Base plate anchor rods 2

[JStructsteel]

So I am working on anchor rod design. Is there specifications on washer size for baseplates? Also, do you have to have 2 nuts(one under baseplate, one on top?

At the concrete end, I am just showing a nut jambed to the end of the thread. Does it have to have a washer? I know the design is based on diameter, do you folks use a washer? My detail so far.

 

[phamENG]

For washer size, look at the steel manual. There's a section for minimum plate washer diameter and thickness for normal baseplate hole sizes. That information is also listed in DG1.

No, a leveling nut is not required. The three 'typical' options are: leveling nut, shim packs with grout, or hire a magician of a concrete guy that can give you a true and level foundation surface at the perfect elevation and set it base plate directly on the footing.

Washer is not required. Again, DG1 has all the bearing area already done for you in one of the tables. You can get a little more area with a washer, but I think its contribution is a bit dubious if it's not really thick, considering relative stiffness of a thin washer, the nut, and the concrete itself.

 

[Once20036]

The size of the washer on the below the top nut is based on the size of the hole in the base plate. Check out AISC Design Guide 1 for recommended washer sizes.

There are three ways that I know of to level a base plate. 1) Leveling plate where a bed of grout is placed, then a thin plate that matches the base plate dimensions is set into the grout bed - level (hopefully) and to the correct elevation. 2) Leveling nuts where a nut is placed on each anchor rod to the correct elevation. 3) Shim stack to the correct elevation.
Only method #2 requires a nut below the base plate.

We don't use a washer at the bottom for gravity columns, only when required due to a pullout failure mechanism for lateral columns.

 

[dvd]

excerpt from The Grouting Handbook -

 

[dvd]

AISC anchor rod dos and don'ts -

 

[cliff234]

Keep in mind that if you’re using oversized holes in your baseplate per Table 14-2 (AISC 15th edition Manual) then you’ll need oversized washer plates per Table14-2 on both the top and bottom of the baseplate (because the holes are almost as big as the nuts).

 

[Eng16080]

At the concrete end, I am just showing a nut jambed to the end of the thread. Does it have to have a washer? I know the design is based on diameter, do you folks use a washer? My detail so far.

It might be better to tack weld the nut to the rod at the embedded end, or provide a double nut, or damage the threads around the nut. Basically, you want to eliminate the risk of the rod turning independent of the embedded nut when you tighten the upper nut at the baseplate. I wouldn't be confident that jambing the nut at the end of the threaded length is necessarily adequate.

I wouldn't normally use a washer at the embedded end, unless the strength was limited by anchor pullout and I needed more capacity. The pullout capacity (per ACI 318, Section 17.4.3.4) is a function of the bearing area of the anchor, so a washer can potentially increase that area leading to a greater capacity. You would want to check that the washer is sufficiently thick in this case, as a thin washer may provide no real benefit if it fails in bending. Also, DG1 recommends keeping the washer as small as possible so as not to diminish the concrete strength.

 

[WesternJeb]

To piggyback off of cliff234, using plate washers to transfer the load from the base plate into your anchor bolt (plus grout under the base plate) will induce bending moments into your anchor bolts. You need to use AISC DG1 to account for this, as your anchor bolt size might go up substantially due to unity checks.

Another thing to look out with double nuts... Once your anchor rods get over 1-1/4" dia., you need to start looking at your clearance your are specifying from the bottom of your base plate to the top of concrete to allow space for the nuts, which might cause you to raise your base plate, and ergo increase your moment arm into your anchor rods and increase their size more. It can be a very iterative process when you have substantial shear loads.

Just some tips on stuff that has caught me before.

 

[bones206]

This PIP is my go-to reference for anchor rod detailing:

 

[Eng16080]

WesternJeb, I agree with everything you wrote, except that raising the base plate (and presumably increasing the grout thickness below it) shouldn't increase the moment acting on the rod, at least per DG1. Per DG1, top of p. 29: "... no bending of the anchor rod within the grout need be considered." I've heard this be disputed, but at least this is the stance from DG1.

 

[WesternJeb]

Eng16080, Not disagreeing with you and appreciate you bringing that point up! However, I have seen too many a various light poles and columns without any grout underneath the base plate, and too many lazy workers that I have caught not compacting the grout well to rely on the grout for anything except for developing a force couple during bending (edit: bending from the column that induces tension into the base plates).

I usually take the route of assume everyone does their job correctly and don't overdesign, but this is one I can't justify... Just a personal preference item and CYA on myself more than anything because after something has failed, there is no proof if the grout was or was not properly placed around the anchor bolt to prevent any excess of bending.

 

[JStructsteel]

Thanks all. I am trying to set up a standard detail. I will make the leveling nut optional. I always detail with 1" grout space.

 

[phamENG]

I always detail with 1" grout space.

AISC recommends 1-1/2" to 2" if your on a footing or pier as surface tolerances are much more lax than a typical slab.

 

[Eng16080]

WesternJeb, those are good points. I've always assumed the grout is being placed correctly. I also normally use a leveling plate detail, though, which should make this more likely to be true. I was considering switching my standard detail to using a leveling nut instead, but improper grout placement seems like a big downside to that.

One question though: if you assume the grout might not be placed correctly, do you design the anchor rods to support the vertical load from the column?

 

[WesternJeb]

ENG16080, I think that is the point I was trying to make that I don't rely on the grout for anything except developing the force couple due to bending/overturning of the column (ie bearing). In a forensic study it will be shown if there is grout or not, just not how well it was placed. It is a "risk analysis" that grout might at least take the bearing for myself to not be majorly conservative on this issue.

 

[JStructsteel]

How in the weeds do you get with AB details. Do you call out the ASTM of the nuts, etc?

 

[WesternJeb]

JStructSteel, I include this in my spec. If we don't have specs then I don't go through the trouble of putting it on my drawings though.

 

[phamENG]

Do you call out the ASTM of the nuts, etc?

Not in details. Depending on the size and complexity of the project, I either include that in the specs or in my general notes.

 

[WinelandV]

I note that the embedded nut(s) should be heavy hex, as that's the nut size we use in our anchorage checks.

Please note that is a "v" (as in Violin) not a "y".

 

[dik]

Further to that... some grout suppliers restrict the thickness their products should be used for...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik