Problem 2.4.1 Structural Design
Introduction
Structural engineering is the design of structural
elements and their connections that work together
to support loads and maintain stability within a
system.
Structures vary by application and can range in
scale from complex bridge designs to massproduced cell phone enclosures. Regardless of the
structure’s scale or purpose, all structures are
designed to meet specific design criteria, including
operational environment, durability, aesthetics,
internal and external load handling, and cost. To
ensure that the optimal structural design is achieved
engineers with diverse backgrounds (e.g., material
science, statics, etc.) work together throughout the
design process. To aid engineers in the
development of complex structural design,
computer-aided design packages are used for
design analysis and verification.
Equipment
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Engineering notebook
Research sources
Computer loaded with West Point Bridge Designer software
Procedure
Your team will design and create a bridge utilizing West Point Bridge Designer
software. West Point Bridge Designer is a simplified and scaled down computeraided design tool developed by Colonel Stephen Ressler, Department of Civil and
Mechanical Engineering, U.S. Military Academy, West Point, New York. The
software will allow you to apply engineering design, material science, and statics to
the design of a truss bridge carrying a two-lane highway that spans a riverbed.
Design Constraints
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Minimization of Cost (Design success will be evaluated based upon structural
stability and overall cost—decrease the cost and improve the design.)
Bridge Configuration
o The bridge may cross the valley at any elevation from high water level to
24 meters above high water level.
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 1
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o If the elevation of the bridge deck is below 24 meters, excavation of the
riverbanks will be required to achieve the correct highway elevation.
o To provide clearance for overhead power lines, the highest point on the
bridge may not exceed an elevation 32.5 meters above the high water
level (8.5 meters above the top of the riverbanks).
o The bridge substructure may consist of either standard abutments (simple
supports) or arch abutments (arch supports). If necessary, the bridge may
also use one intermediate pier, located near the center of the valley. If
necessary, the bridge may also use cable anchorages, located 8 meters
behind one or both abutments.
o Each main truss can have no more than 50 joints and no more than 120
members.
o The bridge will have a flat, reinforced concrete deck. Two types of
concrete are available:
 Medium-strength concrete requires a deck thickness of 23
centimeters (0.23 meter).
 High-strength concrete requires a deck thickness of 15 centimeters
(0.15 meter).
 In either case, the deck will be supported by transverse floor beams
spaced at 4-meter intervals. To accommodate these floor beams,
your structural model must have a row of joints spaced 4 meters
apart at the level of the deck. These joints are created automatically
within West Point Bridge Designer.
o The bridge deck will be 10 meters wide, such that it can accommodate two
lanes of traffic.
Member Properties
o Materials—Each member of the truss will be made of either carbon steel;
high-strength, low-alloy steel; or quenched and tempered steel.
o Cross Sections—The members of the truss can be either solid bars or
hollow tubes. Both types of cross sections are square.
o Member Size—Both cross sections are available in a variety of standard
sizes.
The bridge must be capable of safely carrying the following loads:
o Weight of the reinforced concrete deck.
o Weight of a 5-cm thick asphalt wearing surface, which might be applied at
some time in the future.
o Weight of the steel floor beams and supplemental bracing members
(assumed to be 12.0 kN applied at each deck-level joint).
o Weight of the main trusses.
o Either of two possible truck loadings:
1. Weight of one standard H25 truck loading per lane, including
appropriate allowance for the dynamic effects of the moving load.
Since the bridge carries two lanes of traffic, each main truss must
safely carry one H25 vehicle, placed anywhere along the length of
the deck.
2. Weight of a single 480 kN Permit Loading, including appropriate
allowance for the dynamic effects of the moving load. Since the
Permit Loading is assumed to be centered laterally, each main
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 2
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truss must safely carry one-half of the total vehicle weight, placed
anywhere along the length of the deck.
The bridge will comply with the structural safety provisions of the 1994 LRFD
AASHTO Bridge Design Specification (Load and Resistance Factor Design), to
include:
o Material densities
o Load combinations
o Tensile strength of members
o Compressive strength of members
Cost Calculations
The cost of the design will be calculated using the following cost factors:
 Material Cost:
o Carbon steel bars—$3.78 per kilogram
o Carbon steel tubes—$6.30 per kilogram
o High-strength steel bars—$4.62 per kilogram
o High-strength steel tubes—$7.03 per kilogram
o Quenched and tempered steel bars—$5.70 per kilogram
o Quenched and tempered steel tubes—$7.95 per kilogram
o Connection cost—$300.00 per joint
o Product cost—$1000.00 per product
 Site Cost:
o Reinforced concrete deck (medium strength)—$4,850 per 4-meter panel
o Reinforced concrete deck (high strength)—$5,500 per 4-meter panel
o Excavation—$1.00 per cubic meter (see the Site Design Wizard for
excavation volume)
o Supports (abutments and pier)—cost varies (see the Site Design Wizard
for specific values)
o Cable Anchorages—$6,000 per anchorage
Explore West Point Bridge Designer Software
1. Launch
West Point
Bridge Designer
Application.
2. Select
Create a New
Bridge Design.
Select OK.
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 3
3. Read the
design
requirements
overview.
Select Next.
4. Under local
contest code,
select No.
Select Next.
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 4
5. Explore and
investigate the
impact of deck
elevation and
support
configurations
related to the
“Site Cost” by
completing the
deck elevation,
arch abutment,
pier, and cable
anchorages cost
impact tables.
Deck
Elevation
24 meters
20 meters
16 meters
12 meters
8 meters
4 meters
0 meters
Deck Elevation Cost Impact
Abutments
Pier
Cable
Anchorages
Standard
No Pier
No
Standard
No Pier
No
Standard
No Pier
No
Standard
No Pier
No
Standard
No Pier
No
Standard
No Pier
No
Standard
No Pier
No
Deck
Elevation
24 meters
24 meters
24 meters
24 meters
24 meters
24 meters
Arch Abutment Cost Impact
Arch
Pier
Cable
Abutments
Anchorages
24 meters
No Pier
No
20 meters
No Pier
No
16 meters
No Pier
No
12 meters
No Pier
No
8 meters
No Pier
No
4 meters
No Pier
No
Deck
Elevation
24 meters
24 meters
24 meters
24 meters
Pier Cost Impact
Abutments
Pier
Cable
Anchorages
Standard
24 meters
No
Standard
20 meters
No
Standard
16 meters
No
Standard
12 meters
No
Site Cost
62,700
72,500
86,300
97,100
108,900
121,700
131,500
Site Cost
91,200
85,300
82,500
81,400
80,100
81,000
Site Cost
107,000
104,200
101,400
98,600
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 5
24 meters
24 meters
24 meters
Deck
Elevation
24 meters
24 meters
24 meters
6.
Standard
Standard
Standard
8 meters
4 meters
0 meters
No
No
No
Cable Anchorages Cost Impact
Abutments
Pier
Cable
Anchorages
Standard
No Pier
None
Standard
No Pier
One
Standard
No Pier
Two
95,800
93,000
90,200
Site Cost
62,700
68,700
74,700
Select:
Deck Elevation: 24 meters
Support Configuration:
Standard Abutments
No Pier
No Cable Anchorages
Note that total site cost should
be $67,350.00.
Select Next.
7. Explore and investigate the
impact of deck material and
truck loading configurations
related to the “Site Cost” by
completing the deck material
and truck loading cost impact
tables.
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 6
Deck Material and Truck Loading Cost Impact
Deck Material
Loading
Medium-Strength
Standard 25kN
Medium-Strength
480 kN Permit
Loading
High-Strength
Standard 25kN
High-Strength
480 kN Permit
Loading
8.
Site Cost
62,700
62,700
66,000
66,000
Select:
Deck Material: Medium Strength
Loading: Standard 225kN Truck
Select Next.
9. Under Select a Template,
select none.
Select Next.
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 7
10. Type your engineering
team name into the Designed
By field.
Type “Exploring” into the Project
ID field.
Select Finish.
11. Explore the design window.
12. Explore the toolbars.
13. Investigate specific
member properties.
Select the Member Properties
Report icon.
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 8
14. The Member Properties
window provides you with
detailed information related to
the currently selected member.
Notice that the material type,
cross section type, and cross
section size relate to the
selected material in the toolbar.
If you change the member
properties within the toolbar, the
Member Properties Report will
also change. Investigate the
different member properties by
completing the member Material
selection comparison, member
Cross Section Type comparison
and member Cross Section
Size comparison.
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 9
Material
Member Material Selection Comparison
Cross
Yield
Modulus
Mass
Section
Stress
of
Density
Size
Elasticity
160
2.00E+08
mm
250000
Kn
7850 kg
Carbon
Steel
Cross
Section
Type
Solid
Bar
HighStrength
Solid
Bar
160
mm
Quenched
Solid
Bar
160
mm
Material
Carbon
Steel
Carbon
Steel
Material
Carbon
Steel
Carbon
Steel
Carbon
Steel
Carbon
Steel
Cross
Section
Type
Solid
Bar
Hollow
Tube
345000
485000
2.00E+08 7850 Kg
Kn
2.00E+08
kN
7850kg
Member Cross Section Type Comparison
Cross
Yield
Modulus
Mass
Section
Stress
of
Density
Size
Elasticity
160
250000
2.00E+08
mm
Kn
7850 kg
160
mm
250000
2.00E+08 7850 kg
kn
kN
Member Cross Section Size Comparison
Cross
Cross
Yield
Modulus
Mass
Section Section
Stress
of
Density
Type
Size
Elasticity
Solid
30 mm
Bar
250000
kN
2.00E+08 7850 kg
kN
Solid
160 mm 250000
Bar
kN
2.00E+08 7850 kg
kN
Solid
360 mm 250000k 2.00E+08
Bar
N
kN
7850 kg
Solid
500 mm 250000 2.00E+08
Bar
kN
kN
7850 kg
Moment
of Inertia
Cost per
Meter
5.46E-05
meters
904.32
5.46E-05
1004.80
5.46E-05
1115.33
Moment
of Inertia
Cost per
Meter
5.46E-05
904.32
1.88E-05
240.55
Moment
of Inertia
Cost per
Meter
6.75E-08
31.79
5.46E-05
904.32
1.40E-03
4578.12
5.21E-03
8831.25
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 10
15. Specify carbon steel, solid
bar, 100mm. Select the Joint
design tool and create a
series of joints above the
bridge road deck.
16. Select the Member draw
tool and draw members
between each joint.
17. After your bridge design is
complete, select the load test
icon from the toolbar.
18. A simulated load test will
play for your bridge design.
Notice as the truck (load)
goes over the bridge, member
forces can be seen by the
change of color in each
member.
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 11
19. Examine the forces within
each truss member by
expanding the member list
located on the right side of the
screen. This detailed list will
allow you to optimize your
design. A completely
optimized design will have
member compression and
tension reading < 1. When the
member reaches 1, it will fail.
20. Spend time optimizing your current truss design by altering material properties.
When complete save your design as “Exploring”.
Begin Your Own Design
Utilizing West Point Bridge Design Software and the engineering design process, create
the lowest cost possible bridge design that meets all design constraints. When you have
completed your final design, register your design team for the official West Point Bridge
Design Challenge and upload your team’s design at https://bridgecontest.org/
Documentation Deliverables (Written or multimedia format)
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Title Page: Include the title of the project, a picture of your final bridge design
and team members, team member names, course title, name of your school, and
the date.
Design Brief: Include a description of the problem and constraints.
Research Summary: Summarize your research related to material selection and
bridge truss design. The research summary should be less than one page.
Brainstorming Sketches/CAD Designs: Include copies or originals of your
team’s brainstorming sketches and CAD designs.
Modification Sketches: Include copies or originals of all major modifications.
Final Bridge Design: Include copies or originals of the final design, including the
following reports: load test results report, member property reports for all member
styles used, and cost calculations report.
Final Design Justification: Include justification for material selection and truss
configuration.
References: Use APA format to list all sources that were used to complete this
activity.
Conclusion Questions
1. How does the type and direction of stress applied affect the selection of the
material type and the cross-sectional area?
The wider the beam the more stress is applied and thus the selection of material type
and cross-sectional area must be higher.
2. How can the forces of compression and tension work together to make a
stronger bridge?
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 12
The forces of compression and tension work together to create a better bridge by
spreading the forces evenly to make a safe and stable bridge.
© 2012 Project Lead The Way, Inc.
Principles of Engineering Problem 2.4.1 Structural Design – Page 13