Ad Hoc Construction Joint in Waffle Slab?

[TarikHKJ]

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 slab problem.dwg?dl=0

PDF if you dont have ACAD at the moment:

https://www.dropbox.com/s/t7s9t1urlkrsr3h/waffle slab problem.pdf?dl=0

not sure if this is possible for my case

http://www.concretenetwork.com/concrete-joints/construction-joints.html

http://www.sefindia.org/forum/files/construction_joints_119.pdf

Much appreciated!

 

[hokie66]

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?

 

[TarikHKJ]

@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!

 

[hokie66]

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.

 

[TarikHKJ]

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

 

[hokie66]

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.

 

[rapt]

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.

 

[TarikHKJ]

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.

 

[calvinandhobbes10]

"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.

 

[TarikHKJ]

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/0B3Q6BZ33FuxBMU9Hb1otZUR6dk0/view

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.

 

[hokie66]

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.

 

[TarikHKJ]

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

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

 

[hokie66]

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.

 

[TarikHKJ]

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.

 

[TarikHKJ]

This a PDF of the Drawing

https://www.dropbox.com/s/m9l26n8fy47g46j/B3 RIBS 22-5-2016-Model.pdf?dl=0

 

[hokie66]

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.

 

[TarikHKJ]

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.

 

[hokie66]

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.

 

[rapt]

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.

 

[hokie66]

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