck dead load results in
tion, resulting in a significant reduction original design wind pressure was proba- increased tensile force in the bottom
of axial forces calculated in the bottom- bly much lower than today’s require- chords, and secondly, the use of deck
chord eye-bars as compared to those ment, as discussed above. Bottom chords models instead of skeleton models also
calculated using a traditional skeleton that are eye-bars — the most common results in increased tensile force in the
model. The increased lateral stiffness due type — were originally designed for net bottom chords. Increased tensile force in
to the deck’s contribution to the total tension under the load combination of the bottom chords reduces, or in many
structure reduces forces otherwise deter- self-weight plus wind.) Thus, the bottom cases eliminates, the problematical com-
mined by skeleton analysis in many chords are of particular importance to a pression in the windward bottom chord.
members. A measure of this effect can be truss bridge. The upper chords, subject The diaphragm models, in which the
found in the midspan bottom-chord to compression under self-weight, will plane of the deck was locked to prohibit
members. These members have been have increased compression on the wind- deformation, were included, not because
selected as an example of this effect be- ward side as well. The construction of they are considered realistic, but because
cause: they respond to wind by develop- upper chords was originally designed for they represent theoretical upper bounds,
ing relatively greater force than other compression; for the bridges discussed least compression or highest tension, on
members; analysis of skeleton structures here the increased net compression, even bottom chord force (i.e., on deck lateral
often indicates undesirable compression for the skeleton analyses, fell within the stiffness).
in these members; and these members capacity of the as-built upper chords. Figure 13 shows the relationship of
were selected for instrumentation in field Figure 11 shows the relationship of the force in the windward bottom chord
tests and thus field measurements could the force in the windward bottom chord for the deck models versus those for the
be compared to analytical results. As this for decks of different dead loads. Forces diaphragm models. The greatest differ-
paper focuses on modeling, the field test from analyses with the original timber ence between the deck and diaphragm
results are not presented; they can be decks, with relatively light dead loads, models was at Fruita Bridge, where the
found in the references.16 are plotted next to forces from analyses deck was discontinuous. This condition
Wind pressure against a truss bridge made using the current and higher deck demonstrates an advantage in using
results in tension forces being developed dead load. The problem of compression continuous decks, although that struc-
in the leeward members and compres- in the windward bottom chords is clearly tural advantage must be balanced against
40 A P T B U L L E T I N : J O U R N A L O F P R E S E RVAT I O N T E C H N O L O G Y / 3 8 : 1 , 2 0 0 7

the use of historically accurate timber maintenance, and repair of infrastructure; non- 5. Squire Whipple, A Work on Bridge Building:
deck planks. destructive evaluation and failure analysis of Consisting of Two Essays, The One Elementary
infrastructure; and properties of cement and and General, The Other Giving Original Plans,
Further information on the authors’ concrete. and Practical Details for Iron and Wooden
work on modeling of this behavior can Bridges (Utica, N.Y.: H. H. Curtis, 1847).
be found in the references.17 VERONICA R. JACOBSON is a graduate student
6. Joseph Pullaro, “Rehabilitation of Two 1890s
in structural engineering and a research assistant
in the Department of Civil Engineering, Univer- Metal Truss Bridges,” in International Engineer-
Conclusions sity of Colorado at Denver. ing History and Heritage, 215 (Reston, Va.:
ASCE, 2001). Pullaro offers a reason for these
Use of skeleton models will lead to arti- SHOHREH HAMEDIAN is a structural engineer observations: “Trusses generally experience
at JR Engineering, Greenwood Village, Col- lower stresses than shown by analytical methods
ficially low bottom-chord forces (i.e., due to the overall composite action of trusses
orado, and a graduate student in structural
artificially high compressive forces). The engineering at the University of Colorado at and the deck which is not accounted for in
problem of high calculated compression Denver. analytical methods.”
in windward bottom-chord eye-bars 7. David Fischetti, “Conservation Case Study
under the combination of dead load plus KAZWAN M. ELIAS, EI, is a structural engineer for the Cornish-Windsor Covered Bridge,” APT
and a graduate student in structural engineering Bulletin 23, no. 1 (1991): 22–28. While the
wind load can be addressed in two ways: at the University of Colorado at Denver. article discusses reconstruction work on the
• Account for the stiffening effect of the main lattice trusses, it does not describe the in-
deck. As the deck stiffens the struc- WILLIAM B. SWIGERT, PE, SE, is a senior struc- genious use of the deck in the lateral force-re-
tural engineer at Schmueser Gordon Meyer, sisting system.
ture, the windward bottom-chord Glenwood Springs, Colorado, and a graduate
force decreases in compression or student in structural engineering at the Univer- 8. See Table 1 for wind pressures recommended
increases in tension. Use of deck sity of Colorado at Denver. by J. A. L. Waddell in The Designing of Ordi-
models will more accurately predict nary Iron Highway Bridges (New York: John
Wiley & Sons, 1884). For bridges in “unusually
the actual bottom-chord forces. De- Acknowledgements exposed” situations, Waddell recommended
pending on the deck dead load, this increasing these pressures by 10 psf (480 Pa). He
change may be sufficient to remedy The University of Colorado at Denver has con- advises applying this additional pressure to the
ducted this work funded in part by Grant No. vertical projected area of the floor and stringers
the problem of eye-bar compression. MT-2210-04-NC-12 from the National Center and to twice the area of the vertical projection of
The series of linear regression curves for Preservation Technology Transfer and Grant the windward truss, railings, curbs, and ends of
shown in Figure 12 illustrate this ef- No. 2004-M1-019 from the State Historical floor beams. It is assumed that his use of “twice
fect. Note that there is no construc- Fund of the Colorado Historical Society. Further, the area” was meant to include the respective
the cooperation of the City of Fruita, Summit areas on the leeward side.
tion cost; this approach is entirely County, Bent County, Montrose County, and Fourteen years later, Waddell, writing in De
analytical. Garfield County is gratefully acknowledged. Pontibus, had developed his “General Specifica-
• Add dead load to the deck. Increasing tions Governing the Designing of Steel Highway
Bridges and Viaducts.” Three notable develop-
the deck dead load increases the ten- Notes ments had occurred in the intervening years:
sile force in all bottom chords. There • With the commercial development of the
1. Eric DeLony and Terry H. Klein, “Rehabilita-
is an upper bound to the amount of tion of Historic Bridges,” ASCE Journal of Bessemer and open-hearth processes, steel
additional dead load: at some point Professional Issues in Engineering Education had come of age. Steel was now the material
and Practice 131, no. 3 (2005): 178. of choice, having replaced wrought iron.
member stresses under live and dead
• Numerous highway-bridge failures had oc-
loads will be limiting. Figure 11 illus- 2. Frederick Rutz, “Lateral Load Paths in Hist- curred. Wind had contributed to some of
trates this effect. There will be con- oric Truss Bridges” (PhD diss., University of them.
struction costs. Colorado at Denver, 2004), 127–130.
• New bridges were virtually always of the
An analytical model that includes the 3. American Association of State Highway and Pratt or Warren types.
deck with the skeleton can account for Transportation Officials, Guide Specifications By 1916, when Waddell’s classic Bridge En-
for Design of Pedestrian Bridges (Washington, gineering was published, he had further refined
the stiffness of the bridge in the lateral D.C.: AASHTO, 1997). his thoughts on wind loadings. He offered a
direction, resulting in a significant reduc- more theoretical basis:
4. Frederick Rutz inspected 16 pin-connected
tion of axial forces calculated in the bot- P = K V2
truss bridges in 2001. All were found all to
tom-chord eye-bars compared to those suffer from varying degrees of deterioration and where P is pressure in psf, K is a coefficient dis-
calculated using a traditional skeleton- damage, but there was no evidence of wind-in- cussed below, and V is wind speed in mph.
only model. duced distress at any of the bridges. Fifteen Waddell explained that the coefficient K
bridges were in Colorado: Keystone Bridge, “cannot be given with any certainty, but is
Bailey; Blue River Bridge, Summit County; generally considered to lie between 0.003 and
FREDERICK R. RUTZ, PhD, PE, is a senior
Silverthorne Pedestrian Bridge, Silverthorne; 0.005, with most of the later writers assuming it
project manager at J.R. Harris & Company in
South Canyon Bridge and Hardwick Bridge, as 0.004 or less.” Today we would treat K as
Denver and has been a practicing structural
Garfield County; Fruita Bridge, Fruita; Paonia equal to 0.00256 only for the stagnation pres-
engineer since 1972. His 2004 doctoral disserta-
Bridge, Paonia; San Miguel Bridge, Montrose sure at sea level, not the design pressure. For
tion research at the University of Colorado at
County; Lado del Rio Bridge, Archuleta County; objects with a drag coefficient of 2, which is an
Denver focused on the stiffening effect of timber
Costilla Crossing Bridge, Conejos and Costilla approximate average for common structural
decks for truss bridges.
counties; Timpas Bridge, Timpas; Smith Hollow shapes today, the resulting K would be 0.00512,
Bridge, Manzanola; Nyberg Bridge, Avondale; or slightly greater than Waddell’s upper bound
KEVIN L. RENS, PhD, PE, is an associate profes-
Sante Fe Avenue Bridge, Pueblo; and Larimer recommendation.
sor in civil engineering at the University of
County Fairgrounds Bridge, Loveland. The For railroad structures, Waddell would com-
Colorado at Denver, where he has taught since
sixteenth, Ft. Laramie Bridge, is at the Ft. bine the wind pressures listed in Table 1 on both
1995. His specialties are inspection, rating,
Laramie National Historic Site in Wyoming. the structure and the train. For highway bridges,
 STRUCTURAL MODELING FOR HISTORIC TRUSS BRIDGES 41

however, he would not combine them for the 12. Frederick Rutz, Kevin Rens, Veronica Load Paths in Historic Truss Bridges: New Ap-
reason that “no person would ever venture upon Jacobson, Shohreh Hamedian, Kazwan Elias, proaches for Preservation,” Proceedings of the
the structure when there exists a wind pressure and William Swigert, Load Paths in Historic 2004 Structures Congress, Structural Engineer-
per square foot of anything like thirty (30) Truss Bridges, No. 2004-25, prepared by Dept. ing Institute of the American Society of Civil
pounds.” of Civil Engineering, University of Colorado at Engineers, May 22-26, Nashville, Tenn., CD-
Denver for National Center for Preservation ROM (ASCE, 2004).
9. Frank Hatfield, “Engineering for Rehabilita- Technology and Training, Natchitoches, La.,
tion of Historic Metal Truss Bridges,” in Pro- under Grant No. MT-2210-04-NC-12, 2005, 13. Jacobson, 18–23.
ceedings of the 7th Historic Bridges Conference, 14–56. Veronica Jacobson, “Analytical Tech-
7–11 (Cleveland: Cleveland State University, 14. The versions of RISA 3D and RAM Advanse
niques and Field Verification Method for Wind apply the modulus of elasticity in all directions.
2001). Loading Analysis of the Historic Prowers These software packages were used intentionally
10. RISA-3D, version 4.5, RISA Technologies, Bridge” (master’s thesis, University of Colorado because of their widespread availability and use.
Foothill Ranch, Calif., 2001. RAM Advanse, at Denver, 2006), 10–25. Rutz, “Lateral Load More sophisticated (and expensive) software
version 7.0, RAM International, Carlsbad, Paths in Historic Truss Bridges,” 165–180. (SAP, for example) has the ability to accept dif-
Calif., 2005. Shohreh Hamedian, “Analysis and Testing of the ferent values for the modulus of elasticity in
Historic Blue River Bridge Subjected to Wind” different directions.
11. These elements are four-joint (quadrilateral) (master’s thesis, University of Colorado at
elements. They are called “mixed interpolation Denver, 2006), 46–60. Frederick Rutz, Kevin 15. Jacobson, 74–77.
elements” because they are based on plate as- Rens, Veronica Jacobson, Shohreh Hamedian,
sumptions with added interpolating functions Kazwan Elias, and William Swigert, “Response 16. Rutz et al., Load Paths in Historic Truss
for out-of-plane shear. This approach is analo- of Pin-Connected Truss Bridges to Wind,” Bridges, 60–63, 68–71, 76–79, 83–86, 91–93.
gous to incorporating shear deformation with Proceedings of the 2006 Structures Congress, Jacobson, 53–67. Rutz, “Lateral Load Paths in
flexural effects from beam theory, resulting in an Structural Engineering Institute of the American Historic Truss Bridges,” 181–225. Rutz et al.,
element that can be used for thin- and thick- Society of Civil Engineers, May 17-20, St. Louis, “Response of Pin-Connected Truss Bridges to
plate applications. See “Plate/shell element form- Mo., CD-ROM (ASCE, 2006). Teby Hererro, Wind,” 6–9. Hamedian, 24–40, 61–72.
ulation,” RISA-3D, in the Help menu on CD- Frederick Rutz, and Kevin Rens, “Field Testing 17. Rutz et al., Load Paths in Historic Truss
ROM. A reference for this element is K. J. Bathe, and Data Acquisition of Historical Truss Bridges Bridges, 57–59, 64–68, 72–76, 80–82, 87–89,
Finite Element Procedures (Englewood Cliffs, using Modular Strain Transducers,” Proceedings 94. Jacobson, 68–78. Rutz, “Lateral Load Paths
N.J.: Prentice-Hall, 1996), 420–449. of the 2006 Structures Congress, CD-ROM. in Historic Truss Bridges,” 226–284. Hamedian,
Frederick Rutz and Kevin Rens, “Alternate 61–72.