Load And Resistance Factor Design (LRFD) For Highway .

FHWA LRFD Steel Bridge Design ExampleDesign Step 1 – General Information / IntroductionDetailed Outline and FlowchartsEach step in this design example is based on a detailed outline and a series offlowcharts that were developed for this project.The detailed outline and the flowcharts are intended to be comprehensive. They includethe primary design steps that would be required for the design of various steel girderbridges.This design example includes the major steps shown in the detailed outline andflowcharts, but it does not include all design steps. For example, longitudinal stiffenerdesign, girder camber computations, and development of special provisions areincluded in the detailed outline and the flowcharts. However, their inclusion in the designexample is beyond the scope of this project.SoftwareAn analysis of the superstructure was performed using AASHTO BrD software. Thedesign moments, shears, and reactions used in the design example are taken from theBrD output, but their computation is not shown in the design example.Organization of Design ExampleTo make this reference user-friendly, the numbers and titles of the design steps areconsistent between the detailed outline, the flowcharts, and the design example.In addition to design computations, the design example also includes many tables andfigures to illustrate the various design procedures and many AASHTO references. Italso includes commentary to explain the design logic in a user-friendly way. A figure isgenerally provided at the end of each design step, summarizing the design results forthat particular bridge element.Tip BoxesTip boxes are used throughout the design example computations topresent useful information, common practices, and rules of thumb forthe bridge designer. Tip boxes are shaded and include a tip icon, justlike this. Tips do not explain what must be done based on the designspecifications; rather, they present suggested alternatives for thedesigner to consider.1-3

FHWA LRFD Steel Bridge Design ExampleDesign Step 1 – General Information / IntroductionDesign Step 1.1 - Obtain Design CriteriaThe first step for any bridge design is to establish the design criteria. For this designexample, the following is a summary of the primary design criteria:Design CriteriaGoverning specifications:AASHTO LRFD Bridge Design Specifications (Seventh Edition, 2014), including the2015 Interim SpecificationsDesign methodology:Load and Resistance Factor Design (LRFD)Live load requirements:S3.6HL-93Deck width:Wdeck 45.375 ftRoadway width:Wroadway 42.5 ftBridge length:Ltotal 240 ftSkew angle:Skew 0 degStructural steel yield strength:STable 6.4.1-1Fy 50ksiStructural steel tensile strength:STable 6.4.1-1Fu 65ksiConcrete 28-day compressive strength:S5.4.2.1f c′ 4.0ksi1-4

FHWA LRFD Steel Bridge Design ExampleDesign Step 1 – General Information / IntroductionReinforcement strength:S5.4.3f y 60ksiSteel density:STable 3.5.1-1Ws 0.490kcfConcrete density:STable 3.5.1-1Wc 0.150kcfParapet weight (each):W par 0.53KftFuture wearing surface:STable 3.5.1-1W fws 0.140kcfFuture wearing surface thickness:t fws 2.50in(assumed)Design Factors from AASHTO LRFD Bridge Design SpecificationsS1.3.2.1The first set of design factors applies to all force effects and is represented by the Greekletter η (eta) in the Specifications. These factors are related to the ductility, redundancy,and operational importance of the structure. A single, combined eta is required for everystructure. When a maximum load factor from STable 3.4.1-2 is used, the factored loadis multiplied by eta, and when a minimum load factor is used, the factored load isdivided by eta. All other loads, factored in accordance with STable 3.4.1-1, aremultiplied by eta if a maximum force effect is desired and are divided by eta if aminimum force effect is desired. In this design example, it is assumed that all eta factorsare equal to 1.0.Ductility:η D 1.0Redundancy:η R 1.01-5

FHWA LRFD Steel Bridge Design ExampleDesign Step 1 – General Information / IntroductionImportance:η I 1.0For loads for which the maximum value of γi is appropriate:η η D η R η Iandη 0.95SEquation 1.3.2.1-2For loads for which the minimum value of γi is appropriate:η 1andη D η R η Iη 1.00SEquation 1.3.2.1-3Therefore for this design example, use:η 1.00The following is a summary of other design factors from the AASHTO LRFD BridgeDesign Specifications. Additional information is provided in the Specifications, andspecific section references are provided in the right margin of the design example.Load factors:STable 3.4.1-1 &STable 3.4.1-2Limit StateStrength IStrength IIStrength IIIStrength VService IService IIFatigue IFatigue IILoad Combinations and Load FactorsLoad FactorsDCDWLLIMWS WLMax. Min. Max. Min.1.25 0.90 1.50 0.65 1.75 1.751.25 0.90 1.50 0.65 1.35 1.351.25 0.90 1.50 0.651.401.25 0.90 1.50 0.65 1.35 1.35 0.40 1.001.00 1.00 1.00 1.00 1.00 1.00 0.30 1.001.00 1.00 1.00 1.00 1.30 1.301.50 1.500.75 0.75-Table 1-1Load Combinations and Load FactorsS3.4.2.1The abbreviations used in Table 1-1 are as defined in S3.3.2. Also, S3.4.2.1 states thatprimary steel superstructure components are to be investigated for maximum forceeffects during construction for an additional special load combination consisting of theapplicable DC loads and any construction loads that are applied to the fully erected1-6

FHWA LRFD Steel Bridge Design ExampleDesign Step 1 – General Information / Introductionsteelwork. The load factor for force effects caused by DC loads and construction loads,including dynamic effects (if applicable), is not to be less than 1.4 for this additionalspecial load combination.The extreme event limit state (including earthquake load) is not considered in thisdesign example.S5.5.4.2 & S6.5.4.2Resistance factors (See S5.7.2.1 for a detailed description of tension and compressioncontrolled concrete sections):Resistance FactorsType of ResistanceFor flexureMaterialStructural steelReinforcedconcreteResistance Factor, φφf 1.00For shearφv 1.00For axial compressionφc 0.95For bearingφb 1.00Tension controlledφ 0.90For shear and torsionφ 0.90Compression controlledφ 0.75Bearing on concreteφ 0.70Table 1-2Resistance FactorsMultiple presence factors:STable 3.6.1.1.2-1Multiple Presence FactorsNumber of Lanes LoadedMultiple Presence Factor, m11.2021.0030.85 30.65Table 1-3 Multiple Presence Factors1-7

FHWA LRFD Steel Bridge Design ExampleDesign Step 1 – General Information / IntroductionDynamic load allowance:STable 3.6.2.1-1Dynamic Load AllowanceDynamic LoadLimit StateAllowance, IMDeck Joints - All Limit States75%Fatigue and Fracture Limit StateAll Other Limit States15%33%Table 1-4 Dynamic Load AllowanceDesign Step 1.2 - Obtain Geometry RequirementsGeometry requirements for the bridge components are defined by the bridge site and bythe highway geometry. Highway geometry constraints include horizontal alignment andvertical alignment.Horizontal alignment can be tangent, curved, spiral, or a combination of these threegeometries.Vertical alignment can be straight sloped, crest, sag, or a combination of these threegeometries.For this design example, it is assumed that the horizontal alignment geometry is tangentand the vertical alignment geometry is straight sloped.Design Step 1.3 - Perform Span Arrangement StudySome clients require a Span Arrangement Study. The Span Arrangement Studyincludes selecting the bridge type, determining the span arrangement, determiningsubstructure locations, computing span lengths, and checking horizontal clearance forthe purpose of approval.Although a Span Arrangement Study may not be required by the client, thesedeterminations must still be made by the engineer before proceeding to the next designstep.For this design example, the span arrangement is presented in Figure 1-1. This spanarrangement was selected to illustrate various design criteria and the establishedgeometry constraints identified for this example.1-8

FHWA LRFD Steel Bridge Design ExampleEDesign Step 1 – General Information / IntroductionFECL BearingsAbutment 1CL BearingsAbutment 2CL Pier120'-0”120'-0”240'-0”Legend:E Expansion BearingsF Fixed BearingsFigure 1-1 Span ArrangementDesign Step 1.4 - Obtain Geotechnical RecommendationsThe subsurface conditions must be determined to develop geotechnicalrecommendations.Subsurface conditions are commonly determined by taking core borings at the bridgesite. The borings provide a wealth of information about the subsurface conditions, all ofwhich is recorded in the boring logs.It is important to note that the boring log reveals the subsurface conditions for a finitelocation and not necessarily for the entire bridge site. Therefore, several borings areusually taken at each proposed substructure location. This improves their reliability as areflection of subsurface conditions at the bridge site, and it allows the engineer tocompensate for significant variations in the subsurface profile.After the subsurface conditions have been explored and documented, a geotechnicalengineer must develop foundation type recommendations for all substructures.Foundations can be spread footings, pile foundations, or drilled shafts. Geotechnicalrecommendations typically include allowable bearing pressure, allowable settlement,and allowable pile resistances (axial and lateral), as well as required safety factors foroverturning and sliding.For this design example, pile foundations are used for all substructure units.Design Step 1.5 - Perform Type, Size and Location StudySome clients require a Type, Size and Location study for the purpose of approval. TheType, Size and Location study includes preliminary configurations for the superstructureand substructure components relative to highway geometry constraints and siteconditions. Details of this study for the superstructure include selecting the girder types,determining the girder spacing, computing the approximate required girder span anddepth, and checking vertical clearance.1-9

FHWA LRFD Steel Bridge Design ExampleDesign Step 1 – General Information / IntroductionAlthough a Type, Size and Location study may not be required by the client, thesedeterminations must still be made by the engineer before proceeding to the next designstep.For this design example, the superstructure cross section is presented in Figure 1-2.This superstructure cross section was selected to illustrate selected design criteria andthe established geometry constraints. When selecting the girder spacing, considerationwas given to half-width deck replacement.45'- 4 ½"9 '-3”Shoulder12'-0”Lane12'-0”Lane9 '-3”Shoulder1'-5¼"3'-6” (Typ.)3'- 2 ¼"4 Spaces @ 9’-9” 39’-0”3'- 2 ¼"Figure 1-2 Superstructure Cross SectionDesign Step 1.6 - Plan for Bridge AestheticsFinally, the bridge engineer must consider bridge aesthetics throughout the designprocess. Special attention to aesthetics should be made during the preliminary stages ofthe bridge design, before the bridge layout and appearance has been fully determined.To plan an aesthetic bridge design, the engineer must consider the followingparameters: Function: Aesthetics is generally enhanced when form follows function. Proportion: Provide balanced proportions for members and span lengths. Harmony: The parts of the bridge must usually complement each other, and thebridge must usually complement its surroundings. Order and rhythm: All members must be tied together in an orderly manner. Contrast and texture: Use textured surfaces to reduce visual mass. Light and shadow: Careful use of shadow can give the bridge a more slenderappearance.1-10

FHWA LRFD Steel Bridge Design ExampleDesign Step 2 – Concrete Deck DesignConcrete Deck Design ExampleDesign Step 2Table of ContentsPageDesign Step 2.1 - Obtain Design Criteria .2Design Step 2.2 - Determine Minimum Slab Thickness .5Design Step 2.3 - Determine Minimum Overhang Thickness .5Design Step 2.4 - Select Slab and Overhang Thickness .5Design Step 2.5 - Compute Dead Load Effects.5Design Step 2.6 - Compute Live Load Effects.7Design Step 2.7 - Compute Factored Positive and NegativeDesign Moments .9Design Step 2.8 - Design for Positive Flexure in Deck .15Design Step 2.9 - Check for Positive Flexure Cracking underService Limit State .16Design Step 2.10 - Design for Negative Flexure in Deck .20Design Step 2.11 - Check for Negative Flexure Cracking underService Limit State .21Design Step 2.12 - Design for Flexure in Deck Overhang .24Design Step 2.13 - Check for Cracking in Overhang underService Limit State .42Design Step 2.14 - Compute Overhang Cut-off LengthRequirement .42Design Step 2.15 - Compute Overhang Development Length .43Design Step 2.16 - Design Bottom Longitudinal DistributionReinforcement .45Design Step 2.17 - Design Top Longitudinal DistributionReinforcement .46Design Step 2.18 - Design Longitudinal Reinforcement overPiers .472-1

FHWA LRFD Steel Bridge Design ExampleDesign Step 2 – Concrete Deck DesignDesign Step 2.19 - Draw Schematic of Final ConcreteDeck Design .502-2

FHWA LRFD Steel Bridge Design ExampleDesign Step 2 – Concrete Deck DesignDesign Step 2.1 - Obtain Design CriteriaThe first design step for a concrete bridge deck is to choose the correct design criteria.The following concrete deck design criteria are obtained from the typical superstructurecross section shown in Figure 2-1 and from the referenced articles and tables in theAASHTO LRFD Bridge Design Specifications, Seventh Edition (2014), including the2015 interims.Refer to Design Step 1 for introductory information about this design example.Additional information is presented about the design assumptions, methodology, andcriteria for the entire bridge, including the concrete deck.S4.6.2 and S9.7.2The next step is to decide which deck design method will be used. In this example, theequivalent strip method will be used. For the equivalent strip method analysis, thegirders act as supports, and the deck acts as a simple or continuous beam spanningfrom support to support. The empirical method could be used for the positive andnegative moment interior regions since the cross section meets all the requirementsgiven in S9.7.2.4. However, the empirical method could not be used to design theoverhang as stated in S9.7.2.2.Overhang WidthThe overhang width is generally determined such that the momentsand shears in the exterior girder are similar to those in the interiorgirder. In addition, the overhang is set such that the positive andnegative moments in the deck slab are balanced. A common rule ofthumb is to make the overhang approximately 0.28 to 0.35 times thegirder spacing.2-3

FHWA LRFD Steel Bridge Design ExampleDesign Step 2 – Concrete Deck ulder10'-0”Shoulder1'-5¼"3'-6” (Typ.)Bay 3Bay 2Bay 1Bay 44 Spaces @ 9’-9” 39’-0”3'-11¼"3'-11¼"Figure 2-1 Superstructure Cross SectionThe following units are defined for use in this design example:K 1000lbkcf K3Kksi 2inftDeck PropertiesGirder spacing:S 9.75ftSTable 5.12.3-1Number of girders:N 5STable 5.12.3-1Deck top cover:Covert 2.5inDeck bottom cover:Coverb 1.0inReinforced Concrete density:Wc 0.150kcfConcrete 28-daycompressive strength(Type A (AE)):f’c 4.0ksiSTable 5.4.2.1-1Reinforcement strength:fy 60ksiS5.4.3Future wearing surface:Wfws 0.140kcfSTable 3.5.1-12-4

FHWA LRFD Steel Bridge Design ExampleDesign Step 2 – Concrete Deck DesignParapet properties:Weight per foot:Width at base:Moment capacity at base*:Parapet Height:Critical length of yield line failure pattern*:Total transverse resistance of the parapet:* Based on parapet properties not included in this design example. See PublicationNumber FHWA HI-95-017, Load and Resistance Factor Design for Highway Bridges,Participant Notebook, Volume II(Version 3.01), for the method used to compute theparapet properties.STable 5.12.3-1Deck top cover - The concrete top cover is set at 2.5 inches since the bridge deckmay be exposed to deicing salts and/or tire stud or chain wear. This includes the 1/2inch integral wearing surface that is required.STable 5.12.3-1Deck bottom cover - The concrete bottom cover is set at 1.0 inch since the bridgedeck will use reinforcement that is smaller than a#11 bar.S5.4.2.1-1 and STable C5.4.2.1-1Concrete 28-day compressive strength – The compressive strength for decks shall2-5

FHWA LRFD Steel Bridge Design ExampleDesign Step 2 – Concrete Deck Designnot be less than 4.0 KSI. Also, type “AE” concrete should be specified when the deckwill be exposed to deicing salts or the freeze-thaw cycle. Class A (AE) concrete has acompressive strength of 4.0 KSI.STable 3.5

Factor Design (LRFD) to highway bridge design. One design example is a two-span steel plate girder bridge, and the other is a two-span prestressed concrete girder bridge with simple span