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Maximum Steel Ratio

Maximum Steel Ratio

Is there any place other than the PCA notes on ACI 318 to find a chart for the allowable maximum steel ratios that insure a ductile failure.  I forget the table designation and my newly ordered manual is still a few days away.  Unfortunalty I need an answer either today or early tomorrow if possible.  Thanks for any help.

RE: Maximum Steel Ratio

If you are working off a 1999 or earlier version, the max ratio is 0.75 or 0.5 (seismic zones) of the balanced ratio. Balanced ratio is ((0.85?1f’c)/fy)(87000/(87000+fy)). The reinforcement limit of 0.75 of the balanced ratio results in a net tensile strain at nominal strength of 0.00376. 2002 and later versions require a net tensile strain at nominal strength of 0.004 for flexural members with an axial load less than 0.10f’cAg. I haven’t read 318-05, but I do know the ACI has proposed newer ways to actually measure ductility. For low rise and uniform-shape buildings, I typically use a reinf ratio of 0.01. This results in a fairly ductile and economical design.

RE: Maximum Steel Ratio

Sections 10.3.4 and 10.3.5 of ACI 318-02 require different values of steel strains at nominal strength and reduce the strength reduction factor for the lower values.
As a rule of thumb, if your steel ratio is greater than .017, you're probably over-reinforced and should explore a deeper member.

RE: Maximum Steel Ratio

Thanks for the response.  I am sorry I was late in writing a reply.  I was actually trying to save myself a little work by using the pre-calculated values given in the PCA table.  For instance, if I remember correctly, according to the table when fy = 60,000 psi and f'c = 4,000 psi the maximum allowable steel ratio is 0.02125.  They had it tablulated for all the combinations.  Anyway, I did it the long way now so all is well.  Thanks again for the help.  I'll give you an exact Table number when my manual arrives in the mail.

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