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## Part 1

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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 • 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**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**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)**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**Prestressed Girders**I - GIRDER BULB TEE**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**Triple Box Culvert**Box Culvert**Voided Slab Bridge**Solid Slab**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**Pile Cap Column Footing**Footing**Preliminary Design**• Bridge location • Hydraulic design to determine required bridge length and profile grade • Bridge type selection**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)**Stream Valley Cross-sections**n1 n2 n3 Right Overbank Left Overbank Channel**Manning’s Equation**n = Roughness Coefficient A = Area R = Hydraulic Radius = A / P P = Wetted Perimeter S = Hydraulic Gradient (channel slope)**Stream Valley Cross-sections**n1 n2 n3 Right Overbank Left Overbank Channel**1**2 EGL Headloss hl Velocity V12/2g HGL V22/2g Velocity Pressure y1 y2 Pressure z1 Elevation z2 Datum Energy Equation Elevation**Opening Length**Constriction of Valley by Bridge Bridge Deck/Roadway**Encroachment by Roadway Fill**Encroachment Bridge Opening Encroachment Fill Fill Flood elevation before encroachment on floodplain Backwater**Affect of Bridge on Flood Elevations**Design High Water Surface (DHW) Backwater Normal Water Surface Water Surface through Structure**Part 2**Slab Design**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**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**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)**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**Part 3**Steel Beam Design**Live Load = HS20 Truck x Distribution Factor**Distribution Factor = S/5.5**Steel Girder Design**• Design Moment = 2358 k-ft • Design Shear = 214 kips • Limit Bending Stress Due To Moment • Limit Shear Stress Due to Shear