Shear Key
Shear Key
(OP)
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.
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
RE: Shear Key
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Shear Key
RE: Shear Key
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
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
DaveAtkins
RE: Shear Key
But I agree there isn't much true engineering rationale with them.
I usually specify - 1/4" roughened surface.
RE: Shear Key
I don't like framing concrete beams into existing concrete structures. Why not provide another column for the new beam?
RE: Shear Key
In wall construction, just specify an unfinished surface or roughen the surface. Forget the shear key.
RE: Shear Key
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