Shear on insert rebar 1

[Italo01]

Hello,

I started recently designing for a company and was asked to design some shear connections between steel beams and precast concrete columns. The connection is very simple, composed of a two plates welded as a T at which are welded weldable rebars to be inserted in the precast columns, as shown in the image below.

I'm used to design only steel so i have no questions about the bolt and plate designs, so my question is about the rebar design, since the rebar is subjected to shear. Is the rebar strength equal to 0,577*Φ*fy*As? Can i use a all round fillet weld to the bar or i must bend the bar to fillet weld? Is there another concert on this type of connection?

I think that the strength of the bar is given by this equation but since i've never designed rebar in this situation, it'd be good to hear your opinions.

Thank you.

 

[ENGIRL]

Are you sure that the rebar is subjected to shear only? Could some of the rebars experience tension caused by the combinasion of shear and eccentricity? if so, I would verify the rebar strength in shear, tension and also verify the concrete cone failure.

 

[KootK]

You'll want to dig into the anchorage provisions of ACI or whatever you local standard is. This stuff has gotten pretty complicated in recent times with all manner of breakout failures to be worried about and such. It might also be fruitful to look into the literature on this in the precast space as then tend to be a bit less conservative with it at times.

 

[Italo01]

Thanks ENGIRL and KootK for the answers.

Are you sure that the rebar is subjected to shear only?

In this case, i can design the bolts or the anchors for the moment created by the eccentricity. I consider best to design the bolts for the eccentricity.

You'll want to dig into the anchorage provisions of ACI or whatever you local standard is.

Yeah, KootK, i'm aware of the ACI provisions but and will consider them but my question is specific about the use of the rebar because the provisions that i know of do not consider it. I had recently a discussion with a professor who showed that the brazilian code doesn't allow the use of the rebar because of the low ductility, so although not necessarily a problem, the use of the rebar is not covered by the code.

It might also be fruitful to look into the literature on this in the precast space as then tend to be a bit less conservative with it at times.

Do you suggest any?

 

[KootK]

Do you suggest any?

The PCI Design Handbook, and it's Canadian counterpart, would be logical places to start. I believe that you can get older editions of PCI for free and the latest edition of CPCA for free. If you don't find what you need there, chase down the many references in those docs.

I had recently a discussion with a professor who showed that the brazilian code doesn't allow the use of the rebar because of the low ductility, so although not necessarily a problem, the use of the rebar is not covered by the code.

The "rebar" used on precast anchorages is usually deformed bar anchor. Deformed bar anchor is weldable and, thus, relatively ductile. Regardless, from a calculation perspective, I don't usually find that deformed bar anchors offer much of an advantage over headed studs so, nowadays, I mostly just use headed studs.

 

[Italo01]

Thanks Kootk.

Just for context, when you say "deformed bar anchor", is it a bar produced specifically for the purpose of anchorage and different from the bars used for concrete reinforcement, or they are the same bar but the word rebar is applicable only in the case of reinforcement?

As for the studs, they are much more expensive than the rebars here. You may find weird but rebars are used a lot for shear connectors on composite beams due to the reduced cost compared to headed studs and this is even more complicated. The code which governs the productions of rebars is called NBR 7480 and the required ductility for these bars is very low.

 

[KootK]

Just for context, when you say "deformed bar anchor", is it a bar produced specifically for the purpose of anchorage and different from the bars used for concrete reinforcement, or they are the same bar but the word rebar is applicable only in the case of reinforcement?

It's a specific produce where the rebar knurls project into the bar rather than out from it so that it can be welded in place with a gun welding machine.

I am surprised that studs are prohibitively expensive in your region. Do you know if the cost is the studs themselves or the machinery used to weld them?

 

[calvinandhobbes10]

Hand calc- I often treat top two anchors in tension and bottom 4 (or even just 2, extra-conserv) in shear. It’s oversimplification of the statics but good envelope check. Use ACI.

Supporting software- Hilti Profis. You can get some results free, subscription is a very cheap must have if you are planning to do concrete anchorage work moving forward. It wont have the DBAs but if you input headed studs instead you’ll get a sense for the stress ratio. And maybe you’ll just want to spec headed studs anyway—-they are better.

My hunch is, if the precast has confining ties and/or reasonable edge distance, your connection will work easily for the type of load a 3-bolt shear tab typically takes.

If load is too high— I have used a double-embed plate connected by studs/rods— sort of like an embedded through bolt. Seems to suit your precast situation.

Good luck

 

[Italo01]

Thanks again, Kootk.

Do you know if the cost is the studs themselves or the machinery used to weld them?

Both. The machinery is very expensive and most companies won't have it, except for big companies which build a lot of tall buildings and there are not much of them, since most tall buildings here are made of concrete. Some companies will use fillet welds for the studs but the cost of the studs will still be much greater (more than double) than the cost of the rebars.

 

[Italo01]

Thanks for the suggestions, calvinandhobbes10.

 

[Italo01]

I have another question about this connection related to the hinge location. For steel connections, we have a lot of freedom to determine the flow of loads, since we guarantee equilibrium and nowhere the structure is above plastic limit, according to the lower bound theorem.

In the software Idea Statica, for example, i have the capability of considering the hinge at the steel-concrete interface and at the bolt line. I think that consider it at the steel-concrete interface is usually better because i have only shear on the anchors and don't have plate bending.

My question is if both approaches are valid or if in this case i must consider the hinge at the bolt line, since the bolts have tolerances which cause some slip and the anchors don't have.

Thanks.

 

[KootK]

1) Personally, I would put the hinge at the bolt line.

2) It is entirely possible that the hinge will actually wind up beyond the bolt line and into the supported member.

3) Even if the hinge were at the steel-concrete interface you would still have tension in the upper parts of the connection (per ENGIRL) because the column concrete resists the applied shear inboard of the steel-concrete interface.

4) The lower bound methodology of steel connections has to be applied judiciously and only where ductility allows effective stress redistribution. In steel only connections, it cannot cause catastrophic buckling en route to the assumed stress distribution. In a connection involving concrete, the method is usually junk because most of your failure modes in the concrete are brittle (tension breakout, prying breakout, etc).

 

[Italo01]

Kootk, you are correct in all you points. Thanks for your help.

 

[KootK]

Kootk, you are correct in all you points.

Really? Are you sure that you don't want to disagree just a little bit?

Thanks for your help.

You're most welcome Italo01.