corner r/f in concrete wall 3

[Structures33]

Which of the two (A or B) is the correct way to detail the interior horizontal bars of a concrete corner/ intersection? Assume that the walls are exposed to lateral on both sides.

I've uploaded a pdf -- CORNER_RF.pdf

Thanks

 

[lkjh345]

Always have done in like 'A'.

 

[DaveAtkins]

Either is acceptable, but I never use the diagonal bars.

DaveAtkins

 

[ron9876]

I agree with Dave. Would only need diagonal if there was a problem with development length for hooks. I haven't used a diagonal before either.

 

[miecz]

Of the two details, I would use "B", and I use diagonal bars for liquid retaining structures.

 

[JAE]

I don't think the direction of the hook is important to developing the hook.

 

[ron9876]

Thats right. I normally show the hooks like B but the direction doesn't matter in my opinion. I guess if the tail is long enough it will dictate the direction.

 

[hokie66]

Doesn't matter at intersections, but I have always used "A" as a typical detail at corners. Often, the wall thickness is not sufficent to allow placement of standard 90 degree cogs within the wall width at corners. But if any of the corner joints are opening joints and are supposed to carry a lot of moment, neither detail is actually very efficient because there is no reinforcing to resist diagonal cracking. Think of the truss analogy, and without diagonal reinforcement from outside to inside corner, the compressive force cannot go around the corner. Hairpins in both directions have been shown by European testing to be best for opening joints, and I think double hairpin corners is common practice there in highly loaded joints.

 

[miecz]

Any comments on the attached detial? It uses hairpins and eliminates the exterior L bar from Type B above.

 

[haynewp]

I remember seeing tests done on walls that included the diagonal bars which showed they added a lot more resistance to the corner opening up than without them.

 

[bigmig]

The diagonal bars will blow out your interior corners if they ever go into tension. The vector addition of the two directions of the bar points right into the corner. We always put the bars as far from the corners as possible.

 

[miecz]

I remember reaing literature on this many years ago, but I've lost it. I hate relying on my increasingly failing memory. If anyone has a reference, I'd like to see it.

bigmig-

I'm not seeing how tension in the diagonal blows out the interior corner. It's force is perpendicular to the corner.

 

[OldPaperMaker]

The twelfth edition of the Aurthur H. Nilson book, "Design of Concrete Structures", on pages 350 - 353, has a section 10.6 Corners and T Joints.It talks about the efficiency of various corner joints subject to bending moments.

 

[haynewp]

bigmig:

"Nilsson and Losberg have shown experimentally that a joint reinforced as shown in Fig 18-41b (with diagonal bar) will develop the needed moment capacity wihtout excessive deformations". (MacGregor). Graphically, it is shown as having 4 times greater joint efficiency than joints without the diagonal bars and only 90 degree hooks.

 

[miecz]

hokie66-

Thanks for the truss analogy. I had never thought of it that way.

 

[hokie66]

miecz,

And thanks to you for attaching the detail. Exactly what I was trying to describe, except I think some of the Europeans use hairpins which are a semicircle at the ends rather than two 90 degree bends. The research I have seen suggests that 80% efficiency can be achieved with this joint, and with the addition of stirrups and diagonal ties, 100% is possible in critical areas. With the details given in the OP, the research says that the joints are only 10 to 30% efficient.

Yes, Nilsson et al is the source. For those in Australia, the information is addressed in Warner, Rangan et al.

 

[msquared48]

If you can get tension across the inside of the joint, I can see the use of the hairpin. Otherwise, I would eliminate them to help relieve possible congestion of the rebar, affecting concrete placement and rock pockets.

I would definitely lean to "A" though to avoid possible congestion from the vertical steel placement at the corner considering lap aplices. I can visualize four vertical bars there as normal reinforcing.

Mike McCann
MMC Engineering

 

[miecz]

I agree that bar congestion is a consideration, but, looking at the sketches, Type "A" looks more congested at the corner to me.

 

[hokie66]

Why do you say that, miecz? The only difference between the two is where the cogs go, and in "A" they go out into the wall, while in "B" they are all together in the corner.

 

[youngstructural]

Depending on if you are facing Seismic detailing concerns, A is definately a no-no for the hook development and cyclical loading.

While I am not an expert, I do undertand the mechanism involves several problems, not least of which is the fact that as you column rotates away from your beam, your hooks can punch out. Also, the compression forces involved serve to further confine and strengthen your hooks in B, while successfully confining A is very difficult since sliding full hoop stirrups over becomes terribly difficult with out-of-beam column joint projecting bars involved.

I have never used diagonal bars and am not certain of their potential effects in an earthquake. Normally the rule is to make your system simple and straight-forward in order to ensure it is easily predictable and critical shear forces can be addressed directly. Bringing the diagonal accross may give rise to an undesirable (and potentially stiffer?) load path, leading to early concrete loss and a reduced strength.

I would be very keen to hear other people's opinions regarding the details in seismic zones.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...