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Special reinforcement around small openings @ concrete roof slab

Special reinforcement around small openings @ concrete roof slab

Special reinforcement around small openings @ concrete roof slab

(OP)
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

RE: Special reinforcement around small openings @ concrete roof slab

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

RE: Special reinforcement around small openings @ concrete roof slab

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.

RE: Special reinforcement around small openings @ concrete roof slab

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

BA

RE: Special reinforcement around small openings @ concrete roof slab

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.

RE: Special reinforcement around small openings @ concrete roof slab

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.    

RE: Special reinforcement around small openings @ concrete roof slab

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.

RE: Special reinforcement around small openings @ concrete roof slab

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.

RE: Special reinforcement around small openings @ concrete roof slab

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

RE: Special reinforcement around small openings @ concrete roof slab

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.  

RE: Special reinforcement around small openings @ concrete roof slab

Okie dokie hokie.

BA

RE: Special reinforcement around small openings @ concrete roof slab

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?

RE: Special reinforcement around small openings @ concrete roof slab


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

RE: Special reinforcement around small openings @ concrete roof slab

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.

RE: Special reinforcement around small openings @ concrete roof slab

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.

RE: Special reinforcement around small openings @ concrete roof slab

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

RE: Special reinforcement around small openings @ concrete roof slab

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

RE: Special reinforcement around small openings @ concrete roof slab

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

RE: Special reinforcement around small openings @ concrete roof slab

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.

RE: Special reinforcement around small openings @ concrete roof slab

"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."

I Agree.  

RE: Special reinforcement around small openings @ concrete roof slab

Interesting discussion.

I am curious though when a "small opening" becomes a "big opening" that requires consideration in the analysis stage. I understand for small openings, the steel blocked by the opening is placed on both edges of the opening. That seems simple enough, but when would I know that I have a large enough opening that I have to take into account the opening when I am determining my bending moments when performing my analysis.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

RE: Special reinforcement around small openings @ concrete roof slab

I think ACI has something on opening around support regions. But judgement could be the most important factor.
Depending on the location, the strips on sides of the opening have to rely on adjacent members to pickup the loads, what is the picking up capacity of those members, what are the local effects...quite a few things to look at. There maybe rule of thumb I don't know about. I wouldn't worry about 1'x1', or 2'x2' openings, but for a small panel, these could be problemic too.

RE: Special reinforcement around small openings @ concrete roof slab

BARetired-

I like our idea of using the orthogonal bars.  They are there anyway.  It would relieve the congestion, allow for better concrete placement and so result in a stronger corner.  I will just increase the size a wee bit to replace the diagonals.  

Clansman-

Sometimes I see slabs designed by others and submitted to me for review that are riddled with large openings.  These are always designed as though the openings weren't there, with additional bars added to the sides of the openings.  These get a "revise and resubmit".
 

RE: Special reinforcement around small openings @ concrete roof slab

miecz,

I don't think you need to increase the bar size to replace the diagonals.  A #5 bar has an area of 0.31 in2.

If placed diagonally across a corner of the opening, it provides 0.31 in2 at right angles to a potential crack.

Two #5 bars placed orthogonally provide 1.414*0.31 = 0.44 in2, not only at right angles to the potential crack but in every other direction as well.

I conclude that 4-#5 bars placed orthogonally, one each side of the opening are more effective in resisting corner cracking than 4-#5 bars placed diagonally, one at each corner of the opening.

BA

RE: Special reinforcement around small openings @ concrete roof slab

BAretired,

In your experience, have you noticed better performance using orthogonal bars as opposed to diagonal bars for suspended slabs. We have always called up 2-N12 diagonal bars 1500 long around each re-entrant corner (2 x 1/2" bars (72ksi) 5' long) for slabs on ground and typically provide  orthogonal bars for suspended slabs.

All,

When the opening is located in lowly-stressed regions of the slab (i.e. regions common to both middle strips), I would simply place the additional reinforcement either side of the opening that was affected by the opening.

If the opening was a large opening in a highly stressed region of the slab (and I would classify a large opening as an opening which affects more than 1-bar), than I would definitely perform an analysis to ensure that the reduced cross-section can deal with the increase stresses.

dcceey,

Has the slab been poured and the client wants to core these openings.

RE: Special reinforcement around small openings @ concrete roof slab

asixth,
No difference in performance noted.  Both systems work fine.  It is largely a matter of preference.

BA

RE: Special reinforcement around small openings @ concrete roof slab

(OP)
The concrete has not been poured yet. But it will be a change of order. So cost will be more if we make too much change.

RE: Special reinforcement around small openings @ concrete roof slab

Back to the original post, I hope this roof slab has a membrane.  The typical reinforcement is quite light and will not control restraint shrinkage cracking.

RE: Special reinforcement around small openings @ concrete roof slab

Asixth,
what if the slab was 400thick? would you still supply 2-N12?

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

RE: Special reinforcement around small openings @ concrete roof slab

No, would only call up 2-N12 trimmer bars for slabs on ground which are reinforced with mesh. Don't generally call up 400 thick slabs on ground.

RE: Special reinforcement around small openings @ concrete roof slab

ok Asixth so you may not have done many container slabs in the middle of nowhere, but not meaning to be picky but the point is that 2-N12's are not sufficient for every situation. I doubt 2-N12s would have much impact on a thicker slab, I believe the house code specifies (AS2870)3-N12's as a minimum by the way.

I think the reinforcement whether diagonal or orthogonal should be in ratio to the situation, we could argue semantics about the differences but at the end of the day you need reinforcing to keep the crack small, this reinforcement needs to be or a size to match the problem. The thicker the slab the more reo required.

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

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