Lam. Elastomeric Bearings - Horizontal Des. Loads

[justhumm]

I am designing a simple span (about 70 ft) bridge, using prestressed concrete box girders; and laminated elastomeric bearing pads.

I am looking for a specification that will tell me the (min. or max.) horizontal load that will theoretically be transferred from the superstructure to the concrete pier.

The only actual spec. I have found so far called for a min. of 10% of the vertical design load, but that was for a seismic design spec.

Can anyone give me a decent reference? Thanks.

 

[cooperDBM]

The shear force, which is due to the shear deformation of the bearing, is dependent on the shear modulus (stiffness) of the elastomeric material (i.e. durometer of the rubber or neoprene) and the design of the bearing (i.e. net elastomer thickness). You generally need to get this information from the bearing manufacturer (or potential manufacturer if not yet known).

Note that the shear modulus is dependent on temperature. Finding the shear force resulting from a drop in ambient temperature will require knowing how the shear modulus changes with the change in temperature. Most elastomeric bearing manufactuers publish tables or graphs showing this variation in shear stiffness.

To get the force required to shear the bearing over a temperature range will require a rational method to account for this change such as integration, or simply averaging the stiffness if the curve is roughly linear. It would be too conservative to simply take the maximum stiffness of the range since the force required to hold a certain shear deformation doesn't increase with a drop in temperature, only the force required to shear it further.

 

[JAE]

This is standard fare calculations for bridge engineers. Use the AASHTO specification and/or consult with your local DOT as they have bridge standards that can help you determine this load.

Essentially, you have thermal movements of the bridge, longitudinal live loads from truck loading, seismic, and possibly some shrinkage of the precast girders that will all add and/or subtract from the longitudinal force on the bearings.

In the lateral direction of the bridge, you have possibly lateral seismic and wind loads across the bridge on both the bridge and vehicles.

 

[jimaitken]

Look at 14.5 .3.1 in the 17th edition of AASHTO. This is the force required to deform a thickness of elastomer. If you know what your superstructure movement is (delta s)you should be able to determine the force on the bearing.

 

[jimaitken]

One other thing to be aware of is that you should to take the stiffness of the bent into account when you apply the horizontal force. If you have a relatively low stiffness (Pile bent) this will have the effect of reducing the horizontal force. The formula would be :

Hforce= ([Δ] total)/[(elastomer thick/(G*Apad))+((H column)^3/# column*E*I column*3)].

where [Δ] total is the total thermal movement of the bridge.