Bridge Loading - Unknown

azlanfka/utm06/mab1053 1

Bridge Loading BS5400 Part 2 & BD37/01 Part 14


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 (vehicles, locomotives, rolling stock and pedestrians).


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 .Qk)

= f3 . fL .Qk


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 combinations


Partial 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. centrifugal, braking, skidding & collision loads)


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

Combination 5. Permanent Loads + Friction at bearings


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 for deriving the intensity of the live loads.


Carriageway Dimension


Carriageway


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




Notional Lane Determination (Sabah).ppt



Notional Lanes (Clause 3.2.9.3 BD37/01)

Notional lanes shall be taken to be not less than 2.50m wide. 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






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.

Loaded Length & Influence Line (Sabah).ppt


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

67.01

336

LW



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.

1.01

36

LW


Intensity of HA Load (BD37/01)


HA Load Intensity


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 mid-

span.


Application of HA Load

HA-UDL

HA-KEL

span width

= Total HA Loading


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

c) Design LTAL combined with design SV


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/m2.

Load combinations similar to BS5400.


JKR UDL Loadings (LTAL)


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.


HA Lane Factors (BD37/01)


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.


Truck Load (Denmark)


JKR Malaysia SV Load


British HB Load


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 markings.


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

cL of bridge

A

A

Section 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.

See Example.



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


JKR Lane Loadings (Controlled SV)


JKR Lane Loadings (Uncontrolled SV)


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 sidewalks should be considered.


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

where L is the loaded length (in m).



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.



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.

See Example.



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.

Loading & PSC Beam.xls


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 divide it into concentrated loads.


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 bridges and 3.70m above rail level in railway bridges.


Lateral Wind Effects (Unloaded Case) (BS153 : Part 3A : 1972) Clause 12.2.2.1

Unloaded case : wind pressure 1.4 kN/m2 taken as acting 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 be taken 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.

See example

Loading & PSC Beam.xls


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 should indicate when adjustments have to be made.


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

equivalent static force.


Dynamic Behaviour of Bridge


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 = KV2

Where, P = pressure (lb/ft2)

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 K=1.


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, cross bracing and slabs


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.



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.



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.

Local slab and deck analysis is carried out separately.

Documents

Bridge Loading - Unknown