Could you tell me about the values used in interactive concrete frame design?
Extended Question: I am trying to compare the moment required by ASCE 31-03 load combinations to the moment capacity of reinforced-concrete columns. How do I find moment capacity?
Answer: Section 3.4.2.3 of the ETABS Concrete Frame Design Manual (ACI 318-08) describes capacity-ratio formulation in detail.
Additional notes are as follows:
- Moment capacity may be back-calculated from capacity ratio and design moment. Ensure that the capacity ratio is less than 1.0. If the column is failing, increase reinforcement until this condition is met. As an alternative, you may use the maximum probable moment, but keep in mind that maximum probable capacity is calculated using phi=1.
- Design Mu2 and Mu3 are factored moments which result from the higher of governing load combinations or Mumin. Capacity moment (maximum probable moment) is for columns defined as special seismic.
- The shear in special seismic columns is checked according to maximum probable moments (Mpr+ and Mpr-) under the influence of axial force Pu using phi=1, as required by code. Maximum probable moment is per shear detailing for special frames.
- To factor moment (per ACI 318 Section 9.3.2), phi varies from 0.7 for compression control to 0.9 for tension control.
- Minimum Mu is based on minimum eccentricity and is calculated according to ACI 318 10.10.6.5.