Steel Bearing - bridge Anchor Rod Design

[Enjoy learning_not working]

Hi Everyone,

I would appreciate any help in trying to understand how to tackle this problem I have with the design of steel bearings for a railroad bridge. I will start with the admission that my current understanding might be flawed. So, the question in topic is as follows, but first I will frame how I came across this problem.

Design is for railroad bridge (AREMA dictates). The fixed bearing assembly for this RR bridge consists of beveled rocker plates - pintles connecting the rocker plate to a base plate, base plate connected to masonry plate that rests on a 3/8" neoprene pad. There are 4 anchor bolts in the masonry plate connecting to the drilled shaft cap beam (see attached).

So, for the particular design of the bearing, I followed these steps: design for the min thickness of the rocker plate required for the vertical reaction bearing stress - check the shear capacity of the pintles for the horizontal shear from the superstructure (shear being the longitudinal / frictional reaction) - design the base and masonry plate for the min thickness required for vertical reaction bearing (followed ASIC min thickness base plate formula) - check the shear capacity of the 4 anchor bolts - check whether the anchors are subjected to net uplift due to the moment arm created from the shear acting at the pintle level. Next step is where I have been stumbling upon and have no clue how to solve or do an analysis.

The anchor bolts are 2 1/8" dia and the oversize required as per AISC is 3/8" larger. I am assuming that the horizontal shear is transferred to the substructure through the anchor bolts and as such due to the presence of this oversize, the bolts will be subjected to bending prior to being mobilized to transfer the shear. AISC Design Guide 1 (Base Plate and Anchor Rod Design) states that this is a particular check that needs to be performed if trying to transfer shear through anchors (they suggest to only consider half the anchor bolts if we want to bypass the bending check). unfortunately, the horizontal shear is too large, and 2 bolts is not sufficient. A caveat being, at the moment I am considering all the horizontal reaction from the longitudinal forces from the superstructure analysis. AREMA does state that the longitudinal forces (braking / traction) must be distributed as per the relative stiffness. The superstructure design does not do this and transfer all the forces to the beam and the floorbeam (if anyone has an idea how to reduce this longitudinal force, please do provide some reference).

And, following the bending stress check, that fails for the anchor bolts as well. And this is where I am stuck. I am trying to figure out if this approach is sound, if not how to tackle this or is there anything else that can be done.

I know the post ended up being too long but felt it was necessary to explain. I have attached some of my calcs, the bearing detail for you all to look at.

Thanks.

 

[BridgeSmith]

Not sure whether it's allowed for railway bridges, but on a highway bridge, if we used that configuration, we would consider the resistance provided by shear deformation of the neoprene pad, up to where it first slips under minimum loads. That would help some.

For the bolts in sign structure and high mast light tower foundations, the AASHTO sign spec allows us to ignore bending and just check shear where the exposed bolt length (between the leveling nut and the top of the foundation) is less than 1 bolt diameter. Does AISC have a similar exception?

 

[Enjoy learning_not working]

Not sure whether it's allowed for railway bridges, but on a highway bridge, if we used that configuration, we would consider the resistance provided by shear deformation of the neoprene pad, up to where it first slips under minimum loads. That would help some.

For the bolts in sign structure and high mast light tower foundations, the AASHTO sign spec allows us to ignore bending and just check shear where the exposed bolt length (between the leveling nut and the top of the foundation) is less than 1 bolt diameter. Does AISC have a similar exception?

Re the deformation in the neoprene. I believe that it probably cannot be used for this case. The neoprene thickness is minimal, and we don't want any slip of pad itself at the bearing which would compromise the integrity. The highway bridge elastomeric pads are designed to account for all the shear deformation within the element itself. However, the use of thin neoprene (3/8") is only to avoid direct bearing of steel on concrete substructure (AASTHO / NSBA Guidance document G9.1 Steel Bridge Bearing Guidelines, C3.3.6).

Also, from what I have seen, ignoring bending is also not an option due to the hole oversize. I have come across numerous references (the most helpful being NYSDOT and NY Thruway design guide, section 8 on bearings) where it is stipulated that bending must be accounted for if trying to transfer shear through anchor rods. They do provide some guidance that in such case, interaction of Shear and tension (from bending) must be checked.

And, that's where I am stuck now. The saving grace might be to reduce the Longitudinal forces from braking / traction as per elements relative stiffness (RC abutment, soil backfill behind the abutment backwall, Arema 2023 Ch 15, Sec 1.3.12.b), but I have no reference on how to proceed with this.

 

[BridgeSmith]

For anchor bolts that I've seen where the bearing needed to be fixed and not move, but oversized holes were needed for installation, a heavy plate washer with a standard sized hole was installed over the anchor bolt and welded to the masonry plate. Not sure how or if that that would impact the design for bending.

Btw, what grade of anchor rods are you using?

 

[Enjoy learning_not working]

Oh boy, I am already at Grade 105 (F1554) for the anchor rod and can't increase the capacity any further, if that's what you were trying to ask

 

[BridgeSmith]

Oh boy, I am already at Grade 105 (F1554) for the anchor rod and can't increase the capacity any further, if that's what you were trying to ask

Yeah, darn it!

 

[Lomarandil]

Yeah, the bolt bending is a killer.

If you can't weld washers, can you oversize the holes even more (or provide a port from the bottom) and inject epoxy or grout to get positive shear engagement at all anchors?

I agree with Rod, you ought to be able to count on some shear transfer through the neoprene at a level low enough to have no risk of slip or loss of integrity. Might be just enough to get you over an edge.