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Hanger Stirrups in Beam-to-Girder Joints

Hanger Stirrups in Beam-to-Girder Joints

Hanger Stirrups in Beam-to-Girder Joints

(OP)

My co designers at office just determine if the girder portion where beam frames into it has sufficient V to resist the shear of the beam. They assume the reaction from the floor beam is more or less uniformly distributed through the depth of the interface between beam and girder. But in references, there is this compression strut forming from beam to lower part of girder which has to be provided for by means of hangers stirrups inside the joint that serve as tension ties to transmit the reaction of the beam to the compression zone of the girder, where it can be equilibrated by diagonal compression struts in the girder. And so these hanger stirrups are supposed to be put inside the joint itself (see illustration provided). Do you put them too, as not all follow it?

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)

No one designs beam-beam joints with internal stirrups here? Maybe most misses the crucial aspect of it. Here are the exact details.

According to the book "Design of Concrete Structures" by Arthur Nilson, David Darwin Charles Dolan (and illustration attached):

"Commonly in concrete construction, secondary floor beams are supported by primary girders, as shown in Fig. 11.13a and b. It is often assumed that the reaction from the floor beam is more or less uniformly distributed through the depth of the interface between beam and girder. This incorrect assumption is perhaps encouraged by the ACI code "Vc+Vs) approach to shear design, which makes use of a nominal average shear stress in the concrete, vc=Vc/Bw d, suggesting a uniform distribution of shear stress through the beam web.

The actual behavior of a diagonally cracked beam, as indicated by tests, is quite different, and the flow of forces can be represented in somewhat simplified form by the truss model of the beam shown in Fig. 11.13c (Ref. 11.7). The main reaction is delivered from beam to girder by a diagonal compression strut mn, which applies its thrust near the bottom of the carrying girder. Failure to provide for this thrust may result in splitting off the concrete at the bottom of the girder followed by collapse of the beam.

Proper detailing of steel in the region of such a joint requires the use of well-anchored "hanger" stirrups in the girder, as shown in Fig 11.13 a and b, to provide for the downward thrust of the compression strut at the end of the beam (Refs. 11.8 and 11.9). These stirrups serve as tension ties to transmit the reaction of the beam to the compression zone of the girder, where it can be equilibrated by diagonal compression struts in the girder. The hanger stirrups, which are required in addition to the normal girder stirrups required for shear, can be designed based on equilibrating part or all of the reaction from the beam, with the hanger stirrups assumed to be stressed to their yield stress fy at the factored load stage".

Is this mentioned in the ACI or Eurocode at all? How many implement it?

RE: Hanger Stirrups in Beam-to-Girder Joints

What you are referencing, I believe, is "Hanger" reinforcement. It is not covered in the code, but should be. Check out MacGregor for more information. He actually provides design guidance.

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)

It's not in the codes and even my colleagues haven't designed internal stirrups in the beam-beam joint. But I noticed hairline cracks underneath it. Please see.

http://img713.imageshack.us/img713/7444/zdz1.jpg

Owing to unfamiliarity with 45 degree compression struts in the joint. Colleagues can't state definitely whether it is lack of internal stirrups. The beam was just under service load and I'm sure it's not overload.

Has anyone noticed cracks like it too? Is it common?

RE: Hanger Stirrups in Beam-to-Girder Joints

I don't think this is really an issue in design or construction. There's been thousands/millions? of beam to beam joints designed without hanger stirrups all over the world and I've yet to hear of or see any shear failure at the joints.

The mechanism for failure that is implied is a rupture through the side of the supporting beam. Usually supporting beams have a lot of longitudinal reinforcement where a shear friction mechanism can develop and avoid a "bite out of the side of the beam" failure.

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)

Quantify "a lot of longitudinal reinforcement". For instance, in midspan of a supporting girder are 3 #3 (3/8") main bars with a beam with 2 lower bars of same size framing into it. Supposed the 2 beam bars only rest on 2 of the girder lower bars. Won't the 2 girder bars reach yielding strength in horizontal orientation or buckle.

You may reason the girder top bars are also connected to the beam framing into it with bent up bars. But the bent up bar will not help to share the load since the load distribution of secondary beam shear on main beam is assumed to be dispersed at 45 degree on either side .

The top bent is usually for bond and anchorage requirement. So how would the 2 lower girder bars support the load of the entire beam and supported slabs?

We usually compute the yielding of a bar in vertical orientation. How about sideways or horizontal. Won't the yielding be the same as vertical oriented?

RE: Hanger Stirrups in Beam-to-Girder Joints

Quote (NeilL286)

It's not in the codes and even my colleagues haven't designed internal stirrups in the beam-beam joint. But I noticed hairline cracks underneath it.

Quote (JAE)

I don't think this is really an issue in design or construction. There's been thousands/millions? of beam to beam joints designed without hanger stirrups all over the world and I've yet to hear of or see any shear failure at the joints.


To quote from the Australian Bridge Code, AS 3600:

Quote:

8.2.11 Suspension reinforcement
If forces are applied to a beam in such a way that hanging action is required, reinforcement or tendons shall be provided to carry all of the forces concerned.
Where, for some situations, suspension reinforcement is required, it shall be in accordance with the procedures given in Appendix D.

If I recall correctly, similar provisions were introduced around 25 years ago, in response to real cracking problems.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Hanger Stirrups in Beam-to-Girder Joints

AS 3600 (Australian Concrete Structures code) on the same subject:

Quote:

8.2.11 Hanging reinforcement
Loads applied to a member other than at the top chord of the member shall be transferred to the top chord, within the load application region, by the provision of hanging reinforcement of area consistent with strut-and-tie modelling.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Hanger Stirrups in Beam-to-Girder Joints

This depends a lot on the geometry of the connection. Arthur Nilson is certainly a respected source, and he is correct in pointing out the potential problem. I agree with JAE that hanger stirrups within the width of the supported beam is not common practice, except perhaps in high seismic areas. Maybe the reason they don't crack in practice is the clamping action of the longitudinal bars in the supporting beam as JAE suggested, although there might be little top reinforcement near midspan. Or maybe the flanges, along with the stirrups just outside the beam, carry the load into the supporting beam. Structures are often smarter than their designers.

RE: Hanger Stirrups in Beam-to-Girder Joints

IDS - I'm not familiar with your code - but those quoted provisions appear to be talking about load applications along the length of a beam - such as a spandrel beam with a ledge for brick veneer where the load along the beam is applied on the lower half of the beam. That doesn't appear to apply to a simple intersecting beam condition. I would agree that for spandrel/ledge load applications that hanger steel is needed along the beam length.

My experience with many beam-beam connections tells me this isn't a problem.

NeilL - I never would use #3 bars in a beam for flexural reinforcement - for main girders carrying other joists or beams you would typically see heavier reinforcement such as #6 to #9 bars (3/4" to 1 1/8" bars).

RE: Hanger Stirrups in Beam-to-Girder Joints

That appears to be more in depth than anything ACI 318 has provided...in fact I don't think that 318 has any reference to hanger stirrups for conditions of beam to girder connections that I'm aware.

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)

I think we missed something. Internal Hanger stirrups are only required if V (shear) at the end of the supported beam is beyond a certain threshold that can cause diagonal cracking. Dolan Et Al mentioned "Hangers will also be unnecessary if the factored beam shear is less than 0.85Vc (as is usually the case for one-way joists, for example), because in such a case diagonal cracks would not form in the supported member. The predictions of the trust model would thus not be valid, and the reaction would be more nearly uniform through the depth."

Another author Mcgregor stated " These provisions can be waived if the shear, Vu2, at the end of the supporeted beam is less than 3*sqrt(fc')bd, because inclined cracking is not fully developed at this shear.

What I wish to know is what is the provision if the supported beam (beam framing into girder) has stirrups and Vs resistance is added to Vc. Would the provisions for hanger stirrups still hold?

In other words. Would compression fan still be form for beam with sufficient stirrups that prevent the diagonal crack widths from opening, or would the mere presence of the diagonal crack widths irregardless of stirrups produce compression fan that dispensed 45 degrees to the girder bottom?

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)
The drawing of the compression fan attached (from Mcgregor presentation of the 1984 Canadian Concrete Code 6-29) illustrated my description above. If there is no compression fan (no diagonal inclined cracks). Would the load still be dispensed 45 degrees toward the bottom of the girder or would the reaction be more or less uniformly distributed through the depth of the interface between beam and girder? This is important because if it is, then if our loads are below the concrete Vc strength, then we can avoid the hanger stirrups. Or would the beam framing into the girder stirrups Vs add to its total V (but then stirrups are said to be functional only after the concrete cracks)?? Would someone be so kind as to clear this up?



or

http://imageshack.us/a/img13/8156/gdyd.jpg

RE: Hanger Stirrups in Beam-to-Girder Joints

A side loaded beam as you are discussing is treated as a bottom loading as most of the shear comes in at or near the bottom. Hanging shear reinforcement is required for the full load in these cases. And it is required by most design codes. It is definitely required by AS3600, BS8110 and Eurocode 2. ACI discusswes the need for inclusiuon of any load applied below the inclined crack for loads applied near supports. It does not mention it otherwise.

Draw your truss analogy diagram withn the load applied at the bottom and you will see why you need it. Same as you do for structural streel members.

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)
Where exactly in ACI is this provision mentioned? Nonetheless, according to fellow poster JAE, millions of buildings are built without any hanger reinforcement in beam-girder joints. Maybe millions of buildings are really in danger? Why is ACI not distinct on this requirement?

How do you differentiate between the compression fan forming in inclined cracks and uncracked beam?

RE: Hanger Stirrups in Beam-to-Girder Joints

NeilL286,

R11.1.3.1.

Remember also that a design code is not a design manual or textbook. Engineers are supposed to understand and apply proper design theory. Codes are supposed to define Limits etc, not teach you how to design.

RE: Hanger Stirrups in Beam-to-Girder Joints

I think this may be a solution in search of a problem. The truss analogy is obvious, but as in many cases of strut-tie theory, there is more than one applicable model. Typical details for stirrup placement show one stirrup in close proximity to the support. In the case in discussion, this stirrup would serve as a tie to "hang" the load nearer to the top of the supporting member.

As JAE pointed out, many structures are standing without purpose designed hanging reinforcement in beam to beam joints, and I don't think there have been reports of in service failures of these joints.

Hanging reinforcement is crucial in some connections, e.g. corbels, halving joints, but in full depth joints like this, I think the required reinforcement is typically there as a matter of course. Needs to be thought about, but usually can be deemed to comply by inspection.

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)
Hookie, please see images of the joint I designed which gave me great concern as it doesn't have the internal hanger stirrups. Earlier in this thread, I posted the hairline crack below the joint. Here is the file photo of exact bar details of the joint. Have you designed anything like it? How does your design differ from it? Do you believe the top 4 bars of the beam framing into the joint can carry load or only for moment requirements with the load dispensed 45 degrees downward? What are others opinions of this (and how do you design such joint)?



or

http://img849.imageshack.us/img849/9636/8mrd.jpg

RE: Hanger Stirrups in Beam-to-Girder Joints

Why is there more top reinforcement than bottom in the supported beam? No, I don't think those bars carry the shear. You have two (four legs) #3 stirrups in close proximity to the girder. Those can act as tension ties in a truss analogy, delivering the load at the top, if they are strong enough.

As to the crack, I couldn't see it in the photo, but will have another look.

RE: Hanger Stirrups in Beam-to-Girder Joints

Looking at your first picture now, those cracks are across the supporting beam on either side of the supported beam? Could be flexural cracks, but hard to say without more information.

RE: Hanger Stirrups in Beam-to-Girder Joints

There must be a compression strut in the truss analogy to the bottom of the main beam as I understand it. This force must be "lifted" somehow. The shear that is being carried by the stirrups in the beam could be justified as being applied at the top and does not require "lifting" as the secondary beam stirrups are already lifting it. The remainder needs to be lifted to the top. Top tension reinforcement cannot do this. Whether the stirrups to do the lifting are within the main beam or in the secondary beam I am not sure. I would have thought a combination would be possible, or increase the number of stirrups in the main beam at the connection, rather than secondary beam stirrups.

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)

Quote:

Looking at your first picture now, those cracks are across the supporting beam on either side of the supported beam? Could be flexural cracks, but hard to say without more information.

Yes, hairline cracks across the supporting beam on either side of the supported beam. If it is flexural cracks. Why form right there at the interface instead of at middle or elsewhere? Have you seen anything like it before in your beam-joint design, Hookie? What cracks have you commonly encountered?

I invited two expert structural engineer colleagues to see the cracks personally and they stated they see it form in 10% of joints they saw and think it's hardless because it is just hairline cracks and don't know exactly the cause. They don't use internal hanger stirrups either in their thousands of projects because of lack of this provision in the code. Therefore I'm appealing to those whose code require this to comment on it so I can decide whether to have I-beam put below the beam welded to metal plates on the columns to support the joint. But this would be very expensive for the owner so need tips and advice on this before taking this drastic retrofit action. Thank you.

RE: Hanger Stirrups in Beam-to-Girder Joints

Yes, hairline cracks across the supporting beam on either side of the supported beam. If it is flexural cracks. Why form right there at the interface instead of at middle or elsewhere? Have you seen anything like it before in your beam-joint design, Hookie? What cracks have you commonly encountered?

I have seen a lot of similar types of cracking at beam-to-girder joints (most of my experience is with PT beams and girders of same depth but different widths). Such cracking occurs across the supporting beam (girder) interface due to the abrupt section prop change, coupled with the stirrup placement.

I am going to guess if your look real close there will be additional cracking on the supporting beam (both sides, away from the supported beam) directly under the next few stirrup locations/s too.

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)

Quote:

I have seen a lot of similar types of cracking at beam-to-girder joints (most of my experience is with PT beams and girders of same depth but different widths). Such cracking occurs across the supporting beam (girder) interface due to the abrupt section prop change, coupled with the stirrup placement.

I am going to guess if your look real close there will be additional cracking on the supporting beam (both sides, away from the supported beam) directly under the next few stirrup locations/s too.

Ingenuity, How does such cracking from your description "Such cracking occurs across the supporting beam (girder) interface due to the abrupt section prop change, coupled with the stirrup placement." actually occur? related to Torsion? Shrinkage from temperature? What is the step by step process where the cracking takes place? What if there is abrupt section prop change? Were you referring to section "properties" change in your "prop", or section "formworks" change? The cracks didn't appear after pouring so it is not the formworks that caused it.

RE: Hanger Stirrups in Beam-to-Girder Joints

Flexural cracks in concrete beams always occur, if they are loaded. Without cracking, the reinforcement does't do much. As Ingenuity advised, flexural cracks often form at beam junctions because there is a stress riser due to the abrupt change in the section properties...the section goes from beam width to "very wide", then back to beam width.

Provided the beams are appropriately sized and reinforced for the loads, the flexural cracking is inconsequential.

I would still like an explanation as to why the supported beam has so much top reinforcement.

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)

The reason the supported beam has so much top reinforcement is just redundancy. It only costs a little for the two extra bars. I know just 2 bars is sufficient as the moment there is little as it is the end joint of a continuous beam.

Going back to the cracks. If is is flexural cracks, why the cracks only occur at either bottom side of the beam-girder joint and not on other parts of the supporting or girder beam. Other structural engineers told me they also see it in their projects too and still a mystery how the isolated crack formed. If someone has come up with the explanation to this big mystery of isolated crack formation just as the picture earlier shows. Let us know. Thanks a whole lot.

RE: Hanger Stirrups in Beam-to-Girder Joints

I thought that Ingenuity and I had explained the location of cracking quite well. But you haven't explained the extra top bars well. They could have been used better elsewhere.

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)
I'll explain the extra top bars in detail. First please look at my layout:



I designed a 2-storey warehouse, the girder at center is 16.4 feet side to side of building and a vertical secondary beam that frames at the center of it. The secondary beam from back girder to center girder is 18 feet and from center girder to front girder 18 feet. Have you designed anything like it?

Now here are the details of the bars at center girder to secondary beam.

There are 4 top bars in secondary beam resting on center girder because of moment demand. To avoid construction error. I use same 4 top bars at end of beam instead of 2 so it is same as center girder. This answer your question.

The center girder bottom bars is 5 pcs of #6 grade 60 as shown in the following file picture.



The top bars is 2 #6 grade 60.
Concrete is 4000 psi. All beams have similar size of 11.8" width and 19.6" depth.
The 4" thick slabs are one-way with retail load requirement of 100 psf.

Have you or others designed anything like it? Based on the bar details. Is the center girder joint stable or strong? Can it just fall? Notice one of the #3 stirrup is a little far from the secondary beam edge. So not only it didn't have internal hanger stirrups but the one outside it is a little far (4" from it).

If you have to design the bars based on the layout and requirements. What could you have done to the joints detailing to ensure the secondary beam won't fall down (from lack of internal hanger stirrups)?

RE: Hanger Stirrups in Beam-to-Girder Joints

Yes, I have designed similar floors, although your solution would not be my first choice. I won't try to check your design, but it won't "just fall". The placement of the reinforcement appears to be typical of industry practice, and I wouldn't be concerned about the hanging forces in this connection.

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)
Hokie66. I shared details of the center girder connection due to the fact I noticed a hairline crack below it yesterday. It just formed this week and gave me the horrors. Prior we were discussing about the beam-girder connection at one end only. This time, there is a crack at the center girder as the file photo in the following show.


or
http://img16.imageshack.us/img16/6186/rt8k.jpg

Have you seen anything like it too? At center girder and not just at the secondary end? I checked the hairline crack and it extends to half of the beam in one side of it (close to the neutral axis).

If you or anyone have time. I give permission for peer review of my design as the lack of internal hanger stirrups gave me huge concern and regrets. Luckily it is not in the ACI codes so can't be accused of not following code that does have the exact provision of internal hanger stirrups.

RE: Hanger Stirrups in Beam-to-Girder Joints

Wow - I think you are over-reacting. "horrors" and "huge concern".

Is the crack near the midspan of the supporting girder? If so this is probably a very VERY common flexural crack in the beam and no concern what-so-ever.

Has the beam even remotely been loaded to its factored level of loads? I doubt it.

RE: Hanger Stirrups in Beam-to-Girder Joints

JAE has it. As I tried to explain earlier, flexural cracks near midspan are to be expected, and the intersection of two beams creates a stress riser due to the reentrant corner which is formed.

No one here is going to do a peer review for you, but the comments of several of us should give you some comfort.

All stirrups in beams are "hanger stirrups", and you have stirrups adjacent to the joint, just not within the joint. I see no problem.

RE: Hanger Stirrups in Beam-to-Girder Joints

(OP)
Thanks for the comfort. Me and my colleagues usually design only maximum of 13 feet beam span connected directly to columns. I didn't have prior experience working with long beam span connected beam to beam. We don't have confidence when beam span go above 13 feet as the deflections, moments, shear and axial loads go up and construction error become possible that escape our inspection.

Flexural cracks are usually in the bottom of the beam only. How often does anyone encounter flexural cracks that reach the neutral axis in the middle depth of the beam. Please see this high resolution close up of the cracks on the side of the girder. Is this typical too?

http://imageshack.us/a/img19/8190/g0rv.jpg

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