ASCE 7-16 12.4.2.2: Vertical Seismic Exception - Mat Slab Foundation

[Electric_Steel]

(Apologies for a repost, but trying again in the general group to hopefully reach more eyes)

Hello all,

Long time reader, but first time posting a question on Eng-Tips. I have a project with significant seismic forces (PGA.m > 2.0) and we need to support a quite heavy piece of equipment (~400kips) on a reinforced concrete mat slab. Currently we are trying to avoid deep foundation elements if possible and economize where it makes sense. As you might expect, overturning and sliding (i.e. - stability) is controlling the size of the slab.

ASCE 7-16 section 12.4.2.2 seems to allow the engineer to exclude vertical seismic effects during foundation design, but we are trying to determine how far does this exception goes, and if this can be applied to overturning/sliding checks. This exception would certainly help us economize the design, but several folks in my office are hesitant to rely on this code section. I think there are at least a few reasons for this reticence:

1) The commentary doesn't explain the "why" behind this section. If we understood the logic of this exclusion for foundation design, we could better use our engineering judgement. Does anyone have any insight into the "why" of this exception?

2) The section states that vertical seismic may be neglected for the purposes of "determining demands on the soil-structure interface of foundations". What exactly is meant by "soil-structure interface"? This term is not clearly defined in Section 11 (as far as I can tell interface is different from soil structure interaction). Should we be interpreting this section to only apply to some foundation checks and not others? Perhaps this exclusion is just when determining contact pressures (i.e. - bearing capacity checks)?

Let me know what you think or if I might have overlooked a good resource. Always curious to hear others' perspectives. For the client's sake, I'd like to get more comfortable with this section and leverage this exclusion if possible.

--Cheers!

 

[cec17]

Does the vertical seismic effect influence your overturning and sliding of the foundation? My understanding of this exception is that it would only apply to the vertical seismic effect E[sub]v[/sub] - Any overturning or sliding would be from your E[sub]h[/sub] force, where this exception would not apply.

 

[WesternJeb]

Let's back up here to what I think you are asking.. The horizontal component, Eh, is what creates your vertical forces that lead to overturning issues. These are still components of Eh and shouldn't be reduced. The vertical component, Ev, is a gravity only component (usually downward) that is what I believe the code section refers to.

In my mind, it is permitted to ignore vertical seismic forces for overturning checks, as including them would be added basically dead load, and ergo INCREASE your overturning safety factor if included. Include and exclude Ev as necessary to create your maximum force demand.

 

[canwesteng]

Vertical earthquake loads are meant to be taken to act down or up, so if included are always critical for overturning and sliding. I personally read that clause as meaning you can exclude vertical seismic uplift when calculating bearing pressures - think of the case where you have only a very small portion of the footing in contact with soil with the resultant outside the kern. You might be stable but end up with very large bearing pressures.

 

[Electric_Steel]

Hey all, apologies if my question was a bit vague. I think it might be helpful to look at this with some numbers. Some more context: this is specifically for a high voltage electric transformer anchored to a mat slab inside a substation.

Sliding is my biggest headache - so let's focus on that. Essentially if I consider the vertical seismic component as an uplift load opposing my dead load (i.e.: [0.6- 0.2Sds]*DL), my normal force is not significant enough to generate sufficient sliding resistance. Specifically let's assume the following numbers (these are close to what I actually have):

Coeff. of friction between sub-base and concrete = 0.55
E.h = 473k

Scenario 1 - "Normal" uplift condition:
[ul]
[li](0.6)*DL = 907k[/li]
[li]Fr.sl = 0.55*907k = 499k[/li]
[li]FOS = 499k/473k = 1.05 --> O.K.[/li]
[/ul]

Scenario 2 - Uplift condition with seismic working against us:
[ul]
[li](0.6-0.2S.ds)*DL = 733k[/li]
[li]Fr.sl.uplift = 0.55*733k = 403k[/li]
[li]FOS.uplift = 403k/473k = 0.85 --> No Good[/li]
[/ul]

As you can see, having 0.2*S.ds oppose the dead load is making or breaking the design. If I can comfortably leverage section 12.4.2.2 to ignore scenario 2, it would make all the difference. Thoughts?

 

[WesternJeb]

Let's back this up in that I have never included it in my calculations for this exact scenario, right or wrong. I assume that the tributary area is large enough that it, "cancels out" for some areas being positive and some areas being negative. I think of this localized Ev as a positive or negative load similar to components and cladding in wind. Once you get a large enough area, i.e. at the soil-foundation interface, it all equals out and can be ignored.

 

[cec17]

@Electric_Steel thanks for the clarification.

IMO, you are still evaluating the soil-structure interface using E[sub]h[/sub] in the sliding case you described. Per the exception, you would not need to reduce the DL.

Guess I'm interested in seeing what others have done here, but I have also done several mat foundations in higher seismic areas and never reduced the DL using E[sub]v[/sub].

 

[Electric_Steel]

@WesternJeb & @cec17 - I am leaning towards excluding the vertical component too. It seems like I'd be following the "letter of the law" I just wish ASCE gave me a little more to work with here - I never like blindly applying codes/standards without understanding the background.

Anyhow, it's nice to hear how others approach these concepts. Thanks for the feedback! Always open to other opinions if anyone else has an opposing point of view.

 

[JStephen]

A couple of days ago, I was doing some continuing ed- one of those items points out that structural elements are designed for 100% of vertical force, but for overturning stability and sliding, it is only 100% of horizontal plus 30% of horizontal in the other direction plus 30% of vertical- but not 100% of vertical. Not sure if that is factored in above.