Bridge Loading
Objective
To identify the principal actions on bridge
structures and to describe how they are
considered in design.
Why Bridge Loading is Important
Bridges, particularly larger structures, are
substantial investments of public funding for
which a high level of safety is required.
Loads may be determined with greater
precision than with many other types of
structure.
Load paths are usually well defined - some
bridge structures are effectively iso-static.
Strength, static or fatigue, is more frequently
the governing design condition.
Definitions of Loads
‘Loads’ includes external forces applied to the structure and imposed deformation such as caused by restraint of movement due to changes in temperature.
Dead Loads are the weights of the parts of the structure
that are structural elements.
Superimposed Dead Loads are the weights of all
materials on the structure that are not structural
elements - road surfacing, ballast, parapets, ducts etc. Live Loads are the vertical loads due to the traffic
Loads & Factors
Nominal loads specified in the code.
Design loads. Nominal loads should be multiplied by the
appropriate value of γfL to derive the design load to be used in the calculation of moments, shears, total loads and other effects for each of the limit states under
consideration.
Additional factor γf3. Moments, shears, total loads and
other effects of the design loads are also to be multiplied by γf3 to obtain the design load effects.
Loads to be considered. The loads to be considered in
different load combinations, together with the specified values γfLare given in the code.
Design Load Effects
Moments, shears etc must be resisted at a
particular limit state
Design Load Effect:
S
*=
f3(effects of design load Q
*)
=
f3(effects of
fL.Q
k)
Partial Safety Factors
f3 takes account of any inaccurate assessment of effects of loading, unforeseen stress distribution in the structure & variation in dimensional accuracy in construction.
f3 ~ 1.1 to 1.2 for imposed load
f3 is always 1.15 for dead load For simplicity,
f3= 1.15 for all loads and all types of analysis, provided the percentage redistribution is not more than 20%.
fL values are given in the code for different types of loads & load combinationsPartial Safety Factors
fL
(Clause 4.4, Table 1)
Load Classification
Classification of loads. The loads applied to a structure
are regarded as either permanent or transient.
Permanent loads include dead loads, superimposed
dead loads, loads due to filling material, differential settlement and loads derived from the nature of
structural material (e.g. creep & shrinkage)
Transient loads include wind loads, temperature loads,
erection loads, primary & secondary highway loadings, footway & cycle track loadings.
Primary loadings are vertical live loads. Secondary loadings are due to changes in speed or direction (e.g.
Load Combinations
Combination 1. Permanent Loads + Appropriate
Primary Live Loads
Combination 2. Combination 1 + Wind Load + Erection
Loads
Combination 3. Combination 1 + Temperature Load +
Erection Loads Combination 4.
For highway bridges : Permanent Loads + Secondary LL with associated Primary LL
For footway/cycle bridge : Permanent Loads +
Secondary LL of a vehicle colliding with a support
Application of Loads
Arrangement of loads on a bridge depends on the load effects and the critical section being considered.
Code requires that when the most severe effect on a
structural element can be diminished by the presence of a load on a certain portion of the structure, then the load is considered to act with its least possible magnitude. (i) In case of DL, γfL = 1.0 is applied to all parts of the DL (ii) In the case of SDL & LL, these loads should not be
applied to those portions where their presence would diminish the load effect.
In the use of influence line, the SDL & LL should be
applied to the adverse parts and not the relieving parts of the influence line.
Highway Definitions
Carriageway Width
- Width includes all traffic
lanes, hard shoulders, hardstrips and marker
strips. It is the width between raised kerbs or the
distance between safety fences minus the
‘set-back
’ for the fences.
Traffic Lanes
- Lanes marked on the running
surface of the bridge. They have a maximum
width of 3.65 metres.
Notional Lanes
- Parts of the carriageway road
Notional lanes (BS5400 Part 2)
Clause 3.2.9.3 : Notional lanes are part of the
carriageway used solely for the purpose of applying the specified live loads.
Notional lanes shall be taken to be not less than 2.3m & not more than 3.8m wide. For carriageway ≥ 4.6m,
Carriageway width m Number of notional lanes
4.6m up to and including 7.6 2
above 7.6 up to and including 11.4 3
above 11.4 up to and including 15.2 4
above 15.2 up to and including 19.0 5
Notional Lanes (Clause 3.2.9.3 BD37/01)
Notional lanes shall be taken to be not less than 2.50mwide. Where the number of notional lanes exceeds two, their individual widths should be not more than 3.65m. The carriageway shall be divided into an integral number
of notional lanes have equal widths as follows:
Carriageway width m Number of notional lanes
5.00 up to and including 7.50 2
above 7.50 up to and including 10.95 3
above 10.95 up to and including 14.60 4
above 14.60 up to and including 18.25 5
Loaded Length & Influence Line
Bridges are very load position sensitive. The effect of the applied loads will vary with their position on the bridge. The UDL is to be applied to a loaded length (see notes)
corresponding to either the positive or negative portion of an influence diagram relevant to the effects being
considered.
For a two-span bridge, the loaded length should be
positioned in the span for worst span moments but should be applied over the central pier for maximum support
reactions. Simply applying a UDL across the whole bridge, with a load intensity appropriate to the whole length, will not necessarily be the worst case.
Traffic Loads (Live Loads)
Traffic loads on bridge decks are used to simulate the effects of vehicles and/or pedestrian loads. Some traffic loads represent the weight of real vehicles that can
travel over the bridges; other values and distributions are chosen in such a way that they produce maximum internal forces in bridge structures similar to the ones produced by real vehicles.
Four types of loads are specified in the many codes:
a) Uniform distributed loads b) Knife-edge load
c) Single wheel loads d) Truck load
UDL Live Load
This load simulates the effects of normal permitted vehicles. In some national codes its value is constant and independent of the loaded area. In other codes the load value decreases with the area occupied by the load. Distributed load is applied on the traffic lanes and over the lengths that give the extreme values of the stress resultant (or internal force) being considered. It may be continuous or discontinuous.
Highway Bridge Live Loads
(BS 5400, BD 37/01)
Loads to be considered. The structure and its elements
shall be designed to resist the more severe effects of either:
a) design HA loading or
b) design HA loading combined with design HB loading All road bridges shall be designed to carry HA loading. In
addition, a minimum of 30 units of type HB loading shall be taken for all road bridges except for accommodation bridges which shall be designed to HA loading only.
Motorways/trunk roads : 45 units HB, Principal roads : 37.5 units HB; Other public roads : 30 units HB (min.)
Type HA Loading (BD37/01)
Nominal uniformly distributed load (UDL). For loaded
lengths up to and including 50m the UDL, expressed in kN per linear metre of notional lane, shall be derived from the equation,
where L is the loaded length (in m) and W is the load per metre of notional lane (in kN).
See Example 67 . 0
1
336
L
W
Type HA Loading (HA UDL)
BD37/01
For loaded lengths in excess of 50m but less than
1600m the UDL shall be derived from the equation,
For loaded lengths above 1600m, the UDL shall be
agreed with the appropriate authority. Values of the load per linear metre of notional lane and the loading curve are given in the code.
1 . 0
1
36
L
W
KEL Live Load
This load is usually associated with the uniform distributed load. It does not represent a single axle load, but is a device to ensure that, together with the uniform distributed load, the vertical shear and the longitudinal moments that may occur in real bridge elements are produced.
Type HA KEL (Knife Edge Load)
The
HA-KEL
is a line load acting across the
width of the notional lanes. It is a movable load
along the span and is placed is such a position
so as to cause the most adverse effect. Intensity
of HA-KEL is
120kN/width (kN/m).
In the design of abutment or pier, the HA-KEL
must be positioned over the abutment. In beam
design, HA-KEL is usually positioned at
Application of HA Load
HA-UDL
HA-KEL
span width
JKR Specification for Live Loads
Read in conjunction with BS5400: Part 2:
1978 with loaded lengths not exceeding
50m. All references to HA & HB loadings
are replaced with LTAL & SV loadings.
Loads to be considered :
a)
Design LTAL loading
b)
Design SV loading
JKR Specification for Live Loads
Notional Lanes : fixed as 2.5m for LTAL
loading.
The width of SV is taken as 3.5m.
Areas of carriageway not covered by
notional lanes are loaded with the
minimum pedestrian loading of 5.0kN/m
2.
Multiple Lanes
Full HA loading should be applied in up to two lanes on the bridge. When there are more than two lanes, the extra lanes should be loaded with 60% (or 1/3 or
specified lane factor) of HA loading.
The choice of which lanes are loaded with full HA and which are loaded with 60% (or 1/3 or lane factor) HA should be made such that the maximum bending
moment or shear force is produced in the part of the structure which is being designed.
Except where otherwise specified, the HA lane factors
for HA UDL & KEL shall be applied and the values are given in Table 14 BD37/01 Part 14.
Single Nominal Wheel Load
Single nominal wheel load alternative to UDL and
KEL. One 100 kN wheel, placed on the carriageway and
uniformly distributed over a circular contact area
assuming an effective pressure of 1.1 N/mm2 (i.e.340mm
diameter), shall be considered.
Alternatively, a square contact area may be assumed, using the same effective pressure (i.e. 300mm side).
Dispersal. Dispersal of the single nominal wheel load at
a spread-to-depth ratio of 1 horizontally to 2 vertically through asphalt and similar surfacing may be assumed, where it is considered that this may take place.
Dispersal through structural concrete slabs may be taken at a spread-to-depth ratio of 1 horizontally to 1 vertically down to the neutral axis.
Single Wheel Load
Some national codes
specify the
application of a single
heavy wheel load
placed anywhere on
the carriageway, with
a
circular
or
rectangular
contact
area.
Truck Load
This load is intended to represent the
extreme effects of a single heavy vehicle.
In some countries it consists of a specified
number of wheel loads and arrangements.
Other codes indicate only the distances
between axles, the spacing of wheels in
each axle, and the minimum number of
axles.
BS5400 Type HB Loading
Nominal HB loading. The plan and axle arrangement
for one unit of nominal HB loading is given in the code. One unit shall be taken as equal to 10 kN per axle (ie 2.5 kN per wheel).
The overall length of the HB vehicle shall be taken as 10, 15, 20, 25 or 30 m for inner axle spacings of 6, 11, 16, 21 or 26 m respectively, and the effects of the most severe of these cases shall be adopted. The overall
width shall be taken as 3.5m. The longitudinal axis of the HB vehicle shall be taken as parallel with the lane
Depends on judgement of designer. ~400mm Maximum moment occurrs here 1.8m 1.0m 1.0m 1.0m 1.8m 1.5m 1.5m 3.0m cL of HB cL of bridge 1.0m 1.0m 1.0m A A Position of HB Load to produce Maximum Moment
Type HA & HB Loading Combined
Type HA UDL determined for the appropriate
loaded length and type HA KEL loads shall be
applied to each notional lane in the appropriate
parts of the influence line for the element or
member under consideration.
Type HB loading shall occupy any transverse
position on the carriageway, either wholly within
one notional lane or straddling two or more
notional lanes.
HB Vehicle within One Lane
BD 37/01
HB Vehicle straddling 2 Notional Lanes
(BD 37/01)
HB Vehicle straddling 2 Notional Lanes
(BD 37/01)
JKR Lane Loadings (LTAL)
see Example
Sidewalks/Footway
Many highway bridges, in urban and non-urban areas, have
sidewalks (footpaths) for pedestrian traffic and/or cycle tracks. On these areas a uniform distributed load is usually considered.
Some codes indicate also that one wheel load applied on the
Nominal Pedestrian Live Load
Elements supporting footways or cycle tracks only.
The nominal pedestrian live load on elements supporting footways and cycle tracks only shall be as follows:
(a) for loaded lengths of 36 m and under, a uniformly distributed live load of 5.0 kN/m2.
(b) for loaded lengths in excess of 36m, k x 5.0 kN/m2
where k is the
nominal HA UDL for appropriate loaded length (in kN/m) x 10 L+270
Nominal Pedestrian Live Load
Where the footway (or footway and cycle track together) has a width exceeding 2m these intensities may be
further reduced by 15% on the first metre in excess of 2m and by 30% on the second metre in excess of 2m. No further reduction for widths exceeding 4m shall be made. These intensities may be averaged and applied as a uniform intensity over the full width of the footway or cycle track.
Special consideration shall be given to the intensity of the pedestrian live load to be adopted on loaded lengths in excess of 36m where exceptional crowds may be
expected. Such loading shall be agreed with the appropriate authority.
Nominal Pedestrian Live Load
Elements supporting footways or cycle tracks and a carriageway. The nominal pedestrian live load on
elements supporting carriageway loading as well as footway or cycle track loading shall be taken as 0.8 of 5.0 kN/m2 or k(5.0) kN/m2 as appropriate, except for
loaded lengths in excess of 400m or where crowd loading is expected.
Reduction for footway exceeding 2m width is similar to the previous case. Other reduction conditions are given in the code.
Parapets
Parapets of footpaths and cycle tracks that are protected from
highway traffic by an effective barrier are designed to resist horizontal distributed force applied at a height of 1m above the footway. The nominal value of this force is about 1.5kN/m.
When footways and cycle tracks are not separated from the highway
traffic by an effective barrier, design loads have to recognise the need to contain traffic in the case of an accident. These loads are considerably higher and include an alternative concentrated load.
Traction & Braking Forces
These forces result from the
traction or braking of vehicles and they are applied to the road surface, parallel to the traffic lanes.
BS use 100kN HA for span up
to 3m, & plus 17kN each metre of span over 3m but not
exceeding 253kN. For HB, 450kN for all spans.
JKR use a predetermined
maximum value of 253 kN for both HA and HB loading.
Loads due to movement of beam caused by
Temperature, Shrinkage and Creep.
These are horizontal forces acting longitudinally on a
bridge generated by movement of beam caused by
effects of temperature, shrinkage and creep.
Temperature and shrinkage coefficients are often
assumed to be universal values. Creep coefficients
are dependent on concrete cube strength and cube
strength at transfer for prestressed concrete beams.
When the actual movement of beam is known and the
plan area of elastomer and its shear modulus found,
the horizontal forces due to temperature, shrinkage
and creep can be determined.
Beam Movement due to S,T,C
Forces
Movement due to shrinkage = (shrinkage coefficient x beam length)
Movement due to temperature = (Temp. Coefficient x beam length x Temp. difference)
Movement due to creep = (creep coefficient x beam length)
JKR assumes ⅔ shrinkage & ½ creep have taken place at time of beam placement.
Total beam movement = (Movement due Temp.) +
⅓(movement due to shrinkage) + ½(movement due to creep)
Horizontal Force due to STC Forces
The horizontal forces due to S,T,C produce beam movement which are taken up by the elastomeric
bearing. The size and properties of the elastomer must be known to calculate the total shear force due to S,T,C. See Example.
Centrifugal Forces
Curved bridges are subject to centrifugal forces applied by the
vehicles that travel on them. These forces are related to the traffic loads by a coefficient, a, whose value depends on the radius of the curve, R, and on the design speed, v .
Some codes consider a uniform distributed radial load and others
Wind Loads
The wind actions on a bridge depend on site conditions and geometrical characteristics of the bridge. The
maximum pressures are due to gusts that cause local
and transient fluctuations about the mean wind pressure. Design gust pressures are derived from the design wind speed defined for a specified return period.
BS153 : Part 3A : 1972 gives the values of wind
pressure and wind speeds for the ‘Loaded Case’ and ‘Unloaded Case’.
Exposed area of traffic on bridges has the length
corresponding to the maximum effects and in general a height of 2.50m above the carriageway in highway
Lateral Wind Effects (Unloaded Case)
(BS153 : Part 3A : 1972) Clause 12.2.2.1
Unloaded case : wind pressure 1.4 kN/m2 taken asacting horizontally & normal to the sides of the bridge on a total exposed area of the superstructure made up of the these areas :
a) Windward girder, deck & bracing : the net exposed
area in normal projection elevation of the windward girder, deck construction, bracing & parapet.
b) Leeward girders : the fraction n/16 of the net exposed
area in normal projected elevation of the leeward girder (when the windward girder is a plate girder).
n is the ratio of distance c-c between windward &
outermost leeward girder, to the depth of the windward girder.
Lateral Wind Effects (Loaded Case)
(BS153 : Part 3A : 1972) Clause 12.2.2.2
Loaded case. A wind pressure of 0.7kN/m2 shall betaken as acting horizontally & normal to the sides of the bridge on the exposed area of the superstructure and live load taken as a single vertical plane surface having a continuous height of 2.50m above the carriageway.
Longitudinal Wind Effects
(BS153 : Part 3A : 1972) Clause 12.3
For Plate Girder Bridges
Unloaded Case : A quarter of the total
lateral wind forces on the superstructure
Loaded Case : A quarter of the total
lateral wind forces on the superstructure
and half of the total lateral wind forces on
the live load.
Settlement of Foundation
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
Earthquake Effects
The behaviour of a structure during an
earthquake depends on its dynamic behaviour,
namely its natural vibration modes and
frequencies, and damping coefficients.
When the bridge has a simple dynamic
behaviour, for instance when the first vibration
frequency is much lower than the other ones,
the seismic action may be reduced to an
Forces due to Water Current
All piers and other portions of the bridge should
be designed to resist the forces induced by
flowing water or debris.
Effect of stream current = KV
2Where, P = pressure (lb/ft
2)
V = velocity of stream flow (ft/s)
K = constant
(K = 4/3 for square end; 2/3 for circular end)
Effect of debris is calculated as above but with
Collision Force
In structures where essential load-carrying elements may be subject to impact by vehicles, ships or aircraft, the
consequences should be considered as accidental load cases - unless the risk of such collisions is evaluated as being so small that it can be neglected.
It is necessary in many cases to allow partial destruction or damage of the element which is directly hit. This element then has to be repaired after the collision. It should, however, be shown that the partial destruction of a single element will not lead to a total collapse of the entire structure.
To reduce the consequences of collisions it may be necessary to limit the movements of movable bearings so that only the
movements due to temperature effects can take place without restraint.
Friction in Bearings
It should be checked whether the unavoidable friction
in bearings can induce forces or moments that have to
be considered in the design of the structural elements.
In a continuous beam with a fixed bearing at the
centre and longitudinally movable bearings on either
side, expansion (or contraction) of the beam induces
symmetrical frictional forces.
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.
Erection Loads
Erection loads are especially important for the design of composite and long-span bridges.
In long-span bridges the internal forces existing when
the construction is completed are frequently adjusted by movements of supports or, in the case of cable stay
bridges and suspension bridges, by adjustment of the cable forces.
In composite bridges the formwork for the deck is
usually supported by the steelwork alone, and is not removed until after the deck becomes composite. The stresses induced in the composite deck by the removal of the formwork may be small enough to neglect, but in principle, they are a form of permanent prestressing, which can be considered in load combinations.
Critical Variable Action Effects
positive longitudinal moments within the span
negative longitudinal moments at internal supports
greatest longitudinal moments at changes of girder cross-section
maximum shears at supports
maximum shears at changes of web resistance maximum reactions
critical combinations of moment and shear (usually at supports)
maximum torsions (usually most critical for box sections) maximum moments, shears and torsions on cross girders,
Analysis for Load Moments, Shears
and Torsions
Most global analysis is carried out by grillage
analysis.
Influence lines are still used; sometimes just to
identify critical locations for heavy vehicles and
knife-edge loading and sometimes for the
determination of numerical values. They may be
developed by the use of coefficients for
transverse distribution or they may be
determined by grillage analysis
.
Analysis for Load Moments, Shears
and Torsions
Most countries have one or two heavy vehicles,
usually with defined axle and wheel layouts. (e.g. JKR’s SV Load). They govern global effects for
medium and short span bridges. They are applied at specific positions on the structure.
These positions may be determined by general
inspection, or by examination of influence lines.
Some modern computer programmes have automatic
load stepping facilities, both along and across the bridge with search routines to determine relevant maxima and minima.
Analysis for Load Moments, Shears
and Torsions
Knife Edge Loads are applied at specific locations,
usually at midspan or close to supports.
Distributed loads are applied over the full lengths of
positive, or negative, influence lines. For example,
both neighbouring spans are loaded to determine
governing support moments, only one span is loaded
to determine governing mid-span moment
Software routines for automatic summation are
becoming more popular to determine governing
values of action effects.
