Seismic R value less than 5

[TheLinker]

I am looking through a design example in the NEHRP (FEMA 451) Recommended Provisions Aug 2006 and it states that “if a value of R in either direction is less than 5, the smaller value of R must be used in both directions”.

It references NEHRP 2000 sec 5.2.2.2.1 (2003 sec 4.3.1.2) for this provision. I cannot find any such provision in ASCE 7-05 chapter 12.2. Does anyone know if this is / was an ASCE seismic design requirement?

For the project I am currently looking at, this provision will not govern, but I am just curious for future situations if I should be aware of the R less than 5 rule.

 

[JAE]

Never seen anything like that in ASCE 7.

 

[Qshake]

I can't say i've seen it either in ASCE 7 but conceptually it makes sense. No sense in having a more ductile (more displacement) support in one direction so that it creates bracing or support loss for the less ductile direction.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[msquared48]

If the floor and roof diaphragms are rigid, I can see the logic considering the torsional aspects, but not for flexible diaphragms where there is no torsion.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JoshPlumSE]

The 2003 NEHRP says that this only applies to systems other than dual systems. In addition, the commentary says the following:

4.3.1.2 Combinations of framing systems. For those cases where combinations of structural systems are employed, the designer must use judgment in selecting appropriate R, ?0, and Cd values. The intent of Sec. 4.3.1.2.1 is to prohibit support of one system by another possessing characteristics that result in a lower base shear factor. The entire system should be designed for the higher seismic shear as the provision stipulates. The exception is included to permit the use of such systems as a braced frame penthouse on a moment frame building in which the mass of the penthouse does not represent a significant portion of the total building and, thus, would not materially affect the overall response to earthquake motions.


Lastly, it does not apply to single and two family light frame construction. At least not in the 2003 version.