Shear Key

[slickdeals]

Folks,
I have seen plenty of details for construction joints, retaining wall bases etc. which shows a shear key with reinforcing running through it.

It appears that using shear friction (even on a smooth formed surface), you would get considerably more capacity than the shear key.

However, the mechanics of dowel action require some surface roughness, but has testing been done on dowel action in perfectly smooth surfaces?

What are the reasons for providing one even if it is not as effective as the reinforcing bars passing through the joint.
My question becomes more pertinent when trying to frame a concrete beam/slab into an existing concrete structure. The effort for roughening/installing shear keys are quite significant.

 

[slickdeals]

"stem wall bases" in retaining walls.

 

[paddingtongreen]

I thought that ACI 318 had values for different concrete surfaces, including smooth, in the shear friction section.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[KBVT]

I never recommend the use of shear keys in my concrete details. ACI provides coefficient of friction values for concrete placed against hardened concrete not intentionally roughened in the shear-friction part of the code. Obviously, you need sufficient clamping reinforcement through the joint to utilize the shear-friction load path.

 

[DaveAtkins]

Shear keys are worthless.

And how can you create a shear key with reinforcing through it? I think that would be difficult to construct. Would the reinforcing run through the blockout?

Use shear friction.

DaveAtkins

 

[slickdeals]

@Dave:
I did not express myself correctly. For example, if there is a 10" thick wall, providing a 2x4 keyway in the middle with reinforcing bars on either side just seems worthless.

But I have seen plenty of details that show a continuous keyway at the interface of the stem wall and toe/heel slab.

 

[DaveAtkins]

OK--but I still say a keyway is worthless, and more work for the contractor.

DaveAtkins

 

[JAE]

Keyways are "TRADITION!!!".

But I agree there isn't much true engineering rationale with them.

I usually specify - 1/4" roughened surface.

 

[hokie66]

Keyways in thin concrete elements like slabs are not effective because the outside parts can break off. Full thickness keyed elements make sense, e.g. setting down the whole width of a wall below the inside of the slab. Dowels in the centre of an element carry shear, the amount depending on the steel strength and the concrete bearing. Shear friction works by clamping one element to another along a surface in friction, so roughening this section is advantageous. Use of shear friction depends on developement of the bars.

I don't like framing concrete beams into existing concrete structures. Why not provide another column for the new beam?

 

[weab]

JAE hit the nail on the head. In the past, it was thought to be a good idea, and this concept is still being passed on from project to project.

In wall construction, just specify an unfinished surface or roughen the surface. Forget the shear key.

 

[DTGT2002]

Unless a formed surface can be stripped and hit with a pressure washer while green, attaining the 1/4" amplitude can be quite challenging, though 1/4" amplitude can be quite subjective.

For those who specifiy 1/4" amplitude, how do you provide acceptance criteria for the amplitude or is it a statement of direction without verification?

All in all, the keyway is not that challenging to install, but can be a challenge to remove, but it is certainly not productive work with no capcity - I assume it is still common as a historical reference?



Looking to the shear capacity across a joint, with typical tensile steel passing through the interface, how often is the shear friction the difference in an adequate design? The 40% reduction for an as cast surface is a big hit, but a number of times, I have encountered a roughening requirement but upon further exploration, the full shear friction capacity was not needed.


Thanks for the discussion!


Daniel Toon