Is Sliding Allowed per IBC?

[TJW]

Have had some technical discussions with colleagues regarding the requirements in IBC regarding anchorage and sliding. Putting aside the argument of whether or not it is a great idea, does IBC preclude you from allowing a structure to slide? Section 1604.9 "Counteracting Structural Actions" says ".... where sliding is used to isolate the elements, the effects of friction between sliding elements shall be included as a force". Where reference is made to "elements", would that include between the structures base plate and foundation?

Since the question may come up, to be specific about the structure:
It falls under ASCE 7 Chapter 15 "Non-Building Structure Similar to a Building"
It is a multiple story (2 above ground) work platform with people working on it. Approx 16' to upper deck with a footprint of 20' x 30'.
Seismic Design Category D
Located indoors (manufacturing facility)
The coefficient of friction between steel base plates and concrete floor has been accounted for.
It has greater than 1.5 safety factor on tipping using ASCE 7 load combinations from Chapter 12 (ASD).
There is no concern for the structure impacting anything if it slides in an earthquake
It is not a temporary structure

So, the obvious question is "why would you NOT anchor it down", which is a legitimate question but again, putting that aside does the Code REQUIRE this structure to be anchored?

I would really appreciate how you all interpret the Code in this regard

Thanks,
TW

 

[msquared48]

Have you considered the friction force going to zero during an earthquake due to uplift?

Mike McCann, PE, SE (WA)

 

[KootK]

In this case, I'd think that you'd actually want your friction force going to zero as it would minimize the force needing to be dealt with in the sliding structure. Based on my read of it, it sounds as though you are allowed to slide. Poor man's base isolation I suppose.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[cvg]

downward ground movement would cause the structure to "jump" off the floor, friction and lateral force both go to zero.

upward ground movement would restore the contact with the floor

in my experience riding out several earthquakes, you will have both horizontal and vertical ground movement depending on the type of wave. jumping and bouncing might be a worse consequence than sliding...

 

[WARose]

So, the obvious question is "why would you NOT anchor it down", which is a legitimate question but again, putting that aside does the Code REQUIRE this structure to be anchored?

I've seen it a few times with equipment in high seismic zones (or with thermal expansion issues): it sits on "slide plates". (While still being able to accommodate over turning forces.) Most commonly it's done with teflon plates. (I.e. a teflon plate epoxy bonded to a steel backing plate, and then that backing plate welded to structural steel/equipment support. (I.e. teflon on teflon) Teflon has a low coefficient of friction.)

Basically what I read that section of the code as saying is: you have to account for that (minimal friction) force in the supporting structure. (Like any other load path.)

 

[msquared48]

And whatever goes up, comes down with more than the static force of gravity alone.

Even if the IBC does not address the anchorage to resist sliding, I think OSHA, and good engineering judgment does.

You can use friction in some structural situations as in shear friction, but I still use it sparingly as friction is an unreliable and variable force, sometimes not easily quantifiable.

Mike McCann, PE, SE (WA)

 

[JAE]

Folks I think the OP is asking if he can NOT anchor and NOT depend on friction for anything in the design.

I think a concern I'd have is the mezzanine sliding in some parts (some columns slip) while others do not and you have a strange set of forces in the columns that might be hard to predict.

This is similar to the requirement for building columns to have interconnected foundations to avoid the building doing the splits.

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[haynewp]

I think you would have to meet the requirements of ASCE Chapter 17 for a base isolated structure unless there is adequate friction or anchorage to resist sliding. Sliding has the safety factor included when meeting the 7-10 load combination via the reduced dead load. The commentary to IBC section 1604.9 mentions the friction or anchorage requirement.

 

[TJW]

Thank you all for your input. I really appreciate you taking the time and providing your insight.
TW

 

[XR250]

Another thing to consider is it hitting a nearby occupant who may be leaning against a column during a smoke break. I guess this falls under your original, "not a great idea" statement.