Anchor Bolts for Moment Frames

[MacGruber22]

Our firm always provides over-sized base plate hole for anchor bolts, which is accepted industry practice as far as I am aware. However, our details indicate the same condition for columns part of moment frames with our any modification. So, I modified the detail for the moment frame base plates to include heavy washers welded to the base plate. However, when the project was reviewed internally, they said that "we don't do that", but they weren't clear why it was OK not to provide that detail. I am a bit confused by this, as I am not aware that there is a standard for pretensioning ASTM F1554 rods to develop shear in the faying surface. The detail has no other provisions (shear lugs, etc.) to develop shear, so I don't know what I am missing.

 

[KootK]

Based on the information you've provided, I agree with you MacGrubber. If you have AISC design guide #1, you might want to review that with your colleagues to see what they think the shear resistance mechanism is. I'd be wary of relying on pretension either way. Relaxation and construction quality make that method questionable from a reliability standpoint.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[KootK]

Could they be counting on friction at the side of the base plate that carries the compressive component of the moment?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[MacGruber22]

I do have design guide #1.

Not so sure about the friction part. I understand, but I didn't think that is was a code-acceptable practice.

 

[paddingtongreen]

Why do you think the basis should be different? Moment frames and braced frames both have vertical and horizontal forces to transmit. This is not clockwork, neither the column nor the anchor bolts go in exactly as drawn!

The only time I have seen your detail used was when the vertical load was too small to develop enough friction to hold.

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

 

[KootK]

My friction comment wasn't so much a recommendation as a suggestion for what your employer might be thinking. I find it a bit sketchy too. Unfortunately, I'm not sure that there is a "code approved" solution. You've got appendix D and strut and tie as approved tools. The rest is up to the designer.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[KootK]

There are some jurisdictions that forbid the use of friction for seismic loads.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Lion06]

I'm surprised to hear how many do NOT weld the plate washers to the base plate for moment frame columns. That seems like the most direct and reliable load path. For those who do not weld the plate washers to the base plate, are you actually checking the frictional resistance to sliding?

 

[KootK]

I usually check friction only to find that it doesn't work for me. I'm conservative with it and, for low rise buildings, I find that the layout often doesn't attract enough dead load.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[MacGruber22]

Lion: That is what I was thinking, as well. I know that none of our engineers are checking shear resistance from friction between the base plate and the concrete. Besides, the details have 2" +/- of grout and leveling plate as well. That means, you need to develop shear-friction between the grout and the concrete. Besides, the quality of the grout base is usually in question.

 

[Hokie93]

I always provide a positive connection to transfer shear forces from the column base plate to the foundation. For small to moderate shear froces, this typically involves welding a plate washer, with a standard-size hole, to the base plate to transfer the shear force from the base plate to the anchor rods. This, of course, then results in larger anchor rods since the shear force results in bending stress in the anchor rods along with shear and, quite possibly, axial tension. For large shear forces, I use a shear lug. I do not rely on friction for shear force transfer. Another option is to recess the base plate into the foundation but I rarely employ this practice. AISC Design Guide #1 provides some guidance on shear transfer via friction though, again, I don't use it and do not recommend it.

 

[MacGruber22]

Thanks, Hokie. That seems consistent with everything I have learned. I am assuming that the only two limit states to check for the washer are the weld shear and bolt bearing? I imagine 5/16" hardened steel washer is usually enough for bearing and a 1/4" weld.

 

[Hokie93]

MacGruber: That is correct. I typically use a 1/2" thick plate washer of sufficient size to cover the "oversize" anchor rod hole (from Table 14-2 of the AISC Steel Manual) with the anchor rod fully off to one side. If your column is subject to net uplift, the top plate washer is in the load path and would need to be sized accordingly but the minimum thicknesses in Table 14-2 have accounted for that. Note that AISC Design Guides #1 and #7 recommend that the plate washer thickness be approximately 1/3 of the anchor rod diameter.

 

[MacGruber22]

Thanks, Hokie.

 

[darthsoilsguy2]

in North Carolina USA region, i've seen welded plate washers with oversized holes on details by lots of other SE firms.... it was more rare to cross it 10 yrs ago than it is now. I wouldn't know if those designs assume Moment reactions, but certainly it's a safe bet there are horizontal reactions that egrs want the rod to be max 1/16" away from contacting the plate without setting an impossible target for the concrete crew to hit.

10-20 years from now, there will be someone posting about how their firm doesn't specify to fill flush the rest of the over-sized baseplate hole with _____________, before placing the washer and nut. "Those guys still have an air gap between the grout and welded washer elevations.... at the very point where the plate wants to load the rod... craziness?!"

While you are discussing anchor bolts, try to work in specifying 2" projection over the nut into details to cut down on the short bolt field problems.

 

[paddingtongreen]

"While you are discussing anchor bolts, try to work in specifying 2" projection over the nut into details to cut down on the short bolt field problems."

I have always seen and used the projection above concrete, it seem like nonsense to do anything else.

Going back to the friction thing. In general, the force is applied high enough on the structure that the tension in the bolts and the increase in compression force under the plate are multiples of the applied force, providing ample friction without even considering the DL and LL contribution.

I did use shear lugs where needed.

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

 

[KootK]

It's too bad that one can't get the same mojo working for braced frames somehow.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[darthsoilsguy2]

most SE plans i've reviewed don't force the fabricator to supply longer-than-needed rods. i wasn't specifying, but communicating that we put the rod length in the fabricators hands after embedment/grout/baseplate/nut. The grounds for acceptable bolt length is 1-thread, but if it were 2" or 3", we would have a lot less anchor bolt discrepancies and retrofits done in the world.

projection over concrete sounds good for concrete foreman, but if i called out to the internet a #-inch projection above the concrete, i would have fished out a few "why does this (geotechnical) feel like he has something to say here" comments because of all the variables.

 

[delagina]

my company use 1/2 of bolt shear capacity to take into account the oversize of base plate hole. no, we don't also weld the plate washer.

 

[paddingtongreen]

@Darth. I don't quite understand why it should be anywhere except on the concrete drawings or why a Geotech would be involved in this function. It is part of the structural design.

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