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MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

(OP)
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.

Quote (MSC)


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?

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

bones, I read that yesterday and had the same thought. I was also pretty surprised by the "not recommended" language. I was taught anchor bolts first, shear lugs only if absolutely necessary.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

Quote (bones206)

..but surely anchor rods are the most commonly used method to resist column base shear, are they not?

Agreed.

Quote (phamENG)

I was taught anchor bolts first, shear lugs only if absolutely necessary.

Me too. Shear lugs as the default will land you in the soup kitchen line in no time.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

I believe response is accurate, but only in seismic regions where the shear force is generally large enough where you'd have a difficult time getting the anchors to work in shear anyways; using supplemental shear reinforcement and/or shear lug as the next design consideration.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

IMO they were not clear, I think they are talking about MFRS anchors not storefront, stairs or other lightly loaded columns. IMO if you have a braceframe, moment frame, primary lateral systems etc. you should not have the anchors transfer the shear into the foundation system. We use reinforcing, HSS, Cx or embedded the columns to transfer shear.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

I'm with sandman, but in industrial so take it with a grain of salt I guess. I just hate to think that there are steel high rises out there taking shear through anchors, although I've also seen people use shear lugs and ignore concrete breakout in the pier completely, so it could be worse.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

I have heard of this opinion, but always having to do with 2" of grout. I've also seen other engineers respond to this problem by blocking out space in the concrete footing so that the base plate is grouted level with the footing. The manual even shows details where the base plate and column are not welded because the base plate is a compression member only.

However, I continue to do as I have always seen done.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

It seems another layer of safety concerns. My guess is:

The misalignment of anchor bolts in over sized bolt holes can cause a few bolts to resist the entire lateral load, the other bolts will see action only when those few bolts either have deformed, or failed, due to over stress.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

If you see a highrise out there taking its shear through anchor bolts, somebody's botched the math.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

I'd actually be quite curious to know the cost difference between welded plate washers and shear lugs now. I'd think field welding 4 plates per columns is worse than a little bit of forming and filling a grout pocket. Although if you block out space for the base plate to be level with the TOC you've kind achieved worst of both worlds.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

Bones206:
I suspect that the thinking behind those answers is that as we allow larger and larger A.B. holes in the base pls. for contractor convenience, and then weld down the pl. washer, the base pl. and column have to move quite a bit to really engage A.B. shear and bearing. In the meantime, the A.B’s. are cantilevering an inch or two, in bending to pick up any lateral loads at the base pl. to found. interface. Remember, they are writing codes and advice for people who don’t know enough to come in out of the rain, but want to pretend to be real engineers, thus the, “use with caution.” But then, friction and potential uplift is an equally dubious solution.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

Here in Australia I've never seen welded plate washers for column base plates. Mind you that might just be to do with the industry I work in, but I've never seen them or seen them recommended. Plate washers sure, but not welded ones. That said if your anchor bolts are taking shear then I can see the good reasons why you'd do this.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

Quote (KootK)

If you see a highrise out there taking its shear through anchor bolts, somebody's botched the math.

The Hard Rock Hotel design team would like to have a talk. ;)

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

Quote (sandman21)

The Hard Rock Hotel design team would like to have a talk. ;)

I'm not abreast of that one. Did that actually have something to do with MWFRS shear being transferred through the bolts??

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

Does anybody know the reason(s) to weld washers to the base plate? I don't see the benefit(s), but to add erection cost. And per my experience on industrial settings, this practice was ineffective in structures subjected to dynamic loadings, such as crane runway support columns. We sometimes specify tack weld the nut to the bolt to prevent back-out in applications with vibratory environment though.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

Retired, the idea is that the washers, with holes sized to fit the bolts, will transfer the horizontal load to the bolts...because the holes in the base plate are too large.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

(OP)
Great points so far. I think I’ve been successfully recalibrated a bit on anchor bolts.

@sandman21 - when you say you sometimes use reinforcing to take the shear, what does that detail look like?

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

hokie66,

Thanks.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

You provide oversized holes to comply with ACI 117 tolerances for anchor bolt locations, which are BS, as they are far greater than a standard connection. Hole size are very large and the columns could move inches prior to hitting the anchor rod. Welding the washer will transfer shear into the anchor.

Quote (KootK)

I'm not abreast of that one. Did that actually have something to do with MWFRS shear being transferred through the bolts??

Likely nothing to do with anchor shear but just very bad design. No drags to MWFRS, W10x19 going 25'+, deck spanning that far as well. Someone botched alot of math on that one. Enjoy Hard Rock Hotel under construction....

Quote (bones206)

@sandman21 - when you say you sometimes use reinforcing to take the shear, what does that detail look like?

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.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

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"

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

Quote (canwesteng)

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.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

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.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

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.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

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.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

(OP)

Quote (sandman21)

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.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

Quote (bones206)

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.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

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.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

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/aisc/research-li...

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

slickdeals,

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

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

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??

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

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

Quote:

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.

RE: MSC Magazine - Shear in Anchor Bolts and Welded Plate Washers

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.

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