×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Related Articles

Jobs

Girder Anchorage Hook forces
9

Girder Anchorage Hook forces

Girder Anchorage Hook forces

(OP)
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?

RE: Girder Anchorage Hook forces

Those top bars are nominal, as the secondary beams are normally designed as pinned at that end. I like to place the hooks inside the main girder top bars, and that helps with the spalling you are concerned with. That spalling is generally due to inadequate cover rather than bending of the bar.

RE: Girder Anchorage Hook forces

(OP)
BAretired stated somewhere that "If the joint is pinned, none of the members can have any moment at that joint. If the joint is fixed, none of the members can have any rotation at that joint. If the joint is free to rotate, then all members meeting at that joint will rotate by the same amount." A secondary beam is moment connected to the girder owing to the secondary top and bottom bars so it can't be pinned. So why do you model it as pinned?

RE: Girder Anchorage Hook forces

Because it is safe to do so. We don't always match design and analysis. The top bars are necessary, as there is some moment, but standard practice can deal with that moment.

RE: Girder Anchorage Hook forces

Certainly not. The beam is continuous through the girder, so its maximum moment is there.

RE: Girder Anchorage Hook forces

(OP)
The reason the central girder with beam continuous through it has maximum moment is because it is much more rigid, right?

And the secondary beam framing into end girder has small moment because the edge girder can twist? If so, how do you prevent the twist in the edge girder to create more moments in the secondary beam framing into the end girder?

Just confirming because the books didn't state the above clearly. Thanks very much.

RE: Girder Anchorage Hook forces

Not necessary to prevent the edge girder from twisting, but you should have appropriate torsional reinforcement in all spandrel beams. The twist of the edge girder will be restrained by the stiffness of the supported beam. I don't know why you would want to "create more moment in the secondary beam" at that point.

The maximum moment I addressed was in the secondary beam, because it is continuous at that point, so tension in the top. The supporting girder also has its maximum moment there, but tension in the bottom. And depending on the stiffness of the two elements, there may also be deflection compatibility issues.

You can't learn everything from a book. You need an experienced mentor, which I hope you have.

RE: Girder Anchorage Hook forces

(OP)
Just verifying and double checking things the mentor says and things he didnt say. And he is not perfect... even mentors are learning. He is not so familiar with settlement behaviors in columns and beams. If the columns supporting the edge girder settles.. and there is big deflection... most of the loads can transfer to the central girder... how do you handle this? My mentor doesn't want to think of this as he has no experiences about settlements. Do you think beams directly connected to columns would have less drastic loading transfer to the unsettled columns than edge girders settlement issues? Any experiences?

RE: Girder Anchorage Hook forces

Lots of problems in the superstructure can arise from settlement of the footings. The key is to ensure uniform settlement, and that generally requires involvement of a geotechnical engineer.

RE: Girder Anchorage Hook forces

(OP)
My mentor was tied up designing a high rise now and has no time for theoretical discussions.. but I just wanna know.. see attached etabs output of the secondary beams on edge girders and central girders. The secondary beams on central girders have obviously huge moments compared to the edge secondary beams. Short of extending the beam at the secondary beam on edge girders. What kind of modification can you make to the edge girders to make the moment maximum too and equal to the central? Would a beam made of diamond do that? What kind of restrain (theroetical and hypothetical) should you do to create maximum moment at the edge secondary beams? This is just to learn the concept of controlling moments at will and not for any practical design.

RE: Girder Anchorage Hook forces

(OP)

Quote:

The maximum moment I addressed was in the secondary beam, because it is continuous at that point, so tension in the top

This is the part I need clarification.. there is maximum moment at the secondary beam framing into central girder because it is continuous and tension in the top as you said. But at the edge beam. You also have tension at the top. So why is the moment almost minimum at edge. I reasoned it was because the edge girder twist but make the secondary beam almost pinned.. but you said the twist of the edge girder will be restrained by the stiffness of the supported beam. But why is the moment of the secondary beam at edge girder minimum? I tried asking this from my mentor and he said it is because the program outputs it that way. I want to know the theories behind it which most designers who relied on software just took for granted nowadays.

RE: Girder Anchorage Hook forces

2
If most designers just take computer outputs for granted nowadays, that is a sad commentary on our profession. I won't comment on your model, but follow the stiffness.

RE: Girder Anchorage Hook forces

(OP)
I just want to know why is the edge girder less stiff than the central girder? couldn't they be made similar in stiffness? if you were to make them identically stiff.. what would you do to the edge girder?

RE: Girder Anchorage Hook forces

(OP)
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.

RE: Girder Anchorage Hook forces

2
Hokie is spot on, loads will follow stiffness in indeterminate structures. Your computer model is very good at keeping track of stiffness and will distribute the loads accordingly. However, it might not know some things that make things stiffer than normal or otherwise will be very dumb about the actual construction. Garbage in = garbage out. We pin ends of beams in models to tell the program to "assume this connection has no flexural stiffness" and thus it will transfer loads to stiffer connections. In reality the connection is not pinned but if it sees some load then it reduces load elsewhere. If it sees too much load then (if your system is sufficiently ductile) it will begin to yield, reducing stiffness, and the load gets transferred to the stiffer elements as your idealized model originally predicted.

Thus, for ductile structures; as long as your load path is complete and all loads are accounted for, if all limit states have sufficient capacity then any redistribution of forces due to member or connection stiffness not accounted for will be acceptable and the structure safe as it can accommodate this redistribution by inspection.

For your setup, yes the edge girder can attract moment even though it's idealized as pinned. However, torsion of the edge girder is not as stiff as flexure in your center girder beam. Thus, the majority of the load will follow the stiffer path and go toward the center beam. We conservatively force more load toward this center beam by idealizing the connection of the edge girder and secondary beam as pinned.

Try this, take your pinned ends out of your computer model and compare the moments between the idealized model with pinned ends and the "realistic" model with fixed ends. I suspect you'll find the moment diagrams fairly similar. If you want, design the floor for both conditions (fixed ends and pinned ends); this is called "enveloping the design" and is used to ensure that your system works regardless of what actually happens in your structure.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Girder Anchorage Hook forces

By the way; I applaud you for asking these questions. Your mentor sounds like a successful engineer and it's good to learn from him, but it's equally important to find your own answers and develop your own understanding of why your mentor does what he does.

Quote (OP)

My mentor doesn't want to think of this as he has no experiences about settlements.

Quote (OP)

I tried asking this from my mentor and he said it is because the program outputs it that way.

I'm sure you're paraphrasing but my original mentor was the same way. Smartest engineer I know but he had worked for long enough that he just stopped asking "why?" anymore. If presented with new methods, software, codes, etc. he just accepted things at face value. My mentor had the experience to backup their rote acceptance of whatever the black box spit out but, when I was in your shoes, I also asked "why?" a lot and had to crack a few books to get the answers. This has paid huge dividends in my career.

So, bravo for asking; knowing why the answer is correct is far more important than knowing the correct answer.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Girder Anchorage Hook forces

Quote (OP)

but by themselves I think structures can feel their own stiffness and where there are more stiffness there are more moments

This is true. It generally would only not be true in uncommon cases like when small-deflection theory is invalid or you otherwise get non-linear or non-elastic effects (e.g. seismic).

Quote (OP)

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.

Correct, making the edge girder bigger will attract more moments from the secondary beam. However, this isn't really an advantage. The flexure in the center girder is far stiffer than the torsion and flexure in the edge girder. Thus, it takes far more concrete and steel to "move" the flexure from the center beam to the edge. The end result is even though you distribute the forces more evenly the cost of your structure is greater than before.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Girder Anchorage Hook forces

Excellent summary, TME. Enveloping some designs as you suggest, and closely scrutinizing the results, is a great way for someone less experienced to develop the judgment for when it is or isn't necessary to go through the process.

I don't know how well it would work for a structure like the one under consideration, but I have attempted to quantify where a beam falls between fully rigid and fully flexible by forcing the same deflection on the beam and the supports. It's akin to virtual work (it may be exactly that, but it's been so long since Structural Analysis class, I can't say for sure). I calculated the force to deflect a CIP abutment cap 1/8" with one of the supporting piles missing, and then forced a 1/8" axial deformation on the pile. I used the ratio of the forces required for each deformation to conclude the abutment cap was 95% rigid. Is that a valid way to determine rigidity and adequate justification to ignore the flexible abutment case?

RE: Girder Anchorage Hook forces

Thanks for the help, TME. I am just not patient enough to follow through as you have done.

RE: Girder Anchorage Hook forces

(OP)
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.

RE: Girder Anchorage Hook forces

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.

RE: Girder Anchorage Hook forces

(OP)
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)

RE: Girder Anchorage Hook forces

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

RE: Girder Anchorage Hook forces

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

RE: Girder Anchorage Hook forces

(OP)
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.

RE: Girder Anchorage Hook forces

(OP)

Quote:

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..

RE: Girder Anchorage Hook forces

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?

Quote (OP)

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.

Quote (OP)

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/

RE: Girder Anchorage Hook forces

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.

RE: Girder Anchorage Hook forces

(OP)
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.

RE: Girder Anchorage Hook forces

(OP)
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?

RE: Girder Anchorage Hook forces

Quote (OP)

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.

Quote (OP)

pls. see attached picture..

You need a newer camera; I've seen better photos taken with a potato. afro2

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Girder Anchorage Hook forces

Quote (OP)

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

RE: Girder Anchorage Hook forces

(OP)
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.

RE: Girder Anchorage Hook forces

Quote (OP)

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

RE: Girder Anchorage Hook forces

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.

RE: Girder Anchorage Hook forces

Quote (OP)

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/

RE: Girder Anchorage Hook forces

(OP)
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.

RE: Girder Anchorage Hook forces

(OP)
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?

RE: Girder Anchorage Hook forces

The secondary beam has a large negative moment at the central girder because it is continuous, not because the girder is stiff. At each end, the moment is nearly zero because the beam terminates there and the edge girder can provide very limited torsional resistance.

The edge girder cannot resist a large moment from the beam. Making it as stiff as the central girder won't help either. Why would you want to? There is no benefit in doing so. Much better to reinforce for the moments as they occur naturally.

Your proposed framing plan is not recommended. It would be better to replace the E/W continuous beam with a couple of N/S simple spans...easier to analyze and easier to form.

BA

RE: Girder Anchorage Hook forces

(OP)
But in Aug 2, 13:45 message by TehMightyEngineer.. why did he emphasize it was the central girder and edge girder that dictates the moments... he wrote "For your setup, yes the edge girder can attract moment even though it's idealized as pinned. However, torsion of the edge girder is not as stiff as flexure in your center girder beam. Thus, the majority of the load will follow the stiffer path and go toward the center beam. We conservatively force more load toward this center beam by idealizing the connection of the edge girder and secondary beam as pinned."

RE: Girder Anchorage Hook forces

I don't believe TehMightyEngineer intends to suggest that the central girder is responsible for a high moment in the beam. The peak negative beam moment at the middle support results from continuity of the beam, not from stiffness of the central girder. TehMightyEngineer may want to correct me if he disagrees.

In summary, the torsional stiffness of the edge beam dictates a near zero moment at the ends of the beam whereas the peak moment at the central support results from the continuity of the beam.

BA

RE: Girder Anchorage Hook forces

(OP)
While we await TehMightyEngineer defense. I'd like to know one thing before we conclude this thread. Supposed for sake of discussion (no one would do this), the top bars are cut right at the middle of the secondary beam at the center of the central girder.. would this make it no longer continuous beam and you would have two pinned loading conditions with maximum moment in middle of the pinned beams (imagine the central girder becomes like separately pinned each to the respective edge girder at its side). Or would you still have continuous beam and maximum moment at the cut secondary beam at the central girder (imagine the tension bars are only developed in one half the width of the central girder because it was cut at middle).

RE: Girder Anchorage Hook forces

Simply cutting the top steel on the beam would tend to make the beam act like two simple beams, but would result in undesirable cracking each side of the central girder. Unless the slab reinforcement was also cut, there would still be some continuity of the tee beam.

Casting the central girder as two completely separate members would change the continuous beam into two simple beams, each with small end moments.

BA

RE: Girder Anchorage Hook forces

(OP)
BA. If you have 3 columns instead of girders and continuous beam going to the 3 columns. The edge column would contribute to significant moment such that in etabs frame analysis you have a 45% reaction at edge and 55% reaction at beams in center column. But in actual construction or in your model. Do you just assumed the edge column is pinned and have few moments (instead of what software frame analysis shows?) and most of the reaction goes to the central column?

RE: Girder Anchorage Hook forces

A frame consisting of three columns and a continuous beam would be analyzed by ETABS taking into account the stiffness of each member. It would be wrong to assume a pinned condition at the end columns if the members were rigidly connected together.

BA

RE: Girder Anchorage Hook forces

(OP)
BA. In malls and hotels or even ordinary buildings in my place (see attached picture of an actual mall).. you can often see over 12 meter long secondary beams that is 1 meter deep that span over edge girders (without any central girders). In the malls or buildings in your place, how common is this? If not common (and why?), you mean you would have very thick slabs instead that spans 14 meters x 6 meters?

RE: Girder Anchorage Hook forces

It's pretty common here too. Why do you ask?

BA

RE: Girder Anchorage Hook forces

(OP)
There are no details on secondary beam-to girder behavior in seismic in most books. They are mostly about column-beam joints. Have you seen books that details it? As the secondary beam sway left and right, up and down against the edge girder.. what behavior or failure mode that can occur aside from the common diagonal shear crack. Or just the same failure mode as a column-beam joint? But the anchorage in edge girders are just poorer compared to a column that has restrained top and bottom so there must be different or more adverse behavior in edge girder joints in lateral movement.

RE: Girder Anchorage Hook forces

I live in a place with virtually no seismic activity, so I am not the best person to ask. Using the search function of Eng-Tips, I found a number of threads on this subject. I have not read them all, but this one appears to be of some interest:

thread507-347413: Hanger Stirrups in Beam-to-Girder Joints

If you want more, try the search function.

BA

RE: Girder Anchorage Hook forces

(OP)
I have read all the above threads. Thank you. But my question is how come most books don't details how to properly pin the end of secondary beams to girder. Im not talking about hanger reinforcement which is easy to implement and another issue. I'm talking of the details of the pinning especially the bottom bars. For example. In most structural drawings where the secondary beams are connected to edge girder.. you only have two bottom bars that connects to the edge girder. The drawing has more bottom bars at midspan of the secondary beam to handle big moment at midspan. But in the anchorage. Are 2 bottom bars sufficient? How do you compute for the contribution of the 2 bottom bars in addition to aggregate interlock and compression friction in the vertical component of the interface (i'm not talking of diagonal tension check but vertical shear capacity check per kootk definition.. see attachment).

RE: Girder Anchorage Hook forces

"But my question is how come most books don't details how to properly pin the end of secondary beams to girder."

Because generally there's no reason to create a truly pinned connection. The connection is fairly rigid, but the edge beam doesn't have the torsional stiffness to create a fixed condition for the secondary beam.

"Are 2 bottom bars sufficient?"

Assuming a competent engineer designed it, likely it is sufficient. Except for possible seismic loading, the bottom will generally be in compression.

"(i'm not talking of diagonal tension check but vertical shear capacity check..."

That isn't checked, because monolithic concrete sections do not fail along a vertical failure plane.

RE: Girder Anchorage Hook forces

Quote (HotRod10)

That isn't checked, because monolithic concrete sections do not fail along a vertical failure plane.

In reference to the KootK sketch, HotRod is correct. Concrete does not fail in pure shear. If pure shear exists on one plane, to maintain equilibrium, it also exists on a plane at right angles. Planes at 45o experience either pure tension or pure compression. Concrete is weak in tension, so it tends to crack on the tension plane.

BA

RE: Girder Anchorage Hook forces

(OP)
So what governs in secondary beam-edge girders design is the strength of the edge girder.. meaning if it can carry the load of the secondary beam.. then no problem with the interface. And also you can treat that joint as like the rest of the secondary beam in strength and continuity.. meaning if vertical shear won't occur in any part of the secondary beam.. then it won't occur in the interface. Anyway. What is the heaviest secondary beams to edge girder has anyone ever designed here? (like a 2 meter (about 80 inches) depth beam spanning 20 meters)? Doesn't it make you nervous? Can you at least share the drawing details of the interface?

RE: Girder Anchorage Hook forces

"Can you at least share the drawing details of the interface?"

There is no interface in a monolithic cast section.

"Doesn't it make you nervous?"

Why should any competent engineer be nervous about designing using methods that have many years of success, both in testing and actual construction, using the principles of structural mechanics going back many decades?

"...meaning if vertical shear won't occur in any part of the secondary beam.. then it won't occur in the interface."

Failure can occur anywhere if the load exceeds the capacity, but it will occur along a failure plane dictated by the mechanics of the material. Concrete is strong in compression and shear but weak in tension. It fails in tension, thus what we term as a shear failure of concrete is actually a tension failure, but it occurs along a diagonal line because of the interlock of the materials (primarily aggregate).

RE: Girder Anchorage Hook forces

(OP)
in lateral movement (seismic) where the secondary beam sway back and forth, up and down against the edge girder.. can it goes plastic and how does it behave? Can anyone share technical papers of the behavior of such joint? I saw one..

https://ascelibrary.org/doi/10.1061/%28ASCE%29ST.1...

RE: Girder Anchorage Hook forces

(OP)
I've been googling for an hour about the seismic behavior of secondary beam to edge beam joints and couldn't seem to find any. The above is about beam to column. Does anyone know why there is almost none existent study or research about it. And if there is a few rare studies, can anyone share the valuable reference or site or any discussion about it? Thank you.

RE: Girder Anchorage Hook forces

Contrary to popular belief, google does not have all the answers. The behavior of a joint is a response to the loading applied. Just like any other connection, you have to determine the loads applied and model the behavior according to the configuration and materials that make up the joint. In other words, sometimes you have to do the engineering yourself using basic principles of analysis.

RE: Girder Anchorage Hook forces

(OP)
I mean I want to know to what extend can the beam-girder joint encounter plastic rotations in addition to that occurring in column-beam joints. Or since a column forms the major lateral resistance system and hence the major joint at issue is column-beam joint which has numerous references. What residual lateral movement can occur to the beam-girder joint. I'm seeking research papers about it.

RE: Girder Anchorage Hook forces

That particular joint is no different than any other similarly-configured joint, regardless of the orientation. The plastic deformations have the same limitations as any joint reinforced in a similar way. If plastic deformations are expected, it must be reinforced to remain ductile when a plastic hinge forms and other members must be designed accounting for the reduced stiffness.

RE: Girder Anchorage Hook forces

(OP)
attached is typical behavior of column-beam joint in seismic.. see the distortions profile.. how does the distortion occurs in beam-girder joint? You see. It's not easy to predict.. BA is one of the world leading experts in structural and if he couldn't comment on its seismic performance.. we need seismic experts who research such to give us the dynamical behavior of such beam-girder joint...

RE: Girder Anchorage Hook forces

Since I last looked at this thread, it has morphed into a discussion of seismic joint behavior. And you are right to pay special attention to all joints potentially subject to load reversal in seismic events. This would include beam to girder joints, in which both the top and bottom reinforcement needs to be well anchored.

RE: Girder Anchorage Hook forces

(OP)
While awaiting seismic behavior papers on beam-girder joints. I'd like to clarify the following statement by HotRod10:

"Because generally there's no reason to create a truly pinned connection. The connection is fairly rigid, but the edge beam doesn't have the torsional stiffness to create a fixed condition for the secondary beam.".

In Etabs.. If you moment connect the edge girder and secondary beams.. you have torsion shear failure in the edge girder. But if you release the moments. You don't have torsion failure in the edge girder. So must one release the moment in the edge girder in Etabs or does the program compute the stiffness of the edge girder and use torsion values that can become big if you moment connect the secondary beam and edge girder which should be the default?

RE: Girder Anchorage Hook forces

Quote (BA)

I don't believe TehMightyEngineer intends to suggest that the central girder is responsible for a high moment in the beam. The peak negative beam moment at the middle support results from continuity of the beam, not from stiffness of the central girder. TehMightyEngineer may want to correct me if he disagrees.

Took the weekend off from internet, just getting back to reading this.

Yes, I poorly explained that. The center girder is simply a lever fulcrum point; what really gives you the stiffness is the continuity of the beam. I was trying to use too few words to explain that. You could replace the center beam with a small column and as long as the beam was continuous it would have the same affect.

Quote (OP)

So must one release the moment in the edge girder in Etabs or does the program compute the stiffness of the edge girder and use torsion values that can become big if you moment connect the secondary beam and edge girder which should be the default?

This is a good question. In my understanding these torsional forces are real and should be designed for. However, I'm surprised you're getting a torsional failure; the torsion transferred to the edge girder should be small. Also, remember that the edge girder should be supported laterally for torsion by the slab and thus has a lot more torsional capacity than just the beam alone. Pinning the end of the beam will ensure that the beam is properly designed but this is where enveloping the design comes in handy to ensure that you have sufficient support if the beam connection does not act as pinned.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Girder Anchorage Hook forces

(OP)
I made the beam small and the loadings large and there is huge torsion created at the edge girder. When I moment release it (or pinned it). The torsional failure is gone. Here are the sizes of the members (see attached capture of etabs). The span is 12 meters. The girder and secondary beams are 0.3 width and 0.5 depth. There are slabs present. The is point loads of 100 kN in the 3 meter and 9 meter length of the 12 meters secondary beam. At default Etabs use moment connection between the secondary beam and edge girders. So why is the torsion large (equal to 83.162 (see attachemnt values) whereas if it is moment release, torsion is negligible and torsional failure. Did you expect that even moment connected, the torsion should be small and not 83.162? What do you make of it? Thank you.

RE: Girder Anchorage Hook forces

"In my understanding these torsional forces are real and should be designed for."

I was just about to write that. It is my understanding as well. If you are going to have a moment-resisting connection in the actual structure, it should be modeled as such. If the edge beam fails in torsion in the model, then one of two things is going on: 1) The model is inaccurate in some way, or 2) The edge beam will actually fail in torsion under the loading. Just releasing the moment connection in the model DOES NOT solve the problem, unless you're going to provide a truly pinned connection in the actual structure.

I'll reiterate my previous comment: The model has to reflect the actual behavior of the structure, or it may be worse than worthless.

If the stiffness of the members cannot be accurately modeled, it must be modeled CONSERVATIVELY. It's the responsibility of the engineer to understand the mechanics involved so that they know, for sure, that they've done that. If you're not sure, bring it to someone who has the experience to analyze it properly, then watch and learn. Posting some sketches here does not qualify. As much experience and knowledge as there is here, these guys do not have enough details to do that.

RE: Girder Anchorage Hook forces

"I made the beam small and the loadings large and there is huge torsion created at the edge girder."

Of course. Large deflection of the secondary beam applies large torsion to the edge beam.

"When I moment release it (or pinned it). The torsional failure is gone."

Again, of course. No moment from the secondary beam = no torsion in the edge beam. You solved the problem in the model, but if the connection in the actual structure isn't pinned, the edge beam may actually fail in torsion, assuming the edge beam is modeled accurately.

RE: Girder Anchorage Hook forces

(OP)
Ok. Actually there is a central girder in the etabs model. I just removed it exploring what would happen if the loads suddenly transfered to the 2 edge girders when the central girder is gone (this is used by Navy where there is redundant load paths in case the central girder was blasted). Whatever morale is do not put so much loads into beam and designed it as moment connected to edge girder to address torsion. Can anyone share illustration how to actually create pinned vs moment connected joint in actual construction? I won't attempt any pinned connection but just curious how they differ.

RE: Girder Anchorage Hook forces

Quote (OP)

Can anyone share illustration how to actually create pinned vs moment connected joint in actual construction?

If it's poured monolithic then there's almost no practical way to get a true pin other than by allowing it to flexurally crack and ensuring it's fully supported even cracked. What you should do (generally) is ensure that when you load the assumed pin connection you get a ductile yielding of the top (negative moment) reinforcement which allows the beam to distribute forces as if it was pinned.

If you want true pinned with concrete then my preference is go precast using elastomeric bearing pads; but that's my industry so I'm biased in my preference.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Girder Anchorage Hook forces

(OP)

Quote:

If it's poured monolithic then there's almost no practical way to get a true pin other than by allowing it to flexurally crack and ensuring it's fully supported even cracked. What you should do (generally) is ensure that when you load the assumed pin connection you get a ductile yielding of the top (negative moment) reinforcement which allows the beam to distribute forces as if it was pinned.

Does JAE have the above in mind too about allowing it to flexurally crack or have ductile yielding at the top (negative moment) (both in my mind dangerous) or does he have another pinned method when he confidently stated in

https://www.eng-tips.com/viewthread.cfm?qid=235070

"OK, Clansman, I'll take a crack at a few:
CONCRETE:
I use ACI 318 (you didn't state your own location/code) and within that code the ACI explains that if you have secondary beams framing into a primary edge beam, as long as you design the secondary beams for pinned ends, you can then design the primary beam for a minimum torsion loading instead of a full analysis which would assume fixed ends and calculated torsion.

They even have a couple of 3D sketch-views of two structures showing the difference between a structure which doesn't need the torsional resistance for stability and one that does need the torsional resistance for stability.

For a typical exterior bay of a building, where the interior joists or beams are designed with assumed pinned exterior ends, then the exterior beam's torsional resistance is not theoretically needed for the structural stability of the floor. "

I don't have ACI 318. can anyone share his two 3D sketch views or give more details?

RE: Girder Anchorage Hook forces

I think you may be confusing the design assumptions for the secondary beam with those of the edge beam. What I think he's saying is that if you design the secondary beam as if it's pinned, then the restraint to rotation of the secondary beam provided by the torsional stiffness of the edge beam does not need to be quantified, since the secondary beam would be adequate without it.

If the rotation of the secondary beam is excessive (as in your extreme example where the main center beam disappears), then the torsional deformation capacity of the edge beam would have to be evaluated, and possibly enhanced, to accommodate the rotation imposed on the joint by the secondary beam. Alternately, the secondary beam would have to be hinged.

RE: Girder Anchorage Hook forces

(OP)
If you design the secondary beam as pinned.. it means simply putting more midspan bottom bars for moments or also creating semi yielded top bars at the joint as described by TEH to create truly pinned connection? Can anyone share exactly the chapter and passage in ACI318 so I can look at it directly to resolve what Jae might mean?

RE: Girder Anchorage Hook forces

Quote (OP)

or also creating semi yielded top bars at the joint as described by TEH to create truly pinned connection

It's worth noting two things. One is having the yielding of the top bars is not a true pin, it's just close enough as the ability to transmit torsion will be small relative to other stiffer elements.

Secondly, I believe JAE, HotRod, and myself are all saying the same thing with different words. I agree 100% with hotrod's statement:

Quote (HotRod10)

What I think he's saying is that if you design the secondary beam as if it's pinned, then the restraint to rotation of the secondary beam provided by the torsional stiffness of the edge beam does not need to be quantified, since the secondary beam would be adequate without it.

This is exactly what I mean; by assuming the beam is pinned, and providing only minimal top bars at the beam to edge girder joint, you ensure those top bars yield easily and thus load has to transfer to stiffer elements just as if the joint was pinned. Allowing the actual structure to behave just as how you modeled.

Quote (OP)

I don't have ACI 318.

Wait, what? You're in the US right? Your office doesn't have a copy?

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Girder Anchorage Hook forces

(OP)
I was thinking you had to intentionally yield the top bars before casting the beams but in truth you just used smaller bars to ensure it would yield should loadings or torsion in the edge girder are being engaged. This is a clever idea. Many thanks guys.

But my mentor and his team who have designed over 100 structures never use pinned secondary beams. His reasoning is that construction crews can make mistakes so in the plan he put the same numbers of top bars in the central girder as in the edge girders.. this is to make sure they won't be reversde in actual connection. So in the company's desgiend buildings.. all edge girder are fully moment connected and torsion is controlled by having central girders and make sure the loadings are computed to ensure no torsion exceeded in the edg girder. Last question. How many percentage of engineers (approx) created pinned secondary beams like you guys and how many use the method of my mentor where the edge girder has maximum top bars without thinking of the pinned thing.

About ACI 318.. i was asking what chapters so I can specifically look at it in the library (with different codes) since it is very thick book.

RE: Girder Anchorage Hook forces

Quote (OP)

About ACI 318.. i was asking what chapters so I can specifically look at it in the library (with different codes) since it is very thick book.

Gotcha; I'll reference ACI 318-14. Check out these sections:

  • 9.7.7.1
  • 9.7.7.2
  • 22.7.1.1
  • 22.7.3
  • R22.7.3
  • Especially R22.7.3b
  • 22.7.4

Quote (OP)

Last question. How many percentage of engineers (approx) created pinned secondary beams like you guys and how many use the method of my mentor where the edge girder has maximum top bars without thinking of the pinned thing.

No idea. This would be hard to determine.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Girder Anchorage Hook forces

"How many percentage of engineers (approx) created pinned secondary beams like you guys and how many use the method of my mentor where the edge girder has maximum top bars without thinking of the pinned thing."

I think most probably envelope the design by modeling it both ways:

Pinned to design the secondary beam for positive moment.
Fixed to design the connection for moment and the edge beam for reaction and torsional deformation capacity.

RE: Girder Anchorage Hook forces

(OP)
Thanks for the sections. I'd like to inquire specifically about R22.7.3b. It is stated there that

"(b) The torsional moment can be reduced by redistribution
of internal forces after cracking (22.7.3.2) if the torsion
results from the member twisting to maintain compatibility
of deformations. This type of torsion is referred to
as compatibility torsion.
For this condition, illustrated in Fig. R22.7.3(b), the
torsional stiffness before cracking corresponds to that of
the uncracked section according to St. Venant’s theory.
At torsional cracking, however, a large twist occurs under
an essentially constant torsional moment, resulting in
a large redistribution of forces in the structure (Collins
and Lampert 1973; Hsu and Burton 1974). The cracking
torsional moment under combined shear, moment, and
torsion corresponds to a principal tensile stress somewhat
less than the 4λ f  c used in R22.7.5."

Nowhere is it mentioned about the secondary beam being pinned. So when the torsional cracking occurs in the edge girder.. where would it redistribute the forces? In the edge girder itself? How? If the secondary beam is not pinned (and it doesn't assume it's pinned).. it won't get redistributed there because the default assumption is its moment connected.

Another thing. And this is separate concept from above. What can you say about deflection and torsional stiffness? When I put lighter edge girder in Etabs.. the deflection is more and torsion is minimum yet there is almost zero moment at the joint (even though the setting is full moment connection).. so it seems deflection governs more than torsion stiffness in the edge girder?

RE: Girder Anchorage Hook forces

Quote (OP)

Nowhere is it mentioned about the secondary beam being pinned. So when the torsional cracking occurs in the edge girder.. where would it redistribute the forces? In the edge girder itself? How? If the secondary beam is not pinned (and it doesn't assume it's pinned).. it won't get redistributed there because the default assumption is its moment connected.

The forces get distributed in other members than the edge girder.

Imagine the torsional cracking as a torsional "pin". If the edge beam twists itself apart (but somehow still supports the vertical loads of the supported beams) does the structure still stand up? The answer should be yes; the loads go to all the other members holding up the structure. This can be easily modeled in ETABS. Put a torsional release in your edge girder; you just redistributed the forces and you still have a moment connection between the edge girder and the supported beams.

Quote (OP)

What can you say about deflection and torsional stiffness?

They're mostly unrelated. A deep steel wide flange beam will not deflect much along it's strong axis (high flexural stiffness) but has low torsional stiffness relative to the flexural stiffness. This is why an unbraced W8 floor beam can be stronger than an equal weight W12 floor beam, the lateral-torsional stability of the W12 is low compared to the more compact W8 beam.

Quote (OP)

When I put lighter edge girder in Etabs.. the deflection is more and torsion is minimum yet there is almost zero moment at the joint (even though the setting is full moment connection).. so it seems deflection governs more than torsion stiffness in the edge girder?

This gets back to the earlier comments; your edge girder's relative stiffness is small compared to the continuous span of the floor beam; making the edge girder even lighter makes this disparity even greater.

Try this; make the interior span of the floor beam small (or delete it altogether). Your torsion to the edge girder should significantly increase and now it's the stiffest path for the load to go.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Girder Anchorage Hook forces

(OP)
If the torsional cracking in the edge girder occurs.. won't it engage the torsion stirrups increasing it's fy until yield? Without yielding.. there would be no redistribution of forces.

And you assumed the torsion stirrups have yielded? Assuming it does. Let's imagine you are hanging on tree branch with your arms twisting the front branch.. then the torsion exceeds in the branch and it cracks. Won't your hold of it continue until the branch just breaks? How does redistribution here works?

Quote:

try this; make the interior span of the floor beam small (or delete it altogether). Your torsion to the edge girder should significantly increase and now it's the stiffest path for the load to go

When I removed the secondary beam at Etabs, the slabs are connected to the edge girder by tributary loading and it is just small so the torsion didn't increase. Anyway how do you make torsion release in Etabs?
Appreciated your help so much. Thanks.

RE: Girder Anchorage Hook forces

Quote (Quence)

Nowhere is it mentioned about the secondary beam being pinned. So when the torsional cracking occurs in the edge girder.. where would it redistribute the forces? In the edge girder itself? How? If the secondary beam is not pinned (and it doesn't assume it's pinned).. it won't get redistributed there because the default assumption is its moment connected.

The secondary beam is not pinned. Before torsional cracking, the secondary beam has an end moment equal to 2*Mt where Mt is the torsional moment in the edge girder. After torsional cracking, 2*Mt is suddenly reduced. The reduction is redistributed to the secondary beam, creating an increase in positive and negative moment at the central support (if it exists). If it does not exist, then the positive moment increases by precisely the reduction in 2*Mt if girder cracking occurs simultaneously at both ends of the beam.

BA

RE: Girder Anchorage Hook forces

There is a very good reason why engineers don't rely on torsional moments in indeterminate structures. If the girder suddenly cracks in torsion, the secondary beam has lost its end moments.

When torsional moments are required for stability of the structure, it cannot be ignored. Personally, I don't like to design structures which rely on torsion for stability of the structure.

In the Canadian code, if the torsional moment exceeds 0.25Mcr (cracking moment), even if it just compatibility torsion, then minimal torsional reinforcement must be provided. I believe ACI is the same but I'm not certain.

BA

RE: Girder Anchorage Hook forces

(OP)
BA. You haven't mentioned the torsion stirrups.. if the girder suddenly cracks in torsion, and there are sufficient torsion stirrups.. would the secondary beam lose its end moments?

Let's review the possible scenerios.

1. If the secondary beam end is designed as pinned with weak top bars... the top bars would yield first before the torsion failure in the edge girder? Or would the edge girder failed first in torsion before the top bars in the secondary beam end yielded?

2. If the secondary beam end is designed with many top bars and significantly moment connected.. after torsion failure in the edge girder.. would the secondary beam lose its end moment or is it still moment connected to the failed edge girder.. what would occur then in this case? How would the redistribution occur?

RE: Girder Anchorage Hook forces

Quence, you still don't seem to be able to see the distinction between conservatively designing the secondary beam ignoring the (small) moment restraint provided by the edge beam, and actually providing a pinned connection. It's a typical simplification in such cases to provide a monolithic connection, but ignore the reduction in positive moment in the secondary beam.

RE: Girder Anchorage Hook forces

Quote (Quence)

BA. You haven't mentioned the torsion stirrups.. if the girder suddenly cracks in torsion, and there are sufficient torsion stirrups.. would the secondary beam lose its end moments?

Read your own post which says in part:
At torsional cracking, however, a large twist occurs under an essentially constant torsional moment, resulting in a large redistribution of forces in the structure (Collins and Lampert 1973; Hsu and Burton 1974).

Quote (Quence)

1. If the secondary beam end is designed as pinned with weak top bars... the top bars would yield first before the torsion failure in the edge girder? Or would the edge girder failed first in torsion before the top bars in the secondary beam end yielded?

If the secondary beam is designed as pinned, it makes no difference which fails first. The point is, you cannot rely on the torsional moment at the end of the secondary beam.

Quote:

2. If the secondary beam end is designed with many top bars and significantly moment connected.. after torsion failure in the edge girder.. would the secondary beam lose its end moment or is it still moment connected to the failed edge girder.. what would occur then in this case? How would the redistribution occur?

After torsional cracking, the midpoint of the girder rotates. The beam is still moment connected to the midpoint of the girder, but they both rotate which means the end moment of the beam is reduced. The redistribution would occur by applying a corrective moment to each end of the secondary beam.

BA

RE: Girder Anchorage Hook forces

Quence: If you're struggling with these concepts you're going to really have trouble with P-delta second order effects, prestressing, and high-seismic design.

At this point I feel like we're kind of going around in circles. Not saying anything by it but it probably would be good to take these questions to someone who can discuss these is person. Drawing a few diagrams or otherwise visually explaining these concepts might help you out more than what we can do over a forum.

Either way, keep at it. Eventually things will start clicking into place.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Girder Anchorage Hook forces

(OP)
After reading it over and over again. I understood it already.

For normal residental and not so large span loadings. The difference in positive midspan moments between forced pinned secondary beam and full moment connection is not so large.. so even if you don't make it pinned.. the positive moment is accounted for.

It is only in heavy loadings and long secondary beams that there is big difference.. this is when the torsion stiffness of the edge girder is being engaged at overdrive.. here a pinned secondary beam design would be safer due to possible torsion cracks occuring suddenly in the edge girder.. this is because the torsion stiffness is weak there.
I got the general idea. Many thanks for all of you (BA, TEH, Rodrods, Hokie).

Unfortunately. My mentor team ignore all of this as his computer operators only rely on Etabs output of their 50 storey building and they are designing many buildings simultaneously. My mentor is president of the company and dont spend all time analyzing each design.. the operators don't understand the pinned arguments you guys were explaining or concepts about moment redistribution. The operators said their job is only to find the steel reinforcement in etabs and it is up to the contractor to implement the design. But in another company. I saw the structural drawings as having only 2 top bars in the secondary beam framing into the edge girder.. while in my mentor's company.. it's heavily loaded. So I guess i'll change mentor soon.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close