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?

 

[phamENG]

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.

 

[KootK]

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

Agreed.

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.

 

[spieng89]

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.

 

[sandman21]

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.

 

[canwesteng]

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.

 

[RPMG]

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.

 

[r13]

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.

 

[KootK]

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

 

[canwesteng]

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.

 

[dhengr]

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.

 

[human909]

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.

 

[sandman21]

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.

 

[KootK]

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??

 

[r13]

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.

 

[hokie66]

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.

 

[bones206]

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?

 

[r13]

hokie66,

Thanks.

 

[sandman21]

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.

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....

@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.

 

[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"