Rebar pattern in a slab with circular openings

[Chandan321]

Hi Guys,

I am trying to design an 8 ft thick table top slab (52 ft x 15 ft rectangular) standing on concrete columns 8 ft x 8 ft. The columns stand on an 8 ft thick mat.

--- The table top slab has 4 circular openings of 26 ft dia each.

--- Very heavy circular drums will sit on anchor bolts around the openings. Its a very heavy seismic zone.

--- The concrete columns are at 4 corners, as well as on the perimeter between drums, totaling 10 columns.

--- The rebar pattern that I have decided for the slab is: longitudinal rebars between columns in N-S and E-W directions. For the circular openings, I have decided for rebars in circular pattern all around the openings, and in concentric layers in plan.

Guys, I want your opinions on the rebar layout, specially the circular pattern, as this is something which I have no precedent of.

Thanks in advance.

 

[JStephen]

Check the posted dimensions again- the openings are larger than the table top right now.

 

[asixth]

I hope the slab thickness is 8" opposed to 8'.

 

[rday]

Agreed, the dimensions look suspect. You should check them.

 

[JStephen]

If he's putting heavy 26' diameter drums on a slab with corner supports, it won't be 8".

 

[Chandan321]

hey guys,
sorry to catch your posts a little late.
the columns are 43 ft apart center to center. so the drum openings are OK. the 8 ft thickness is also OK. the slab dimensions are 170 ft x 50 ft. sorry for giving the dimensions in m instead of ft in my earlier post.

 

[JStephen]

My first thought for the layout of a mat would be to run bars lengthwise and crosswise leaving a big square opening at each of the circular openings (think that's what you were looking at.) Then run straight bars at 45 degrees to the others to fill in the corners.

 

[hokie66]

JStephen is right. Using circular bars around the openings is needlessly complicated.

 

[Chandan321]

the opening is actually a 16 sided polygon. so, running rebars parallel to each side of the polygon will create so many layers of rebars that they will fill most of the slab thickness (as i have 4 layers of rebars top and bottom, for both the straight rebars as well as for rebars at 45 degrees). that is the reason I am inclined towards circular pattern of rebars.

 

[miecz]

I wouldn't run bars parallel to each side of your polygon. I would follow JStephen's suggestion for a circular hole, with bars lengthwise, crosswise and at 45[°]. This pattern never has more than six layers at any point. With this reinforcing pattern, one can visualize how a load travels from any point on the slab to the support. With a circular rebar pattern, it's harder to visualize the load path.

 

[rday]

I agree with the pattern proposed by JStephen giving an octagonal opening. Save the circles for another day.

 

[Chandan321]

I tried to place rebars in lengthwise, crosswise and at 45 degrees, and at a clear cover of 2 inches from the circular concrete edge. This leaves an unreinforced area near the opening with one dimension as 1.2 ft. How can we take care of this area?

 

[cooperDBM]

With a 26' diameter, 45 degree bars will leave some unreinforced concrete at the 1/8 points (i.e. 22.5 degrees). In combination with 45 degree bars you could consider an embedded steel collar (16 sided) with studs around the opening (perhaps top and bottom). Your anchor bolts could be attached to the collar.

Otherwise I would look at 45 degree bars and hoop bars. The hoop bars could be inside the 45 degree bars so that you don't need an additional layer. You could then hook the longitudinal and transverse bars over the hoops to tie them into the cage.

Have you considered post-tensioning this thing?

 

[miecz]

I can see why you're concerned. Why not use 2 layers top and bottom by using another set of bars offset 4 inches radially?

 

[Chandan321]

thanks for all your responses.

cooperDBM, post-tensioning is not an available option.

miecz, actually, due to flexure requirement, I need 4 layers (top and bottom) of rebars at any section. even with this, do you think that I can offset the rebars radially? if so, can you please explain in detail.

 

[hokie66]

In addition to the orthogonal main bars and bars at 45 degrees, it is probably a good idea to have one layer of circular bars around the hole like side face reinforcement in any deep beam. These should go around the perimeter of the slab also. But I see no need for multiple layers of the rings.

 

[miecz]

I'm imagining that, by four layers, you mean four bars stacked vertically at some clear spacing of one or two bar diameters. These sets are spaced horizontally at some spacing. Let's say it's 8 inches. I was merely thinking that, in the vicinity of the opening, you could use two bars stacked vertically at a spacing of 4 inches.