Elastomeric Bearing in Bridges


Elastomeric Bearing in Bridges 

The use of elastomeric bearing in bridges is not uncommon in Hong Kong. However, not all designers get a full picture of various aspects  of elastomeric bearing. Hence, this article is intended to introduce the  design of elastomeric bearing to graduate engineers and to refresh the
knowledge of experienced bridge engineers.

Basic Design Consideration 

For elastomeric bearing, the shear stiffness is an important parameter  for design because it influences the force transfer between the bridge  and   its   piers.   In   essence,   elastomers   are   flexible   under   shear  deformation   but   it   is   relatively   stiff   in   compression.   However, elastomeric bearings should not be used in tension.

Elastomeric  bearing  should  be  designed  in  serviceability  limit  state  only.   The   cross   sectional   area   is   normally   determined   by   the  compressive  stress  limit  under  serviceability  limit  state.  The  shape  factor, i.e. plan area of the laminar layer divided by area of perimeter  free  to  bulge,  affects  the  relation  between  shear  stress  and  the  compressive  load.  In  essence,  higher  capacity  of  bearings  could  be obtained with higher shape factor.  

The long side of the bearing is usually oriented parallel to the principle  axis   of   rotation   because   it   facilitates   rotational   movement.  The  thickness   of   bearings   is   limited   and   controlled   by   shear   strain  requirements. In essence, the shear strain should be less than a certain  limit  to  avoid  the  occurrence  of  rolling  over  at  the  edges  and  delamination due to fatigue. Hence, it follows that higher rotations and  translations  require  thicker  bearing.  On  the  other  hand,  the  vertical stiffness of bearings is obtained by inserting sufficient number of steel  plates. In addition, checks should be made on combined compression  and  rotation  to  guard  against  the  possible  occurrence  of  uplifting  of  corners of bearings under certain load combinations.  

 Steel Plates Inside the Bearings

In the design of elastomeric bearing, the bearing should be allowed for  bulging laterally  and the compression stiffness could be increased by limiting  the  amount  of  lateral  bulging.  To  increase  the  compression stiffness  of  elastomeric  bearings,  metal  plates  are  inserted.  After  the
addition  of  steel  plates,  the  freedom  to  bulge  is  restricted  and  the deflection is reduced when compared with bearings without any steel plates under the same load. Tensile stresses are induced in these steel plates during their action in limiting the bulging of the elastomer. This in turn would limit the thickness of the steel plates.

However,  the  presence  of  metal  plates  does  not  affect  the  shear stiffness of the elastomeric bearings.

Anchor Dowel 

Elastomeric  bearing  is  normally  classified  into  two  types:  fixed  and free.  For  fixed  types,  the  bridge  deck  is  permitted  only  to  rotate  and the horizontal movements of the deck are restrained. On the other hand, for  free  types  the  deck  can  move  horizontally  and  rotate.  To  achieve
fixity,  dowels  are  adopted  to  pass  from  bridge  deck  to  abutment. Alternatively, in case there is limitation in space, holes are formed in the  elastomeric  bearings  where  anchor  dowels  are  inserted  through these  holes.  It  is  intended  to  prevent  the  “walking”  of  the  bearing
during its operation.

Limitation of Elastomeric Bearing   

Elastomeric  bearing  has  the  potential  advantage  of  low  cost  when compared  with  other  bearing  types.  In  particular,  it  requires  little long-term  maintenance  to  enhance  its  performance  during  servicing. Moreover,  it  demonstrates  good  performance  in  seismic  condition because  of  its  relatively  large  plan  areas  with  low  height  and  the natural dampening effect of elastomer.

In  the  event  of  high  vertical  loads  combined  with  large  angle  of rotations,  rubber  bearings  are  undesirable  when  compared  with  pot bearings.  For  instance,  elastomeric  bearings  require  large  bearing surfaces  so  that  compression  can  be  maintained  between  the  contact
surfaces  between  the  bearings  and  piers.  Moreover,  it  also  leads  to uneven  distribution  of  stress  on  the  piers  and  some  of  these  highly induced stresses may damage the structure. Consequently, pot bearings are better alternatives than elastomeric bearings in such an scenario. 
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