Cover Slab Design 6

[Joe Grant]

Looking for examples/help for designing a concrete cover slab. Attached is an example picture of a cover slab. My thoughts would be to design for punching shear around the missing area where the manhole cover is to be place however I am unsure how to make up for the missing concrete.

Any tips/advice is welcome

Thanks

 

[TehMightyEngineer]

KootK, I believe I'm going to agree with you 100% after running this FEA model. It seems that there's really wont be any two-way shear action at the edge of the cover and it will be a one-way shear failure at the frame edge.

I guess the only question I have remaining now is what would you say the shear "depth" would be for the shear calculations. Using a depth of "d" for the positive reinforcement doesn't seem quite right but it seems like using the full thickness might also not be correct. What say you KootK?

By the way, I've come to a similar conclusion that 9" seems to be the limit for this 36" dia. frame and cover. It's a cast-in frame with an 8" depth so 1" clear cover to the bottom at 9" and those both seem to match up nicely with what you came up with.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

KootK, I believe I'm going to agree with you 100% after running this FEA model. It seems that there's really wont be any two-way shear action at the edge of the cover and it will be a one-way shear failure at the frame edge.

Well it's nice to get some agreement on something. As you know, that's not always the case for me. That said, I'm curious, how can you tell from FEM results whether or not your shear stress is more appropriate as one way or two? I believe you. I just haven't done this myself and am curious to know how the determination is made.

I guess the only question I have remaining now is what would you say the shear "depth" would be for the shear calculations. Using a depth of "d" for the positive reinforcement doesn't seem quite right but it seems like using the full thickness might also not be correct. What say you KootK?

I would say that the shear depth would be the distance from the top steel to the underside of concrete. I feel like that would always be the case at a gravity loaded free edge. I'm assuming that the frame delivers its load to the top of the concrete in some fashion.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

That said, I'm curious, how can you tell from FEM results whether or not your shear stress is more appropriate as one way or two? I believe you. I just haven't done this myself and am curious to know how the determination is made.

Well, my big sticking point was that the metal cover wouldn't just distribute the load more or less evenly around the rim. This convinced me that there is a "kern" as you put it. I've also modeled the concrete cover in RISA and put an approximate eccentric cover load into it as well. Then, I've taken all this and concluded that, all things equal, I concede that you're definitely better at I am with the minutia of reinforced concrete design and detailing. It's also more conservative so if you're not correct then at least I've erred in the conservative direction.

I'll probably ponder this some more and report back if I have additional thoughts.

I would say that the shear depth would be the distance from the top steel to the underside of concrete. I feel like that would always be the case at a gravity loaded free edge.

Do you mean bottom steel because this particular cover wont need much (if any) top reinforcement? My lifting inserts are going to put it primarily into positive bending and it's a simple-span two-way slab in all other respects.

I'm assuming that the frame delivers its load to the top of the concrete in some fashion.

Yeah, it's got a 42"OD lip on the frame and I'll probably get some additional bond strength out of the vertical walls cast into the concrete.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

Do you mean bottom steel because this particular cover wont need much (if any) top reinforcement?

Definitely top steel I'm afraid. Think of it like this:

1) A free edge must, in all cases, sort of cantilever out from some perpendicular support akin to a beam.

2) Cantilever = top steel in tension.

The Canadian code requires hooked top bars at all slab edges, presumably for this same reason. Granted, I'm sure that they didn't have manhole lids in mind. I can't remember if there's an analogous US provision. Some alternatives:

1) Weld some little dowels to the underside of the lip if that doesn't cause cover issues.

2) Use shear values for plain concrete.

3) Consider the shear depth to be from the bottom steel to the bottom of the slab. I don't love this. Seems like you'd be asking for some serious top side shear cracking.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

1) A free edge must, in all cases, sort of cantilever out from some perpendicular support akin to a beam.

Makes sense. I was thinking top steel as well was required for the shear but wanted to make sure.

I can't remember if there's an analogous US provision

The only one I can think of is the requirement that bars be developed properly for the shear/moment at simple supports. For a slab cover over large wall supports this typically isn't an issue and I can use straight bars without hooks AFAIK.

1) Weld some little dowels to the underside of the lip if that doesn't cause cover issues.

Guys in the shop would hate me for this as they've never had to do it before for manhole frames, something tells me they don't want to hear about one-way shear and cantilevers.

2) Use shear values for plain concrete.

This is what I've done in the past but that was assuming two-way shear values. For one-way shear I have insufficient capacity, without going thicker on the slab it doesn't seem like the way to go.

3) Consider the shear depth to be from the bottom steel to the bottom of the slab. I don't love this. Seems like you'd be asking for some serious top side shear cracking.

Definitely don't like this either.

I could hook the bars up into the top of the slab but I think the guys in the shop would like it better if I just provided a 2nd top mat of bars around the opening.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com