Hanger Stirrups in Beam-to-Girder Joints 1

[NeilL286]

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?

 

[hokie66]

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.

 

[rapt]

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.

 

[NeilL286]

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.

 

[Ingenuity]

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.

 

[NeilL286]

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.

 

[hokie66]

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.

 

[NeilL286]

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.

 

[hokie66]

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.

 

[NeilL286]

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)?

 

[hokie66]

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.

 

[NeilL286]

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.

 

[JAE]

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.

 

[hokie66]

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.

 

[NeilL286]

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