Excessively Oversized Basplate Holes

[UnusedHandle]

I have am currently working on a design for columns that have moment resisting baseplates. The original installation was to use post-installed, epoxy anchors. Due to geometric and design constraints, the anchor rods were sized so that they just met the requirements for the loading. However, the foundations have changed, and the concrete contractor now wants to use cast-in-place head studs. The original anchors were 1" diameter. As the anchor capacity is no longer dependent on bond failure, and in turn rod diameter, we have changed the anchors to 3/4" to ensure ductile failure of the rods before brittle failure of the concrete. The new problem I have is that the baseplates have already been cut and drilled with 1 13/16" holes, sized for 1" anchors per AISC Design Guide 1. This seems to be an excessively large hole for the 3/4" anchor. Is this allowable, and if so are there any special modifications that I need to incorporate? I've already sized up to PL 1/2" x 3 1/2" x 3 1/2" washers.

 

[KootK]

The plate washers is a good way to go. Weld them to the base plate if the bolts are the shear transfer mechanism for your base plate.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

Are you actually getting full moment capacity out of only a 1/2" thick plate if your anchors need to be that large?

I would expect the plate deformations to play a significant role in the fixity of that base.

 

[paddingtongreen]

Control the failure mode by limiting the rebar holding the bolt's pull out block with the 1" bolts, or use your washer plates with the 3/4" bolts.

Do you have to transmit shear? That can change thing.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[boo1]

plug weld and re-drill?

 

[UnusedHandle]

@jayrod, That is exactly my concern. I don't have a design standard for determining the plate washer thickness. The original hole/washer sizes were pulled right out of the Design Guide. The only thing I could quickly think of was using Roark & Young for a circular plate with a "concentrated" load at the middle by using the r'0 approximation or using r0 of the anchor rod. However, this will only get me to the stress in the washer, not the fixity of the base.

@paddington, minor shear in this case, the friction of the baseplate on the grout is sufficient without lugs or the anchor rods bearing. The floor and foundation are the responsibility of another company, who we have been working with, but have not been directing them how to design. Since I have no control over the rebar sizing/placement, I sized the rods with the assumption that the rebar won't contribute to the capacity of the anchor rod. This is conservative, but I don't want to incorporate rebar into my portion of the design if I have no influence over the foundation design other than providing baseplate layout and loading information.

 

[UnusedHandle]

@boo1, 1 13/16" hole, 2" thick baseplate, 56 baseplates, 8 holes per baseplate. We considered, but not very long. We have a deadline with penalty clause for being late, it would probably be cheaper and faster to by all new plates with the right size holes.

 

[KootK]

No point messing around. Just throw some 3/4 washer plates on and never think of it again. If you search for "washer plate" here you'll find that they're not exactly designed per se. Some folks say that they're just specified thick enough that they can't deform all to hell and slip through the hole. The washers themselves ought not have a large impact on your base plate fixity.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

My bad. I thought you had said your baseplate was 1/2" thick. A 2" thick plate with 1/2" washers would do the trick in my book