Bridges With MSE Wall Supported Abutments

14d ago
36 Views
0 Downloads
201.32 KB
7 Pages
Transcription

Design MemorandumTO:FROM:DATE:SUBJECT:All Design Section StaffBijan KhaleghiJuly 1, 2013Bridges with MSE wall supported abutmentsThis design memorandum defines WSDOT policy for the use of Mechanically Stabilized Earth(MSE) wall supported abutments for single or multi-span bridges. This design memorandumsupersedes the design memorandum issued on June 2012.For the purpose of this policy memorandum, MSE walls shall be taken to include geosyntheticretaining walls (with and without structural facing) and structural earth walls.MSE wall supported abutments shall be designed in accordance with the requirements of thispolicy memorandum, as well as following documents in order of hierarchy: WSDOT Geotechnical Design Manual section 15.5.3.5 for MSE Wall SupportedAbutments AASHTO LRFD Bridge Design Specifications, 6th Edition 2012. FHWA NHI-10-024 Volume I and NHI-10-025 Volume II, “Design and Construction ofMechanically Stabilized Earth Walls and Reinforced Soil Slopes,” (Berg et al., 2009)Bridges with MSE wall supported abutments shall be categorized as one of two types describedbelow, and shall meet the associated design requirements:1) Single span bridges constructed with a precast slab superstructure supported directly onreinforced soil, as shown in Figure 1.2) Single span bridge constructed with a precast slab superstructure supported directly onreinforced soil two stage wall with full height concrete panels, as shown in Figure 2.3) All other bridges with spread footings supported directly supported by an MSE wall, asshown in Figure 3.These limitations supersede the limitations of AASHTO LRFD Section 11.10.111. Single span bridges constructed with a precast slab superstructure supporteddirectly on reinforced soil.oThe span length shall not exceed 60 feet.oThe superstructure shall include a 5” thick C.I.P. composite topping.

oMSE walls shall be 30 feet or less in total height, which includes the retained soilheight plus embedded depth, measured from bottom of superstructure to top of thereinforced soil foundation.oThe end of the precast superstructure shall be at least 4ft from the face of the MSEwall. Minimum seat width requirements shall be provided on the reinforced soilbearing area.oA foam board detail shall be used to create a 1 ft. horizontal buffer between thebearing area and the MSE wall facing.oThe vertical gap between top of wall facing and bottom of superstructure shall be4” or 2% of the abutment height, whichever is greater.oPrestressing strands in the zone bearing on reinforced soil shall have a minimumconcrete cover of 2”. Transverse reinforcing steel within this zone shall have aminimum concrete cover of 1½”. All prestressing strand shall be removed to a 2”depth from the end of the slab. The voids shall be patched with an approvedepoxy grout.oWhere voided slab superstructures are used, the slab section shall be solid fromthe end of the slab to at least 1 ft. in front of the fascia.oThe abutment shall be designed for a bearing pressure at service loads not toexceed 2.0 TSF and a factored load at strength and extreme limit states not toexceed 3.5 TSF. The bearing pressure may be increased to 3.0 TSF at serviceloads and 4.5 TSF at strength and extreme limit states if a vertical settlementmonitoring program is conducted in accordance with WSDOT GDM section15.5.3.5.oMSE walls supporting bridge abutments shall be special designed wall systems,and shall be one of two types:o2. Geosynthetic and structural earth walls with concrete facing. Geosynthetic walls with a stacked dry-cast modular concrete block facing.The top 3 rows of dry-cast modular concrete blocks shall be grouted with#4 rebar.Bridge approach slabs may be omitted.Bridges with spread footings supported directly on an MSE wall.oMSE walls directly supporting spread footing bridge abutments shall be 30 feet orless in total height, which includes the retained soil height up to the bottom of theembedded spread footing.

oFor SE walls, the front edge of the bridge footing shall be placed 4 ft. minimumfrom the back face of the fascia panel. For geosynthetic retaining walls with awrapped face, the front edge of the bridge footing shall be placed 2 ft. minimumfrom the back face of the fascia panel.oThe abutment footing shall be covered by at least 6 inch of soil for frostprotection.oThe superstructure of continuous span bridges shall be designed for anydifferential settlements between piers.oAbutment spread footings shall be designed for bearing pressure at service loadsnot to exceed 2.0 TSF and factored load at strength and extreme limit states not toexceed 3.5 TSF. The bearing pressure may be increased to 3.0 TSF at serviceloads and 4.5 TSF at strength and extreme limit states if a vertical settlementmonitoring program is conducted in accordance with WSDOT GDM section15.5.3.5.oMSE walls supporting bridge abutments shall be special designed wall systems.Only precast concrete panel faced MSE walls or cast-in-place concrete facedMSE walls may be used for permanent bridge installations. Dry-cast modularconcrete block faced MSE walls and welded wire faced SE walls may be used fortemporary bridge installations.oTo provide bridge inspection access, the bottom of the bridge superstructure totop of slope protection surface shall be 3 feet minimum for I-girder type bridges,and 5 feet minimum for non I-girder, slab and box girder type bridges. Fallprotection shall be installed as required by the WAC.Proprietary SE walls supporting abutments shall not be considered preapproved, and shall not beused beyond the limits described herein unless approved by the State Geotechnical Engineer andthe Bridge Design Engineer.Background:This memorandum is intended to update WSDOT design policies on bridge abutments to includethe use of systems similar to the Geosynthetic Reinforced Soil – Integrated Bridge System (GRSIBS). The GRS-IBS method has been used increasingly around the country with success. TheFHWA has developed a manual for this type of bridge abutment, provided on the followingFHWA hnology/grs ibs/

However, this memorandum and the referenced manuals provided at the beginning of thismemorandum shall be considered to supersede the FHWA GRS-IBS manual with regard todesign and material requirements.Using MSE structures as direct bridge abutments can be a simplification in the design andconstruction of bridge abutments and may lead to faster construction of bridges. Additionally, itcould result in construction cost savings due to elimination of tall abutments. This solutionwould also contribute to better compatibility of deformation between the components of bridgeabutment systems, thus minimizing the effects of differential settlements and the undesirable“bump” at bridge / embankment transitions, reducing the life cycle cost of the bridge.For water crossings with potential for scour, the structural design must be closely coordinatedwith the hydraulic design requirements.If you have any questions regarding these issues, please contact Tony Allen at 360 709-5450([email protected]) or Bijan Khaleghi at 360-705-7181 ([email protected]) .cc: Mark Gaines, Bridge Construction - 47354Craig Boone, Bridge and Structures – 47340

Precast voided or slabsuperstructure (voidis min. 1 ft from facing)4 in. min.vert. clearanceMin. 3 ftbehind concretebearing beamSurfacing8 in. high by 12 in.wide precastconcrete beam fullwidth of slabGeotextile for Underground Drainage,low survivability, Class A per Std. Specs.9-33.2(1) with 1 ft min. horizontaloverlap (only needed of geogrid isused for reinforcement)Bridge approach soil reinforcement(min. length of 12 ft or to back of wallreinforcement, whichever is greater,and vert. spacing of 8 in.)Compressible material (provide min. 4 in. thickness)Bearing bed reinforcement (max. vertical spacing of 8 in.,min. length of 2 ft beyond flat slab) if vertical spacing ofprimary reinforcement is greater than 12 in. for min. of 5ft below structural slabJoint filler#4 rebar, groutedin placeSecondary reinforcement (max. vertical spacing of8 in., min. length of 4 ft behind facing), if primaryreinforcement spacing is greater than 12 in.Dry cast concretemodular blockfacingMax. spacing between primaryreinforcement layers 16 in.Primary reinforcementConcrete leveling padFigure 1. Typical Section – Single span bridge constructed with a precast slab superstructuresupported directly on dry-cast concrete modular block faced reinforced soil wall.

Precast voided or slabsuperstructure (voidis min. 1 ft from facing)4 in. min.vert. clearanceMin. 3 ftbehind concretebearing beamSurfacingBridge approach soil reinforcement(min. length of 12 ft or to back of wallreinforcement, whichever is greater,and vert. spacing of 8 in.)Compressible material (provide min. 4 in. thickness)8 in. high by 12 in.wide precastconcrete beam fullwidth of slabBearing bed reinforcement (max. vertical spacing of 12 in.for 5 ft below structural slab)Facing extends 8 in.above bottom ofcompressible materialGeotextile for Underground Drainage,low survivability, Class A per Std. Specs.9-33.2(1) with 1 ft min. horizontaloverlap (only needed of geogrid isused for reinforcement)Joint fillerPrecast of CIPconcrete facingMax. spacing between primaryreinforcement layers 16 in.Primary reinforcementConcrete leveling padFigure 2. Typical Section – Single span bridge constructed with a precast slab superstructuresupported directly on reinforced soil two stage wall with full height concrete panels.

A. 4 ft min for SE Walls (precast concrete panel face or cast-in-place concrete face) and 2ft min for special designed Geosynthetic retaining walls with wrapped faceB. 3 ft min for I-girder bridges and 5 ft min for non-I-girder, slab, and box girder bridgesC. 30 ft maxFigure 3. Typical Section – Bridge with spread footing supported directly on an MSE wallSemi-integral abutment shown, L-abutment similar. Wing/Curtain Wall not shown