Step down in slab detail

[ENG_33]

Can anyone tell me how wide the soffit has to be for step downs in one way RC slab and why?
Also do you see any issue with the connection below?
Thank you.

 

[Manitou90]

There has been some good discussion regarding this.

https://www.eng-tips.com/viewthread.cfm?qid=477967

https://www.eng-tips.com/viewthread.cfm?qid=408814

 

[ENG_33]

Thank you. I have checked the links you provided as well as the paper for large steps. However I believe that the 800mm step I have here is way too large compared to the 250mm steps in that paper although it is near the support instead of mid span. Is tie model mentioned in the links is applicable to my case?

 

[KootK]

However I believe that the 800mm step I have here is way too large compared to the 250mm steps in that paper although it is near the support instead of mid span. Is tie model mentioned in the links is applicable to my case?

I disagree. Much of the theory discussed in those thread will still be valid for your situation, particularly the strut and tie models.

Can anyone tell me how wide the soffit has to be for step downs in one way RC slab and why?

The rule of thumb that I was taught was to make [w] twice the dimension of the overall step height. As for the why, that was discussed extensively in those other threads. In short, the goal is to provide enough space to smooth out stress concentrations around the corners, particularly rebar bond stresses.

Also do you see any issue with the connection below?

1) I suspect that the dimension [w] is too small as discussed above.

2) You've got a tendency towards unrestrained bursting as shown in the sketch below.

 

[KootK]

Also, as a subtler matter, I feel that your detailing ought to reflect that the setup may well behave as I've shown below regardless of how we'd like it to behave. This will have implications for crack control and shear capacity.

 

[ENG_33]

Thank you Kootk!
The below replies are based on positive moment assumption.

twice the dimension of the overall step height]

I have seen 1.5D or 2D from many firms too but wondering if this is specified in any standards? I was looking at the paper "one way slab with large steps" mentioned in the reference that Manitou90 provided and they tested increased step thickness 'W' specimens and came to a conclusion from the results that "It is not possible to have the same maximum moment strength as that of flat one-way slabs by simply increasing the thickness of the step". The results also show that it doesn't increase the displacement capacity too compared to the smaller step thickness. I wonder if I really need 1.5D or 2D here because it means I will have 1.6m wide step, which will add more loads.

I used strut-tie model but I got the tension tie as below instead of strut. Can you explain the unstrained bursting from strut in more details? Thanks.

The paper also mentions that the below below supplementary reo in step has the best performance in both strength and displacement.

The U & inverted U bars are used to take the diagonal tension shown above. But I wonder with the detail I made in the very first pic I uploaded in the thread, can these vertical bars (with lap anchorage) work as U & inverted U bars to resist diagonal tension instead of adding additional U bars?

Also, if the main bars extend and lap like I detailed, are these additional L & inverted L bars necessary? Looks like the they extended and lapped the main bars and provided L bars too in their tests.

 

[ENG_33]

Can you please further explain how this will affect the shear? I would expect the shear in the step increases when the stiffness of horizontal restraint increases instead of the rotation fixity?

 

[dik]

Something like:

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik