CMU h/r ratio use r-net or r-avg?

[azureblue83]

This should be a quick question for the experienced CMU forks.

In ACI 530, par 3.3.4.1.1, to determine the axial strength, we need the h/r ratio. I tho the r should be the r-net, because usually net sectional properties are used for stress/strain computation, while average sectional properties are used for deflection/stiffness computation.

But then in TEK 14-1, the tables for CMU properties, they listed r-average but does not list r-net, which seems to imply you should use r-average in h/r. (They listed A-net, I-net, S-net, but did not list r-net. They listed A-avg, I-avg, S-avg, and r-avg).

Please help me clarify.

Thanks.

 

[RPMG]

Calculate r = sqrt(I/A) for net sectional properties. Include grout.

 

[concretemasonry]

If you are doing concrete masonry design, it would be prudent to get accurate information on the f'm of the units you might be using and specific shape configurations and basic thicknesses (face shell thickness and web thickness). The f'm of the units you actually get will probably be at least 25% higher than the woeful ASTM C90 requirements and may be 50% to 60% above ASTM. The manufacturing process has surpassed the changes in ASTM. It is not the simplistic concept of adding cement. Cement is not the most costly cost element in the cost of a concrete block. Adding more water is more effective way to get more strength.- It is not like ready-mix concrete, since it is a factory produced product and sold from inventory.

Finding the actual properties is far easier than requiring a stronger unit despite the low cost of doubling the for a specific project. I have seen an f'm of over 4500 psi as opposed the usually assumed 1900 psi. That also goes a long way to toward a realistic idea of the action of the structure.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.