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Diagonal Reinforcement around opening in Pile Mat 1

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allimuthug

allimuthug

Civil/Environmental
Oct 5, 2014
142
Hi,

I have a Pile Mat of 10 meter x 15 meter supported on Pile. It has a opening of 5M x 3M as shown in the picture attached.
Can any body explain me how to provide the diagonal reinforcement around the opening, and why it is provided.
MAT_SLAB_uuod7p.png
 
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Diagonal reinforcing at re-entrant corners if often provided. I usually don't and usually use reinforcing parallel to the opening and extending a couple of feet past the opening (normally done) and the reinforcing parallel to the opening can be increased to reflect the area of steel provided by the diagonal bars. Having diagonal bars only congests the corners and makes it more difficult to consolidate concrete in areas that may be subject to cracking.

Dik
 
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Allimuthug, to confirm, this is a slab-on-grade condition where the mat is continuously bearing on ground, and therefore you don't expect significant tension loads in any reinforcing bars, is that correct? If this is the case, then I have nothing to add to Dik's post because the reinforcing functions just for temperature/shrinkage.

In the event you expect flexural/tensile stress on the mat, then you would want to analyze the loads closer and make sure you have full load transfer, comparable to a wood header which supports joists at an opening, and then the header frames into a larger joist/beam that spans the full length.
 
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NorCal...
"In the event you expect flexural/tensile stress on the mat, then you would want to analyze the loads closer and make sure you have full load transfer,"

Even for flexural/shear design, I would not normally use diagonal bars at re-entrant corners... cheaper if you don't and minimises congestion.

Dik
 
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I agree about not using diagonal bars for the congestion reason you've mentioned. Even if you're counting on bars to resist loads, you could still get proper load transfer around the opening via "trimmed" bars to "header" bars to "full-length" bars parallel to the trimmed bars.
 
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Seems to me that we need to know the thickness of the "Pile Mat" before commenting about the reinforcement. I assume it is thick.
 
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Hokie... thick or thin, no diagonal bars for me...

Dik
 
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1) The use of diagonal bars seems to be a matter of personal, designer choice these days.

2) Like other trim bars, we're usually attempting to develop them on both sides of the presumed crack that we're trying to keep closed.

3) If there's a way to calculate the quantity needed, I don't know it. I use 2-#4 top and bottom for slabs around 8" thick and scale up and down from there.

4) Don't forget about your re-entrant corners as well.

Diagonal_Bars_szwgdo.png


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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If it is 650 thick, I wouldn't bother with diagonal bars. But if you want them, there is certainly no congestion issue. With a thick slab like that, you will already have a lot of orthogonal reinforcement adjacent to the openings and extending past.

With slabs on grade, diagonal bars at reentrant corners serve a purpose in controlling the stress riser type cracks there. Get the bars as close as cover conditions allow.
 
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First of all thanks for the suggestions.
However I still have some clarification.
(1) Can the slab can crack as shown in the figure attached below, and if it cracks what is the solution or remedy to avoid it.
CRACK_ueexpg.png


(2) For this Pile Mat of 15 meter x 10 meter, do I need to provide expansion joint. Please ex pain me in detail regarding the expansion joint for a Pile Mat.
 
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I've never seen a crack start from a corner as shown... maybe mid way between the corners and at right angles to the opening...

Dik
 
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Can you explain me why the crack doesn't starts from there. I want to know the reason why the crack forms at right angles and not for the above one as shown.
 
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Cracks form at the reentrant (interior) corners where you show diagonal bars, not at the projecting corners as your last sketch shows. Rather than us explaining to you why cracks don't start at the tips, maybe you can explain to us why you think they would, as in practice they don't.
 
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The main reason I cannot see them start is that there is no lack of restraint where there is with re-entrant corners...

Dik
 
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All the sharp edges in concrete would crack due to expansion and contraction.
However there is no restraint in the re-entrant corners for expansion and contraction(as mentioned earlier by Dik).
Since, there is restraint for the interior corners (where diagonal bars are shownby KootK)the cracks are formed.
Please correct me if I am wrong.
 
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maybe have interior and re-entrant corners mixed...

Dik
 
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Reentrant and interior mean the same thing, in my lingo. Those are the ones which will crack. The external corners don't.
 
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