LOADS and FORCES on BRIDGES

cypriotcamelUrban and Civil

Nov 29, 2013 (3 years and 11 months ago)

77 views

Seminar:
1

of 27

COPYRIGHT
© AREMA 2008

LOADS and FORCES on
BRIDGES

Seminar:
2

of 27

COPYRIGHT
© AREMA 2008

Loads and Forces on Bridges

Carrying the trains over the gap.

Seminar:
3

of 27

COPYRIGHT
© AREMA 2008


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


Loads and Forces on Bridges

Subject Outline


Seminar:
4

of 27

COPYRIGHT
© AREMA 2008

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

Seminar:
5

of 27

COPYRIGHT
© AREMA 2008

Tension will tend to make
an object longer.


It might also cause cracks
to open up and spread.


Tension
-

Pull

Seminar:
6

of 27

COPYRIGHT
© AREMA 2008

Stress
-
Strain Curves for Steel and Douglas fir

Vertical (Y) Axis is Stress (from Force)

Horizontal (X) Axis is Strain (Deformation)

Stress
-
Strain Curves

Seminar:
7

of 27

COPYRIGHT
© AREMA 2008

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.)

Seminar:
8

of 27

COPYRIGHT
© AREMA 2008

Shear


Slice

Shear forces are generally greatest at the ends of beams.

Seminar:
9

of 27

COPYRIGHT
© AREMA 2008

Shear


Transverse and Horizontal

Seminar:
10

of 27

COPYRIGHT
© AREMA 2008

Shear


Transverse and Horizontal

Seminar:
11

of 27

COPYRIGHT
© AREMA 2008

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.

Seminar:
12

of 27

COPYRIGHT
© AREMA 2008

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

Seminar:
13

of 27

COPYRIGHT
© AREMA 2008

Deflection


Span deflects in a downward curve


Free ends rotate under load

Seminar:
14

of 27

COPYRIGHT
© AREMA 2008

Deflection

Seminar:
15

of 27

COPYRIGHT
© AREMA 2008

Forces in Bridges


Where the forces come from:


Mass


Acceleration


Thermal (heat) expansion and contraction.


Wind


Stream flow


Earthquakes


Vehicle and boat strikes

Seminar:
16

of 27

COPYRIGHT
© AREMA 2008

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.

Seminar:
17

of 27

COPYRIGHT
© AREMA 2008

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


Seminar:
18

of 27

COPYRIGHT
© AREMA 2008

Forces in Bridges


Where
independent forces

come from
-


Dead load


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)

Seminar:
19

of 27

COPYRIGHT
© AREMA 2008

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.

Seminar:
20

of 27

COPYRIGHT
© AREMA 2008

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
lateral loading caused by:


Wind.


Nosing.


Rocking.


Centrifugal (curve).

Deck Girder Spans

Seminar:
21

of 27

COPYRIGHT
© AREMA 2008

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

Seminar:
22

of 27

COPYRIGHT
© AREMA 2008


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

Seminar:
23

of 27

COPYRIGHT
© AREMA 2008

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?

Seminar:
24

of 27

COPYRIGHT
© AREMA 2008

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

Seminar:
25

of 27

COPYRIGHT
© AREMA 2008

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.

Seminar:
26

of 27

COPYRIGHT
© AREMA 2008

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:
27

of 27

COPYRIGHT
© AREMA 2008

Loads and Forces on Bridges

Questions?