Bad Bridge Bearing Design Details 3

STrctPono · Apr 9, 2020

I need your guys' help in finding issues with these details. See attached. I have not been around long enough to have seen this design implemented in older concrete bridges or to see how it has performed. From what I hear from the older Engineers, corrosion, maintenance, and replacement have made this bearing type obsolete.

I have a list of issues and want to see if you guys have any more concerns.

Thanks

OSUCivlEng · Apr 10, 2020

That looks typical to bearings I design all the time. It looks like these bearings use elastomeric pads? The polished stainless steel sheet bonded to the bearing plate at the expansion bearing isn't needed if you are using elastomeric bearing pads.

Stainless steel should help with corrosion issues. I would use stainless steel anchor rods along with the bearing plates.

BridgeSmith · Apr 10, 2020

I believe the stainless steel plate is the mating surface to to the TFE (Teflon) sliding surface. I've heard that the friction of these 'low friction' surfaces doesn't stay low in the long-term, due to accumulation of dust between the surfaces. We try to avoid them when possible, opting instead for taller, steel reinforced elastomeric bearings, even if we have to design them by AASHTO Method B, which has higher design limits, but requires more testing and has tighter tolerances on the elastomer properties.

We also hold the anchor bolt nuts 1/2" clear of the sole plate to allow free longitudinal movement of the girder.

Rod Smith, P.E., The artist formerly known as HotRod10

r13 · Apr 10, 2020

The advancement in newer bridge bearing pad has rendered the old Teflon-stainless steel bearing pad design/detail outdate, as the former posses many structural qualities that the latter lacks, as simple as elastic vs linear behaviors. I wouldn't call the old detail "bad" though, as it still has importance in certain industry applications.

Guest090822 · Apr 10, 2020

I've only used that type of bearing for temporary situations during construction.

STrctPono · Apr 10, 2020

Thanks for the input so far. To give a bit more information, this is the current design for a bridge that we are trying to Value Engineer. Not sure if that is a sensitive subject for some.

1. The current design shows hot-dip galvanized sole plate, stainless steel bearing plate, hot-dip galvanized shear plate, and hot-dip galvanized anchor bolts. All welded together. Doesn't that seem strange that they are welding these different types of steel together? Why? Why not use all stainless steel? Due to galvanic corrosion, I can't imagine those anchor bolts are going to last more than 5 years. There is no deck on this bridge so the bearings are unprotected from the environment. Not to mention field welds are to be protected via zinc rich paint. Bah!

2. Anchor bolts are called out as ASTM A490 hot-dip galvanized!? This is hardware galvanizing 101. If they needed the higher strength, why wouldn't they have just gone with F1554 Grade 105 threaded rods? I checked the shear due to seismic and the current anchor bolts are slightly deficient in shear which means even if they were to use stainless steel they would then be severely deficient.

3. As OSUCivlEng mentioned... what is the point of the polished stainless steel sheet if the whole assembly is sitting on a laminated elastomeric bearing pad?

4. Why are they welding the plate washer to the bearing plate at the expansion bearing?

5. I very much dislike using slotted holes as justification for allowing expansion. I'll use it on smaller ancillary items, and I recently did on a small 50ft aluminum dock, but no way would I use that detail on a bridge.

6. Don't even get me started on the eccentric bearing location with the centerline of the drilled shaft cap.

In short, due to the corrosive environment, galvanic action, and poor detailing, I believe this bearing assembly is going to have a very short service life which is going to be very difficult to replace in the future. Jacking the bridge 6 - 10", disconnecting a 42" dia. waterline, busting out all the concrete and coupling on new anchor bolts. Not even sure how they would replace the sole plate. This is going to be an expensive maintenance item for the owner and they are none the wiser.

Am I being irrational here?

r13 · Apr 10, 2020

I think BridgeSmith's response provided reason for use stainless plate on bearing pad, not to repeat here. It puzzles me too why weld the washers to the bearing plate instead of the nuts. It wasn't a drafting mistake as thought originally, as note 2 addressed the touch up galvanization after the field weld. I don't see problem with the slot holes at expansion end. No comments on other concerns.

STrctPono · Apr 10, 2020

retired13, you're right. BridgeSmith did provide reason for this. But do you even need an elastomeric bearing pad with this type of system? Could the pad just as easily be an embed plate on the top of the cap for which the TFE was bonded to? I guess what I am saying is it seems as if they are mixing systems. Either go with a stainless steel and TFE slider bearing or go with a laminated elastomeric bearing. Not sure why both are used.

r13 · Apr 10, 2020

Rod,

Do you agree?

BridgeSmith · Apr 10, 2020

First question after reading the additional information, including the welded plate washer part - is this supposed to be an expansion bearing or fixed? I'm struggling to reconcile how it provides for movement if the plate washer is welded, and if it's fixed, what would be the purpose of the TFE/stainless surface.

The 1" elastomeric pad will only accommodate about 1/2" of lateral movement by internal deformation, and then it has to slip, or it will tear the pad apart. As I said, our elaomeric bearings are typically tall enough to accommodate the relative lateral displacement by deformation of the pad, so no stainless steel of TFE or PTFE is required.

The tricky part for this situation would seem to be how to galvanize the sole plate after it's welded to the embed plate, or how to restore protection to the area of the weld after welding.

Rod Smith, P.E., The artist formerly known as HotRod10

bridgebuster · Apr 10, 2020

An embedded plate field welded to a sole plate sitting on elastomer is common in the northeast. However, they’re mixing systems. PTFE and stainless is one system; elastomer is another.

I see some strange things, PTFE bonded to elastomer; stainless sheet bonded to stainless plate. Section A calls out 1-5/8” elastomer but I only see 1” & 1-1/4”; a single 1” layer is a little to thick by some DOT standards.

490 anchor bolt? Maybe they meant 449?

BridgeSmith · Apr 10, 2020

490 anchor bolt? Maybe they meant 449?

Now that you say that, I realize A490 is indeed not the right spec anchor bolts. A490 for 150 ksi high strength structural bolts, and I don't think they can be galvanized. A449 or F1554 would be correct for anchor bolts.

Rod Smith, P.E., The artist formerly known as HotRod10

STrctPono · Apr 10, 2020

Rod,

Details B1 and C4 show the hinged bearing. Details B2 and C5 show the Expansion Bearing. They do correctly show the PTFE only at the expansion bearing but they incorrectly show the welding of the washer to the bearing plate at the expansion bearing as well. In our location we are supposed to design for a 30 degree temperature drop/rise so I am calculating 0.37 inches of movement at the expansion bearing due to Service I level TU. Our proposed VE bearings are much taller. Shrinkage is strange since this designer is casting concrete around a structural steel wide flange truss. My opinion is the structural steel is going to restraint the shrinkage of the concrete and you will see cracks all along the length of the bridge. Our PT VE girder design will alleviate this. You are correct, they should have just called out F1554 Grade 105 if they wanted the higher strength galvanized rod.

Bridgebuster,

Mixing systems indeed. Thanks for clarifying my suspicions that this was not typical. I didn't even catch the mistake for the 1-5/8" callout on the elastomeric. Trust me, that is pretty par for the course on this design. The plans don't match the specs. The plans don't match themselves. The design is offensive. I know of the designer and they are not bridge guys. They have no idea how to design a concrete girder over 100 ft so their solution was to erect a steel truss and cast concrete around that.

Do you guys have any experience with replacing these types of bearings? Assuming they are replaced in kind. The argument we need to make is that this is a serviceability issue for the owner and that it is going to cost them "X" amount of dollars in so many years due to the unavoidable corrosion issues. I previously mentioned having to jack the bridge a considerable amount to replace the necessary components which would disrupt service to the utilities on the bridge.

BridgeSmith · Apr 10, 2020

If there's any thought of replacing these bearings, I would recommend dispensing with the adhesive bonding the pad to the abutment and provide some other means of keeping the pad in place, which in this case would have to be keeper bars on the sole plate (the other common way is a recess in the bearing seat).

With only 0.37" of expected movement, there is no need for the stainless steel sheet and PTFE topping for the elastomeric. We have done bearing systems with PTFE sheets bonded to the top of elastomeric pads (the elastomeric pad accommodates the rotation), with a polished stainless steel mating surface, but we didn't use a stainless steel sole plate.

I've never seen a concrete-encased steel truss, so I have no idea if this would work, but could the sole plate be welded to the steel girder and galvanized before casting the concrete?

Rod Smith, P.E., The artist formerly known as HotRod10

bridgebuster · Apr 11, 2020

STrctPono - PTFE/stainless/elastomer system is something you would see nowadays on a multi-rotational bridge bearing or a pipe support; Fabreeka makes something similar. The designer should be able to make everything work with elastomer.

One way to replace the proposed bearing is to jack the bridge about 1/8", cut the anchor bolts, cut the sole plate as close to the truss as possible in order to burn off the weld between the embedded plate and the sole plate. An alternate method would be to jack the bridge a considerable amount and try peeling the elastomer off; even then they're not going get a clean enough surface to bond new elastomer. Elastomer can easily have a 50+ year life; PTFE is questionable because it wears out due to movement; dust and debris particles will accelerate the deterioration

Also, if you haven't done so already, point out to the owner that the drawing doesn't call out field welding. If they shop weld everything there's no field adjustment. I know plenty of contractors who, after a job is bid, would be happy to say "oh, you wanted this field welded, no problem, $2000 extra per bearing." [colorface]

Rod - I've seen trusses that had the floorbeams and stringers encased but never the whole magilla. These were built in the early 1900's; typical practice for bridges over railroads in the northeast back then.

STrctPono - kind of strange they can't get a 100' precast concrete beam to work. What sort of bridge is this? Encasement sounds crazy. Couldn't they galvanize or metalize? That would last 75 years.

BridgeSmith · Apr 11, 2020

Yeah Bridgebuster, I was confused as to why a normal, everyday prestressed girder wasn't used here. Bulb tees are very common for applications in this span range.

If they want to replace the bearings without jacking the girders at least 5/8", then the only practical way I know to accomplish that is to provide a bolted keeper bar in front of the bearing and use an unbonded pad. Replacement can be accomplished then by unbolting and removing the keeper bar, lifting the girders an 1/8", sliding the old pad out, and sliding the new one in. However, it's not something we typically are concerned about because, as Bridgebuster pointed out, the elastomeric pads are very durable.

Rod Smith, P.E., The artist formerly known as HotRod10

bridgebuster · Apr 11, 2020

A little off-topic but if you haven't heard AISC is offering free access to 2020 NASCC: The Virtual Steel Conference. On April 22 there's an afternoon session on bridge bearings.

Link

STrctPono · Apr 12, 2020

BridgeSmith,

The project has been bid and the Contractor that we are working for certainly isn't going to take any bearing modifications by the designer lying down. Part of our premise to get this VE accepted is the poor detailing of the bearing design.

I spoke with one of the former DOT Highways Bridge Engineers and he told me that last time our State used this bearing design was in 1978. That bridge had its bearings replaced with elastomeric bearings due to issues they were having with it. But this is not a DOT job. Still, this is a viable argument to use for our case. I too am predicting 50 years on the elastomeric bearing.

Bridgebuster,

Thanks for the replacement scenarios. I think your 2nd scenario is the more likely case since replacement of that sole plate is going to require some more space. With the bearing detailed as is, I plan on telling the Owner that I would predict 10 years of service life out of it before it will need partial or full replacement. I think the galvanic corrosion will make quick work of those anchor bolts. The sole plate might get a few more years of life out of it, but when that goes, it will be expensive.

The bridge is a twin girder 141 ft long utility bridge, no deck, no truck loads. 100 psf LL. They have W12x279 T&B chord members encased in concrete to form a girder! You are right, it is crazy. Our VE girder design is 60% of the weight of the current design. Due to the location, they will not be able to deliver a single piece pre-tensioned girder for this project, so we have designed a 3 piece post-tensioned girder that they will splice on-site. Most firms in my State do not know how to do this. Steel has gotten a very bad stigma by the design community (with good reason). 90% of all buildings and bridges are concrete. The only time the DOT would ever accept a steel bridge design would be if it were a replacement of an existing historic steel bridge.

Thanks for the NASCC link. I will check it out on the 22nd. We do a lot of integral abutment designs so my knowledge of bridge bearings is pretty limited. I will check it out and hopefully learn some new things.

Thank you both for your insight on this subject.

bridgebuster · Apr 12, 2020

Below is a link to an article by the American Galvanizers Association on stainless steel and galvanized steel in contact, if you scroll to the bottom they provide some guidance as to when it's acceptable. One thing they forgot to do in the on-line version was define the terms "S" & "G" in the table.

S = Heavy corrosion of metal in the row
G = No or only slight corrosion of metal in the row

Link

I wanted to post the original article from 1999 (which is a little more detailed) but the file upload isn't working.

r13 · Apr 12, 2020

Very interesting. I think the surfaces separation technic can be applied here.

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