Engineering Design Process: 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 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 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. 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 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 Site Cost Explore West Point Bridge Designer Software 1. 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 Impact Deck Elevation Abutments Pier Cable Anchorages 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 Site Cost
0 meters Standard No Pier No Arch Abutment Cost Impact Deck Elevation Arch Abutments Pier Cable Anchorages 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 Abutments Pier Cable Elevation Anchorages 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 24 meters Standard No Pier None 24 meters Standard No Pier One 24 meters Standard No Pier Two Site Cost Site Cost Site Cost
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 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
8. Select: Deck Material: Medium Strength Loading: Standard 225kN Truck Select Next. 9. Under Select a Template, select none. Select Next. 10. Explore the design window.
11. Explore the toolbars. 12. Investigate specific member properties. Select the Member Properties Report icon. 13. 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.
Material High- Strength Quenched Cross Section Type Member Material Selection Comparison Cross Yield Modulus Mass Section Stress of Density Size Elasticity 160 mm 160 mm 160 mm Moment of Inertia Cost per Meter Material Cross Section Type Hollow Tube Member Cross Section Type Comparison Cross Yield Modulus Mass Section Stress of Density Size Elasticity 160 mm 160 mm Moment of Inertia Cost per Meter Material Cross Section Type Member Cross Section Size Comparison Cross Yield Modulus Mass Section Stress of Density Size Elasticity 30 mm 160 mm 360 mm 500 mm Moment of Inertia Cost per Meter
14. Specify carbon steel, solid bar, 100mm. Select the Joint design tool and create a series of joints above the bridge road deck. 15. Select the Member draw tool and draw members between each joint. 16. After your bridge design is complete, select the load test icon from the toolbar. 17. 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.
Begin Your Own Design After seeing how the software works, brainstorm and research with your partner and Utilize West Point Bridge Design Software and the engineering design process, create the lowest cost possible bridge design that meets all design constraints. Documentation Deliverables (Written or multimedia format) Title Page: Include the title of the project, a picture of your final bridge design 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? 2. How can the forces of compression and tension work together to make a stronger bridge?