MORGAN BRIDGE                                                                                                                   HAERVT-33
(Upper Bridge)                                                                                                                                      VT-33
Spanning Nortli Brancli Lamoille River on IVlorgan Bridge Road
Belvidere Junction
Lamoille County
Vermont
PHOTOGRAPHS
WRITTEN HISTORICAL AND DESCRIPTIVE DATA
REDUCED COPIES OF MEASURED DRAWINGS
FIELD RECORDS
HISTORIC AMERICAN ENGINEERING RECORD
National Park Service
U.S. Department of the Interior
1849 C Street NW
Washington, DC 20240-0001
HISTORIC AMERICAN ENGINEERING RECORD
INDEX TO PHOTOGRAPHS
MORGAN BRIDGE                                                                                        HAER VT-33
(Upper Bridge)
Spanning North Brmich Lamoille River on Morgmi Bridge Road
Belvidere Junction
Lamoille County
Vermont
INDEX TO BLACK AND WHITE PHOTOGRAPHS
Jet Lowe, photographer, June 2003
VT-33-1                             WEST PORTAL.
VT-33-2                             PERSPECTIVE FROM SOUTHWEST.
VT-33-3                             PERSPECTIVE OF EAST PORTAL.
VT-33-4                             EAST PORTAL.
VT-33-5                             INTERIOR, VIEWED FROM EAST.
VT-33-6                             INTERIOR, EAST PORTAL.
VT-33-7                             NORTH PANEL FROM SOUTHWEST.
VT-33-8                             WINDOW DETAIL, LOOKING NORTH.
VT-33-9                             BELOW DECK VIEW OF BRACING SYSTEM.
VT-33-10                           SOUTH ELEVATION.
VT-33-11                           NORTH ELEVATION.
HISTORIC AMERICAN ENGINEERING RECORD
MORGAN BRIDGE
HAERNo.VT-33
LOCATION:
DATE OF
CONSTRUCTION:
BUILDER:
PRESENT OWNER:
PRESENT USE:
SIGNFICANCE:
AUTHORS:
PROJECT
INFORMATION:
Morgan Bridge Road spanning the North Branch of the Lamoille
River, Belvidere, Lamoille County, Vermont
UTM: 18.0679866E.4956962N, Johnson, VTQuad.
Possibly 1886, rebuilt 1898-99
Attributed to Lewis Robinson, Charles Leonard, and Fred Tracy
Town of Belvidere
Vehicular bridge
A good example of the once common queenpost, Morgan Bridge is
most remarkable in that it is one of the few that still functions as a
timber load-bearing truss bridge. One of about sixteen queenposts
remaining in Vermont, it was entered in the National Register of
Historic Places in 1974.
Dr. Mark M. Brown, August 2003
Megan Reese, Engineering Technician, and Dario A. Gasparini,
Ph.D., Professor of Civil Engineering, Case Western Reserve
University, Summer 2003
The National Covered Bridges Recording Project is part of the
Historic American Engineering Record (HAER), a long-range
program to document historically significant engineering and
industrial works in the United States. HAER is administered by
the Historic American Buildings Survey/Historic American
Engineering Record, a division of the National Park Service, U.S.
Department of the Interior. The Federal Highway Administration
funded the project.
MORGAN BRIDGE
HAERVT-33
(Page 2)
Description
The Morgan Bridge is a subdivided queenpost covered bridge with an overall bottom chord
length of 62'-l 1/4", clear span of 54'-10", and a road deck width of ll'-8 5/8" supported by
concrete abutments. It connects a back road to State Road 109 along a north northwest-south
southwest axis and in doing so rises I'-IO" as it crosses the North Branch of the Lamoille River
(sometimes known as Kelley, or Kelly, River).
The bottom chords are three 4" x 11" timbers with bolted shear blocks spaced an average of
every 4'. Timbers 7-1/2" x 10" x 23'-6" to 24' brace the 9-1/2" x 13-1/2" x 15" queenposts
against the 10" x 10" x 12' top chords. Counter braces measuring 5" x 10" and two struts, 4" x
10" each, respectively, support the queenpost braces and the top chord against buckling. Seven
vertical rods also resist buckling forces in the queenpost braces and top chord of each truss. The
three-centermost rods are 1-1/4" diameter steel while the outermost ones are 1" diameter
wrought iron.
These metal rods also serve the function of hanging the 9-1/2" x 11-1/2" deck beams against the
bottoms of the bottom chords. Three of the seven deck beams have mortises used in earlier
lower lateral bracing system and as such are clearly older. The current lower lateral bracing
system consists of 4" x 4-1/2" to 5" timbers arranged in a double Warren pattern perpendicular to
the longitudinal axis of the bridge. A single Warren patternjust two diagonalsis used
adjacent to the abutments. Stringers, 4" x 7" and 7" x 7", support the 3" thick deck planking,
which in turn are topped with 3" thick wheel path runners.
Some of these tie beams are supplemented with metal rods. There are also knee braces that
augment the tie beams. Most of them are secured with toenails. Notable exceptions are those at
the queenposts. Unlike the arrangement elsewhere at Morgan, there are three knee braces at each
queenpost. Two of them are mortise and tenon connections, while the third is toenailed. An
interior photograph taken by noted covered bridge authority Richard Sanders Allen about 1940
shows this configuration.
The fabric of Morgan Bridge has many anomalous features that suggest repairs or rebuilding
with previously used material. All of the tie beams, except the two that connect the tops of the
queenposts, have step-lapped rafter notches of the sort used in the rafter beam and also common
in bams. Were these timbers reused from Morgan Bridge, from another bridge or from a bam?
Most of the posts that support the rafter beams have some sort of currently non-functioning
notching. Perhaps most striking of all are unused connection points in the upstream queenpost
braces. Virtually identical to the framing cuts for the hanger rods and stmts connection on the
braces, they are however, oriented in a way that indicated they were never used in their current
^ Terminology in this section follows that in Joseph C. Nelson, Spanning Time: Vermont's Covered Bridges
(Shelbume, VT: New England Press, 1997) whenever possible. See especially pp. 230, 240, and 244-245.
^ The measured drawings do not show that at least two of the four connections between the queenpost braces and the
lower chords have been strengthened with steel plates.
^ A photocopy of Allen's image is in the field notes. The originals are part of the Allen Collection, National Society
for the Preservation of Covered Bridges, Westminster, VT.
MORGAN BRIDGE
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location. Are they the resuh of mistakes or are the timbers reused from another structure?
Unfortunately, the historical record does little to clarify how this complicated fabric acquired the
present configuration.
History
A Belvidere selectman, Clyde Lanpher, told the writer that [Morgan] bridge, which was
posted against trucks the fall of 1971, also may soon have to have steel girders installed
to sustain for continued use.
-Robert L. Hagerman, Covered Bridges of
Lamoille County, Vermont, 1972.
The North Branch of the Lamoille River descends through the Town of Belvidere in a roughly
southwesterly direction. The narrow and sparsely populated valley is defined by the foothills of
Cold Hollow Mountain and dominated by the 2,280-foot Laraway Mountain on the right and left
banks, respectively. Only a limited amount of land is suitable for farming and consequently
logging and sawmills long dominated the local economy. During the nineteenth-century,
however, there was some manufacturing of small-scale wood products such as butter tubs and
maple sap buckets.   State Route 109 parallels the North Branch and connects the Town's three
hamlets, Belvidere Junction, Belvidere Center, and Belvidere Corners. A back road, between
Belvidere Junction and Belvidere Center, connects residents and property owners along the right
bank to route 109 and the rest of the Town. Two covered bridges, the Mill Bridge (near
Belvidere Junction) and Morgan Bridge, increase access to the back road.
In 1833, Nathan Morgan purchased a plot of land "supposed to contain 60 acres" in Town Lot
17probably in the general area of Morgan Bridge. Morgan and any decedents seem the most
likely source for the bridge's name as the town records list few Morgans and maps from 1859
and 1878 show no Morgans in the town.
Unfortunately, while it is clear that the Morgan crossing is very old, the history of the current
bridge and any predecessors is as confused and muddled as the bridge's fabric. In March 1886,
-7
Belvidere "voted to build Morgan Bridge & put in stone abutments with sufficient wings."   The
4
Robert L. Hagerman, Covered Bridges of Lamoille County, Vermont (Morrisville, VT: by the author, 1972), 20.
^ Historical Records Survey (Vermont), Inventory of the Town, Village and City Archives of Vermont No. 8,
Lamoille County, Vol. I, Town of Belvidere (Montpelier, VT: The Survey, 1940), 11.
^ Town of Belvidere, DeedBooks (Town Clerk's Office, Town of Belvidere Offices, Belvidere Center, Vermont)
Vol. 2, May 9, 1833, p. 40, Nelson, Spanning Time, 65; Henry Francis Walling, Map of the Counties of Orleans,
Lamoille and Essex, Vermont: from actual surveys under the direction ofH.F. Walling (New York: Loomis & Way,
1859); F. W. Beers, Atlas of the Counties of Lamoille and Orleans, Vermont (New York: F. W. Beers & Co., 1878),
18.
^ Hagerman, Covered Bridges of Lamoille County, 19, gives the date for this as 188*7*, which surely is a
typographic error. See Town of Belvidere, Town Records, (Town Clerk's Office, Town of Belvidere Offices,
MORGAN BRIDGE
HAERVT-33
(Page 4)
Morrisville and Hyde Park, Vennont, News Citizen's coverage of the Town meeting reports the
vote to raise taxes for highway and bridge repair, but makes no mention of the vote to build the
bridge.   In September 1898, ihsNews Citizen reported, "new abutments have been put under the
covered bridge a mile north of the Junction."   It has not been possible to confirm or dispute
Hagerman's report that in 1899, ihQNews Citizen records that "the Morgan Bridge is being
rebuih."^'^
Hagerman does correctly report that Richard Sanders Allen believes that a Lewis Robinson built
the bridge, but Hagerman does it in the way that obscures the fact that Allen dates the bridge to
1895.     Hagerman concludes his account of Morgan Bridge with an oral source indicating that
the bridge's construction as taking place in 1898 by three men, Lewis Robinson,
Charles Leonard[,] and Fred Tracy. The day the bridge was raised, ... workers at
the nearby Page mill left their jobs to help in the work. This account, it would
1 ^
appear, refers to the later rather than the original construction of Morgan Bridge.
To Hagerman, then, Morgan Bridge was originally constructed in 188[6] and underwent a
"major, and possibly complete, reconstruction" in the late 1890s by Robinson, Leonard, and
Tracy.
More recently. Nelson misreads Hagerman by stating the bridge "was built by Lewis Robinson,
Charles Leonard, and Fred Tracy in 1887."     Even carefi
accounts might not resolve this confused historiography.
Charles Leonard, and Fred Tracy in 1887."     Even careful reading of a decade of newspaper
In the summer and early fall of 1979, the Vermont Agency of Transportation planed extensive
renovations to the Morgan Bridge.     Apparently the southeast stone abutment collapsed in the
course of giving it a concrete facing.     The project was expanded to stabilizing the abutments
with concrete facings and providing for deck beams at an average of 8' intervals. In dealing with
the latter, the agency's engineers installed five steel vertical rods in each truss. This probably
required new holes in the queenpost braces and in the top chord. The new deck beams would
place new vertical loads on the braces and top chord, and thus increase the likelihood of
buckling. To prevent this, the engineers added new struts between the queenposts as well as
Belvidere Center, Vermont), Vol. 2, p. 101. Also of interest is the wording of the article in the warning (on page 99):
"Art 10'^ to see if the Town will vote to build the Morgan Bridge so called."
^ ''Belvidere," Morrisville and Hyde Park (Vt.) News Citizen, 13, no. 51 (March 4, 1886): 3.
^ "Belvidere," Morn5v;7/e and Hyde Park (Vt.) News Citizen, 17, no. 46 (September 7, 1898): 5.
^^ Hagerman, Covered Bridges ofLamoille County, 20.
^^ Hagerman, Covered Bridges ofLamoille County, 19; Richard Sanders Allen, Covered Bridges of the Northeast,
rev. ed. (Brattleboro, VT: Stephen Greene Press, 1974), 109.
^^ Hagerman, Covered Bridges ofLamoille County, 20.
^^ Nelson, Spanning Time, 64-65.
^'^ Drawings 2, 3, 4, Belvidere, Cov. Br. 48, Proposed Improvement, Bridge Project, Town of Belvidere, County of
Lamoille, July 1979, Records of the Vermont Agency of Transportation, Project Development Division, Montpelier,
Vermont.
^^ Hugh Tallman, Road Commissioner, Town of Belvidere, Vermont, Personal conversation with author, August 20,
2003.
MORGAN BRIDGE
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(Page 5)
vertical struts parallel to the outer most rods. While not executed with great elegance, as
witnessed by the poor alignment of the mid-span vertical rod and struts on the upstream truss, the
work was accomplished without a steel deck. The new deck beam system also required new
lower lateral bracing. In 2001, a crack in the counter brace on the northwestern most comer of
the bridge prompted the use of a threaded steel bolt to secure the counter brace against the
underside of the queenpost brace. A green standing seam roof, one of many placed on covered
bridges throughout Vermont was installed in 2002.     Despite its many trials and murky history,
Morgan Bridge still functions as a working queenpost truss.
Morgan Bridge was placed on the National Register of Historic Places in 1974.
The Queenpost Truss
The queenpost truss, a horizontal top chord between two vertical posts supported by two
diagonal braces that are tied together by a horizontal bottom chord that creates an overall
1 7
trapezoidal shape, dates back at least to the Italian Renaissance.     Compared to the kingpost
truss with its single vertical and two inclined end-braces, the queenpost can span somewhat
greater distances. As the span of a queenpost truss is increased, the distance between the bottom
chord panel points also increases. At some point the longer and longer bottom chords start to sag
from dead loads (self and bridge deck weight). One simple solution to this phenomenon that also
retains the overall truss configuration is to add additional members to the truss  a process called
subdividing  to support the middles of the long bottom chord members. There are limits both
to subdivision and to the maximum length of a particular truss type; the longest queenpost in
1 Q
Vermont measures 88'.
Essentially, the twentieth-century repairs and alterations to the Morgan Bridge represent
subdivisions of the original queenpost configuration by the addition of kingposts. That is, the
struts and the vertical rod added to the panel between the two queenposts is a kingpost. The
struts and hanger rods between the braces and the queenposts is one-half a kingpost with the
balance of the kingpost form provided by the existing brace. Tom Peters has observed that the
overlaying of familiar truss types, as seen at Morgan, in order to create longer and stronger
bridges was common in at least the eighteenth and nineteenth centuries.
Conclusion
^^ Hugh Tallman, August 20, 2003.
^^See any of the numerous reprints of Andrea Pahadio's / Quattro Lihri dell'Architettura of 1570.
^^ Nelson, Spanning Time, 245.
^^ Tom F. Peters, "Bridge Technology and Historical Scholarship," Proceedings of the First International Congress
on Construction History, ed. Santiago Huerta (Madrid: Instituto Juan de Herrera, 2003), Vol. 1, 62-64; Likewise, a
reverse procedure of peeling off simple truss forms was used in the nineteenth century to analyze certain statically
indeterminate forms. See Historic American Engineering Record (HAER), National Park Service, U.S. Department
of the Interior, "Structural Study of Pennsylvania Historic Bridges," HAER No. PA-488, 11-13.
MORGAN BRIDGE
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While it may have been academically trained engineers who planned the 1980 renovations, and
while the execution of the plan does not show the care associated with the craft tradition, the
forms selected for the work were very consistent with the craft tradition. More importantly,
these changes avoided the use of steel beams such as found immediately down stream at the Mill
Bridge while keeping the trusses of an increasingly rare type ftally ftanctional.
^^ According to information compiled by David W. Wright and Joseph Conwill of the National Society for the
Preservation of Covered Bridges, Personal conversations with author, August 6 and 19, 2003, there are sixteen old
queenposts left in Vermont plus two others that are hard to classify.
MORGAN BRIDGE
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(Page 7)
APPENDIX A, Engineering Report
Introduction
The Morgan Bridge is currently one of the few remaining covered wooden bridges
making use of the queenpost truss form. This report investigates the engineering aspects of
queenpost bridges in general, and the Morgan Bridge in particular, including the effect of
modifications made to the structure in 1979. It will also compare the use of experimental load
testing with the more common practice of computational modeling as a means of analyzing the
structural behavior of a unique, timber bridge.
Historical Perspective
The history of structural engineering is not only an interesting topic to engineers, it is also
essential for a complete understanding of structures. Engineers must understand the reliability of
materials, design details, and construction techniques used in the past so that they can make
appropriate, informed decisions about ageing structures in the future. Ever changing building
codes, methods of construction, and material strengths add difficulty and confusion to projects
involving structures that were built generations ago. The ability to make accurate assessments of
the condition of existing structures may help prevent unnecessary replacements, and also assist
engineers to make appropriate decisions about rehabilitation. To fully appreciate and therefore
fully understand any structure, engineers must be able to identify not only its structural behavior,
but also its place in society, both socially and symbolically.
There are 101 known queenpost covered wooden bridges left today, which account for a
little over 10 percent of all remaining covered bridges in the United States.     Similar, but
generally shorter, kingpost bridges account for a little over 3 percent. These are fairly low
numbers, considering the relative ease with which both types could be constructed, and the
consequently large number that were likely once in use. It is not surprising, however, that the
simplest bridges were not among the first to be protected, as they were short spans that broke no
new technical ground, and typically treasured only locally.
Nevertheless, this study of the most basic truss forms should prove valuable, since they
established many of the principles that made the more-complex designs possible. These two
forms date back to at least the sixteenth century. The kingpost is unarguably the simpler form of
the two, consisting of a bottom chord, a center vertical member (the kingpost) and two diagonal
bracing members that extend from the top of the kingpost to the ends of the bottom chord. The
bottom chord and kingpost act in tension, and the diagonal braces are in compression under
typical loading conditions. The forces in the structural members are illustrated in Figure 1,
where compression members are labeled "C," and tension members are labeled "T."
World Guide to Covered Wooden Bridges, 1989.
MORGAN BRIDGE
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Figure 1. Typical kingpost configuration
The queenpost is a variation of the kingpost form that was most likely developed to span
longer distances. The queenpost form contains three panels, separated by two vertical tension
members (queenposts). As in the kingpost, the bottom chord is under tension and the diagonals
are in compression, as shown in Figure 2. The center panel, between the two queenposts, is an
open box formed with the top and bottom chords. Having no diagonal braces, it would be
unstable, except that the truss must have an effectively continuous bottom chord capable of
resisting moment loads at the bottom of the queenposts.
Figure 2. Typical queenpost configuration
The Morgan Bridge
An inspection of the Morgan Bridge made it clear that, while its design was carefully
planned, its purpose was purely utilitarian. Many of the timber members exhibited unused
notches, indicating that the bridge may have been built of reclaimed timbers. As detailed in the
HAER historical report, records clearly indicate the rebuilding of the bridge during 1898-99, but
the original bridge may well have been constructed at this location as early as 1886. Its design
may have differed from the 1899 form, and original timbers could have been reused, leaving the
notches as remnants from its original construction.
When the Morgan Bridge was rebuilt in 1899, it most likely resembled the structure
illustrated in Figure 3. In typical queenpost form, there were no members in the middle panel,
but both diagonal braces and counterbraces in the side panels. Only three floor beams were used
MORGAN BRIDGE
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to transfer loads from the deck to the truss. Metal rods in the side panels doubled as tension
members (posts) in the truss and as a means to transfer load from the floor beams. Since the
bridge had no post at its center, short rods were used to connect the floor beam to the lower
chords at mid-span. For reasons lost to time, an odd post was inserted in the north side panel, but
not quite at the intersection of the two diagonal members. The south side panel had no
counterpart.
Figure 3. 1899 Configuration of the Morgan Bridge
In 1979, the Vermont Agency of Transportation rehabilitated the Morgan Bridge to the
configuration illustrated in Figure 4. Four floor beams were added to bring the total to seven,
and as in the original construction, metal rods were used to attach the new floor beams to the
truss. It appears that the engineers' primary concern at the time was the floor, rather than the
truss itself. Except for the addition of diagonals in the center panel to create a new panel point
for the center floor beam support rod, there is no indication that individual truss members were
strengthened. One area of particular interest is whether these new diagonals significantly altered
the truss's performance.
Figure 4. Current (post-1979) Configuration of Morgan Bridge
The 1979 rehabilitation opened up another interesting area for exploration as well.
Comparing experimental load tests with computer models of the current configuration should
provide evidence and feedback to determine if the selected models can be used to evaluate other
configurations of timber bridgesin this case, the 1899 designwith confidence, and to
understand the limits of the models. Since rehabilitation is often necessary if these structures are
to remain in service, assessing the positive and negative impacts of alterations can help engineers
and owners to make better decisions when questions arise about the condition of other existing
bridges.
MORGAN BRIDGE
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(Page 10)
EXPERIMENTAL LOAD TESTING OF THE MORGAN BRIDGE
Testing Procedure
The vehicle used to load test the Morgan Bridge was a Chevrolet Astro Van. Figure 5 is
a photograph of the vehicle and Figure 6 illustrates the assumed axle weights, based on the curb
weight data obtained from the manufacturer's specifications.
Figure 5. Chevrolet Astro van used in experimental load testing
2150 lb. 2150 lb.
Figure 6. Assumed axle weights of load test vehicle
Instrumentation used to measure the bridge's response to live loading was placed on the
bridge prior to testing. The truck was driven across the bridge from north to south several times,
stopping at predetermined locations along the span so that data could be recorded from the
instrumentation. These selected stopping points were chosen based on preliminary computer
modeling done prior to field work. Tape was used to mark the exact axle locations during the
first run of the truck across the bridge so that the same locations could accurately be found in
each subsequent run. The arrows in Figure 7 indicate the locations at which data were recorded.
MORGAN BRIDGE
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(Page 11)
FB#1
FB#2
FB#7
Figure 7. Typical front axle locations along the west truss (view from inside bridge)
Instrumentation
Field testing of the Morgan Bridge involved the measurement of displacements, or
structural movement under load, and strains in selected members. Several types of
instrumentation were employed to acquire this data, including:
Linear Variable Differential Transformer (LVDT)
Position Transducers
Extensometer
Strain Gages
Surveying Equipment
An LVDT may be used to measure the relative movement between any two points. Using an
LVDT alone is somewhat limiting in fieldwork, so a linear spring-cable system was introduced
to the assembly. The cable allowed measurements between two points spaced far apart, and the
spring allowed the LVDT to capture both positive and negative movements. The LVDT
assembly shown in Figure 8 had one end attached to the bridge at mid-span and other end fixed
to a point in the stream bottom.
Figure 8. Linear variable differential transformer (
jVDT) assembly
MORGAN BRIDGE
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Two Position Transducers were used in the load-testing program. Their assembly also
employed a linear spring-cable assembly similar to that used with the LVDT, as shown in Figure
9. The difference in the two assemblies is that the position transducers capture relative
movement through a different mechanism than the LVDT. Additionally, the position
transducer's range of motion is only 4 centimeters, as opposed to 6 centimeters for the LVDT
used. In Figure 9, the position transducer is mounted underneath the top chord of the west truss,
adjacent to the north queenpost, and the cable runs diagonally to a position on the east truss,
adjacent to the south queenpost. This was done to examine out-of-plane effects of live loading
on the bridge.
Figure 9. Position transducer assembly
An extensometer also measures the relative movement between two points. It is a small
instrument with only a %-inch range of motion, but it is highly accurate and a good instrument
for measuring movements across connections. For these tests, an extensometer was mounted at
the connection between one of the new diagonal members in the center panel and a queenpost, as
shown in Figure 10.
MORGAN BRIDGE
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Figure 10. Extensometer spanning the joint between a diagonal and queen post
Strain gages were used to measure the axial strain in bridge members when subjected to
live loading. While it is possible to mount them to wood members, they are more accurate when
mounted to metal. Figure 11 shows one of the strain gages mounted to a metal rod. The gages
used were capable of capturing up to 5 percent strain.

Figure 11. Strain gage mounted to a metal rod on the west truss
Surveying equipment, normally used for land surveying, was used to collect overall
deflection data. The equipment included a total station and several reflective prism targets.
MORGAN BRIDGE
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These are shown in Figures 12 and 13, respectively. Prisms were securely attached to the
structure using custom-built brackets, and their initial positions were measured and recorded by
the total station. These positions corresponded to the bridge in its dead load condition. The
positions of these prisms were again measured and recorded by the total station under the various
live load conditions to allow calculation of the deflection at each target location.
Figure 12. View of total station with respect to the bridge
Figure 13. Typical reflective prism target and bracket
STATIC ANALYSIS OF THE MORGAN BRIDGE
MORGAN BRIDGE
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(Page 15)
Geometry and model construction
Based on infonnation collected in the field, a computer model of the Morgan Bridge truss
was developed using the MASTAN2 structural analysis software.     A three-dimensional model
will be presented briefly later in the report, but a majority of the analysis, both in the field and
using the model, was performed in two dimensions, and on the west truss alone. Since the two
trusses appeared to be alike, it was believed that they would perform the same way.
Figure 14. Single-line diagram of west truss model
As the diagram in Figure 14 shows, the model of the bridge used had forty-six nodes
connecting sixty-six beam elements. The elements represent the centerlines of the actual
member dimensions, as measured in the field. While a typical truss model assumes pinned
connections at all nodes, in this case only the ends of certain members are pinned, while others
are modeled as fixed connections. A fixed connection allows the transfer of moment between
members, but a pinned connection is capable of transferring only axial forces. The circles shown
in the model indicate the connections modeled as pins. (The actual bridge has fixed connections
wherever a member, such as the top or bottom chord, is continuous across a connection, but there
is limited rotational flexibility between the chords and the posts, so pin connections were used.)
One location where all members were modeled as fixed was at the north (right) end of the west
truss, where a triangular metal plate is securely fastened between the bottom chord and top
diagonal bracing. The plate was modeled as a series of vertical steel elements, fixed at the top
and bottom where they connected to the timber truss members. This plate can be seen in the
Figure 15 below.
MASTAN2, version 2.0, developed by R. D. Ziemian and W. McGuire, 2000.
MORGAN BRIDGE
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(Page 16)
Figure 15. Triangular metal plate in northwest corner of west truss
In addition to capturing the varying joint stiffnesses between members, an attempt was
also made to capture any significant eccentricities that might affect the structural behavior of the
bridge. One of these eccentricities was at the connection between the new (1979) bracing in the
center panel and the original top chord. The eccentricity of this connection can be seen in
Figures 4, 14, and 16.



/
"IX.jX{{{{v!HH!5^

.: * ':       iiiSiiii:-"       "i^^ils-lJ

Figure 16. Eccentricity of connection at mid-span of top chord.
MORGAN BRIDGE
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(Page 17)
Another variable thought to influence the bridge was the composition of the bottom
chord. Since the bridge is approximately 60' long, the builders used three parallel pieces of
wood, staggered and spliced along their lengths, to act as one continuous piece with a varying
cross section. The full cross section showing all three pieces is shown in Figure 17, and Figure
18 is a plan view that shows the scarf joints and member end locations. The highlighted areas
along the chord illustrate individual members that were included in the model. The basis for this
selection is the conservative assumption that anywhere an individual member terminates,
whether at a butt joint or scarf joint, it is not likely to contribute significantly to the overall
strength of the chord.
Figure 17. Cross section of lower chord
Figure 18. Plan view of lower chord with contributing members highlighted
The external boundary conditions at the ends of the bridge were modeled as simple
supports, with the exact point of support located at the intersection of the bottom chord and
queenpost brace centerlines. If it is assumed that the reaction force from the abutment beam
seat is distributed evenly over the portion of the bolster beam supported by the abutment, then
the resultant of the distributed reaction may be assumed to act at the midpoint of this supported
length.
Mechanical Properties of Structural Members
One of the most challenging aspects of modeling the structural behavior of a nineteenth-
century timber bridge is determining how best to quantify the mechanical properties of its
members, especially its wooden members. Experimental means exist for determining certain
mechanical properties, but they are expensive and, consequently, unavailable in most instances.
The only definitive information regarding the species of wood used in the Morgan Bridge came
from the U. S. Forest Products Laboratory, which tested a small piece of wood from one
queenpost and determined that it was spruce.
The laboratory can usually identify the genus of a sample through microscopic
examination and its knowledge of the anatomical characteristics of different woods. So, while
the exact species of spruce could not be provided, it was assumed for the purposes of this
MORGAN BRIDGE
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analysis that it is eastern spruce, the only species that would have been readily available in New
England. For such a small bridge, it is unlikely that wood would have been shipped from the
southern states, or the Rockies, where other varieties of spruce are prevalent. Table 1 lists the
mechanical properties assumed for the timber members in Morgan Bridge, as well as the
mechanical properties assumed for the metal rods.
Table 1. Mechanical Properties used in Computer Model
Element
Material
E (psi)
V
Density (pcf)
Wood members      Eastern spruce            1,400,000             0.4                27.9
Metal rods             Mild steel                  29,000,000           0.3                490
Panel Point Loads due to Live Loads
MASTAN2 computes dead loads internally using user inputs for cross-sectional area and
material density. For vehicular live loads, it was desired to apply the wheel loads to the
computer model in such a way that appropriate comparisons could be made between the model
results and the field data. Using known axle weights and distances, it was possible to determine
the distribution of loads from the tires to the deck, thence to the stringers, floor beams, and,
finally, the truss. It was assumed for these calculations that the stringers were simply supported
between the floor beams, and that the floor beams were simply supported between the east and
west trusses. Table 2 summarizes the panel-point loads resulting from the truck located at the
positions shown in Figure 7.
Table 2. Panel Point Loads Resulting from Live Load Positions
Floor Beam			Front Axle Location Reaction				
	A	B	C	D	E	F	G
FB No. 1	586	420					
FBNo. 2	903	90 i					
FB No. 3	65')	827	1075	142			
FB No. 4			894	932	993	554	
FB No. 5			180	1075	924	1206	155
FB No. 6					232	389	9!9
FB No. 7							1075
MORGAN BRIDGE
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Influence Lines
Engineers often use graphical methods to describe the behavior of a structure. Common
methods include shear, moment, and axial force diagrams, which are often used to demonstrate
the effect of fixed or dead loads. Bridge live-load analysis requires the use of another graphical
method due to the moving live loads. The curves generated are called an influence lines.
Influence lines differ from the other diagrams in that they represent the effect of a moving load at
one particular location on the bridge, rather than the effect of all loads at all locations on the
bridge.  Since the field test data on the Morgan Bridge were taken at specific points while the
vehicle was moved to several different locations, influence lines provide a useful way to present
and compare test data and computed values.
Figures 19 and 20 form an elementary example showing the influence line of quarter-
point deflection on a simply supported span.
t
5(2.1)
&C2,2)
t
S(2,3)
6(2,4)
t
5(2,5)
Figure 19. Series of loading conditions on one beam used to develop an influence line
MORGAN BRIDGE
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(Page 20)
(2,1)
(2,5)
CM
"(3
c
,o
|5 (2,4)
5 (2,2)
5 (2,3)
2                          3                          4
distance along span
Figure 20.   Influence line for quarter point deflection of a beam with simple supports
TRUSS BEHAVIOR UNDER DEAD LOAD
The dead load of each structural member was computed using member sizes obtained
from field measurements and the weight densities given in Table 1. The dead load is the sum of
the weights of all truss members plus the weights of all floor and roof components. The existing
floor system was well documented so its weight could be computed and applied to the
appropriate panel points. With the exception of fewer floor beams and panel points, it was
assumed that the original floor system was similar to the existing one. Detailed measurements
were not made of the roof, but it is most likely similar to the original structure, so a weight
comparable to the floor system was used and applied to the top chord where it connects to the
roof truss. The only dead load neglected in the analysis was that of the siding.
Figures 21 and 22 show the axial force diagrams for the existing and original
configurations, respectively. The thickness of each member in these diagrams is indicative of its
stress, with the shaded members in compression and the unshaded ones in tension. While the
diagrams are not labeled for simplicity, they are drawn to the same scale. The compression
forces in the top chord and tension forces in the bottom chord are similar for both configurations.
Specifically, the maximum compression in the top chord is approximately 15,200 pounds for the
existing configuration (Figure 21) and 12,500 pounds for the original configuration (Figure 22).
The maximum tension in the bottom chord is 14,000 pounds for the existing configuration and
11,300 pounds for the original configuration. The forces due to dead loads in the existing
configuration are approximately 20 percent higher than those in the original configuration, which
is due to the addition of four floor beams in 1979, as well as the significant change in the floor
system load distribution that resulted from this change. Prior to the rehabilitation, the bolster
beams likely distributed more load from the floor system to the abutments, but with addition of
floor beams so close to the bolster beams, their role has likely been reduced significantly. The
forces in both configurations are typical for the queenpost form, with only the bottom chord and
queenposts in tension (plus the floor beam hanger rods in the existing configuration).
MORGAN BRIDGE
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7
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O
Pn
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MORGAN BRIDGE
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(Page 22)
TRUSS BEHAVIOR UNDER LIVE LOAD
Mid-Span Deflection and Overall Bridge Stiffness
Due to the considerable variability in geometry and materials, it is difficult to draw
comparisons among different covered wooden bridges. Stiffness can be a convenient,
normalizing tool with which to make this comparison, as it reflects the behavior of the bridge
system, rather than that of individual members. The formula for stiffness is:
F
k= 
5
where: k = flexural stiffness of the system
F = force applied to the system
S = overall displacement of the system
In the field, an LVDT was used to take mid-span deflection measurements of the bridge
under vehicular live loading. Figure 23 shows two photographs of this test assembly. On the left
is the position of the LVDT at mid-span, as seen from inside the bridge. It was firmly connected
to the lower chord, near the center vertical rod. The cable ran down through a space between the
chord and deck, and was securely fastened to a tripod that sat in the water, held down by rocks
and laboratory weights. An initial displacement was induced into the assembly by stretching the
spring, to allow for both positive and negative deflections under live load. The cable in Figure
23 is darkened for clarity.
Eleven trials were performed with this instrumentation in place. Figure 24 shows the
results of the mid-span deflection tests. The independent (horizontal) axis demonstrates the
location of the vehicle's center of gravity along the west truss, with zero at the south end, and the
dependent (vertical) axis is the mid-span bridge deflection measured by the LVDT. A negative
deflection value indicates that the bridge was moving downward. The dashed line shown on the
plot is an interpolation of the likely influence line for mid-span deflection.
Mid-span deflections obtained from the computer model are provided in Table 3, along
with the mean and standard deviation of the experimental data plotted in Figure 24.
MORGAN BRIDGE
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(Page 23)
cable
from
LVDT
'imvwjv.vj

^f
"    ::  'i i

cable
from
LVDT
Figure 23. Measuring mid-span bridge deflections with LVDT
O     -0 03    
20                                  30                                  40                                   50
distance along west truss (ft)
Figure 24. Influence line for mid-span deflection as a result of live load
MORGAN BRIDGE
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Table 3. Comparison of mid-span deflection data
Truck Position		Field Data		Computer	Model
Description	Location (ft.)	Hs (in.)	as (in.)	S(in	1.)
A	11.05	-0.0315	0.0025		-0.0360
C	22.80	-0.0506	0.0026		-0.0523
D	29.80	-0.0499	0.0027		-0.0525
F	34.80	-0.0412	0.0035		-0.0485
G	45.80	-0.0119	0.0014		-0.0292
fi-s = mean
as = standard deviation
Generally, the model predicted the data within one standard deviation. The only
exception occurred at the north end of the bridge, vv'here the prediction -was nearly 50 percent
greater than the actual deflection. This, the quarter-point truck location, -was, hovv'ever, close to
the triangular steel gusset plate. Given such small deflections, it is possible that the plate -was
providing more strength in that corner of the bridge than the model, vv'hich simulated the plate
-with five vertical, fixed-connection members, predicted.
The foUovv'ing calculations compare the overall stiffness of Morgan Bridge, first as
calculated using the computational model results, and then using the field results. For these
calculations, it -was assumed that the overall stiffness -was equal to the mid-span stiffness -with the
live load at location D.
4300 kips
Ko,ei = ^^.^..    =81,900 k/in.
0.0525 in.
versus
4300 kips                                         4300 kips
(0.0499-F0.0027) in.     '^p'^'"''""^^    (0.0499-0.0027) in.
81,750 k/in.<^__,,<91,100k/in.
The predicted stiffness -was vv'ithin the possible range of stiffness determined from field
measurements (considering instrument error). Given the practical assumptions needed for the
model, this -was good agreement.
Lower Chord Scarf Joint Behavior
Scarf joints have been used in many timber bridges as a solution to the problem of
transferring tension forces across a connection betvv'een t-wo pieces of vv'ood. The Morgan Bridge
MORGAN BRIDGE
HAERVT-33
(Page 25)
is no exception, and it employs three scarf joints on each bottom chord to "splice" together two
pieces of wood. The bottom chord appears to be part of the original structure, as evidenced by
the scarf joints that are worn down and cracked from years of use. The following diagram
illustrates how scarf joints would have originally worked.
Figure 25. Scarf joint mechanism
Field tests were performed using an extensometer in an attempt to determine the extent to
which the scarf joints were still functioning. The scarf joints in both the interior lower chord
member near the quarter point and the exterior lower chord member at mid-span were tested.
The extensometer was positioned to span the top of the joint. Figure 26 is an illustration of the
lower chord, showing the positions of the scarf joints. Three trials were performed for each joint
location, and for each trial data was recorded with the truck at six different positions along the
span: A, B, C, D, F and G (see Figure 8).
QUARTER POINT                                                             MID-SPAN
SCARF JOINT
Symmetrical about CL
Figure 26. Plan view of southern half of lower chord showing locations of scarf joints
MORGAN BRIDGE
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(Page 26)
Figures 27 and 28 show the extensometers in place at the two scarf joints.
Figure 27. Extensometer spanning the scarf joint at mid-span
Figure 28. Extensometer spanning the scarf joint at quarter point
MORGAN BRIDGE
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(Page 27)
Results of the data collected from the extensometer are provided in Figures 29 and 30.
For both plots, the independent axis demonstrates the location of the vehicle's center of
gravity along the west truss, with zero at the south end. The movement of the scarf joint is
shown on the dependent axis.
20                             30                             40
location of load along west truss (ft.)
60
Figure 29. Results of extensometer placed at the mid-span scarf joint
0.0006
0.0005    -
.E     0.0004    -
=     0.0003    -
E
g     0.0002    -
0.0001   -
0.0000
    -0.0001  I-
O
(A
*    -0.0003    -
-0.0004
0
20                             30                             40
location of load along west truss (ft.)
50
60
Figure 30. Results of extensometer placed at the quarter-point scarf joint
MORGAN BRIDGE
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(Page 28)
Initially, it was thought that the scarf joint would act as a "pointer," such that a positive
result would indicate the two members were pointing upward ( a ) and a negative result would
indicate that they were pointing downward (v). According to the influence line in Figure 29,
however, this would mean that the mid-span scarf joint consistently pointed up, even though the
deflected shape at mid-span suggested just the opposite. Rather, it seems the results indicate that
the scarf joint was not rotating at all, but was simply opening up as a result of tension in the
bottom chord. Still, it is not clear why the mid-span joint would open more with the truck near
the south end than it did with the truck near the north end. Nor is it clear why the scarf joint at
the quarter point closed with the truck near either end of the bridge, since the bottom chord was
always in tension, regardless of the truck's position.
One thing is clear from the findings^the assumption in the model that the areas around
scarf joints did not contribute to the strength of the bottom chord was justified. If a stronger
correlation between magnitudes of force in the bottom chord and the magnitude or direction of
scarf joint movements existed, it would have been plausible to assume that some force was being
transferred through that connection. For instance, if the mid-span scarf joint had opened its
maximum amount with the truck at mid-span and less with the truck near the ends, it might well
have indicated that the two sides of the spliced bottom chord member were reacting to the
increased force in the chord, engaging more as the tension force increased and less as it
decreased. As the results show, there was no such correlation to suggest that the movements are
in any way related to the magnitudes of the tension forces.
Wood Joint Behavior
For covered wooden bridge builders, one of the main design goals was to minimize the
number of tension connections. Unlike compression joints, wooden joints that functioned
efficiently and reliably in tension were difficult to fabricate and maintain. This goal was
apparent from the two earliest truss designs, the kingpost and queenpost. As discussed above,
tension connections occur only where the vertical posts meet the top and bottom chords, and
between adjacent bottom chord members. In the Morgan Bridge, the bottoms of the queen posts
were notched to transfer tension force in the form of bearing pressure to the bottom chord, while
mortise-and-tenon joints were employed forthe same purpose at the tops of the posts. Where
adjacent timbers were needed to make up the length of the bottom chord, the scarf joints just
discussed were used. These joints also transfer tension by way of bearing pressure between the
interlocking knuckles.
If the application of live load causes a member in compression under dead load to see
tension instead, its compression joints cannot transfer this tensile load to the rest of the bridge,
rendering the member useless. For early bridges^those built before prestressing became
commonthe usual way to avoid this kind of stress reversal was to be sure the compression
force from the dead load was great enough to maintain a net compressive force, even when live
loads tended to reduce it. Thus, the joints would remain tight. This was the case for the original
configuration of Morgan Bridge.
When Morgan Bridge was rehabilitated in 1979, two counterbracing timber members
were added to the center panel. While apparently installed to give better support to the center
MORGAN BRIDGE
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(Page 29)
floor beam rods, their orientation is such that under dead load they are in compression.
Interestingly, these members were toe-nailed in place, something that should not have been
necessary in compression joints. (Exactly when this was done is not known.) When the live
load was positioned with a significant portion within the exterior panels, however, the model
predicted, and field testing confirmed, that these new counterbracing members experienced
tension. Closer inspection of the structure revealed that one of these members has recently
been replaced, possibly indicating excessive stress as it at some point.
This behavior is apparent in the axial force diagrams of Figure 31. The first two
diagrams show the axial forces in the original bridge, first with live load alone (Figure 31a), then
with the combined dead and live load (Figure 3 lb). The second two diagrams show the axial
forces in the existing bridge, first with live load alone (Figure 31c), then with the combination of
dead and live load (Figure 3 Id). The live load for all four diagrams in the model was placed at
location A, where the maximum effect was observed in the field (see Figure 7). Note, however,
that due to the difference in the number of floor beams between the original and existing bridges,
the load distribution varied between the two models.
(a)
(b)
(c)
(d)
Figure 31. Axial force diagrams for (a) live load at A in original configuration, (b) dead
and live load at A in original configuration, (c) live load at A in existing configuration, and (d)
dead and live load at A in existing configuration
Experimental field tests were performed to determine the validity of the computer model
with respect to the existing configuration. With the difficulties of measuring strain in timber, it
was decided instead to measure displacement across the joint between the south counterbrace in
the center panel and the south queenpost. An extensometer was positioned along the
MORGAN BRIDGE
HAERVT-33
(Page 30)
longitudinal axis of the counterbrace, spanning its connection with the queenpost (see Figure 32).
Three trials were performed, stopping the vehicle at five locations along the span each time to
collect data.
Figure 32. Extensometer in place at the connection
The results of the field test and computational model are shown in Table 4 and Figure 33.
Table 4 provides a comparison of the observed field data with the values predicted by the
computer model. Figure 33 illustrates the data with the x-axis denoting the vehicle's location
along the west truss (zero represents the south end), and the y-axis showing the amount of
movement recorded in the joint along the longitudinal axis of the counterbracing member. A
positive movement correlates to the joint opening up due to a net tensile force, while a negative
movement correlates to its closing under net compression. The dashed line provides an
approximate solution of the influence line for joint movement.
MORGAN BRIDGE
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(Page 31)
Table 4. Comparison of experimental results and predicted results for joint movement at
queenpost - counterbrace connection
Truck Position	Field Data	Computer Model
Description      Location (ft)	^15 (in)       CT6(in)	5 (in)
	A	11.05	0.0106	0.0006	0.0014
	C	22.80	0.0005	0.0001	-0.0001
	D	29.80	-0.0030	0.0001	-0.0020
	F	34.80	-0.0038	0.0001	-0.0021
	G	45.80	-0.0023	0.0000	-0.0010
1^5   =	mean				
cfs	= standard deviation				
While the calculated and experimental magnitudes differ, their matching trends are
clearly evident in Figure 33. The greatest variation between the experimental and computational
results was when the live load was very close to the south abutment. The errors in the computed
values were due to the model having a pinned joint at this location instead of one that suddenly
became disconnected. In the field, the extensometer was observed to widen steadily as the truck
moved past the queenpost location. It was apparent from both the experimental and
computational analysis that the addition of these center panel diagonals added nothing to the
performance of the truss system. In fact, the bridge is doing its best to remove them!
0.012
0.010 -
0.008 -
0.006    -
I     0.004    -
<v
O     0.002    -
E
c   0.000 i^
-0.002 -
-0.004 -
-0.006
0
4       Experimental
Q-----Computational
.-Ek
4.
El----------Q-

20                          30                         40
distance along west truss (ft)
50
60
Figure 33. Comparison of influence lines for joint movement at queenpost
counterbrace connection
MORGAN BRIDGE
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(Page 32)
Influence of Metal Rods
Each truss (east and west) employs seven metal rods to convey vehicular live loads from
the floor beams to the truss. Two of the rods are part of the original configuration, but the rest
were added as part of the 1979 rehabilitation. Without definitive information, it has been
assumed throughout this study that all of these rods were made from steel. In the field, strain
gages were mounted to three of them to get a better understanding of the loads they carried. The
locations of the instrumented rods are illustrated in Figure 34.
Figure 34. Strain Gage Locations along West Truss
Twelve trials were performed for each rod: three with the truck at location C, six at D,
and three at location G (see Figure 7). The results of the data collected from the strain gages are
shown in Table 5. A positive strain indicates axial tension, and a negative strain indicates
compression. Strain gage data was read in microstrains from a strain indicator unit that was
connected to all three strain gages.
To test the predictions of the computational model, the strains were computed using the
initial and final lengths of the corresponding metal rod elements and the following equation.
These results are also shown in Table 5.
L.._,.,-L
s._, =
AL
L
L
MORGAN BRIDGE
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(Page 33)
Table 5 Comparison of experimental and computation results for strain in metal rods
Member
Truck Position
Description     Location (ft)
Field Data
Computer Model
e(in)
Rod 1
Rod 2
Rods
27.6	27.6667	1.1547	21.3135
34.6	20.8333	3.2506	22.2804
50.6	-9.6667	1.5275	0.6946
27.6	7.3333	1.5275	5.8743
34.6	22.0000	9.0111	13.2619
50.6	1.3333	1.5275	-1.0333
27.6	1.0000	0.0000	-0.6908
34.6	7.8333	0.7528	-0.4979
50.6	39.0000	0.0000	53.0203
)j.s = mean
a, = standard deviation
In addition to the Table 5, Figures 35, 36, and 37 show the position of the model
prediction within the range of experimental data for each rod. Considering the sensitivity of
strain gage instrumentation, the results are reasonable. For the most part, the model predicted
values that followed the general trend of the strains, even if the predictions were outside the
range of error.
It is interesting to note the number of times a negative value crept into the results, which
indicated a rod was being compressed. It was surely not the intention of either the original
builders or the engineers who designed the rehabilitation for this to happen, but while the
magnitudes differ, both the experimental and computational results predict such behavior. Of
course the computational results could be affected by limitations on connection modeling, but in
the field, the rods should be free to slide up and down through their connections to the truss
members. Since the instrumentation picked up compression, the connections must be preventing
this free movement.
35
(JT	20
c	
2	15
(A	10
c	5
	
	0
	
	-5
	-10
	-15
1	:	1-^	
r    1        ^'^          1    ,,.			?     experimental -o-    computational
			
			X \ s X X X X X
			1
10                   20                  30                  40
distance along west truss (ft)
50
60
Figure 35. Influence line for strains in Rod 1
MORGAN BRIDGE
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(Page 34)
35
(A
60
50
--    40
(A
10
0
-10
1	:ti.	? 1 ?	
			?     experimental "D"   computational
-			X X X ? t                     ,r
			*--A.......^
10
20                   30                   40
distance along west truss (ft)
Figure 36. Influence line for strains in Rod 2
50
60
?     experimental
-H-   computational
.-Q-.
^--------
%
*-H'
10                   20                  30                  40
distance along west truss (ft)
50
60
Figure 37. Influence line for strains in Rod 3
The variations between experimental and computational data are interesting; since the
model overestimated how much load a particular rod was carrying in some instances, but
underestimated it in others. In particular, the model underestimated the load sharing of Rod 2,
and overestimated the load sharing of Rod 3. In the case of Rod 3, it is likely that a more
significant portion of the load was transferred back to the abutment by way of the stringers and
bolster beams than the model predicted, as this is a well-documented occurrence in truss bridges.
A greater concern was the gross underestimation of the load carried by Rod 2. Rod 2 was
added in 1979 along with the addition of a floor beam adjacent to the queenpost. While it was
most likely intended for the rod to transfer load to the bottom and top chords, thence to the
queenpost, two possibilities seem likely. Either the queenpost was no longer capable of carrying
a majority of the load, or the chords were no longer transferring this load to the queenpost, thus
requiring the rod to carry a significantly larger force.
MORGAN BRIDGE
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(Page 35)
Panel Shear
The role of the web in a steel beam is much like the role of the vertical and diagonal
members between two chords of a truss. That is, they are both a means of transferring shear
forces and maintaining stability in the beam or truss. Depending on the orientation of the
diagonal, shear forces in a truss may be transferred through tension or compression. A simple
example to describe this mechanism is a square box, shown in Figure 38a. When subjected to
shear forces, the box will tend to deform as shown in (b). To resist this deformation, diagonal
members may be added as shown in (c) or (d). The orientation of the diagonal member does not
affect the magnitude of the force that it resists, but it does determine whether the method of
resistance is tension or compression.
(a)
(b)                              (c)                              (d)
Figure 38. Shear force in a square truss panel
A queenpost truss like the Morgan Bridge has such a square center panel, and diagonals
were added to it in the 1979 rehabilitation. To investigate the distribution of shear forces in this
panel, two position transducers were arranged as shown in Figure 39. This location was chosen
based on preliminary computational model results predicting the maximum deflection to be in
this panel. Figure 40 shows the influence line for panel shear developed from the field test data.
position
transducer
#3
position
transducer
#4
Figure 39. Location of Position Transducers on the West Truss
MORGAN BRIDGE
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(Page 36)
o
"i. -0.0100 -I
-0.0200 -I
-0.0300 -
-0.0400
20                      30                      40
distance along west truss (ft)
Figure 40. Influence line for panel shear
60
As in the previous graphs, the x-axis indicates the location of the vehicle's center of
gravity along the west truss, with zero at the south end. The y-axis denotes the expansion and
contraction between the ends of the two position transducer cables. A positive mean
displacement correlates to an expansion along the line of the transducer, and a negative mean
displacement correlates to a contraction. Since the bridge was primarily loaded on its northern
half for this test, position transducer 4 consistently contracted from its unloaded position and
position transducer 3 consistently expanded. At the position closest to the south abutment
(0 feet), the results of the transducers are just beginning to cross each other. Had the experiment
been carried out closer to the south abutment, it is likely that position transducer 4 would have
expanded and position transducer 3 contracted, as shown by the proposed influence line in
Figure 41.
Initial comparisons of the model to the experimental results indicated that the model was
too stiff with regard to panel shear deformations. The "computational - with diagonals" line in
Figures 44 and 45 describes these results. It was determined that there were two possible reasons
for this error in model prediction. The first is that the queenpost, to which one end of both
position transducers were attached, was sharing more of the load with the adjacent steel rod than
predicted. This was previously suggested as a reason for discrepancies in model prediction of
strain gage results related to this rod. Another possibility is that the counterbracing diagonals in
the center panel were not accurately modeled due to their cyclical loading. With different
structural analysis software, it might have been possible to describe these members as gap
elements (members released under tension forces), as that is what happens in a system such as
this. To illustrate an "upper-bound" of the computational data in Figures 41 and 42, the
deformations with both counterbracing members removed is shown with the other data. These
results demonstrate that the actual effectiveness of the bridge in transferring shear forces is
somewhere between these two extremes.
MORGAN BRIDGE
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(Page 37)
XI
^ -0 04(1
0 10 20 30 40 50 60
distance along west truss (ft)
Figure 41. Influence line for panel shear displacement predicted compared with experimental
statistical data (Position Transducer 4)
-Oi
o
I
Ocomputational-'
0     compirtational -
3or
0 10 20 30 40 50
distance aiong west truss (ft)
60
Figure 42. Influence line for panel shear displacement predicted compared with experimental
statistical data (Position Transducer 3)
Three-Dimensional Behavior under Live Load
In general, truss bridges are analyzed as two-dimensional structures, with the
assumptions that there are no significant eccentricities in either the structure or the loads, and
that the trusses are adequately braced against lateral forces. For the Morgan Bridge, differences
between the east and west trusses, such as the triangular gussets in two corners, make the
occurrence of significant eccentricities a realistic possibility. To investigate this, two position
transducers and an LVDT were connected between the east and west trusses to measure relative
out-of-plane displacements under vehicular loading. The locations of the instruments are
illustrated in Figure 43. Figure 44 is a photograph of the assembly.
MORGAN BRIDGE
HAERVT-33
(Page 38)
^
LVDT
^
F
position transducer #4
position transducer #3
N
Figure 43. Plan view of bridge, showing locations of the position
transducers and LVDT
Figure 44. Photograph of test assembly (looking south)
Tests were performed with the truck at positions C and G, although there were no out of
plane movements detected with the truck at G, so only C will be discussed here (see Figure 7). It
is likely that position C provided the most significant results because it placed the truck in the
same panel as the instrumentation. In fact, the axle positions are nearly symmetrical about the
mid-span of the bridge. Three trials were performed, with significant variation among results.
Both the mean and standard deviation are reported in Table 6.
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Table 6. Comparison between experimental and computational out-of-plane displacements
Cable
Location
Field Data
H5(in.)             a8(in.)
Computer Model
_____Slinj_____
A-D        -0.0009333      0.001617
A-C            0.003192      0.001689
B-D          -0.001490      0.002052
-0.00000065
-0.00098
-0.0022
)j.5 = mean
Qg = standard deviation
To evaluate the experimental results, a three-dimensional computer model was
developed, based on the two-dimensional model, but with the addition of floor beams and roof
truss members. Field measurements were not made of the east truss, but a triangular steel plate
was observed at its south end, identical to the one at the north end of the west truss. Assuming
that this would be the most significant contributing factor to the results of the model, the east
truss was modeled as a 180-degree rotation of the west truss about the vertical axis. The
transverse members of the roof truss were included in the three-dimensional model. To account
for the apparent stiffness of the knee braces, the joints between the roof members and the trusses
were assumed to be fixed. The floor beam connections to the trusses were modeled as pins,
assuming that no moment could be transferred through the metal rod connection. The three-
dimensional model is shown in Figure 45.
Figure 45. Three-dimensional MAST AN model of Morgan Bridge with truss loading
shown for truck at position C
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The model was loaded just as for the two-dimensional model, with the appropriate
concentrated forces applied to the panel points. Since the floor beams were pin-connected to the
truss, loading the truss anywhere along the floor beam would be suitable, since no moment from
the floor beam could be transferred to the truss. Figure 46 illustrates the undeflected shape of the
roof truss (as viewed from above) and the deflected shape of the roof truss when loaded at
position C. The darkened lines represent the LVDT and position transducer locations. Table 6
contains the results of both the experimental tests and the computational model.
D
C
(a)
(b)
Figure 46. (a) Undeflected and (b) Deflected shape of roof truss
with live load at position C
While the two sets of results are not at all close to the same magnitude, they do both
represent a similar type of deformation at the top of the truss. As the deflected shape in Figure
46(b) shows, the position transducer connecting points A to C should be in tension, or positive,
while the position transducer connecting points B to D should be in compression, or negative.
This was demonstrated nicely by the experimental results, but not quite as well by the model.
Both results indicate that the transverse members in the roof truss were compressed, which is
somewhat intuitive since the live load was applied to the floor.
While neither method (experimental or computational) resulted in large
displacementsin fact, they were quite small the experimental displacements were
significantly higher in magnitude than the model predicted.
Live Load Survey
Part of the experimental testing included surveys of the bridge before and after the
application of live load to get an image of its overall deflection. The reflective prism targets
were positioned in two ways. The first was done to yield the deflection at the top of each
panel point, and the second to compare relative deflections between the east and west trusses.
The results of the first test are illustrated in Figure 47, which shows a comparison of deflected
shapes between the computer model loaded at mid-span, and the computer model with
settlements equal to measured deflections.
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. r--.
(a)                                                             (b)
Figure 47. Deflected shapes with live load at E as indicated by (a) computer model and
(b) field survey
There were clearly errors in the deflections obtained, as evidenced by the above figure.
This was not surprising due to a number of factors, not the least of which was human error. The
survey equipment required the operator to line up the prism, as viewed through a lens, with cross
hairs in the total station. The process was tedious, and since the deflections were occurring only
in the vicinity of the highest possible tolerance (1 mm = 0.03937 in), they could easily have been
affected by the slightest human error. Accordingly, this experience indicated that surveying is
not a useful tool for this type of experiment, especially when the expected deflections are within
the range of equipment tolerance.
CONCLUSIONS
This analysis of the Morgan Bridge demonstrated that the combination of experimental
load testing and computational modeling is of significant value in the structural analysis of a
timber structure. This combination provided an understanding of the behavior of a unique,
nineteenth-century timber structure that could not have been gained with the use of either method
alone. Considerable insight was gained into the bridge's behavior, both as it exists today, and as
it may have looked 100 years ago. Insight was also gained into what works and what does not in
a computational model of a timber structure.
In general, the computational and experimental results were in good agreement, which
was encouraging since bridge design engineers in the United States do not extensively use
experimental load testing. The only significant difference between the experimental evidence
and computational model prediction was in the shear panel deformation test. Further testing of
the counterbracing members in the center panel, as well as the queenpost, would be required to
fully understand the disparity.
Other definitive results of the analyses indicated that the rehabilitation work done in 1979
did not appreciably change the load-carrying behavior of the main trusses, but it did introduce
members, the center panel diagonals, that are unlikely to ever perform well due to cyclical forces
in their connections under typical live loading. Even with some of the unusual and asymmetrical
detailing done during the rehabilitation, the system remarkably displays little out-of-plane
displacement.
The most noticeable contribution of the rehabilitation, though difficult to accurately
model, was the stiffening of the floor system and subsequent alteration of live load distribution.
With only three floor beams, as opposed to the seven that exist today, it is likely that a more
significant portion of the load was at one time carried through the floor system to the abutments.
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The load-sharing role of the hanger rods, along with the additional floor beams, was not
accurately determined, but there was some indication that they are more active in carrying the
load than the simplified computational model might suggest. Finally, in comparing the
computational model with a series of load tests performed on the scarf joints, it was determined
that they are no longer effective at transferring axial forces between lower chord members.
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