Braking force at Piers with Elastomeric or HLMR Expansion Bearings?

[jbuening]

I'm just curious if other bridge designers on here consider elastomeric/HLMR bearings and fixed bearings to take their respective contributing span length for braking force equally, or if all braking force of the bridge is transmitted at the fixed bearing down into the pier? For example, a 4 span bridge with 100' spans and the center pier is fixed with all other piers and abutments on elastomeric/HLMR bearings. Do you consider the the longitudinal tributary length for the fixed pier to be 400' or 100'?

In reality a particular longitudinal force will transmit to the substructure up until the point the coefficient of friction is reached, at which point the bearing will slide on the concrete. Typically the piers we've done were relatively short, but I have a project with braking force moment arms approaching 100' with 3 lanes one-way, so the effect on the column moments are huge. Due to the large loads on this project, it will need guided-HLMR bearings. They utilize PTFE discs or silicone grease between the piston and base plate, so I'd expect the coefficient of friction to be very small compared to elastomeric bearings with rubber against the concrete. In this case I'm leaning toward transferring all braking forces through the single fixed pier.

Per FHWA Design Example, they consider all braking force to be taken by the fixed pier:

http://www.fhwa.dot.gov/bridge/lrfd/us_ds8.htm

Per Iowa DOT, they say to consider braking force at elastomeric bearings as well as fixed bearings:

http://www.iowadot.gov/bridge/policy/66pierLRFDJa10.pdf

It seems some DOTs specify to consider it at elastomeric bearings and others choose to not mention anything about bearing type.

Thoughts?

 

[ishvaaag]

A pair of months ago I read an issue of the magazine Hormigón y Acero that dealt with a number of high speed railway bridges and invariably took the braking force at the ends if feasible, and only when convenient at the fixed point, usually at the apex or one inverted V arch support amidst the bridge, fixed to the horizontal superstructure. So the elastomeric bearings etc were thought to slip (longitudinally). These bridges were all -that I remember- incrementally launched from one end, and most of them were 1 order of magnitude over the spans you are dealing with ... the longest was I think to remember over two miles long on a number of piers.

 

[ishvaaag]

errata

... at the apex OF ...