MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers 1

[bones206]

In the Q&A portion of the latest issue of AISC's Modern Steel Construction magazine, someone asks if they should be welding plate washers on column base plates. The answer (quoted below with emphasis added by me) surprised me.

It is generally not recommended to transfer shear through the anchor rods. Note that if the base plate is transferring axial load only, then it is not necessary to weld the plate washers to the base plate. If there is a shear force, you could consider a few options for transferring this load:

• Friction​

• Bearing of the base plate and/or shear lug against concrete​

• Shear in the anchor rods—plate washers not welded (not recommended; use with caution)​

• Shear in the anchor rods—plate washers welded (not recommended; use with caution)​

It's generally not recommended to transfer shear through the anchor rods?? I know we have to pay attention to detailing and make prudent assumptions about load distribution to the anchors, but surely anchor rods are the most commonly used method to resist column base shear, are they not?

 

[sandman21]

The tolerance for anchor bolts is not BS, you'll make a mess of everything if you oversize your AB holes only 1/16", or 1/8"

The tolerance does not reflect modern construction practices and is a band-aid for poor contractor work. For years we only allowed 1/8" with little issue with locating anchors of all types.

 

[sbisteel]

I don't know what contractors you're working with, but we have issues with anchor bolt placement on almost every project we work on. The AISC code of standard practice calls out the expected tolerance of anchor bolt locations, but it is often missed.

 

[canwesteng]

There's no way I've seen a single job go by where a 1/8" oversize hole wouldn't cause some issue, that only leaves them a 1/16" to move from the correct position in any direction. A competitor tossed 1/8" oversize holes on a conveyor job and the contractor ended up having to ream out almost all the holes.

 

[bootlegend]

1/8" oversize holes in the baseplate would be a field disaster in my experience. Even the good contractors would have some interference with that tolerance. I think the ACI placement tolerance even exceeds that.

 

[bones206]

Lets say you have a braceframe you would weld reinforcing to the baseplate that transfers the shear into the slab and right into the grade beam. SEAOC seismic design manual has a similar concept but with an HSS located in volume four design example 9.

I don't have that design guide but I think I can picture what you're describing. Have you found that detail to be fairly constructible? I assume it would require 2 separate concrete placements. One to set the baseplate on and another to encapsulate and tie in the rebar.

 

[structSU10]

I don't have that design guide but I think I can picture what you're describing. Have you found that detail to be fairly constructible? I assume it would require 2 separate concrete placements. One to set the baseplate on and another to encapsulate and tie in the rebar.

the separate pour is the slab on grade.

 

[r13]

Welding rebars to the base plate shall be done in the premises that the rebar is weldable, and the welding of two dissimilar metals should be qualified by a welding engineer.

 

[slickdeals]

One of the other things to consider is that we are not just talking about anchor rods in shear but also in bending because of the standoff distance (grout bed). It doesn't take much force in the combined interaction scenario to start needing crazy big bolts. If you are only considering shear with no standoff, it's not correct.
AISC has done considerable research in this topic.

https://www.aisc.org/globalassets/a...ar-transfer-in-exposed-column-base-plates.pdf

 

[r13]

slickdeals,

Thanks for the excellent research report on column baseplate anchorages. All structural guys shall have a peek, at least.

 

[RWW0002]

It looks like the anchor rod shear cases in this were run with the "superoversize" holes per AISC T14-2 and used the same 2 1/16" holes for both anchor sizes. As noted above 1/8" oversize is asking for trouble, but I have successfully spec'd "green book" hole sizes for anchor rods per footnote 3 of T14-2 without too much trouble for a long time (5/8" oversize for 3/4"Ø-1"Ø, 1/2" oversize for 1"Ø-2"Ø).

I wonder how anchors with less oversize (while still remaining somewhat constructable) would perform under similar testing.. Hopefully better than those in the report. sheesh.

As noted above, it seems like transferring shear through anchor bolts has been the preferred method for low-rise construction for a long time in my area with little consideration for anchor rod bending. While anchor rod construction issues are far to common and no fun to deal with, are we causing more problems than we are preventing by allowing "superoversize" holes? Or is this a serious design discrepancy that needs to be addressed regardless the (fairly recent) increase in anchor rod tolerance??

 

[bones206]

This statement from the first paragraph of the study report sums up the issue:

This investigation is the first phase of a broader study whose aim is to develop improved design guidelines for column base plates. A review of existing design guidelines and published research reveals that shear transfer mechanisms in exposed base plates are not well understood, and methods to characterize strength capacities in shear are not adequately supported by experimental data.

Edit: ...and that study was published in March 2009, yet here we are almost 11 years later and not much has changed.

 

[canwesteng]

1/2" oversize isn't too bad for construction, although I fail to see how it's any different than much larger holes for shear transfer. Either way I'd think typically you would weld washers down, and for low rise structures the shear probably isn't working the bolts too hard.