Stress concentration under packer/shim 1

[bugbus]

General question about using packers and shims between precast concrete elements to provide a gap for forming a grout or mortar bed.

Is there any risk with using a stiff (i.e. steel) packer that there would be a concentration of force through the packer under applied loading or post-tensioning (and potentially causing issues with bearing failure or splitting in the concrete)?

Is it better to use a softer material (e.g. plastic) to ensure the grout or mortar takes up the load?

And to what extent does the shrinkage compensation or expansive behaviour of the grout/mortar have on alleviating this?

 

[HTURKAK]

Lets be more specific.. your thread implies pc column supports with socket ( pocket provided in foundation ) or with base plate and anchor rods.. For both cases you will need shims for adjustment of levelling and to provide space for grout .

There are welded and bolted base connections also used at some localities.. one of them is which I found with web search,

Pls post a sketch to get better responds....

 

[r13]

No. The packer or shim shall have a compressive strength at least the same as the grout. The concentrate force is short lived, and usually the intensity of the load is much less than the fully loaded structure. At erection, only member weight will be there.

No effect, between two hard surfaces, the grout will deform side ways that releases much of the pressure.

 

[KootK]

Is there any risk with using a stiff (i.e. steel) packer that there would be a concentration of force through the packer under applied loading or post-tensioning (and potentially causing issues with bearing failure or splitting in the concrete)?

Yes, I think so. In most common situations the load path through the shims will be stiffer than than through the grout. Two things may mitigate this:

1) The concrete in contact with the shims benefits from the bearing stress increases associated with improved confinement and;

2) If the mode of failure were concrete crushing locally, redistribution would presumably take place.

Is it better to use a softer material (e.g. plastic) to ensure the grout or mortar takes up the load?

Probably but:

3) I've not seen that done and;

4) I'm not sure how to go about doing that reliably with respect to matching stiffnesses etc.

And to what extent does the shrinkage compensation or expansive behaviour of the grout/mortar have on alleviating this?

5) If the grout is expansive, you probably lift the member clear off of the shim stack.

6) If the grout shrinks, all of the weight goes to the shim stack until crushing and redistribution occur.

5) I don't see how one could reliably tune the grout expansion just right.

The situation is almost identical with steel columns. They too have shim stacks and, what's worse, common thickness base plates mostly suck at being stress distribution elements until after local crushing and redistribution occurs.