Aci 318 Stiffness Modifier 3c5y71

ACI 318-11 Code Sections for Stiffness Modifiers Factors: --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------There are two seperate analysis you will be doing of your 3D Concrete Frame: 1) At Service Load Combinations:-For Frame Drift, Member Deflections and Foundation Pressures (Soil Bearing Stress): Based on section 8.8.1: a) For Columns Use Iyy and Izz = 1.4X0.7Ig = 0.98Ig = Ig (at Service Load combinations you can use Ig (uncracked) for Columns. b) For Beams (Flexure) Use Iyy and Izz = 1.4X0.35Ig = 0.49Ig (say 0.5Ig or 50% of gross Moment of Inertia for beams. Think of this even at Service Loads analysis ACI Code wants you use only 50%Ig for Beams in Frame Analysis. This will change your frame Drift, Member Deflections and may impact your Frame Member Sizes to get to the allowable Drift, Member Deflections Values. STAAD/PRO Tech. Reference Manual Section 3.3 has the same values of Concrete Cracked Section Properties for Beams (50% Ig) for many years. 2) At Factored Load Combinations:-- For Member Design Forces and Reinforcement Design. a) For Columns use Iyy and Izz = 0.7Ig No reduction in Ixx (Torsion) and Ax, AY , AZ b) For Beams (Flexure) use Iyy and Izz = 0.35Ig No reduction in Ixx (Torsion) and Ax, AY , AZ If you want Code also allows you to use Iyy and Izz = 0.5Ig for both Columns and Beams based on section 8.8.2(b). Point is using Gross Moment of Inertia Ig for Concrete Frame Analysis at Factored Load Combinations is simply wrong. Some of us engineers still have not caught up with this change and use Gross Moment Of Inertia (Ig) for beams at Service Load and Factored Loads. Based on ACI 318 Code we can not and should not do that.