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Re: CMU piers & columns vs WALL

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Andrew,
I ran into this problem on a job where another engineer criticized my design. So I sought advice from the Canadian Portland Cement Assoc'n ( now Concrete Assoc of Canada) and their advice was that if the column or wall can take all of the axial and shear loads as plain masonry or concrete then it does not need to meet the code requirements for reinforced concrete columns. The moment forces just make it a beam. Best of luck.
Gary

Andrew Kester, P.E. wrote:
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

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