Bridge Articulation and Bearing Specification

7/25/2019 Bridge Articulation and Bearing Specification

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SteelConstruction.infoThe free encyclopedia for UK steel construction information

Bridge articulation and bearing specification

Articulation is the term for the configuration of bridge supports and choice of structural bearingsthat provides the

necessary restraints to the superstructure whilst at the same time providing freedom to some displacements androtations in order to avoid unnecessary forces on both the superstructure and substructure due to constraint.

A range of different bearing types is available. Selection of the appropriate type and specificationof the forces they

must resist and displacements they must permit is a key responsibility for the structural designer.

Clear communication of the requirements for the bearingsand for their installationis essential for proper functioning of

the structure and for avoidance of unanticipated maintenanceissues.

Typical elastomeric pot bearing under a steel girder

Articulation

Bearingsare used to transfer forces from the superstructure to the substructure whilst either tolerating or constraining

relative movement. The principal actions that give rise to displacements and rotations at supports are:

Temperature change (uniform and temperature difference)

Shrinkage of concrete deck slab

Permanent actions (dead loads and superimposed dead loads)

Variable actions mainly traffic loads

Vertical loads

Horizontal loads

http://www.steelconstruction.info/Bridge_articulation_and_bearing_specification - 1st December 2015

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Settlement of supports

Accidental actions - vehicular collision

Movements can be either permanent (irreversible) or transient (reversible).

Generally, the structure will rotate about longitudinal and transverse axes at its supports and these rotations musteither be accommodated in the bearingsor the bearingsmust be designed to resist them (in which case the effects on

the structure must also be considered). In some cases, there is also a rotation about a vertical axis (associated with

plan bending of the structure) but this is usually small in magnitude.

Horizontal displacements at supports arise both from an overall change in length of the structure and due to bending in

a vertical plane (since the centroidal axes are above the levels of the supports). Since it is necessary to resist

horizontal forces at at least one position, it is usual to do so by preventing horizontal displacement at that position; this

means that horizontal displacements at other positions are due to the expansion/contraction of the length from the

fixed bearingand to the (vectorial) sum of the movements due to bending rotation.

The following recommendations relate to the articulation arrangements for typical highway bridges.

As bearingsand expansion jointsintroduce a maintenance liability, it is good practice to limit the number of bearings

required and to minimise the movement to be accommodated by an expansion joint. Spans should be arranged so as

to avoid uplift at bearingpositions (it is a very complex and costly matter to provide restraint against uplift in a bearing),

particularly when dealing with skewedstructures.

The designer should avoid locking in forces that would hinder bearingreplacement. Restraint against longitudinal

forces should be provided at one support, with guided restraints aligned to allow movement at the other supports.

Similarly, restraint against transverse forces should be provided at only one bearingat each support. The construction

sequence of the structure should also be considered, to establish the permanent displacements.

The articulation scheme that the designer chooses should be shown on the drawings and will form the basis of a

bearingschedule.

The convention for illustrating the movements and constraints in bearingsis given in Table 1 of BS EN 1337-1[1]. A

selection of common symbols is given below.


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Symbols for common bearing types (from Table 1 of BS EN 1337-1[1])

Symbol in

plan

Type of bearing Relative movements

X

directio

n

Y

directio

n

Z

directio

n

About

X axis

About

Y axis

About

Z axis

Elastomeric

bearing

deformin

g

deformin

g

small deformin

g

deformin

g

deformin

g

Elastomeric

bearing with

restraints for one

axis

deformin

g

none small deformin

g

deformin

g

deformin

g

Pot bearing none none very

small

deformin

g

deformin

g

deformin

g

Pot bearing with

unidirectional

sliding

sliding none very

small

deformin

g

deformin

g

deformin

g

Pot bearing with

multidirectional

sliding

sliding sliding very

small

deformin

g

deformin

g

sliding

Note: Some symbols are due to be modified in a future amendment of BS EN 1337-1[1]. The symbols for elastomeric

pot bearings will be solid black, rather than an open square and circle.

Single span bridges

Floating articulation

If a bridge deck is relatively small and the associated horizontal forces are not too big, the deck can effectively float

on bearingsthat will each accommodate rotational and translational displacements and will each provide part of the

resistance to horizontal forces. The bearingsfor this articulation arrangement will be elastomeric bearings. All

horizontal forces and movements are then accommodated by shear deformation of the bearings.

Floating bearing layout (single span on elastomeric bearings)


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Articulation from a fixed point

Most bridges will require some form of mechanical restraint to resist the horizontal forces and ensure that thermal

expansion and contraction occurs in the right direction. This is most easily achieved using pot bearings.

Simple bridge bearinglayout (single span on pot bearings)

The arrangement above shows the layout for a simple single span structure with skew. The bridge deck is fixed in one

corner and horizontal movements are controlled by the use of guided (unidirectional) bearings. A free (multidirectional)

bearingis provided for the diagonally opposite corner to the fixed bearing. Longitudinal forces are taken by both the

fixed and guided bearingat the fixed end of the span. In a wider deck, it would be preferable to locate the fixed

bearingcloser to the centre of the deck so as to minimise the relative transverse movement and thus limit the

movements to be accommodated by the expansion joint.

Continuous multi-span decks

For longer spans, the magnitude of the movements increases and therefore these should be minimised by locating the

fixed bearingat the centre of the bridge to ensure the thermal expansion is split between each end of the bridge. Care

should be taken to ensure that the pier is designed for the resulting horizontal forces, particularly from braking and

acceleration actions.


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Examples of bearinglayouts for a 2-span bridge (on pot bearings)

On multi-span structures, care should be taken to ensure movements are not restrained, however the use of slender

piers that are able to flex may allow for load sharing between bearingsat a support location.

Further examples of bridge articulation arrangement are given in Guidance Note 1.04.

Curved bridge decks

Radial alignment on a curved bridge

Curved deckscan be guided either radially from a fixed point or tangentially to the radius of curvature. If the deck is

guided radially, then the accuracy of the geometry becomes critical for the bearingsfurthest from the fixed point.

For structures with a constant curvature it is best to align the bearingstangentially to effectively guide the deck around

the curve as it expands and contracts. The resulting horizontal forces are often accommodated by the use of specific

guide bearingswhich may not be vertical load carrying.

Tangential alignment on a curved bridge

Expansion joints


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Most historic bridges had no specific means of articulationbetween the deck and substructure: movements would

produce local cracking at the abutments. In modern bridges, articulationarrangements, such as those described

above, accommodate thermal and other movements by selecting appropriatebearingsas fixed and allowing

controlled movements to take place elsewhere. At road level, these movements are accommodated by an expansion

joint, which isolates the abutments from the displacements and maintains the integrity of the surfacing at the end of the

bridge.

The bridge designer should specify the expansion joints in a similar manner to bearings, giving details of characteristic

and design values of displacements to the joint designer.

Annex B of BS EN 1993-2[2]contains guidance for the preparation of a technical specification for expansion joints.

By introducing bearings, and particularly by introducing expansion joints at road level, a significant maintenance

liability is created. To reduce such liability, integral constructionis often considered for short bridges. For example,

BD 57[3]currently recommends that integral constructionbe considered for all bridges up to 60m overall length and

less than 30 skew. This reduces, and in some cases eliminates, the need for maintenancebut the designer must still

consider the movements (displacements and rotations) that are induced by traffic and thermal actions and make

appropriate allowances.

Bearings

The product standard for bearings is BS EN 1337 and this is the standard referred to in the Eurocodes. BS EN 1337

comprises 11 Parts, of which the most relevant are:

1. Part 1: General rules[1]

2. Part 2: Sliding elements[4]

3. Part 3: Elastomeric bearings[5]

4. Part 5: Pot bearings[6]

5. Part 8: Guided bearings and Restrained Bearings[7]

The choice of bearing will be governed by both the values and directions of the actions and also by the magnitude and

directions of the allowed and restrained displacements. Typical load bearing capacities (at ULS) are tabulate below.

Further guidance on the types of bearings and their usage can be found in Guidance Note 3.03.


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Load capacity of bearing types

Type Load capacity (kN) Comments

Elastomeric Strip 200 1,000 Limited translation and

rotation, and used only

for very short spans and

light loadsPad 10 500

Laminated 100 1,000 Widely used for short

spans

Pot 500 30,000 Proprietary product,

widely used on steel

bridges

Line rocker 1,000 10,000 No lateral rotation, fixed

bearings, rail bridges,

large longitudinal rotation

Spherical 1,000 12,000 Expensive, large rotation

capacity, used on major

steel bridges

Elastomeric bearings

Elastomeric bearing


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Elastomeric bearings normally consist of a number of rubber layers separated by steel plates. These are normally laid

in pads or strips and are ideally suited for small structures. They accommodate movements by deformation. It is not

normally required to fix the bearing in place as friction between the rubber and the support surfaces will normally be

adequate.

Elastomeric bearings provide an excellent economic solution for applications where structure movements, longitudinal,transverse and rotational are small. They provide vibration isolation and are generally simple to install. Elastomeric

bearings are relatively maintenance free but will degrade over time and require replacement.

Larger movements require taller bearings and possibly additional mechanical means of preventing the bridge deck

from effectively floating from the desired position. When used on steel bridges, elastomeric bearings can be positively

located using perimeter keep strips weldedto the underside of the bottom girder flange.

Pot bearings

The elastomeric pot bearing consists of a confined disk of elastomer within a short cylinder (the pot). Loading is thenapplied via a close fitting steel piston. This puts the elastomer under high pressure, making it behave like a liquid,

permitting rotation in any direction with very little resistance.

A sliding surface can be included to accommodate translational movement, which can be in any direction or

constrained by guides. The rotations and the translations, as well as the loads carried, can be greater than for

elastomeric bearings.

Elastomeric pot bearing with multi-directional sliding part

Other bearing types

Spherical bearings

Spherical bearings are used to accommodate large rotations by the use of a lower spherical surface. This is normally

lined with dimpled PTFE and matched to an upper stainless steel surface. These types of bearings are more

expensive than pot bearingsdue to the increased machining and would only be used on major structures, to

accommodate increased deck rotations. Generally, these bearings require a minimum co-existent vertical load to

prevent instability.

Spherical bearings


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Plain spherical bearing

Spherical bearing with sliding guided element

Rocker bearings

Line rocker bearing

These bearings allow rotation about a single axis (usually transverse to the girder). The advantage of these bearings is

that torsional restraint is provided about the axis orthogonal to the line of contact and therefore can be useful in U

frame bridges. They are often used when impact loading is high, such as on railway bridges.

Guide bearings


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As the name suggests, these bearings are used to ensure the structure maintains the correct location or

expansion/contraction path and take no vertical load. These types of bearings are occasionally used on heavily

skewedor multispan structures.

Guide bearing

Bearing specification

It is the bridge designers responsibility to prepare the bearingschedule. The schedule should contain the following

information:

A list of forces on the bearingsfrom each action

A list of movements of the bearingsfrom each action

Other performance characteristics of the bearings

The bearingdesigner (normally the manufacturer) will then use this information to determine the design values andtherefore the full specification. There are currently two alternative templates given for the bearingschedule, one is

given in Table A.3 of Annex A of BS EN 1993-2[2]and the other in Table B.1 Annex B of BS EN 1337-1 [1].

Table A.3 of BS EN 1993-2[2]requires the designer to give characteristic values due to the separate actions, which

then need to have partial and combination factors applied to them to give the design value for the bearings. Generally,

the bearingdesigner will be unaware of the relevant design combinations and will thus not be able to determine design

values for the bearingsfrom these characteristic values.

Table B.1 of BS EN 1337-1[1]simply expects the designer to give the relevant design values of loads (forces on the

bearings) and displacements. This schedule also requires reference data, maximum dimensions and fixing details to

be indicated. This is more informative for the bearingdesigner but still does not give the full range of coexisting

combination of forces and displacements for each bearing. (This deficiency will be addressed in the plannedAmendment to BS EN 1337-1[1], which will give new schedule tables.)


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Bearing schedule (as in Table B.1 of BS EN 1337-1)[1]

Structure Name or Reference

Bearing Identification Mark

Type of Bearing (see Table 1 of BS EN 1337-1)[1]

Number off

Seating Material Upper Surface

Lower Surface

Average Design Contact

Pressure (N/mm2)(Capacity of structure)

Upper Face Serviceability

Ultimate

Lower Face Serviceability

Ultimate

Design Load Effects

(kN)

Serviceability Limit State Vertical Max.

Perm.

Min.

Transverse

Longitudinal

Ultimate Limit State Vertical

Transverse

Longitudinal

Displacement (mm) Serviceability

Limit State

Irreversible Transverse

Longitudinal

Reversible Transverse

Longitudinal

Ultimate LimitState


Longitudinal


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Longitudinal

Rotation (Radians) Serviceability

Limit State


Longitudinal


Longitudinal

Maximum Rate

(Radians / 100kN)

Transverse

Longitudinal

Maximum Bearing

Dimensions (mm)

Upper Surface Transverse

Longitudinal

Lower Surface Transverse

Longitudinal

Overall Height (mm)

Tolerable movement of bearing under transient loads (mm) Vertical

Transverse

Longitudinal

Maximum acceptable reaction to displacement under serviceability

limit state (kN)

Transverse

Longitudinal

Maximum acceptable reaction to rotation under serviceability limitstate (kNm) Transverse

Longitudinal

Type of fixing required Upper Face

Lower Face

The designer must be aware of the difference between the two schedules and ensure that adequate information is

supplied to the bearingsupplier. It is also important, for correct installation, that the orientation of the bearingis clear;

see advice in Guidance Note 2.09.


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It is important to note that, for steel bridges, the bearingsare normally installed before completion of the bridge deck

and therefore bearingswill have to accommodate additional thermal displacements and also movements due to

construction activities. A common situation that must be considered is rotation due to pre-camber and the drop-out

during construction, particularly in heavily skewedstructures where may be large transverse rotations at the supports.

These rotations are a function of the plan geometry and are related to the magnitude of the dead load effects and the

pre-camber provided, they cannot be avoided.

Bearing installation

Bearingsare normally bolted to the girders above and the substructure below to allow replacement. Normally the

bearingsurface is set to be horizontal and therefore taper plates are normally required to follow the geometry of the

steelwork above. These taper plates should be designed along with the main girders, taking into account the final

geometry of the bridge post camber. The bearingsare normally bolted through the girder bottom flange though

difficulties do arise with thick flanges and moderate to large gradients since it is only feasible to drill square to the

flange surface. A common solution to this problem is to use tapped holes in the taper plate, which is then welded to the

underside of the girder; when using this detail, the horizontal forces on the bearingneed to be minimised. Refer to

Guidance Note 2.08for more information.

Skew ladder deckbeing lowered onto an elastomeric

pot bearing

(Image courtesy of Arup)

Attachment of bearingby bolting through girder flange

Initial temperature and temperature range

An estimate for the initial installation temperature for the installation of the bearingshould be given by the designer to

the constructor enable the bearingto be set correctly prior to installation, in order to allow the full expansion and

contraction displacements to be accommodated. This is not explicitly stated in Annex B of BS EN 1337-1[1]but is

stated in Annex A of BS EN 1993-2[2]. Some guidance for this installation temperature and the associated temperature

range is found in the Eurocodes but there remains some potential confusion. The following is an attempt to guide the

designer through the relevant parts of the Eurocodes relating specifically to bearingsand expansion joints as the onus

is on the designer to specify the range of displacement at the ultimate limit state.


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An initial bridge Temperature T0is given in the National Annex to BS EN 1991-1-5[8], clause NA.2.21 states that "In the

absence of specific provisions to control the temperature at which a bridge is restrained, the initial temperature T0should be taken as 0C for expansion and 20C for contraction." This would then be taken in conjunction with BS EN

1991-1-5[9]clause 6.1.3.3 (3) Note 2 for bearingswhich adds 20C to both the expansion and contraction range of the

uniform temperature component if no further information is available. This may be reduced to an additional 10C if an

initial installation temperature is specified. However, clause NA.2.6 of the National Annex to BS EN 1991-1-5

[8]

thenpoints the designer to BS EN 1993-2[2].

Annex A of BS EN 1993-2[2]requires a reference T0to be calculated as above. The uncertainty of the position of a

sliding bearingat installation should be accounted for by adding T0as described in Table A.4. The design value for

temperature difference is then determined by adding T0to TKand including a safety term Ty, which is given as

5C.

It is sensible to give the assumed installation temperature, so as to reduce the temperature range of the bearings a

value should be selected to be such that the temperature expansion and temperature contraction are similar (i.e. in the

middle of the range), a value for T0of 10C would be reasonable.

Using this installation temperature T0of 10C as the reference temperature will give similar but not identical results forboth methods. As the designer should use the temperature ranges given to estimate the maximum reversible

displacements, there is scope for conservatism here without undue cost.

Verification of the initial installation temperature on site will need to be made in accordance with BS EN 1337-11[10].

Further guidance on how designers should calculate the movement range to be specified for bridge bearings, taking

account of both thermal change and uncertainty in the relative positioning of bearings on the sub- and superstructures,

is available in SCI P406.

Maintenance of bearings

Abutment gallery for a composite bridge


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Clause A.3.1 (6) of BS EN 1993-2[2]states that bearingsand supports should be designed in such a way that they can

be inspected, maintained and replaced if necessary. To achieve this, access for inspection must be provided, there

must be means to relieve the bearingsof load, and it must by physically possible to extract the old and insert a new

bearing.

At an abutment, a common design feature that facilitates inspection and maintenance of the bearingsis the abutment

gallery. An example of the arrangement for a composite bridge is shown right.


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References

1. ^ 1.01.11.21.31.41.51.61.71.81.9BS EN 1337-1:2000. Structural bearings. General design rules. BSI

2. ^ 2.02.12.22.32.42.52.6BS EN 1993-2:2006 Eurocode 3 - Design of steel structures: Steel Bridges. BSI

3. ^BD 57/01 Design for Durability. The Design Manual for Roads and bridges. TSO

4. ^BS EN 1337-2:2004.Sliding elements. BSI

5. ^BS EN 1337-3:2005. Structural bearings. Elastomeric bearings. BSI

6. ^BS EN 1337-5:2005. Structural bearings. Pot bearings. BSI

7. ^BS EN 1337-8:2007. Guided bearings and Restrained Bearings. BSI

8. ^ 8.08.1NA to BS EN 1991-1-5. UK National Annex to Eurocode 1: Actions on structures: General actions

Thermal actions. BSI

9. ^BS EN 1991-1-5. Eurocode 1: Actions on structures: General actions Thermal actions. BSI

10. ^BS EN 1337-11:1998. Structural bearings. Transport, storage and installation. BSI

Further reading

Hendy, C.R.; Murphy, C.J. Designers Guide to BS EN 1993-2 Eurocode 3: Design of steel structures. Part 2

Steel bridges. Thomas Telford Ltd.

Hayward, Alan; Weare, Frank. (1989) Steel Detailers Manual. BSP.

Ray, S.S; Barr, J.; Clark, L. (1996) Bridge detailing guide. (Report R155) CIRIA.

Souby, M. (2001) Bridges design for improved durability. (Report C543) CIRIA.

Resources

Iles, D.C. (2010) Composite highway bridge design. (P356 including corrigendum, 2014). SCI

Section 8.8

Hendy, C.R.; Iles, D.C. (2015) Steel Bridge Group: Guidance Notes on best practice in steel bridge

construction (6th Issue). (P185). SCI

Guidance Note 1.04 Bridge articulation

Guidance Note 2.08 Attachment of bearings

Guidance Note 2.09 Alignment of bearings

Guidance Note 3.03 Bridge bearings

Steel Bridges: A practical approach to design for efficient fabrication and construction. (51/10). BCSA

Section 3.10: Bearings

Iles, D.C. (2015) Determining design displacements for bridge movement bearings. (P406, 2015). SCI

See also

Integral bridges

Half-through bridges


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Skew bridges
http://www.steelconstruction.info/Skew_bridgeshttp://www.steelconstruction.info/Skew_bridges

Bridge Articulation and Bearing Specification

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