1OVERVIEW OF ESSENTIAL DESIGN AND CONSTRUCTION ASPECTS OF PT SPLICED GIRDER BRIDGE Teddy Theryo, P.E.Bryce Binney, P.E., S.E.Parsons BrinckerhoffSegmental Bridge GroupPCEF COMMITTEE MEETING IN DOVERAUGUST 25, 2009
2Presentation Outline 1 2 3 4 5 Definition & History Construction Process3Design Aspect4Segment Detailing5Grouting, Anchorage Protection
3Outline Definition Reasons for Use History (Florida Specific) 1Definition & HistoryDefinitionReasons for UseHistory (Florida Specific)Current Practices
4Definition of Spliced Girder: NCHRP Report 517 “Extending Span Ranges of Precast Prestressed Concrete Girders” Defines as:Section “…a precast prestressed concrete member fabricated in several relatively long pieces (i.e., girder segments) that are assembled into a single girder for the final bridge structure. Post-tensioning is generally used to reinforce the connection between girder segments.”Reasons for Use:Increasing span lengthsImproving safety by eliminating shoulder piers or interior supports (beginning applications)Economic considerations (cost savings over other materials)
5History in Florida:Growing concern over highway safety during middle 1960’s1967 AASHTO Traffic Safety Committee Report“…adoption and use of two-span bridges for overpasses crossing divided highways…to eliminate the bridge piers normally placed adjacent to the shoulders”
6History: PCI published information to extend spans: PCI, Prestressed Concrete for Long Span Bridges, 1968.PCI Bridge Bulletin, “Long Spans with Standard Bridge Girders”, 1967.Publications addressed splicing using P/T and falsework~115'Florida used this approach on bridges in the late 1970’s:Chipola Nursery Bridge (Northwest Florida)I-75 Overpasses (Southwest Florida)Both used AASHTO Type IV with spans of approx. 115’
7Typical Usage Original concept evolved to bulb-tees with haunches Intracoastal Waterway Crossings (navigational clearance)Spans up to 300’ can be accommodated200’ to 300’65’150’Haunch Girder is used for span equal or larger than 250’
8Spliced Girder Project Photos EscambiaRiver BridgePascagoulaBridge, MSRt.33 Bridge, VA
9Outline Design/Construction Interdependency Erection Process 2Construction ProcessDesign/Construction InterdependencyErection ProcessDesign Concept
11Temporary Support During Construction Robust temporary structuresStable structure at all stagesAvoid using PT bars coupler if possibleRobust temporary bracing systemLock girder Supports against longitudinaland lateral sliding
14Falsework on Pier Footing 1Falsework on Pier FootingBleed lenses form as a result of the separation of water from the cement. Ducts with vertical rises will typically experience more bleed due to the increased pressure within the grout column. Intermediate bleed water lenses may form in tall vertical ducts, leaving a void through the cross-section of the duct exposing the tendon. This sedimentation process is accentuated by the addition of seven-wire strand. The spaces within the individual twisted wires that form the strand are large enough to allow easy passage of water but not cement.
233Strongback DetailMasterflow 816 is a highly fluid grout which contains no aggregate of any kind. It is a Grade C grout with excellent flow retention designed for superior pumpability. It is typically used for grouting of post tensioned cables and bars, and protects the highly stressed steel from corrosion by giving complete fill, with absolutely no bleeding or segregation.
25Prepare for Drop-in Girders 4Prepare for Drop-in Girders
26Placing Drop-in Girder 4Placing Drop-in GirderThis slide builds on both sides, showing the process of measuring flowable grouts on a flow table and fluid grouts using the flow cone. Describe the test procedure, and the relationship between the two tests. 145% flow on the table represents the limit of the edge of the table. After that the grout runs off the edge, and you have to use the flow cone.
28Significant Stages of Construction: Design ConceptSignificant Stages of Construction:Girders erected & supported by falsework or strongbacks
29Significant Stages of Construction: Design ConceptSignificant Stages of Construction:C.I.P. diaphragms or splices poured, Stage 1 P/T applied
30Significant Stages of Construction: Design ConceptSignificant Stages of Construction:Deck poured, Stage 2 P/T applied (deck compression)Longitudinally, bridge is fully serviceable. Deck and girders held to an allowable stress (usually 3√f’c).
31Assumed Erection Sequence Construction Sequence to be included in contract documents
32Outline Span to Depth Ratios Staged Construction Loading 3Design AspectSpan to Depth RatiosStaged Construction LoadingPost-Tensioning LayoutsDeck Replaceability
33Design Philosophy Two Stages of Post-Tensioning Stability ServiceabilityTime Dependant AnalysisUltimate Limit States
34Span to Depth Ratios General Guidelines: Simple span AASHTO prestressed girders (L/20)L/25 – L/28Continuous post-tensioned spliced girder (L/25 to L/28)L/20 – L/25Haunched pier continuous post-tensioned spliced girder (L/20 to L/25)** See AASHTO LRFD Article Agency may adhere to these requirements.
35Example of Dead Load Staging: Staged Dead LoadsExample of Dead Load Staging:Erect girders+After P/T, removal of temp. supports**Deck pouring** Notice change in static scheme in this stage to a continuous unit
36Staged Construction*Forces/stresses in structure are sum of all load casesFinal analysis program should account for:Incremental summation of all loadsChanges in static system throughout constructionTime dependant creep & shrinkage, including beam/slab differential (CEB-FIP Model Code Capability Preferable)Temporary loads & supportsPreferably ability to pour deck in stagesA few available software packages:BD2TANGOADAPTConsplice (LEAP)MIDASRMLARSA 4D
37Typical Tendon Profile Post TensioningTypical Tendon ProfileParabolic profileInflection points at 0.9 to 0.95 of span length90% to 95% of SpanParabolic
38Post Tensioning 3 to 5 tendons per girder 9 to 15 strands per tendon Typical Tendon Sizes3 to 5 tendons per girder9 to 15 strands per tendonNormally 50% of the tendons stressed on non-comp. structureDouble End StressingFor tendons with excessive lengths, dual end stressing may be required to overcome large friction loss.
39Miscellaneous Considerations Other ConsiderationsMiscellaneous ConsiderationsNon-Linear Temperature GradientCheck Principle Web Stress (composite section complicates this procedure, perhaps check at non-composite & composite C.G.) If stresses are high, 3.5√f’c to 4.0√f’c, consider using a thicker web.
40Vertical Geometry Spliced FBT-78 Navigational Unit Finished Grades vs. Top of Beam Elevations75.575.074.574.073.5Finished Grade Elevation73.072.572.071.571.00.0100.0200.0300.0400.0500.0Distance From Begin Main UnitFinished GradesTop Of Beam
41Deck Replaceability Common Practice to include the deck as part of composite section in supporting SDL and LiveLoadsCan the deck be replaced ?Answer: YesHow can the deck be replaced ?It must be designed for deck replacement
42Design Strategy for Deck Replacement Deck ReplaceabilityDesign Strategy for Deck ReplacementUse light weight Concrete DeckUse Precast Deck Panel (ABC)Stress all tendons prior to deck placementAdd temporary / permanent straight top tendonDeck and girder is a composite section for SDLand LL (no prestress in the deck section)
43Outline Web Thickness Pier Segment End Span Segments 4 Segment DetailingWeb ThicknessPier SegmentEnd Span Segments
49Pier Segment Considerations Erection Load CaseSupport self-weight during transport and erectionSupport weight of “drop-in” girders with prestressing (other loads: diaphragms, strongbacks, construction loads)Prestressing ConfigurationPositive and negative moments during constructionUse eccentricity vs. 1/P diagrams to satisfy both conditions
52Ult. Negative Moment Capacity Neg. MomentOver-reinforced(AASHTO Formulas)Under-reinforced(AASHTO Formulas)32”52”40”20”
53Ult. Negative Moment Capacity Local Widening of Compression Flange
54End Segment Considerations Anchorage BlockUse web tapering to transition to typical beam cross-sectionBursting ForcesUse local thickening of beam to accommodate anchoragesConsider staging of P/T in bursting modelsIf beam reaction is > approx. 10% of P/T force, consider in S & T
65Outline5Grouting & Anchorage ProtectionRecent research has reveled the importance of good grouting practicesAnchorages shall receive multiple layers or protection to avoid ingress of contaminantsFlorida DOT has implemented a stringent / robust PT and grouting spec. to enhance durability
66Grout from lowest point and one direction grout flow Grout Tube and Grout Vent LocationsVentVentGroutFlowGroutFlowGrout Tube/DrainSimply Supported BeamStrandBundleVentVentDuctVentMin.3 ft.Grout FlowGrout FlowStressingAnchorageGroutDead EndAnchorageGrout Tube/DrainGrout Tube/DrainContinuous BeamGrout from lowest point and one direction grout flow
68Highlights of Florida Grouting Grout MaterialZero Bleed Water, Pre-Bagged, Pre-ApprovedDuct MaterialPlastic Duct, Light ColorSealed & Pressure TestedGrouting ProcedureInjected Solely from Extreme Low PointHigh Points & Anchorages Inspected After GroutingSecondary (Vacuum) Grouting if Needed
69Anchorage Protection (FDOT Standard) Four Levels of Protection:Grouted TendonPermanent Grout CapEpoxy Grout Pour-BackEncapsulating DiaphragmPermanentGrout CapEpoxy GroutPour-backDiaphragmDeck Slab PourFull WidthPermanentGrout CapEpoxy GroutPour-backDrain Hole (extendOutside of Beam – fillWith Epoxy Grout)Diaphragm
70Summary Topics Presented Spliced girder explanation Construction processDesign philosophySegment detailing/designAddressed durability and groutingFinal Statement:Concrete spliced girders provide an economical and durable alternative for medium span ranges. With increasing familiarity to owners and contractors, they will likely become a common solution…..
71ReferencesAnon., “Highway Design and Operational Practices Related to Highway Safety,” Report of the Special AASHTO Traffic Safety Committee, February 1967.Anon., Prestressed Concrete for Long Span Bridges, Prestressed Concrete Institute, Chicago, 1968.Anon., “Long Spans with Standard Bridge Girders,” PCI Bridge Bulletin, March-April 1967, Prestressed Concrete Institute, Chicago.Castrodale, R. W., and C. D. White “Extending Span Ranges of Precast, Prestressed Concrete Girders.” NCHRP Report 517. TRB, National Research Council, Washington, D.C.“Detailing for Post-Tensioning”, VSL International, Bern Switzerland, 1991.FDOT, New Directions for Florida Post-Tensioned Bridges, Volumes 1 & 4.Podolny, Jr., Walter, and Jean Muller, “Construction and Design of Prestressed Concrete Segmental Bridges,” John Wiley & Sons, Inc., New York, 1982.PTI Post-Tensioning Manual, 6th ed., Post-Tensioning Institute, U.S.A., Ch. VIII, Anchorage Zone Design.Ronald, H. D “Design and Construction Consideration for Continuous Post-Tensioned Bulb-Tee Girder Bridges.” PCI Journal, Vol. 46, No. 3, Prestressed Concrete Institute, Chicago, IL, May-June 2001, pp