Stirrup pattern in a concrete beam

[Chandan321]

I am designing a slab 180 ft x 51 ft in plan and 8 ft thick. The slab is supported by columns 8 ft x 8 ft (total 8 columns). The columns are 100 ft long and standing on 8 ft mat foundation. On the top slab, there are 4 nos. of 26 ft diameter cutouts. So, I think essentially, the slab is divided into beams running from column to column.

Now, during the design of these beams, construction guys ask me to keep some space at the center of the beams for workers to work easily. So, there is around 4 ft of horizontal space at the center of the beams where I did not put any stirrups.

I need your opinion about this empty space at the center of the beam where there is no stirrup. Is it Ok to keep that space empty and assume that the stirrups at the sides of the sections will be able to take the shear stress acting on the beam?

You input will be valuable to me in taking decision in this context.

 

[hokie66]

I think the "construction guys" are being very reasonable in asking for working space, and I see no reason why you can not place the stirrups as described. Just curious, did you reduce the number of columns from your earlier post from 10 to 8? Quite an interesting structure. Can you share with us what it is for?

 

[Chandan321]

hokie66, thanks for your interest in the problem. I am sorry, the no. of columns is still 10, it was a typo. I am glad you pointed it out.

Each concrete beam has 10 legs of stirrups(5 nos. of 2 legged closed tie stirrups). I placed the 2 legged closed ties symmetric about the vertical center line: one all around close tie, and 2 on each side, thus keeping a horizontal space of 4 ft at the center.

I fear that since there are heavy vertical and torsional loads on the beams, there might be local cracking at the center where there are no stirrups. what do you think about it.

The structure is used to support refinery drums.

 

[hokie66]

The vertical loads are best resisted by structure as close to the loading as possible. So in that respect, having more reinforcement near the edges is preferable.

As to the torsion, the center portion of the beams contribute very little. Consider the beams acting like giant tubes, where the torsional shear travels around the perimeter. So again, having the preponderance of your reinforcement around the perimeter is better. What you should check is that enough reinforcement is provided across the top and bottom to resist the torsion. As you will have only one of the stirrups going completely across the bottom, it may be necessary to add some additional transverse bars.

Having said all this, based on the geometry of your structure, I don't think torsion will be a controlling case.

 

[rapt]

Chandran321,

The Australian code has transverse shear leg spacing limits. The absolute maximum is 600mm (2'). You would be violating this!

Don't forget that all of your torsion reinforcement has to be in the outside tie!

 

[hokie66]

Chandan,

Rapt is correct for the Australian Standard, but I doubt that you are using that standard. Your code may have a differing provision.

Even so, I don't think the Australian Standard contemplates such a massive structure as you have, and I see no logical reason why the core of your beam needs a vertical leg. I would stick to the 600 max (or 24") in the outside parts, but the 4' wide working gallery is appropriate.

 

[Chandan321]

hokie66 and rapt,
thanks for your valuable inputs. The open gallery of 4 ft is right in the zone from column to column. from what hokie66 said, I believe this portion of the beam will not take any torsion. But will this portion take substantial beam shear. If yes, then how will this open gallery affect the ability of the beam to take beam shear.

your inputs have been very valuable on such a specific topic. what are your opinions on the above point.

 

[oneintheeye]

uk code requires a max spacing between longitudinal reinforcenment and vertical leg of a link. Not sure if your is same.

 

[kslee1000]

Per your latest post, it looks like you have a deep beam (4' clear span, 8' total depth?)situation, for which I don't think you can get away without shear reinforcemt - both horiz & vert. Talk to the construction guys, they can place the required bars just prior to pouring concrete. The designer may need to provide some fabrication/installation details, such as splices if necessary.

 

[hokie66]

kslee,

It is not a deep beam, it is a massive slab with holes in it. Suggest you read the OP.

Chandan,

I don't consider the working gallery to be a problem. The outside sections are reinforced as two beams, with the 4' wide interior section just filling in the structure. You could probably leave a big void there, but that is probably impractical.

 

[kslee1000]

ok, the overall "slab" has a length 180' with 4-26' dia holes, say equal spacing on edges and the interior sections, the width in between holes = (180-4*26)/4=18' with total span length (from edge to edge)=51'. The part I don't quite get is "The open gallery of 4 ft is right in the zone from column to column". A sketch may explain all.

 

[kslee1000]

Please ignore my posts, I appologize for my mistakes after further review.

 

[Chandan321]

thank you for all your responses.

hokie66, let me know your opinion about my reasoning here:
since the openings in the slab are huge, the structure is effectively beams running from column to column, and added mass around them from the slab portion.

If this resoning is OK, then I think that the portion of the beam running from column to column will effectively take the shear arising from earthquake forces. In that case, can I leave the column to column portion of the beam without any transverse reinforcement.

 

[hokie66]

No, I didn't mean that. Your main longitudinal reinforcement will be spread over the width between the openings, and you will need transverse reinforcement both bottom and top. Part of this transverse reinforcement can be the horizontal legs of the shear stirrups, but as I suggested before, you probably need to add some transverse bars where only one stirrup horizontal continues across.