What every bridge and structural engineer should know about selecting and specifying structural bearings.

It is important to define the demands to which the bearings will be subjected. This assessment should not be limited to just the six degrees of freedom, in particular the deck forces, to be carried and resisted, but should consider all other relevant factors such as the cumulative movements and rotations during the lifetime of the bearings. 

The selection of bearing type (e.g. pot, spherical, disc, elastomeric, cylinder, shear key, ILM or a seismic isolator) will be largely defined by the principal functions the bearing is required to fulfil for the bridge – in particular, in terms of resisting forces and accommodating movements. Long-term performance should be maximised and maintenance effort should be minimised, as should the frequency of bearing replacement works and the required effort when replacement becomes unavoidable. 

It is most important that the ability of the selected bearing, as designed and fabricated by its manufacturer, to withstand the loads and facilitate the movements to which it will be subjected during a long life on a structure, should be verified in advance of its use. The best verification of this is a strong track record on the part of the bearing supplier, with evidence of satisfactory performance of similar bearings over many years on comparable structures. Laboratory testing also serves a useful purpose.

Once the type of bearing which can optimally satisfy the structure’s needs has been identified, it is important, at an early stage, to ensure that the bridge deck is designed to receive the selected bearings, with proper access, adequate clearance between substructure and superstructure, correctly sized and reinforced connecting surfaces of the appropriate concrete strength (e.g. for high-performance RESTON® bearings we recommend a contact strength of at least 50 N/mm2), and allowance for suitable anchorage. Inadequate access to the bearings may cause difficulties with inspection and maintenance at a later stage, and incorrectly dimensioned main structures may necessitate changes to approved plans, or even adaptations to the constructed bridge structure on site.

A key factor in maximising durability is the proper application of a suitable corrosion protection system (e.g. painted, galvanised etc.), appropriate to the bridge’s  environment. An appropriate level of protection must be specified, and properly applied, with adequate verification of quality and particularly layer thickness and adhesion. Another important factor is the design and orientation of bearings to protect sensitive sliding surfaces.

The integration in a bearing’s design of structural health monitoring (SHM) functionalities can help to optimise inspection and maintenance work, thereby ensuring that important data about a bearing’s condition and performance are efficiently recorded and notified in real time, maximising the bearing’s service life. And of course, as noted previously, the design of the main structure must ensure adequate access for inspection and maintenance activities.

When the time comes to replace a bridge bearing that is anchored to a concrete structure, the effort required will be greatly reduced if the existing bearing, when installed, was equipped with anchor plates. The bearing can then be removed without any breaking out of concrete and with only minimal lifting of the bridge deck. The design of the bridge should also consider future bearing replacement needs, including lifting of the superstructure as required and temporary transfer of functionality (e.g. resisting horizontal forces) to a neighbouring bearing where appropriate.