t6 bridges tubbs jb
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Light Rail Transit Facilities
Design Course
Bridges & Structures05.11.2010
Joel Tubbs, P.E., S.E.
2
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Light Rail Transit Facilities
Design Course
Bridges & Structures 3
OVERVIEW
1. Structure Layout and Type Selection2. LRT Loading Requirements3. Special Considerations4. Constructability Considerations
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Light Rail Transit Facilities
Design Course
Bridges & Structures 4
Structure Layout & Type Selection
Basic types of structures- Bridges
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Light Rail Transit Facilities
Design Course
Bridges & Structures 5
Structure Layout & Type Selection
Basic types of structures- Buildings
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Light Rail Transit Facilities
Design Course
Bridges & Structures 6
Structure Layout & Type Selection
Basic types of structures• Tunnels
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Light Rail Transit Facilities
Design Course
Bridges & Structures 7
Structure Layout & Type Selection
Basic types of structures• Stations
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Light Rail Transit Facilities
Design Course
Bridges & Structures 8
Retaining Walls Sound Walls
Structure Layout & Type Selection
Basic types of structures- Walls
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Light Rail Transit Facilities
Design Course
Bridges & Structures 9
Structure Layout & Type Selection
What does a structure do?- Provides infrastructure for system- Separates facility from other features
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Light Rail Transit Facilities
Design Course
Bridges & Structures 10
Structure Layout & Type Selection
What does a structure do?- Solves safety concerns
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Light Rail Transit Facilities
Design Course
Bridges & Structures 11
Structure Layout & Type Selection
Crossing Types- Bridge over road
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Light Rail Transit Facilities
Design Course
Bridges & Structures 12
Structure Layout & Type Selection
Crossing Types- Bridge over water
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Light Rail Transit Facilities
Design Course
Bridges & Structures 13
Structure Layout & Type Selection
Crossing Types- Tunnel under road
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Light Rail Transit Facilities
Design Course
Bridges & Structures 14
Structure Layout & Type Selection
Crossing Types- Tunnel under geographic feature
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Light Rail Transit Facilities
Design Course
Bridges & Structures 15
Structure Layout & Type Selection
Modern Bridge Types- Pre-stressed Concrete
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Light Rail Transit Facilities
Design Course
Bridges & Structures 16
Structure Layout & Type SelectionModern Bridge Types
- Pre-stressed Concrete
Advantages- Lowest cost bridge alternative- Good for shorter crossings- No falsework required in roadway or
stream- Fast, simple installation, saving
construction time- Shallow depth providing greater
clearance to stream or roadway surfaces below
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Light Rail Transit Facilities
Design Course
Bridges & Structures 17
Structure Layout & Type Selection
Modern Bridge Types- Cast-in-Place Concrete
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Light Rail Transit Facilities
Design Course
Bridges & Structures 18
Structure Layout & Type Selection
Modern Bridge Types- Cast-in-Place concrete
Advantages - Good for longer spans - Resistance to seismic forces - Accommodating horizontal
curves, gradelines, or superelevations
- More aesthetically pleasing
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Light Rail Transit Facilities
Design Course
Bridges & Structures 19
Structure Layout & Type Selection
Modern Bridge Types- Concrete
~ Segmental
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Light Rail Transit Facilities
Design Course
Bridges & Structures 20
Structure Layout & Type Selection
Modern Bridge Types- Segmental Concrete
Advantages- Good for longest spans- Highly aesthetic- Limited surface-level disturbance- Geometric flexibility
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Light Rail Transit Facilities
Design Course
Bridges & Structures 21
Structure Layout & Type Selection
Modern Bridge Types- Steel
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Light Rail Transit Facilities
Design Course
Bridges & Structures 22
Structure Layout & Type Selection
Modern Bridge Types- Steel
Advantages- Longer spans- Can accommodate track
geometry- Lighter foundation & seismic
loads
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Light Rail Transit Facilities
Design Course
Bridges & Structures 23
Structure Layout & Type Selection
Modern Bridge Types- Signature Bridges
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Light Rail Transit Facilities
Design Course
Bridges & Structures 24
Structure Layout & Type Selection
Modern Bridge TypesTypical span ranges (order by superstructure
cost)- Precast concrete slabs up to 80
feet- Precast concrete box beams up to 120
feet- Precast concrete girder up to 180
feet- CIP post-tensioned box girder 100-600 feet- Steel plate girder 60-300 feet- Steel box girder 60-500
feet- Segmental concrete
~ Span-by-span 80-150 feet
~ Balanced Cantilever up to 800 feet
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Light Rail Transit Facilities
Design Course
Bridges & Structures 25
Structure Layout & Type Selection
Modern Bridge TypesTypical Span-to-Depth ratios (structure thickness
only)- Precast concrete slabs/boxes
Span/33- Precast concrete girder
Span/23- CIP post-tensioned box girder
~ simple span Span/26~ continuous, uniform depth
Span/29~ continuous, variable depth
Span/35 - Steel plate girder
~ simple span Span/25~ continuous
Span/31
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Light Rail Transit Facilities
Design Course
Bridges & Structures 26
Structure Layout & Type Selection
Modern Bridge TypesStructure Depth
Don’t Forget!!
Overall Structure Depth = Structure thickness + Superelevation +
Track section depth
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Light Rail Transit Facilities
Design Course
Bridges & Structures 27
Cut WallsFill Walls
Structure Layout & Type Selection
Retaining Walls
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Light Rail Transit Facilities
Design Course
Bridges & Structures 28
Structure Layout & Type Selection
Retaining WallsCommon Types
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Light Rail Transit Facilities
Design Course
Bridges & Structures 29
Structure Layout & Type Selection
Retaining WallsCommon Types
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Light Rail Transit Facilities
Design Course
Bridges & Structures 30
Structure Layout & Type Selection
Retaining WallsCommon Considerations
- Excavation for reinforcement/footings- Easements for subterranean elements- Increased design height on slopes- Proper consideration at wall terminations- Drainage conveyance
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Light Rail Transit Facilities
Design Course
Bridges & Structures 31
Structure Layout & Type Selection
Common bridge layout considerations:- Site conditions- Bent locations and required span lengths- Cost- Material availability- Aesthetics- Vertical clearance- Horizontal alignment- Schedule- Seismic resistance- Maintenance, future widening, and more...
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Light Rail Transit Facilities
Design Course
Bridges & Structures 32
Structure Layout & Type Selection
Bent location considerations:- Proximity to facilities- Right of way- Span length- Constructability- Required clearances- Environmental concerns
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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LRT Loading Requirements
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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LRT Loading Requirements
Load effects of DL- Not much variance
in stresses over time
Load effects of LL - Transient loads
produce variable stresses
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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Vehicle Bending Moments on Simple Spans
HL93
LRT
Cooper E80
0
1000020000
30000
40000
5000060000
70000
80000
0 50 100 150 200 250
Span Length (ft)
Mid
span
Mom
ent (
k-ft)
LRT Loading Requirements
General Design Criteria:- Agencies allow both AASHTO & AREMA- Most light rail loads are greater than the HL93 used
by AASHTO LRFD, but much less than AREMA’s Cooper E80
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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LRT Loading Requirements
General Design Criteria:AREMA
- Restrictive for light rail transit structures due to the great differences in loading
- Wheel spacings don’t correspond to those found on LRV’s
- Impact criterion is not consistent with the suspension and drive systems used
on LRV’s- Types of loading not consistent with LRV’s
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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LRT Loading Requirements
General Design Criteria:AASHTO
- Ratio of LL to DL more closely approximates that of highway loadings than heavy rail loadings
- Axle loads and car weights are similar to LRV’s
- Results in conservative design that is not overly restrictive or uneconomical
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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LRT Loading Requirements
Loads and Load Combinations (TriMet 2010):• Dead load• Live load
- LRV-specific- Highway Pedestrian- Seismic loads- Earth loads- Wind loads- Thermal Loads
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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Loads and Load Combinations (TriMet 2010):Dead loads (DC)
- Superstructure weight- Superimposed loads- Cross Beam weight- Column weight- Footing weight- OCS poles- Ductbanks- Plinths/Ballast- Rail
LRT Loading Requirements
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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Loads and Load Combinations(TriMet 2010):• Live Loads (LL)
- Highway (AASHTO)- Pedestrian (AASHTO)- LRV-Specific
~ 1 to 4 car train~ Single or multiple tracks loaded
LRT Loading Requirements
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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Loads and Load Combinations (TriMet 2010):Other LRV-specific live loads:
- Vertical impact (Iv or IMv)~ Max between AASHTO and AREMA but
generally not exceeding 30%
- Horizontal impact (Ih or IMh)~ 10% of each axle load applied transversely
at 4 ft above TOR- Impact applies generally only to structural
elements above ground for trains that are not stationary
LRT Loading Requirements
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Light Rail Transit Facilities
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Bridges & Structures:LRT Loading Requirements
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Loads and Load Combinations (TriMet 2010): Other LRV-specific live loads:
- Longitudinal forces (BR):~ Acceleration = 16% of LRV load~ Deceleration = 21% of LRV load~ Combine as necessary to obtain max force
effect(e.g., one track accelerating while other
track decelerating)
LRT Loading Requirements
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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Loads and Load Combinations (TriMet 2010): Other LRV-specific live loads:
- Centrifugal forces (CE):~ 10% of axle load for track CL radius <=
2450 ft~ Axle load*0.0875*(V^2)/R for larger
radius~ Applied transversely at 4 ft above TOR
LRT Loading Requirements
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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Loads and Load Combinations (TriMet 2010): Special LRV-specific live loads:
- Emergency Braking (EB)~ 46% of LRV on one track~ Combine with BR loads on other tracks as necessary to obtain max force effect~ Considered only for Strength II limit state,
and is not combined with derailment loads
LRT Loading Requirements
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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Loads and Load Combinations (TriMet 2010): Special LRV-specific live loads:
- Derailment Loads (DR)~ Vertical – 100% impact applied for any
truck~ Horizontal – 10-30% of single LRV
vehicle applied at 2 ft above TOR over 10 ft
distance~ Only one track assumed to derail, other
tracks unloaded or loaded with stationary train
~ Considered only for Strength II limit state, and is not combined with EB loads
LRT Loading Requirements
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Light Rail Transit Facilities
Design Course
Bridges & Structures:LRT Loading Requirements
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Loads and Load Combinations(TriMet 2010):
Other Special LRT Loads:- Thermal forces
~ Radial rail forces~ Rail break
LRT Loading Requirements
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Special Considerations
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Light Rail Transit Facilities
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Bridges & Structures:Special Considerations
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Ballasted Track versus Direct Fixation (DF)
Special Considerations
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Light Rail Transit Facilities
Design Course
Bridges & Structures:Special Considerations
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Ballasted track versus direct fixation on structures
Ballasted track- Greater DL requires larger structural
members- Flexible track structure support- Most prevalent track type used at grade- Must contend with electrical isolation & acoustic attenuation- Results in deeper bridge structure
Special Considerations
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Light Rail Transit Facilities
Design Course
Bridges & Structures:Special Considerations
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Ballasted track versus direct fixation on structures
Direct Fixation- High initial cost- Rail interacts with structure- Standard method of construction on aerial
structure- Much stiffer vertically than ballasted track- Lower maintenance costs
Special Considerations
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Light Rail Transit Facilities
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Bridges & Structures:Special Considerations
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Continuously Welded RailRail Break Gap
- Occurs when a thermally induced tensile force
exceeds the ultimate tensile strength of the rail.
- Likely to occur at or near~ Bridge expansion joints~ At a bad weld~ A rail flaw~ Weak spot in rail
Special Considerations
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Light Rail Transit Facilities
Design Course
Bridges & Structures:Special Considerations
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Continuously Welded RailRail Break Gap
- Established limits on gap size~ Usually based on LRV’s wheel diameter~ Decreasing the fastener’s longitudinal
stiffness results in increased gap size
Special Considerations
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Light Rail Transit Facilities
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Continuously Welded Rail- Rail-Structure Interaction
~ Thermal deformations of bridge induce stress on rails
~ Restraint of CWR and DF fasteners induce stresses on rails and structure
~ Rail break forces transferred through DF fasteners to structure and to remaining unbroken rails according to relative stiffnesses
Special Considerations
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Light Rail Transit Facilities
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Continuously Welded RailRail-Structure Interaction
DF Fasteners:~ Proprietary devices that allow differential
movement between structure and rail
~ Full lateral restraint~ Provide varied levels of longitudinal
restraint
Special Considerations
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Light Rail Transit Facilities
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Bridges & Structures:Special Considerations
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Continuously Welded RailRail-Structure Interaction
DF Fasteners:- Lower restraint fasteners often used at
locations of highest structure thermal deformation (i.e., near expansion joints)
- Higher restraint fasteners used near middle of frame
Special Considerations
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Light Rail Transit Facilities
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Mixed modes on bridge
Special Considerations
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Light Rail Transit Facilities
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Stray current protection- Stray currents are leaking current from the rails that
return to the ground grid of the substation- Corrosion is the most common result of stray
currents
Special Considerations
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Light Rail Transit Facilities
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Bridges & Structures:Special Considerations
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Stray current protectionTo minimize stray currents:
- Insulate rails from their fastenings and encase rails in embedded track with extruded boot
- Continuously weld reinforcement in underlying slab
- In ballasted track areas the ballast should be clean, well-drained and not in contact with the rail
- Conduct corrosion surveys and perform regular monitoring and maintenance
Special Considerations
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Light Rail Transit Facilities
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Pedestrian considerations:- Restriction to trespassing- Emergency access/egress
Special Considerations
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Light Rail Transit Facilities
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Bridges & Structures:Structure Layout & Type Selection
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Design Course
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Constructability Considerations
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Light Rail Transit Facilities
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Basic bridge construction issues
Constructability Considerations
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Light Rail Transit Facilities
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Basic bridge construction issues- Maintenance of traffic- Adequate easements for construction equipment
and laydown areas- Detailing with construction tolerances in mind- Staged construction- Concrete pour sequences- Work site access
Constructability Considerations
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Light Rail Transit Facilities
Design Course
Bridges & Structures:Structure Layout & Type Selection
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Deck/Plinth Construction- Method of plinth construction can have significant impact on cost and constructability
Constructability Considerations
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Bridges & Structures:Structure Layout & Type Selection
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CWR welding and setting track
Constructability Considerations
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Bridges & Structures:Structure Layout & Type Selection
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Downdrag on foundations & long term settlement:
- Downdrag~ Occurs as layers of soil consolidate~ Causes: Additional fill, liquefaction,
secondary compression~ Can introduce substantial vertical load on
piles~ Can create settlements in shallow foundation
systems- Mitigation
~ Coat piles to create slip-plane~ Design for additional loads~ Surcharge prior to construction to pre-
consolidate soils
Constructability Considerations
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Bridges & Structures:Structure Layout & Type Selection
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Shoring existing facilities
Temporary works
Constructability Considerations
Temporary work bridge
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Temporary worksFalsework
Constructability Considerations
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Foundation construction in water (cofferdam)
Constructability Considerations
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Bridges & Structures:Structure Layout & Type Selection
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Cofferdam subject to high water
pressures
Foundation construction in water (cofferdam)
Constructability Considerations
Pile driving through template in flooded
cofferdam
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Bridges & Structures:Structure Layout & Type Selection
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Subgrade stabilization below concrete seal
Subgrade excavation of footing in dry
cofferdam
Foundation construction in water (cofferdam)
Constructability Considerations
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Bridges & Structures:Structure Layout & Type Selection
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Foundation construction in water (drilled shaft)- Drilled shaft with temporary casing negates need
for cost prohibitive cofferdam and reduces environmental impacts
Constructability Considerations
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Bridges & Structures:Structure Layout & Type Selection
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Temporary Work Bridge Covered With Plastic to Keep Dredged Materials from Entering Slough
Dredged Materials Removed Safely From Site
Foundation construction in water (drilled shaft)
Constructability Considerations
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Girder shipping and setting- Crane placement- Girder delivery- Shipping/handling weights
Constructability Considerations
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QUESTIONS?