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BadgerEngineer (Structural) (OP)
23 Sep 08 14:24
We often get asked by contractors if its acceptable to pour deep concrete beams in two pours.  This usually occurs when the slab ties into the side of the beam.  (Top of slab elevation is lower than top of beam elevation) And GC wants to pour the slab to outside edge of beam in one pour.

Is this something others are allowing? Typically we do not, however, this has been met by much resistance.

If you were to allow a cold joint in a concrete beam - what is the primary failure mode that you'd need to check.  I understand longitudinal shear at the cold joint may be an issue.  However, if the hooks are embedded in the lower section of the beam you will have rebar continually crossing the cold joint.  In addition, the concrete can be left rough at both faces.  Concrete friction and your #3 hooks (or similar) should provide enough strength to resist this load.  

Another issue for exterior beams may be water penetration - if a cold joint is left in the beam, water could easily find its way into the beam and corrode the rebar.  

Does anyone have any references or design suggestions for me on this topic?  What should I be checking to approve this method?

I greatly appreciate your input.  
DaveAtkins (Structural)
23 Sep 08 15:18
What you mention in your post is what should be checked--horizontal shear at the interface.

I have used such a detail on occasion with no problems.

DaveAtkins

msquared48 (Structural)
23 Sep 08 15:33
Shouldn't really be a problem if you leave a roughened concrete surface and have shear steel projecting through the interface.

Mike McCann
MMC Engineering

civilperson (Structural)
23 Sep 08 15:56
Ask the price for a cold joint bulkhead between the slab and the beam in order to place the beam monolithically.  A second placement can be used to make the slab.
JAE (Structural)
23 Sep 08 16:06
Keep in mind that to use shear friction across the joint, the ACI code requires the reinforcement to be developed to achieve full fy.  This means that you need to have enough distance on either side of the joint to at least have full hook development lengths.  (the reduction using As(req'd)/As(provided) is not allowed in this case)

For a 4 1/2" slab or such it is impossible to provide this development and you would therefore ignore the top, second pour and count on the lower portion of beam alone.

 
hokie66 (Structural)
23 Sep 08 20:48
I would have no problem with casting the upstand part of the beam later, provided that the mating surface is rough and clean.  But depending on the exposure condition, this joint could be an area for water penetration, as you indicated.

The other thing to consider is that the upstand will shrink later than the beam below, so some additional crack control reinforcement close to the joint should be considered.

JAE, I don't see that this joint involves shear friction.  It is just horizontal shear.
JAE (Structural)
23 Sep 08 21:18
hokie66 - I guess, based on ACI 318, that there is no other means of determining the shear "capacity" of horizontal shear conditions without using Shear Friction.  To me, this is exactly the sort of condition that shear friction covers.

Vs + Vc shear values found in chapter 11 deal with shear in beams with flexure, an entirely different shear behavior than that produced by horizontal shear.

Since the shear plane is due to concrete cast at different times, and the surface condition can vary a lot (troweled surface, as-cast surface, or intentionally roughened surface) shear friction is the way to go.

The spacing of stirrups across this horizontal shear plane would be dependent on the shear and these would be added to the flexural shear stirrups.

Thoughts?

 
hokie66 (Structural)
23 Sep 08 21:47
I am not versed in the current ACI Code, but to me shear friction applies to brackets, corbels, etc.  Horizontal shear can be developed between a structural topping and precast slabs without reinforcement crossing the joint.  There would probably be requirements for shear steel crossing the horizontal joint in the beam, but I think that is different from shear friction provisions.
DaveAtkins (Structural)
24 Sep 08 6:37
I agree with JAE that shear friction is the mechanism by which the horizontal shear in this beam is resisted.

However, the ACI Code has a specific section on horizontal shear.  You don't follow the shear friction section of the Code for this situation.

DaveAtkins

BadgerEngineer (Structural) (OP)
24 Sep 08 7:56
The beam for this case is 36" deep. (The upstand portion is 10")  The hooks in this beam will certainly be able to acheive full development length on the lower side of the joint (26"), however, the upper 10" section would need to be checked.

JAE - can you tell me where this statement the "ACI code requires the reinforcement to be developed to achieve full fy." in ACI?

I agree with JAE regarding checking shear friction.  Per ACI 11.7.1 "Provisions of 11.7 are to be applied where it is appropriate to consider shear transfer across a given plane such as an existing or potential crack.... OR an interface between two concretes cast at different times"

Based on the shear friction design method Vn=A*fy*coef friction.  (Friction Coef for normal weight roughened concrete = 1.0 per ACI 11.7.4.3)

For this condition I simply plan to check VQ/Ib<Vn.   

Does anyone disagree with the above?   
JAE (Structural)
24 Sep 08 8:16
DaveAtkins - hey if you can, give us the ACI reference for horizontal shear that you mention?  I can look it up too, just thought having it on this thread would be good...thanks.

Badger - I'm away from my office right now but in ACI 318 there is a paragraph that states something like this:  "When the reinforcement is required to develop full yield, then the provisions for As(req'd)/As(provided) cannot apply."   I'll look it up later and post it here unless someone beats me to it.

 
Lion06 (Structural)
24 Sep 08 8:27
It's in 17.5.
It does reference minimum ties in accordance with 11.5.6.3 (which is the minimum stirrups for beam shear).  Does this mean the stirrups can do double duty?
WmacG (Structural)
24 Sep 08 10:04
Would this situation not be the similar to a beam with a composite slab?
JAE (Structural)
24 Sep 08 10:06
ACI 12.2.5 (the As/As provision for development length) states that you can use this reduction "except where anchorage or development for fy is specifically required..."

If you look in the shear friction section, 11.7.8 states that you must use reinforcing that is developed to fy.

StructuralEIT - I think 17.5 probably does apply here more-so than the shear friction section.  But it looks like they are similar.  17.5.3.4 actually refers you back to shear friction when Vu exceeds a set limit.

 

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