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

[ajk1]

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?

 

[KootK]

@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

 

[TLHS]

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.

 

[ajk1]

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.

 

[Ron]

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.

 

[ajk1]

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.

 

[BAretired]

A ring around the column sounds like the best idea.

BA

 

[ajk1]

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