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

[Electric_Steel]

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!

 

[ggcdn]

My understanding is that Ev can be excluded from only the ASD combinations, which you would only be using for checking soil demands. The concrete design would be using USD combinations which then need to include Ev. This can lead to a bit of inconsistency for things like checking uplift and overturning.

NEHRP has a pretty good free document "Seismic Design of Reinforced Concrete Mat Foundations" which doesn't fully address the question but provides some additional information.

https://www.nehrp.gov/pdf/nistgcr12-917-22.pdf

Another component of load combinations for mat foundation
design is the vertical seismic load effect (Ev). Per ASCE 7-10
§12.4.2.2, Ev is permitted to be taken as zero when determining
demands on the soil-structure interface of foundations.
However, this is applied only to the basic allowable strength
design load combinations provided in ASCE 7. Similarly, per
IBC 2012 §1605.3.2, Ev is permitted to be taken as zero when
using the alternative basic allowable strength design load
combinations to proportion footings. Therefore, the vertical
seismic load effect does not need to be included in either
allowable strength design load combination for mat foundation
design.

Therefore, provided that a mat foundation
is able to maintain equilibrium under the applicable allowable
strength design load combination and not exceed the allowable
bearing pressure of the soil, the mat would be considered stable
and acceptable.

 

[ZiggyKS]

In §12.4.2.2, Exception 2a. the ±E[sub]v[/sub] vertical load is removed (neglected) when designing for Seismic Design Category 'B' so it is still applicable for SDC C and higher. Note SDC 'A' structures are already exempt from seismic design so it is not listed. I've never read this before but it will certainly help with analysis of SDC 'B' "building" structures to neglect vertical seismic load effects in Chapter 12. These exclusions do not show up in Chapters 13 & 15 but might be applicable where they tie back to Chapter 12. (I didn't dive that deep into it.) The lateral E[sub]h[/sub] force is still applicable for SDC 'B' and higher.

Exception #2b. excludes only the -E[sub]v[/sub] (upward) component in Eqn 12.4-2 E = E[sub]h[/sub] -E[sub]v[/sub] for demands on bearing pressure (soil/structure interaction) but not for the design of the building. I take soil/structure interaction to include bearing pressure, overturning and sliding cases. Exclusion 2b does not exclude (neglect) -E[sub]v[/sub] in Eqn 12.4-6 E[sub]m[/sub] = E[sub]mh[/sub] -E[sub]v[/sub] for load cases including seismic overstrength used for anchorage calcs. Since using -E[sub]v[/sub] reduces the vertical resistance to uplift on anchor bolts/rods, it makes sense. It also does not exclude +E[sub]v[/sub] in Eqns 12.4-1 and 12.4-5 so the +downward seismic force needs to be included in the analysis.

With regards to Factors of Safety (FoS) for sliding and overturning. If you are using ASCE 7, §2.4 ASD Load combinations #7 and #10 with 0.6D then you do not apply the "old school" FoS for Sliding and Overturning since the value 0.6D inherently reduces the load. Or put another way, using the inverse of 0.6D provides an equivalent FoS = 1/0.6 = 1.67. The FoS's of 1.5 or 2.0 can be used with Alternative Load Cases using 1.0D if you want to do that in place of ASCE 7 Load Combinations. If you are using 0.6D loads then checking O'Turn/Resistance > 1.5 you are in essence doubling up on your factor of safety. If you include Passive Soil pressure for sliding resistance, then the value 0.6H is used in accordance with §2.4.1 where it talks about H lateral earth pressures. H is not used in vertical bearing pressures.

In your case it sounds like you are higher than SDC 'B' so Exclusions 2a is not applicable.

Also, and more importantly, why are you using Chapter 12 for the design of a foundation for a Component which is covered by Chapter 13 for components and Chpater 15 for "Other" structures? If you are SDC 'C' or less you might be exempt from seismic design altogether.

 

[DanielSgeo]

Out of curiosity where are you that you have PGA.m > 2g?

 

[RDR89]

Jumping on this thread as I am dealing with something similar. @Electric_Steel were you able to dig any deeper?

@ZiggyKS §12.4.2.2 also has exception 2B which specifically allows for "soil-structure interface of foundations"... I guess my main question is, do you design an equipment foundation using chapter 13/15 but invoking the exception for sliding/overturning/bearing pressure (presumably the soil-structure interface conditions as you stated)? Or do you not consider the exception at all?

 

[Electric_Steel]

Sorry all - I ended up cross posting on the general section and forgot to come back here and provide responses. If you want to check out the other post, you can find it here:

Link

A few things specific to this thread though:

@ggcdn - I like this document. I think it's at least evidence of industry consensus on the use of this exception for mat slabs.

@ziggyKS - I appreciate the thoughtful response. Generally we feel that 15.1.3 points back to section 12.8. Similar to what you mention, we feel that this suggests that the exceptions in 12.4 are "tied" to chapter 15. That said, I definitely see how that could be up for debate. Which is why I really wish the commentary explained the "why" so I could use some engineering judgement. We are using 0.6DL + 0.7E and therefore only checking against a 1.0 "safety factor". We're still not safe even when considering that. We are ~ SDC E, so no way to get out of it there.

@DanielSgeo - We're in the bootheel of Missourah. Specifically the New Madrid fault region. Having worked in both... the seismicity in this region can put the SF Bay area to shame - especially given the relatively poor soils in the region.

@RDR89 - No luck going deeper. I've functionally hit a dead end from a literature perspective. The NEHRP document referenced by ggcdn is the best I've seen thus far. That said, I might be looking in the wrong places. For our projects I've been advocating for using this exception since it's pretty clear at this point that sliding and overturning is a "soil-structure" interaction. From a practical perspective, I don't see how calculating the stability of a mat slab under a building vs a piece of equipment is meaningfully different. Not iron clad, but enough for me to sleep at night especially for a foundation that poses little risk to life safety.

--ES

 

[RDR89]

Thanks for sharing the link to the other thread. I’ve noticed an older engineer here include the -Ev term for sliding and overturning and I’ve been digging deep into the code to try to figure out why, besides “it’s conservative”. That thread was a very helpful read just to see how others have handled it. Appreciate everyone’s input on it!