Lever arm on bolts with Mortar

[Liam1009]

Hi,

This is quite specific, but does anyone have any experience specifically to code of seemingly conservative estimates of bolt shear strength, where mortar is used?

This is specifically in clause 6.2.2.3 in BS EN 1992-4:2018:

6.2.2.3 Shear loads with and without lever arm
(1) Shear loads acting on fastenings may be assumed to act without a lever arm if all of the following conditions are satisfied.
a) The fixture is made out of steel and is in contact with the fastener over a length of at least 0,5 ⋅ tfix .
b) The fixture is fixed:
1) either directly to the concrete without an intermediate layer; or
2) using a levelling mortar with a thickness t grout ≤ 0,5d under at least the full dimensions of the fixture on a rough concrete surface (see EN 1992-1-1:2004, 6.2.5) as intermediate layer; the strength of the mortar shall be at least that of the base concrete but not less than 30 N/mm2.


This essentially means if you use any leveling mortar of any significant thickness, you will induce bending into your bolts, and with any shear loading, stresses that will result in using large bolt sizes. This seems extremely conservative, when a high strength mortar will provide continuity from the concrete platform.

Is this clause just absurdly conservative? Would be interesting to hear a wider viewpoint.

Liam

 

[r13]

I see differently (note I am in the US), if the leveling grout is kept within 0.5d, then no bending needs to be considered. For grout thickness greater than that limit, yes, you need to consider bending of the bolt, caused by the shear time the lever arm. Quite reasonable to me.

 

[strucbells]

I think it's generally expected that the nonshrink grout will crack, especially under cyclic loading. I'm in a high seismic area, so I always assume this to be the case. Not sure if wind-controlled areas make this same assumption.

In the US- AISC Design Guide 1 has some good information about anchor rod bearing for shear transfer through a grout pad.

There's also a couple of research papers (free PDFs online) with varying conclusions on this:

"Steel Shear Strength of Anchors With Stand-off Base Plates". Study commissioned by Florida Department of Transportation.

"Shear Transfer in Exposed Column Base Plates". Study commissioned by AISC.

Generally my recollection is that they confirmed the AISC Design Guide 1 approach is reasonably conservative, and that flexure does need to be considered for design of grouted anchors.

 

[r13]

During my years in the steel mill, I have seen many crushed grout, the column survived only by the bolts, and adjacent structures that tied into it.

 

[dauwerda]

In the utility industry in the US it is typical to ignore bolt bending as long as the distance between the top of concrete and bottom of base plate is less that 2 x diameter of anchor rod. Note, it is also typical to install these structures with a leveling nut under the base plate and without any grout. It should be noted that these are also typically fixed moment connections, supporting cantilevered columns.

 

[Liam1009]

Thanks for replies all!

The bending forces should be small, even for reasonable shear loads (100-200kN).

What are the appropriate stress formulas to use, subject to the resultant bending, to ensure the yield strength of the bolt is not exceeded?

Liam

 

[JLNJ]

Hilti has a toggle for this in their Profis program. The standoff toggle essentially neglects the grout and wreaks havoc with the anchor size.

So, yes, this seems overly (and maybe absurdly) conservative to me.

 

[r13]

Doesn't BS code provides a method, if it requires the bending to be considered? AISC basically is against use of anchor bolt to resist shear, and it neglects bending of anchor bolt in grout as others have mentioned. Otherwise, you may consider leveling nut, if helps.

 

[strucbells]

The bending forces should be small, even for reasonable shear loads (100-200kN).

What are the appropriate stress formulas to use, subject to the resultant bending, to ensure the yield strength of the bolt is not exceeded?

The papers & design guide I mentioned above go more in depth, including calculation examples and recommended procedures. For typical cast-in-place anchors with oversized holes & welded washers, they describe the following failure progression:

1. At low base plate slip, the grout remains uncracked and the anchor bending length is equal to the base plate thickness + 1/2 the depth of the welded washer, consistent with the current AISC DG1 recommendations.
2. As the base plate slips more, the grout cracks, and now the bend length is equal to the grout pad depth + base plate thickness + 1/2 the depth of the welded washer.
3. However, as deformations increase, there is tension stiffening of the anchor rods and the rods are more likely to bear on the base plate itself, reducing the bend length, increasing the strength of the connection.

The effective length factor (k = 0.5) as the bolt goes into reverse curvature.

I'm not sure what British/Euro code approaches are for this.