Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Shear Key

Shear Key

Shear Key

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.

RE: Shear Key

"stem wall bases" in retaining walls.

RE: Shear Key

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

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

RE: Shear Key

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.

RE: Shear Key

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.


RE: Shear Key

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.

RE: Shear Key

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


RE: Shear Key

Keyways are "TRADITION!!!".

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

I usually specify - 1/4" roughened surface.


RE: Shear Key

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?  

RE: Shear Key

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.

RE: Shear Key

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close