Splicing steel column to concrete column?

[jpw2913]

We are working on an existing concrete framed building and the owner wants to add a floor to the building, but wants to use steel framing. The existing roof level framing and concrete columns were designed for the added future loading (we have the building drawings). But what about splicing a new steel column to an existing concrete column? The existing drawings show a splice connection for a future column extension, but it's a concrete to concrete column splice detail (I've attached a pic of that detail). If the future column is steel wide flange is it just a matter of designing a moment base connection to the existing 1" steel plate that's cast into the top of the existing column? Thanks for any thoughts/help!

 

[jayrod12]

I would design it as pinned base steel column and provide your lateral resistance via cross bracing or the stair/elevator cores. Analysis is simpler and then you can breathe easier knowing you've got some leftover capacity there since there will be some fixity even with a pinned base.

 

[bones206]

I think you should look at it as having an existing embed plate on the top of concrete column. I don't think the new steel column must have a moment connection at the base - it can be whatever boundary condition is dictated by your framing scheme. So if you have shear/uplift/overturning reactions based on your framing scheme, just apply those as loads to the existing embed plate and evaluate the tension and shear capacities of the embed.

 

[Brad805]

You could use threaded rebar as anchor bolts. Then you can design it for a moment if you need. The base plate design for the steel column will be a tad unique if your steel column is significantly smaller, but it could work. If you are not familiar with those anchors, you might be surprised at the embedment. You will need to use the grout specified for the anchor.

 

[dik]

I'd just be using HILTI adhesive anchors... and go from there. Steel is a good choice because of added weight. Just a caution, whenever there is a change in load regime, you can have some unexpected results.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[bones206]

If I understand the detail correctly, the top of the column, as it exists today, is basically capped with a solid steel plate that is welded the the verts of the column cage. So I’m not sure threaded rebar or adhesive anchors would be viable options here. jpw please correct me if I’ve misinterpreted the detail.

 

[dik]

Steel plate secured in place... then just field fillet weld.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[jpw2913]

Bones - Correct, the top of the columns have the 1" steel plate already attached with the threaded bars shown in the detail. So we basically have a column base plate already existing that we can weld the new column to.

 

[KootK]

What's the lateral system for the existing building?

How many stories?

Is this a high seismic environment?

Will the existing roof be getting a topping in order to turn it into a floor?

Is there any indication that the designer of the original building had a certain lateral scheme in mind for the future extension?

I like the concept of using pinned based column connections in many respects. However, if you add braces at bays that don't already have stiff braces or shear walls below in a high seismic environment, you may wind up paying a pretty stiff penalty for that and creating new problems for yourself.

A scheme that I've used before in such situations, and feel good about, is to continue the columns up in concrete and then switch to steel at the roof level (beams, joists, etc). Particularly with a moment frame building, this allows one to extend the vision of the original lateral system upwards without having to deal with transfer diaphragms, discontinuities, and localized axial forces getting driven through columns and footings not intended for it. Of course, maybe your client simply wants to not have to deal with any concrete at all, particularly at height. If that's the case, then concrete columns are probably no go.

If the original lateral system was discrete braced bays or shear walls then you're in great shape with pinned column bases if you can just extend those elements upwards.

One drawback of just welding steel columns to the existing plates is that it doesn't provide much of a mechanism for plumbing the columns. That's not insurmountable but does introduce a wrinkle into the mix. One solution might be to weld a short, column stub to the tops of the columns and then install the rest of the column on top of that with a conventional column splice that would facilitate column plumbing. It doe introduce an additional step of course.