Weldable anchor bolts 8

[BigBakwas77]

I provided a normal HSS102x102x6.4 lightly loaded column (axial 50kN, shear 2kN) with a 20 thk base plate and 4-20Ø A325 anchor bolts with Hilti HIT HY200 on a project where the base building is existing. Now the architect has come back to tell me that he cannot accept the base plate and bolts/nuts exposed above the concrete surface. I am proposing as per the attached sketch. My questions are,
1. Is it acceptable to remove nuts and plug weld the hole 10mm (half the plate thickness)?
2. What type of bolts will be weldable? ( I have seen F1554 grade 36 as weldable.)
3. To what extent the welding (heat) affect the Hilti connection to concrete?
4. Would mechanical fasteners be better solution than chemical ones?

 

[phamENG]

I'm not excited about the plug welds. May work, and I can't point to a specific reason why not to do it, just doesn't feel right.

F1554 Gr. 36 are only weldable if they have a dual cert as A36 (or some other steel spec with acceptable weld chemistry). F1554 alone does not provide the right chemistry. You can go up to F1554 Grade 55 and require the weldable supplement certification (I think it's S2), and that will give you a chemistry that can be welded.

I think the more important concern is following the installation directions for the hilti product. You'll have zero visibility to verify proper filling the hole, etc. Any chance some sort of architectural cover could be placed over the base plate? See the pictures here for an example: [URL unfurl="true"]http://www.colorcraftsigns.com/Pylon-Base-Plate-Covers-Pylon-Signs_Colorcraft-Signs.htm[/url]

If that's not an option, I'd say cut the current plate off, have the fabricator make a new one with the anchors welded to the bottom, set it in epoxy (maybe leave an inch standoff to get some visibility below for the epoxy to be grouted later) and then field weld your column to it in place. Probably easier that swinging the whole column up and making the four anchors land in the holes.

 

[r13]

Since load is light, I think you can consider provide a footing of its own, but then you need to open up the slab and do some digging.

 

[Ingenuity]

You can go up to F1554 Grade 55 and require the weldable supplement certification (I think it's S2)

Supplement S1.

BigBakwas77:

I have done what phamENG is suggesting, with setting in the base plate (with welded anchors) into the 4 pre-drilled holes. I had a concrete filled stainless steel bollard that got hit and we had to re-connect.

But you cannot use a paste epoxy (like Hilti HY200 or SST etc) because installation of anchor rods into paste epoxy requires a twisting-action of each rod to get coverage to the all the threads and eliminate air pockets. With 4 rods (studs) pre-welded to a base plate you only have the option of installing vertically downwards, whilst keeping it in alignment and level etc. I therefore used a liquid epoxy (like Dayton superior PROPOXY 100).

1. fabricate your base plate with pre-welded rods to u/s plate;
2. drill 4 x holes to concrete;
3. clean holes with wire brush and oil-free compressed air;
4. repeat step 3 - cause your hole can never be too clean
5. dry-run install the base plate/rd assembly for fit - adjust (bend) rods to suit;
6. mix epoxy and grout 4 x holes to 2/3 of hole depth;
7. insert base plate/rod assembly and align/level with wedges.
8. field weld column to base plate.​

I have used a hydraulic cement 'dam' around the perimeter of the base plate, so when the base plate/rod assembly is installed it 'floods' the area with epoxy and is contained so you get a bit of resin under the base plate and then when it cures simply chip the hardened excess resin from the plate perimeter.

Bit of a tedious and messy process - hopefully you do not have too many on these to do.

This is the finished condition for the bollard repair - the cementitious grout was only applied because the surface was sloped (ramp) and had to taper the edges for appearance:

Also, Duane Miller (welding guru from Lincoln Electric) does NOT recommend plug welding of base plates to anchor rods.

 

[phamENG]

Ingenuity - thanks for the correction.

 

[BigBakwas77]

All,
Thank you all for your quick responses.
I must agree that I did not think of welding bolts under the plate and then Hilti them in as suggested by @Ingenuity and @phamEng. I will go with this suggestion. @retired13 This is a suspended slab and not SoG.

 

[Ingenuity]

This falls under the contractor's 'means and methods', but it is handy for an engineer to be 'armed' with such methods so the contractor does not try and say 'it cannot be done', or some other excuse.

A variant of grouting the base plate/anchor rod assembly is to drill a small dia angled pilot/inlet hole that intersects the anchor rod holes, clean the holes, then install the assembly, then inject low viscosity epoxy into the inlet hole and 'bleed' the resin out under the base plate. I have done this many times and works great, even for difficult access rebar dowels, anchor rods, through-rods, etc...but tedious work.

You could delay the grouting until the column welding was completed, if there was concern about heat input and epoxy, but I think in your case your welding is probably far enough away from the anchor rods.

For my project, the previous bollard repair (by others) used HILTI RE500 paste epoxy (which is great epoxy used in the right application) and due to the difficulty of installing all 4 rods of the assembly in the pre-placed epoxy they did not achieve the required full-bond, and there was no real way of knowing how good or bad it was until it failed next time...which was shortly after, when it got 'nudged' by a vehicle.

 

[BridgeSmith]

Have you considered embedding the post into the concrete? You mention axial and shear, but no moment. If there's no moment, do you need anchors, or would having the base plate (or just the post) embedded in the concrete be sufficient?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

Ingenuity: "I therefore used a liquid epoxy (like Dayton superior PROPOXY 100)"

Thanks for the info on the Propoxy line... several variations... no mention if creep resistant.

Dik

 

[r13]

This might work.

The problem is if the architect doesn't want baseplate above the floor, then you might not have adequate thickness to install the twist in anchor (with plate embedded in concrete). But you can consider bolt through the slab with an additional plate at the bottom of the slab, if acceptable.

 

[BigBakwas77]

@Ingenuity- Good to know about this solution for future projects. In my case it is a short 9'-0" column. So the contractor has decided to pre-weld column to baseplate before installing it into concrete.
@BridgeSmith- I have only top 1" of concrete (cover thickness) which I can chip off for baseplate. So, embedding the base plate in concrete does not sound like an acceptable solution.
@retired13- To follow your advice I will need a baseplate much thicker than 1" (the thickness of concrete I am allowed to chip-off)

But all your suggestions are appreciated and may help me in future!

 

[Ingenuity]

Thanks for the info on the Propoxy line... several variations... no mention if creep resistant.

The creep resistant version is called PROPOXY UNOBTAINIUM

 

[r13]

BigBakwas77,

Depends on location of the column to be placed, you might have to chip out an area larger than the baseplate, and splice the rebars been cut in the process. I am more concerned with the depth of the remaining slab, it might not thick enough to allow for chemical/mechanical anchors. Then the only option left is to bolt through the slab as suggested.