Problem 2.4.1 Structural Design

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 mass-produced 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

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 computer-aided 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

Minimization of Cost (Design success will be evaluated based upon structural stability and overall cost—decrease the cost and improve the design.)
Bridge Configuration
The bridge may cross the valley at any elevation from high water level to 24 meters above high water level.
If the elevation of the bridge deck is below 24 meters, excavation of the riverbanks will be required to achieve the correct highway elevation.
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).
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.
Each main truss can have no more than 50 joints and no more than 120 members.
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.
The bridge deck will be 10 meters wide, such that it can accommodate two lanes of traffic.
Member Properties
Materials—Each member of the truss will be made of either carbon steel; high-strength, low-alloy steel; or quenched and tempered steel.
Cross Sections—The members of the truss can be either solid bars or hollow tubes. Both types of cross sections are square.
Member Size—Both cross sections are available in a variety of standard sizes.
The bridge must be capable of safely carrying the following loads:
Weight of the reinforced concrete deck.
Weight of a 5-cm thick asphalt wearing surface, which might be applied at some time in the future.
Weight of the steel floor beams and supplemental bracing members (assumed to be 12.0 kN applied at each deck-level joint).
Weight of the main trusses.
Either of two possible truck loadings:
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.
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 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:
Material densities
Load combinations
Tensile strength of members
Compressive strength of members

Cost Calculations

The cost of the design will be calculated using the following cost factors:

Material Cost:
Carbon steel bars—$3.78 per kilogram
Carbon steel tubes—$6.30 per kilogram
High-strength steel bars—$4.62 per kilogram
High-strength steel tubes—$7.03 per kilogram
Quenched and tempered steel bars—$5.70 per kilogram
Quenched and tempered steel tubes—$7.95 per kilogram
Connection cost—$300.00 per joint
Product cost—$1000.00 per product
Site Cost:
Reinforced concrete deck (medium strength)—$4,850 per 4-meter panel
Reinforced concrete deck (high strength)—$5,500 per 4-meter panel
Excavation—$1.00 per cubic meter (see the Site Design Wizard for excavation volume)
Supports (abutments and pier)—cost varies (see the Site Design Wizard for specific values)
Cable Anchorages—$6,000 per anchorage

Explore West Point Bridge Designer Software

Launch West Point Bridge Designer Application.

Select Create a New Bridge Design.

Select OK. /

Read the design requirements overview.

Select Next. /

Under local contest code, select No.

Select Next. /

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.

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Deck Elevation Cost Impact
Deck Elevation / Abutments / Pier / Cable Anchorages / Site Cost
24 meters / Standard / No Pier / No
20 meters / Standard / No Pier / No
16 meters / Standard / No Pier / No
12 meters / Standard / No Pier / No
8 meters / Standard / No Pier / No
4 meters / Standard / No Pier / No
0 meters / Standard / No Pier / No
Arch Abutment Cost Impact
Deck Elevation / Arch Abutments / Pier / Cable Anchorages / Site Cost
24 meters / 24 meters / No Pier / No
24 meters / 20 meters / No Pier / No
24 meters / 16 meters / No Pier / No
24 meters / 12 meters / No Pier / No
24 meters / 8 meters / No Pier / No
24 meters / 4 meters / No Pier / No
Pier Cost Impact
Deck Elevation / Abutments / Pier / Cable Anchorages / Site Cost
24 meters / Standard / 24 meters / No
24 meters / Standard / 20 meters / No
24 meters / Standard / 16 meters / No
24 meters / Standard / 12 meters / No
24 meters / Standard / 8 meters / No
24 meters / Standard / 4 meters / No
24 meters / Standard / 0 meters / No
Cable Anchorages Cost Impact
Deck Elevation / Abutments / Pier / Cable Anchorages / Site Cost
24 meters / Standard / No Pier / None
24 meters / Standard / No Pier / One
24 meters / Standard / No Pier / Two

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. /

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.

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Deck Material and Truck Loading Cost Impact
Deck Material / Loading / Site Cost
Medium-Strength / Standard 25kN
Medium-Strength / 480 kN Permit Loading
High-Strength / Standard 25kN
High-Strength / 480 kN Permit Loading

Select:

Deck Material: Medium Strength
Loading: Standard 225kN Truck
Select Next. /

Under Select a Template, select none.

Select Next. /

Type your engineering team name into the Designed By field.

Type “Exploring” into the Project ID field.
Select Finish. /

Explore the design window.

Explore the toolbars.

Investigate specific member properties.

Select the Member Properties Report icon. /

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.

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Member Material Selection Comparison
Material / CrossSection Type / CrossSection Size / Yield Stress / Modulus of Elasticity / Mass Density / Moment of Inertia / Cost per Meter
Carbon Steel / Solid Bar / 160 mm
High-Strength / Solid Bar / 160 mm
Quenched / Solid Bar / 160 mm
Member CrossSection Type Comparison
Material / CrossSection Type / CrossSection Size / Yield Stress / Modulus of Elasticity / Mass Density / Moment of Inertia / Cost per Meter
Carbon Steel / Solid Bar / 160 mm
Carbon Steel / Hollow
Tube / 160 mm
Member CrossSection Size Comparison
Material / CrossSection Type / CrossSection Size / Yield Stress / Modulus of Elasticity / Mass Density / Moment of Inertia / Cost per Meter
Carbon Steel / Solid Bar / 30 mm
Carbon Steel / Solid Bar / 160 mm
Carbon Steel / Solid Bar / 360 mm
Carbon Steel / Solid Bar / 500 mm

Specify carbon steel, solid bar, 100mm. Select the Joint design tool and create a series of joints above the bridge road deck.

Select the Member draw tool and draw members between each joint.

After your bridge design is complete, select the load test icon from the toolbar.

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.

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.

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

Documentation Deliverables (Written or multimedia format)

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

How does the type and direction of stress applied affect the selection of the material type and the cross-sectional area?

How can the forces of compression and tension work together to make a stronger bridge?

Principles of Engineering Problem 2.4.1 Structural Design– Page 1