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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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DIFFERENTIAL SETTLEMENT
An example of a differential settlement.
In this continuous beam, settlement occur at support B.
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DIFFERENTIAL SETTLEMENT
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.