Resolving Moments on Steel Anchors

[jreit]

I have a loading situation where some of my anchors are more heavily loaded than others.
I'm trying to take advantage of the less loaded anchors to share the load but want to make sure my approach is correct.

There's a moment on the bent plate connecting two columns - one column is more heavily loaded than the other (M[sub]A[/sub]).
Converting the moment to a force couple and moving that to the centroid of the anchor bolt group does help to reduce the maximum tension on the anchors - I'm relying on the bent plate to redistribute forces but don't have a great way to quantify that.

Also, it appears that the d[sub]3[/sub] value is not critical which seems counter-intuitive.

 

[KootK]

I'm trying to take advantage of the less loaded anchors to share the load but want to make sure my approach is correct.

1) I think that you rely on the vertical leg of the bent plate, if it is substantial, to redistribute the moment.

2) I'd still be assigning all of the flexural tension to a single anchor, the most extremely located one relative to the compression reaction of the connection. The horizontal plate leg will be too flexible to mobilize the interior anchors in my mind.

3) I see this as a situation where you will develop prying forces on your anchors.

 

[jreit]

Thanks for replying KootK.

1) The moments are not that large and the bent plate is designed for them so I think that's fine.

2) I was thinking all 4 anchors would go into tension and the bent plate bearing on the edge of the concrete slab to be in compression.

3) That's a good point. I thought tension breakout might govern but there will be pryout also.

The anchor bolts are through bolted - I'm struggling to find literature that would help me quantify the effects of that.
ACI provisions don't cover that and not sure if any other codes too.

 

[JLNJ]

Are the moments about the long axis of the plate or the short axis?

 

[jreit]

The moment is about the short axis.
I've attached a sketch showing the side elevation.
The column is attached to a cantilever beam which creates the moment.
The restraint is at the bent plate connection so I'm resolving the force at that location.

 

[KootK]

I was assuming moment the other way per the torque arrow in the original sketch.

For this setup, I wouldn't count on any more bolts helping either post than the two immediately behind it. Again, there's just not enough stiffness in the horizontal plate leg to justify redistribution in my opinion.

 

[jreit]

That was my original approach also - just could not get that to work with the geometric constraints.
Would it be appropriate to take the moment arm to the outside edge of the steel column or should it be till the end of the concrete slab?
Not sure how else to try and reduce the tension in the anchors.

 

[SWComposites]

The moment arm should be to the inside (left) side of the column.
Ideas:
Make the anchor plate an L section
Add bolts in the middle of the 9" spacing.
Make the bent plate and anchor plate thicker if required.

 

[jreit]

Yes agreed the inside of column/edge of slab is the way to go.
The steel anchors/bent plate/anchor plate are adequate - it's the concrete failure modes that're giving me issues. The moment is about 9 kip-ft on one column and 3 kip-ft on the other.
I like the idea of making the anchor plate L-shaped - offers confinement of the concrete but that only helps with concrete shear breakout and shear pryout.
The controlling limit state is concrete tension breakout failure so adding more anchors in the same row would engage the same concrete area.
I could add another row of bolts behind the first one - engages more concrete but the bent plate would then start to get large (~20" from the current 14").
I've never specified something that size for fabrication but I don't think that's unfeasible.