Column Base Plate Anchor Bolts

[Eng_Struct]

Hi Group,

I had a column with a 10"x10"x1/2" base plate with 2-3/4" sitting on 1.5" grout for a project. The contractor somehow did not get the top of footing and anchor bolt location correct. The underside of the baseplate is 3.5" from top of pier. They ended up slotting existing holes to fit the anchor bolts (now have a bigger hole) and provided a thick washer plate welded to the top of base plate.

Typically, I consider the grout to provide lateral support to the anchor bolt and check the anchor rods in pure shear at the top of the grout (I take 0.8 times the shear strength of the anchor rod considering the reduction due to grout).

My problem is now is that the hole at the bottom will not bear against the anchor rod and load will transfer from washer plate to top of grout by bending the anchor bolt. I do not have sufficient bending capacity and have also checked to approach this by checking the interface shear (friction) and cannot get the required capacity.

I am looking for suggestions how to best connect the anchor rod to the underside of the base plate so that the rod does not undergo any bending and only go into shear. Note that the column is already up with the base plate and we have 3.5" of space to work with. Bolts in shear alone and concrete pier are OK for the applied loads. I was thinking of adding stiffener plates (shown in cyan in the sketch below) adjacent to the bolts at the bottom welded to the baseplate and anchor rod to prevent bending of the bolts.



Appreciate the help in advance!

 

[human909]

Sorry I can't help. But I can give you solace that your problem is a common one.

This happens regularly all over the world and yet then number of failures due to anchor bolts failing from bending does not seem to be hitting any radars.

Friction, steel ductility, never reaching ultimate loads and uplift probably all play a part. Unless these are grossly underdesigned then they are likely good.

Your hand wringing is likely more of a display of diligent engineering than a real concern.

As far as solutions go. I don't have many.

 

[HTURKAK]

a 10"x10"x1/2" base plate with 2-3/4" sitting on 1.5" grout for a project. The contractor somehow did not get the top of footing and anchor bolt location correct. The underside of the baseplate is 3.5" from top of pier.

- 2 in is substantial difference . Are the Anchor Projections adequate ?

My problem is now is that the hole at the bottom will not bear against the anchor rod and load will transfer from washer plate to top of grout by bending the anchor bolt. I do not have sufficient bending capacity and have also checked to approach this by checking the interface shear (friction) and cannot get the required capacity.

- Did you evaluate the need for the anchor to resist shear ? . Perhaps friction due to net compression and to compression due to moment couple will carry the total shear . As an alternative , shear transfer can be accomplished by filling the annular space between the anchor rod and anchor hole with epoxy grout.

 

[XR250]

I'm curious in similar situations, how much credit can we give to grout preventing bending in an anchor bolt?. Seems like something that is commonly assumed but never checked. Most of my situations involve low shear as they are typically residential flagpoled columns or moment frames.

 

[TRAK.Structural]

Is there enough room to drill new holes and post install anchors?

In my experience it is pretty common to weld the washer to the baseplate for columns that resist lateral loads. I've never really accounted for this extra height in where the load application to the bolts occurs with this detailing.

 

[DaveAtkins]

If there is room at the edges of the base plate, you could use steel angles (5" vertical leg and 3" horizontal leg), with the vertical leg welded to the base plate and the horizontal leg connected to the pier with adhesive anchors.

 

[ANE91]

I'm curious in similar situations, how much credit can we give to grout preventing bending in an anchor bolt?. Seems like something that is commonly assumed but never checked. Most of my situations involve low shear as they are typically residential flagpoled columns or moment frames.

20mm is about the lever-arm threshold. Grout absolutely crushes and so does not provide full bending restraint to anchors.

Refs:
- Section 4.1.2.2 of “Anchorage in Concrete Construction” by Eligehausen and Silva
- https://files-ask.hilti.com/original/pu/pu5ebwyxp4.pdf
- https://www.eng-tips.com/threads/hilti-profis-anchor-in-shear.493294/#google_vignette