Concrete Columns K factor in Precast Buildings 2

[Gus14]

In Multi-story precast concrete buildings, where the columns are cast on site, while the beams and the slabs are precast. The beams simply rest on the corbels, so the columns are not restrained by the beams or the slab, therefore what K factor should be used ?

Also. What height should we design the columns for, the story height as we do in ordinary concrete building ?

 

[KootK]

I think that k=1.0 makes sense so long as:

1) The columns are not part of moment frames.

2) The CIP columns, as individually cast elements, are not multi-floors tall (and, perhaps, even if they are).

The beams simply rest on the corbels, so the columns are not restrained by the beams or the slab

I would normally consider that detail to laterally restrain the tops of the columns.

What height should we design the columns for, the story height as we do in ordinary concrete building ?

The design height should realistically represent the distance between points of lateral column support. Or a conservative approximation of that. So maybe the distance from the top of your foundations to the top of the uppermost corbel, assuming that you're not claiming lateral support from a SOG etc.

 

[BAretired]

This article from the 1992 May-June PCI Journal is worth reading:

 

[BAretired]

Following are excerpts from the above article:

 

[BAretired]

The beams simply rest on the corbels, so the columns are not restrained by the beams or the slab, therefore what K factor should be used ?

If that is true, you are relying on friction to hold everything together. I don't believe that is good practice. The beams must at least be pin connected to the corbels. As seen in the sketches above, the k factor can, in some cases, exceed 1.0.

 

[JohnRwals]

BA,
What do you mean by 'pin' connection?
Do you think simple connection between beam and column is not enough?
Is connection more positive than simple lateral/erection stability required?
-JRW

 

[BAretired]

What do you mean by 'pin' connection?

A pin connection is one which prevents translation, but permits rotation. Fig. 9a above is one example of a pin connection. There are many other ways to achieve it.

If "the beams simply rest on the corbels", that is a simple or roller connection in which the only force resisting translation is friction, which is not enough.

I do not understand your last sentence, but a positive link between corbel and beam should be mandatory.

 

[KootK]

If "the beams simply rest on the corbels", that is a simple or roller connection in which the only force resisting translation is friction, which is not enough.

A very common beam to column connection in precast construction is this:

1) Beam rests on column / corbel on a bearing pad designed to facilitate lateral movement. Like a small scale bridge girder bearing.

2) Beam is secured a bit by way of a coil rod running through an oversized sleeve cast into the beam end. Nut and anchor plate at the top. This is for erection stability as much, or more, than any other purpose.

From a lateral perspective, this is a friction connection. At the least, it is until you run out of play in the coil rod sleeve.

I've designed connections such as these where, inadvertently, a positive lateral connection was made between the beams and columns. In these cases, post installation shrinkage in the beams has sometimes resulted in the tops of the columns shearing off / spalling. Hence the preferred detailing that does allow for some movement.

 

[Gus14]

Thank you KootK and BAretired for replying.

I think that k=1.0 makes sense so long as:

1) The columns are not part of moment frames.

What if they are a part of sway frames ( cantilever column action )? K will be large and the columns design height should be the entire building height not just one story.

If the beams were cast on site as we do in typical concrete structures, but the slab was left as precast, maybe a sway frame would be more realistic, but still K will be large because I will likely only have a one beam stiffness framing into the column from only one direction, while the slab would still not be connected to the columns.

In these cases, post installation shrinkage in the beams has sometimes resulted in the tops of the columns shearing off / spalling. Hence the preferred detailing that does allow for some movement.

Thank you for sharing your experience.

This article from the 1992 May-June PCI Journal is worth reading

Seems straightforward, thank you.

 

[BAretired]

If usual practice is followed, N[sub]u[/sub] = 0.2V[sub]u[/sub], which is presumably adequate in most situations, but I would not rely on friction to provide it. For a multistory structure, N[sub]u[/sub] from wind or seismic forces could be larger than 0.2V[sub]u[/sub].

 

[KootK]

What if they are a part of sway frames ( cantilever column action )? K will be large and the columns design height should be the entire building height not just one story.

If they are part of sway frames then I assume that they are also part of the primary lateral force resisting system and, therefore, receiving loads from some manner of diaphragm collector. In that case, I also would not rely on friction alone.

For cantilevered sway columns I would expect:

1) K > 2.0, giving some account to base rotational flexibility and;

2) The column height to remain the distance from the foundation to the beam bearing. That said, one would normally use the elevation from the base to the diaphragm for the purpose of calculating overturning.

 

[Gus14]

Again thank you Kootk and BAretired.

The column height to remain the distance from the foundation to the beam bearing. That said, one would normally use the elevation from the base to the diaphragm for the purpose of calculating overturning.

I understand now thank you.

When the time comes to design one of these buildings, maybe it would be best to use braced frames with shear walls and tell the Architect to deal with it.