Bridge: Fixed and guided bearing (pryout)

[Kaniou]

Greetings to everyone. First time posting!

I have been asked to design an abutment and along with designing the reinforcement, check the connection of a fixed/guided bearing on the abutment. The aforementioned bearing has to be able to transfer 5000kN (1125kips) of shear force transverse to the bridge axis. I know bearings such as these exist and have found some approximate geometries (diameter, embedment depth, distance of anchors etc.), but looking at the pryout check I cannot seem to make it work.

I feel that reinforcement should be the answer and having searched older eng-tip posts I know a lot of you agree, but neither the European nor the American Codes seem to allow for such a solution to my problem.

Having said all this and since bearings able to transfer this kind of (or even bigger) loading exist, I wonder what am I missing? Is pryout not an issue in such a case and if so, how would this be justified?

Thanks for any input!

 

[KootK]

I feel that reinforcement should be the answer and having searched older eng-tip posts I know a lot of you agree, but neither the European nor the American Codes seem to allow for such a solution to my problem.

What makes you say that? Certainly, ACI 318 in the US allows for rebar to be used to help address anchorage breakout issues.

 

[Kaniou]

What makes you say that? Certainly, ACI 318 in the US allows for rebar to be used to help address anchorage breakout issues.

Nice to meet you KootK and thank you for your answer.

Having taken a look at ACI 318-19, Table 17.5.2, reinforcement can be taken into account instead of Concrete edge breakout resistance (same as the Eurocode). It does not mention anything about being able to use reinforcement in the case of pry-out failure. I have seen that you have made this exact point in the past. I have also seen, that a limit of "heff>(4.5~5)*d" in order to ignore pry-out, was supposed to be included in ACI-318 but I have not seen such a thing.

Am I wrong in the above?

 

[BridgeSmith]

That must be a really big bridge in a high seismic zone...If I may ask how is the abutment resisting the force? Also, has seismic isolation been considered to reduce the lateral demand? I can't imagine the size of the guides required for that force. The last one I did with large lateral seismic load incorporated a sacrificial (controlled capacity) concrete shear key.

Anchoring the bearing into concrete to resist pryout is not my area, but I suggest trying a wider base plate for the bearing to minimize the overturning force.

I'd also suggest getting this moved to the Bridge Engineering forum, but I'm not really familiar with how to do that.

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

 

[Kaniou]

The bridge is indeed in a seismic zone and also has a large curvature. A seismic isolated solution had been considered but was rejected due to more than one reasons.

Indeed shear keys is where my thought process is taking me if I can't justify the ignoring of the pry-out check. The necessary (big) widening of the base plate is what makes me say that I cannot continue with this solution (attached are quick calculations for anyone interested).

I did not know that there is a bridge forum! Searching the list for ones to subscribe to, I did not stumble upon a bridge forum, which was odd.

 

[STrctPono]

No kidding! 1125 kips is no joke. I have never designed for a shear force at a bearing that big so you are already out of my league.

A few general comments. American bridge code (AASHTO) is terrible for anchorage design as they really don't provide any guidance. They essentially default to ACI.

My understanding is that ACI prescribed anchorage reinforcement DOES NOT help you with pryout in shear but does help with breakout in shear.

Your anchorage embedment is close to 21 inches. That is pretty shallow for the bearing size that I am picturing in my head. Why can't you go deeper? Going deeper with your anchorage pretty much starts to negate many of the anchorage failure modes.

I really think you need to be working with the bearing manufacturer on this one. Some bearing manufacturers already have their own specific anchorage details for their bearings and the expected lateral loads. I've seen bolts that thread into couplers that connect to anchorage rebar embedded deep in the abutment. Others have specific shear lugs or sleeves.

 

[jayrod12]

This is outside my experience, but why wouldn't you be able to account for reinforcing in pryout? Provided you've got it detailed such that the pryout failure can't occur? I'm envisioning hairpins that extend down deep enough below that it prevents that piece of concrete from lifting out in combination with ties or something of that ilk to prevent the horizontal movement of the chunk of concrete.

 

[Kaniou]

Going deeper with your anchorage pretty much starts to negate many of the anchorage failure modes.

This was another thought since I have seen that from research it is said that if heff/D>4.5~5 pryout would not be a factor, but I could not justify it by code.

why wouldn't you be able to account for reinforcing in pryout?

Both the American Code and the Eurocode, even though they do not prohibit it clearly, they do not mention that it is allowed even though it's allowed on other checks.

 

[HTURKAK]

5000 kN is a substantial amount of shear force and not sure about the figure is correct.. I live in high seismic zone and the transversal shear force is transferred to pier or abutment with shear keys. I will look to my labtop for past example but the concept is similar to the following picture...If the figure is correct, it is not reasonable to design the abutment for concrete prying..

Some options, for curved bridges could be ;
i= The use of the middle pier monolytic with superstructure,

ii= Provide shear key for piers also ,

iii= consider seismic dampers ,

I will suggest you to post this thread at Bridge Engineering group together with more descriptive info. ( seismic loads, a sketch showing the cured bridge together with spans, pier heights....)

Good lUck..

 

[Kaniou]

Indeed if I were to use shear keys a pryout check would not be necessary. But the first idea was guided bearings so that is what I started with.

Do you know if it is possible to move this thread or I would have to post anew?

Thanks for your wishes..

PS. We could be neighbours.

 

[HTURKAK]

O.K... What is hindering you to use shear keys?.. You can use bearings free to move in two orthogonal directions + shear keys for transversal directions and seismic buffers for longitudinal direction..

Never forget, Murphy's Law : Never Play Leapfrog with a Unicorn..

You are the owner of the post so, you may delete the post and write new thread at bridge group.. bu in this case, the other posts will also disappear and IMO ,this will not be fair..
May be better next time write your bridge threads at bridge group..I will suggest you provide more details to get better responds..

 

[STrctPono]

I too would be interested in seeing a plan view and section of this bridge to see the source of such a large load. A curved bridge will still naturally have an out of plane movement (perpendicular to the abutment face) for everyday service level use which will need to be accounted for in the design and detailing of either the slider bearing or transverse shear keys so as not to lock up the bridge.

HTURKAK, I've never heard that saying before... That's good and I'll have to remember that.

 

[KootK]

Am I wrong in the above?

Nope, you're not wrong. Now I understand the particular nature of your concern. Thanks for clarifying. By the sound of it, your load is too high for this kind of anchorage setup anyhow. I'll set that aside in what follows, however, as I feel that there is still some interesting ground to cover in your original question.

What follows is speculation on my part. I can't point to a code clause to echo my opinion.

1) There are many anchorage situations that are predominately shear. So, rationally, one might allow the shear frustum cracking to develop and resist the load with anchored rebar instead. Check.

2) There are many anchorage situations that are predominately tension. So, rationally, one might allow the tension frustum cracking to develop and resist the load with anchored rebar instead. Check.

3) What if there is significant shear AND tension and one or both of those exceeds the frustum cracking value? Can you crack up in shear but then still assume that your concrete is uncracked for tension? That seems dubious to me, even if the individual failure frustums may be different.

4) If you assume that you're cracked for both tension and shear, can you deal with both of those things with anchored rebar? Maybe... but, again, it's dubious in some situations. Rotational equilibrium of the disengaged concrete frustum would be one concern as shown below.

5) I table all of this because of a unique feature of shear pryout: by definition, it results in a situation where anchors are subject to significant shear and tension simultaneously. I feel that, in such situations, anchorages should be designed without the benefit of reinforcing unless one essentially reverts to a an alternate method like strut and tie that ensures equilibrium and a complete load path. Conspicuously, as you've pointed out, ACI 318 also does not seem to condone the use of reinforcement for pryout.

 

[KootK]

In a way, I feel that pryout failure and reliance on shear reinforcement are mutually exclusive. In that case, there would seem to be a rational argument for using rebar to provide rotational restraint to the concrete mass attempting to pry out. Perhaps it's an omission in ACI or something that simply has yet to be researched convincingly.

 

[Kaniou]

What is hindering you to use shear keys?

Nothing because after I raised my concern to my seniors they hinted that if a solution is not found through the bearing manufacturer, shear keys will be the way to go. Thank you very much for your input HTURKAK.

I too would be interested in seeing a plan view and section of this bridge to see the source of such a large load.

The bridge has multiple spans of around 40m each, with a deck width of more than 15m and a plan radius of less than 80m. I am sorry but as much as I would like to share more details, I do not think I am allowed.

Now I understand the particular nature of your concern. Thanks for clarifying. By the sound of it, your load is too high for this kind of anchorage setup anyhow. I'll set that aside in what follows, however, as I feel that there is still some interesting ground to cover in your original question.

Exactly. I wonder if I am missing something, because I was provided with an approximate sketch of a bearing that I was told could withstand 5000kN and if I were to check pryout failure on it, the concrete fails.

Don't you think that something like the following sketch would theoretically help in preventing a pryout failure?

 

[KootK]

Don't you think that something like the following sketch would theoretically help in preventing a pryout failure?

I don't see it helping the pryout mechanism shown below. The rebar that you've got would be oriented the wrong way, right?

What may well make pryout a moot point the possibility of a substantial vertical point load coming into the failure frustum from the reaction of the bridge deck above. Is there such a vertical point load? If so, I'd expect it to restrain pryout in a similar fashion to how vertical reinforcing would accomplish that.

 

[Kaniou]

I see what you mean. By implying that it is a purely rotational failure the reinforcement I sketched would not really help. Only if used to anchor vertical legs.
No, the vertical loads are transfered through free sliding bearings on each side of the guided bearing. Thank you very much for your input KootK!

 

[BridgeSmith]

the vertical loads are transfered through free sliding bearings on each side of the guided bearing.

That's where the problem comes in. You need the vertical load on the guided bearings to prevent the uplift that's causing it to have pryout forces instead of simple lateral shear.

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

 

[HTURKAK]

Dear Kaniou ,

I looked one of the famous bearing manufacturer to see the range , and the following is the snap for the bearing which can support horizontal loads perpendicular to the direction of movement and vertical loading..

Apparently if you insist for this type of bearing, i suspect the product will be out of standard range and special for this project and surely with special price..

You do not want to share some details .. O.K. I got it.. But we are free to speculate and make guesses;

I guess,
- This is a fly over bridge above a round about or an interchange,
- There are two bearings at each pier/abutment and vertical loading for each bearing in the range of 20000 kN.
- You have calculated the horizontal seismic load 5000 kN in transversal direction but you did not consider uplift that the bearing will experience during the design seismic event..

If these are true, you should think again for the use of shear key and external hold down devices.

If you write (bridge seismic external hold down ) and search , one of the free of charge outcomes;

http://civilwares.free.fr/ACI/MCP04/3412R_97.pdf

Dear Kaniou ; When you decide upon your final solution for the bearing type , please share your solution together with the approx,imate price of the bearings and please keep us involved ...



The original say as far as i know , is for Never Play Leapfrog with a Unicorn RHINO ..

You can be a sport man and jump over the rhino but when the rhino views you as a competitor, can somebody really trust it to bow down and let leap over 6000 lb animal?

In this case, if the seismic horizontal loads are too high to resist, better not to resist with pot bearing . That is the reason i used this say..