Seismic R Value 1

[msquared48]

Doing a remodel on a 100,0000 sq foot concrete tilt structure done under the 1994 UBC where an "Rw" of 6.0 was used for a concrete tilt-up wall structure, classed as a bearing wall wywtem with a concrete shear wall lateral system. All well and good.

Now move ahead to the 2009 IBC and ASCE 7-05 where, there is an apparent distinction now between precast and reinforced concrete shear walls.

In Table 12.2-1 of ASCE 7-05:

B5 and B6 refer to Special and Ordinary reinforced concrete shear walls with respective R values of 6 and 5. But for precast concrete walls, under B9 and B10, the value for Intermediate and Ordinary precast walls are 5 and 4.

So, I guess the question is, is a Concrete Tilt-Up wall system considered Precast or not? To me, it looks like a code change with slightly higher lateral values under the newer codes.

Also, what is considered "Intermediate" as opposed to "Ordinary". How would the detailing be different? Closed shear reinforcing?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Nonlinearer]

I don not know much about Tilt-up wall system but I think as the name implies, it is precast system of walls. Regarding ordinary & Intermediate walls, refer ACI-318-11, chapter 21.
Generally, minimum strength of Intermediate precast wall is equivalent to Ordinary Cast-in-Place reinforced wall. Therefore, you can find their 'R' values are also same in ASCE 7-10. Whereas for Special walls, the Code demands same strength, ductility and toughness, for Cast-in-Place and Precast walls.

IR

 

[DST148]

Yes. Tilt-up wall would be considered precast for determining seismic design parameters.
Per ASCE 7-05, Ordinary Precast Structural Wall - A precast wall complying with the requirements of Chapters 1 thru' 18 of ACI. (ACI does not include definition of Ordinary Precast Structural Wall)
Per ACI 318-05 - Intermediate Precast structural wall - A wall complying with applicable requirements of Chapters 1 thru' 18 in addition to Section 21.13.
Moreover, UBC-94 gave seismic forces at service level, IBC gives at strength level.

 

[HuckleberryFinn]

Mike,

It's been a bit since I was taught about these walls but I think it has to do with the level of detailing of the walls boundary elements.

A quick google search revealed a FEMA document that looks pretty good (although bear in mind they are referencing 318-05 instead of 08 when they hit on the detailing information):

http://www.nibs.org/client/assets/files/bssc/Topic11-SeismicDesignofReinforcedConcreteStructures.pdf

Personally, I vote for any wall system that is modular in construction as being classified as "precast" but that's just me.

-Huck

 

[nac521]

Some clarity here from a 2008 StructureMag article: Tilt-Up Building Seismic Design

http://www.structuremag.org/article.aspx?articleID=514