Bearings
The articulations of the civil engineering structures
Bridges and large building are not so immovable as they can appear to the non-professional eye. They are subjected to continuous bending and torsion due to the live loads, the wind and human actions. They are subjected to expansion and shrinkage due to temperature variations and other long term effects.
Thanks to the structural bearings they can accommodate all these movements and transfer the applied loads granting the safety and a long service life.
Structural Bearings are elements allowing rotation between two members of a structure and transmitting the loads defined in the relevant requirements as well as preventing displacement (fixed bearings), allowing displacement in only one direction (guided bearings) or in all directions of a plane (free bearings) as required.
Elastomeric bearing can be considered as fixed with forces limitations or movable with displacements limitations. Sliding bearing are designed and manufactured in order to minimize the friction coefficient.
The upper 95% fractile of the adverse friction μa to be considered is 0.03. This can be reduced to 0.02 if the temperature never drops under -5°C.
The lower 95% fractile of the friction is equal to zero (favorable friction μf)
If more than 4 sliding bearings are considered together the upper fractile of the adverse friction and the lower fractile of the favorable friction may be modified as shown in the graph.
F = fixed bearing
M = movable bearing
Fx = Fxa + μaΣiFzir + μr ΣjFzjl
Fx = longitudinal reaction fixed bearing
Fxa = longitudinal live load (Wind and braking)
Fzir, Fzjl= vertical reaction i-pier right, left
