Eliminating the weld washer for High Strength Anchor Bolts at - Field Issue

[oengineer]

Is it ever acceptable to eliminate welded washers for a high strength anchor bolts for brace frame columns? The contractor has installed the anchor bolt at two baseplates that does not have the required projection for seating of the nut. Both locations have the same condition due to improper elevation of the anchor bolts but all other anchor bolts at these locations have proper connections. The contractor is asking if it is okay to eliminate the weld washer at each of these locations. The anchor bolts are 1.75" diameter and the tension reactions at the brace frames are over 200 kips.

Based on what I read in AISC Design Guide 1 for base plates and anchor bolts, washer are recommended for high strength bolts subjected to resist tension loads. My 1st thought is to reject the contractor's request and have them install the welded washers. To deal with the projections issue I am think of suggesting Anchor rod extension using coupling nut or directly welding the rods together is to join the extra length of threaded rod using lap plates.

Please let me know any other thoughts. Suggestions/comments are appreciated.

 

[phamENG]

The welded washers are usually there for shear (tension wouldn't require the weld EDIT: IT WOULD REQUIRE THE WASHER, THOUGH). If you have adequate friction resistance or shear keys under the baseplate, you don't need welded washers. If you designed the connection to resist lateral loads through shear in the bolts, it depends. If you designed for 2 of the 6 bolts (I'm guessing at your layout) to take all of the shear, you probably don't need the welded washers as long as your ok with movement of the base of the column equal to the diameter of the oversized hole (again, assuming it's oversized) minus the anchor diameter (Not half, but all - what if the anchor is slightly off and hard against the opposite side of the hole? There's a reason oversized holes are used in baseplates.) If you need all of the bolts to resist the shear, then you need to have welded washers at all of the bolts. If you don't, you could have a couple bolts closer to the sides of the holes than others, so they'll load first and prevent the others from loading until they have deflected and/or failed - and then the others engage with two fewer than needed and the rest fail.

Coupling nuts could work, but aren't they pretty big? Do you have enough room? I've never used them. For welding, you need to look at the chemistry of the anchor rods. Are they F1554 Gr 36? You may be ok - make sure they're dual certed as A36 to be sure of their weldability. Are they Gr. 55? Make sure they meet the chemistry requirements of Supplement 1 (usually marked as S1 on the mill cert). Are they Gr. 105? Don't bother - the heat from the weld will embrittle the rod and do more harm than good.

 

[WinelandV]

Hold on, you ABSOLUTELY need those plate washers for tension if you used over-sized holes in the baseplate.

 

[phamENG]

windelandv - not sure if that directed at me. If so, I went back and edited my post in case it wasn't clear before I saw your post. In tension, the washer wouldn't need to be welded - the weld is there for shear transfer.

 

[WinelandV]

Sorry, phamENG. since the OP mentioned removing the bolts due to space, I thought your post was discussing going down that route. I'll readily admit that I tend to read posts too quickly...

At any rate, if tension is needed, you'll still need the washers, just not welded to the BP, as phamEng said.

 

[phamENG]

No worries. I regretted what I said as soon as I hit submit - that's why I went back and edited it immediately!

 

[dold]

oengineer,

Is this the same rod projection issue that you had another thread going for?

Whatever you end up deciding, make sure the special inspector out there for continuous inspection of the entire process. Generally this would be covered in the notes but I usually put a giant 'can't miss it' note on the sheet/RFI response/sketch/whatever. Particularly for a critical connection like this. These repairs do not lend themselves to multiple attempts to get it right.

 

[oengineer]

@winelandv

The base plate holes shown on the shop drawings are 2 1/16" for the six 1 3/4" dia anchor bolts. I believe with this size hole this means we are not using over-sized holes.

@ phamENG

The base plate design guide states, " For anchor rods designed to resist moment or axial tension,the hole and washer sizes recommended in Table 2.3 should be used." and "The addition of plate washers or other similar devices does not increase the pullout strength of the anchor rod, and can create construction problems by interfering with reinforcing steel placement or concrete consolidation under the plate. Thus,it is recommended that the anchorage device be limited to either a heavy hex nut or a head on the rod. As an exception, the addition of plate washers may be of use when high strength anchor rods are used, or when concrete blowout could occur (see Section 3.2.2 of this Guide)."

Based on the statement above I was inclined to believe the washers are need for the brace frame. The baseplate does have a shear lug to resist shear forces and is using six 1 3/4" dia anchor bolts. The anchor bolts are grade 55. Even if the washers are not welded, I would think that the washers are still required.

 

[phamENG]

I think you're confusing two different things - or maybe I am?

When you talk about welded washers and projection issues, it sounds to me like we're talking about small pieces of steel plate with a standard sized hole in the middle that are placed over the bolt and welded to the base plate. This does a couple things - it allows you to use oversized holes (d+5/16, which is exactly what you have) or larger if needed for erection tolerance, and then also allows you reliable shear transfer through those holes by providing a load path from the plate to the bolt without the connection needing to slip to engage, and also gives the nut enough "bite" to hold the plate down.

You're quoting a section about washers on the rod embedded in the concrete. Pullout is a function of embedment and bearing area - a plate washer is rarely stiff enough to actually matter. Also, why would a plate washer on top of the base plate have an effect on reinforcement?

Which one are we talking about? Embedded in the concrete, or between the standard washer and the baseplate?

 

[phamENG]

Also, is this at the brace connection to the foundation, or is at a column without a brace directly attached? In either case you need the plate washer above the base plate, if the former it needs to be welded and the welds need to be sized for the shear load at the base of the frame.

 

[oengineer]

@phamENG

It is a column with a brace directly attached to it at a foundation. So, you're saying the washer needs to be welded in this situation.

This situation is a base plate on top of 2" of grout for a foundation.

See Image: [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1572372615/tips/Doc1__ffgrnv.pdf[/url]

 

[bones206]

The baseplate does have a shear lug to resist shear forces

If there is a shear lug, I don't believe the plate washers need to be welded. The load path for shear is brace--> gusset--> baseplate--> shear lug--> pedestal--> foundation--> earth. The shear load path bypasses the anchors so there is no need to weld the washers to the baseplate.

This is a completely separate and distinct issue from whether or not plate washers are required.

 

[oengineer]

I just found this statement in AISC Design Guide 1:

"Washers should not be welded to the base plate, except when the anchor rods are designed to resist shear at the column base (see Section 3.5)"

Based on the comments in this thread, it appears that the washer is required, but it does not necessarily need to be welded to the baseplate due to shear lug on the base plate. Would this a be correct statement?

 

[phamENG]

Correct - sorry, I missed the shear lug statement.

 

[dauwerda]

Plate washers are required when over-sized holes are used. In Table 14-2 of the 15th edition of the AISC steel construction manual there is a footnote that standard (ASTM F844) washers may be used if hole clearances are limited to 5/16" for rods up to 1"Ø, 1/2" for rods up to 2"Ø, and 1" for rods over 2"Ø. The dimensions you gave (1-3/4" rod with 2-1/8"Ø hole) meet these criteria so a standard washer could be used rather than an over-sized plate washer.
PIP STW05121 "Anchor Bolt Design Guide" also provides maximum hole sizes for anchors with the use of hardened (ASTM F436) washers. According to this table (below), you would also be able to use hardened washers rather than standard or plate washers.

EDIT *15th edition of the SCM, not the 14th*

 

[phamENG]

I'm not familiar with the PIP document, but my SCM, Table 14-2 doesn't have a foot note so detailed. The closest is #3 - "When base plates are less than 1-1/4" thick, punching of holes may be an economical option. In this case, 3/4" anchor rods and 1-1/16" diameter punched holes may be used with ASTM F844 washers (USS Standard) in place of fabricated plate washers."

Going back through section 14, under "Washer Requirements," it states "The thickness must be suitable for the forces to be transferred." Then it mentioned minimum sizes, though these minimum sized don't apply in this situation as they're for larger than oversized holes, and in this case I believe you have "standard" oversized holes (hope that's not as confusing as it sounds...).

In any case, for tension, you have to make sure that what is there is sufficient to transfer the loads. If you have access to Roark's Formulas for Stress and Strain, you can use Table 11.2, Case 1a (Annular Plate with Uniform Annular Line Load, Outer edge Simply Supported, Inner Edge Free). Another thing to consider - did you specify heavy hex head nuts? F1554 anchors can be fitted with standard hex head, which has a smaller bearing area and would increase the cantilever distance of the plate washer. Even if you have heavy hex head, your smallest width across the flats will be 2", which is less than your hole diameter. I wouldn't dismiss the plate washer based on a rule of thumb for significant uplift loads.

 

[dauwerda]

I meant to write the 15th edition of the SCM has that footnote in the table (it was added in the 15th edition), I have gone back and edited my original post. For reference, here is a summary of all of the changes made from the 14th to the 15th edition:

 

[phamENG]

Neat. Thanks, dauwerda. I guess I should start thinking about buying the new one.

That said, I'd still be cautious about reducing it based on that. Those are minimum requirements. In the OP's case, he has a significant tension load that should be considered in an analysis of the baseplate/washer/nut/anchor connection. My opinion, anyway.