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?

 

[BAretired]

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

 

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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."

 

[BAretired]

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

 

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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).

 

[BAretired]

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

 

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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?

 

[BAretired]

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

 

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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?

 

[BAretired]

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

BA

 

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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.

 

[BAretired]

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

If you want more, try the search function.

BA

 

[BAretired]

Here are a couple more that may be of interest:

thread507-235070
thread507-407547

BA

 

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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).

 

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This is kookt attachment which fails to appear above.

 

[BridgeSmith]

"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.

 

[BAretired]

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 45[sup]o[/sup] experience either pure tension or pure compression. Concrete is weak in tension, so it tends to crack on the tension plane.

BA

 

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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?

 

[BridgeSmith]

"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).

 

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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/(ASCE)ST.1943-541X.0001995

 

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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.