Limitations of ASCE 7-10 12.13.4 25% reduction in overturning effects

axiomata · Jul 14, 2016

ASCE reads:

12.13.4 Reduction of Foundation Overturning. Overturning
effects at the soil–foundation interface are permitted to be
reduced by 25% for foundations of structures that satisfy both
of the following conditions:
a. The structure is designed in accordance with the equivalent
lateral force analysis as set forth in Section 12.8.
b. The structure is not an inverted pendulum or cantilevered
column type structure.
Overturning effects at the soil–foundation interface are per-
mitted to be reduced by 10% for foundations of structures
designed in accordance with the modal analysis requirements of
Section 12.9.

NEHRP explains:

2009 NEHRP provisions:
C12.13.4 Reduction of Foundation Overturning. Since the vertical distribution of forces prescribed for use with the
equivalent lateral force procedure is intended to envelope story shears, overturning moments are exaggerated. (See
Section C12.13.3.) Such moments will be lower where multiple modes respond, so a 25 percent reduction is permitted for
design (strength and stability) of the foundation using this procedure. This reduction is not permitted for inverted pendulum
or cantilevered column type structures, which typically have a single mode of response.
Since the modal response spectrum analysis procedure more accurately reflects the actual distribution of shears and
overturning moments, the permitted reduction is only 10 percent.

Question:
For the 25% reduction, are the requirements that the structure be analyzed per ELF and that it not be an inverted pendulum or cantilevered column type structure sufficient to meet the intent of the NEHRP commentary? Specifically, consider a single story, single DOF moment frame set on top of a couple spread footings. I believe the ELF procedure is pretty accurate in this case with all the load applied at the top at a single DOF and without multiple stories to envelope. Can you still take 25% off in this case? What about a series of single story moment frames on spread footings that are tied together at the top?

mike20793 · Jul 15, 2016

Yes, you are correct about the fist part. Moment frames aren't single DOF systems, but yes, you can apply the reduction to these footings. The commentary you quoted explains why the reduction is possible; it's due to the ELF enveloping story shears and being inherently conservative.

A single DOF system would include cantilever columns or inverted pendulums, which is prohibited because it is inherently not as conservative to use ELF for these systems.

KootK · Jul 15, 2016

OP said:
Specifically, consider a single story, single DOF moment frame set on top of a couple spread footings...Can you still take 25% off in this case? What about a series of single story moment frames on spread footings that are tied together at the top?

It's an interesting point that you raise. My take:

1) Based on a strict reading of the ASCE provision, the 25% reduction would apply.

2) Rationally, I would say that one story moment frames would not meet the intent of the NEHRP commentary and should not qualify for the 25% reduction. Single story moment frames will exhibit single mode responses to lateral loads, just like inverted pendulums and cantilevered columns. Moment frames embody greater redundancy but doesn't seem to have anything to do with these particular clauses.

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.

WillisV · Jul 15, 2016

That reduction is going the way of the Dodo bird very soon.

axiomata · Jul 15, 2016

WillisV-

How do you know it's going away? It's in the 2015 NEHRP and I couldn't find mention of it going away in ASCE 7-16. Also "very soon" is relative. Even if it gets in the 2016 ASCE 7, that'll be adopted in 2018 IBC which will be adopted by jurisdictions in 2019 if you're lucky.

KootK-

That's pretty much the page I'm on. It's just tough imagining having this discussion with the client:

Engr: "You will need a bigger foundation due to excessive seismic overturning bearing pressures"
Client: "Why? Can't you use that 25% knockoff I know you guys keep in your back pocket?"
Engr: "Well technically per the plain reading of the code yeah, but in this particular case I don't think it's a good idea."
Client: "Do you think you're smarter than the code committee?"
Engr: "... No ... but I can explain my rationale if you like."
Client: "That's not necessary. You know it won't be too hard to find another engineer to approve the foundation per the code."
Engr: "You're probably right."

Any documentation of it going the way of the dodo would help.

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Limitations of ASCE 7-10 12.13.4 25% reduction in overturning effects

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