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RE: ACI 530 R=1.5

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This is interesting, but I would interpret this to be a little different.  The provision is an exception to the requirement that at least 80% of the stiffness is provided by walls for the story level.  The other 20% has to be something.  If 80% of the lateral stiffness is not walls, then an R of 1.5 would allow you to use piers and columns which is essentially a cantilevered system and therefore makes sense.  But if 80% of the lateral stiffness is walls, and you happen to have a couple piers between openings that require design for their contribution to the overall wall, I would not take that as requiring an R of 1.5. 
 
Also, as you point out, if you are designing to ASD, this provision does not apply.  You are not required to use 1.5V in strength design either, so each system has different mechanisms to limit design and set minimum levels.  If it is a cantilever system you would already be using the lower R factor from the IBC.
 
What are you designing?
 
Paul Feather PE, SE
pfeather(--nospam--at)SE-Solutions.net
www.SE-Solutions.net
 
 


From: Rick [mailto:mohr2circle(--nospam--at)yahoo.com]
Sent: Thursday, February 05, 2009 8:43 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: ACI 530 R=1.5

Look at ACI 530 3.1.3.1... it limits the R to 1.5, but only in the strength section.  City of LA code references 530 to answer the other section.
 
I saw 3.3.3.5, and thought thats what they were origianlly referring to, but 3.1.3.1 is pretty clear. But nothing in ASD
 
Rick

--- On Thu, 2/5/09, Paul Feather <PFeather(--nospam--at)se-solutions.net> wrote:
From: Paul Feather <PFeather(--nospam--at)se-solutions.net>
Subject: RE: ACI 530 R=1.5
To: seaint(--nospam--at)seaint.org
Date: Thursday, February 5, 2009, 8:35 AM

ACI 530 does not set or establish R factors.  There are only two possible explanations that I am aware of: first if using strength design there is a provision for maximum area of flexural tensile reinforcement (3.3.3.5) where M/Vd greater or less than 1 and IF the design used an R of less than or greater than 1.5 is a trigger for additional design requirements.  This is not an R factor limitation, just a trigger for other provisions.  Second would be if they are somehow mis-interpreting the shear force required of 1.5V in ASD design, but this is an IBC requirement and not in ACI 530 either.
 
I would ask the Plan reviewer to specify which code provision he / she is referencing with his comment.
 
Paul Feather PE, SE
 
 


From: Rick [mailto:mohr2circle(--nospam--at)yahoo.com]
Sent: Thursday, February 05, 2009 7:58 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: ACI 530 R=1.5

I have a City of LA Plan check comemnt that R = 1.5 "must" be used for masonry walls piers per ACI  530.
 
Can someone point me to the specific reference.