Problem 2.4.1 Structural Design



Problem 2.4.1 Structural DesignIntroductionStructural 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.EquipmentEngineering notebookResearch sourcesComputer loaded with West Point Bridge Designer softwareProcedureYour 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 ConstraintsMinimization of Cost (Design success will be evaluated based upon structural stability and overall cost—decrease the cost and improve the design.)Bridge ConfigurationThe 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 PropertiesMaterials—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 CalculationsThe cost of the design will be calculated using the following cost factors:Material Cost:Carbon steel bars—$3.78 per kilogramCarbon steel tubes—$6.30 per kilogramHigh-strength steel bars—$4.62 per kilogramHigh-strength steel tubes—$7.03 per kilogramQuenched and tempered steel bars—$5.70 per kilogramQuenched and tempered steel tubes—$7.95 per kilogramConnection cost—$300.00 per jointProduct cost—$1000.00 per productSite 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 anchorageExplore West Point Bridge Designer SoftwareLaunch 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. Deck Elevation Cost ImpactDeck ElevationAbutmentsPierCable AnchoragesSite Cost24 metersStandard No PierNo$62,70020 metersStandardNo PierNo$77,40016 metersStandardNo PierNo$88,40012 metersStandardNo PierNo$100,7008 metersStandardNo PierNo$110,4004 metersStandardNo PierNo$123,7000 metersStandardNo PierNo$134,000Arch Abutment Cost Impact Deck ElevationArch AbutmentsPierCable AnchoragesSite Cost24 meters24 metersNo PierNo$89,70024 meters20 metersNo PierNo$83,50024 meters16 metersNo PierNo$81,80024 meters12 metersNo PierNo$80,80024 meters8 metersNo PierNo$83,30024 meters4 metersNo PierNo$80,200Pier Cost Impact Deck ElevationAbutmentsPierCable AnchoragesSite Cost24 metersStandard 24 metersNo$104,00024 metersStandard20 metersNo$101,20024 metersStandard16 metersNo$98,40024 metersStandard12 metersNo$95,60024 metersStandard8 metersNo$92,80024 metersStandard4 metersNo$90,00024 metersStandard0 metersNo$87,200Cable Anchorages Cost Impact Deck ElevationAbutmentsPierCable AnchoragesSite Cost24 metersStandard No PierNone$62,70024 metersStandardNo PierOne$65,70024 metersStandardNo PierTwo$68,700Select:Deck Elevation: 24 metersSupport Configuration: Standard AbutmentsNo PierNo Cable AnchoragesNote 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. Deck Material and Truck Loading Cost ImpactDeck MaterialLoadingSite CostMedium-StrengthStandard 25kN$62,700Medium-Strength480 kN Permit Loading$62,700High-StrengthStandard 25kN$67,100High-Strength480 kN Permit Loading$67,100Select:Deck Material: Medium StrengthLoading: Standard 225kN TruckSelect 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.302260028702000The 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 MaterialCross Section TypeCross Section SizeYield StressModulus of ElasticityMass DensityMoment of InertiaCost per MeterCarbon SteelSolid Bar160 mm250,00 kN per sq. meter2.00E+08 kN per sq. meter7850 kg per cubic meter5.46E-05 meters4$864.13 High-StrengthSolid Bar160 mm345,00 kN per sq. meter2.00E+08 kN per sq. meter7850 kg per cubic meter5.46E-05 meters4$1125.38QuenchedSolid Bar160 mm485,000 kN per sq. meter2.00E+08 kN per sq. meter7850 kg per cubic meter5.46E-05 meters4$1205.76CarbonHigh-StrengthQuenchedMember Cross Section Type ComparisonMaterialCross Section TypeCross Section SizeYield StressModulus of ElasticityMass DensityMoment of InertiaCost per MeterCarbon SteelSolid Bar160 mm250,00 kN per sq. meter2.00E+08 kN per sq. meter7850 kg per cubic meter5.46E-05 meters4$864.13Carbon SteelHollowTube160 mm250,00 kN per sq. meter2.00E+08 kN per sq. meter7850 kg per cubic meter1.88E-05 meters4$240.55Hollow TubeMember Cross Section Size ComparisonMaterialCross Section TypeCross Section SizeYield StressModulus of ElasticityMass DensityMoment of InertiaCost per MeterCarbon SteelSolid Bar30 mm250,00 kN per sq. meter2.00E+08 kN per sq. meter7850 kg per cubic meter6.75E-08 meters4$30.38Carbon SteelSolid Bar160 mm250,00 kN per sq. meter2.00E+08 kN per sq. meter7850 kg per cubic meter5.46E-05 meters4$864.13Carbon SteelSolid Bar360 mm250,00 kN per sq. meter2.00E+08 kN per sq. meter7850 kg per cubic meter1.40E-03 meters4$4374.65Carbon SteelSolid Bar500 mm250,00 kN per sq. meter2.00E+08 kN per sq. meter7850 kg per cubic meter5.21E-03 meters4$8438.75Specify 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 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 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 QuestionsHow 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? ................
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