Reduction in Strength of epoxy anchors based on reduced anchor spacing

[VSS Engineering]

My team is working a project where the concrete contractor misaligned a few cast-in place anchors. We need to remedy this by replacing the cast-in place anchors with threaded rod epoxy anchors. The issue we're having is that the column base plates do not allow us to meet the required anchor spacing for epoxy anchors.

Required spacing between anchors = 3-3/4"
Actual spacing between anchors = 3"

I have spoke with HILTI engineers to see if they have any literature for a reduction in strength based on a reduced anchor spacing, they do not. Tapcon allows for said strength reductions based on reduced spacings, our thought was to use a similar reduction factor to justify the use of the epoxy anchors we're trying to use.

Looking to get other professional opinions on this or any alternative ideas on how to combat this anchors spacing issue.

Thanks.

 

[Eng16080]

I think there's something in Chapter 17 of ACI 318 that allows the calculations to be based on an anchor of lesser diameter than what's actually used. So, if you were using 3/4" anchors, for example, you could base the design on 5/8" or 1/2". Doing so should allow a lesser anchor spacing.

Not sure if that helps. Also, don't take my word for it. I only vaguely remember reading this and haven't used this provision. It seems somewhat logical, though.

 

[EngDM]

I think there's something in Chapter 17 of ACI 318 that allows the calculations to be based on an anchor of lesser diameter than what's actually used. So, if you were using 3/4" anchors, for example, you could base the design on 5/8" or 1/2". Doing so should allow a lesser anchor spacing.

Not sure if that helps. Also, don't take my word for it. I only vaguely remember reading this and haven't used this provision. It seems somewhat logical, though.

CSA A23.3 has a clause for this, D.9.5. And if CSA has it, it's likely sourced from ACI.

You reduce all values for diameter to a lower value that satisfies spacing requirements. This is only permitted for anchors that won't be torqued, and for anchors that don't produce a splitting force I.E. no wedge anchors.

Minimum spacing specified by Hilti is also lower than the code since they have tested in accordance with ACI A355.2 or ACI A355.4.

 

[Eng16080]

CSA A23.3 has a clause for this, D.9.5. And if CSA has it, it's likely sourced from ACI.

You reduce all values for diameter to a lower value that satisfies spacing requirements. This is only permitted for anchors that won't be torqued, and for anchors that don't produce a splitting force I.E. no wedge anchors.

I just checked, and this is in Section 17.7.4 of ACI 318, nearly verbatim as what you wrote.

 

[SandwichEngine]

I'm in a hurry but I believe the ACI code gives the required spacing but makes exceptions for products with tested values with closer spacing. If you free download Simpson Strong Tie anchor designer, I know it will allow about 4db for many of the epoxy anchors, depending on diameter. I presume that these results are consistent with the allowable testing that the code specifies.

 

[VSS Engineering]

CSA A23.3 has a clause for this, D.9.5. And if CSA has it, it's likely sourced from ACI.

You reduce all values for diameter to a lower value that satisfies spacing requirements. This is only permitted for anchors that won't be torqued, and for anchors that don't produce a splitting force I.E. no wedge anchors.

Minimum spacing specified by Hilti is also lower than the code since they have tested in accordance with ACI A355.2 or ACI A355.4.

Thanks, I'll look into this.

 

[TRAK.Structural]

Second what others have said.

I would also check all the manufacturers as they all have specific testing data and their own set of minimum spacing/edge distance requirements. Not sure if they are much different between brands but worth a check.

Instead of using the method from ACI as others have noted, can you actually just use smaller diameter anchors?

As a last resort you could modify the baseplate to get the spacing you need.