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MoreCowbell 11-04-2011 10:04 PM

Preparing footing surface for steel column
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Hello. I am getting ready to construct my new column footings for my crawlspace-to-basement project. The footings will support steel columns resting on 1/2" think baseplates as shown in the attached graphic.

My assumption is that it is critical that I get the footing surface as flat and as parallel to the baseplate as I can so that the entire baseplate surface is resting on the concrete. I'm thinking of several ways to do this:

Method #1: Form the concrete surface as level and flat as possible using a bubble level. Install the welded column assembly on top after curing and hope that I got it exactly right.

Method #2: Form the footing as in #1 above, but position the baseplate and column separately and weld together in place (this allows the baseplate to adapt to the footing surface.

Method #3: Install the welded column assembly on the cured footing as in #1, but apply a thin layer of epoxy or grout under the baseplate to provide an even surface for the baseplate to rest on.

I'm curious how others would handle this. Is this really as critical an issue as I'm thinking or am I getting way too anal about this unnecessarily? Thanks in advance for any replies.

BridgeMan 11-04-2011 11:50 PM

Yes, you definitely are approaching anal-hood. If it were mine, I'd just pour a simple footing (without the step-up as shown in your sketch--that's only extra work, and accomplishes nothing), making sure it's completely level and a true plane surface. Then install the pre-assembled column and base plate assembly by ever-so-slightly jacking the load-carrying member(s) above the footing an eighth-inch or so to enable you to "talk" the welded assembly into position with a few hefty pokes with a maul.

Should you choose to advance to the next level of anal-hood, you could place a thin layer of non-shrink mortar on the footing surface immediately before placing the base plate and column assembly. Some state DOTs still routinely use that method prior to installing bridge girder bearing plates, although I think just as many don't as do. I've always preferred the far simpler method of making sure my concrete bearing pads are "true", even if that means doing a bit of careful grinding with a large masonry wheel on an angle grinder. In the old bridge building days, we used large pieces of sheet lead under each plate to make up for any small imperfections. Over time, the stuff would actually flow out and curl from under the bearing plates as live loads were applied.

joecaption 11-05-2011 07:38 AM

Most important to have that footing the correct size and depth for you soil conditions. If it's wrong it's just going to tip or sink.
If the top is as level as you can get then there is no real need to all that morter or other stuff. Those 1/2 thick pads at the bottom and top will be more then enough contact to support it.
A much simpler way then a welded one shot deal coloum is an adjustable lally coloum.
Steel Post Shore, House Jack, Screw Jack, Screw Jack, Scaffolding Screw
Lowes and Home Depot carry them.

MoreCowbell 11-05-2011 07:46 PM

BridgeMan, thanks for the reassurance. I'll definitely just get the footing surface as level as I can and install the fully assembled column.

Joe caption, I'm actually replacing preexisting masonry columns but with deeper footings. I'll be building the new footings with the same dimensions as the originals, so I should be OK. Regarding the actual columns, I'm using 4" schedule 40 steel piping for peace of mind.

Regarding the extra "curb" section at the top of the footing, I'm doing that for two reasons. One reason is to raise the base of the steel column to the level of the slab surface (to reduce rusting and facilitate future repairs). The other reason is because I think it would make leveling the footing surface easier. The main part of the footing form will be cut into the thick clay. The curb form will sit on top and should be easier to adjust to level (at least that is the plan).

Any suggestions or criticism is welcome. Thanks again!

BridgeMan 11-07-2011 01:40 AM

I'm not following the reasoning for raising the column base elevation (to avoid corrosion on the steel members)--is the crawlspace frequently full of water? If that's the case, you need to correct that problem before doing anything else.

But here's something to think about, if you're determined to install a small step-footing on top of the larger one. Properly consolidating the fresh concrete usually involves beating on the forms, or spading the mud with a wide float or trowel. Either way, there's a very good chance that you will loosen and move (raise) the forms slightly, allowing mortar to get underneath them. Once that happens, forget about things being level--you'll be fighting the silly thing and talking to yourself, trying to get the concrete even reasonably level.

Here's a hint--after you strike off the finished surface, let the bleed water come up and evaporate (or help it along, by laying down a few large rags and gently removing them). Then do your final finishing with a steel float, bearing down hard to bring up the fines. And then, take a torpedo level and check for level with it both ways (at right angles to each other, each check), where your column base will be placed. Works best if you have a strong light source, as it makes it easier to see glimmers of light coming from under the level from any low spots present. Work in a dab of mud with the float at any low spots, and check again with the level. After stripping forms and some cure time, check with a level again, and use a rubbing brick to knock down any small lumps that might have sneaked in.

MoreCowbell 11-07-2011 07:57 PM

That is right BridgeMan. My intention with that curb section is to move the base of the steel column away from any moisture that may exist beneath the vapor barrier. I agree that any obvious water problem should be addressed before I convert the crawlspace to a basement, but my concern was the affect of small amounts of moisture under the slab over time.

You make a good point on the how difficult the curb forming will be in practice. I never thought of having to bang the forms to get the concrete to compact. That would certainly disrupt any fine tuning I did with leveling the forms.

Nevertheless, I'm pretty much locked into that plan at this point since I already bought the custom cut steel piping and plates late last week. I think I'll end up just leveling the curb as best I can, then use your suggestion on the non-shrink grout to account for any error.

Thanks again for the tips!

nealtw 11-07-2011 08:42 PM

It is standard to build a curb to floor level on top of the footing, building it is simple. If you have built you footing form and staked it in place 3 1/2 " below floor level, just use two 2x4s on edge across to top with two spacers between them. Use screws for easy removal.
You could also have stood you post on the footing and then layed the floor around the post.

MoreCowbell 11-11-2011 06:33 AM

That is pretty convenient that the width of a 2x4 matches the depth of my slab / footing curb. That does sound like the easiest way to form the curb. I just picked up my building permit yesterday, so nothing is standing in my way now.

The only thing I'm considering now is if there is a way to avoid drilling my bolt holes for the column plates into the footings after they are cured (I'm hesitant to use J-bolts in the wet concrete as I'm thinking it will be difficult to locate them properly without the column sitting on the footing). However, that is a topic for another thread I suppose. :D

Thanks again for the replies!

nealtw 11-11-2011 03:59 PM

Make a plywood template for the bolt pattern and fit it to the forms where you want it and mark where it goes and remove it when the concrete is still wet place the template with the bolts back on top. Use bolts one size smaller than the holes in the post base.

BridgeMan 11-11-2011 05:17 PM

Unless you're planning to weld the steel column to the base plate in place, you cannot embed the anchor bolts in the wet concrete. Projecting above the concrete, they will be in the way, when you want to slide the column/base plate assembly into place.

If you decide to weld things together in place, make sure to allow at least 4 or 5 days' cure time on the concrete, such that the welding heat passing through the plate doesn't blow out the moisture-laden concrete underneath it. I've seen green concrete literally explode when in contact with hot steel (water in concrete turns into steam).

If you don't trust your field-welding skills, there are other options besides drilling anchor bolt holes in the hardened concrete. Speaking of which, I wouldn't do it (too much like work, and using a hammer-drill is likely to vibrate and debond your thin curb concrete away from the footing beneath it). Far better plan is to use what are called ferrule-loop inserts, embedded flush in the curb concrete (temporarily bolted to a plywood template across the top of the forms). They're simply a deep hex nut (or coil nut) that has a loop welded to it. You won't find them at a typical big-box store, but concrete accessory places carry them. Or you can easily build your own, using doubled hex nuts and No. 9 wire, and your welder.

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