Seismic response modification coefficient, R, for walls in bearing vs building frame structure 3

[Ingenuity]

For a floor framing system consisting of a PT flat plate supported by RC columns and structural walls, in seismic design category C, where the walls are designated as "special reinforced concrete shear walls" with an R of 6 (building frame system) should not this framing type be classified as a "bearing wall system" according to ASCE 7-2005, and hence R of 5 is appropriate?

Building Frame System: A structural system with an essentially complete space frame providing support for vertical loads. Seismic force resistance is provided by shear walls or braced frames.

 

[KootK]

Not the easiest question to answer these days. Some excellent discussion here: Link. And a blurb from one of the NEHRP design briefs..

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.

 

[Teguci]

You know you're in trouble when the answer is "ask the building official."

This is from FEMA Pub 695 - "Precision of Seismic Performance Factors
In general, there is no practical difference in the collapse performance of
systems designed with fractional differences in the response modification
coefficient, R. For example, collapse performance of structures designed for
R = 6 and R = 6.5 is essentially the same, all else being equal. There is a
discernible, but modest difference in collapse performance for systems
designed for moderately different values of R, for example R = 6 and R = 8.
There is, however, a significant difference in collapse performance for
systems designed using different multiples of R, as in R = 3 versus R = 6.
Current values of R provided in Table 12.2-1 (e.g., 3, 3-1/4 and 3-1/2) reflect
a degree of precision that is not supported by results of example collapse
evaluations." -

http://www.fema.gov/media-library-data/20130726-1716-25045-9655/fema_p695.pdf

I've put this question to several experts and have received varying answers. I would not expect a P/T deck on a bearing wall system (a mess of slip joints) and just from your little blurb, I'd lean towards calling this structure a "Building frame system."

For SDC C I'd think this voluntary extra detailing (Special) is being required just to reduce seismic loads on a deep foundation.

 

[Ingenuity]

A star to you both.

Excellent references.

Much appreciated.

 

[wannabeSE]

In chapter 11 of ASCE 7-10, the definition for bearing wall is more definitive (See Wall, Bearing wall on page 61)

Bearing Wall: Any wall meeting either of the following classifications:
1. Any metal or wood stud wall that supports more than 100 lb/linear ft (1,459 N/m) of vertical load in addition to its own weight.
2. Any concrete or masonry wall that supports more than 200 lb/linear ft (2,919 N/m) of vertical load in addition to its own weight.

 

[Ingenuity]

Thanks, wannabeSE. I did notice that definition in ASCE 7 based upon applied loading.

My experience with the definition of "building frame" dates back to a hospital I designed in Canada back in the late 80's/early 90's where we used RC frames with ductile detailing, checking joint capacities, rebar congestion etc, so I was surprised when I recently saw this project where the EoR noted the use of an increased R with what we used to call a "bearing wall" system of years past. I guess, like Teguci's reference quotes there is not much difference between a 5 and a 6 in terms of performance.

 

[KootK]

I don't think that the 200 lb definition applies here. See the last sentence of the blue box in my previous post.

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.

 

[Ingenuity]

I don't think that the 200 lb definition applies here. See the last sentence of the blue box in my previous post.

Ok, thank you for the note.

Damn, you would think the code writers would get together and unify the nomenclature and definitions. What is an extra few pages to the current 658 pages that make up ASCE 7-10!