LOADS ON BRIDGES

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Chapter 3

• LOADS ON BRIDGES

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contents

• 3.1 Loads on bridges
• 3.2 Dead loads on highway and railway bridges
  Fig(3-1)
• 3.3 Live loads on bridges
• 3.4 Impact loads
• 3.5 Centrifugal force Fig(3-4)
• 3.6 Temperature effect ((5-5),(6-5)) Fig(3-4)
• 3.7 Wind pressure ((5-9),(6-9)) Fig(3-5) , Fig
  (3-6)

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• 3.8 Braking force Fig (3-7)
• 3.9 Lateral shock effect (6-7) Fig (3-8)
• 3.10 Frictional resistance of bearings
  ((5-10),(6-10))
• 3.11 Settlement of supports ((5-11),(6-11)) Fig
  (3-9)
• 3.12 Forces due to erection ((5-14),(6-14))

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3.1 Loads on bridges
Loads acting on bridges are divided into- 1.
Primary loads. 2. Secondary loads. A load is
considered primary or secondary according to the
part of the bridge which shall be designed. Wind
loads are secondary loads in designing the main
girders and primary loads in designing the wind
bracings.
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3.1.1 Primary loads on highway and railway
bridges 1-Dead loads. 2-Live loads.
3-Impact loads (dynamic effect).
4-Centrifugal forces. 3.1.2 Secondary loads on
highway and railway bridges 1-Wind pressure
or earthquake. 2-Braking force. 3-Lateral
shock effect. 4-Temperature effect.
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5-Frictional resistance at movable bearing.
6-Forces due to settlement of supports.
7-Effect of shrinkage and creep of concrete.
8-Forces due to erection. 3.2 Dead loads on
highway and railway bridges Fig(3-1) It consists
of the weight of steel structure and the bridge
floor. The weight of the floor is found from the
dimensions and the unit weight of the different
materials. Weight of an open timber floor for a
single track railway bridge (250350)600 kg/m
(9.3.4(p149)).
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The weight of the steel structure is first
approximately determined from similar existing
bridges or from empirical formula. Approximate
weight of the steel structure for single track
Railway Bridge with open timber floor (standard
grade steel) - For through bridge W 0.75
0.50 L t \ m - For deck bridge W 0.50
0.50 L t \ m.
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For deck bridge without stringers and cross
girders W 0.25 0.50 L t \ m Where, W
weight of total steel in (t) for one meter of
bridge, L effective span (leff) of bridge in
meters, (for continuous bridge, leff (0.70
0.80) L the distance between two sequence
points with zero total moment. We have to
increase the above steel weight by 90 for
double track and 80 more for triple track For
a ballasted floor the weight of steel is 20 40
greater.
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For roadway bridge with heavy traffic the
weight of steel structure is approximately for 1
m2 of roadway and side walks as follows-
Outside side walks W1 200 4 L 0.03 L2 kg/
m2 for roadway W2 100 3 L kg/ m2 for
side walks W W1 W2 kg/ m2 of bridge
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Inside side walks W 200 4 L 0.03
L2 for roadway W2 100 3 L for side
walks W W1 W2 kg/ m2 of bridge L
effective span of bridge in meter
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Fig(3-1)
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3.3Live loads on bridges
Railway Bridge (p6-1) Fig(3-2) The type of trains
is different for different countries according to
the importance of lines. In Egypt we shall
consider one type of three train types (D, H, and
L), train type D is the heaviest train is used
in Egypt. Train type D consists of a two
locomotives and two tenders followed on one side
only by an unlimited number of wagons
Locomotive Tender Locomotive Tender
unlimited number of wagons
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Where total weight of one Locomotive 100 ton,
and its length 10.50 m(p6-3). While total
weight of one Tender 80 ton, and its length
8.40 m. If two tracks are loaded at the same
time, only 90 of loads are used. In case of
three tracks only 80 of loads are used while in
case of four tracks we use 75 of loads are
used(p6-2).
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Fig(3-2)
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Roadway Bridges Fig(3-3) Within the kerb to kerb
width of the roadway, the standard vehicles are
assumed to travel parallel to the length of
bridge, and to occupy any positions, which
produce the maximum stress. For the standard
vehicle, all the axles of a unit of vehicle are
considered as acting simultaneously in a position
causing maximum stresses. The vehicle in adjacent
lanes is taken as headed in the direction
producing maximum stresses. The maximum bending
moment and maximum shear force on the plate
girders are found by longitudinal location of
loading. For main roadway bridge, the L.L shall
be that type of vehicular rolling load and/ or
distributed load representing it.
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a.The main girder. As well as the floor
system shall be designed for truck concentrated
axle loads the standard type shown together with
distributed load of 500 kg/m2 cover the main lane
of three meter width lane(p5-1), the second lane
of three meter width each shall be covered with
one truck moving in the same direction and
parallel to the axes of the bridge. The remaining
parts of the floor are covered with a uniform
load of 300 kg/ m2 (p5-3). Also, side walks shall
be covered by the same distributed load. The
impact will be considered for the loads on the
main lane only. b.The elements of side walks.
Fig(3-4) It shall be designed for 500 kg/ m2, and
then we check
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also for a vertical concentrated load of (5 t,
acting without uniform loads) in the position
giving maximum stresses(5-2-2). The handrail,
shall be designed for line distributed load not
less than 150 kg/m' at top level of the
handrail(5-2-6).
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Fig(3-3)
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Foot Bridges It shall be designed for uniform
load of 500 kg/ m2, without impact(5-2-4). 3.4
Impact loads Impact is the dynamic effect on the
bridge due to the moving loads. If we measure the
deflection at a certain point of the bridge for
slowly moving train (static L.L), and for rapidly
moving train (static L.L Impact), the increase
of deflection in the later case is due to
impact.
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The chief factors causing Impact are-
1.Roughness and unevenness of the track of a
railway bridge or of the roadway surface of a
roadway bridge. The smoother of the surface the
smaller will be the impact. In Railway Bridge the
joints of rails increase the impact. It is
recommended to use long rails on bridges or to
weld the joints. 2.Irregular and eccentric
wheels are defective springs.         The
proportion
is called Impact coefficient This
coefficient depends on the loaded length and on
the type of
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  This coefficient depends on the loaded
length and on the type of the bridge.   In
case of bigger loaded length we have smaller
Impact coefficient. Rigid parts are more
affected by impact than elastic parts.    
For main truss member, the impact decreases as
the loaded length increases, since the time
necessary to cover a greater length is more and
the load is applied less suddenly.     
Impact formula for Railway Bridges (6-3) is-
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where, 0.25 ? I ? 0.75 Where I is the
factor by which the live load is to be multiplied
to give the addition due to dynamic
effect         L loaded length in meters of
track or the sum of loaded lengths of double or
multiple tracks producing maximum stresses in
members. EXAMPLE Span of bridge 50 m. Distances
between X.G. 5.0 m
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1. Single Track Bridge For stringer
use I 0.75 For
X.G. use I
0.704 For M.G.
use I 0.325 2. Double Track Bridge For stringer
use I
0.75 For X.G.
use I 0.546
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For M.G.
use I 0.25 Impact formula for Roadway
Bridges (5-2-3) is- ,
L loaded length         Where I is
the impact coefficient (for the main lane only)
due to vertical concentrated loads (60 t) and
uniform distributed live load (500 kg/
m2).         L loaded length in meters of
traffic main lane producing maximum stresses in
members.
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3.5Centrifugal force Fig(3-4) For bridges in
curves, the stresses due to the centrifugal
action and the super elevation of the track must
be considered in designing the members. A
vertical load w moving in a curve of radius R and
a speed V. For Railway bridges,
(6-4) W axle load in tons
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(R 400 600 m) (V 70 90 km/ hr) C is a
horizontal force acting at the center of gravity
of masses 2.00 above the top of rail (6.4.1) . It
produces an increase of the vertical reaction on
the outside rail and a decrease of the vertical
reaction on the inside rail. For Roadway bridges,
(5-4) R
radius of curve in m C centrifugal force in
tons every 50 m
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3.6 Temperature effect ((5-5),(6-5))
Fig(3-4) When steel structure is not free to
expand or contrast under variation of
temperature, the stresses due a variation of ?
30? C. From local main must be considered. The
coefficient of expansion for steel and concrete
is 0.00001. If we consider unequal variation of
temperature, in some structures which are not
affected by equal changes, we allowed only for ?
15? C. In two hinged arches and suspension
bridges the equal change of temperature has an
effect on the internal forces. In continuous
bridges the equal change of temperature has no
effect because the girders are free to expand,
but the unequal change has an effect. The
horizontal displacement at point b in the main
system
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?10 F1???t?L 1??10 ? S1???t?L ? get ?10
?10 X1??11 0
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        Temperature has large effect.        
The modulus of elasticity E 2100 t/cm2 for
steel. E 1000 t/cm2 for cast iron. E 210
t/cm2 for concrete
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Fig(3-4)
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3.7 Wind pressure ((5-9),(6-9)) Fig(3-5) , Fig
(3-6) For bridges we consider either the case of
unloaded bridge with a wind pressure of 200 kg/m2
or the case of loaded bridge with a wind pressure
100 kg/m2 on exposed surfaces of bridge and
train. The effective height of a train in railway
bridges is 3.50 m from the rail level, and that
for crowds or road vehicles is 3.00 m. The train
is considered as having on single vertical plane
only.
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Fig(3-5)
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Fig(3-6)
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3.8 Braking force Fig (3-7) In railway bridges
(6-6) we have to consider the stresses resulting
from the application of brakes to the live load
while passing on the bridge. The braking force is
equal to 1/7 of the maximum Live Load, without
impact, supported by one track only. In case of
several tracks, the braking force on the second
track is equal to 1/14 maximum L.L (of the second
track). The braking force has a great effect on
the design of the towers and also on the
abutments and piers supporting the fixed bearing
of bridges (hinged bearing). In roadway bridges
(5-6) the braking forces (? 90 t) 0.25 Loads on
main lane (L) ( (L - 6)? 3?0.50
60?0.25)
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3.9 Lateral shock effect (6-7) Fig (3-8) In
railway bridges a single force 6t (without
impact) is taken normal to the track at top rail
level and in position giving maximum stresses.
The stresses due to the lateral shock of
locomotive wheel are considered in the design
of- 1-Stringer. 2-Stringer bracing.
3-Wind bracing. 4-End X-frame. 5-The
bearings. 6-Rail connections.
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7-The piers, the foundation. (If there is My
(due to lateral shock) use B.F.I.B, for stringer,
to support My.) For railway bridges on a curve,
only the greater of the centrifugal force or the
lateral shock must be considered.
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Figure (3-7)
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Fig(3-8)
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3.10Frictional resistance of bearings
((5-10),(6-10)) Forces due to friction at the
expansion bearing under dead load only must be
considered and the coefficient is- F
??RD.L. For roller bearing with one or two
rollers ? 0.03 For roller bearing with three or
more rollers ? 0.05 For sliding of steel on
hard copper ? 0.15 For sliding of steel on cast
iron or cast steel ? 0.25
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3.11 Settlement of supports ((5-11),(6-11)) Fig
(3-9) Stresses due to unequal settlement of
continuous structures supported on piers or
abutments shall be added for all members. (Fig.
) (Maximum allowable settlement is 2.50
cm.) Settlement may be lead to the continuous
structure to be simple structure hence the
internal forces will be increased.
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Figure (3-9)
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3.12 Forces due to erection ((5-14),(6-14))
1-Erection by cantilever method. Additional
stressed will be exists due to erection by
cantilever method, so it must be considered
during the design of bridge, also the allowable
stresses are increased by percentage of 25 ,
(0.58Fy ? 1.25 0.73 Fy) (2.5P8). If the
erection of the bridge is done by the cantilever
method, the biggest possible forces in the
members during the erection must be considered in
the design of these members. A higher working
stress may be used (or 0.73 ?y) than for the
complete bridge.
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2-Erection by floating method. It is used in
cases of simple beam bridges. Where a loaded ship
carries the structure up to the site of erection,
then the loads are removed slowly till the
structure has in its required erection level.
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