Elastomeric Bearing Rotatons 1

[bridgeman8]

I have a question about elastomeric bearing rotations. I'm designing steel reinforced elastomeric bearings for a single span steel railroad bridge (transit). I'm using AREMA and AASHTO Standard Specifications to check the rotation requirements. Do I need to check bearing rotations during construction? The bearing rotation due to the camber of the steel girder produces a large bearing rotation with a small compressive force (before deck is placed). This could cause lift-off at one edge of the bearing, increasing stresses and strains. This case would increase the required thickness of the bearing considerably. Some references state the the design provisions were developed for repeated cycles of service load, so they are not applicable to a single application of a construction load combination. Is this true? Do I only need to consider rotations due to service loads (rotations due to live load deflection)? Any comments would be appreciated.

 

[VoyageofDiscovery]

Hi bridgeman8,

Good question, I only check for my live load and superimposed dead load for rotation. I have not checked the construction "cambered steel only" situation. Does anyone else have thoughts on this?

Regards

VOD

 

[koo]

Bridgeman 8,

I usually use AASHTO Method B. It requires a 0.005rad allowance and have an uplift check. Although the check is for DL+LL, I think the formula is still valid for a construction load only check. I think AASHTO indeed says "bearings shall be designed so that uplift does not occur under any combination of loads...", but I don't know how many people wound actually check construction phase load. My thoughts is camber ratation is mostly covered in the 0.005 rad. Say a 2" camber on a 100' beam, end rotation is probably about 0.0035 rad. It would be interesting to know how other people tackled on this.

Koo

 

[jakemal]

I use AASHTO Method B for elastomer bearing design.
My question is below
1. Do we need to design the outside face of elastomer layer in order to satisfy uplift base on fatigue.
2. Also, do we need to design load base on superload such as 8 Axles, 19 Axles.

Thanks

 

[Dinosaur]

Greetings,

I design laminated elastomeric pads for 0.005 radians fabrication tolerance and the range of movement & load likely to be experienced during the application of HS loads during normal service. In my opinion, rotation during construction is the contractors problem. If it doesn't satisfy routine checks during deck placement, then the contractor can deal with it in his means and methods. However, for a railroad bridge, the dead load to live load ratio may be a problem during service.

Good Luck

 

[oldhand]

Rotations and deflection movements due to dead load have to be added or subtracted in obtaining the total rotational and deflection movements of the bearing. Failure to do this could lead to premature failure of the bearing. Uplift on a corner of a bearing under dead load is not a problem as it is of short duration. If you are concerned about it put timber wedges under the beam in the temporary condition and remove these once the load is large enough to cancel the uplift.

 

[maxmozo]

Take a look at AASHTO LRFD Commentary section C14.4.2. There is a statement in the second paragraph saying that I interpret to mean that the rotations during construction can effectively be overlooked. In talking about elastomenric bearings AASHTO makes this staement "...As a result such bearings are permitted temporary overstress during construction. If this were not so, temporary local uplift caused by light load and a large rotation might unreasonably govern the design..."

Thats what I've always understood. Does anyone else have an opinion?

Thanks,

Max

 

[bridgeman8]

Thanks for your responses. I agree with maxmozo. The AASHTO LRFD Commentary section C14.4.2 allows for temporary overstress during construction. I do not have to consider the case of temporary uplift caused by light load and large rotations immediately after the girder is placed on the bearings.