Bridge 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:
- Part 1: General rules
- Part 2: Sliding elements
- Part 3: Elastomeric bearings
- Part 5: Pot bearings
- Part 8: Guide bearings and restraint bearings
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.
Load capacity of bearing typesTypeLoad capacity (kN)CommentsElastomericStrip200 – 1,000Limited translation and rotation, and used only for very short spans and light loadsPad10 – 500Laminated100 – 1,000Widely used for short spansPot500 – 30,000Proprietary product, widely used on steel bridgesLine rocker1,000 – 10,000No lateral rotation, fixed bearings, rail bridges, large longitudinal rotationSpherical1,000 – 12,000Expensive, large rotation capacity, used on major steel bridges
Elastomeric bearings
Elastomeric bearing
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 welded to 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 then applied 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 bearings due 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
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
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 skewed or multispan structures.
Guide bearing
Bearing specification
It is the bridge designer’s responsibility to prepare the bearing schedule. The schedule should contain the following information:
- A list of forces on the bearings from each action
- A list of movements of the bearings from each action
- Other performance characteristics of the bearings
The bearing designer (normally the manufacturer) will then use this information to determine the design values and therefore the full specification. There are currently two alternative templates given for the bearing schedule, 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 bearing designer will be unaware of the relevant design combinations and will thus not be able to determine design values for the bearings from 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 bearing designer 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 planned Amendment to BS EN 1337-1[1], which will give new schedule tables.)
Bearing schedule (as in Table B.1 of BS EN 1337-1)[1]Structure Name or ReferenceBearing Identification MarkType of Bearing (see Table 1 of BS EN 1337-1)[1]Number offSeating MaterialUpper SurfaceLower SurfaceAverage Design Contact Pressure (N/mm2)
(Capacity of structure)Upper FaceServiceabilityUltimateLower FaceServiceabilityUltimateDesign Load Effects (kN)Serviceability Limit StateVerticalMax.Perm.Min.TransverseLongitudinalUltimate Limit StateVerticalTransverseLongitudinalDisplacement (mm)Serviceability Limit StateIrreversibleTransverseLongitudinalReversibleTransverseLongitudinalUltimate Limit StateIrreversibleTransverseLongitudinalReversibleTransverseLongitudinalRotation (Radians)Serviceability Limit StateIrreversibleTransverseLongitudinalReversibleTransverseLongitudinalMaximum Rate
(Radians / 100kN)TransverseLongitudinalMaximum Bearing Dimensions (mm)Upper SurfaceTransverseLongitudinalLower SurfaceTransverseLongitudinalOverall Height (mm)Tolerable movement of bearing under transient loads (mm)VerticalTransverseLongitudinalMaximum acceptable reaction to displacement under serviceability limit state (kN)TransverseLongitudinalMaximum acceptable reaction to rotation under serviceability limit state (kNm)TransverseLongitudinalType of fixing requiredUpper FaceLower Face
The designer must be aware of the difference between the two schedules and ensure that adequate information is supplied to the bearing supplier. It is also important, for correct installation, that the orientation of the bearing is clear; see advice in Guidance Note 2.09.
It is important to note that, for steel bridges, the bearings are normally installed before completion of the bridge deck and therefore bearings will 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 skewed structures 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
Bearings are normally bolted to the girders above and the substructure below to allow replacement. Normally the bearing surface 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 bearings are 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 bearing need to be minimised. Refer to Guidance Note 2.08 for more information.
Skew ladder deck being lowered onto an elastomeric pot bearing
Attachment of bearing by bolting through girder flange
Initial temperature and temperature range
An estimate for the initial installation temperature for the installation of the bearing should be given by the designer to the constructor enable the bearing to 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 bearings and expansion joints as the onus is on the designer to specify the range of displacement at the ultimate limit state.
An initial bridge Temperature T0 is 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 T0 should be taken as 0°C for expansion and 20°C 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 bearings which adds 20°C 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 10°C if an initial installation temperature is specified. However, clause NA.2.6 of the National Annex to BS EN 1991-1-5[8] then points the designer to BS EN 1993-2[2].
Annex A of BS EN 1993-2[2] requires a reference T0 to be calculated as above. The uncertainty of the position of a sliding bearing at installation should be accounted for by adding ΔT0 as described in Table A.4. The design value for temperature difference is then determined by adding ΔT0 to ΔTK and including a safety term ΔTy, which is given as 5°C.
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 T0 of 10°C would be reasonable.
Using this installation temperature T0 of 10°C as the reference temperature will give similar but not identical results for both 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.