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Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

(OP)
I am designing a 24"x24" steel end plate to an HSS 4"x4" section that I am connecting to an existing concrete column.
The HSS is in pure tension.
I require 16 drilled-in epoxy adhesive anchors (3/4" diameter threaded B7 rods, as determined by the PROFIS program), assuming that each rod anchor into the concrete takes the same load.
The anchors are arranged in 4 vertical rows, with 4 anchors per row.
The plate has to be designed to meet 2 critera, namely:
a) the bending strength due to the applied load;
b) that each anchor rod is about equally loaded.

Question: How can I determine the plate thickness to ensure that each of the 16 anchors is about equally loaded?

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

a) is just a matter of determining the plate bending and sizing the thickness of the plate to suit your allowable stress. I assume you know how to do that.

b) is more difficult. The anchors closest to the tie will be loaded first, then the more remote. As you have a 24" x 24" plate, I assume the anchors are proposed to be on a 7" grid. I would look at using a smaller number of larger anchors, or preferably through bolts.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

Size the plate such that bending is irrelevant...that's the only way you'll get the same load in each of the fasteners.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

I don't know that you can. For that to work, the bolts have to be considerably more flexible than the plate. Otherwise, the relative flexiblity of the plate between inner and outer bolts would be about the same, regardless of plate thickness. Possibly put the bolts in a sleeve, so a longer body acts in tension?

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

Using plate stiffeners in each direction would help, but would not completely eliminate the inequality of loading.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

To load each anchor equally, you either have very difficult and indeterminate figuring of plate bending, anchor strain, etc. or you have a single ring of anchors equidistant from both the HSS and the outer edge of your plate. Or you make a swag & do 2 different sizes/spaces/something for the 2 concentric rings & make sure you have more than enough total capacity.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

Deep stiffener plates so that your deflection is low, with each bolt being about equidistant from a stiffener if possible and design the bolts so that the concrete can develop the full yield strength of the bolt, so you have a ductile failure mechanism? You could check the total deflection you could expect through bolt yielding and use that to come up with a differential deflection criteria for the plate/stiffeners. If this is a possible fatigue scenario, you'd want to be careful with this.

Alternately, you could investigate by modelling it in finite element software as a plate with a bunch of rigid pin supports. Then spring them assuming the bolts are yielding. Then throw in some stiffeners and see what happens.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

It's a long shot but, if the anchors could be made to fail by a ductile failure mode, you could assume redistribution.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

Stiffeners will certainly help.

If you are good with FEA and can model the anchors, concrete, and plate + stiffeners you can get pretty accurate results.

If you have a 4x4 HSS that requires that many 3/4 diameter rods, stiffeners might help you connect the HSS to the plate anyway. Sounds like you have a helluva lot of load to develop there.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

(OP)
Thanks for all the comments. It is significant that there is little to answer the question of how to calculate, except perhaps by FEM. Too bad. the response seemed to be more at how to avoid the problem than how to solve it.
The reason for the large number of anchors is not so much the load as because we only have a limited embedment before the anchors hit the spiral reinforcing in the column, so we had to keep the embedment fairly shallow (at the minmium allowable embedment published in the Hilti catalogue for the anchor). That is also the reason the that larger anchors are not the answer here.
I had considered stiffeners of course, but did not think that solved the problem because the plate still has to bend to get the load back to the stiffener from the bolts closest to the stiffener and the bolts farthest from the stiffener. The stiffener would work if there were say just 2 rows of bolts, but we are bolting to a round coulmn, so this would require extending the plate around much of the column perimeter.
What I did was use a plate about double the thickness required for flexure (2 x 0.75" = 1.5") and this has very little deflection based on calcualtion. The only reason I asked the question was becasue the contractor asked if we could use a thinner plate. Since there is only one of these, I don't think the thicker plate should be a problem, and think I will just stick with that.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

Just a question: Using 16-3/4" anchors in a 24" x 24" plate will require these to be placed at 5.5" to 6" o.c., at which point your interaction effects due to overlapping cones start to control significantly.

What failure more is controlling your anchor design?

Instead of drilling 16 holes into the column, hitting rebar and creating Swiss cheese, you could probably drill 4 through-holes in the column and put a plate on the back to transfer the loads.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

A scheme where the anchors are limited in depth to the extent where they stop "before the anchors hit the spiral reinforcing in the column" would not be an acceptable solution to me.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

(OP)
to Slickdeals:
The anchors were designed using the Hilti PROFIS software, so yes the reduction due to interaction definitely has been accounted for where applicable, but due to the shallow embedment it was not hard to space the anchors so that there is no reduction due to this effect;
It is not possible to drill thru the circular column because we will hit the column verticals which are #18 bars at mimimum spacing.

to Hokie66: you usually have well thought out comments but I am puzzled this time as to why this would not be acceptable to you without giving a reason. You seem to be saying that you do not trust the Hilti data for capacity at minimum embedment. Or do you have some other reason? I can think of no reason why this is not accptable, except if the rebar should corrode and spall off the cover concrete, but there is no leakage and no corrosion of the column rebar and there is a waterproofing membrane on all floors. Perhaps in a seismic event the covercrete could spall off but I think this is the least of our worries as the steel we are installing is supplementary to the post-tensioned beam and has the capcaity to support most of the load if more tendons should break (a not liklely event since we are drying and greae injecting all the tendons).

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

it looks to me as though you've got a single row of fasteners, 5 per edge, about 4" pitch as noted above.

the load is being introduced by a 4x4 column, presumably welded to the plate, 16" of weld in tension = 16 3/4" bolts in tension ?

load distribution would not be equal; the corner fasteners would be less loaded, quite likely the corner ones would be loaded in compression (review Roark for square plates).

you'll get closer to a uniform distribution if you have a circular fastener pattern and plate.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

I second hokie's concern regarding the embeddment depth. Fancy maths and software output aside, delivering a large tensile load to only the cover concrete of a column feels "icky".

In a similar situation in the past, I've used two semicircular bands of steel plate wrapping the entire column. Where the bands abutted one another, they were detailed to connect through bolted flange plates. I included a few shallow anchors as well just to hold the thing in place. If access and fire proofing concerns would permit such a solution, it might be worthy of consideration in your case.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

Column reinforcing seems fishy in terms of being capable of handling such a large tensile load (assuming it causes a large bending moment on the column).
Has the column been checked for this load?

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

(OP)
To Kootk and Hokie: I should have been clearer - the embedment depth is 86 mm. Where we have chipped out the concrete to expose the spirals and measure the cover on other columns indicate that in most locations in columns in this structure, that embedment depth is possible. We have already installed dozens of rods at this depth in other columns in the structure, but it is possible that in this particular column we will not be so lucky, in which case we will probably have to use a collar that goes around to the far sdie of the column and bears there.

to Toadjines: The load is applied as a horizontal load within about a foot of the concrete beam soffit at the top of the 36" diameter heavily reinforced columns which were designed origonally in 1975 to accommodate a future hotel; since then another floor of parking has been added but the hotel plans have been abandoned. The column has oodles of excess capacity. In any event, I would expect that a load applied so close to the column top (the column has a large dead load on it from the 4 floors of parking above) will produce relatively very small moments and e/t ratios. We have already checked the column for far greater horizontal force and the effect is very tiny.

Reminder - My original question was how to determine the plate thckness to districute the load equally. I think there amy be some information in the Hilti Manual on this. I will check. I will aslo checck with my academic contacts and see what they say.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

I think that the reason that folks are dodging your question about plate thickness is that it's going to be impossible / impractical to guarantee a uniform anchor force distribution. A very thick plate or the use of stiffeners will improve your situation but not ameliorate it altogether. I looked at this for a base plate condition using FEM software once. The conclusion was that the plate would need to be ridiculously thick (>5") in order to truly approximate a uniform stress distribution. If you really need all of the anchors to be uniformly loaded, wrap the plate around to the sides of the columns so that your anchors will be loaded in shear instead of direct tension.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

review Timoshenko "Theory of Plates and Shells" for the difference between round and square/rectangular plates

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

ajk1,
KootK said it for me...I wouldn't trust anchors just in the cover for this sort of application...don't know if Hilti would recommend that or not, but 86mm is not enough for me. If these anchors were cast in, would that be enough? Of course not. So why accept less embedment for glued in anchors?

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

@ajk1:

Could you share the actual load being considered with us? I think that we're stuck in a "big load" frame of mind because you're proposing so many fasteners. Based on what you've shared with us, I get the sense that:

1) Your load may not be all that large and;
2) You're only using so many fasteners because each one has relatively little capacity as installed.

KootK

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

This is a plate on a curved column face?

Have you taken into account the fact that your effective embedment is reduced because your breakout cone will intersect with the face of the column as it curves? Are you installing them so the bolts are aligned to the tension, or so that they're normal to the face of the concrete? The former would have issues with the breakout cone on the outer side of the plate, as the column curves toward the base of the embedment. It also seems like it would be really irritating to drill properly. The latter puts your bolts into an exciting combination of shear and tension. Either way, if your plate is curved you're going to have a combination of tension and moment effects. You'll see a bunch less deflection than with a flat plate, and it could tend towards arch action.

Personally, in this situation if I couldn't get the bolts pretty close together so that I need a minimal amount of grout to flatten the plate bearing I'd probably do one of these things:

-Build a ring to fully encricle the column and take the load in bearing on the far side. The load can't be more than a few hundred kN with the embedment of those bolts, so this shouldn't be too bad.

-Build a stiffened seat, like a pipe saddle, to attach to the column, with a couple of pieces of plate cut to match the curvature and act as stiffeners that are welded to your bolt plate. It would let you tighten it against the column and keep your plate surface at right angles to the bolt (assuming your bolts are installed along the axis of the tensile load)

You could also embed some rebar and build a more convenient piece of concrete out of the column, but you obviously don't want to embed things very far. However, with rebar it might not be as much of an issue, as you could have a reasonable amount of tolerance for if you hit reinforcement and had to move it slightly.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

(OP)
Now we are getting some information relevant to the questions. Many thanks to all for your positive contributions.

to KootK;
yes you are right on. The un-factored load is 360 kilonewtons. The only reason for so many fasteners is the shallow embedment, as you have noted - I noted that too, somewhere way back in the correspondence.

to TLHS;
yes I have taken into account the curvature of the column by drawing a 35 degree failure cone and where it intersects the curved surface of the column, I draw in the chord and calculate the distance from the chord to the curved surface at the anchor location. This adds about 20 mm to the required embedment; Hilti catalogue minimum embedment = 86 mm; so we would specify 106 mm after allowing for curvature. Not sure if this is entirely right, but it is an attempt.

I like your suggestion of building a ring to encircle the column and take the load by bearing on the far side. This was precisely the thought that I had too. It does though probably require a beam to carry the load across the width of the round column on the near side; the semicicular strap could attach to the beam and carry the load to the far side of the column.

I could not quite follow your stiffened seat connection idea, but sounds interesting...if you have a moment, could you send a free-hand sketch of it? I would be most grateful.

to Hokie66; Hilti Manual says minimum embedment is 86 mm for 19.1 mm diameter threaded rod. Are you saying that you do not agree with the Hilti data? Anyway, I think TLHS has a really good idea that I will adopt.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

One additional suggestion, though it isn't usually done for anchor bolts...pretension them all to the same value, even if it is a low value....helps with initial mobilization and distribution of stress in the plate.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

(OP)
Ron;

To which option are you referring?

If it is to my original option of 16 anchor bolts into the column to fasten the steel plate, then all the anchors (3/4" diameter B7 grade rods threaded full length in Hilti RE500 adhesive) would be torqued to the torque specified in the Hilti Manual for the size and type of anchor. Would that accomplish what you are recommending?

Or are you referring to the latest option, of putting a collar around the column and transferring the force by bearing against the far face? In this case there would not be any bolts into the column except to hold the collar in place.

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

A ring around the column sounds like the best idea.

BA

RE: Plate Thickness in Pure Tension Connection to Uniformly Distribute Load to Fasteners

(OP)
Yes, a ring around the column is what I have decided to do. I have designed and drawn it up. It works quite nicely.

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