Anchor Bolts / Base Plates - Grouted vs Standoff vs None

[JNEnginr]

Hey Everyone,

Just looking for some general information about what is industry standard when specifying anchor bolts and base plate connections. I'm typically working on residential/commercial structures, and specifying base plates being set on a 1/4" leveling plate and non-shrink grout. Recently I did a tilt up structure with interior columns, and all those columns were requested to be constructed on standoffs, then grouted later. Then more recently, I designed a freestanding sign (about 20ft tall) and typical construction I see everywhere is stand offs as well, some with clamps. Hilti Profis has the 4 options (no stand off, stand-off with clamps, stand-off without clamps, and stand-off with grouting). Just looking for feedback on when each type is used in what construction conditions, and why? Maybe just ease in construction that I'm not aware of?

Thank you all!

 

[azcats]

What are 'clamps?'

In telecom, it's most common to set the poles on leveling nuts above top of concrete w/o grout. ABs are designed for the bending effects unless the distance between top of concrete and bottom of baseplate is limited to 2 bolt diameters or less.

 

[JNEnginr]

Clamps are a third nut, usually "clamped" against the face of the concrete.

But any idea, why it's done that way? Corrosion? Too big/awkward to grout under?

 

[azcats]

It's difficult to properly place grout under large baseplates with center openings.

We'll embed a nut in the concrete at/near the top of concrete if the bottom of AB is near bottom of foundation.

 

[dauwerda]

The utility industry (transmission poles and substation structures) also does not typically use grout under base plates.
It is my understanding that the reason for this is to help prevent any corrosion issues that tend to happen with grouted installations.

 

[BridgeSmith]

The utility industry (transmission poles and substation structures) also does not typically use grout under base plates.
It is my understanding that the reason for this is to help prevent any corrosion issues that tend to happen with grouted installations.

That's what we do for high mast lighting towers and signals and sign structures. In our case, as I suspect it is in the utility industry, it's done that way to provide precise geometric control i.e. being able to get the towers and poles exactly plumb.

We don't grout them because it's unnecessary, and provides no advantage in performance or durability. On the bigger structures, we put a screen around it to keep the critters out, but that's it.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dauwerda]

Certainly, the leveling nuts are used to make it easy to plumb (or in some cases rake) the structures.
My second comment was in reference to not using grout. No grout allows for any moisture to freely drain away (and for air circulation to dry it out). I have been in older substations (built in the 70's and 80's) where they were grouted and many structures have the grout broken out around the anchors and the anchors are corroded. Some of the dead end structures had to have line tensions reduced due to anchor cross section loss. (Anchors were sand blasted clean of all corrosion and cold galved for protection.) I have not seen any corrosion issues on anchors without grout.

The building industry will tend to have/need grout under their base plates for a few reasons. First, the columns and base plates are inside the building envelope so corrosion is less of an issue. Second, these columns tend to support much more vertical load than utility structures so they need all of the bearing area they can get to efficiently transfer forces from the column to the foundation. Grout ensures that there is full support as it will form to any irregularities in the concrete and base plate.

No standoff i.e. no grout (to me) is more typical in the residential world where the loads are light so full bearing of baseplate is not as critical. It also eliminates the need of a specialty grout contractor (or depending on an untrained person to do it correctly).

 

[driftLimiter]

Oh the grouted baseplates. The only real structural advantage we get out of them for commercial and residential buildings is that they help achieve complete bearing on the baseplate, other than that they are being used as a means and methods of getting the column plumb and leveled to the correct height.

The downsides of this approach are many, not the least being the impact on the anchorage forces.

Interestingly for the pre-engineering metal building industry, they don't use grout pads or leveling nuts, even for the most massive PEMBS I have seen they just put the frames down right on top of pilaster.

 

[BridgeSmith]

My second comment was in reference to not using grout. No grout allows for any moisture to freely drain away (and for air circulation to dry it out). I have been in older substations (built in the 70's and 80's) where they were grouted and many structures have the grout broken out around the anchors and the anchors are corroded. Some of the dead end structures had to have line tensions reduced due to anchor cross section loss. (Anchors were sand blasted clean of all corrosion and cold galved for protection.) I have not seen any corrosion issues on anchors without grout.

I understand and agree with all of that. I was just adding that without substantial axial load, there is also little, if any, structural advantage to adding grout to a baseplate supported by leveling nuts.

Even when significant axial load is present, it is difficult to achieve a condition where a baseplate with steel leveling nuts is supported by the grout, rather than the anchor bolts. This is sometimes achieved by the use of 'temporary' plastic or nylon leveling nuts, which are only used to align the column during construction, and then strip out to allow the axial load of the column in service to be carried by the grout. Here is an earlier thread on a similar topic.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Struct-007]

Fatigue is a design consideration in pole structures. Grout is not used under the base plate in poles because grout has low fatigue capacity. Therefore, the design is performed so that the Anchor Bolts take both tension and compression.