Girder Anchorage Hook forces 10

[Quence]

Please see attached image. In airports and open hallways and malls.. you can often see long big secondary beam ends framing into girders. I'm concerned about the details of the anchorage and hooks. This is not often mentioned in structural books. Do you make the hook detail at the edge of the girder? I'd like to know the behavior of the vertical part of the hook.. would the forces be to the left or right? Won't it spall the concrete cover to the left?

 

[Quence]

For pinned connection that has minimal negative moment bars and relying only on lower longitudinal bars and aggregate interlock or in other words.. depends on vertical shear friction only in the beam-girder joint... what is the formula to derive at the shear capacity of the secondary beam framing into the end girder? This is not standard formula that you can find in books because you are talking about shear friction and not longitudinal yield strength.

 

[hokie66]

Shear friction? I don't ascribe to that concept, but I think it is for cold joints, not monolithic. But if you want to use shear friction, only the bars in tension would apply a clamping force, so the top bars.

 

[Quence]

Please see attached illustration. Monolithic has aggregate interlock which is important for loading resistance. But supposed it was cold joint at the beam-girder connection and you rely on the top bars only. What happens if the concrete cover of the girder spalls away.. What is the behavior of the top bars of the secondary beam anchoring into the girder with spalled cover? would it still hold.. this is just focusing on the anchorage and not rusts forming because it would not be done (note: Under no circumstances would I do this.. just for theoretical discussion purpose only to learn of the separate forces of the anchorage)

 

[hokie66]

As I said, I am not a believer in the shear friction theory. Therefore, I won't attempt to answer your question.

 

[Quence]

Im reading this famous thread by our shear friction expert kootk..

https://www.eng-tips.com/viewthread.cfm?qid=372294

 

[hokie66]

I'll concede that title to KootK. Perhaps he will respond to your question.

 

[Quence]

In normal beams, flexural reinforcement takes care of positive and negative moments and it is an established manner of connecting members.. but when you rely on pinned connection that doesn't engage those flexural bars and instead on only non moment related anchorage, then shear friction concept becomes an issue. KootK said he thought about this for several years. So I guess it's not a clear cut concept.

 

[Quence]

Shear friction? I don't ascribe to that concept, but I think it is for cold joints, not monolithic. But if you want to use shear friction, only the bars in tension would apply a clamping force, so the top bars.

By the way, can anyone explain why if you want to use shear friction, only the bars in tension (the top bars) would apply a clamping force.. i mean.. if the top bars are in tension.. they are being pulled apart. And so how can it form a clamping force? And what is the context of "clamping force"? And in pinned connection, top bars don't have moments or no tension.. so how can you have shear friction and pinned connection that relies on top tension bars? Perhaps anyone can draw what hokie66 was describing or can describe using different words.. thanks..

 

[TehMightyEngineer]

I share hokie's comments about shear friction; I've always felt it works but that the way we calculate the capacity for shear friction to be dubious. For example, if you look at AASHTO shear friction values they're way higher as they consider cohesion between two pours. Who's right, ACI or AASHTO?

What is the behavior of the top bars of the secondary beam anchoring into the girder with spalled cover? would it still hold.. this is just focusing on the anchorage and not rusts forming because it would not be done

Possibly, right behind shear friction is dowel shear capacity (think anchors). It's not going to be very strong but may get you there. You're correct that losing the cover (and thus development) of the top bar hurts it's ability to contribute to any shear friction. However, I don't believe we're relying on the top bar for shear friction.

By the way, can anyone explain why if you want to use shear friction, only the bars in tension (the top bars) would apply a clamping force

That's not correct by my understanding. In a flexural joint the bars and region in compression provide the aggregate interlock and the shear friction capacity. The tension force created by shear friction is that the surface is not smooth and thus the joint has to open a bit to slide. This opening of the joint is resisted by the reinforcement in tensions which "clamps" the joint shut and prevents movement.

I'd also defer to KootK as the expert in shear friction.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[hokie66]

I won't argue the shear friction theory, but you need to distinguish between the assumption of a pinned end of a member and the actuality, which is that monolithically cast joints are never truly pinned. Neither are they ever truly fixed, or even truly rigid.

 

[Quence]

Thanks. TME. I got the idea now. I'd like to know how common are beam-to edge girder setups.. is this design avoided in other places? In my place, it is so common to see 12 meters span and 1 meter deep beam ending in edge girder. With no book details about the anchorage.. sometimes it gives me chills to walk below them as we are near faultlines and earthquakes are so common.

 

[Quence]

pls. see attached picture.. it's right in my office building.. you can see 12 meters span beam resting on edge girders.. most buildings have this.. There is no standard in books where the anchorage should be put.. at center between bars in the edge girders or across at edge.. now I want to know if there are any among you that avoid any beam-girder joint.. If yes, then you prefer very thick 10 meter x 10 metes slabs instead?

 

[TehMightyEngineer]

I'd like to know how common are beam-to edge girder setups.. is this design avoided in other places?

I see it all the time in precast but we cheat and use bearing pads and loose connections to get a true pinned connection (which sometimes don't work out). For regular CIP construction you see it often enough in my experience. But, I generally have only designed or evaluated concrete floors in older industrial buildings so my knowledge of the standard practice for modern elevated beam/slab CIP construction is limited to what I see on trade magazines and eng-tips and the like.

In short I don't believe it's avoided where it makes sense. Remember, the diagonal shear should be the controlling shear failure mode for these beams. For seismic loads, the lateral force resisting system is taking the majority of the seismic loads and providing dissipation of those loads, there's some vertical seismic load on the gravity system but it's generally not hard to design around.

pls. see attached picture..

You need a newer camera; I've seen better photos taken with a potato.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[BAretired]

When we looked at structures.. they appeared very stiff to our flesh and blood perspective.. but by themselves I think structures can feel their own stiffness and where there are more stiffness there are more moments.. I think by making the edge girder bigger or brace it.. you attract more moments to the secondary beam framing it.. the advantage is to let it have more or less equal reactions in camparison to the central girder.

The reason the secondary beam moment is higher at the central girder has nothing to do with the stiffness of the central vs edge girders. It has everything to do with the continuity of the secondary beam.

A better framing arrangement would be to replace the continuous secondary beam with two simple spans running at right angles to it (shown in red in the attached).


BA

 

[Quence]

BA. Why does continuity of the secondary beam at central girder leads to higher moment?

TME. KootK said shear friction (which means not cold joint but vertical aggregate interlock, friction and dowel action) needs to be satisfied at every part of the beam. Im just concern specifically right at the beam girder joint whether the joint moving back and forth could create vertical crack and not just diagonal tension crack.

 

[BAretired]

BA. Why does continuity of the secondary beam at central girder leads to higher moment?

A two span continuous beam, uniformly loaded, has maximum moment at the central support and zero moment at the ends. Deflection of the girders changes this slightly, but it is still a two span beam.

BA

 

[hokie66]

Quence,
You need to go back to the basics if you don't understand continuity. This site is no substitute for a good engineering education.

 

[TehMightyEngineer]

KootK said shear friction (which means not cold joint but vertical aggregate interlock, friction and dowel action) needs to be satisfied at every part of the beam.

KootK also mentioned that while it needs to be satisfied at every point it might not need to be checked at every point.

In the end you are on to something. By code there are a required number of bars that have to be developed beyond the joint interface of the beam with the supporting edge beam. IIRC there were a number of seismic failures where they had only a few stubbed bars connecting beams to their supports and these bars would easily withdraw during a seismic event.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[Quence]

TME. Do you agree that during seismic event, there is tendency for the edge beam-girder joint to detach by forming vertical crack right at the secondary beam to edge girder interface (see attached illustration).
Can you give references (books, any codes, etc.) of the required minimum numbers of bars to be developed beyond the joint interface of the beam with the supporting edge beam?
My mentor said since the moment is low at the edge.. he put minimal bars.. but since the shear is still significant. I'm worried about the detachment especially for very big beams interface to edge girders in malls and hotels.

 

[Quence]

BA. Ah. you were describing the 3 point support conditions depicted in the attachment and saying that even if in real structures we don't have pinned supports, the continuous beam moments shape still applies in a certain degree. Thanks for this thought. Well. I was asking about how to create fixed or near fixed end support in real structure. If one can do that. Then you can produce maximum moment at edge girder. Not that I'd do it actual but wanting to know the behavior and what you must do to the connections to make it happens. Here I descrdibed edge girder made of diamond or making it bigger or bracing it and then make full anchorage. When you can get it closed to fixed, then you can produce maximum moment at the edge beam and it would act just like the contnuous beam at central girder, right?