Engineering Analysis
of Covered Wooden
Bridges from the
HAER Summer 2002
Project
Dylan Lamar
Undergrad Researcher, Univ. of Arkansas
Ben Schafer
Asst. Professor, Johns Hopkins University
schafer@jhu.edu www.ce.jhu.edu/bschafer
Acknowledgments
• NPS – HAER and summer 2002 team
• Erika Stoddard, Francesca da Porto
Objectives
• To use modern engineering analysis
to better understand historic covered
wooden bridge forms
• To better understand the intent of the
design details selected by the historic
builder through engineering analysis
• To provide a context whereby historic
engineering structures can be
understood and discussed in relation
to modern structures
Pine Grove Bridge
Chester/Lancaster County, PA
Burr truss (arch-truss)
Captain Elias McMellen builder
1884
double span, two at 93 ft
HAER PA-586 (2002)
Brown Bridge
Shrewsbury, VT
Town lattice
Nichols M. Powers builder
1880
single span, 112 ft
HAER VT-28 (2002)
Pine Grove Bridge
Pine Grove Bridge
architectural rendering from summer 2002 HAER documentation team
Longitudinal System
architectural rendering from summer 2002 HAER documentation team
Longitudinal System Model
Understanding Dead Load
Dead Load Response - Separate
(shaded bars proportional to axial forces in bridge members)
33 k
47 k
Dead Load Response - Combined
9k
35 k
Dead Load Subtleties
46 k-in.
axial (F)
moment (M)
(shaded areas indicate magnitude of bending in a member)
Dead Load Member Demand Highlights
F My
s  
A
I
• Arch (at end)
s = -391 (C) psi
(allowable 1000 psi)
• Truss (post 4)
s = -272 (C) psi
(allowable 1000 psi)
• Truss (post 5)
s = 261 (T) psi
(allowable 925 psi)
Arch-truss synergy
Midspan deflection (dead load)
Structural model
truss alone
= 0.96 in.
arch alone
= 0.91 in.
arch
truss
Simple parallel combination
truss+arch
= 0.47 in.
Structural model
truss+arch
= 0.25 in.
arch-truss is far stiffer than a simple addition of the two separate systems
Live Loads?
Live Load Arch Deflections
5k
d = 2.0 in.
5k
d = 4.8 in.
Live Load Results
5k
d = 0.07 in.
5k
d = 0.06 in.
Total Load Member Demand Highlights
(results for total load = dead load + quarter point live load)
F My
s  
A
I
• Arch (at end)
s = -489 (C) psi
(allowable 1000 psi)
• Truss (post 4)
s = -394 (C) psi
(allowable 1000 psi)
• Truss (post 4)
s = 458 (T) psi
(allowable 925 psi)
Pine Grove Bridge Thoughts
• Arch is the dominant load carrying member
• Arch success w/ live loads depends on truss
• Stiffness of system greater than sum of parts
• No overstressed members
• Tension in some diagonals under live load
• Far more enlightening analysis of these forms
is possible through further engineering study
Brown Bridge
Brown Bridge Structural System
architectural rendering from summer 2002 HAER documentation team
Brown Bridge Longitudinal System
architectural rendering from summer 2002 HAER documentation team
Brown Bridge Longitudinal Model
Dead Load Behavior
Dead Load
37 k
25 k
22 k
40 k
Brown Bridge Longitudinal System
(3 x 9 7/8 in.)
(3 x 11 in.)
architectural rendering from summer 2002 HAER documentation team
Dead Load Subtleties
moment
170 k-in.
Influence of Bolster
Member stresses are typically reduced by ¼ and reductions as
high as ½ are possible due to the addition of the bolster.
Stress Demands
Alternate Lattice Forms
Alternate Lattice Forms
Alternate lattice forms with greater structural efficiency are
possible, but constructional efficiency drives the actual design.
Brown Bridge Thoughts
• Structural system = beam behavior
• Stiff! L/1600 for our loading cases
• Sizing of primary bottom chord
member reflects deeper understanding
of member demands
• Bolster relieves stress concentrations
• Form follows construction efficiency
more than structural efficiency