Fixing / Hanger Question (Rapt)

[KootK]

I recently hijacked this thread Link with a discussion about the need for hanger reinforcement for fixing hardware. It's the last four or five posts. I'd like to continue the discussion here. The last statement in that conversation was:

KootK,

I hope if you have a significant load being applied at the bottom of a concrete, you are providing "hanging reinforcement" to transfer that load to the top of the member. Similar to the way you would in a steel member.

The whole concept of the truss analogy for shear requires this. It assumes that all loads are applied at the top surface and if not then they have to be carried to the top surface using reinforcement. It cannot be carried there by concrete in tension.

@ Rapt:

I agree that the truss analogy for shear requires top surface loading. It makes sense to me and is supported by pretty much every concrete design example that I can remember seeing. I have to confess that I've never actually thought of this before in such explicit terms. Thanks for bringing it to my attention and contributing to my engineering education!

I also agree that, in any given scenario, there is a load above which explicit hanger reinforcement would be required. However, I also believe that there is also a load below which explicit hanger reinforcement would not be required. Below a certain level, I believe that bottom applied loads can be transmitted, via some form of diagonal tension / shear, into pseudo top applied loads. This would also be the case for your steel example. Below a certain load, one could forgo stiffeners altogether. Please review the attached sketch for my thoughts on the concrete version of this.

As fuzzy "evidence", I submit the following to support my claim that bottom applied loads can in fact be carried to the top of a member via concrete in tensin / diagonal tension / shear:

1) I can't remember many instances of a fixing supplier's manual specifying the need for hanger reinforcement. Sometimes they ask for local reinforcement to improve breakout capacity slightly, but not explicit "take it to the other side" hanger reinforcement.

2) Much of the self weight of a concrete beam itself is transferred to the top surface without explicit hanger reinforcement. Certainly this is the case for a beam with no stirrups which, admittedly, may not fit into the truss shear model analogy. When stirrups are present, the argument gets somewhat murkier.


The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[rapt]

KootK,

I did say "significant" loads. What is the definition of significant? I an not sure. That one will require some engineering judgement!

Fixing suppliers are only interested in getting the load into the concrete. They are not interested in member design. In a lot of cases, the direction of the load carried by a fixing is not gravity based so no problem for them. In all other cases, the member design is the responsibility of the designer!

Re the beam loading, technically yes, the bottom half of the beam load is a hanging load. This is possibly covered by some design codes always requiring minimum shear reinforcement (BS and EUR). Other codes like ACI and AS do not require this and therefore designers possibly should consider it as hanging loads, but the half beam load is a small percentage of the full SW load normally with the slab dominating.

The more important one is the case of an upstand beam with the slab at the bottom. Then most of the SW and extra loads are hanging loads and must be accounted for as such.

But for significant attached loads, you cannot rely on concrete tension to transfer the load. Either the fixing needs to extend to and develop at the top (plate bearing above the top reinforcement, of hanger bars must be added and they must overlap with the fixing and the bottom reinforcement to make sure there is no possible tension failure plane). In your diagram with the bottom bar, they would have to be stirrups around that bottom bar.

 

[KootK]

Thanks for jumping back in the pool with me Rapt. I believe that we are in substantial agreement with the exception of your very last sentence. For a substantial load, I do not believe that it would be sufficient to have stirrups and a favourably placed bottom bar. Rather, I think that one would have to do one of two things:

1) Positively attach the fixing to what would effectively be drag strut rebar right from the get go.
2) Create a concrete breakout cone that intersected enough developed stirrups to serve as the hanger bars.

As an interesting aside, I got my start as a metal plate connected wood truss designer. There was a time when the design community "discovered" that connector plates might rip members in half even though the plates could tranfere the required loads between members. It's a similar problem.

Check out the attached sketch for a brief summary of my thoughts here.

KootK

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[rapt]

KootK,

I did mean that the stirrups would be hangers (as that is all I have been talking about) and logically hangers for a specific load have to be at the load, not well away from it! Basically your middle diagram, but the hangers are extra to the shear reinforcement requirements and are not dispersed along the beam. Your third diagram has left out the longitudinal and shear reinforcement. These hangers should hook around the longitudinal reinforcement as stirrups to so all need to be shown. They should not finish above the longitudinal bottom reinforcement.

As I said earlier, the best solution is that the fasteners/bolts actually extend to a plate at the top!

Another place where this is important and it often ignored is at the connection between primary and secondary beams! The secondary beam load needs to be lifted to the top by hanging reinforcement in addition to the shear reinforcement.

 

[rowingengineer]

Looks like I missed all the fun stuff, had been keeping an eye open for the thread too. All I will add is that in all the example given extra reo is required to ensure the concrete isn't required to take tension. Concrete in effect has zero tension capacity in our detailing of structures, so tension reo cannot terminate without thought been given to the final force path (strut tie is very helpful here). Other than reiterating this fact I think you and RAPT have covered the subject sufficiently.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[KootK]

Howdy RE. Yeah, I think that Rapt and I have reached substantial agreement on this one. Do you guys both have code provisions for supported beam hanger reinforcing in your jurisdictions? There were none in the US when used to practice there. I first encountered it when I returned to Canada which does have hanger provisions. I fear for the first few hundred concrete beams that I designed.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[rowingengineer]

Yes, but the wording could be better.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[rapt]

KootK,

Yes, Australian and Eurocode both have hanging shear provisions.

Re improved wording required by RE, he should see how hard it is to get any wording approved in code committees! At least the codes make the need obvious, it is then up to the engineer to understand engineering (which RE does) and be able to research the topic fully. A code cannot be a text book explaining every possibility. That is the engineering part of being an engineer. The problems come when a code does not even point out the need and engineers are not trained properly!

I had a rather heated argument once with a principal in a consulting firm when I insisted that hanging shear reinforcement was required to support secondary beams at a time when it was not in the Australian code (late 70's/early 80's). I won in the end simply because I would not accept its removal and in the end a technical group meeting in that company decided I was correct and company policy was changed. Fortunately it was eventually added to the Australian code.

 

[rowingengineer]

True, I shouldn't be too harsh on the code it is in fairly good shape such that most engineers should be able to design safe structure.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."