T6 bridges tubbs jb

Light Rail Transit Facilities

Design Course

Bridges & Structures05.11.2010

Joel Tubbs, P.E., S.E.

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Bridges & Structures 3

OVERVIEW

1. Structure Layout and Type Selection2. LRT Loading Requirements3. Special Considerations4. Constructability Considerations


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Structure Layout & Type Selection

Basic types of structures- Bridges


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Basic types of structures- Buildings


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Basic types of structures• Tunnels


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Basic types of structures• Stations


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Retaining Walls Sound Walls


Basic types of structures- Walls


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What does a structure do?- Provides infrastructure for system- Separates facility from other features


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What does a structure do?- Solves safety concerns


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Crossing Types- Bridge over road


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Crossing Types- Bridge over water


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Crossing Types- Tunnel under road


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Crossing Types- Tunnel under geographic feature


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Modern Bridge Types- Pre-stressed Concrete


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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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Modern Bridge Types- Cast-in-Place Concrete


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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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Modern Bridge Types- Concrete

~ Segmental


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Modern Bridge Types- Segmental Concrete

Advantages- Good for longest spans- Highly aesthetic- Limited surface-level disturbance- Geometric flexibility


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Modern Bridge Types- Steel


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Modern Bridge Types- Steel

Advantages- Longer spans- Can accommodate track

geometry- Lighter foundation & seismic

loads


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Modern Bridge Types- Signature Bridges


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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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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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Modern Bridge TypesStructure Depth

Don’t Forget!!

Overall Structure Depth = Structure thickness + Superelevation +

Track section depth


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Cut WallsFill Walls


Retaining Walls


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Retaining WallsCommon Types


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Retaining WallsCommon Types


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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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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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Bent location considerations:- Proximity to facilities- Right of way- Span length- Constructability- Required clearances- Environmental concerns


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Bridges & Structures:LRT Loading Requirements

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LRT Loading Requirements


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Load effects of DL- Not much variance

in stresses over time

Load effects of LL - Transient loads

produce variable stresses


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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)


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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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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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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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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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



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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



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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



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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)



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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



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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



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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



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Loads and Load Combinations(TriMet 2010):

Other Special LRT Loads:- Thermal forces

~ Radial rail forces~ Rail break



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Bridges & Structures:Special Considerations

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Special Considerations


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Ballasted Track versus Direct Fixation (DF)



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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



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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



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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



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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



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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



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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



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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



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Mixed modes on bridge



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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



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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



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Pedestrian considerations:- Restriction to trespassing- Emergency access/egress



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Bridges & Structures:Structure Layout & Type Selection

60Light Rail Transit Facilities

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Bridges & Structures:Constructability Considerations

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Constructability Considerations


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Basic bridge construction issues



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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



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Deck/Plinth Construction- Method of plinth construction can have significant impact on cost and constructability



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CWR welding and setting track



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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



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Shoring existing facilities

Temporary works


Temporary work bridge


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Temporary worksFalsework



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Foundation construction in water (cofferdam)



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Cofferdam subject to high water

pressures



Pile driving through template in flooded

cofferdam


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Subgrade stabilization below concrete seal

Subgrade excavation of footing in dry

cofferdam




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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



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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)



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Girder shipping and setting- Crane placement- Girder delivery- Shipping/handling weights



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QUESTIONS?

Technology

T6 bridges tubbs jb