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Grout under a steel column plate.

08/22/2008 11:29 PM

I've got a 4.5"OD x 0.237"wt x 7.5' long steel pipe column with a 1/4" steel plate welded on the bottom; to be fastened to a large concrete footing with 4 ea 1/2" dia anchor bolts. I assume it'd be better if I put grout under the plate. How do you calculate how thick? What kind? Do you first level the plate and column on 4 nuts, then place the grout so its slightly above the nuts, then place the plate and column on top the nuts? Thx.

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#1

Re: Grout under a steel column plate.

08/23/2008 2:24 PM

The purpose of grout is to ensure that the base plate makes good contact with the concrete over its entire area. The thickness of grout needs to be only as thick as you require for placing it. Probably one inch would be about right in your case. It should be high strength expanding (non-shrink) grout similar to the product shown here.

A plate thickness of 1/4" seems a bit skimpy but you may not be carrying much load or have any moment to be transferred to the concrete, so it could be okay.

Leveling nuts are sometimes used in plumbing columns but in most cases, steel erectors use shim plates under the center of the baseplate. After the columns are plumbed, the general contractor packs the grout under the plate.

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#4
In reply to #1

Re: Grout under a steel column plate.

08/24/2008 4:59 PM

Good afternoon ba/ael, thx for your post. I changed my spec to 1" of grout. Called for leveling nuts under the plate. Kept the 1/4" plate thickness, because your assumption about minimal moments is correct.

Since this is a retrofit under an existing beam, I'm calling for the contractor to install ~6' of the bottom part of the ~7-1/2' column, then after the grout cures, the top ~1-1/2' of the column; then butt-weld the two together. This should totally eliminate any slack, and produce a solid support. Comment?

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#5
In reply to #4

Re: Grout under a steel column plate.

08/24/2008 5:24 PM

You can do it that way if you wish, but I would prefer to install the column in one piece. The contractor places the column, snugs it up to the underside of beam, connects column to beam then places grout under the baseplate and waits a couple of days until it cures.

The top of column (or bottom of beam) should be laterally braced if the column is to be considered hinged top and bottom. If the column is considered to be a vertical cantilever, the baseplate thickness should be increased to about 5/8".

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#6
In reply to #5

Re: Grout under a steel column plate.

08/24/2008 10:31 PM

Good evening ba/ael, I like your way better, but with cast-in-place anchor bolts I don't see how it'd work. If there's 7'-6" between the beam and the footing, and you subtract the one inch of grout, the column weldment would be 7'-5" long, but with 2" of anchor bolt protruding, you might not be able to get it in place over the bolts.

I guess I could pre-drill holes in the concrete and epoxy some studs, but how would you achieve plumb? Thx.

I was depending on a 12" long bracket welded to the top of the post with two each 1/2" machine bolts thru the neutral axis of the beam to take care of the top loads. I don't expect much side loading on the column, as this building was built on the side of a hill, on solid rock about 100 years ago. The past owners have made it work by installing a number of timbers under the beam.

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#7
In reply to #6

Re: Grout under a steel column plate.

08/25/2008 12:03 AM

Yes, if the anchor bolts are cast-in-place, they will get in the way of placing the column. Probably the best solution is to drill and epoxy the bolts in place after the column has been plumbed. If they are going into new concrete, drilling is not difficult. Another solution is to slot the baseplate from the anchor bolt to the outside of the plate to allow the column to be placed under the beam. If the height of column is 7'-5" (89") and the width of baseplate is 10" the maximum diagonal dimension of the column and plate is 89.6" so it should fit under the beam...still, I think it best to drill and epoxy the bolts.

The top of column needs to be laterally braced whether or not there is side loading on the column. It is a question of stability.

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#9
In reply to #7

Re: Grout under a steel column plate.

08/25/2008 9:39 AM

Good morning, ba/ael, now I'm thinking a good way would be to cast in place the 2 far anchor bolts. Slot the base plate for these two bolts and swing the column into place. Mate the column firmly under the beam, plumb it, bolt it. Using 2 each 5/8" dia holes in the steel base plate for the remaining hold down bolts, drill two 9/16x6" holes into the new concrete footing, blow out the debris, inject Simpson Set adhesive, insert the 2 each 1/2" x 8" thdd rods (let cure 24h before loading), inject the expanding nonshrinking grout, and viola!

The only negative I can see with this method is the plumbing may be somewhat limited, but hey, life's a compromise.

The beam extends from the south wall of the building to the north wall. The walls are steel reinforced 18" thick concrete. Any transverse loading the column might see is carried to identical walls on the east and west by the joists resting on and nailed to the beam. As a precautionary measure, I could call out 2x6x~7' blocking (2 ea, each direction) to assure any transverse load is carried to the side walls.

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Guru

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#10
In reply to #9

Re: Grout under a steel column plate.

08/25/2008 10:38 AM

Good morning, flyinghigh,

Your method will work fine, but if you are going to the trouble of field drilling two bolts, why not drill all four? In that way, you can plumb the column properly. Or alternatively, use only two bolts, one each side of the column. There is no appreciable bending moment at the base of the column and negligible shear, so two bolts would seem to be enough.

I still think your 1/4" baseplate is inconsistent with the axial capacity of the column you have selected. Could you use a 5"x10"x5/8" plate with two 5/8" threaded rods set into adhesive.

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#11
In reply to #10

Re: Grout under a steel column plate.

08/25/2008 5:04 PM

Good afternoon, ba/ael. I think you are onto something. I am going to change to 2 each epoxied studs.

I am however hesitant to increase the thickness of the baseplate, because as you point out, there's little or no moment to be resisted by the base of the column. To my way of thinking, the baseplate in this discussion, is basically only there to hold the bottom of the column in one place. The grout and concrete footing will take the compression load delivered by the column, and distribute it harmlessly to the earth. BTW, the footing is 38.5" square, the baseplate 20" square, while the original 4 bolts were on a 16" square pattern.

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#12
In reply to #11

Re: Grout under a steel column plate.

08/25/2008 6:15 PM

I do not know the magnitude of the column load. Make sure that the baseplate has sufficient bending capacity to spread the column load over an area large enough to avoid excessive bearing stress on the concrete. It would appear that your column selection has far more capacity than needed.

A baseplate of 20 x 20 x 1/4 seems out of proportion to me. Better to us 5 x 10 x 5/8 or, if you need more room to drill the holes, could use 5 x 12 x 5/8.

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#13
In reply to #12

Re: Grout under a steel column plate.

08/25/2008 10:36 PM

Good evening ba/ael, The total column load (LL+DL) is 43,596#. I reviewed ACI 318, Building Code Requirements for Structural Concrete, section 10.17, Bearing Strength, but I could not find anything else that directly relates to a steel baseplate. I would like to keep the baseplate square, to match the footing. The 14" deep concrete footing is f'c 4000 psi. I was thinking of increasing the plate thickness to 3/8", and reducing its area to 16" square, with 2 epoxied bolts 12" apart.

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#14
In reply to #13

Re: Grout under a steel column plate.

08/26/2008 12:47 PM

C = 44,000# (column load)

Cf ≈ 1.5 * 44,000 = 66,000# (factored load)

m = n = (16 - 0.95*4.5)/2 = 5.86 (projection of plate beyond column)

If you assume the load is uniformly distributed over the area of the base plate, the plate thickness required is given by:

tp = √(2*Cf*m2/(AφFy))

A = 16*16 = 256 sq. in. (area of plate)

Fy = yield strength of steel, say 36,000 psi

φ = 0.9 for steel

This gives tp = 0.74" (say 3/4")

If you cut the plate down to 12 x 12, you still have a very acceptable bearing stress and can cut the thickness down to 5/8".

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#15
In reply to #14

Re: Grout under a steel column plate.

08/26/2008 2:34 PM

Good morning Bruce, I think this is just what I need, but I have a couple ?'s. 1) In your "m" formula, where does the 0.95 and 4.5 come from? 2) Is the "m" formula different for a rectangular plate? 3) Where does the tp formula come from?

I have reduced the size of the steel baseplate to 12" square, and increased the thickness to 1/2".

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#16
In reply to #15

Re: Grout under a steel column plate.

08/26/2008 3:11 PM
  1. 0.95 comes from the CISC handbook for a square column. It should maybe be a bit less for a round column. 4.5 is the exterior dimension of the column.
  2. For a rectangular plate, two dimensions are considered, "m" and "n" and are defined in the same way for each dimension. For a plate 16 x 12, for example, the "n" value would be 2" less than the "m" value. Plate thickness would be calculated using the more critical of the two, i.e. the "m" value.
  3. The tp formula comes from the CISC handbook (which would be the same as the AISC handbook). It simply equates the factored bending moment in the plate to the resisting moment and solves for tp.

Here is a reference for calculating base plates. Or you could google "column base plates" and find a number of other references.

I don't quarrel too much with your 12 x 12 x 1/2 selection because one could argue that you have a virtual 10 x 10 x 1/2 plate inside and the outer one inch ring is just decorative. In my practice, I have generally used 5/8" as a minimum thickness because I find that thinner plates tend to curl when welded...but this is not too serious as you can alway fill with grout.

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#17
In reply to #16

Re: Grout under a steel column plate.

08/27/2008 10:37 AM

Good morning Bruce, thx for the great letter and the nice link. It has to be the best I've ever gotten. I made an Excel spreadsheet of your formula. The latest iteration that I'm going to go with, is a 9" square x 1/2" plate with 7.5" between the holddown bolts. That should be close enough for the girls I go with.

You are truly a gentleman and scholar.

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#18
In reply to #17

Re: Grout under a steel column plate.

08/27/2008 10:58 AM

Hi flyinghigh,

Thanks. Happy to help. But 9" square and 7.5" between bolts gives only 3/4" from centre of bolt to edge of plate. That is a bit tight. I suggest 10" x 10" x 1/2".

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#19
In reply to #18

Re: Grout under a steel column plate.

08/27/2008 12:21 PM

Good morning Bruce, that sounds good to me.

Thx for your assistance, I sure appreciate it.

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#20
In reply to #9

Re: Grout under a steel column plate.

01/28/2011 12:52 AM

hi..

can u tell me how to install cast-in u anchor in jetty or wharf?

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#2

Re: Grout under a steel column plate.

08/23/2008 11:18 PM

AS THIN AS POSSIBLE

--just to avoid air space between Steel base and Concrete.

Assuming your Designs are OK.

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#3
In reply to #2

Re: Grout under a steel column plate.

08/24/2008 3:20 AM

When bolting down roll-formers:

  • We had 2@M10 grub screws through the plate also, for height and level.
  • We would only nip up the holding down bolts.
  • Place a dam of steel strip, unjoined in one corner to spring over the column.
  • A hole close to the column, for a funnel helps with grout pouring.
  • Pour the grout and leave for 24 hours.
  • Remove the grub screws and tighten the holding down bolts.

Often the machines would be "nudged" by forklifts, this way the columns would flex and the alignment was not lost.

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#8

Re: Grout under a steel column plate.

08/25/2008 1:28 AM

your calcullation is ok

if your are embedding direct with concrete or making footing putt Anchor bolt 1/3 depth of footing , Anchor bolt 25 mm Dia. with base plate of 6mm to 20mm.

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