ASCE 7-05 Section 15.6.1

[bjb]

I have this also posted in the ASCE Code section, but haven't gotten any responses.

Section 15.6.1 starts by saying that is applies to all earth retaining structures assigned to Seismic Design Category D, E, or F. It then goes on to say that lateral earth pressures due to earthquake ground motions shall be determined in accordance with Section 11.8.3 for Seismic Design Categories B, C, D, E, and F.

Do only earth retaining structures in design categories D, E, or F require seismic design, or is seismic design also requried for earth retaining structures in seismic design categories B and C?

According to Section 15.1.1, it seems that all earth retaining structures require seismic design, so I interpret ASCE 7-05 to require that earth retaining structures in any seismic design category to require a seismic design.

 

[JAE]

I think any structure design must consider seismic. Section 15.6.1 does seem weird in referencing D, E, and F and then immediately in the next sentence referencing also B and C.

 

[DaveAtkins]

The reference to SDC B and C was eliminated in ASCE 7-10.

DaveAtkins

 

[MJB315]

So does that mean the intent is now to not consider seismic loading for SDC B & C?

"We shape our buildings, thereafter they shape us." -WSC

 

[bjb]

It seems to me that retaining walls need to be designed for seismic forces per Sections 15.1.1 and 15.6. The Q&A guide to the 2000 IBS does say that retaining walls are to be desinged for seismic loads.

I think the intent of Section 15.6.1 was to require determination of seismic forces for retaining walls in the more severe seismic design categories by a geotechnical engineer, but that all retaining walls need to be designed for seismic. Presumably retaining walls in the less severe can have seismic loads determined by the structural engineer.

 

[JoshPlumSE]

The calculation of seismic forces for Retaining walls is not very well defined. Which is probably why the code points you back to 11.8.3 (which basically says that the geotech is responsible for defining these forces).

I'm going back some years now, but I remember going through this in detail when studying for the PE or SE exam. Back then, there was very little information about retaining walls and seismic forces. The only method that I found was Monobe-Okabe which was viewed as being extremely conservative. Especially given the lack of observed retaining wall failures in past earthquake investigations. At least the lack of failures in engineerdd retaining walls.

 

[bjb]

I typically use Mononobe-Okabe for cantilevered walls. For walls that are supported laterally top and bottom, I use Wood's method as described in "The Seismic Design of Waterfront Retaining Structures." I have a PDF of this, but I don't know how to upload it.

 

[msquared48]

I base the loads for seismic on the recommendations of the Geotechnical Report. If it does not contain those parameters, I ask for them.

I have seem many pressure prisms of various shapes based on the different site conditions, so I leave the specs to the experts here and do not try to second guess the induced forces.

Mike McCann
MMC Engineering