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?

 

[BridgeSmith]

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.

 

[Quence]

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.

 

[BridgeSmith]

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.

 

[Quence]

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...

 

[hokie66]

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.

 

[Quence]

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?

 

[TehMightyEngineer]

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.

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/

 

[Quence]

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.

 

[BridgeSmith]

"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.

 

[BridgeSmith]

"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.

 

[Quence]

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.

 

[TehMightyEngineer]

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/

 

[Quence]

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?

 

[BridgeSmith]

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.

 

[Quence]

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?

 

[TehMightyEngineer]

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:

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.

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/

 

[Quence]

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.

 

[TehMightyEngineer]

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:

[ul]
[li]9.7.7.1[/li]
[li]9.7.7.2[/li]
[li]22.7.1.1[/li]
[li]22.7.3[/li]
[li]R22.7.3[/li]
[li]Especially R22.7.3b[/li]
[li]22.7.4[/li]
[/ul]

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/

 

[BridgeSmith]

"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.

 

[Quence]

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?