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Re: CMU Infills

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The bottom line is that CMU infill working in conjunction with a concrete moment-resisting frame is not a good lateral-load resisting system.
The stiffness and deformation compatibility between the systems would be a nightmare to capture correctly using linear analysis methods - let alone pushover analysis that Alex inquired about a few weeks ago.  On the construction side of the equation, detailing the appropriate reinforcement to literally force these two disparate systems to work together seems that it would be time consumptive and difficult to build.
The point of the referenced MSJC provisions seem to me to serve as reminders about good seismic detailing, not blank checks that indicate this type of construction is seismically smart.
-Ben Maxwell

"Gerard Madden, SE" <gmse4603(--nospam--at)> wrote:
Good Info Daniel. thanks!

So MSJC is very specific on this particular issue, where as the IBC makes it basically a partition wall to be excluded from the LFRS (3 sides isolated....i.e. only the base is connected to something)


On Jan 28, 2008 3:58 PM, Daniel Popp <drp181(--nospam--at)> wrote:
The code does address the issue of CMU infill walls in several specific ways:

IBC 2006 Section 2106.1 (Masonry - Seismic Design)  ..."All masonry walls, unless isolated on three edges from in-plane motion of the basic structural systems, shall be considered to be part of the seismic-force-resisting system"...

MSJC 2002 Section 1.10.2 (Connection to structural frames)  "Masonry walls shall not be connected to structural frames unless the connections and walls are designed to resist design interconnecting forces and to accommodate calculated deflections."

MSJC 2002 Section (Design of elements that are not part of the lateral force-resisting system)  "Masonry partition walls, masonry screen walls and other masonry elements that are not designed to resist vertical or lateral loads, other than those induced by their own mass, shall be isolated from the structure so that vertical and lateral forces are not imparted to these elements.  Isolation joints and connectors between these elements and the structure shall be designed to accommodate the design story drift."

Ralph has a very good point:  Beware partial-height CMU infill.  I had a professor in graduate school who referred to partial-height infill walls as "baby killers".  It seems that in some countries, this was (or is) a common type of construction for schools, with windows above the masonry.  When an earthquake hits, the inelastic deformation of the column is limited to the portion above the infill, which can lead to collapse.  We were shown a picture of a two-story school building that had formerly been three stories.  If school had been in session...


----- Original Message ----
From: "Gerard Madden, SE" <gmse4603(--nospam--at)>
To: seaint(--nospam--at)
Sent: Monday, January 28, 2008 12:56:11 PM
Subject: Re: CMU Infills

I think it's sort of taken care of by not having it listed as a recognized LFRS in the structural system tables in the code.

Then there are code provisions about interactions of different systems within a same direction of a building, on different and same lines of resistance, and by story.

It's in there (i.e., you need to model the building...if the infill walls are in contact with moment frames, your model better reflect that), but it just takes digging rather than blatantly saying in Code Commandment #  "Thou shall not use CMU infill walls in combination with Concrete Moment Frames.

I guess the point is, rather than try to capture every possible system name, the code attempts to say "you can't do this unless..." in more general terms


On Jan 28, 2008 2:15 PM, Alex C. Nacionales <anacionales(--nospam--at)> wrote:
Designing the frames for forces that will be created due engaging the infill CMU "infill" walls will be expensive. Creating a space between CMU infill  walls seems awkward. I wonder why a dangerous issue such as this was not taken care of by any structural code.
Alex C. Nacionales 
----- Original Message -----
Sent: Saturday, January 26, 2008 6:38 AM
Subject: RE: CMU Infills

I don't recall if there are actual code provisions, but there are certainly FEMA documents that discussion the issue of masonry infill walls in frames and how to retrofit or even design for them.  If I recall correctly, the main thing to try to do is leave enough isolation room between the frame and the infill walls to allow the frame to deflect as "desired" without "engaging" the infill wall...OR design the frame for the types of forces that will be created due to "engaging" the infill walls.
Adrian, MI
-----Original Message-----
From: Alex C. Nacionales [mailto:anacionales(--nospam--at)]
Sent: Friday, January 25, 2008 5:02 PM
To: seaint(--nospam--at)
Subject: CMU Infills

Dear Fellow Engineers,
     My RC design teacher has taught us has to ignore the effect of CMU infills on a RC frame because they will be crushed during a seismic event. Recent seminars I attended shows that cmu infills interferes with  frame action thus attracting more shear. Is there any code provision regarding on how to treat cmu infills in relation with RC frames or steel frames?
       Your assistance will much appreciated. Thanks.
     Alex C. Nacionales.



Benjamin H. Maxwell, S.E.

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