Openings in R.C cover slabs 1

[Edu09]

Hi,

I'm undertaking a design check for a large 12m diameter cover slab. The slab has a number of large openings(2.9 x 2m the largest).

What I'm not sure about is how to determine the reinforcement at the corners of these openings, I see the practice is to provide diagonal bars but I'm not sure how to analyse this.

If anyone could point me in the direction of a thread or reference it would be much appreciated.

Thanks and regards,

Edu

 

[JAE]

Edu09 - can you offer more information?

1. Is the cover slab spanning over air or is it on-grade?
2. What sort of live load or other loads are anticipated?
3. What is the nature of the support around the perimeter if it is spanning?



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[Edu09]

JAE, sure thanks for the reply!

1. It's spanning over air/ or sewage, it's a drop shaft for a combined sewer overflow.

2. there's permanent loads of 40kN/m^2 for the permanent condition and in the maintenance condition it has to support some equipment and crane wheel/ outriggers, so, more like 100kN/m^2

3. It's simply supported around the circumference and there are 2 precast beams on corbels into the shaft wall also supporting across the span. I'll try to attach a sketch, I appreciate how difficult it is to describe this sufficiently.

I suppose I was asking in general, how to design for openings, I see lots of detailing guides- how it should look but not man analysis examples.

But really your thoughts would be welcome, Thanks a lot

Sketch attached, the rectangles are the openings and the lines across show the precast beams

 

[JAE]

100 kN/m^2!! Am I converting that right? That appears to amount to about 2090 psf which is unheard of. Am I doing something wrong here?

In any case, so despite the fact that the precast beams are designed to take their share of the loading from the slab, the opening corners will still experience stress concentrations I would think.

These are hard to manually calculate with custom layouts/geometry but you could certainly attempt to model the slab (and beams) to replicate the anticipated deflections and shear/bending under loads and see what sort of hot spots are generated at the corners of the openings.

It appears that these corner hotspots wouldn't be critical to the load carrying capacity of the slab but you'd still want to control cracking and a good Finite Element model could get you some idea of the magnitude in these areas.



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[Edu09]

JAE, you're right, nowhere near that! I horsed that up not you. That value from the previous issue of the loading they made but it was amended in a later issue.

It's more like 30kN/m^2 in the worst case. I already made a finite element model in fact so I'm glad you thought that was the right approach! Okay, and to control cracking you place bars diagonally at the corner? Then I can pick a section and check it for cracking?

Thanks for this. It's much appreciated.

 

[dik]

That's still over 600 psf. You have to look at the locations of the loads relative to the holes.

Dik

 

[WARose]

[blue](Edu09)[/blue]

What I'm not sure about is how to determine the reinforcement at the corners of these openings, I see the practice is to provide diagonal bars but I'm not sure how to analyse this.

For the corner steel SOP is to provide temperature steel (per reinforcing level) at each corner or 1 #4 bar (at twice the development length).....whichever is greater.

That is (of course) a minimum. In addition (for such a situation) I typically have a FEA model. From that model you can get stress concentrations (that you would normally ignore in a material like steel for static loads) and have that as a reinforcement check as well.

 

[MotorCity]

For openings like these, I usually design beams within the thickness of the slab to distribute the load around the opening. The reinforcing for these "beams" runs parallel to the sides of the opening. The diagonal bars I have seen designed two ways. First, as typical crack control reinforcing with appropriate development length from the corner of opening. The other method is a rule of thumb by providing area in steel equal to area of steel interrupted by the opening.

 

[dik]

I try to avoid diagonal bars... too much congestion. I put more reinforcing parallel to the openings.

Dik

 

[BAretired]

With a live load as high as 30 kPa, you may have some significant concentrated loads (wheel loads) which should be considered separately. This is not a normal situation and there are no simple rules for placing reinforcement. You could use Yield Line analysis or Hillerborg Strip Method for checking the existing reinforcement.

BA

 

[Edu09]

Thanks for the replies chaps, really helpful.

The figure of 30kN/m^2 is right in the worst areas of the slab under certain conditions- i.e maintenance equipment and cranes. The figure does seem a bit high but that's what has been used so for now I'm going with it.

The wheel loads are considered as a separate case causing more local effects but they are spread through a decent amount of fill so, don't appear to be a problem.

Currently what I've done is create an FE model to obtain moments and hand calculate the reinforcement required. In addition to this there is a wood-armer tool in the program which I've used as an extra check although I must say, I'm not as proficient with this method.

Around the openings there are upstand walls which are acting structurally which is helping the load get around these holes.

Again, thanks for the answers.

 

[hokie66]

For a heavy slab of limited dimensions, designed for a live load of 30 kPa, the normal orthogonal bars will surely protect the corners, as you will have substantial reinforcement adjacent to the openings. Corner bars are typically used in much lighter slabs for crack control where stress risers occur, but there is no specific design criteria that I know of.

I wouldn't count on the upstands around the openings, unless they go all the way to the supports.

 

[Edu09]

Thanks hokie66. that's useful to know. And you are quite right, the orthogonal reinforcement is enough to handle the openings.

And regarding the upstands, and framing back to supports. The other openings in the slab either have one edge along the precast beams or quite close to the outer edge of the slab which seems provides adequate framing/ load paths.

A bonus question if anyone doesn't mind answering. For the deflection would a span/d(the whole span ignoring holes) check vs. the calculated deflection be okay? It would be more conservative but if it works, very simple.

Edu09

 

[JoelTXCive]

Our firm often does water treatment plants with submerged tanks/lift stations having concrete caps. Often times they will have all sorts of holes in them for piping or mechanical access. We have corner bars, but the main reinforcing runs parallel to the holes.

A conservative quick and dirty design is to assume the hole is not there. Then take a 1ft strip of the slab/cap along the hole with an appropriate tributary area and treat the 1ft strip as a beam. If its simply supported you can just use wl^2/8 and design for steel in that 1ft strip.

Attached is a detail we have used before. It's a below ground valve box with a concrete cap.

 

[Edu09]

JoelTXcive,

Thanks a lot for this, it's a pretty similar project to what you describe. Good to see your standard detail for comparison.

Cheers