Application of seismic forces

[hayeska]

I have a question regarding the application of seismic forces to earth retaining structures (gravity, cantilever, SRW etc.) used in non-building applications, commercial site application.

Building Code IBC 2006

Location: Northeast

Ss=0.232

S1=0.061

Specifically, should seismic forces be applied for such applications?

Thanks

 

[moe333]

Yes, use Mononobe-Okabe with 1/3 site PGA.

 

[hayeska]

Please see the following. It was taken from a RetainPro newsletter.

Also, see IBC 1613 below. If the wall is used for site grading, it is non-structural and not attached to any building structure.

Seismic kh factor, again
This was discussed in an earlier newsletter, but here is an update: IBC '06 requires retaining walls to be designed for seismic forces if Seismic Design Category D, E, or F applies (see Table 1613.5.6-1), and if SDS accelerations are 0.50g or greater. This is in Section 1802.2.7, which states that unless a site-specific report is provided by a geotechnical engineer, the value of kh (design acceleration) should be a ground acceleration equal to SDS / 2.5. This is further clarified in the NEHRP (FEMA) Handbook, 2003, Part 2, Commentary, Section 7.5.1. Retain Pro uses the Modified Mononobe-Okabe equation (modified Coulomb) to calculate seismic forces for cantilevered retaining walls, wherein the theta (?) factor used in the equation is tan-1 kh.. As those of you in high-risk seismic area have discovered, this added seismic force can nearly double the static force and triple the overturning moment, particularly if backfill is sloped. In our opinion this is an overly severe requirement due to the rare instances of seismic problems with retaining walls (waterfront structures subject to liquefaction excepted) and code requirements should be reevaluated. We will report more on this in our Fall newsletter. In the meantime, anyone with an opinion please email us -- we'd like your views and comments on this important issue.

IBC '06 1613 Earthquake Loads
1613.1 Scope
“Every structure, and portion thereof, including nonstructural components that are permanently attached to structures and their supports at attachments, shall be designed and constructed in accordance with ASCE 7, excluding Chapter 14 and Appendix 11A. The seismic design category for a structure is permitted to be determined in accordance with Section 1613 or ASCE 7.

Any thoughts or interpretations greatly appreciated.

 

[moe333]

Try this

http://www.ce.berkeley.edu/~lalatik/hudson.pdf

 

[hayeska]

Moe333,

Thanks for your responses.

I had previously downloaded the link you had posted.

It seems to say that the forces are applicable in SDC D,E,& F. Agreeing with the previous post, unless I am missing something.

My retaining wall structure could be classified as SDC B.

The following are from the link.

"FEMA 369 report (Part 2 –Commentary) contains almost five pages of commentary on the consideration of lateral pressures on earth retaining structures. Section 7.5.1 of the commentary states that “In addition to the potential site hazard discussed in Provisions Sec. 7.4.1, consideration of lateral pressures on earth retaining structures shall be included in investigations for Seismic Design Categories D, E, and F.”

"At the present time, it appears appropriate to use judgment in recommendations of seismic lateral earth pressures, especially for basement walls. For free-standing retaining walls, established methods can be employed to estimate the seismic lateral earth pressures, but the results should also be tempered in light of the response of actual retaining walls."

Thanks

 

[msucog]

i agree with the sds/2.5 (which is the PGA).

if you have a geotech, ask their opinion. they can probably provide you a great deal of assistance. to get a sdc b, the accelerations are probably "fairly low" anyway.

 

[hayeska]

msucog,

Thanks for the response.

The acceleration is low but does effect the results.

Determining the appropriate variables and their application

to the project is not the problem.

The question is really a code question.

Should retaining wall structures in SDC with Ss=0.232

S1=0.061 have seismic forces applied to them, following IBC

2006. I do not see it specifically addressed and some of

the information I have posted supports not applying it.

Thanks

 

[msucog]

you have seismic loads, so i believe that seismic loads should be considered. i don't think there are exceptions in ibc for your case. i might consider going for the 20% reduction (max allowed by ibc) assuming the parameters you listed are pulled from the ibc (psuedo usgs) maps.

i often see these seismic forces disregarded/overlooked since they are relatively insignificant (although they do contribute to some degree).

 

[moe333]

Using the PGA for your seismic coefficient is extremely conservative and is technically incorrect. Have a geotech. recommend an appropriate value (1/3 to 1/2 of the PGA) such as the code allows.

 

[msucog]

for the reduced pga, other "things" should be considered. i think 1/3 pga is too low and not appropriate since you're dealing with too many unknowns (you're not "building a watch...you're playing with dirt"). if you're going to use 1/3 then why bother evaluating it all together. 1/2 might be appropriate in some situations where displacement is acceptable. no reduction is more appropriate in most conditions in my opinion. the decision should be based on the many different aspects of the project, expected performance of the structure, expectations of the owner, permitted permanent deformation, etc. (in other words, the engineer of record must make an engineering judgement based on the project specific details)

i suggest using pga with no reduction unless someone takes a more detailed look at the scenario. for some situations, the geotech may only provide pga...the reduction would ultimately be the decision of the design professional responsible for designing the structure. for instance of an mse wall, the mse wall designer should choose the appropriate reduction...not the geotech unless they are responsible for designing the thing.

here is a very good link

http://www.bssconline.org/2nd09MOballot/Part3/TS-3 proposal 3-6_Geologic Hazards.pdf

that's my take on the topic at hand for what it's worth...

 

[hayeska]

MSUCOG/MOE333,

Thank you for your responses.

I have reviewed much literature regarding the application of the seismic lateral pressures to various structures.

The following is from the link MSUCOG posted.

"Determination of Lateral Pressures on Basement and Retaining Walls Due to Earthquake Motions Paragraph 1 of Section 11.8.3 requires that seismic lateral pressures on basement walls and free-standing retaining walls be determined for structures on SDC D through F, but does not specify the methods for calculating these pressures."

In my situation, I have a retaing wall for grading purposes only. SDC B, no life safety issues, not a highway structure, not connected to a building structure. In IBC 2006 it appears that applying the seismic force is not required. The text above and other posted text appears to support this assumption.

Regarding the application of the code only, what is your opinion of the assesment?

 

[moe333]

It appears you may not need to apply the seismic force based on your SDC, but you should confirm with your local governing agency.

I would reiterate that using PGA for the seismic coefficient is not correct. There is a large concensus on this issue. Try reading papers by Seed and Whitman, the one I referenced above, and a very clear discussion in the California Special Publication DMG 117 which you can download free.

 

[hayeska]

moe333,

Thank you for reference and your help on this topic.

 

[msucog]

interesting read

http://trb.org/news/blurb_detail.asp?id=9631

NCHRP 611

 

[hayeska]

Thanks msucog

 

[STVU]

Here is a spreadsheet for yielding walls.

http://www.soilstructure.com/spreadsheets.html

The original Seed & Whitman paper suggests Kh from 0.10 to 0.15 g. This is inline with moe333 suggestion & about one third of the PGA value.

 

[msucog]

one should realize the "substantial" displacement/deformation associated with that 1/3A assumption before simply using it. for situations where large displacement and deformation is not acceptable (which is quite often from my experience), 1/3 is not appropriate. this is echoed in the building code and nchrp links i posted above. i suggest nchrp 611 (parts 1 & 2) on the link posted.

analogy: the 1/3 assumption is sort of like using a 6000 psf bearing pressure for support of a footing. strength testing may indicate that the soil is fully capable of supporting 6000 psf but the foundation might experience 6 inches of settlement as a result. if the foundation cannot tolerate 6 inches of settlement, then you've got to shave that down to something more in line with the expected performance. if you don't care that the foundation is going to settle out of site and/or move all over the place, then use it. essentially it's a design choice by the wall designer based on the expected performance of the structure...but the client should be made aware by the designer that his wall may look ugly (not necessarily fall down) and anything on top of the wall might also experience movement, cracking, etc. at the end of the day.

 

[moe333]

In the end it comes down to how conservative do you want to be. The TRB link recommends about 1/2*PGA, and codes like these are always very conservative, but this is still much lower than what has previously been recommended in this thread (full PGA). If you want to be more realistic, 1/3 to 1/2 PGA as many of the experts have recommended is probably the number.

 

[msucog]

"more realistic"? the full pga should be used when substantial displacement/bulging is not acceptable. trb doesn't recommend 1/2 unless displacement, bulging, cracks, etc are acceptable. no one is saying that 1/2 can't be used...just simply that everyone should recognize the risks associated with doing so. in some instances, seismic forces may not even need to be considered. and codes are sometimes conservative but then that's where the wall designer can put his stamp saying that the design and associated assumptions are sound relative to the expected performance and actual site conditions.

i think we are both dancing around a similar idea at this point (maybe just approaching it from different directions)

 

[moe333]

Here is my final 2 cents on this thread; I completely disagree that you need to use the full PGA to avoid “substantial displacement/bulging”. As discussed in case histories, walls that were designed using a coefficient as low as 0.1 or 0.15 performed very well. So generally, 0.3 to 0.5 is going to result in very small deformations for the majority of cases.

Seismic deformations are a function of the static factor of safety, the yield acceleration, and the Horizontal Equivalent Acceleration (HEA). Since the soil is not a rigid body, and since the EQ pulses happen in both directions, HEA is much smaller than PGA, and the wall generally only feels HEA.

Deformations only accumulate where the HEA exceeds the yield acceleration, and the higher the static factor safety, the higher the yield acceleration. Most walls are designed with a relatively high factor of safety (1.5-2 at least), therefore small deformations. It’s not all about PGA. I suggest you review the Newmark sliding block type of analyses....I’m done.