Bearing B is much easier to fabricate, construct, and replace the elastomeric pad, if needed. I don't see any advantage to Bearing A.
On the subject of replacing the elastomeric pad, to facilitate that, and allow the sole plate to be shop-welded to the girder, we use keeper bars in front and behind the pad to keep it from walking, rather than vulcanizing it to the sole plate. With your anchor bolts as far away from the pad as they are, you might need keeper bars on the sides, too, if you don't have the shear plates for the anchor bolts.
Rod Smith, P.E., The artist formerly known as HotRod10
BridgeSmith - for want of a better answer, I don't think A is better than B.I prefer B. However, in NY this is what we've been using for about 40 years. I went through some historic information but couldn't find a reason for A vs B. In the early 60's a variation of B was used in NY. In the late 80's I was resident engineer on a viaduct rehab project in CT we were installing Type A bearings; since then ConnDOT went to B.
Ah, the old 'we do it this way because that's the way we've always done it' reason. Seen that alot here, too. It wasn't too long ago we were still putting slots in the pads for the anchor bolts - then we had to replace some, and it was a PITA. Never mind that with the slots the fabrication had been way more expensive for decades, it made the design more difficult, and resulted in larger pads.
Rod Smith, P.E., The artist formerly known as HotRod10
That's the typical reason. The rubber typically used will leach oils when put under pressure, effectively greasing the seat, and then the cyclic rotations cause the pads to 'walk' out from under the girders. We prevent that by using keeper bars welded to the sole plate around the pad.
We use AASHTO Method B for our pad design. Supposedly you end up with a stronger, more durable pad, or that's what I read somewhere.
Same fabrication, but with more stringent testing and verification that the shear modulus is within the specified range. This allows for a tighter design (higher allowable compressive stress, etc.). the extra testing cost money and takes time, and if the pads fail, they get tossed and the fabricator has to start over. We only design per Method B if there are space restraints where we can't get a pad to design out using Method A, such as in rehab work where we're replacing existing bearings of another type.
Rod Smith, P.E., The artist formerly known as HotRod10
We had designed replacement pads with Method A for a rehab project. After the contractor started sand blasting the beam ends he started blowing basketball sized holes through the webs. The fix was to encase the beam ends in concrete, but this obviously added significant load to the bearings. The bearings had already been fabricated and on site. I spoke with the fabricator and he told me that they test Method A and Method B bearings with the same testing except for the compression tests. Method A uses a 15 minute test and the Method B uses 15 hour test. The fabricator also said if the bearings pass the 15 minute test they will pass the 15 hours test. We checked the bearings with Method B. Some still did not pass code checks but several did and were used in the rehab. The ones that failed, had to be redesigned.
Good info, 3Fan. I didn't know what additional testing was involved, just that it verified the shear modulus of the elastomer, rather than relying on the Durometer hardness, which only yields an approximation of the shear modulus. Apparently, getting the right properties in the fabrication process is hit-or-miss, even if they know it has to meet the stricter requirements from the outset.
Rod Smith, P.E., The artist formerly known as HotRod10
Yikes there are areas that prefer detail A... I am very curious why? Are we talking about small to medium size bridges only???
I would be nailed to a cross if I suggested (A) in my area. It is very hard to install and replace (elastomers do not last as long as people think).
In my area we use type B and not vulcanized to the sole plate (but with keeper bars as mentioned above).
