AASHTO:Chapter 14, Bearings-16th Edition vs. 17th Edition 2

[jimaitken]

Is anyone familiar with the changes to the AASHTO Standard Specifications for Highway Bridges Chaper 14, Bearings. Specifically steel reinforced elastomeric pads designed using Method A and the changes between the 16 th(1996)1997 interim through 17 (2002) Editions of the code.

The 1997 interim spec 14.6.6.3.5 used the total thickness of the elastomer when checking rotation stress, but the latest edition has changed the requirements so that you have to check internal and external layers.

My calculations are finding that I have to increase the cover layer thickness in a standard pad that our DOT uses so that it meets the 70% criteria and is more than 1/2 the thickness of the internal layers to take advantage of an additional layer of elastomer to divide the rotation by. I also have found that the internal layers are generally thicker. The result is a thicker pad with more layers of elastomer and reinforcement. The cover layer seems to be driving this change.

My questions are:

1. Why has this spec been changed so often (it has substatially re-written since the 15th edition) and what is the main reason for the most recent change?

2. How critical are the cover layers in overall performance of the bearing?

3. If I neglected the effect of rotation on the cover layers and made my internal layers thicker (smaller shape factor) would this be a way to mitigate any adverse effect on overall performance?

 

[broekie]

I am somewhat familiar with the changes made to the AASHTO specification in regards to bearings. From the conversations that our office has had with other designers and bearing suppliers, there is no one out there who likes the changes. Everyone has a harder time making the geometry of the pads work. Change the width, rotation doesn't work. get rotation to work, compression stresses no longer meet code. It seems to be a viscious cycle. The pads seem to be getting wider and thicker based on the new standards. In answer to your questions:

1. I don't know why the spec has changed so often. Our office has not heard of many bearing failures at all, so I am not really sure what is driving these changes.

2. Not sure.

3. Because we were having trouble making our bearings work, we decided to neglect the rotation of the external layer. We only check rotation of the internal layers.

I certainly don't have all the answers, but wanted to share our experience.

 

[jimaitken]

I appreciate the reply and am glad (or sorry depending on viewpoint) to hear that someone else is having the same difficulties that I am having with the revised spec. I think I am going to neglect the cover layer in my design.

 

[TampaBridgeDesign]

Thought you might be interested in the FDOT's stance on bearing design. They allow you to neglect the combined compression and rotation check. By eliminating this check, your bearing thickness decreases significantly.


6.5.1 Design
A. Specify composite neoprene bearing pads and other bearing devices that have been designed in accordance with LRFD Method B, the Department's Standard Specifications for Road and Bridge Construction, and this document. Until ongoing research is completed, delete Article 14.7.5.3.5 for Combined Compression and Rotation.