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Ad Hoc Construction Joint in Waffle Slab?

Ad Hoc Construction Joint in Waffle Slab?

Ad Hoc Construction Joint in Waffle Slab?

(OP)
Hi,

I'm not sure if this is going to sound a bit stupid but here's the situation:

I work at an 8-story parking garage building project, we already completed the first waffle slab and now the second slab is on its way, awaiting confirmation for the time to pour concrete,

but one of the management has decided (NOW!!) that this building is too big and needs a construction Joint to split the slab in half to prevent shrinkage, etc.

I'm not sure this is possible now, and I came here for help.

this is an EXTREMELY rough sketch and I will post the structural and other CADs later, : https://www.dropbox.com/s/gqwb9xwqwd75383/waffle%2...

PDF if you dont have ACAD at the moment: https://www.dropbox.com/s/t7s9t1urlkrsr3h/waffle%2...

not sure if this is possible for my case http://www.concretenetwork.com/concrete-joints/con...
http://www.sefindia.org/forum/files/construction_j...


Much appreciated!

RE: Ad Hoc Construction Joint in Waffle Slab?

You are talking about a construction joint, but then show a detail of a movement joint. Which is it? What are the overall slab plan dimensions?

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
@hokie66 sorry about the expansion joint link chrome has 20+ tabs things got messed up, original post edited for new links,


I'll get the CADs soon by email, will upload them as soon as possible,
but 40+ meters by 60+ meters, spans between 7m and 10m , slab thickness 50cm , topping 7cm

Thanks for the reply!

RE: Ad Hoc Construction Joint in Waffle Slab?

A construction joint won't materially assist with "shrinkage, etc." The contractor is probably wanting the joint because 2400 m^2 is a large area for placement, and particularly for finishing in one day. This of course is limited by the supply of concrete, craneage, and size of work crew.

A construction joint is common for constructability reasons. Just place it near centre span, where shear is minimal.

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
Thanks again for the reply

sorry for not being so clear, English is not my first language,
the slab reinforcement steel is already built , and I want a section or a detailing of the construction joint, esp. that the primary beams' reinforcement is already built with no consideration for construction joints

RE: Ad Hoc Construction Joint in Waffle Slab?

You would have to seek advice from the project engineer about specifics required for a construction joint, as we can't design it for you here. The primary reinforcement should not change. However, and depending on the location of your construction joint, the engineer may require provisions for carrying shear across the joint. This could conceivably involve adding short lengths of reinforcement, dowels in affect, which go through the construction joint.

RE: Ad Hoc Construction Joint in Waffle Slab?

Talk to the designer. It is his decision as to where the joint is placed and the details.

And make sure it is ALL propped until the second pour has reached stripping strength.

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
Was hoping you could give me an idea on how to design construction joints in waffle or ribbed slabs, I know that per ACI code we place construction joints at areas of minimum shear (typically in middle third of span) or point of contraflexure, but would be very helpful if someone could help me get starting in making the detailing of the waffle slab construction joint, or at least refer me to resources, as I have only couple of months of structural design experience and have not designed construction joints before.


Kindest regards,
T. K.

RE: Ad Hoc Construction Joint in Waffle Slab?

"Design" of CJ's by the structural engineer usually entails providing a typical detail and giving permissible locations (midspan, third-span, etc) for the contractor to utilize them. If this info is not present on the structural drawings, the structural engineer needs to supply/confirm the joint. Construction joint layouts are also often required as submittals for A/E review.

Shear is transferred by added rebar dowels as hokie suggests, or forming a shear key with wood nailed to the bulkhead in each waffle rib perpendicular to the span. I think dowels are easier for the slab since it's so thin. Pick whichever is easiest to build, and send RFI asking if it is OK along with the location (I recc third-span of your center bay). Owner sounds very capable of alerting the A/E of its urgency.

Also---waffle slab, cool! Figured those were out of style these days but guess not.

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
so should the rib reinforcement disconnect at the CJ, or should it continue through? and should the construction joint cut the primary beams too?

These are the CADs : https://drive.google.com/file/d/0B3Q6BZ33FuxBMU9Hb...

it would be really helpful if you could illustrate your answer calvin on this whiteboard https://whiteboardfox.com/66238-7956-7446 since I am not as familiar with some of the terminology you used, I work in an Arab country and technical English terminology isn't widely used.



Kindest regards,
T. K.

RE: Ad Hoc Construction Joint in Waffle Slab?

I can't open any of your CAD stuff, but the slab must function as it would without the joint. That means the reinforcement must be continuous through the joint. You have to build bulkheads which allow the reinforcement, including all flexural reinforcement and shear dowels, to pass through. This will be a tedious job in a waffle slab.

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
Thanks hokie! you couldn't be more clear and helpful.

I may get back here for more help if that's ok..

RE: Ad Hoc Construction Joint in Waffle Slab?

You are welcome, and good luck. When the contractor finds out how much work is involved in providing a construction joint, stripping it, cleaning out and preparing for the next pour, he may reconsider and instead increase the resources available. That is assuming that he is reasonable.

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
The strange thing is, he came to us and said that he thinks its better to make a construction joint since he's not sure that its possible to pour 1000+ m3 in one go,

Thanks again.

T. K.

RE: Ad Hoc Construction Joint in Waffle Slab?

I can see the problem with casting that all in one go. But as I am sure you agree, the decision on construction staging should have been made much sooner, so that the formwork and reinforcement could be coordinated with the joint. But if you place the joint at midspan, running vertical on your drawing, there should be less top bars to contend with, which you will probably have to remove and then reinstate. You should be able to slot the bulkhead plywood over the bottom bars.

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
As per ACI 222.3R-95 , 3.2.2.1 : "Beams and slabs—Desirable locations for joints
placed perpendicular to the main reinforcement are at points
of minimum shear or points of contraflexure. Joints are usually
located at midspan or in the middle third of the span, but
locations should be verified by the engineer before placement
is shown on the drawings. Where a beam intersects a
girder, ACI 318 requires that the construction joint in the
girder should be offset a distance equal to twice the width of
the incident beam."

I understand that the point of contraflexure is the zero moment where moment changes sign, but does this point also coincide with the points of minimum shear?

Kindest regards,

T. K.

RE: Ad Hoc Construction Joint in Waffle Slab?

No, minimum shear is at maximum positive moment, thus very close to centre span in most cases. Draw your shear and moment diagrams to convince yourself. In my opinion, locating joints at contraflexure is poor practice, so I think ACI is wrong about this.

In your case, with a waffle slab, I don't think the part about a beam intersecting a girder is applicable.

The most important part of the clause you quoted is the need to verify with the engineer. You have stated that you have limited experience, so these decisions should not be made by you alone.

RE: Ad Hoc Construction Joint in Waffle Slab?

With joints in PT slabs I would always put them at the point of contraflexure (or about 1/4 point in the span. Tendon profiling and anchorage placement do not work at midspan.

I would tend to do the same with RC slabs. As long as the short end is poured first, it allows the builder to "accidently" remove the formwork before the second section is poured without problems.

Remember that even though the full load shear diagram is 0 at midspan (in a symmetric internal span), once you consider partial loadings it could be anywhere in about the middle 1/3 of the span.

RE: Ad Hoc Construction Joint in Waffle Slab?

I do agree with rapt about the contraflexure with PT slabs, but not with deformed bars, and I assumed this waffle slab is PT.

RE: Ad Hoc Construction Joint in Waffle Slab?

Quote (hokie66)

I assumed this waffle slab is PT.

I assume you meant NOT PT.

RE: Ad Hoc Construction Joint in Waffle Slab?

Yep, I left out the key word. Thanks.

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
Its a dire situation, the seniors are convinced that we must place a construction joint at the span in the middle of the slab, solely to assist with the problem of thermal expansion of concrete. Opposite to what hokie said in his second post that a CJ will not materially assist with that.

Kindest regards,
T. K.

RE: Ad Hoc Construction Joint in Waffle Slab?

I don't know why it is dire. The joint won't hurt if properly constructed, whether made for good reasons or not.

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
They want to do something different, to pour areas 1 and 2 and leave a 1 meter strip (area 3) to be poured 1 week later...
The 1 meter strip they will pour later is at third of span in the middle span of the slab

Reference sketch : http://s19.postimg.org/ogsh5p0n7/screenshot_41.jpg

do you think this will help with thermal expansion at all?


Kindest regards,
T. K.

RE: Ad Hoc Construction Joint in Waffle Slab?

Quote (OP)

do you think this will help with thermal expansion at all?

It won't help with thermal but it will help with shrinkage which is probably the primary concern. In my neck of the woods, we'd call this a "pour strip" and I think that it's an excellent solution here. If I were the contractor here, I think that I'd want to create the construction joint by removing a line of waffle slab forming pans anyhow. That way you'd make the joint in solid (not ribbed) concrete and, I think, have a much easier time installing the bulkheads and creating a shear key(s).

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.

RE: Ad Hoc Construction Joint in Waffle Slab?

Yes, shrinkage is an issue, not expansion. And a delayed pour strip is helpful, but a week is not long enough to make a big difference. Agree with KootK that removing a line of pans would make the construction joint much easier to build.

Hard to tell from your plan how much restraint to shrinkage you have. Are there perimeter walls, or are the shaded parts just balustrades?

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
There are perimeter walls from 3 sides.

RE: Ad Hoc Construction Joint in Waffle Slab?

Then shrinkage is certainly a big issue. Those long side walls will restrain the slab from shrinkage. I hope you have a lot of reinforcement to constrol the size of cracks.

RE: Ad Hoc Construction Joint in Waffle Slab?

I imagine that's the two short side and one of the long sides. As such, a delay strip across the short dimension will probably make pretty good sense.

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.

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
The two long sides are perimeter walls, and the fourth shorter side does not have walls to give access to the interfloor ramp.

RE: Ad Hoc Construction Joint in Waffle Slab?

Agree with Hokie66, a one week delay will have little effect on shortening restraint. And with the walls along the 2 long sides, you will need a lot of reinforcement for crack control. Australian code would say at least .6% and possibly more.

RE: Ad Hoc Construction Joint in Waffle Slab?

I know it is too late now, but this structure should have been designed so that restraint is minimized, rather than provide a lot of crack control reinforcement. With adequate crack control reinforcement, there is still a lot of cracking, but the cracks are hopefully not very wide.

There will also be restraint cracking perpendicular to the shorter end wall.

RE: Ad Hoc Construction Joint in Waffle Slab?

Good to see you have a sense of humour.

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
is this acceptable?
not sure, but maybe people with more expertise than mine can say their opinion?

Kindest regards,
T. K.

RE: Ad Hoc Construction Joint in Waffle Slab?

If it is truly a construction joint (or delay strip), the entire cross-section of the concrete waffle slab needs wood bulkheads. The ribs, with wood cut to fit tightly against the pans; and the top slab itself, also cut tightly. You're probably seeing why it was suggested to remove one of the rows of waffle pans completely---a little more concrete but a more straightforward joint.

RE: Ad Hoc Construction Joint in Waffle Slab?

(OP)
The bulk heads they made did not hold up to the concrete weight, and they ended up cancelling it,
Follow up question, waffle slab formwork removal is 28 or 14 days ?

RE: Ad Hoc Construction Joint in Waffle Slab?

It should be in the contract drawings. It's typically expressed in a length of time or a percentage of the design strength.

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