column reinforcement - crank on top or bottom? 1

[BauTomTom]

Hi guys

I would like to ask you if it does make a difference by column reinforcement when you crank the bars on top or on bottom. See the attached sketch, this expalins it better what I mean.

I see in books both cases but what is the better solution?

And which levels whoul you actually give in the bending schedule. Top and bottom floor or rather beam or suffit level?

Tom Tom


Your file's link is:

http://files.engineering.com/getfile.aspx?folder=15b55a31-41fb-443c-9fdb-3022d0d0d78e&file=crank.jpg

 

[LiloSA]

As you said, both ways can be used. I prefer the lower bars being cranked into a position inside the upper bars (bottom crank according to your sketch).

The crank is then located in the beam/slab and not in the column. Also, it helps a bit in reducing the clashing with beam reinforcement.

Different strokes for different folks.

 

[BauTomTom]

to LiloSA

Ja I would also say but I have a case where I actually need to specifie the other case, where the crank is in the upper bars. My problem is that am afraid that the crank actually creats a kind of a weak spot. Just imagine this bar woudl be in compression and not fixed then it would buckel out at the bottom crank

Tom

 

[JAE]

In the US it has been typical to see the lap splice occur just above the floor, with the 1:6 slope happening essentially in the floor depth.

This method has a couple of problems:
1. The sloping bars are within the floor space and thus tend to interfere more with the beam or slab reinforcement that occurs, usually quite heavily, in this depth.

2. The maximum moment in a concrete column is at the top and bottom of the column, with cross-over moment curves producing zero moment somewhere within the column length - close to mid-height. This means that if you splice the bars just above the floor, with the 1:6 sloping upward, inward to lap with straight upper bars, your column "d" is reduced at the floor line, just where you need it most.

Therefore, I tend to like crank the upper bars into the lower bars such that the lower bars come through the floor straight and vertical, keeping you "d" where its at, and have the upper bars crank into the lower bars.

 

[LiloSA]

It'll be fine - just make sure the crank is max 1:10 and it's lap with the bottom bar is sufficient as per relevant code.

The moment will be taken down by the crank bar and transferred into the bottom bar if you create a slow bend for the crank and allow enough lap length.

 

[JAE]

Do remember that where the crank begins (i.e. where the vertical bars just begin to slope inward, you have to add additional ties per the code. This bending in the vertical bars creates additional outward bursting forces in the bars.

 

[BauTomTom]

to JAE

so you mean in the grafik the right side solution is the best for the strenght and the Moment transfer?

but why is then the left on more common?

http://files.engineering.com/getfile.aspx?folder=15b55a31-41fb-443c-9fdb-30

Do I really need to add more links at the crank, even if it is very flat for example 1:10 ?

Then it should be actually specified in the drawing or bending schedule as well. Somehow I never noticed it hhmm....

 

[BauTomTom]

what about to take the bars of the column and make them double so long and cover two floors with it. In case you woul splice them with mechanical couplers so you would need only half of them, half fixing work and good force transfer

Tom

 

[JAE]

BTT: Yes - the right detail is the one which gives more strength and stiffness at the base of the column.

On left detail, your "d" value (distance from compression face of column to vertical bar is = col. dimension minus cover minus one bar diameter, minus 1/2 bar diameter.

On the right detail, your "d" value = col. dimension minus cover minus 1/2 bar diameter.

Double story bars have been done before but you'd better check with the contractor as these would have to be laterally braced during the lower column concrete placement since the bars would be loosely dangling above the formwork.

Yes - extra ties are required - See 7.8.1.3. if you use a 1:10 vs. a 1:6, the amount of extra ties will diminish.

 

[hokie66]

I'd like to say something, but JAE has said it all. Just because a solution is the more common way of doing it doesn't make it the best.

 

[BauTomTom]

TO HOKIE66:

yes yes this is totally right but I just wanted to find out why is it more common? Are there some reasons which I oversaw?


TO JAE:

first of all thanks, I really appreciate our discussion. You refer to…..- extra ties are required - See 7.8.1.3. ….. which code?

I had a look now in different codes and the what I found is, that the EC nothing about the extra ties says, the SABS just a double link wants and the BS only says additional links but not how many. But at least it gives the area where to place them max 8xdia up and down from the crank. See attached graphic “crank 2”

To be honest makes actually the left solution ind the first graphic for me more sense in terms of bigger “d”. Your loads are coming from the top of the column and will be transferred till the slab nicely with the constant and big ‘”d” somehow I don’t get the idea hhmmm……
It would be really nice to see this two cases in a FE program how the loads behave isn’t it?

Tom

 

[hokie66]

I think it is more common in the US to crank the lower bars "because that is the way it done around here". In Australia, the Concrete Institute of Australia publishes a handbook on reinforcement detailing which shows the upper bars cranked, so that way is more common here.

The number and size of ties required where the bar cranks depends on the force in the bar. Some design offices provide a table on the drawings showing the ties required for all the column bar sizes, but I prefer the simpler and less risky approach of showing a detail for the worst case.

Look again at the two approaches, and read JAE's description again. With straight bars from below, the full d is available right at the floor level, while at the top of the splice, the d is less, and the reverse is the case with bent bars from below.

FE analysis of the joint? Why? It is simple statics.

 

[JAE]

BTT - sorry - my US-centric mind blew it. I was referring to the ACI 318 code for the extra ties.

The moment "curve" in a column, in a building with lateral forces, is a "bow tie" type of diagram with the maximum moments at the top and bottom of the column and the minimum (0) moment somewhere near the mid-height of the column.

Thus, by using your right diagram, the bars at both the top and bottom of the floor strucuture (where moments are maximum) are placed at a full "d".

 

[ishvaaag]

I agree with JAE in the right option being the one that provides more moment strength where a higher value of required moment to be counteracted usually develops, the root at the bottom of the column. I think the reason because we invariably see (at least here in Spain) the other detail is constructive. There is a moment where the upper rebar cage has to stand afoot alone without any other support (typically) than itself and the bigger arm between legs provides for better stability.

As well, at least here the detail is so common that no book or code doubts its validity; hence we might assume there is safety enough in the code to deal with this usual loss of mechanical arm.

Note also that the detail must be somewhat improved by keeping all bars to be overlapped, both of the upper and bottom columns, at the same planes at the faces, in svastika-like setup; this will keep the mechanical arms untouched for moments parallel to the faces, yet wil fail to completely deal with such thing for moments asking for bigger reinforcement at corners; in more than creating an asymmetrical setup, generally hated by constructive minds except by intent.

 

[BauTomTom]

TO hokie66:

Australia? hhmm.. What confuses me is that I spoke to some german engineer and he told me that the german DIN code even vorbits to provide the crank by the upper bars. So now am totaly confuse

number of stirrups depending on the forces this is true but how to calculate this now?



TO JAE:

ok ACI 318 hhmmmm iwill tray to find it in the internet

ja it makes sensse with the max moments on top and bottom and the "full d"


TO ishvaaag:

so you say in Spain the upper bars beeing cranked is common? do you have maybe some site photos?



and what with the idea to skip the problem and provide double longer bars which will cover up two floors
and expirience with this? this would mean that the column bar will be around 6m long

Tom

 

[hokie66]

BTT,
If you want to calculate the force, just use the yield force of a given bar size as the axial force, then provide a horizontal force to counterract the offset. Truss analogy. Double storey column bars are quite common in Australia in high rise construction, generally 8 metres long or a bit more. Bracing for safety is an issue, but sometimes contractors prefer it.

 

[TXStructural]

US practice is 1:6 maximum offset of lower bars, offset which completes 2" from the top of beam/slab, and for construction efficiency, the upper bars are designed to rest on top of slab/beam. (See ACI 315 / CRSI Detailing Manual for details.)

In the western US, it is common to have 60 foot (18 meter) long column cages erected in one piece and guyed to stabilize them. Fewer splices, less weight, and less cost, if the contractor is capable. Mechanical splices are excellent if you need maximum strength in a region where splices are needed.

I could see how offsetting the upper bars would be easier, but changing from an industry standard risks unnecessary construction errors. Please stay with the practices for your region.

 

[JAE]

I've offset the upper bars in many projects in Texas and in the midwest. As long as you discuss this with the contractor and detailer - I've had no problems with reversing these.

TXStructural, I think you are correct that most projects in the US have the lower bars offset rather than the upper bars.

But once you show them that the congestion from the diagonal offset bars in the beam zone is eliminated, most prefer it.

 

[ishvaaag]

BauTomTom, I was referring to the upper column having its bars straight at the bottom, this is the common practice, the other is almost unseen. As TXStructural notes, here the practice is also to go for 1:6 máx within the slab depth (or column depth if exceeding the slab depth etc) and the upper colum rebar resting on the upper surface of the structural floor level.

I must correct myself and precise that when describing the "svastika-like" trick this obviously is not going to work to preserve the maximum depth in one of the two corners.

But you can still with this idea preserve such maximum depth by making the overlap happen in the same plane normal to the one that holds the moment for beamcolumns with the flexure predominantly in one plane. In such case be the overlap either inwards or outwards, you can keep the the notional distance between opposed rebars as mechanically are being considered. It is surmised obviously that building such way the rebar the distances between them are still aceptable.

The svastika-like pattern of overlap may still be interesting when on xx and yy bending moments are present for some depth gain at the critical point.

 

[BauTomTom]

TO HOKIE66:
....then provide a horizontal force to counteract the offset.... exactly there am not sure how to do it correctly


TO TXSTRUCTURAL:
18meter cages? guys with cables? fixed to the shuttering? but how do you pour concrete then in? Also in one lift?

but why staying with the practices of the region if there are more efficient solutions?

TO JAE:
lower bars offset? so you mean the crank in the lower bars? in the beam or slab? ja this is also here in southern Africa the case and was also in Germany the case.
I actually really newer sow the upper bars cranked. maybe I should google it to see it at least on photos

does it really make such an big difference regarding congestion in the beam? you could still go little bit higher and have the crank in the slab.

TO ALL:
Did you guys ever used mechanical couplers to rebar slicing? I never did and never saw it but am kind of forced to use it now because the steel/concrete ration will be to high by some slim columns in the overlapping zone.

BauTomTom