RC Shear Keys in Building Structures 1

[hannis]

All,

I'm currently reviewing structural drawings for a code compliance check and the RC details indicate a 'Shear Key' at the interfaces between floor slabs, walls and columns etc. I understand that this is a standard way of detailing concrete pour breaks by North American practice. And for the purpose of design the joint is considered continuous for moment and shear transfer. This is a method in the UK that we are not familiar with. Am I correct in understanding that the joint is effectively a smooth finish and effectively forms a separation joint with no aggregate interlock across the joint? If so does the joint not effectively become a hinge since the only thing continuous across the joint is the rebar.

Any comments would be appreciated.

Many thanks

Hannis

 

[TFL]

a sear key is simply a "notch" in the concrete. typical aplication would be a joint between a wall and a footing. your the footing first with a piece of wood in the center of were the wall will be. this 2x4 piece of wood is removed and when the wall is poured it will "key" into the void providing shear resistance. the key does not provide moment resistance. that must be provided( and designed) by steel passing thru the joint.

 

[TFL]

i find it hard to belive this method is not common in the UK. what would you do instead?

 

[AggieYank]

the joint isn't treated as a hinge. the top steel and bottom steel are continuous through the construction joint. the key, known as a shear key, is responsible for carrying the shear through the joint. Since the bottom steel is continuous and concrete isn't designed to act in tension, the steel will still carry the required tension. The top of the beam will be in compression, and the concrete will still act the same. The beam is still a working beam, even with the construction joint. Having said that, most engineers require the joint to be at a 1/3 point of the beam, where neither the moment nor the shear is at a maximum.

 

[haggis]

Question:

Not being Structural in expertise, I have seen it done two ways in pits within the confines of a plant where the floor was poured then the walls later. One with the 2 x 4 key and bent bars from the floor and with bent bars only without the key. Is the method without the key acceptable practice? There is no soil swelling or anything against the walls.

 

[miecz]

The use of a shear key at the interface of a wall and a footing or a column and a slab seems to be a carryover from unreinforced concrete (residential) construction. It seems redundant in reinforced concrete where reinforcing crosses the joint. In environmental structures where a waterstop is required at the joint, a shear key can make the construction a real pain.

 

[jike]

The use of whether a shear key is necessary can be determined by the amount of shear to be transfered across the joint. It has been the practice of many engineers in the US to just use shear keys at all joints.

Additional dowels crossing the joint may also be used to transfer the shear using the shear friction concept defined by ACI.

ACI 318-02, 6.4.4 requires construction joints in structural floors to be located in the middle third of the span. The commentary states that "when shear due to gravity load is not significant, as is usually the case in the middle of the span for flexural members, a simple vertical joint may be adequate. Lateral force design may require special design treatment of construction joints. Shear keys, intermittent shear keys, diagonal dowels or shear transfer method of 11.7 may be used whenever a force transfer is required."

 

[Zambo]

TFL the answer to your question is that in the UK the surface of the concrete would have the laitence removed to expose the aggregate thereby ensuring the joint integrity. Hannis appears to be asking whether the prescence of the shear keys means that the laitence does not need to be removed and therefore the joint has a smooth finish. This seems unlikely to me as if rebar is passing through the joint I would expect the joint to be formed so as to ensure that there will not be rebar corrosion.

Zambo