The effect of grout under steel column base on base plate thickness

[shnider22]

I have a fixed column base of straight built up steel column. i want to know if I provided a non shrinkable grout under the base plate will this help in reducing the base plate thickness in the compression side of the plate or even general reduction.

 

[WARose]

Thickness/type of grout is pretty much meaningless as far as base plate thickness goes. The biggest impact of grout thickness is anchor bolt size. (I.e. the cantilever length.)

 

[BAretired]

I agree. Non shrink grout would have no effect on base plate thickness.

BA

 

[spats]

If grout under the base plate means there are leveling nuts beneath the plate, then anchor bolts can also act in compression. This would change he load distribution below the plate, and could certainly affect the base plate thickness. Normally the anchor bolts are ignored on the compression side. I would think this is conservative, but don't know for sure.

 

[Lomarandil]

For the base plate geometries I've seen, considering the anchor bolts as the sole source of the compression reactions is typically the more conservative option (vs considering a bearing compression block).

If you're talking about considering combined anchor bolt compression and grout compression... tread carefully.

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The name is a long story -- just call me Lo.

 

[spats]

Lo... it's interesting that you use the bolts on the compression side, not the grout. With what AISC allows these days for oversized holes in base plates (there must have been more than a few contractors on that committee), you would need proper plate washers both above and below the base plate.

 

[Lomarandil]

I'm not saying that's the physical behavior, I'm simply stating that it's a (typically) conservative assumption that simplifies the analysis significantly.

Otherwise, I'm not aware of an officially sanctioned method to determine the distribution and limits of the compression reaction. Do you take a triangular pressure distribution (like soils)? or a Whitney stress block like RC? something different? All reasonable in the right circumstances, but it requires some justification (and then typically an iterative solution).

(If I'm simply not aware -- I'd love to know!)

----
The name is a long story -- just call me Lo.

 

[BAretired]

spats,
I thought that washers were required with oversize holes when the bolt was in shear, not when the bolt is in tension or compression. Leveling nuts on the bottom and nuts on top of the plate should be sufficient to transmit axial forces.

BA

 

[Lomarandil]

BA, I had the same thought, but at some point an anchor-rod "super-oversize" hole won't have much bearing area on the nut. Depending on the magnitude of load, I could see an argument for plate washers there too.

----
The name is a long story -- just call me Lo.

 

[Hokie93]

Per AISC Design Guide 1 Section 2.6, washers are required on top of the anchor rod hole for anchor rods subject to moment or axial tension. The washer thicknesses in Table 14-2 of the AISC Steel Construction Manual have been sized at approximately 1/3 the anchor rod diameter, as recommended in AISC Design Guide 7. This provides adequate strength and stiffness for plate washers for all grades of ASTM F1554 anchor rods. The plate washer size in Table 14-2 is based on the washer covering the hole with the anchor rod located at the edge of the hole. The plate washers need only be welded to the base plate if shear is being transferred from the base plate to the anchor rod.

 

[spats]

Good information Hokie. The way I look at it, though, is that they're not addressing using the anchor rods for compression. It makes logical sense that for a largely oversized hole you would need a plate washer below to assure proper transfer of compression load into the anchor rod.

 

[Amir Hasanpour]

Hello,
You can check the effects of the installed grout below the base plate by using this computer software: ColumnBase Software
For base plate thickness, column design, stiffener results and the design of the footing and grout for bearing stresses.