Jumpform Columns

[rscassar]

Hi guys

I have a contractor who wishes to jumpform columns. What this means is that the negative moment steel is fixed thru the column with couplers installed in the column and a construction joint is formed all around the column.

Is this a common form of construction and is it feasible to have moment continuity thru this section despite that fact that we have a cold joint around the column?

Also, how would shear transfer into the column be analysed. Punching shear will not be an issue because there is no column projection into the slab. The shear equations given in the code for beam shear are not applicable because the slab-column joint is not in-situ. I image that shear transfer needs to be analysed by shear friction. What are the equations for shear friction and where can I find some resources on designing for this?

A sketch of the section has been attached.

All help and input on this one will be appreciated.

Thanks and regards.

 

[ishvaaag]

Punching shear would still be an issue within the slab realm; and longitudinal rebar related to such punch shear must be engaged with the shear reinforcement if such is needed. Then, at the faces you will need to stand for the hiifhest shear, beyond the other values in punch shear within the slab.

If you want to go conservative, the shear friction reinforcement could be dimensioned entirely separated from required longitudinal reinforcement for negative (or eventually positive moments in inversion of moments at the joint).

A more rational scheme would consider a strut and tie scheme at the joint. The struts, in the case above, and in this, must not exceed the permitted limit stresses. Transfer of shear in the strut and tie scheme would be trough inclined struts, the inclination of which not to exceed the allowed friction angle for the cold joint.

So in my view the more rational way to dimension the thing is trough strut and tie scheme.

If you persist to dimension it through a shear friction scheme I would add the required shear friction requirement for a cold joint to the required longitudinal rebar from moments. I would check by some rational mean that the struts are strong enough to transfer the shear without violating limit stresses. And I would check that within the slab itself, punch shear stays controlled, be it reinfored or not.

Respect the equations I would search in ACI-318 in chapter 11 for shear friction. 11.6.4.3 assigns 0.6=tan(phi) for such cold joint in normal weight concrete, or normalized angle of about 31 deg to longitudinal steel for the strut you may draw to pass the shear.

 

[hokie66]

I would not allow this form of construction. The moment development is not an issue, but shear certainly is. Rebates are always provided in slipformed walls, so the same principle applies to columns supporting beams, even moreso.

 

[pwht1]

An interesting proposal, what are the column dimensions?

Would a ripout-box be an option instead of couplers? That'd give you 30mm of bearing to help with shear transfer...although the column reo cage would be reduced.

 

[rapt]

Punching shear is your major problem. I would treat it like a lift slab detail, with all of the punching shear taken by a bracket below the slab attached to the column.

 

[bookowski]

Is this only on perimeter columns or interior also? Part of the lateral system?

ishvaag - why do you say punching shear is the problem? If the beams can be made to deliver all of the slab load through beam shear how is there punching shear?

Even if you can show that this works by ACI shear friction (11.7) it doesn't seem like a good detail. You are going to be left with a very smooth surface along that plane. Why can't they provide a box out in the form to give you a seat? You have 1.5" to your cage. You'd still have to show that it works by strut and tie or shear friction but you'd probably sleep better at night if there was a nice seat there as well.

 

[rapt]

bookowski,

What beams?

 

[hokie66]

I first read it like bookowski, thought beams were landing on the column. Now I see that it is a slab. The misleading thing was that kikflip said punching shear was not an issue due to no projection of the column into the slab...but the column projects 100% through the slab in his detail. Shear friction is black magic, and I would never depend on it in this case.

 

[rowingengineer]

I agree with rapt about supporting brackets and with hokie in regards to shear friction being black magic.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[ishvaaag]

Of course, if the beams are enough to carry the shear, punching shear won't be of concern. I commented that because the graphical detail seemed to correspond to just a slab and column. But punching shear may be a problem within the slab side.

The question here is that friction shear is not a concept for reinforcement in shear, but a connection concept.

Hence, shear friction is used to transmit shear between two parts, both able themselves to carry the interface shear, and full development of the friction-shear rebar is assumed to be well anchored to each side of the interface.

On the other hand, except that support is different, brackets also rely in adequate tensile strength and anchor enough to the receiving side of the interface, so it is at least on these regard some kind of variation of the shear friction concept, and one can even visualize the compression strut that would be outside one side in the pure shear-friction concept within the receiving side. Really it is about the same, just the "capacity and anchor enough" in the bracket side is ensured by the weld or cast integrity.

 

[RHTPE]

This is not an unusual request from a contractor trying to systemize his formwork. I have experienced this on many mid & high-rise building to permit the concrete core walls to advance ahead of the floor system. Makes for speedy & economical concrete construction.

That said, the vertical element(s) should be cast with blockouts to form a shear key to receive the horizontal element. The elevations of the shear key blockouts will require careful coordination during construction to insure that they match the intended floor elevation(s). This is probably where things get screwed-up the most, and there is no easy fix for misplaced keys and dowel bar subs.

This construction methodology is not always permissable and should be carefully examined by the EoR for connection compliance with design assumptions.


Ralph
Structures Consulting
Northeast USA