Reference Needed: Proper detailing of concrete wall 4

[HeavyCivil]

I have always thought that proper detailing of a reinforced concrete wall with two layers of vertical reinforcing was for the hooked portion of the dowels in the footing to point towards one another.

However, I have seen this detailed the opposite more often than not and my supervisor and senior designers here show it as such.

Does anyone have a reference supporting the idea that detailing bars as shown in the attached sketch is indeed the proper way to do it? I think I may have seen this in ACI SP55 or the "Detailing Corner" section of their monthly publication, neither of which I have at my current company.

Thanks.

 

[csd72]

I would also detail them legs facing outward though I cannot think of any strong reasons to do it either way.

 

[HeavyCivil]

Also -

Who details shear keys? I've never seen the need if you can prove that the bar takes the shear.

 

[dik]

If I use two layers of dowels, I detail the bars so hooks overlap, ie. they 'criss-cross'. This is, I believe, the recommended method from one of the reinforcing steel placing institutes (don't recall which one, but will try to dig up the paper). In this fashion, the bar that is stressed in tension benefits from the compression in the opposite face, 'squeezing' down on the bar in tension.

Dik

 

[dik]

VTEIT...

I sometimes use shear keys, if there is no slab or whatever to resist lateral loading... often happens with retaining walls, or the slab will be placed at a much later date, etc.

I split a 2x4 or whatever and cut it on a slope in the middle, I then rotate the members so that I have a key with sloped edges and a space in the middle to accommodate any dowels, or if doubly reinforced, trim off one edge (and cut down the middle) of a 2x? member to permit removal. The dowels are then placed at the outside edges of the 2x?.

I often use a bentonite rod in the keyway to prevent water passage.

Dik

 

[Teguci]

The "ACI Detailing Manual" shows the reinforcement detailed both ways.
- For small strip footings under walls with 2-layers of rebar, the manual shows outward bound hooks (I typically read this as pin joints).
- For wall to mat connections, the manual shows the hooks towards each other (these details are usually for water retaining walls). In addition, the hooks continue and form the bottom layer of rebar in the mat (I've never done this).

I think either way is fine, just note the following:
1. Hooks towards each other will add to local congestion for tightly spaced wall rebar.
2. Outward bound hooks do not pass through the compression zone of the wall during overturning. Therefore, Ldh is critical.

Additionally, I like to detail a bar along the inside bend of a hooked bar.

 

[JoshPlumSE]

When I was starting out 10 or 15 years ago, I had the same question about footing pedestal connections. My take on it:

1) If you bend the bars towards each other, that provides for bettter "confinement". This is a real consideration for seismic design when that connection must take significant moment and have significant ductility.

2) The construction guys in the field will very much prefer that the bars be bent away from each other. It's just a lot easier to build.

Since my college professors were seismic guys, I started out detailing everything with the bends towards each other.... One of my first supervisors put a stop to that.

Josh

 

[HeavyCivil]

Thanks for the feedback. Since Trans bars in footings will be their prior to tying dowels I don't see the 'inward' detail as much more difficult to tie. The only thing is one dowel is on one side of the Trans bar and the other dowel is on the other side.

Good points on this detail placing the hook in the compression area of the footing - that had not occurred to me.

This will probably get red-lined back to 'bars facing outward' but now I do have some justification here and for my piers which carry a substantial base moment. If anyone can find a document to support that would still be appreciated.

 

[dik]

If you have lots of them and dwls required for each face, use 'U' bars... cut down on the congestion as well as rebar length...

I've started (couple of years ago) using these for corner bars...

Dik

 

[HeavyCivil]

I do the same for corners. Works great for 90's... pretty tough to achieve with a 4 way intersection as I've found out...

 

[hokie66]

U-bars are the best option, as they better develop the moment capacity of the joint. Refer to work by Nillson. U-bars are standard in European construction. If two separate bars, it doesn't matter which way they are placed.

 

[TXStructural]

What is the purpose of the bars? If they are dowels and are not going to be taking axial force (like a cantilever wall or other requiring a fixed end) then it would matter little. The practical consideration is usually whether the bar bend radius clears the inside corner with proper cover.

Typically, a wall-footing connection is designed as pin connected, since a strip footing is usually a gravity device (in cross section). If there is uplift, and the footing is designed properly, it shouldn't matter which way they turn.

For a wall where moment connection is required, you should treat it as a cantilever wall, placing bars at the bottom of the footing and turning them across the wall, developing them into the opposite end of the footing. Detailing should conform to the CRSI Design Handbook or ACI/CRSI 315 (they should show the same information.)

Continuous U-bars make placement difficult in taller walls, if the verticals are not spliced. Practice is often to form one side of the wall and then place the bars in layers as they move away from that form. Also, bending tolerances in u-bars may cause encroachment into cover (this can be alleviated by angling the U until the proper wall thickness is reached.)

I do not see this among our Detailing Corner articles or Tech Notes, which are available free on the crsi.org website.

 

[msquared48]

I've always bent the bars away from each other.

For smaller strip footings, the legs of these bars can serve as extra transverse reinforcing to the footing.

If bent out, for uplift, they can effectively serve to pick up more of the footing without additional steel.

Mike McCann
MMC Engineering

 

[dik]

TXS...
If providing dwls on both sides, then U bars work fine and you may want to add the detail to your list. This is sometimes required for tunnel construction, etc. U bars have the same concrete cover spacing problems as a double line, except you only have to concern yourself once... not two lines to be off.

I often use a single layer of dwls in the middle and these are commonly spaced at twice the vertical wall reinforcing spacing.

Mike...
For small footings, I often don't use any reinforcing in the footing (even for commercial/industrial), just treat them as plain concrete. In most cases, as noted, there is not a moment to deal with. For larger footings, normally with large cantilever retaining walls, I wrap the dowels underneath the wall.

 

[TXStructural]

dik, the issue of using U-bars is strictly for constructability. When used solely in short dowels, they present no problem. The problem comes when the U is long, as required for large diameter bars for lap, or to allow unspliced verticals.

As you mention, since the connection is essentially pinned at most footings, a single row of dowels should be sufficient.

 

[OzEng80]

Where i'm from its common practice to have hooks at the top of wall starters/dowels (for development and WH&S - impaling protection). As these would be bending inwards towards each other it lends itself to having the feet pointing towards each other (or U bars) as you have shown - easier bend than a 'z' bar type arrangement.

 

[HeavyCivil]

OzEng08 - I'm curious if the contractor would get still get an OSHA fine for not capping those hooked bars!

 

[OzEng80]

VTEIT - they seem to get away without them. I personally make an effort not to fall on any pieces of protruding steel be it hooked, capped or sharpened!