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RE: CMU piers & columns vs WALL

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I agree with you on a column being described as an isolated member with no walls tying in to it.  I see no provisions for designing walls for axial loads so it looks like you have a pilaster by default.  Code commentary Fig. 2.1-13 shows a "flush" pilaster which would cover a regular wall without additional masonry protruding from the wall.  Section 2.1.7.2 for pilasters states that "Where vertical reinforcement is provided to resist axial compressive stress, lateral ties shall meet all..........etc".   So, if your masonry can carry the load by itself, I interpret that to mean there is no need for reinforcement for axial load and thus no ties required.  Commentary 2.1.7 also says that pilasters can serve one of several purposes.  So if your wall (flush pilaster) has vertical steel for wall bending, this is not for axial loads and no ties required.

 

Anyway, that's my interpretation.  I'm doing some CMU design now myself and I'm using the CMD 06 programs from CMACN.  They have a module for designing walls for axial and out of plane loads so that's what I am using to analyze some walls with concentrated beam loads on them.  This program module considers the axial/bending stress interaction.  It considers that masonry must take all the out of plane shear with no ties or shear reinforcing.

 

Bob Garner, S.E.

 


From: Andrew Kester, P.E. [mailto:akester(--nospam--at)cfl.rr.com]
Sent: Thursday, July 10, 2008 10:30 AM
To: seaint
Subject: CMU piers & columns vs WALL

 

This topic came up a few weeks ago, and I have some situations I am reviewing that have caused me to thoroughly study ACI 530...

 

Simply defined by geometry:

column: isolated vertical member with a width< 3*t

pier: 3*t < width < 6*t

 

So for 8" block, an 8x8 thru 8x24 member is column, and up to an 8x48 member is a pier, if it is an ISOLATED member, of which I have not found a definition. My thinking is if the member is used as a column to support a carport, canopy, porch, etc. and is out there by itself, it is ISOLATED. But if you have a wall with two openings to each side, and it is 16" wide and supports pre-cast lintels to each side as part of a wall system, then is this isolated? My opinion is no.

 

What I am getting at is that if it is considered a column you have to supply (4) vert bars and lateral ties, tough to do in a 8x16 or 8x24 block column, since you would have to have ties at 8" oc vert also. Really tough to build. The commentary says the column requirements are based on ACI concrete column requirements, and are there to provide confinement to prevent the vert bars from buckling, and for shear reinforcement. But what if I am not using the rebar for compression and the grouted masonry easily can carry the axial load, and there is no shear going into the pier? Is it really a column or acting as a column as I believe the Code's intention to be??

 

Besides my isolated member example, which is a cause of concern, are wall elements that meet the geometry requirements required to be designed as columns? If so, this means a min of 4 bars and ties inside a wall. I have never seen this done in Florida (non seismic), and was never instructed to design it this way. In most of my projects, CMU bearing walls are normally rather lightly loaded in axial compression (low rise), the rebar is there for out of plane wind loading and eccentric gravity loading, and thus they would not be treated as columns or piers but as wall elements....

 

My calcs in all cases show the wall segment, pier, or column (whatever it is) works for combined axial (light and carried by masonry only) and flexural compression, and I do not use the steel for anything but flexural reinforcement so I do not see a need for 4 bars, lateral ties, or shear reinforcement.

 

Thoughts and input would be appreciated!

 

Andrew

 

 

 

Andrew Kester, P.E.
Principal/Project Manager
ADK Structural Engineering, PLLC
1510 E. Colonial Drive, Suite 301
Orlando, FL 32803