Cantilevered Concrete Slab Analysis

[Aytacoglu]

Hi Everyone,


I made a post yesterday but I wasn't very clear of the details for it so I thought I put another post with more details.

I am basically looking to assess an existing slab that will have a cantilevered end. We don't have any archive information for the slab and there are no survey information. Hence we don't know the details about the reinforcement inside the concrete. That's why my initial thought was to just analyse it as a plain concrete, and if it passes the analysis, surely it will pass in real life as I am sure there are rebars inside that section.

I have attached the section that I am looking to analyse below. I don't know how to go with the analysis on this as the slab is continuous towards the left side of the page and towards the bottom as there is a concrete wall going all the way down roughly 8m.

I can calculate the bending moment action on the interface between the wall and the cantilevered part M = (Load * 0.260m + 0.287m). But I don't really know how to check the tensile strength or how to convert this moment into stress to compare with the strength of concrete.

Is it the case that I need to assume a 1000mm length of that section and 878mm depth (from the image below). So it will give me a section modulus of 1000*700^2/6. Then I can divide my moment with this value to get my stress. Is this how to go with it?

I would appreciate any help on this to enable me to analyse it.

Thanks everyone!

 

[Tomfh]

That's why my initial thought was to just analyse it as a plain concrete, and if it passes the analysis, surely it will pass in real life as I am sure there are rebars inside that section.

I don't agree with analysing as plain concrete. If you are sure it has rebar, then check that. Some investigation eg slab scanning may be in order to confirm.

 

[JAE]

This seems to be more of a corbel action than a cantilever (flexural) action. My initial thought is that shear is the primary failure mode here - either through the 300 section or the deeper 878 section.

Corbel design (per ACI that I'm familiar with) does include a moment component with flexural reinforcement - not sure whether you are into ACI or some other specification but your particular applicable code would provide some guidance with respect to design against moments.

I'm not sure I'd be relying on a plain concrete (i.e. non reinforced concrete) check here - I wouldn't feel comfortable about it.

I've attached a sketch on how I would think traditional reinforcement might be positioned in a wall/slab like this. The 287 wide section might be a panform extra where there would typically not be any reinforcement at all.

If you cut the slab off you'd have that red rebar I've drawn in but per ACI that rebar end must be somehow developed or hooked or welded to a cross bar to do any good.

You might have some localized chipping to reveal some rebar to see at least what you have.