# LOADS and FORCES on BRIDGES

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29 Νοε 2013 (πριν από 4 χρόνια και 5 μήνες)

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Seminar:
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BRIDGES

Seminar:
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Carrying the trains over the gap.

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The nature of forces

How forces work

Where they come from

Where they go to

The concept of stress

How bridges carry loads and forces

Effects of loads and forces on bridges

Some indications to watch for

Subject Outline

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Basic Statics

Tension

Pull

Compression

Push

Shear

Horizontal and Transverse

Flexure

Bend

Torsion

Twist

(Statics is the study of forces that don’t move)

Forces
-

how they pull, push, slice, bend
and twist

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Tension will tend to make
an object longer.

It might also cause cracks

Tension
-

Pull

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Stress
-
Strain Curves for Steel and Douglas fir

Vertical (Y) Axis is Stress (from Force)

Horizontal (X) Axis is Strain (Deformation)

Stress
-
Strain Curves

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Compression
-

Push

Compression tends to make an object shorter.

It can also cause a slender object to buckle.

(This illustration is not a slender object.)

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Shear

Slice

Shear forces are generally greatest at the ends of beams.

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Shear

Transverse and Horizontal

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Shear

Transverse and Horizontal

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Flexure
-

Bend

Where the load is applied downward and the reactions act
upward, the top of the beam is in compression, and the bottom
is in tension.

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Torsion
-

Twist

Actually, torsion, or twisting, is
found in other bridge members,
too. It is not desirable, but it
happens
.

Torsion is often caused by “out
-
of
-
plane bending,” such as when
the ties on an open deck deflect
downward in the center, and the
ties bear on the inside edges of
the top flanges of girders or
stringers. That makes the
girders and stringers twist
sideways

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Deflection

Span deflects in a downward curve

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Deflection

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Forces in Bridges

Where the forces come from:

Mass

Acceleration

Thermal (heat) expansion and contraction.

Wind

Stream flow

Earthquakes

Vehicle and boat strikes

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Forces in Bridges

Where the forces come from:

Train forces.

Forces in a bridge when it’s carrying a train.

Independent forces.

Forces in a bridge that develop whether or not a
train is on the bridge.

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Forces in Bridges

Where
train forces

come from
-

Mass of locomotives and cars

Acceleration

Gravity

Longitudinal (lengthwise) accelerations

Speeding up (traction)

Slowing down (braking)

Lateral (sideways) accelerations

Centrifugal acceleration

curves

Nosing of cars and locomotives

Rolling (Rock and Roll!)

Wind forces on the train

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Forces in Bridges

Where
independent forces

come from
-

Weight of the bridge and its attachments

Wind forces on the bridge

Thermal (heat and cold) expansion

Ice and snow

Stream flow (water, ice and debris)

Soil pressures

Outside impacts (Trucks, Boats, Earthquakes)

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Forces in Bridges

Force paths

Carrying the load to the ground.

Typical force paths by structure type:

Deck beam and girder spans.

Through girder spans.

Trusses.

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Forces in Bridges

Girders
carry forces by bending (flexure).

Top flange in compression.

Bottom flange in tension.

Web in shear, plus compression over bearings.

Girders

are wider apart than the
rails
, so the bridge won’t tip over with

Wind.

Nosing.

Rocking.

Centrifugal (curve).

Deck Girder Spans

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Stiffeners
prevent web from buckling.

Bearing stiffeners are critical in this regard

high compression at bearings.

Top laterals
prevent top flanges from buckling as horizontal columns.

Cross frames
prevent individual girders from toppling.

They carry lateral loads to the bottom lateral bracing system.

Bottom laterals
carry lateral loads into the bearings and substructure.

Forces in Bridges

Deck Girder Spans

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Through girder
spans have
floor systems

Typically floorbeams and stringers.

Through girders act basically the same as deck girders, except:

Loads are applied to the girders through the floor system.

Knee braces:

Connect floorbeams to girders.

Brace top flanges of girders against buckling in
compression.

Forces in Bridges

Through Girder

Spans

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Arch

Points up
toward center

Compression.

Chain

hangs down
toward center

Tension.

Forces in Bridges

Trusses, generally

What’s in tension? What’s in compression?

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Forces in Bridges

Trusses, generally

What’s in tension? What’s in compression?

Arch and Chain analogy:

Arch

Points up toward center
-

Compression.

Chain

Hangs down toward center
-
Tension.

Through Pratt Truss

Red

Compression. End Post, Top Chord, Interior Posts

Blue

Tension. Lower Chord, Interior Diagonals

Purple

Hangers in Tension

Black

Strut, serves as a brace

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Forces in Bridges

Trusses, generally

What’s in tension? What’s in compression?

Arch and Chain analogy:

Arch

Points up toward center
-

Compression.

Chain

hangs down toward center
-
Tension.

Through Warren Truss

Red

Compression. End Post, Top Chord, Interior Diagonals.

Blue

Tension. Lower Chord, Interior Diagonals.

Purple

Hangers in Tension.

Black

Struts, serve as braces.

Note that some interior diagonals see stress reversals between tension
and compression, depending on live load position.

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Forces in Bridges

Trusses, generally

What’s in tension? What’s in compression?

Arch and Chain analogy:

Arch

Points up toward center
-

Compression.

Chain

hangs down toward center
-
Tension.

Deck Pratt Truss

Red

Compression. End Posts, Interior Posts

Blue

Tension. Lower Chord, Interior Diagonals.

Seminar:
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