Dowel Bar Design (Slab to Wall)

[Callum_S1]

Can anyone please tell me how dowel bar is designed in the following scenarios:
1. RC Slab running on top of premier concrete basement wall with dowel bars.
2. RC Slab connecting to the side face of concrete wall.

Some examples will be very helpful for me. Thank you.

 

[Trippelsewe]

1. Depends what force is being transferred? If none then just nominal ties, normally codes have some sort of clause on what is required for robustness.

2. It is a shear friction calc, check ACI318.

 

[Callum_S1]

Thank you.
For scenario 1, the force I can think of is the lateral shear transfer from floor diaphragm to wall for both wind & earthquake. Also the out-of-plane shear from wall to slab due to the soil pressure. Not sure if I miss anything else.

For scenario 2, I often see a small rebate being used in the wall, if the shear is carried by the dowel bars alone, what is the purpose for the rebate? Is it just for construction?

For dowel bars, do I follow reinforcement rule or anchor's?

 

[Trippelsewe]

The rebate increases coefficient of friction but I always specify the interface to be roughened as well.

For the connection you need to check both anchorage capacity to wall and make sure you have enough developed reinforcement crossing the crack interface.

 

[Callum_S1]

Thank you

 

[Callum_S1]

Actually there is one thing I don't understand about this. I have seen many engineers specify short development length. What is the reason for that? Shouldn't we follow reinforcement rule here for dowel bars?

 

[Trippelsewe]

I reckon it just depends on application. Not much point in fully developing a bar on one side when it is cogged into the wall (and only developing 50% as a result).

But if you are talking about short pins I think there is a method where you actually check the bearing stresses and bending capacity of the pin, but I haven't done that for years, maybe someone else can comment on that.

 

[Callum_S1]

Thanks. What about when slab connects to the face of the walls? I have seen dowel bars with epoxy into wall. In this case is it still considered as reinforcement and where does the Code say epoxy is allowed to shorter the development length? If it is designed as anchor then wouldn't the edge distance (ie concrete breakout etc) an issue when slab is thin?

 

[hokie66]

That is not the way to connect a slab to the face of a wall. The sequence of construction is faulty if you are trying to do that. And depending on so called 'shear friction' is not the way either. The slab needs to bear on the wall, sometimes achieved by a designed rebate.

 

[Trippelsewe]

Hi Hokie, I have read on this forum that shear friction cannot be relied upon. How do I reconcile that when it is an industry standard in many parts of the world and I would not get much work if I ignored it. Eg see below snips from NPCAA Precast Handbook and Reid Bar Design Guide, both connection types rely on shear friction.

 

[hokie66]

Use a cast in rebate connection with pullout bars. They give some slab bearing, and reinforcement to prevent sliding off the rebate. Reid, Ancon...

As far as I know, the Australian concrete code still does not recognize shear friction theory as a design option, for good reason.

https://www.ancon.com.au/products/reinforcement-continuity-systems/keybox-system

 

[Trippelsewe]

I will keep an open mind but like I said using shear friction is very common in Australia and the methods are described in Australian textbooks. You are correct it is not in AS3600.

 

[Callum_S1]

If the rebate is what we reply on to transfer the shear (or bearing in this case) then how do you explain the connection of shotcrete walls to piers using dowel bars where no rebate is used?

 

[hokie66]

Aren't the wall and pier both vertical elements? Not sure where the shear comes from in that case. With slab to wall, the shear is from gravity.

 

[Callum_S1]

Lateral loads from soil to shotcrete walls and shotcrete walls to piers.

 

[hokie66]

If dowel action works, it works. But that is dowels in bearing across the joint, not providing a clamping action as required to make 'shear friction' work. But I would have to know your details and how the walls span to give further opinion.

 

[Callum_S1]

Does gravity or lateral matter here? Aren't they both shear to transfer here?

 

[Callum_S1]

So if shear friction is not recognized how do you design these dowel bars here for shotcrete walls to piers since there is no rebate

 

[hokie66]

I can only speak for projects I have been involved in. It they used spaced piles similar to that, the shotcrete was behind the centre line of the piles, so delivered the horizonal load in bearing. Then if a flat surface had to be achieved, another wall was built in front. But most projects with deep excavations use contiguous or secant piling.

 

[Settingsun]

the methods are described in Australian textbooks. 

Could you post the reference, please.