Special reinforcement around small openings @ concrete roof slab

[dcceecy]

The concrete roof slab will have 1'x2' and 1'x4' openings.

The slab is 8" thick two-way slab supported by concrete beams. the typical reinforcement is #4 @12" OC each way at bottom. some additional reinforcement at top along beam.

So around those openings, what kind of special reinforcement are needed?

some other engineer designed the slab but now the client wants openings. Any suggestions?

Thanks

 

[miecz]

See thread507-170996. In addition, you should add some diagonal reinforcing at the corners.

 

[BAretired]

Some engineers add diagonal bars at the four corners. I prefer to add 1-#5 top and bottom parallel to each side of the opening extending 2'-0" beyond each edge.

BA

 

[Ltwine]

It was our standard details instructing contractors for small opening without reinf details:

1. Add half of the interrupted bars to each side of opening(1 min) with splice length measured from the edge of opening.

2. Add two diagonals on the inner layer of tension steel. The diagonal shall be same size as the slab reinf, and to be centered on the corner with development length each side.

 

[BAretired]

Would anyone care to defend the need for diagonal bars? Orthogonal reinforcement accomplishes the same thing doesn't it?

BA

 

[nackra]

Option to consider:
Problem: A contractor "enlarged" the fabricated opening to match dimensions of the HVAC equiment base. The opening was sized fo the smaller ductwork.
Solution: The precast concrete panel manufacturer reviewed design loads and had the contractor add steel angles around a roof opening to transfer and support the loads. This was much quicker than refabricating and replacing these panels.

 

[Ltwine]

BAretaired:

This wasn't from study, but past on in the company:

Crack is likely to form at the corners in direction align with two diagonal corners. The crack sujjects to tensile stress, which is to be resisted by the diagonals to close the gap/minimize the crack width.

 

[paddingtongreen]

If the concrete around the opening is not properly cured, it shrinks more than the surrounding concrete, causing diagonal cracks to form at the corners, diagonal bars are more effective at resisting the formation of these cracks, at least, I think they are.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

BA,
I defend the diagonal bars also. Shrinkage cracks form at the corners of openings in suspended slabs just as they do in slabs on ground, due to the stress riser effect of the sharp corner.

 

[BAretired]

hokie,
I agree that there is a stress riser at each corner, but a single 15M bar placed orthogonally around an opening is more effective than a single 15M bar placed diagonally at the corner. It is, in fact about 40% more effective.

For years, I added diagonal bars at corners because it seemed to be accepted practice, but now I question the validity of doing that.

BA

 

[hokie66]

I didn't mean not to put in the orthogonal bars. I replace in kind whatever is interrupted by the opening. Then I also add the diagonals, within 40 mm of the corner. I hate cracks at corners.

 

[BAretired]

Okie dokie hokie.

BA

 

[weab]

BA,
This conversation came up at my company some time back and some older more experienced engineers, including my boss, agreed that orthogonal bars are sufficient. Perhaps you'd need 1.414 times as much bar area as you'd use with diagonals. It is certainly easier for thin slabs that don't have much extra thickness to place additional layers of reinforcement required with diagonals.

What this method would tell me is this. If I have several interrupted bars that should then be placed at the edges of the opening anyway, then additional longitudinal bars would seem unnecessary. Right?

 

[BAretired]

jsdpe25684,
First, you would not need 1.414 times as much bar area as those in the diagonals. In fact, you would need 0.707 times as much bar area. That is easily proved.

Bars interrupted by an opening must be replaced, 50% each side of the opening. That is a given. Beyond that, the only question is:

Would it be better to provide diagonal bars or orthogonal bars to prevent corner cracks from occurring? As I have stated earlier in this thread, my preference is to use orthogonal bars. That is what I would do now if I were still practicing engineering, which I am not. So far, I have not heard any legitimate argument to the contrary.

BA

 

[hokie66]

BA,
Why do you say you need only 0.707 times the diagonal bar area as othogonal reinforcement? The crack can go either way or diagonal, so you need bars both ways.

 

[paddingtongreen]

If I was still working, I would add the diagonal #5s just as I always have. They are such a small part of the total cost and the openings in my designs have not cracked.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

hokie,
I believe I was mistaken when I said you would need only 0.707 times the bar area. You could blame it on one too many beers. In actual fact, you would need precisely 1.0 times the bar area if you placed the bars orthogonally. In other words, two bars at right angles with each other provide a resistance in EVERY direction equivalent to the resistance of one bar in a particular direction.

paddingtongreen,
Throughout my practice, I have provided 1-15M diagonal bar at each corner just as you and hokie66 have. It was standard practice. But now, I am questioning the wisdom of doing so.

BA

 

[rowingengineer]

If the slab is less than 250mm thick I wouldn't add diagonal bars, it causes congestion especially if you provided this diagonal reinforcing top and bottom. If the slab is great than 250, I may consider the diagonal reinforcement if the slab is reinforced with mesh. The way I see it is the stress is greatest either at the top or bottom of the slab depending moment distribution. Thus the diagonal bars being in the 3&4 layers of 6 won’t be the most effect of the reinforcement having a lesser d than all other reinforcement trimming the opening.

The only reason you would be providing diagonal reinforcement is for tensile stresses built up be shrinkage and similar effects to stop ripping of the slab, just like paper. These effects could be in high effect during the early ages of the slab while restrained by the formwork or later in life. The diagonal reinforcement would make sense if it was a ground slab where moments in the slab are minimal due to dead weight thus the shrinkage stress are greater than the moment stress. In a suspend slab you always have the moment due to the dead weight of the slab, If these forces are greater than the shrinkage/similar forces than it makes sense to bulk up the orthogonal reinforcement, however if the shrinkage forces are greater than the moment forces than diagonal reinforcement makes more sense.

If the shrinkage & similar force are huge then it may be smarter to start to add joints to the building. If you are concerned about the cracking at early age due to restraint from formwork, the diagonal reinforcement would make sense, but this type of restraint and shrinkage is better handled using good curing methods. Thus in my opinion diagonal bars are not the best way to enhance the performance of the slab.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[dik]

I use BAretired's approach with the edge bars equivalent to the rebar removed and a 15M T&B minimum. The diagonal bars only clutter up the corners.

If a crack forms normal to the tension, the diagonal bars are most effective and efficient. I just don't use them; the normal bars on all faces provide an equivalent reinforcing to replace the diagonal ones...

Dik

 

[ishvaaag]

If the holes are as small as 10 cm and rebar not closer than 30 cm between centers yo may not need in practice do anything, and a FEM analysis might prove that almost anywhere sufficiently apart from a support. In design one woul look for coordination to try to avoid rebar cuts, yet the FEM analysis might still prove yo don't even need to put a substitutory supplement. This functions well enough here, where slabs are between 20 and 35 cm by now and doubly reinforced top and bottom with a minimum.

Anyway, surrounding with orthogonal and diagonal reinforcement is structurally sound practice that only has the problem of poor constructability, due to excess of rebar around small holes.