Ch2 Design Loads on Bridges (Steel Bridges تصميم الكباري المعدنية & Prof. Dr. Metwally Abu-Hamd)

Chapter 2: Design Loads on Bridges
CHAPTER 2
DESIGN LOADS ON BRIDGES

Steel Bridges
CHAPTER 2
DESIGN LOADS ON BRIDGES
2.1 INTRODUCTION
Bridge structures must be designed to resist various kinds of loads: vertical
as well as lateral. Generally, the major components of loads acting on bridges
are dead and live loads, environmental loads (temperature, wind, and
earthquake), and other loads, such as those arising from braking of vehicles
and collision. Vertical loads are caused by the deadweight of the bridge itself
and the live load, whereas the lateral loads are caused by environmental
phenomena such as wind and earthquakes.
Bridge structures serve a unique purpose of carrying traffic over a given
span. Therefore, they are subjected to loads that are not stationary; i.e., moving
loads. Also, as a consequence, they are subjected to loads caused by the
dynamics of moving loads; such as longitudinal force and impact and
centrifugal forces.
Various kinds of bridge loads are shown in Fig. 2.1 and are described in the
following sections.
2.2 ROADWAY DESIGN LOADINGS
a) Dead Load
Dead load on bridges consists of the self-weight of the superstructure plus
the weight of other items carried by the bridge such as utility pipes which may
be carried on the sides or underneath the deck. The self-weight of the
superstructure consists of the deck, including the wearing surface, sidewalks,
curbs, parapets, railings, stringers, cross girders, and main girders. Depending
on the bridge type, the self-weight of the superstucture may be significant, as
in the case of long span bridges, or it may be a small fraction of the total
weight, as in the case of short span bridges. In any case, the dead load can be
easily calculated from the known or the assumed sizes of the superstructure
components.

Fig. 2.1 Design Loads on Bridges
In the case of bridge decks consisting of reinforced concrete slabs, it is a
common practice to apply the wearing surface and pour curbs, parapets, and
sidewalks after the slab has hardened. The weight of these additional
components is usually referred to as the superimposed dead load.
An important consideration in dead-load computation is to include, in
addition to the a.m. components, weights of anticipated future wearing surface
and extra utilities the bridge has to carry.
b) Live Loads
Live loads on bridges are caused by the traffic crossing the bridge. Design
live loads are usually specified by relevant design codes in the form of
equivalent traffic loads. Some traffic loads represent the weight of real vehicles
that can travel over the bridge; other values and distributions are chosen in
such a way that they produce maximum internal forces in bridge structures
similar to those produced by real vehicles.
According to the Egyptian Code for design loads on roadway bridges, the
roadway is divided into traffic lanes of 3 m width; the most critical lane for the
design of a structural member is called the main lane. Two types of loads are
specified in the Code for design:
LOADS ON BRIDGES
LONGITUDINALTRANSVERSALVERTICAL
Wind
Earthquake
Lateral Shock
Centifugal
Wind
Earthquake
Braking
Thermal
Friction
Dead Loads
Live Loads
Impact

Steel Bridges
i) Truck loads:
This load is intended to represent the extreme effects of heavy vehicles. It
consists of a 60-ton truck in the main lane and a 30-ton truck in a secondary
lane, which is taken next to the main lane. The arrangement of wheel loads is
shown in Fig. 2.2a. The locations of the main and secondary lanes are chosen
so as to produce maximum effect on the member considered.
For main girders with spans longer than 30 meters, an equivalent uniform
load of 3.33 t/mP
2
P and 1.67 t/mP
2
P may be used instead of the 60-ton and 30-ton
trucks for the design of Umain girdersU only.
ii) Uniform distributed load:
This load simulates the effects of normal permitted vehicles. It is applied on
the traffic lanes and over the lengths that give the extreme values of the stress
(or internal force) being considered. It may be continuous or discontinuous. It
consists of a 500 kg/m2 uniform load in the main lane in front and back of the
main truck and 300 kg/m2 in the remaining bridge floor areas, as shown in Fig.
2.2 b.
The interaction of moving loads and the bridge superstructure results in
dynamic amplification of the moving loads, resulting in vibrations and
increased stresses. This amplification was found to depend mainly on the
natural frequency of the structure which is a function of its length.
Consequently, the dynamic effect of moving loads is considered in the design
by increasing the static values of the main lane loading by the impact factor I
computed as:
I = 0.40 – 0.008 L > 0 (2.1)
where L = loaded length of main traffic lane giving maximum effect and is
evaluated as follows:
a) For directly loaded structural members, L is taken equal to the span length
of loaded span or the cantilever length of loaded cantilevers.
b) For indirectly loaded structural members, L is taken equal to the span
length of the directly loaded member transmitting the load or the span
length of the indirectly loaded member, whichever is greater.
c) For two-way slabs, L is taken equal to the short span length.

For the assessment of the bridge fatigue strength, the prescribed live load
and impact values on roadway bridges shall be reduced by 50 %.
1.40
1.50
6.00
60 t Truck
(a) Wheel Arrangement
Main
300 kg/m2
Sec.
Lane
500 kg/m2
Lane
1.50
0.60
1.501.50
0.50
1.501.50 1.501.50
0.50
30 T TRUCK = 6 x 5 T
60 T TRUCK = 6 x 10 T
(b) Loading Plan
30 t Truck
3.00
2.00
3.003.00
300 kg/m2
500 kg/m2
300 kg/m2
300 kg/m2
6.00
0.20 0.200.20
0.60
Fig. 2.2 Live Loads on Roadway Bridges

Steel Bridges
c) Longitudinal Tractive Forces
The term longitudinal forces refer to forces that act in the direction of the
longitudinal axis of the bridge; i.e., in the direction of traffic. These forces
develop as a result of the braking effort ( sudden stopping) , or the tractive
effort (sudden acceleration). In both cases, the vehicle’s inertia force is
transferred to the bridge deck through friction between the deck and the
wheels.
These forces are applied to the road surface parallel to the traffic lanes as
shown in Figure 2.3. According to the Egyptian Code, they are taken equal to
25 % of the main lane loading without impact, with a maximum value of 90
tons.
Main Lane
Fig. 2.3 Braking Forces on Roadway Bridges
d) Centrifugal Forces
When a body moves along a curved path with a constant speed, the body is
subjected to a horizontal transversal force due to centrifugal acceleration and
acts perpendicular to the tangent to the path. Curved bridges are therefore
subjected to centrifugal forces applied by the vehicles that travel on them.
According to the Egyptian Code, these forces are taken as two concentrated
forces applied horizontally spaced at 50 m at the roadway surface level at the
bridge centerline as shown in Fig. 2.4. The value of each force is computed
from the equation:

C = 3000 / (R + 150) (2.2)
Where C = centrifugal force, ton
R = radius of curvature, m
A vertical load of 30 tons distributed on a roadway area of 6 m long and 3
m wide is assumed to act with each force.
50m
C
C
Fig. 2.4 Centrifugal Forces on Curved Roadway Bridges
e) Sidewalks
Many highway bridges, in urban and non-urban areas, have sidewalks
(footpaths) for pedestrian traffic. On these areas a uniform distributed load of
300 kg/mP
2
P shall be considered in addition to the main bridge loads.
Alternatively, a uniform load of 500 kg/mP
2
P acting alone shall be considered.
Sidewalks not protected from vehicles cross over (parapet height less than 35
cm) shall be designed for a single wheel load of 5 tons acting on a distribution
area 30*40 cm.
Handrails for sidewalks that are protected from highway traffic by an
effective barrier are designed to resist a horizontal distributed force of 150
kg/m applied at a height of 1m above the footway. When sidewalks are not
separated from the highway traffic by an effective barrier (parapet height less
than 35 cm), The elements of the sidewalk shall also be checked for the effect
of a vertical or horizontal concentrated load of 4 tons acting alone in the
position producing maximum effect. The working stresses for this case are
increased by 25 %.

Steel Bridges
2.3 RAILWAY DESIGN LOADINGS
a) Dead Load
Superimposed dead loads on railway bridges usually include the rails, the
sleepers, the ballast (or any other mean for transmission of train loads to the
structural elements), and the drainage system.
b) Train Loads
Train loads for railway bridges correspond to Train-type D of the Egyptian
Railways as shown in Figure 2.5. Two 100 ton locomotives with 80 ton tenders
are to be assumed, followed on one side only by an unlimited number of 80 ton
loaded wagons. Different live load positions shall be tried to arrive at the
specific position giving maximum effect. If two tracks are loaded at the same
time, only 90 % of the specified loads for one track are used for both tracks. In
case of three tracks, only 80 % of the specified loads are used. In case of four
tracks or more, 75 % of the specified loads are used.
Train loads specified in the code are equivalent static loading and should be
multiplied by appropriate dynamic factors to allow for impact, oscillation and
other dynamic effects including those caused by track and wheel irregularities.
Values of dynamic factors depend on the type of deck (with ballast or open-
deck) and on the vertical stiffness of the member being analyzed. For open-
deck bridges values of dynamic factors are higher than for those with ballasted
decks. Consideration of the vertical stiffness is made by adopting formulae in
which the dynamic factor is a function of the length, L, of the influence line for
deflection of the element under consideration. According to the Egyptian Code
of Practice, impact effects of railway loads are taken into consideration by
increasing the static values by the impact factor I computed as:
I = 24 / (24+ L) (2.3)
Where L (in meters) = Loaded length of one track, or the sum of loaded
lengths of double tracks. For stringers L is taken equal to the stringer span.
For cross girders L is taken equal to the sum of loaded tracks. For the main
girders L is taken equal to the loaded length of one track for single track
bridges or the sum of loaded lengthes of two tracks only in multiple track
bridges.
The value of I in this formula has a minimum value of 25 % and a maximum
value of 75 %. For ballasted floors with a minimum ballast thickness of 20 cm,
the value of I computed from the given formula shall be reduced by 20 %. For
bridges having multiple number of tracks, the dynamic effect shall be
considered for the two critical tracks only.

80TWAGON80TTENDER100TLOCOMOTIVE80TTENDER100TLOCOMOTIVE
12.008.4010.508.4010.50
3.003.00
Fig. 2.5 Live Loads on Railway Bridges (Train Type D)

Steel Bridges
c) Longitudinal Braking and Tractive Forces
These forces, which equals 1/7 of the maximum live loads (without impact)
supported by one track only, are considered as acting at rail level in a direction
parallel to the tracks, Figure 2.6. For double track bridges, the braking or
tractive force on the second track is taken as one half the above value. For
bridges with more than two tracks, these forces are considered for two tracks
only.
B/2
B/2
Fig. 2.6 Braking Forces on Railway Bridges
d) Centrifugal Forces
When the railway track is curved, the bridge elements shall be designed for a
centrifugal force “C” per track acting radially at a height of 2 m above rail
level. Its value is obtained as:
C = ( V2 / 127 R ) W (2.4)
Where C = centrifugal force in tons
V = maximum speed expected on the curve in Km/hr
R = radius of curvature in meters
W = maximum axle load in tons.

e) Lateral Forces From Train Wheels
To account for the lateral effect of the train wheels, the bridge elements are
designed for a single load of 6 ton (without impact) acting horizontally in
either direction at right angles to the track at the top of the rail, Figure 2.7. This
force should be applied at a point in the span to produce the maximum effect in
the element under consideration.
For elements supporting more than one track, only one lateral load is
considered. For bridges on curves, design shall be based on the greater effect
due to the centrifugal forces or the lateral shock.
6t
Fig. 2.7 Lateral Shock Forces on Railway Bridges

Steel Bridges
2.4 OTHER LOADS ON BRIDGES
a) Wind Loads
The wind actions on a bridge depend on the site conditions and the
geometrical characteristics of the bridge. The maximum pressures are due to
gusts that cause local and transient fluctuations about the mean wind pressure.
Because steel bridges have a low span-to-weight ratios, wind effects on
bridges is very important and, if not properly considered, can lead to failure,
see Fig 2.8.
Fig. 2.8 Failure of a Suspension Bridge due to Wind loads

Design wind pressures are derived from the design wind speed defined for a
specified return period. The wind load shall be assumed to act horizontally at
the following values:
1) When the bridge is not loaded by traffic: the wind pressure, on the
exposed area of the bridge, is equal to 200 kg/mP
2
2) When the bridge is loaded by traffic: the wind pressure, on the exposed
area of the bridge and the moving traffic, is equal to 100 kg/mP
2
P.
Exposed area of traffic on bridges has the length corresponding to the
maximum effects and in general a height of 3.00 m above the roadway level in
highway bridges and 3.50 m above rail level in railway bridges, Figure 2.9.
The exposed area of the bridge before the top deck slab is executed is taken
equal to the area of two longitudinal girders. Wind pressure during
construction can be reduced to 70 % of the specified values.
3.50
3.00
LOADEDUNLOADED
200kg/m2
200kg/m2100kg/m2
100kg/m2
100kg/m2
Fig. 2.9 Design Wind loads on Bridges

Steel Bridges
b) Thermal Effects on Bridge Structures
Daily and seasonal fluctuations in air temperature cause two types of
thermal actions on bridge structures:
a) Changes in the overall temperature of the bridge (uniform thermal actions),
b) Differences in temperature (differential thermal actions) through the depth
of the superstructure.
The coefficient of thermal expansion for steel may be taken as 1.2 x 10-5° C.
According to the Egyptian Code; bridge elements shall be designed for:
a) a + 30° C uniform change of temperature, Fig. 2.10 a, and
b) a + 15° C difference in temperature through the superstructure depth,
Fig. 2.10b.
The mean temperature of the bridge shall be assumed at 20° C.
Fig. 2.10 Thermal Loads on Bridges
If the free expansion or contraction of the bridge due to changes in
temperature is restrained, then stresses are set up inside the structure.
Furthermore, differences in temperature through the depth of the superstructure
cause internal stresses if the structure is not free to deform. A differential
temperature pattern in the depth of the structure represented by a single
continous line from the top to the bottom surface does not cause stresses in
statically determinate bridges, e.g. simply supported beams, but will cause
stresses in statically indeterminate structures due to reatraints at supports. If
differential temperature is not represented by a single continous line from the
top to the bottom surface, then thermal stresses are caused even in simple
spans.

c) Shrinkage of Concrete
In principle, shrinkage gives a stress independent of the strain in the
concrete. It is therefore equivalent to the effect of a differential temperature
between concrete and steel. The effect of shrinkage can thus be estimated as
equivalent to a uniform decrease of temperature of 20° C.
In composite girders the effect of concrete shrinkage is considered by using
a modified value of the modular ratio that is equal to three times of the normal
value. Generally, shrinkage effects are only taken into account when the effect
is additive to the other action effects.
d) Settlement of Foundations
The settlements of foundations determined by geotechnical calculations
should be taken into account during design of the superstructure. For
continuous beams the decisive settlements are differential vertical settlements
and rotations about an axis parallel to the bridge axis. For earth anchored
bridges (arch bridges, frame bridges and suspension bridges) horizontal
settlements have to be considered.
Where larger settlements are to be expected it may be necessary to design
the bearings so that adjustments can be made, e.g. by lifting the bridge
superstructure on jacks and inserting shims. In such a case the calculations
should indicate when adjustments have to be made.
e) Friction of Bearings
It should be checked whether the unavoidable friction of bearings can induce
forces or moments that have to be considered in the design of the structural
elements.
According to the Egyptian Code, the force due to friction on the expansion
bearings under dead load only shall be taken into account and the following
coefficients of friction shall be used:
a. Roller Bearings: One or two rollers 0.03
Three or more rollers 0.05
b. Sliding Bearings: Steel on Cast iron or steel 0.25
In a continuous beam with a hinged bearing at the center and longitudinally
movable bearings on both sides, expansion (or contraction) of the beam

Steel Bridges
induces symmetrical frictional forces. These forces are in horizontal
equilibrium if a constant coefficient of friction is assumed, and they normally
result in moderate axial forces in the main girders. However, to take into
account the uncertainty in the magnitude of frictional forces it may be
reasonable to assume full friction in the bearings on one side of the fixed
bearing and half friction on the other side.

Ch2 Design Loads on Bridges (Steel Bridges تصميم الكباري المعدنية & Prof. Dr. Metwally Abu-Hamd)

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