settlement in bridges

8/3/2019 Settlement in Bridges

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EFFECT

OF

SETTLEMENTIN

BRIDGE

DESIGN

By:

Noman Ahmed

CE-10


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SETTLEMENT

The downward movement of a footing, or structure due todeformation or change in the soil properties is known as

settlement.

Settlement can be initiated by a number of factors which include,

Overloading the earth at the site

Lowering the water table Vibrations from live loads or seismic loads

Loading embakments

Changes in soil properties (Shrinkage & Swelling)


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DIFFERENTIAL SETTLEMENT

Distribution of bending moment & shear in the deck.

Cracking of substructure elements.

Instability in superstructure joint & support point.

In extreme cases , entire components such as abutments can be titled over.

Relative movement of different parts of a structure caused byuneven sinking of the structure.

Such settlements can reults,

When settlement occurs in a consistent or uniform manner the effects

are often less severe.


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An example of a differential settlement.

In this continuous beam, settlement occur at support B.


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Due to settlement at B fixed end moments are,

So, moments due to settlement are inversely proportion to sq root of span length.


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Line Model

Free Body Diagrom

of 2 span beammodel

BMD of beam modelw/o settlement

BMD of beam model

with unit settlement

@ B


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Line Model

From above results of moment diagram differential settlement has the

effect of generating sagging moment at the support which settle.

This is important as support in continuous beams are generally subjected

to hogging moment and are often not designed to resist significant sag.


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Effect of Span

Free Body

Diagrom of 2span beam model

BMD of beam model

with settlement @

B

Free Body Diagrom

of 2 span beam

model with larger

span than previous

one.

BMD of beam model

with unit settlement

@ B


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Effect of Depth

Free Body

Diagrom of 2span beam model

BMD of beam modelwith settlement @B

BMD of beam model

with unit settlement@ B with increase

in depth


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Effect of Foundation Settlement

1 Structural.

The differential settlement of substructures causes the development of force

effects in continuous superstructures. These force effects are directly proportional to

structural depth and inversely proportional to span length.

2 Joint Movements.

A change in bridge geometry due to settlement causes movement indeck joints that should be considered in their detailing, especially for deep

superstructures.

3 Profile Distortion.

Excessive differential settlement may cause a distortion of the

roadway profile that may be undesirable for vehicles traveling at high speed.

4 Appearance.

Viewing excessive differential settlement may create a feeling of lack of

safety.

settlement in bridges

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