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Problem 2.4.1 Structural DesignIntroduction

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

ProcedureYour 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

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 –

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 Propertieso 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


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 densitieso Load combinationso Tensile strength of memberso Compressive strength of members

Cost CalculationsThe cost of the design will be calculated using the following cost factors:

Material Cost:o Carbon steel bars—$3.78 per kilogramo Carbon steel tubes—$6.30 per kilogramo High-strength steel bars—$4.62 per kilogramo High-strength steel tubes—$7.03 per kilogramo Quenched and tempered steel bars—$5.70 per kilogramo Quenched and tempered steel tubes—$7.95 per kilogramo Connection cost—$300.00 per jointo 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 Software1. Launch West Point Bridge Designer Application.

2. Select Create a New Bridge Design.

Select OK.


3. Read the design requirements overview.

Select Next.

4. Under local contest code, select No.

Select Next.


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 Cost ImpactDeck

ElevationAbutments Pier Cable

AnchoragesSite Cost

24 meters Standard No Pier No $62,70020 meters Standard No Pier No $77,40016 meters Standard No Pier No $88,40012 meters Standard No Pier No $100,7008 meters Standard No Pier No $110,4004 meters Standard No Pier No $123,7000 meters Standard No Pier No $134,000

Arch Abutment Cost Impact Deck

ElevationArch

AbutmentsPier Cable

AnchoragesSite Cost

24 meters 24 meters No Pier No $89,70024 meters 20 meters No Pier No $83,50024 meters 16 meters No Pier No $81,80024 meters 12 meters No Pier No $80,80024 meters 8 meters No Pier No $83,30024 meters 4 meters No Pier No $80,200

Pier Cost Impact Deck


AnchoragesSite Cost

24 meters Standard 24 meters No $104,00024 meters Standard 20 meters No $101,20024 meters Standard 16 meters No $98,40024 meters Standard 12 meters No $95,600


24 meters Standard 8 meters No $92,80024 meters Standard 4 meters No $90,00024 meters Standard 0 meters No $87,200

Cable Anchorages Cost Impact Deck


AnchoragesSite Cost

24 meters Standard No Pier None $62,70024 meters Standard No Pier One $65,70024 meters Standard No Pier Two $68,700

6. 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.


Deck Material and Truck Loading Cost ImpactDeck Material Loading Site Cost

Medium-Strength Standard 25kN $62,700Medium-Strength 480 kN Permit

Loading$62,700

High-Strength Standard 25kN $67,100High-Strength 480 kN Permit

Loading$67,100

8. Select:

Deck Material: Medium Strength

Loading: Standard 225kN Truck

Select Next.

9. Under Select a Template, select none.

Select Next.


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.

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.


Member Material Selection Comparison Material Cross

Section Type

Cross Section

Size

Yield Stress

Modulus of Elasticity

Mass Density

Moment of Inertia

Cost per Meter

Carbon Steel Solid Bar

160 mm

250,00 kN per

sq. meter

2.00E+08 kN per sq.

meter

7850 kg per cubic

meter

5.46E-05 meters4

$864.13

High-Strength

Solid Bar

160 mm

345,00 kN per

sq. meter

2.00E+08 kN per sq.

meter

7850 kg per cubic

meter

5.46E-05 meters4

$1125.38

Quenched Solid Bar

160 mm

485,000 kN per

sq. meter

2.00E+08 kN per sq.

meter

7850 kg per cubic

meter

5.46E-05 meters4

$1205.76

Carbon High-Strength Quenched

Member Cross Section Type ComparisonMaterial Cross

Section Type

Cross Section

Size

Yield Stress

Modulus of

Elasticity

Mass Density

Moment of Inertia

Cost per Meter


160 mm

250,00 kN per

sq. meter

2.00E+08 kN per

sq. meter

7850 kg per cubic

meter

5.46E-05 meters4

$864.13

Carbon Steel HollowTube

160 mm

250,00 kN per

sq. meter

2.00E+08 kN per

sq. meter

7850 kg per cubic

meter

1.88E-05 meters4

$240.55

Hollow Tube

Member Cross Section Size ComparisonMaterial Cross

Section Type

Cross Section

Size

Yield Stress

Modulus of

Elasticity

Mass Density

Moment of Inertia

Cost per Meter


30 mm 250,00 kN per

sq. meter

2.00E+08 kN per

sq. meter

7850 kg per cubic

meter

6.75E-08 meters4

$30.38



160 mm 250,00 kN per

sq. meter

2.00E+08 kN per

sq. meter

7850 kg per cubic

meter

5.46E-05 meters4

$864.13


360 mm 250,00 kN per

sq. meter

2.00E+08 kN per

sq. meter

7850 kg per cubic

meter

1.40E-03 meters4

$4374.65


500 mm 250,00 kN per

sq. meter

2.00E+08 kN per

sq. meter

7850 kg per cubic

meter

5.21E-03 meters4

$8438.75

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.

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 DesignUtilizing 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 http://bridgecontest.usma.edu

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.


http://bridgecontest.usma.edu/

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 Questions1. How does the type and direction of stress applied affect the selection of the

material type and the cross-sectional area?

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


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