ACI 350 Parametric Design Guide/Steps


The following is a parametric design guide to the design of concrete tanks in general accordance with ACI 350.

These steps would include:

  • Determine the FACTORED AND THE SERVICE moments in the member.
  • Determine the distance “a” which is the depth of the compression block under service loads.
  • Calculate the moment arm between the reinforcement and the center of the compression block (half-way across dimension a).
  • Calculate the actual fs as the service moment divided by the moment arm, divided by the area of reinforcement.
  • Calculate the maximum reinforcement stress, fs(max) from ACI 350 section 10.6.4. This is based on equations 10-4 or 10-5 and is called the PERMISSIBLE STRESS fs(max)
  • Verify that actual fs is less than the permissible stress fs(max) .
  • Calculate the maximum permissible fs (fs(permissible)) from 10.6.4. (this item is redundant as step 5 already had it)
  • Calculate Sd using the Permissible fs(max), your phi factor, your γ load factor, and fy
  • Your design factored moment is your factored moment times the Sd factor. No other load factors should be used. (I previously had this as service moment and was incorrect - Sd is applied to U and U is which is a factored effect such as Mu)
  • Use your SdMu value with phi = 1.0 and design as you would any other concrete member. Sd varies from a low of 1.0 to perhaps upwards of 3.0 should you use large bars with large spacings.