commentary on the nchrp problem statement on...

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


• Disc bearings have an outstandingperformance history with very few fieldproblems

Disc Bearing Resume


• 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

Multidirectional

Unidirectional

Issues Raised in NCHRPProblem Statement

• Design of polyurethane disc



• Design of shear resisting mechanism(SRM)



• Design of shear resisting mechanism(SRM)

• Material tests on polyurethane

Design of Polyurethane Disc

• Adequately covered in AASHTO



• Section 14.7.8.2 covers materials




• Section 14.7.8.3 covers disc design




• Section 14.7.8.3 covers disc design

• Section 14.7.8.1 covers limiting ring(if required)

Limiting Ring

• Earlier disc bearing designs usedsofter polyurethane compound

Limiting Ring


• Limiting ring controlled spreadingand limited deflection

Limiting Ring



• Harder compound (62D) eliminatesneed for limiting ring

Limiting Ring



• 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


• Function of bending, shear andbearing calculations

Design of SRM


• 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



• Performance is verified by AASHTOtesting requirements



• 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


• Important to keep in mind that it isincumbent upon the manufacture todevelop proper compoundstoichiometry


• 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


• There are many compounds that willgive desired results in accordancewith AASHTO


• 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

commentary on the nchrp problem statement on...

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