COMMENTARY ON THE NCHRPPROBLEM STATEMENT ON
DISC BEARINGS
Ronald J. WatsonPresident
R.J. Watson, Inc.
For Presentation to the AASHTO TechnicalCommittee for Bearings and Expansion Devices
(T2)
• Thousands of disc bearings have beeninstalled on structures all over the worlddating back to the early 1970’s
Disc Bearing Resume
• Thousands of disc bearings have beeninstalled on structures all over the worlddating back to the early 1970’s
• Disc bearings have an outstandingperformance history with very few fieldproblems
Disc Bearing Resume
• Thousands of disc bearings have beeninstalled on structures all over the worlddating back to the early 1970’s
• Disc bearings have an outstandingperformance history with very few fieldproblems
• R.J. Watson manufactures more disc bearingsthan any company in the world
Disc Bearing Resume
Fixed
Multidirectional
Unidirectional
Issues Raised in NCHRPProblem Statement
• Design of polyurethane disc
Issues Raised in NCHRPProblem Statement
• Design of polyurethane disc
• Design of shear resisting mechanism(SRM)
Issues Raised in NCHRPProblem Statement
• Design of polyurethane disc
• Design of shear resisting mechanism(SRM)
• Material tests on polyurethane
Design of Polyurethane Disc
• Adequately covered in AASHTO
Design of Polyurethane Disc
• Adequately covered in AASHTO
• Section 14.7.8.2 covers materials
Design of Polyurethane Disc
• Adequately covered in AASHTO
• Section 14.7.8.2 covers materials
• Section 14.7.8.3 covers disc design
Design of Polyurethane Disc
• Adequately covered in AASHTO
• Section 14.7.8.2 covers materials
• Section 14.7.8.3 covers disc design
• Section 14.7.8.1 covers limiting ring(if required)
Limiting Ring
• Earlier disc bearing designs usedsofter polyurethane compound
Limiting Ring
• Earlier disc bearing designs usedsofter polyurethane compound
• Limiting ring controlled spreadingand limited deflection
Limiting Ring
• Earlier disc bearing designs usedsofter polyurethane compound
• Limiting ring controlled spreadingand limited deflection
• Harder compound (62D) eliminatesneed for limiting ring
Limiting Ring
• Earlier disc bearing designs usedsofter polyurethane compound
• Limiting ring controlled spreadingand limited deflection
• Harder compound (62D) eliminatesneed for limiting ring
• 15 years with no reported fieldproblems
Design of SRM
• Covered in AASHTO Steel DesignSection 6
Design of SRM
• Covered in AASHTO Steel DesignSection 6
• Function of bending, shear andbearing calculations
Design of SRM
• Covered in AASHTO Steel DesignSection 6
• Function of bending, shear andbearing calculations
• Since SRM is designed for the totalhorizontal force the shear resistanceof the polyurethane is not relevant
Material Tests on Polyurethane
• Basic requirements in AASHTOConstruction Specifications Section18.3.2.8 & 18.3.4.3
Material Tests on Polyurethane
• Basic requirements in AASHTOConstruction Specifications Section18.3.2.8 & 18.3.4.3
• Performance is verified by AASHTOtesting requirements
Material Tests on Polyurethane
• Basic requirements in AASHTOConstruction Specifications Section18.3.2.8 & 18.3.4.3
• Performance is verified by AASHTOtesting requirements
• Onus is on manufacturer to come upwith proper compound
Creep of Polyurethane
• Testing and field evaluations havedemonstrated this to be negligible
Design of Polyurethane Disc
• Important to keep in mind that it isincumbent upon the manufacture todevelop proper compoundstoichiometry
Design of Polyurethane Disc
• Important to keep in mind that it isincumbent upon the manufacture todevelop proper compoundstoichiometry
• Characteristics such as creepresistance, environmental durabilityand stiffness all have to be factoredinto the compound design
Design of Polyurethane Disc
• There are many compounds that willgive desired results in accordancewith AASHTO
Design of Polyurethane Disc
• There are many compounds that willgive desired results in accordancewith AASHTO
• Past experience, testing and qualitycontrol measures are required tofine tune design
Conclusions
• Cookbook specification would relievethe liability of the supplier and shiftthat to the State DOT or Owner
Conclusions
• Cookbook specification would relievethe liability of the supplier and shiftthat to the State DOT or Owner
• It would also result in unqualifiedsuppliers entering into the marketresulting in field problems
Conclusions
• Disc bearings have an outstandingfield performance history on bridgesall over the world
Conclusions
• Disc bearings have an outstandingfield performance history on bridgesall over the world
• Why not spend limited NCHRPdollars on testing bearings with ahistory of field problems
Conclusions
• If it is determined that a study iswarranted it should be focused onexpanding the capabilities of the discbearing