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Part 1. Introduction To Bridge Design. How Do Bridge Engineers Decide On What Type Of Bridge To Build?. Bridge Survey flood plain cross sections inspection reports existing bridge (scour, etc) water elevations photos existing roadway profile.

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  1. Part 1 Introduction To Bridge Design

  2. How Do Bridge Engineers Decide On What Type Of Bridge To Build? • Bridge Survey • flood plain cross sections • inspection reports • existing bridge (scour, etc) • water elevations • photos • existing roadway profile • Factors affecting choice of superstructure • location, city or rural • span length • vertical clearance • maintainability • environmental concerns • transportation to site issues • cost • Factors affecting choice of substructure • location and geometry • subsoil conditions • height of column • Geotechnical Report • soil / geological formations • slopes and grading • foundation problems • soil prop.’s - phi angles etc

  3. Bridge Design Process • Preliminary Design Process • Bridge Survey • Geotechnical Report • Determine the most economical type structure and span arrangement • Hydraulic Analysis • Preliminary Cost Estimate • Foundation Borings • Determine Foundation Type • Final Design Process • Top to Bottom Design (twice) • Design methods per AASHTO and MoDOT Bridge Manual • Analysis via • computations • spreadsheets • computer programs • Detail plans are produced by technicians (Micro-Station) • Plans are checked • Quantities computed • Special Provisions written • Plans are advertised for bidding • Low Bid Contractor builds the bridge

  4. Types of Superstructures • Bridges are often referred to by their superstructure types. • The superstructure system of members carry the roadway over a crossing and transfer load to a substructure. • Superstructures are categorized by; • Support type (simply supported or continuous) • Design type (slab on stringer, slab, arch. Rigid frame, etc) • Material type (steel, concrete, timber)

  5. Slab on Stringer Bridges • Most common type of bridge in Missouri. • Consist of a deck, resting on the girders. The deck distributes the loads transversely to the girders. • The girders carry the loads longitudinally (down the length of the bridge) to the supports, (abutments and intermediate bents). • Concrete • Deck Girder • Prestressed I Girder • Prestressed Double Tee • Prestressed Box • Steel • Plate Girder • Wide Flange • Steel Box Girder

  6. Prestressed Girders I - GIRDER BULB TEE

  7. Prestressed Concrete I-Girder

  8. Prestressed Concrete I-Girder Bridge

  9. Prestressed Concrete Panels

  10. Prestressed Double Tee Girders

  11. Steel Plate Girder / Wide Flange Beam / Box Beam

  12. Steel Plate Girder Bridge

  13. Slab Bridges In slab bridges the deck itself is the structural frame or the entire deck is a thin beam acting entirely as one primary member. These types are used where depth of structure is a critical factor. Typical Slab Bridges : Concrete Box Culverts Solid Slabs Voided Slabs

  14. Triple Box Culvert Box Culvert

  15. Voided Slab Bridge

  16. Voided Slab Bridge Solid Slab

  17. Substructures • The substructure transfers the superstructure loads to the foundations. • End Abutments • Integral Abutment - girders on beam supported by piles, girders “concreted” into the diaphragm • Non-Integral Abutment - diaphragms of steel cross-frames, uses expansion devices • Semi-Deep Abutment - used when spanning divided highways to help shorten span • Open C.C. Abutment - beam supported by columns and footings, rarely used • Intermediate bents • Open Concrete Bent - beams supported by columns and footings (or drilled shafts) either a concrete diaphragm (Pre-Stressed Girder) or steel diaphragm (Plate Girder) This is the most common type of Pier MoDOT uses. • Pile Cap Bent - beams supported by piling (HP or C.I.P.) and are used when the column height is less than 15 feet and usually in rural areas. • Hammer Head Bent - single oval or rectangular column and footing. • Spread footings - are used when rock or soil can support the structure. • Pile footings - rectangular c.c. supported by HP or Cast in Place piles • Drilled Shafts - holes drilled into bedrock filled with concrete

  18. Integral End Abutment

  19. Semi-Deep End Abutment

  20. Prestressed I-girder intermediate bent

  21. Steel girders with open intermediate bent diaphragms

  22. Pile Cap Column Footing Footing

  23. Column Footing

  24. Preliminary Design • Bridge location • Hydraulic design to determine required bridge length and profile grade • Bridge type selection

  25. Stream Gage Data

  26. Flood-Frequency Rating Curve

  27. Rational Method Q = discharge (cfs or m3/s) kc = constant (1.0 for English units or 0.00278 for metric units) C = Runoff Coefficient I = Rainfall Intensity (in/hr or mm/hr) A = Drainage Area (acres or hectares)

  28. Drainage Area Delineation

  29. Stream Valley Cross-sections n1 n2 n3 Right Overbank Left Overbank Channel

  30. Manning’s Equation n = Roughness Coefficient A = Area R = Hydraulic Radius = A / P P = Wetted Perimeter S = Hydraulic Gradient (channel slope)

  31. Stream Valley Cross-sections n1 n2 n3 Right Overbank Left Overbank Channel

  32. 1 2 EGL Headloss hl Velocity V12/2g HGL V22/2g Velocity Pressure y1 y2 Pressure z1 Elevation z2 Datum Energy Equation Elevation

  33. Opening Length Constriction of Valley by Bridge Bridge Deck/Roadway

  34. Encroachment by Roadway Fill Encroachment Bridge Opening Encroachment Fill Fill Flood elevation before encroachment on floodplain Backwater

  35. Affect of Bridge on Flood Elevations Design High Water Surface (DHW) Backwater Normal Water Surface Water Surface through Structure

  36. Part 2 Slab Design

  37. Geometry & Loads Deck Weight = Width x Thickness x Unit Weight 1 ft x (8.5in x12 in/ft) x 150 lb/cf = 106 lb/ft 16k 16k

  38. Design Moment • MDL1 = wS2/10 = 0.106 x 82 / 10 = 0.678 • MDL2 = wS2/10 = 0.035 x 82 / 10 = 0.224 • MLL = 0.8(S+2)P/32 = 0.8(8+2)(16)/32 = 4 • MImp = 30% x MLL = 1.2 • Mu = 1.3[0.678+0.224+1.67(4+1.2)] = 12.4 Design For 12.4 k-ft/ft

  39. Statics, Moment, Shear, Stress?

  40. Comp. c = a / b1 c d Tens. Reinforced Concrete Design • Basic Equations For Moment Utilize Whitney Stress Block Concept Design Moment = Capacity • 12.4 k-ft/ft= f As fy(d-a/2) f = 0.90 Compression = Tension 0.85f’cba = As fy Two Simultaneous Equations, Two Unknowns (a & As)

  41. Comp. c = a / b1 c d Tens. Reinforced Concrete Design • (0.85)(4ksi)(12in)(a)=(As)(60ksi) a=1.47As • 12.4k-ft=(0.9)(As)(60ksi)(6in-1.47As/2)/(12in/ft) • 12.4=27As-3.31As2 • ax2+bx+c=0 a=3.31, b=-27, c=12.4, x=As • As = [-b - (b2 - 4ac)1/2]/2a • As = [-27 - ((-27)2-(4)(3.31)(12.4))1/2]/[(2)(3.31)] • As = 0.49 in2/ft • 5/8” rebar at 7.5 in centers

  42. Part 3 Steel Beam Design

  43. Simple Span Beam – 50 ft span

  44. Dead Load = Beam Weight + Deck Weight

  45. Live Load = HS20 Truck x Distribution Factor Distribution Factor = S/5.5

  46. Design Moment = 2358 kip-ft

  47. Design Shear = 214 kips

  48. Steel Girder Design • Design Moment = 2358 k-ft • Design Shear = 214 kips • Limit Bending Stress Due To Moment • Limit Shear Stress Due to Shear

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