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PEMB Anchor Bolt Eccentricity

PEMB Anchor Bolt Eccentricity

PEMB Anchor Bolt Eccentricity

(OP)
All,

Anchor bolts are the PEMB responsibility (to a point), but embedment, etc. is mine as the foundation engineer. Since PEMB building footings are rarely concentric with the footings, I see the anchor bolts being loading eccentrically for uplift. I know that the metal building people are assuming concentric resistance from footings, etc. Any thoughts on this? I can easily design the footing for uplift, but I'm worried about the anchor bolts being underdesigned based on how I design the footing.

My guess is that this is rarely considered based on my experience with PEMB foundations and seeing other designs, etc. But I'm not a big fan of just assuming.

Thanks

Nick

RE: PEMB Anchor Bolt Eccentricity

If the column centroid lines up with the center of mass of the bolts, I'm not seeing eccentric loads on the bolts, only on the footing.

RE: PEMB Anchor Bolt Eccentricity

(OP)
I guess I see it in the prying action with the base plate. If the footing below is offset from the centerline of the bolts then as it was being picked up, I see it prying on the column base plate. Does that make sense?

RE: PEMB Anchor Bolt Eccentricity

I'd worry about that in the footing, not in the steel.

RE: PEMB Anchor Bolt Eccentricity

this is a very interesting question and I have not run ito it before. At the moment, I would have to agree that an isolated footing that is not concentic with the bldg col would create eccenticity in the anchor bolts. The bolts don't know or care if the eccenticity is coming from above or below.

RE: PEMB Anchor Bolt Eccentricity

….it would also create a moment in the columns that it was not designed for. Get the foundation right so you don't worry about the steel.

RE: PEMB Anchor Bolt Eccentricity

Are you sure there is that much prying action with the typical 4x4 bolt pattern combined with the thin plates they use?

RE: PEMB Anchor Bolt Eccentricity

This is the age old question 'is my column baseplate really a pin?' but in reverse.

I wouldn't worry about this at all. Design your foundation such that it doesn't rely on a moment being taken into the bolts or the column and such that there isn't a large rotational displacement required to actually make that happen. If this is true and they've detailed their baseplates and columns in a standard way, you should be fine. In extreme load cases you might see yielding in the plate or bolts or column base, but that small moment will release. If the pin base assumption didn't work, we'd have had structures falling down all over the place for a hundred years.

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