Seismic Anchorage Calculations Question

[Zoletres]

I wasn't sure if this should be in the civil or structural general discussion, but I figured I'd ask here because of the design criteria is usually in the structural part of plans.

I work in a project management position where as part of the submittal packages I have to provide seismic anchorage calculations, I'm in CA. There are a few PEs my company has used, both civil and structural. Each have a different ways of preparing, presenting, etc. their reports.

One big difference I've seen is deciding to use 1 bolt vs all the bolts required when they do the calculations use the Hilti or Simpson softwares.

How does one decide when make this decision for the calculations? Or other choices?

I mostly deal with non-structural equipment in my position (pumps, compressors, tanks) if that helps answer anything.

 

[Greenalleycat]

I regard this as a structural question so you're in the right place
I'm not familiar with US codes but am familiar with the Eurocode requirements for epoxy anchors
I've used the Hilti software a lot but have never used Simpson as their stuff isn't used (is it even available?) here

1) If the anchors are loaded in shear then Hilti will force the load onto the critical anchor(s) (closest to the edge) unless you tick the box to use 'Hilti Seismic Filling Set' or whatever they call it now
This is a code requirement that conservatively assumes the anchors are not all loaded at the same time due to the tolerances around holes
It forces you to put all the load on the critical bolts unless you use a structural epoxy to fill the bolt holes post installation to guarantee equal loading
What you are saying could be a) engineers fudging the system b) engineers not knowing what they're doing c) variances between default settings in the system d) regional/code differences - I think Simpson is way more of a USA thing whereas Hilti is big worldwide

2) Sometimes it suits the engineer to demonstrate that something works using fewer anchors
I've done this sometimes when you want to maintain flexibility or do something a bit faster
It can be a bit niggly to setup a full anchor layout when you know that only 1 bolt is really required

Maybe there are other explanations too but those are the obvious ones that come to my mind

 

[JStephen]

The spacing between bolts relative to the embedment depth is the criteria in ACI 318 to determine if bolts are treated singly or as a group- not sure if this is what you are asking.

 

[beav-eng]

I have modeled concrete anchors both ways. Typically, if the anchors are close enough that they are an anchor group, they should be modeled together to capture the combined failure effects. If I only have one anchor or the anchorage is spaced far enough apart, I would model just one anchor. An anchor group could also be acceptable as long as the loading is correct.