column holding down bolts

Is it acceptable practice to have a steel column resting on nuts which are used to adjust for height and upright then tighten down top nuts. .... then grout in the under gap. While this makes the procedure simple I find it strange that if this is acceptable then the holding down bolts are in compression not tension and tightening the top nut does not ‘pull down’ the column onto the grout.

 

[HTURKAK]

Steve G (Materials) , you are fully right.

The use of nuts on the underside of the column base for the leveling of column and base plate will prevent the preloading of concrete and grout so, not a good idea.
However, it is common practice for lighting mass, architectural columns..

In case of vibration, (for seismic, wind, vibrating machine ..) the use of adjustment nut under the base SHALL NOT BE ALLOWED.
Moreover, sometimes anchor rod with sleeve is used for preloading of the full footing height and to provide more ductility.

 

[TheDirtPounder]

The use of leveling nuts are extremely common in my industry. This is due to the need to achieve exact top of steel elevations across many stand alone structures. I rarely specify grout under the base plate as it tends to trap water and begin rusting the rods. Also, it can trap water inside the pole which can cause corrosion issue, and in some locations the entrapped water will freeze and crack the structure. Start looking at sign posts, light posts, etc when driving down the road and you'll see a lot of un-grouted anchor stand-offs.

You need to utilize proper methods to check the anchors for bending due to the moment arm provided by the stand-off.

 

[Steve G]

Thanks for replies, so if you do not grout under the base plate then the whole load that column is supporting is taken by the 4 threads ?? Do you provide calcs to ‘support’ your no grout principal ?

 

[TheDirtPounder]

The number of threads engaged will vary on the bolt diameter and the UNC of the bolt provided. I do not explicitly check the threads for compression, as I do not explicitly check the threads for tension. The bolt threads do not care which way the load is going (compression or tension). The only major difference between compression and tension on anchors with stand-off would be buckling of the bolt in compression and that will rarely ever be a concern, and this does not concern the threads.

Using a 1" dia. (ASTM A307) bolt as an example, the AISC steel construction manual specifies an allowable tensile strength of 26.5 kips (LRFD), so as long as the compression load on the bolt is less than 26.5 kips, I would not be concerned about the threads not carrying the compression load.

Now what HTURKAK said, I would not recommend stand-off to be used on connections that would be under heavy constant vibrations, but I would not consider wind or seismic a part of that as it is generally a transient load. Machine operation loads would be a concern.

 

[r13]

..., but I would not consider wind or seismic a part of that as it is generally a transient load. Machine operation loads would be a concern.

Couldn't agree less.

 

[Tomfh]

Agree that it’s fairly common to use the bolts only. It’s used a lot on tall freestanding posts, eg lighting towers.

Check bolts for bending too, as they can bend over.

 

[Steve G]

Thanks for clarifying and SandEstructures,
Since you are using for levelling then re buckling, the length is unknown , so a low pour slab could result in an unstable stand off, whereas an extra 10mm of grout has no influence . But accept all should be fine if there is good on site practice.
Obviously there is no issue with light structures but where the column is the support for several levels of a building sitting on a grout base still ‘feels’ better somehow !

 

[r13]

I don't really understand your concern. Shrinkage compensate grout is commonly specified for use in filling column base. The grout volume actually will expand a little, plus the dead weight from level above, the column and the grout will be fully engaged for proper functioning. There is no problem with leveling nuts.

 

[dik]

I've never considered this... and still don't. If concerned, put a neoprene washer under it. If the grout is overloaded, it will yield a bit and the load with be transferred to the base plate. BTW, I don't use this detail often.

About 50 years back, I was involved in a court case when 'shear friction' was introduced by the ACI. A local engineer had used it for securing spandrel beams shear walls for a multi-storey masonry building. The engineering prof I was up against had done a 2D FEM analysis to show that the bearing at the rebar, on the concrete was over 13 ksi. He couldn't answer on the stand what would happen if the concrete locally yielded... one of my first court wins.

Dik

 

[dik]

Steve G:
Some grouts have maximum thicknesses... just a caution.

Dik

 

[human909]

There is no problem with leveling nuts.

There potentially is. This issue has been discussed on this site and in numerous other guidelines.

Though I've seen them used commonly and I haven't heard of any issues arising from them.

 

[r13]

Though I've seen them used commonly and I haven't heard of any issues arising from them.

Same experience here. But I will visit the site, which might twist/change my mind.

Nah, I didn't lose my mind, it stays