Abutments Raad M.dhyiab

7/25/2019 Abutments Raad M.dhyiab

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Bridge Abutments

Raad M.Dhyiab

M.Sc., Civil Engineering

Bridge Designer


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ABUTMENTS.

The Structure upon which the ends of a Bridgerests is referred to as an

Abutment.

The most common type of Abutment Structure is a Retaining Wall,

Although other types of Abutments are also possible and are used.

A retaining wall is used to hold back an earthembankment or water and

to maintain asudden change in elevation.

Abutment serves following functions.

- Distributes the loads from Bridge Ends tothe ground.

-

Withstands any loads that are directlyimposed on it.

- Provides vehicular and pedestrian accessto the bridge.

In case of Retaining wall type Abutment bearing capacity and sliding

resistance, of the foundation materials and overturning stability must

be checked.

TYPES OF ABUTMENTS.

Sixteenth edition of the AASHTO (1996) standard

specification classifies abutments into four types:

- Stub abutments,

- partial-depth abutments,

- full-depth abutments; and

- Integral abutments.

Stub abutment


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Partial-Depth Abutment.

Partial Depth abutments are locatedapproximately at mid-depth

of the front slope of the approach embankment. The higher

back wall and wing walls may retain fill material, or the

embankment slope maycontinue behind the back wall. In the

lattercase, a structural approach slab or end span design must

bridge the space over the fill slope and curtain walls are

provided to close off theFull-Depth.

Full-Depth Abutment.

Full depth abutment are located at the approximate front toe

of the approach embankment ,restricting the opening under

the structure.

Integral Abutment.


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Overview

Abutment description/selection

Integral

Semi-integral

Parapet

Abutment design

Wing wall design

Barrier location

End posts

Abutmentselection

Factors contributing to abutment selection

Bridge length

Bridge skew

Horizontal curves

Wing wall length

Presence of retaining wall which ties into wing wall

Front face abutment exposure

Beam depth/superstructure type

Desired joint location.


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Selection/description: Integral

Selection: Integral

Advantages

More cost effective

Simplified design

Joint less bridge

Disadvantages

Geometric and load restrictions

Must be placed on piling


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Integral abutment restrictions

Length restrictions

Bridges under 300 ft long can have up to a 20 degree skew

Bridges under 100 ft long can have up to a 45 degreeskew

Bridges between 100 and 300 ft can have skew up to:

[45 degrees -0.125*(L-100)]

Integral abutment restrictions

Requires a straight horizontal alignment (Slight

curvature can be allowed on a case-by-case basis)

Length of wing wall cantilevers are 14 ft.

Wing walls do not tie into roadway retaining walls

Minimum front face exposure should be set at 2-0

Depth of beams must be 72 inches


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Selection/description: Semi-integral


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Selection: Semi-integral

Advantages

Can be placed on piling or spread footings

Some (not all) restrictions from integral abutments can

be neglected

No wing wall length limit

No front face exposure height limit

No superstructure depth limit

Joint less Bridge

Disadvantages

More complicated design in comparison to integralabutments

Must still meet all bridge length, skew, and horizontalalignment

criteria from integral abutments.


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Selection/description: Parapet


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Selection: Parapet

Advantages

Works for wide variety of applications

No more length or curvature restrictions

Disadvantages

Expansion joints are on the bridge over the bearings

Creates higher maintenance costs.

Design: Integral

Piles are designed for axial load only

Follow the Integral Abutment Reinforcement

Design Guide found in Chapter 11 of the

MnDOT LRFD Bridge Design Manual

Additional requirements for using the Integral

Abutment Reinforcement Design Guide

Beam spacing 13-0

Pile spacing 11-0

Max abutment stem height 7-0

Deck thickness plus stool height 15.5


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Design: Integral


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Design: Integral

Can also perform specific design for abutments

that do not meet Integral Abutment

Reinforcement Design Guide

Use passive soil pressure that develops when bridge

expands for special design

Back face dowels

Diaphragmhorizontals


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Design: Semi-integral

Skews greater than 30degrees require a guidelug to reduce unwanted

lateral movement

Minimum stemthickness of 4-0

Provide a 3 minimumhorizontal gap betweenthe diaphragm lug and

the stem.


Use pedestals and sloped bridge seat

Requires a detailed bearing design in contrastto elastomeric pad

for integral abutments

Typically a curved plate bearing assembly is used


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Construction Case 1A Construction case 1B

Stem has been Abutment stem and

constructed and backfilled superstructure have

been

but superstructure is notin place constructed and

backfilled


Designed bars

Diaphragm horizontal

Back face vertical stem

Footing

Standard bars

Front face stem

Diaphragm lug stirrupand horizontal.


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Design: Parapet

Low parapet abutment.

Total height (including footing) 15 feet. Use a contraction joint every 32 feet.

Typical abutment has standard reinforcement barsfound in the

MnDOT manual.

Design: Parapet

High parapet abutment

Total height (including footing) > 15 feet

Use a construction joint (w/keyways) every 32 feet

Reinforcement bars designed by engineer

When abutments are higher than 40 feet MSE wallsmay be considered.


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Wing walls

Integral Semi-integral/Parapet

Wing wall design: Integral.

Refer to section 11.1.4 of the MnDOT LRFD

Bridge Design Manual for wing wall design

Wing wall thickness should be 1-6

Back face horizontal reinforcement should be

# 16 @ 12 in. for wing walls 8-0

Consider possible restrictions

Wing walls between the lengths of 8-0 and

14 ft.will need a special design

The back face horizontal reinforcement should bedesigned to resist

passive soil pressure.


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Wing wall design: Layout options.

One footing.

Preferred option for laying out wing wall geometry.

Maximum cantilever beyond footing is 12-0.


Separate footing.

Separate footings may be required for wing wallsover 20-0.

Not recommended for spread footings.

Must have a 1V:1.5H slope or shallower between footings.

Limit cantilever beyond footing to 6-0


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Stepped footing.

Follow maximum step heights set forth by retainingwall standards.

Not recommended for piled foundations.

Can delay the contractor significantly.

Wing wall design: Semi-integral/Parapet.

Assume back face vertical dowels andreinforcement take the entire

moment causedby horizontal loads.

Provide a concrete fillet at wing wall/stemconnection.

Cantilevers under 8-0 can use a standardreinforcement design.

Provide wing wall pile loads in the plan if theyare less than 80% of main

abutment pile loadsAbutments.

Wing wall design: Semi-integral/Parapet

Rebar design consideration areas due to plateaction.

Stem/wing wallhorizontalreinforcement.

Footing/wing wallverticalreinforcement.

Center of thewing wall.

Cantileveredsection.


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Wing wall design: Semi-integral/Parapet.

Many resources available for determiningmoments and shears for plate

action.

United States Department of the Interior

Bureau of Reclamation.

Portland Cement Association.

Barrier location

The barrier should typically be located on theapproach panel

One exception is when wing walls tie intoretaining walls

Then coordination is necessary during thepreliminary design

process with roadway design to determine the barriers correct

location.

Barrier should extend 7-0 onto the approachpanel (previously 5-0) for

TL-4 barriers.


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Barrier location

Typical location When wing wall ties

into retaining wall

End posts.

MnDOT is no longerallowing the use of freestanding end posts because.

Typically end posts areconnected to theabutment:-

-3 ft.minimum lengthrequired.

Width and reinforcementshould be matched toadjoining rail.

Reinforcement runningthrough abutment-endpost interface.

Abutments Raad M.dhyiab

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